The recent discovery of dinosaur blood cells in a 75-million-year-old fossil may shed new light on an old controversy:
Although the cells are unlikely to contain DNA, those extracted from better preserved fossils using the same technique may do so, she says.
And even without DNA, soft tissue cells and molecules could help us learn much more about dinosaur physiology and behaviour, the team says. For example, the physical size of blood cells can reveal insights into metabolism, and the possible transition from a cold to warm-blooded existence. – from New Scientist
But were dinos always warm-blooded?
From ScienceDaily:
“Upon re-analysis, it was apparent that dinosaurs weren’t just somewhat like living mammals in their physiology — they fit right within our understanding of what it means to be a ‘warm-blooded’ mammal,” he said.
There is a backstory here because the researchers reanalyzed a study that suggested that dinosaurs occupied a middle ground between warm- and cold-blooded:
Dr. D’Emic specializes in bone microanatomy, or the study of the structure of bone on scales that are just a fraction of the width of a human hair. Based on his knowledge of how dinosaurs grew, Dr. D’Emic re-analyzed that study, which led him to the strikingly different conclusion that dinosaurs were more like mammals than reptiles in their growth and metabolism.
Dr. D’Emic re-analyzed the study from two aspects. First, the original study had scaled yearly growth rates to daily ones in order to standardize comparisons.
“This is problematic,” Dr. D’Emic explains, “because many animals do not grow continuously throughout the year, generally slowing or pausing growth during colder, drier, or otherwise more stressful seasons.
…
“Therefore, the previous study underestimated dinosaur growth rates by failing to account for their uneven growth. Like most animals, dinosaurs slowed or paused their growth annually, as shown by rings in their bones analogous to tree rings,” he explained.
He added that the growth rates were especially underestimated for larger animals and animals that live in very stressful or seasonal environments — both of which characterize dinosaurs.
The second aspect of the re-analysis with the original study takes into account that dinosaurs should be statistically analyzed within the same group as living birds, which are also warm-blooded, because birds are descendants of Mesozoic dinosaurs.
“Separating what we commonly think of as ‘dinosaurs’ from birds in a statistical analysis is generally inappropriate, because birds are dinosaurs — they’re just the dinosaurs that haven’t gone extinct.”
Feduccia would disagree, but never mind.
He explained that re-analyzing the data with birds as dinosaurs lends more support that dinosaurs were ‘warm-blooded,’ not occupants of a special, intermediate metabolic category. More.
If birds (warm-blooded) are indeed descendants of dinosaurs, the details of the transition become less complex. That is, birds did not need to evolve warm-bloodedness, in addition to everything else, if they simply inherited it.
Abstracts here (the in-between theory):
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. (free full text) – J. M. Grady, B. J. Enquist, E. Dettweiler-Robinson, N. A. Wright, F. A. Smith. Evidence for mesothermy in dinosaurs. Science, 2014; 344 (6189): 1268 DOI: 10.1126/science.1253143
and
here (the warm-blooded theory):
Grady et al. (Reports, 13 June 2014, p. 1268) suggested that nonavian dinosaur metabolism was neither endothermic nor ectothermic but an intermediate physiology termed “mesothermic.” However, rates were improperly scaled and phylogenetic, physiological, and temporal categories of animals were conflated during analyses. Accounting for these issues suggests that nonavian dinosaurs were on average as endothermic as extant placental mammals. (public access) – M. D. D’Emic. Comment on “Evidence for mesothermy in dinosaurs”. Science, 2015 DOI: 10.1126/science.1260061
Also, from the BBC
“If you’re finding soft tissues in these kinds of fossils, maybe this kind of preservation might be more common than we realised, and might even be the norm.”
Well, discovery is better than argument.
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Meanwhile, of course: