Endothermy

ABSTRACT ‘Laminar’ bone, commonly found in artiodactyls and dinosaurs, is described. It consists of a series of bony laminae arranged like tree-rings round the marrow cavity, each lamina being between o-i and o-2 mm thick. Sandwiched between each lamina and the next is an effectively two-dimensional network of blood-vessels. Laminar bone grows in a series of spurts; a sheet of woven-fibred bone is formed very quickly, not actually on the pre-existing sub-periosteal surface, but separated from it by a space. The new bone is held clear of the original surface by an occasional bridge of bone. The space so formed is filled in slowly by parallel-fibred lamellar bone, and while this is going on more woven bone, with spaces, may be formed by the periosteum. The vascularization of laminar bone on the one hand and bone composed of haversian systems on the other is compared, chiefly in cattle. Laminar bone has a more intimate blood-supply than haversian bone, and it has a larger surface area of bloodchannel per unit volume. The volume of channel per unit volume is about equal in the two types of bone. The mean distance between anastomoses is less in laminar than in haversian bone. The cement-line round haversian systems hinders the passage of materials through the canaliculi. The osteocytes in the interstitial lamellae are thus in a worse position for obtaining foodstuffs and disposing of waste products than osteocytes in laminar bone a similar distance from the nearest blood-vessel. It is suggested that the formation of haversian systems in laminar bone interferes with the latter’s blood-supply and leads to the formation of further haversian systems.

@article{doi101242jcss310155351,
    author = "Currey, J.D.",
    title = "Differences in the Blood-supply of Bone of Different Histological Types",
    year = "1960",
    journal = "Journal of Cell Science",
    abstract = "ABSTRACT ‘Laminar’ bone, commonly found in artiodactyls and dinosaurs, is described. It consists of a series of bony laminae arranged like tree-rings round the marrow cavity, each lamina being between o-i and o-2 mm thick. Sandwiched between each lamina and the next is an effectively two-dimensional network of blood-vessels. Laminar bone grows in a series of spurts; a sheet of woven-fibred bone is formed very quickly, not actually on the pre-existing sub-periosteal surface, but separated from it by a space. The new bone is held clear of the original surface by an occasional bridge of bone. The space so formed is filled in slowly by parallel-fibred lamellar bone, and while this is going on more woven bone, with spaces, may be formed by the periosteum. The vascularization of laminar bone on the one hand and bone composed of haversian systems on the other is compared, chiefly in cattle. Laminar bone has a more intimate blood-supply than haversian bone, and it has a larger surface area of bloodchannel per unit volume. The volume of channel per unit volume is about equal in the two types of bone. The mean distance between anastomoses is less in laminar than in haversian bone. The cement-line round haversian systems hinders the passage of materials through the canaliculi. The osteocytes in the interstitial lamellae are thus in a worse position for obtaining foodstuffs and disposing of waste products than osteocytes in laminar bone a similar distance from the nearest blood-vessel. It is suggested that the formation of haversian systems in laminar bone interferes with the latter’s blood-supply and leads to the formation of further haversian systems.",
    url = "https://doi.org/10.1242/jcs.s3-101.55.351",
    doi = "10.1242/jcs.s3-101.55.351",
    openalex = "W1877864904"
}

@article{bakker1972anatomical,
    author = "BAKKER, ROBERT T.",
    title = "Anatomical and Ecological Evidence of Endothermy in Dinosaurs",
    year = "1972",
    journal = "Nature",
    url = "https://doi.org/10.1038/238081a0",
    doi = "10.1038/238081a0",
    number = "5359",
    openalex = "W2021172872",
    pages = "81-85",
    volume = "238",
    references = "doi101001jama196203050110085031, doi101111j155856461971tb01922x, doi101152ajplegacy197021941104, doi101515mamm19673111, doi1023071365733, doi1023071933240, doi1023072406945, doi1023073250, doi1023073799111, doi107208chicago97802267365700010001"
}

@misc{bakker1975dinosaur1,
    author = "Bakker, R. T",
    title = "Dinosaur Renaissance",
    year = "1975",
    howpublished = "Scientific American, v. 232, p. 58- 78",
    note = "talkorigins\_source = {true}; raw\_reference = {Bakker, R. T., 1975, Dinosaur Renaissance: Scientific American, v. 232, p. 58- 78.}"
}

@article{doi101038scientificamerican047558,
    author = "Bakker, Robert T.",
    title = "Dinosaur Renaissance",
    year = "1975",
    journal = "Scientific American",
    url = "https://doi.org/10.1038/scientificamerican0475-58",
    doi = "10.1038/scientificamerican0475-58",
    openalex = "W4236582943"
}

@book{desmond1976the4,
    author = "Desmond, A. J",
    title = "The Hot-Blooded Dinosaurs",
    year = "1976",
    publisher = "New York, The Dial Press",
    note = "talkorigins\_source = {true}; raw\_reference = {Desmond, A. J., 1976, The Hot-Blooded Dinosaurs: New York, The Dial Press.}"
}

@article{doi101038262207a0,
    author = "Seymour, R.",
    title = "Dinosaurs, endothermy and blood pressure",
    year = "1976",
    journal = "Nature",
    url = "https://www.semanticscholar.org/paper/2d312170ded1d99ba0aa823d24434ae9662ca9cd",
    doi = "10.1038/262207A0",
    is_oa = "true",
    number = "5565",
    pages = "207-208",
    semanticscholar_citation_count = "37",
    semanticscholar_id = "2d312170ded1d99ba0aa823d24434ae9662ca9cd",
    volume = "262"
}

@book{seymour1976dinosaurs8,
    author = "Seymour, R. S",
    title = "Dinosaurs, endothermy and blood pressure",
    year = "1976",
    publisher = "Nature, v. 262, p. 207-208",
    note = "talkorigins\_source = {true}; raw\_reference = {Seymour, R. S., 1976, Dinosaurs, endothermy and blood pressure: Nature, v. 262, p. 207-208.}"
}

@article{bouvier1977dinosaur,
    author = "Bouvier, Marianne",
    title = "DINOSAUR HAVERSIAN BONE AND ENDOTHERMY",
    year = "1977",
    journal = "Evolution",
    url = "https://doi.org/10.1111/j.1558-5646.1977.tb01028.x",
    doi = "10.1111/j.1558-5646.1977.tb01028.x",
    number = "2",
    openalex = "W2325766925",
    pages = "449-450",
    volume = "31",
    references = "bakker1972anatomical, doi1010160021929074900645, doi101017s0021859600040491, doi101242jcss310361111, doi1021060000462319705208000005, openalexw2247544283"
}

@misc{marx1978warmblooded6,
    author = "Marx, J. L",
    title = "Warm-blooded dinosaurs",
    year = "1978",
    howpublished = "Evidence pro and con: Science, v. 199, p. 1424-1426",
    note = "talkorigins\_source = {true}; raw\_reference = {Marx, J. L., 1978, Warm-blooded dinosaurs: Evidence pro and con: Science, v. 199, p. 1424-1426.}"
}

The ratio of carnivorous to herbivorous dinosaur skeletons from Dinosaur Provincial Park has been cited as evidence of endothermy in dinosaurs. In living populations of large endothermic mammals, carnivore biomass represents approximately 1% of total biomass. Two models describing energy flow from herbivores to carnivores indicate that tyrannosaurids are three to four times more abundant in the fossil sample than would have been the case if they were endothermic. Either the fossil sample does not adequately reflect relative abundances of large dinosaurs in the ancient community, or large dinosaurs were ectothermic.

@article{béland1979ectothermy,
    author = "Béland, P. and Russell, D. A.",
    title = "Ectothermy in dinosaurs: paleoecological evidence from Dinosaur Provincial Park, Alberta",
    year = "1979",
    journal = "Canadian Journal of Earth Sciences",
    abstract = "The ratio of carnivorous to herbivorous dinosaur skeletons from Dinosaur Provincial Park has been cited as evidence of endothermy in dinosaurs. In living populations of large endothermic mammals, carnivore biomass represents approximately 1\% of total biomass. Two models describing energy flow from herbivores to carnivores indicate that tyrannosaurids are three to four times more abundant in the fossil sample than would have been the case if they were endothermic. Either the fossil sample does not adequately reflect relative abundances of large dinosaurs in the ancient community, or large dinosaurs were ectothermic.",
    url = "https://doi.org/10.1139/e79-024",
    doi = "10.1139/e79-024",
    number = "2",
    pages = "250-255",
    volume = "16"
}

@incollection{bakker1980dinosaur2,
    author = "Bakker, R. T",
    editor = "Thomas, D. K. and Olson, E. C.",
    title = "Dinosaur heresy-dinosaur renaissance: Why we need endothermic archosaurs for a comprehensive theory of bioenergetic evolution",
    year = "1980",
    booktitle = "A Cold Look at the Warm Blooded Dinosaurs",
    publisher = "Washington, D.C., American Association for the Advancement of Science, p. 351-462",
    note = "talkorigins\_source = {true}; raw\_reference = {Bakker, R. T., 1980, Dinosaur heresy-dinosaur renaissance: Why we need endothermic archosaurs for a comprehensive theory of bioenergetic evolution, in Thomas, D. K., and Olson, E. C., eds., A Cold Look at the Warm Blooded Dinosaurs: Washington, D.C., American Association for the Advancement of Science, p. 351-462.}"
}

@incollection{hopson1980relative5,
    author = "Hopson, J. A",
    editor = "Thomas, D. K. and Olson, E. C.",
    title = "Relative Brainsize in Dinosaurs: Implications for Dinosaur Endothermy",
    year = "1980",
    booktitle = "A Cold Look at the Warm Blooded Dinosaurs",
    publisher = "Washington, D.C., American Association for the Advancement of Science, p. 287-310",
    note = "talkorigins\_source = {true}; raw\_reference = {Hopson, J. A., 1980, Relative Brainsize in Dinosaurs: Implications for Dinosaur Endothermy, in Thomas, D. K., and Olson, E. C., eds., A Cold Look at the Warm Blooded Dinosaurs: Washington, D.C., American Association for the Advancement of Science, p. 287-310.}"
}

One testable hypothesis of the theory that dinosaurs were endothermic is the observation that sauropod dinosaurs were too large, their heads were too small, and their food was too indigestible for them to be warm-blooded. Calculations on the daily calorie requirements of the sauropod Brachiosaurus, adjusted for digestibility and the energetic cost of “free-living,” were compared with the caloric density of Late Jurassic food plants and the feeding rates of an elephant and a giraffe. Using Brachiosaurus as a model I concluded that endothermy in large sauropods (greater than 55 metric tons) was impossible. Depending on assumptions about feeding rates and the cost of free-living, endothermy in smaller sauropods ranges from improbable to impossible.

@article{doi101017s0094837300007557,
    author = "Weaver, J. C.",
    title = "The improbable endotherm: the energetics of the sauropod dinosaur Brachiosaurus",
    year = "1983",
    journal = "Paleobiology",
    abstract = "One testable hypothesis of the theory that dinosaurs were endothermic is the observation that sauropod dinosaurs were too large, their heads were too small, and their food was too indigestible for them to be warm-blooded. Calculations on the daily calorie requirements of the sauropod Brachiosaurus, adjusted for digestibility and the energetic cost of “free-living,” were compared with the caloric density of Late Jurassic food plants and the feeding rates of an elephant and a giraffe. Using Brachiosaurus as a model I concluded that endothermy in large sauropods (greater than 55 metric tons) was impossible. Depending on assumptions about feeding rates and the cost of free-living, endothermy in smaller sauropods ranges from improbable to impossible.",
    url = "https://www.semanticscholar.org/paper/073fbdb23031536addf71d3c61fee61a22c88d3e",
    doi = "10.1017/S0094837300007557",
    is_oa = "true",
    number = "2",
    pages = "173-182",
    semanticscholar_citation_count = "48",
    semanticscholar_id = "073fbdb23031536addf71d3c61fee61a22c88d3e",
    volume = "9"
}

@misc{bakker1987the3,
    author = "Bakker, R. T",
    title = "The return of the Dancing Dinosaurs, in Czerkas, S. J., and Olson, E. C., eds., Dinosaurs Past and Present, 1",
    year = "1987",
    howpublished = "Los Angeles, Natural History Museum of Los Angeles County, p. 38-69",
    note = "talkorigins\_source = {true}; raw\_reference = {Bakker, R. T., 1987, The return of the Dancing Dinosaurs, in Czerkas, S. J., and Olson, E. C., eds., Dinosaurs Past and Present, 1: Los Angeles, Natural History Museum of Los Angeles County, p. 38-69.}"
}

This study of the habits of dinosaurs covers how they ate, how they reproduced and how they moved. Robert Bakker's illustrations are based on first-hand studies of dinosaur exhibits in museums throughout the world. This book supports the view that dinosaurs were warm-blooded, intelligent and agile creatures bearing little resemblance to existing reptiles. More than 12 years ago, Robert Bakker provoked a heated controversy on this subject, opposing the widely held belief that dinosaurs were cold-blooded, lumbering and of low intelligence.

@article{doi1023073514623,
    author = "Giffin, Emily and Bakker, Robert T.",
    title = "The Dinosaur Heresies",
    year = "1987",
    journal = "Palaios",
    abstract = "This study of the habits of dinosaurs covers how they ate, how they reproduced and how they moved. Robert Bakker's illustrations are based on first-hand studies of dinosaur exhibits in museums throughout the world. This book supports the view that dinosaurs were warm-blooded, intelligent and agile creatures bearing little resemblance to existing reptiles. More than 12 years ago, Robert Bakker provoked a heated controversy on this subject, opposing the widely held belief that dinosaurs were cold-blooded, lumbering and of low intelligence.",
    url = "https://doi.org/10.2307/3514623",
    doi = "10.2307/3514623",
    openalex = "W2035620759"
}

@misc{morell1987the7,
    author = "Morell, V",
    title = "The birth of a heresy",
    year = "1987",
    howpublished = "Discover, v. 8, p. 26-50",
    note = "talkorigins\_source = {true}; raw\_reference = {Morell, V., 1987, The birth of a heresy: Discover, v. 8, p. 26-50.}"
}

The debate over the origins of endothermy—and whether or not dinosaurs were warm-blooded—has blown hot and cold for years, hampered by the lack of clear-cut evidence. But bones in the nose that are essential for conserving water and heat in endotherms seem completely lacking in dinosaurs, and many researchers say this indicates the extinct animals couldn't have been warm-blooded.

@article{doi101126science2705237735,
    author = "Fischman, Joshua",
    title = "Were Dinos Cold-Blooded After All? The Nose Knows",
    year = "1995",
    journal = "Science",
    abstract = "The debate over the origins of endothermy—and whether or not dinosaurs were warm-blooded—has blown hot and cold for years, hampered by the lack of clear-cut evidence. But bones in the nose that are essential for conserving water and heat in endotherms seem completely lacking in dinosaurs, and many researchers say this indicates the extinct animals couldn't have been warm-blooded.",
    url = "https://doi.org/10.1126/science.270.5237.735",
    doi = "10.1126/science.270.5237.735",
    openalex = "W1641318940"
}

A physical, model-based approach to body temperatures in dinosaurs allows us to predict what ranges of body temperatures and what thermoregulatory strategies were available to those dinosaurs. We argue that 1. The huge range of body sizes in the dinosaurs likely resulted in very different thermal problems and strategies for animals at either end of this size continuum. 2. Body temperatures of the smallest adult dinosaurs and of hatchlings and small juveniles would have been largely insensitive to metabolic rates in the absence of insulation. The smallest animals in which metabolic heating resulted in predicted body temperatures ≥ 2°C above operative temperatures (T e) weigh 10 kg. Body temperature would respond rapidly enough to changes in T e to make behavioral thermoregulation possible. 3. Body temperatures of large dinosaurs (>1000 kg) likely were sensitive to both metabolic rate and the delivery of heat to the body surface by blood flow. Our model suggests that they could adjust body temperature by adjusting metabolic rate and blood flow. Behavioral thermoregulation by changing microhabitat selection would likely have been of limited utility because body temperatures would have responded only slowly to changes in T e. 4. Endothermic metabolic rates may have put large dinosaurs at risk for overheating unless they had adaptations to shed the heat as necessary. This would have been particularly true for dinosaurs with masses > 10,000 kg, but simulations suggest that for animals as small as 1000 kg in the Tropics and in temperate latitudes during the summer, steady-state body temperatures would have exceeded 40°C. Slow response of body temperatures to changes in T e suggests that use of day-night thermal differences would have buffered dinosaurs from diel warming but would not have lowered body temperatures sufficiently for animals experiencing high mean daily T e. 5. Endothermic metabolism and metabolic heating might have been useful for intermediate and large-sized (100–3000 kg) dinosaurs but often in situations that demanded marked seasonal adjustment of metabolic rates and/or precise control of metabolism (and heat-loss mechanisms) as typically seen in endotherms.

@article{doi101017s0094837300021321,
    author = "O′Connor, Michael and Dodson, Peter",
    title = "Biophysical constraints on the thermal ecology of dinosaurs",
    year = "1999",
    journal = "Paleobiology",
    abstract = "A physical, model-based approach to body temperatures in dinosaurs allows us to predict what ranges of body temperatures and what thermoregulatory strategies were available to those dinosaurs. We argue that 1. The huge range of body sizes in the dinosaurs likely resulted in very different thermal problems and strategies for animals at either end of this size continuum. 2. Body temperatures of the smallest adult dinosaurs and of hatchlings and small juveniles would have been largely insensitive to metabolic rates in the absence of insulation. The smallest animals in which metabolic heating resulted in predicted body temperatures ≥ 2°C above operative temperatures (T e) weigh 10 kg. Body temperature would respond rapidly enough to changes in T e to make behavioral thermoregulation possible. 3. Body temperatures of large dinosaurs (>1000 kg) likely were sensitive to both metabolic rate and the delivery of heat to the body surface by blood flow. Our model suggests that they could adjust body temperature by adjusting metabolic rate and blood flow. Behavioral thermoregulation by changing microhabitat selection would likely have been of limited utility because body temperatures would have responded only slowly to changes in T e. 4. Endothermic metabolic rates may have put large dinosaurs at risk for overheating unless they had adaptations to shed the heat as necessary. This would have been particularly true for dinosaurs with masses > 10,000 kg, but simulations suggest that for animals as small as 1000 kg in the Tropics and in temperate latitudes during the summer, steady-state body temperatures would have exceeded 40°C. Slow response of body temperatures to changes in T e suggests that use of day-night thermal differences would have buffered dinosaurs from diel warming but would not have lowered body temperatures sufficiently for animals experiencing high mean daily T e. 5. Endothermic metabolism and metabolic heating might have been useful for intermediate and large-sized (100–3000 kg) dinosaurs but often in situations that demanded marked seasonal adjustment of metabolic rates and/or precise control of metabolism (and heat-loss mechanisms) as typically seen in endotherms.",
    url = "https://doi.org/10.1017/s0094837300021321",
    doi = "10.1017/s0094837300021321",
    openalex = "W2264731691",
    references = "crossref1998encyclopedia, doi1010079781461260240, doi101007bf00024969, doi101017s0022336000036076, doi10106313128494, doi1023071942620, doi1023071948545, doi1023072258713, openalexw1558456135, openalexw3215057009, openalexw656806957"
}

@article{doi1016420004803820021191187badsat20co2,
    author = "Feduccia, A.",
    title = "Birds are Dinosaurs: Simple Answer to a Complex Problem",
    year = "2002",
    journal = "The Auk",
    url = "https://bioone.org/journals/the-auk/volume-119/issue-4/0004-8038\_2002\_119\_1187\_BADSAT\_2.0.CO\_2/Birds-are-Dinosaurs-Simple-Answer-to-a-Complex-Problem/10.1642/0004-8038(2002)119[1187:BADSAT]2.0.CO;2.pdf",
    doi = "10.1642/0004-8038(2002)119[1187:BADSAT]2.0.CO;2",
    is_oa = "true",
    number = "4",
    pages = "1187",
    semanticscholar_citation_count = "33",
    semanticscholar_id = "8b20c3572e364d45bddc39c60232ba77e7e75dc6",
    volume = "119"
}

Despite numerous studies, the thermal physiology of dinosaurs remains unresolved. Thus, perhaps the commonly asked question whether dinosaurs were ectotherms or endotherms is inappropriate, and it is more constructive to ask which dinosaurs were likely to have been endothermic and which ones ectothermic. Field data from crocodiles over a large size range show that body temperature fluctuations decrease with increasing body mass, and that average daily body temperatures increase with increasing mass. A biophysical model, the biological relevance of which was tested against field data, was used to predict body temperatures of dinosaurs. However, rather than predicting thermal relations of a hypothetical dinosaur, the model considered correct paleogeographical distribution and climate to predict the thermal relations of a large number of dinosaurs known from the fossil record (>700). Many dinosaurs could have had “high” (>30°) and stable (daily amplitude >2°) body temperatures without metabolic heat production even in winter, so it is unlikely that selection pressure would have favored the evolution of elevated resting metabolic rates in those species. Recent evidence of ontogenetic growth rates indicates that even the juveniles of large species (3000–4000 kg) could have had biologically functional body temperature ranges during early development. Smaller dinosaurs (45°) could not have had high and stable body temperatures without metabolic heat production. However, elevated metabolic rates were unlikely to have provided selective advantage in the absence of some form of insulation, so probably insulation was present before endothermy evolved, or else it coevolved with elevated metabolic rates. Superimposing these findings onto a phylogeny of the Dinosauria suggests that endothermy most likely evolved among the Coelurosauria and, to a lesser extent, among the Hypsilophodontidae, but not among the Stegosauridae, Nodosauridae, Ankylosauridae, Hadrosauridae, Ceratopsidae, Prosauropoda, and Sauropoda.

@article{doi1016660094837320030290105dbttoo20co2,
    author = "Seebacher, Frank",
    title = "Dinosaur body temperatures: the occurrence of endothermy and ectothermy",
    year = "2003",
    journal = "Paleobiology",
    abstract = "Despite numerous studies, the thermal physiology of dinosaurs remains unresolved. Thus, perhaps the commonly asked question whether dinosaurs were ectotherms or endotherms is inappropriate, and it is more constructive to ask which dinosaurs were likely to have been endothermic and which ones ectothermic. Field data from crocodiles over a large size range show that body temperature fluctuations decrease with increasing body mass, and that average daily body temperatures increase with increasing mass. A biophysical model, the biological relevance of which was tested against field data, was used to predict body temperatures of dinosaurs. However, rather than predicting thermal relations of a hypothetical dinosaur, the model considered correct paleogeographical distribution and climate to predict the thermal relations of a large number of dinosaurs known from the fossil record (>700). Many dinosaurs could have had “high” (>30°) and stable (daily amplitude >2°) body temperatures without metabolic heat production even in winter, so it is unlikely that selection pressure would have favored the evolution of elevated resting metabolic rates in those species. Recent evidence of ontogenetic growth rates indicates that even the juveniles of large species (3000–4000 kg) could have had biologically functional body temperature ranges during early development. Smaller dinosaurs (45°) could not have had high and stable body temperatures without metabolic heat production. However, elevated metabolic rates were unlikely to have provided selective advantage in the absence of some form of insulation, so probably insulation was present before endothermy evolved, or else it coevolved with elevated metabolic rates. Superimposing these findings onto a phylogeny of the Dinosauria suggests that endothermy most likely evolved among the Coelurosauria and, to a lesser extent, among the Hypsilophodontidae, but not among the Stegosauridae, Nodosauridae, Ankylosauridae, Hadrosauridae, Ceratopsidae, Prosauropoda, and Sauropoda.",
    url = "https://doi.org/10.1666/0094-8373(2003)029<0105:dbttoo>2.0.co;2",
    doi = "10.1666/0094-8373(2003)029<0105:dbttoo>2.0.co;2",
    openalex = "W2175225824",
    references = "doi101016s016953470102198x, doi101017s0094837300021321, doi101038scientificamerican126292, doi101086283547, doi101086648217, doi101126science28454232137, doi101126science493968, doi101306c1ea47bb16c911d78645000102c1865d, doi105860choice331556, openalexw2002729176, openalexw3149443718, zhao1998the"
}

Avian and mammalian endothermy results from elevated rates of resting, or routine, metabolism and enables these animals to maintain high and stable body temperatures in the face of variable ambient temperatures. Endothermy is also associated with enhanced stamina and elevated capacity for aerobic metabolism during periods of prolonged activity. These attributes of birds and mammals have greatly contributed to their widespread distribution and ecological success. Unfortunately, since few anatomical/physiological attributes linked to endothermy are preserved in fossils, the origin of endothermy among the ancestors of mammals and birds has long remained obscure. Two recent approaches provide new insight into the metabolic physiology of extinct forms. One addresses chronic (resting) metabolic rates and emphasizes the presence of nasal respiratory turbinates in virtually all extant endotherms. These structures are associated with recovery of respiratory heat and moisture in animals with high resting metabolic rates. The fossil record of nonmammalian synapsids suggests that at least two Late Permian lineages possessed incipient respiratory turbinates. In contrast, these structures appear to have been absent in dinosaurs and nonornithurine birds. Instead, nasal morphology suggests that in the avian lineage, respiratory turbinates first appeared in Cretaceous ornithurines. The other approach addresses the capacity for maximal aerobic activity and examines lung structure and ventilatory mechanisms. There is no positive evidence to support the reconstruction of a derived, avian-like parabronchial lung/air sac system in dinosaurs or nonornithurine birds. Dinosaur lungs were likely heterogenous, multicameral septate lungs with conventional, tidal ventilation, although evidence from some theropods suggests that at least this group may have had a hepatic piston mechanism of supplementary lung ventilation. This suggests that dinosaurs and nonornithurine birds generally lacked the capacity for high, avian-like levels of sustained activity, although the aerobic capacity of theropods may have exceeded that of extant ectotherms. The avian parabronchial lung/air sac system appears to be an attribute limited to ornithurine birds.

@article{doi101086425185,
    author = "Hillenius, Willem J. and Ruben, John A.",
    title = "The Evolution of Endothermy in Terrestrial Vertebrates: Who? When? Why?",
    year = "2004",
    journal = "Physiological and Biochemical Zoology",
    abstract = "Avian and mammalian endothermy results from elevated rates of resting, or routine, metabolism and enables these animals to maintain high and stable body temperatures in the face of variable ambient temperatures. Endothermy is also associated with enhanced stamina and elevated capacity for aerobic metabolism during periods of prolonged activity. These attributes of birds and mammals have greatly contributed to their widespread distribution and ecological success. Unfortunately, since few anatomical/physiological attributes linked to endothermy are preserved in fossils, the origin of endothermy among the ancestors of mammals and birds has long remained obscure. Two recent approaches provide new insight into the metabolic physiology of extinct forms. One addresses chronic (resting) metabolic rates and emphasizes the presence of nasal respiratory turbinates in virtually all extant endotherms. These structures are associated with recovery of respiratory heat and moisture in animals with high resting metabolic rates. The fossil record of nonmammalian synapsids suggests that at least two Late Permian lineages possessed incipient respiratory turbinates. In contrast, these structures appear to have been absent in dinosaurs and nonornithurine birds. Instead, nasal morphology suggests that in the avian lineage, respiratory turbinates first appeared in Cretaceous ornithurines. The other approach addresses the capacity for maximal aerobic activity and examines lung structure and ventilatory mechanisms. There is no positive evidence to support the reconstruction of a derived, avian-like parabronchial lung/air sac system in dinosaurs or nonornithurine birds. Dinosaur lungs were likely heterogenous, multicameral septate lungs with conventional, tidal ventilation, although evidence from some theropods suggests that at least this group may have had a hepatic piston mechanism of supplementary lung ventilation. This suggests that dinosaurs and nonornithurine birds generally lacked the capacity for high, avian-like levels of sustained activity, although the aerobic capacity of theropods may have exceeded that of extant ectotherms. The avian parabronchial lung/air sac system appears to be an attribute limited to ornithurine birds.",
    url = "https://doi.org/10.1086/425185",
    doi = "10.1086/425185",
    openalex = "W2022733476",
    references = "doi101007s0011400304837, doi101038416816a, doi10108002724634199710011027, doi101096fasebj4112199286, doi101139e93179, doi101643004585112002002117020co2, doi1023071292217, doi1023071445584, doi1023071942620, doi105860choice343307, openalexw195142154, reid1984primary"
}

@article{amiot2006oxygen,
    author = "AMIOT, R and LECUYER, C and BUFFETAUT, E and ESCARGUEL, G and FLUTEAU, F and MARTINEAU, F",
    title = "Oxygen isotopes from biogenic apatites suggest widespread endothermy in Cretaceous dinosaurs",
    year = "2006",
    journal = "Earth and Planetary Science Letters",
    url = "https://doi.org/10.1016/j.epsl.2006.04.018",
    doi = "10.1016/j.epsl.2006.04.018",
    number = "1-2",
    openalex = "W2003509095",
    pages = "41-54",
    volume = "246",
    references = "doi1010160012821x73900885, doi1010160012821x83901000, doi1010160012821x83901012, doi101016001670378490259x, doi101016s0012821x99001053, doi101016s0016703796002402, doi101016s0031018297001089, doi101017s0094837300021321, doi101029jb093ib10p11791, doi101111j215334901964tb00181x, doi101126science27352791204, openalexw1546962148"
}

@misc{hillenius2006dinosaur,
    author = "Hillenius, Willem J",
    title = "Dinosaur Physiology: Were Dinosaurs Warm‐blooded?",
    year = "2006",
    booktitle = "Encyclopedia of Life Sciences",
    url = "https://doi.org/10.1038/npg.els.0003323",
    doi = "10.1038/npg.els.0003323"
}

@article{crossref2009palaeontology,
    title = "Palaeontology: Hot-blooded dinosaurs",
    year = "2009",
    journal = "Nature",
    url = "https://doi.org/10.1038/462254f",
    doi = "10.1038/462254f",
    number = "7271",
    pages = "254-255",
    volume = "462"
}

@article{pontzer2009biomechanics,
    author = "Pontzer, Herman and Allen, Vivian and Hutchinson, John R.",
    title = "Biomechanics of Running Indicates Endothermy in Bipedal Dinosaurs",
    year = "2009",
    journal = "PLoS ONE",
    url = "https://doi.org/10.1371/journal.pone.0007783",
    doi = "10.1371/journal.pone.0007783",
    number = "11",
    openalex = "W2152815529",
    pages = "e7783",
    volume = "4",
    references = "doi101001jama196203050110085031, doi101038346265a0, doi101111j109600311988tb00514x, doi101111j146979981983tb04266x, doi101126science2740914, doi101126science28454232137, doi101126science493968, doi101146annurevnutr191247, doi1023071366368, openalexw2611511275"
}

Endothermy has evolved at least twice, in the precursors to modern mammals and birds. The most widely accepted explanation for the evolution of endothermy has been selection for enhanced aerobic capacity. We review this hypothesis in the light of advances in our understanding of ATP generation by mitochondria and muscle performance. Together with the development of isotope-based techniques for the measurement of metabolic rate in free-ranging vertebrates these have confirmed the importance of aerobic scope in the evolution of endothermy: absolute aerobic scope, ATP generation by mitochondria and muscle power output are all strongly temperature-dependent, indicating that there would have been significant improvement in whole-organism locomotor ability with a warmer body. New data on mitochondrial ATP generation and proton leak suggest that the thermal physiology of mitochondria may differ between organisms of contrasting ecology and thermal flexibility. Together with recent biophysical modelling, this strengthens the long-held view that endothermy originated in smaller, active eurythermal ectotherms living in a cool but variable thermal environment. We propose that rather than being a secondary consequence of the evolution of an enhanced aerobic scope, a warmer body was the means by which that enhanced aerobic scope was achieved. This modified hypothesis requires that the rise in metabolic rate and the insulation necessary to retain metabolic heat arose early in the lineages leading to birds and mammals. Large dinosaurs were warm, but were not endotherms, and the metabolic status of pterosaurs remains unresolved.

@article{doi101111j1469185x201000122x,
    author = "Clarke, Andrew and Pörtner, Hans‐Otto",
    title = "Temperature, metabolic power and the evolution of endothermy",
    year = "2010",
    journal = "Biological reviews/Biological reviews of the Cambridge Philosophical Society",
    abstract = "Endothermy has evolved at least twice, in the precursors to modern mammals and birds. The most widely accepted explanation for the evolution of endothermy has been selection for enhanced aerobic capacity. We review this hypothesis in the light of advances in our understanding of ATP generation by mitochondria and muscle performance. Together with the development of isotope-based techniques for the measurement of metabolic rate in free-ranging vertebrates these have confirmed the importance of aerobic scope in the evolution of endothermy: absolute aerobic scope, ATP generation by mitochondria and muscle power output are all strongly temperature-dependent, indicating that there would have been significant improvement in whole-organism locomotor ability with a warmer body. New data on mitochondrial ATP generation and proton leak suggest that the thermal physiology of mitochondria may differ between organisms of contrasting ecology and thermal flexibility. Together with recent biophysical modelling, this strengthens the long-held view that endothermy originated in smaller, active eurythermal ectotherms living in a cool but variable thermal environment. We propose that rather than being a secondary consequence of the evolution of an enhanced aerobic scope, a warmer body was the means by which that enhanced aerobic scope was achieved. This modified hypothesis requires that the rise in metabolic rate and the insulation necessary to retain metabolic heat arose early in the lineages leading to birds and mammals. Large dinosaurs were warm, but were not endotherms, and the metabolic status of pterosaurs remains unresolved.",
    url = "https://doi.org/10.1111/j.1469-185x.2010.00122.x",
    doi = "10.1111/j.1469-185x.2010.00122.x",
    openalex = "W2110669729",
    references = "amiot2006oxygen, doi101016s1095643302000454, doi101017s0094837300007557, doi101017s0094837300021321, doi10103835007527, doi101038nature07447, doi101038nature07856, doi101086283249, doi101086284325, doi101086425185, doi101093oso97801985464120010001, doi101126science1061967, doi101126science24248841403, doi101146annurevph57030195000441, doi101152physrev1997773731, doi101371journalpone0003303, doi1016660094837320030290105dbttoo20co2, doi1016660094837320030290243vpasat20co2, doi101890039000, doi1023071223169, doi105860choice295104, openalexw2983381470"
}

Pneumatic (air-filled) postcranial bones are unique to birds among extant tetrapods. Unambiguous skeletal correlates of postcranial pneumaticity first appeared in the Late Triassic (approximately 210 million years ago), when they evolved independently in several groups of bird-line archosaurs (ornithodirans). These include the theropod dinosaurs (of which birds are extant representatives), the pterosaurs, and sauropodomorph dinosaurs. Postulated functions of skeletal pneumatisation include weight reduction in large-bodied or flying taxa, and density reduction resulting in energetic savings during foraging and locomotion. However, the influence of these hypotheses on the early evolution of pneumaticity has not been studied in detail previously. We review recent work on the significance of pneumaticity for understanding the biology of extinct ornithodirans, and present detailed new data on the proportion of the skeleton that was pneumatised in 131 non-avian theropods and Archaeopteryx. This includes all taxa known from significant postcranial remains. Pneumaticity of the cervical and anterior dorsal vertebrae occurred early in theropod evolution. This 'common pattern' was conserved on the line leading to birds, and is likely present in Archaeopteryx. Increases in skeletal pneumaticity occurred independently in as many as 12 lineages, highlighting a remarkably high number of parallel acquisitions of a bird-like feature among non-avian theropods. Using a quantitative comparative framework, we show that evolutionary increases in skeletal pneumaticity are significantly concentrated in lineages with large body size, suggesting that mass reduction in response to gravitational constraints at large body sizes influenced the early evolution of pneumaticity. However, the body size threshold for extensive pneumatisation is lower in theropod lineages more closely related to birds (maniraptorans). Thus, relaxation of the relationship between body size and pneumatisation preceded the origin of birds and cannot be explained as an adaptation for flight. We hypothesise that skeletal density modulation in small, non-volant, maniraptorans resulted in energetic savings as part of a multi-system response to increased metabolic demands. Acquisition of extensive postcranial pneumaticity in small-bodied maniraptorans may indicate avian-like high-performance endothermy.

@article{doi101111j1469185x201100190x,
    author = "Benson, Roger and Butler, Richard J. and Carrano, Matthew T. and O’Connor, Patrick M.",
    title = "Air‐filled postcranial bones in theropod dinosaurs: physiological implications and the ‘reptile’–bird transition",
    year = "2011",
    journal = "Biological reviews/Biological reviews of the Cambridge Philosophical Society",
    abstract = "Pneumatic (air-filled) postcranial bones are unique to birds among extant tetrapods. Unambiguous skeletal correlates of postcranial pneumaticity first appeared in the Late Triassic (approximately 210 million years ago), when they evolved independently in several groups of bird-line archosaurs (ornithodirans). These include the theropod dinosaurs (of which birds are extant representatives), the pterosaurs, and sauropodomorph dinosaurs. Postulated functions of skeletal pneumatisation include weight reduction in large-bodied or flying taxa, and density reduction resulting in energetic savings during foraging and locomotion. However, the influence of these hypotheses on the early evolution of pneumaticity has not been studied in detail previously. We review recent work on the significance of pneumaticity for understanding the biology of extinct ornithodirans, and present detailed new data on the proportion of the skeleton that was pneumatised in 131 non-avian theropods and Archaeopteryx. This includes all taxa known from significant postcranial remains. Pneumaticity of the cervical and anterior dorsal vertebrae occurred early in theropod evolution. This 'common pattern' was conserved on the line leading to birds, and is likely present in Archaeopteryx. Increases in skeletal pneumaticity occurred independently in as many as 12 lineages, highlighting a remarkably high number of parallel acquisitions of a bird-like feature among non-avian theropods. Using a quantitative comparative framework, we show that evolutionary increases in skeletal pneumaticity are significantly concentrated in lineages with large body size, suggesting that mass reduction in response to gravitational constraints at large body sizes influenced the early evolution of pneumaticity. However, the body size threshold for extensive pneumatisation is lower in theropod lineages more closely related to birds (maniraptorans). Thus, relaxation of the relationship between body size and pneumatisation preceded the origin of birds and cannot be explained as an adaptation for flight. We hypothesise that skeletal density modulation in small, non-volant, maniraptorans resulted in energetic savings as part of a multi-system response to increased metabolic demands. Acquisition of extensive postcranial pneumaticity in small-bodied maniraptorans may indicate avian-like high-performance endothermy.",
    url = "https://doi.org/10.1111/j.1469-185x.2011.00190.x",
    doi = "10.1111/j.1469-185x.2011.00190.x",
    openalex = "W2003924744",
    references = "doi101002jez513, doi101002jmor10470, doi101002sici1097018520000215261125aidar630co27, doi101007s0011400804883, doi101007s001140090614x, doi101017s0094837300021308, doi101038nature07856, doi101073pnas0708903105, doi10108002724634199710011018, doi101086284325, doi101093auk12041206, doi101093bioinformaticsbtg412, doi101093sysbio41118, doi101098rstb19890106, doi101111j10963642200600245x, doi101111j10963642200900569x, doi101126science1180219, doi1012066481, doi101371journalpone0003303, doi101371journalpone0007390, doi10167102724634200727127tpasom20co2, doi1023071292217, doi1023071441916, doi105281zenodo16171435, doi105860choice392183, doi105860choice434677, doi105962bhltitle60562, openalexw2611511275, openalexw3086315876, ostrom2019osteology, owen1857monograph, owen2015monograph"
}

To evaluate the possible physiology of dinosaurs, comparisons must be made with their closest living relatives: birds and crocodilians. Although crocodilians maintain ectothermic metabolic rates and have anatomy reflective of this, modern birds achieve high, endothermic metabolic rates through specialised soft tissues supported by unique skeletal attributes. Finding similar shared characters in dinosaurs that are functionally linked to metabolic rates in birds or crocodilians allows plausible reconstruction of dinosaur physiology. Examinations of dinosaur remains reveal no structures with clear functional association with bird‐like respiratory or metabolic physiology, and in some cases indicate crocodilian‐like anatomy. Consequently, dinosaurs were most likely ectothermic, with resting and maximal metabolic rates that were lower than those of modern mammals or birds. However, given the favourable Mesozoic climatic conditions, most dinosaurs were probably able to maintain high, constant body temperatures through behavioural or inertial thermoregulation. Key Concepts: Reconstructing the biology of extinct forms relies on comparison with living taxa that share the same specialised features linked to specific function. Stable body temperature can be achieved through behavioural mechanisms or through virtue of large mass, and need not rely on a particular metabolic strategy. The closest living relatives of dinosaurs are birds and crocodilians, which have widely different metabolic rates supported by different respiratory and skeletal anatomy. Some dinosaur remains preserve evidence, such as postcranial pneumaticity, that may be superficially suggestive of modern bird‐like respiratory anatomy, but they lack other features critical for the ability to ventilate bird‐like lungs or achieve bird‐like aerobic capacity. No dinosaur remains show evidence of respiratory turbinates, a skeletal character functionally associated with modern endothermy. Endothermy was not likely achieved in dinosaurs, but was first present in mid‐Cretaceous birds. Some dinosaurs may have increased aerobic capacity using a crocodilian‐like ventilatory mechanism.

@article{doi1010029780470015902a0003323pub2,
    author = "Hillenius, W. J.",
    title = "Dinosaur Physiology: Were Dinosaurs Warm‐blooded?",
    year = "2013",
    booktitle = "Encyclopedia of Life Sciences",
    abstract = "To evaluate the possible physiology of dinosaurs, comparisons must be made with their closest living relatives: birds and crocodilians. Although crocodilians maintain ectothermic metabolic rates and have anatomy reflective of this, modern birds achieve high, endothermic metabolic rates through specialised soft tissues supported by unique skeletal attributes. Finding similar shared characters in dinosaurs that are functionally linked to metabolic rates in birds or crocodilians allows plausible reconstruction of dinosaur physiology. Examinations of dinosaur remains reveal no structures with clear functional association with bird‐like respiratory or metabolic physiology, and in some cases indicate crocodilian‐like anatomy. Consequently, dinosaurs were most likely ectothermic, with resting and maximal metabolic rates that were lower than those of modern mammals or birds. However, given the favourable Mesozoic climatic conditions, most dinosaurs were probably able to maintain high, constant body temperatures through behavioural or inertial thermoregulation. Key Concepts: Reconstructing the biology of extinct forms relies on comparison with living taxa that share the same specialised features linked to specific function. Stable body temperature can be achieved through behavioural mechanisms or through virtue of large mass, and need not rely on a particular metabolic strategy. The closest living relatives of dinosaurs are birds and crocodilians, which have widely different metabolic rates supported by different respiratory and skeletal anatomy. Some dinosaur remains preserve evidence, such as postcranial pneumaticity, that may be superficially suggestive of modern bird‐like respiratory anatomy, but they lack other features critical for the ability to ventilate bird‐like lungs or achieve bird‐like aerobic capacity. No dinosaur remains show evidence of respiratory turbinates, a skeletal character functionally associated with modern endothermy. Endothermy was not likely achieved in dinosaurs, but was first present in mid‐Cretaceous birds. Some dinosaurs may have increased aerobic capacity using a crocodilian‐like ventilatory mechanism.",
    url = "https://www.semanticscholar.org/paper/5a5c618261f52787333e3a0d397a50797bfdabbf",
    doi = "10.1002/9780470015902.A0003323.PUB2",
    is_oa = "true",
    semanticscholar_citation_count = "1",
    semanticscholar_id = "5a5c618261f52787333e3a0d397a50797bfdabbf"
}

To evaluate the possible physiology of dinosaurs, comparisons must be made with their closest living relatives: birds and crocodilians. Although crocodilians maintain ectothermic metabolic rates and have anatomy reflective of this, modern birds achieve high, endothermic metabolic rates through specialised soft tissues supported by unique skeletal attributes. Finding similar shared characters in dinosaurs that are functionally linked to metabolic rates in birds or crocodilians allows plausible reconstruction of dinosaur physiology. Examinations of dinosaur remains reveal no structures with clear functional association with bird‐like respiratory or metabolic physiology, and in some cases indicate crocodilian‐like anatomy. Consequently, dinosaurs were most likely ectothermic, with resting and maximal metabolic rates that were lower than those of modern mammals or birds. However, given the favourable Mesozoic climatic conditions, most dinosaurs were probably able to maintain high, constant body temperatures through behavioural or inertial thermoregulation. Key Concepts: Reconstructing the biology of extinct forms relies on comparison with living taxa that share the same specialised features linked to specific function. Stable body temperature can be achieved through behavioural mechanisms or through virtue of large mass, and need not rely on a particular metabolic strategy. The closest living relatives of dinosaurs are birds and crocodilians, which have widely different metabolic rates supported by different respiratory and skeletal anatomy. Some dinosaur remains preserve evidence, such as postcranial pneumaticity, that may be superficially suggestive of modern bird‐like respiratory anatomy, but they lack other features critical for the ability to ventilate bird‐like lungs or achieve bird‐like aerobic capacity. No dinosaur remains show evidence of respiratory turbinates, a skeletal character functionally associated with modern endothermy. Endothermy was not likely achieved in dinosaurs, but was first present in mid‐Cretaceous birds. Some dinosaurs may have increased aerobic capacity using a crocodilian‐like ventilatory mechanism.

@misc{quick2013dinosaur,
    author = "Quick, Devon E and Hillenius, Willem J",
    title = "Dinosaur Physiology: Were Dinosaurs Warm‐Blooded?",
    year = "2013",
    booktitle = "Encyclopedia of Life Sciences",
    abstract = "To evaluate the possible physiology of dinosaurs, comparisons must be made with their closest living relatives: birds and crocodilians. Although crocodilians maintain ectothermic metabolic rates and have anatomy reflective of this, modern birds achieve high, endothermic metabolic rates through specialised soft tissues supported by unique skeletal attributes. Finding similar shared characters in dinosaurs that are functionally linked to metabolic rates in birds or crocodilians allows plausible reconstruction of dinosaur physiology. Examinations of dinosaur remains reveal no structures with clear functional association with bird‐like respiratory or metabolic physiology, and in some cases indicate crocodilian‐like anatomy. Consequently, dinosaurs were most likely ectothermic, with resting and maximal metabolic rates that were lower than those of modern mammals or birds. However, given the favourable Mesozoic climatic conditions, most dinosaurs were probably able to maintain high, constant body temperatures through behavioural or inertial thermoregulation. Key Concepts: Reconstructing the biology of extinct forms relies on comparison with living taxa that share the same specialised features linked to specific function. Stable body temperature can be achieved through behavioural mechanisms or through virtue of large mass, and need not rely on a particular metabolic strategy. The closest living relatives of dinosaurs are birds and crocodilians, which have widely different metabolic rates supported by different respiratory and skeletal anatomy. Some dinosaur remains preserve evidence, such as postcranial pneumaticity, that may be superficially suggestive of modern bird‐like respiratory anatomy, but they lack other features critical for the ability to ventilate bird‐like lungs or achieve bird‐like aerobic capacity. No dinosaur remains show evidence of respiratory turbinates, a skeletal character functionally associated with modern endothermy. Endothermy was not likely achieved in dinosaurs, but was first present in mid‐Cretaceous birds. Some dinosaurs may have increased aerobic capacity using a crocodilian‐like ventilatory mechanism.",
    url = "https://doi.org/10.1002/9780470015902.a0003323.pub2",
    doi = "10.1002/9780470015902.a0003323.pub2"
}

The nasal region plays a key role in sensory, thermal, and respiratory physiology, but exploring its evolution is hampered by a lack of preservation of soft-tissue structures in extinct vertebrates. As a test case, we investigated members of the "bony-headed" ornithischian dinosaur clade Pachycephalosauridae (particularly Stegoceras validum) because of their small body size (which mitigated allometric concerns) and their tendency to preserve nasal soft tissues within their hypermineralized skulls. Hypermineralization directly preserved portions of the olfactory turbinates along with an internal nasal ridge that we regard as potentially an osteological correlate for respiratory conchae. Fossil specimens were CT-scanned, and nasal cavities were segmented and restored. Soft-tissue reconstruction of the nasal capsule was functionally tested in a virtual environment using computational fluid dynamics by running air through multiple models differing in nasal soft-tissue conformation: a bony-bounded model (i.e., skull without soft tissue) and then models with soft tissues added, such as a paranasal septum, a scrolled concha, a branched concha, and a model combining the paranasal septum with a concha. Deviations in fluid flow in comparison to a phylogenetically constrained sample of extant diapsids were used as indicators of missing soft tissue. Models that restored aspects of airflow found in extant diapsids, such as appreciable airflow in the olfactory chamber, were judged as more likely. The model with a branched concha produced airflow patterns closest to those of extant diapsids. These results from both paleontological observation and airflow modeling indicate that S. validum and other pachycephalosaurids could have had both olfactory and respiratory conchae. Although respiratory conchae have been linked to endothermy, such conclusions require caution in that our re-evaluation of the reptilian nasal apparatus indicates that respiratory conchae may be more widespread than originally thought, and other functions, such as selective brain temperature regulation, could be important.

@article{doi101002ar23046,
    author = "Bourke, Jason M. and Porter, Wm. Ruger and Ridgely, Ryan C. and Lyson, Tyler R. and Schachner, Emma R. and Bell, Phil R. and Witmer, Lawrence M.",
    title = "Breathing Life Into Dinosaurs: Tackling Challenges of Soft‐Tissue Restoration and Nasal Airflow in Extinct Species",
    year = "2014",
    journal = "The Anatomical Record",
    abstract = {The nasal region plays a key role in sensory, thermal, and respiratory physiology, but exploring its evolution is hampered by a lack of preservation of soft-tissue structures in extinct vertebrates. As a test case, we investigated members of the "bony-headed" ornithischian dinosaur clade Pachycephalosauridae (particularly Stegoceras validum) because of their small body size (which mitigated allometric concerns) and their tendency to preserve nasal soft tissues within their hypermineralized skulls. Hypermineralization directly preserved portions of the olfactory turbinates along with an internal nasal ridge that we regard as potentially an osteological correlate for respiratory conchae. Fossil specimens were CT-scanned, and nasal cavities were segmented and restored. Soft-tissue reconstruction of the nasal capsule was functionally tested in a virtual environment using computational fluid dynamics by running air through multiple models differing in nasal soft-tissue conformation: a bony-bounded model (i.e., skull without soft tissue) and then models with soft tissues added, such as a paranasal septum, a scrolled concha, a branched concha, and a model combining the paranasal septum with a concha. Deviations in fluid flow in comparison to a phylogenetically constrained sample of extant diapsids were used as indicators of missing soft tissue. Models that restored aspects of airflow found in extant diapsids, such as appreciable airflow in the olfactory chamber, were judged as more likely. The model with a branched concha produced airflow patterns closest to those of extant diapsids. These results from both paleontological observation and airflow modeling indicate that S. validum and other pachycephalosaurids could have had both olfactory and respiratory conchae. Although respiratory conchae have been linked to endothermy, such conclusions require caution in that our re-evaluation of the reptilian nasal apparatus indicates that respiratory conchae may be more widespread than originally thought, and other functions, such as selective brain temperature regulation, could be important.},
    url = "https://doi.org/10.1002/ar.23046",
    doi = "10.1002/ar.23046",
    openalex = "W1593320552",
    references = "doi101002ar20984"
}

Were dinosaurs ectotherms or fast-metabolizing endotherms whose activities were unconstrained by temperature? To date, some of the strongest evidence for endothermy comes from the rapid growth rates derived from the analysis of fossil bones. However, these studies are constrained by a lack of comparative data and an appropriate energetic framework. Here we compile data on ontogenetic growth for extant and fossil vertebrates, including all major dinosaur clades. Using a metabolic scaling approach, we find that growth and metabolic rates follow theoretical predictions across clades, although some groups deviate. Moreover, when the effects of size and temperature are considered, dinosaur metabolic rates were intermediate to those of endotherms and ectotherms and closest to those of extant mesotherms. Our results suggest that the modern dichotomy of endothermic versus ectothermic is overly simplistic.

@article{doi101126science1253143,
    author = "Grady, John M. and Enquist, Brian J. and Dettweiler‐Robinson, Eva and Wright, Natalie A. and Smith, Felisa A.",
    title = "Evidence for mesothermy in dinosaurs",
    year = "2014",
    journal = "Science",
    abstract = "Were dinosaurs ectotherms or fast-metabolizing endotherms whose activities were unconstrained by temperature? To date, some of the strongest evidence for endothermy comes from the rapid growth rates derived from the analysis of fossil bones. However, these studies are constrained by a lack of comparative data and an appropriate energetic framework. Here we compile data on ontogenetic growth for extant and fossil vertebrates, including all major dinosaur clades. Using a metabolic scaling approach, we find that growth and metabolic rates follow theoretical predictions across clades, although some groups deviate. Moreover, when the effects of size and temperature are considered, dinosaur metabolic rates were intermediate to those of endotherms and ectotherms and closest to those of extant mesotherms. Our results suggest that the modern dichotomy of endothermic versus ectothermic is overly simplistic.",
    url = "https://doi.org/10.1126/science.1253143",
    doi = "10.1126/science.1253143",
    openalex = "W2010661531",
    references = "doi101017cbo9780511608551, doi101017s1464793106007007, doi101038nature05634, doi101038nature11631, doi101073pnas0708903105, doi101093bioinformaticsbtg412, doi101098rsbl20070254, doi101111j109636422000tb02201x, doi101126science1061967, doi101126science1206196, doi101126science28454201677, doi1012019781420064452, doi101371journalpone0007390, doi101371journalpone0029958, doi101371journalpone0079420, doi101371journalpone0081917, doi1016660094837320030290105dbttoo20co2, doi101890039000, doi1018900814941"
}

@incollection{crossref201614,
    title = "14. HOT-BLOODED DINOSAURS?",
    year = "2016",
    booktitle = "Dinosaurs",
    url = "https://doi.org/10.7312/luca17310-016",
    doi = "10.7312/luca17310-016",
    pages = "255-276"
}

@article{farmer2016hotblooded,
    author = "Farmer, C. G.",
    title = "Hot-blooded lizard illuminates endothermy origins",
    year = "2016",
    journal = "Journal of Experimental Biology",
    url = "https://doi.org/10.1242/jeb.138156",
    doi = "10.1242/jeb.138156",
    number = "7",
    openalex = "W2322300865",
    pages = "909-910",
    volume = "219",
    references = "doi101126sciadv1500951"
}

@article{doi101093bjpsaxw005,
    author = "Currie, A.",
    title = "Hot-Blooded Gluttons: Dependency, Coherence, and Method in the Historical Sciences",
    year = "2017",
    journal = "The British Journal for the Philosophy of Science",
    url = "https://www.journals.uchicago.edu/doi/pdf/10.1093/bjps/axw005",
    doi = "10.1093/bjps/axw005",
    is_oa = "true",
    number = "4",
    pages = "929-952",
    semanticscholar_citation_count = "33",
    semanticscholar_id = "e83c8e6b0bfd5323136896102a1c68f74b918076",
    volume = "68"
}

The evolution of endothermy represents a major transition in vertebrate history, yet how and why endothermy evolved in birds and mammals remains controversial. Here, we combine a heat transfer model with theropod body size data to reconstruct the evolution of metabolic rates along the bird stem lineage. Results suggest that a reduction in size constitutes the path of least resistance for endothermy to evolve, maximizing thermal niche expansion while obviating the costs of elevated energy requirements. In this scenario, metabolism would have increased with the miniaturization observed in the Early-Middle Jurassic (~180 to 170 million years ago), resulting in a gradient of metabolic levels in the theropod phylogeny. Whereas basal theropods would exhibit lower metabolic rates, more recent nonavian lineages were likely decent thermoregulators with elevated metabolism. These analyses provide a tentative temporal sequence of the key evolutionary transitions that resulted in the emergence of small, endothermic, feathered flying dinosaurs.

@article{doi101126sciadvaaw4486,
    author = "Rezende, Enrico L. and Bacigalupe, Leonardo D. and Nespolo, Roberto F. and Bozinovic, Francisco",
    title = "Shrinking dinosaurs and the evolution of endothermy in birds",
    year = "2020",
    journal = "Science Advances",
    abstract = "The evolution of endothermy represents a major transition in vertebrate history, yet how and why endothermy evolved in birds and mammals remains controversial. Here, we combine a heat transfer model with theropod body size data to reconstruct the evolution of metabolic rates along the bird stem lineage. Results suggest that a reduction in size constitutes the path of least resistance for endothermy to evolve, maximizing thermal niche expansion while obviating the costs of elevated energy requirements. In this scenario, metabolism would have increased with the miniaturization observed in the Early-Middle Jurassic (\textasciitilde 180 to 170 million years ago), resulting in a gradient of metabolic levels in the theropod phylogeny. Whereas basal theropods would exhibit lower metabolic rates, more recent nonavian lineages were likely decent thermoregulators with elevated metabolism. These analyses provide a tentative temporal sequence of the key evolutionary transitions that resulted in the emergence of small, endothermic, feathered flying dinosaurs.",
    url = "https://doi.org/10.1126/sciadv.aaw4486",
    doi = "10.1126/sciadv.aaw4486",
    openalex = "W2997429867",
    references = "doi101038272333a0, doi101038nature13272, doi101073pnas251548698, doi101098rsbl20050378, doi101111j1469185x201000122x, doi101111j1469185x201000137x, doi101126science1253293, doi101126science493968, doi101371journalpbio1001853, doi101371journalpone0069361, doi101371journalpone0088834, doi1022179revmacn14372, doi1023071538742, doi107717peerj2159"
}

-ATPase (SERCA) in skeletal muscle, similar to a process seen in some fishes. This similarity prompted our realisation that the capacity for whole-body endothermy could even have pre-dated the divergence of Amniota into Synapsida and Sauropsida, leading us to hypothesise the homology of whole-body endothermy in birds and mammals, in contrast to the current assumption of their independent (convergent) evolution. To explore the extent of similarity between muscle NST in mammals and birds we undertook a detailed review of these processes and their control in each group. We found considerable but not complete similarity between them: in extant mammals the 'slippage' is controlled by the protein sarcolipin (SLN), in birds the SLN is slightly different structurally and its role in NST is not yet proved. However, considering the multi-millions of years since the separation of synapsids and diapsids, we consider that the similarity between NST production in birds and mammals is consistent with their whole-body endothermy being homologous. If so, we should expect to find evidence for it much earlier and more widespread among extinct amniotes than is currently recognised. Accordingly, we conducted an extensive survey of the palaeontological literature using established proxies. Fossil bone histology reveals evidence of sustained rapid growth rates indicating tachymetabolism. Large body size and erect stature indicate high systemic arterial blood pressures and four-chambered hearts, characteristic of tachymetabolism. Large nutrient foramina in long bones are indicative of high bone perfusion for rapid somatic growth and for repair of microfractures caused by intense locomotion. Obligate bipedality appeared early and only in whole-body endotherms. Isotopic profiles of fossil material indicate endothermic levels of body temperature. These proxies led us to compelling evidence for the widespread occurrence of whole-body endothermy among numerous extinct synapsids and sauropsids, and very early in each clade's family tree. These results are consistent with and support our hypothesis that tachymetabolic endothermy is plesiomorphic in Amniota. A hypothetical structure for the heart of the earliest endothermic amniotes is proposed. We conclude that there is strong evidence for whole-body endothermy being ancient and widespread among amniotes and that the similarity of biochemical processes driving muscle NST in extant birds and mammals strengthens the case for its plesiomorphy.

@article{doi101111brv12822,
    author = "Grigg, Gordon C. and Nowack, Julia and Bicudo, J. Eduardo P. W. and Bal, Naresh C. and Woodward, Holly N. and Seymour, Roger S.",
    title = "Whole‐body endothermy: ancient, homologous and widespread among the ancestors of mammals, birds and crocodylians",
    year = "2021",
    journal = "Biological reviews/Biological reviews of the Cambridge Philosophical Society",
    abstract = "-ATPase (SERCA) in skeletal muscle, similar to a process seen in some fishes. This similarity prompted our realisation that the capacity for whole-body endothermy could even have pre-dated the divergence of Amniota into Synapsida and Sauropsida, leading us to hypothesise the homology of whole-body endothermy in birds and mammals, in contrast to the current assumption of their independent (convergent) evolution. To explore the extent of similarity between muscle NST in mammals and birds we undertook a detailed review of these processes and their control in each group. We found considerable but not complete similarity between them: in extant mammals the 'slippage' is controlled by the protein sarcolipin (SLN), in birds the SLN is slightly different structurally and its role in NST is not yet proved. However, considering the multi-millions of years since the separation of synapsids and diapsids, we consider that the similarity between NST production in birds and mammals is consistent with their whole-body endothermy being homologous. If so, we should expect to find evidence for it much earlier and more widespread among extinct amniotes than is currently recognised. Accordingly, we conducted an extensive survey of the palaeontological literature using established proxies. Fossil bone histology reveals evidence of sustained rapid growth rates indicating tachymetabolism. Large body size and erect stature indicate high systemic arterial blood pressures and four-chambered hearts, characteristic of tachymetabolism. Large nutrient foramina in long bones are indicative of high bone perfusion for rapid somatic growth and for repair of microfractures caused by intense locomotion. Obligate bipedality appeared early and only in whole-body endotherms. Isotopic profiles of fossil material indicate endothermic levels of body temperature. These proxies led us to compelling evidence for the widespread occurrence of whole-body endothermy among numerous extinct synapsids and sauropsids, and very early in each clade's family tree. These results are consistent with and support our hypothesis that tachymetabolic endothermy is plesiomorphic in Amniota. A hypothetical structure for the heart of the earliest endothermic amniotes is proposed. We conclude that there is strong evidence for whole-body endothermy being ancient and widespread among amniotes and that the similarity of biochemical processes driving muscle NST in extant birds and mammals strengthens the case for its plesiomorphy.",
    url = "https://doi.org/10.1111/brv.12822",
    doi = "10.1111/brv.12822",
    openalex = "W4200490813",
    references = "cubo2020were, doi101016jgr202008003, doi101016s0092867400814105, doi101017pab201519, doi101038262207a0, doi101038nature11264, doi101038ncomms9296, doi101038s4155901910473, doi101038srep06196, doi1010719781486300679, doi101073pnas1206625109, doi101086283547, doi101093biolinneanblw044, doi101093sysbiosyw033, doi101096fj020367com, doi101098rstb20190136, doi101098rstb20190142, doi101111brv12137, doi101111j10958312201001431x, doi101126sciadvaaw4486, doi101126science1187443, doi101126science493968, doi101126scienceaal4853, doi101152physiol000162016, doi101152physrev000152003, doi1012063521, doi101210er20020012, doi101371journalpone0011613, doi101371journalpone0033539, doi101371journalpone0069361, doi105860choice355657, doi107717peerj1778, doi107717peerj7764, köhler2012seasonal, pontzer2009biomechanics, seymour1976dinosaurs, zhao2019ontogenetic"
}

@incollection{crossref202214,
    title = "14 HOT-BLOODED DINOSAURS?",
    year = "2022",
    booktitle = "Dinosaurs",
    url = "https://doi.org/10.7312/luca20600-018",
    doi = "10.7312/luca20600-018",
    pages = "255-276"
}

Abstract The “Dinosaur Renaissance” is known as a crucial event in the study of dinosaurs. From sluggish and lizard-like, they came to be conceived and represented as more dynamic animals. This paper argues that the “Dinosaur Renaissance” did not only constitute a significant scientific and artistic shift. Indeed, it can also be interpreted as a foundational episode for the historiography of paleoart. During the “Dinosaur Renaissance,” a growing community of artists and paleontologists promoted the integration of artistic processes in paleontology. They began to actively discuss the historical legacy and future of such integration. The itinerant paleoart exhibition Dinosaurs Past and Present, hosted in eight major cities across North America at the end of the 1980s, can be identified as having played a significant role in setting the foundation for the historiography of paleoart. The “Dinosaur Renaissance” did not only result in revised visual representations of dinosaurs, but also spurred some of the first investigations on the historical relationship between visual arts and paleontology. This article concludes by offering some remarks on how the present historiography of paleoart can continue to build on the efforts made during the “Dinosaur Renaissance” while remaining cognizant of their context. To effectively answer the needs of historians, as well as of paleontologists and paleoartists alike, the growing historiography of paleoart has much to gain in clarifying its own history.

@article{doi102478host20230002,
    author = "Monnin, Victor",
    title = "The Dinosaur Renaissance 1960s-80s: A Foundational Episode for the Historiography of Paleoart",
    year = "2023",
    journal = "HoST - Journal of History of Science and Technology",
    abstract = "Abstract The “Dinosaur Renaissance” is known as a crucial event in the study of dinosaurs. From sluggish and lizard-like, they came to be conceived and represented as more dynamic animals. This paper argues that the “Dinosaur Renaissance” did not only constitute a significant scientific and artistic shift. Indeed, it can also be interpreted as a foundational episode for the historiography of paleoart. During the “Dinosaur Renaissance,” a growing community of artists and paleontologists promoted the integration of artistic processes in paleontology. They began to actively discuss the historical legacy and future of such integration. The itinerant paleoart exhibition Dinosaurs Past and Present, hosted in eight major cities across North America at the end of the 1980s, can be identified as having played a significant role in setting the foundation for the historiography of paleoart. The “Dinosaur Renaissance” did not only result in revised visual representations of dinosaurs, but also spurred some of the first investigations on the historical relationship between visual arts and paleontology. This article concludes by offering some remarks on how the present historiography of paleoart can continue to build on the efforts made during the “Dinosaur Renaissance” while remaining cognizant of their context. To effectively answer the needs of historians, as well as of paleontologists and paleoartists alike, the growing historiography of paleoart has much to gain in clarifying its own history.",
    url = "https://doi.org/10.2478/host-2023-0002",
    doi = "10.2478/host-2023-0002",
    openalex = "W4380684871",
    references = "doi1010179781108671996, doi101130mwr218, doi102307428290, doi102478host20230002, doi10268791191, doi1026879145, doi105860choice305605, doi105860choice383924, doi105860choice463047, openalexw1015379046"
}

@article{chiarenza2024early,
    author = "Chiarenza, Alfio Alessandro and Cantalapiedra, Juan L. and Jones, Lewis A. and Gamboa, Sara and Galván, Sofía and Farnsworth, Alexander J. and Valdes, Paul J. and Sotelo, Graciela and Varela, Sara",
    title = "Early Jurassic origin of avian endothermy and thermophysiological diversity in dinosaurs",
    year = "2024",
    journal = "Current Biology",
    url = "https://doi.org/10.1016/j.cub.2024.04.051",
    doi = "10.1016/j.cub.2024.04.051",
    number = "11",
    openalex = "W4396921380",
    pages = "2517-2527.e4",
    volume = "34",
    references = "barta2022osteohistology, doi101002ar24130, doi101016jcub202105041, doi101016jcub202111061, doi101016jgr202008003, doi101017s0094837300004310, doi10103844766, doi101038ncomms12931, doi101038sdata2018214, doi101073pnas2213987120, doi10108003610927808827599, doi101086284325, doi101086426002, doi101093aesa383396, doi101093bioinformaticsbtu181, doi101111j2041210x201100169x, doi101111pala12514, doi101126sciadvaaw4486, doi101371journalpone0235078, doi105281zenodo16171435, doi107717peerj12362, doi107717peerj7764"
}

A new version of the description of thermobiological statuses in vertebrates is proposed: primary and secondary ectotherms, primary and secondary endotherms. Primary ectothermal animals are the first amphibian-like tetrapods (among modern animals – fish and amphibians). They had a low level of metabolism, and most of the body temperature for a number of physiological reasons could not rise above 30°C and almost did not differ from the ambient temperatures. Then they developed a complex of biochemical and physiological aromorphoses, which increased their levels of mitochondrial oxidation and basal metabolism, and began to force them to raise their body temperature. This significantly improved the quality of their activity and other functional characteristics, allowed them to go on land and begin to master it. Already the first terrestrial tetrapods (stegocephalians, seymourians) had an increased metabolism about 330 million years ago. These were basic primary endotherms – mesometabolic animals whose body temperature could hardly rise noticeably more than 30°C; they still had insufficiently developed mechanisms of regulation and control over the levels of metabolism and heat production. In the synapsid line, metabolism gradually increased along with body temperature, and through theriodonts led to the appearance of secondary endothermic animals with constantly high, controlled and regulated tachymetabolism and thermometabolism – mammals. Sauropsids also had an increase in metabolism, and in some archosaurs (dinosaurs, etc.) it sometimes rose to the level of modern birds, and body temperature reached 39–44°C. Some of them developed into secondary endothermic tachymetabolic birds, and some other – into secondary ectothermic bradymetabolic modern reptiles with a periodic increase in body temperature to 30–45°C due to external heat. But secondary ectotherms (mainly modern reptiles) are not a “return” to the state of primary ectothermy, but a powerful evolutionary step forward. Having passed through the mesothermic stage of ancient reptiles in their evolution, they acquired the ability, unlike primary ectotherms, to withstand and use high body temperature (30°C) for their functional and evolutionary benefit. It was by raising their body temperature that vertebrates increased the level of basal metabolism, improved the quality of activity, etc. Thus, the evolutionary function of reptiles is to “teach” primary ectothermic vertebrates to use high body temperature and in this regard become an “elevator” for further evolution of vertebrates. The vast majority of reptiles during their existence were meso- and tachymetabolic endothermic animals, i. e. warm-blooded to varying degrees, and bradymetabolic ectotherms, i. e., classical cold-blooded, turned out to be evolutionarily advanced modern reptiles. In general, ectothermal animals tend in their evolution to “align” with the temperature conditions of the external environment, “fit in” with them, use them. They periodically raise their body temperature due to external heat during periods when it is naturally available, thereby increasing the level of metabolism, the quality of activity and vital activity in the most energetically cheap way. Endothermic animals, on the contrary, try to reliably autonomize themselves from external conditions, raising body temperature mainly due to the endogenous thermogenesis, as a result of which their metabolism reliably and constantly increases, the quality of activity and vital activity improves. This approach is much more energy-intensive, but more reliable, and significantly less dependent on changeable environmental conditions, improving environmental valence and competitiveness. Thus, ectothermy and endothermy are two independent directions of the evolutionary development of vertebrates, each with its own strategy and ways of its implementation. At the same time, ectothermy is not a stage in the development of endothermy, but an independent evolutionary direction of the development of vertebrates, parallel to endothermy.

@article{doi1031857s0044459624030055,
    author = "Cherlin, V. A.",
    title = "The relationship between ectothermy and endothermy in evolution of vertebrates",
    year = "2024",
    journal = "Žurnal obŝej biologii",
    abstract = "A new version of the description of thermobiological statuses in vertebrates is proposed: primary and secondary ectotherms, primary and secondary endotherms. Primary ectothermal animals are the first amphibian-like tetrapods (among modern animals – fish and amphibians). They had a low level of metabolism, and most of the body temperature for a number of physiological reasons could not rise above 30°C and almost did not differ from the ambient temperatures. Then they developed a complex of biochemical and physiological aromorphoses, which increased their levels of mitochondrial oxidation and basal metabolism, and began to force them to raise their body temperature. This significantly improved the quality of their activity and other functional characteristics, allowed them to go on land and begin to master it. Already the first terrestrial tetrapods (stegocephalians, seymourians) had an increased metabolism about 330 million years ago. These were basic primary endotherms – mesometabolic animals whose body temperature could hardly rise noticeably more than 30°C; they still had insufficiently developed mechanisms of regulation and control over the levels of metabolism and heat production. In the synapsid line, metabolism gradually increased along with body temperature, and through theriodonts led to the appearance of secondary endothermic animals with constantly high, controlled and regulated tachymetabolism and thermometabolism – mammals. Sauropsids also had an increase in metabolism, and in some archosaurs (dinosaurs, etc.) it sometimes rose to the level of modern birds, and body temperature reached 39–44°C. Some of them developed into secondary endothermic tachymetabolic birds, and some other – into secondary ectothermic bradymetabolic modern reptiles with a periodic increase in body temperature to 30–45°C due to external heat. But secondary ectotherms (mainly modern reptiles) are not a “return” to the state of primary ectothermy, but a powerful evolutionary step forward. Having passed through the mesothermic stage of ancient reptiles in their evolution, they acquired the ability, unlike primary ectotherms, to withstand and use high body temperature (30°C) for their functional and evolutionary benefit. It was by raising their body temperature that vertebrates increased the level of basal metabolism, improved the quality of activity, etc. Thus, the evolutionary function of reptiles is to “teach” primary ectothermic vertebrates to use high body temperature and in this regard become an “elevator” for further evolution of vertebrates. The vast majority of reptiles during their existence were meso- and tachymetabolic endothermic animals, i. e. warm-blooded to varying degrees, and bradymetabolic ectotherms, i. e., classical cold-blooded, turned out to be evolutionarily advanced modern reptiles. In general, ectothermal animals tend in their evolution to “align” with the temperature conditions of the external environment, “fit in” with them, use them. They periodically raise their body temperature due to external heat during periods when it is naturally available, thereby increasing the level of metabolism, the quality of activity and vital activity in the most energetically cheap way. Endothermic animals, on the contrary, try to reliably autonomize themselves from external conditions, raising body temperature mainly due to the endogenous thermogenesis, as a result of which their metabolism reliably and constantly increases, the quality of activity and vital activity improves. This approach is much more energy-intensive, but more reliable, and significantly less dependent on changeable environmental conditions, improving environmental valence and competitiveness. Thus, ectothermy and endothermy are two independent directions of the evolutionary development of vertebrates, each with its own strategy and ways of its implementation. At the same time, ectothermy is not a stage in the development of endothermy, but an independent evolutionary direction of the development of vertebrates, parallel to endothermy.",
    url = "https://www.semanticscholar.org/paper/0a535d25115678eb2868ab0b07d1b7e81c4917d6",
    doi = "10.31857/s0044459624030055",
    is_oa = "true",
    number = "3",
    pages = "244-266",
    semanticscholar_citation_count = "1",
    semanticscholar_id = "0a535d25115678eb2868ab0b07d1b7e81c4917d6",
    volume = "85"
}