Evidence of a High Metabolic Rate in Archosaurs

Palaeohistological Evidence for Ancestral High Metabolic Rate in Archosaurs

Authors:

Legendre et al

Abstract:

Metabolic heat production in archosaurs has played an important role in their evolutionary radiation during the Mesozoic, and their ancestral metabolic condition has long been a matter of debate in systematics and palaeontology. The study of fossil bone histology provides crucial information on bone growth rate, which has been used to indirectly investigate the evolution of thermometabolism in archosaurs. However, no quantitative estimation of metabolic rate has ever been performed on fossils using bone histological features. Moreover, to date, no inference model has included phylogenetic information in the form of predictive variables. Here we performed statistical predictive modelling using the new method of phylogenetic eigenvector maps on a set of bone histological features for a sample of extant and extinct vertebrates, in order to estimate metabolic rates of fossil archosauromorphs. This modelling procedure serves as a case study for eigenvector-based predictive modelling in a phylogenetic context, as well as an investigation of the poorly known evolutionary patterns of metabolic rate in archosaurs. Our results show that Mesozoic theropod dinosaurs exhibit metabolic rates very close to those found in modern birds, that archosaurs share an higher ancestral metabolic rate than that of extant ectotherms, and that this derived high metabolic rate was acquired at a much more inclusive level of the phylogenetic tree, among non-archosaurian archosauromorphs. These results also highlight the difficulties of assigning a given heat production strategy (i.e. endothermy, ectothermy) to an estimated metabolic rate value, and confirm findings of previous studies that the definition of the endotherm/ectotherm dichotomy may be ambiguous.

Were Dinosaurs Mesotherms?

Evidence for mesothermy in dinosaurs

Authors:

Grady et al

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.

Ectotherms may Have Issues in a Warming World

Climate change may even test lizards' famous ability to tolerate and escape the heat -- making habitat protection increasingly vital -- according to a new study by UBC and international biodiversity experts.

The study, published today in the Proceedings of the National Academy of Sciences, looks at the heat and cold tolerance of 296 species of reptiles, insects and amphibians, known as ectotherms. The researchers discovered that regardless of latitude or elevation, cold-blooded animals across the world have similar heat tolerance limits. However, species in the tropics rely more on behaviour to survive, burrowing or finding shade to shield themselves from the sun.

"By comparing temperature tolerance limits to estimated body temperatures of animals exposed to the sun, we've found that species at low latitudes rely on shade and habitat," says UBC climate-change ecologist Jennifer Sunday, lead author on the paper. "Very few species have any extra heat tolerance."

As the world warms, most ectotherms will rely increasingly on behavioral thermoregulation, making it vital to protect migration corridors and habitats that provide shade, especially in the tropics.

link.

Did DInosaurs Grow Like Very big Ectotherms?

Allometries of Maximum Growth Rate versus Body Mass at Maximum Growth Indicate That Non-Avian Dinosaurs Had Growth Rates Typical of Fast Growing Ectothermic Sauropsids

Authors:

Werner et al

Abstract:

We tested if growth rates of recent taxa are unequivocally separated between endotherms and ectotherms, and compared these to dinosaurian growth rates. We therefore performed linear regression analyses on the log-transformed maximum growth rate against log-transformed body mass at maximum growth for extant altricial birds, precocial birds, eutherians, marsupials, reptiles, fishes and dinosaurs. Regression models of precocial birds (and fishes) strongly differed from Case’s study (1978), which is often used to compare dinosaurian growth rates to those of extant vertebrates. For all taxonomic groups, the slope of 0.75 expected from the Metabolic Theory of Ecology was statistically supported. To compare growth rates between taxonomic groups we therefore used regressions with this fixed slope and group-specific intercepts. On average, maximum growth rates of ectotherms were about 10 (reptiles) to 20 (fishes) times (in comparison to mammals) or even 45 (reptiles) to 100 (fishes) times (in comparison to birds) lower than in endotherms. While on average all taxa were clearly separated from each other, individual growth rates overlapped between several taxa and even between endotherms and ectotherms. Dinosaurs had growth rates intermediate between similar sized/scaled-up reptiles and mammals, but a much lower rate than scaled-up birds. All dinosaurian growth rates were within the range of extant reptiles and mammals, and were lower than those of birds. Under the assumption that growth rate and metabolic rate are indeed linked, our results suggest two alternative interpretations. Compared to other sauropsids, the growth rates of studied dinosaurs clearly indicate that they had an ectothermic rather than an endothermic metabolic rate. Compared to other vertebrate growth rates, the overall high variability in growth rates of extant groups and the high overlap between individual growth rates of endothermic and ectothermic extant species make it impossible to rule out either of the two thermoregulation strategies for studied dinosaurs.

Evolution of Feather Color in Dinosaurs & Birds

Melanosome evolution indicates a key physiological shift within feathered dinosaurs

Authors:

Li et al

Abstract:

Inference of colour patterning in extinct dinosaurs has been based on the relationship between the morphology of melanin-containing organelles (melanosomes) and colour in extant bird feathers. When this relationship evolved relative to the origin of feathers and other novel integumentary structures, such as hair and filamentous body covering in extinct archosaurs, has not been evaluated. Here we sample melanosomes from the integument of 181 extant amniote taxa and 13 lizard, turtle, dinosaur and pterosaur fossils from the Upper-Jurassic and Lower-Cretaceous of China. We find that in the lineage leading to birds, the observed increase in the diversity of melanosome morphologies appears abruptly, near the origin of pinnate feathers in maniraptoran dinosaurs. Similarly, mammals show an increased diversity of melanosome form compared to all ectothermic amniotes. In these two clades, mammals and maniraptoran dinosaurs including birds, melanosome form and colour are linked and colour reconstruction may be possible. By contrast, melanosomes in lizard, turtle and crocodilian skin, as well as the archosaurian filamentous body coverings (dinosaur ‘protofeathers’ and pterosaur ‘pycnofibres’), show a limited diversity of form that is uncorrelated with colour in extant taxa. These patterns may be explained by convergent changes in the key melanocortin system of mammals and birds, which is known to affect pleiotropically both melanin-based colouration and energetic processes such as metabolic rate in vertebrates, and may therefore support a significant physiological shift in maniraptoran dinosaurs.

An Ectothermic Mammal Found?

A prehistoric goat survived for millennia on a resource-poor island by living like a reptile—changing its growth rate and metabolism to match the available food supply, according to a new study of the animal's bones.

The discovery marks the first time scientists have seen this cold-blooded survival strategy in mammals.

[...]

Fossils of the ancient goat, called Myotragus, were first found on Majorca in the early 1900s. The bones show the species lived on the island for more than five million years.

[...]

But the new study, which looked at the bone histories of several Myotragus individuals, revealed that the goats may have fine-tuned their growth and metabolic rates both seasonally and during irregular times such as droughts—just like reptiles.

"This way, it burned only the energy that was available from the environment, slowing down the 'fire of life' in times when resources became scarce," Köhler said.

The lizard-like lifestyle, however, meant that Myotragus's newborns were extremely small—the sizes of large rats—and the young took years to reach adult size.

The goat also saved a lot of energy in its nervous system—among the body's most "costly" tissues—by sporting a brain only half the size of a similar-size hoofed mammal and eyes only a third as large.

The combined effect was that Myotragus was sluggish, with slow reaction times, the bone study suggests.

Like modern-day reptiles, the goats probably "saved as much energy as possible just lying around and basking in the sun," Köhler said.

"The postcranial skeleton indicates that this animal was not able to run, jump, or move fast around, and [would have been] easy prey."

Someone have a link to the paper? I'd like to see if this is really the case that the goat lost endothermy. It has been posited for crocs, but not many other animals.

Note the brain size decrease, folks.