Mammal-bird thermogenesis

[creature386]

So, I'm sure many of you are aware of that paper on mammal and bird endothermy.

onlinelibrary.wiley.com/doi/10.1111/brv.12822

Its central thesis, if you want to call it that way, is that whole-body endothermy in birds and mammals is homologous and that whole-body endothermy is ancestral to Amniota with lepidosaurs, testudines, and crocodilians having lost it secondarily.

This sounds pretty extraordinary, tbh. I haven't found many takes on the paper yet online, maybe because it's so new. I've only seen some discussions on r/Paleontology. It was generally agreed that it made great testable predictions and it was brave in its hypotheses (much like the Ornithoscelida paper), but we'll probably need more research to settle it.

An objection that has been raised was the lack of four-chambered hearts in lepidosaurs and testudines. The authors addressed this and said they were lost over the course of evolution, though that makes me wonder why that didn't happen in crocodiles as well.

Someone also brought Argentine black tegus up, who sometimes display endothermic behavior:

en.wikipedia.org/wiki/Argentine_black_and_white_tegu#Endothermic_behavior

The Reddit thread in question is here:

www.reddit.com/r/Paleontology/comments/rtkx2a/so_theres_a_recent_study_suggesting_that_mammals/

Since the discussion wasn't terribly long, I'm interested in what this forum thinks.

[Infinity Blade]

I love that guy who says "extreme claims require extreme evidence" and just leaves it at that...with no elaboration from himself whatsoever.

The authors address the lack of four-chambered hearts in lepidosaurs and testudines by saying that their hearts are, in fact, more complex than those of their ancestors.

Accompanying the reversion to ectothermy were changes in heart anatomy and function. Although the Crocodylia retain the four-chambered heart of their endothermic ancestors, the extant rhynchocephalians, squamates and testudines do not. Their hearts are in fact more complex, with two atria and a ventricle comprising three chambers, the cavum venosum, cavum pulmonale and cavum arteriosum (Jensen et al., 2010) which, despite being anatomically connected, can achieve significant separation of arterial and venous blood through their internal architecture and timing (Grigg & Simons, 1972; Millard & Johansen, 1974; Shelton & Burggren, 1976; Wang et al., 2003). The embryological development of all amniote hearts is similar, with the final state essentially dependent on the growth of the interventricular septum (for details, see Seymour et al., 2004). Briefly, all amniotes pass through a stage where the septum grows towards the outflow vessels but initially does not separate the ventricle anatomically. In birds, mammals and crocodiles, the septum continues extending and separates the ventricle, but in slightly different ways. In a reversion from endothermy to ectothermy, however, abbreviation of the septum's growth sets up the situation seen in extant (non-avian) reptiles. Interestingly, similar congenital ventricular septal defects occur in humans, including the Tetralogy of Fallot, and are not uncommon, showing that an evolutionary conversion of a completely divided heart into an incompletely divided one is not difficult to imagine. Such a shift would be more detrimental for a tachymetabolic endotherm than a bradymetabolic ectotherm. Hence, if their ancestry did include endothermy, the retention of this ‘defect’ in extant squamates and testudines could be expected, especially since it might confer several postulated advantages (Hicks & Wang, 1996; Burggren, Filogonio & Wang, 2020).

I wonder how we can test the idea that these "lesser-chambered" hearts of lepidosaurs and testudines are actually a derived condition. If this is the case, I feel like we would expect differences between the heart anatomy of lepidosaurs and testudines, reflecting similar, but not identical solutions to the same problem. Or would these differences just reflect derived conditions from their long and separate evolutionary histories?

The discussion on H-H distance as a proxy for endothermy is intriguing, though. If true, it would be further evidence of many non-avian dinosaurs being endothermic.

[creature386]

Jan 5, 2022 20:34:16 GMT 5 Infinity Blade said:

I wonder how we can test the idea that these "lesser-chambered" hearts of lepidosaurs and testudines are actually a derived condition. If this is the case, I feel like we would expect differences between the heart anatomy of lepidosaurs and testudines, reflecting similar, but not identical solutions to the same problem. Or would these differences just reflect derived conditions from their long and separate evolutionary histories?

Interesting thought. I think, as a general rule, if we are to expect homology among bird and mammal thermogenesis features, we should, at the very least, expect a relative absence of homology among lepidosaurs and testudines. It wouldn't be strong evidence, but at least a puzzle piece

Jan 5, 2022 20:34:16 GMT 5 Infinity Blade said:

I love that guy who says "extreme claims require extreme evidence" and just leaves it at that...with no elaboration from himself whatsoever.

Yeah, that's Reddit for you, not far above YouTube. And look at the upvotes...

But seriously though, this made me think: How extreme is the claim really? The standard model assumes one convergence: Bird and mammal endothermy (I will ignore crocodiles here, as they are assumed to have evolved secondary ectothermy in either scenario). The model in the paper also assumed ones convergence: Testudine and lepidosaur ectothermy.

So, it doesn't seem like one is more or less parsimonous than the other. I think part of why people might consider the paper's claim extreme is that it challenges deeply held notions (even among people on a dedicated paleontology subreddit) that mammals are supposed to be above reptiles in the Great Chain of Being and thus, any traits reptiles have that we don't have must be ancestral. That, and there's the fact that the hypothesis is new.

[Infinity Blade]

Jan 6, 2022 22:26:38 GMT 5 creature386 said:

But seriously though, this made me think: How extreme is the claim really? The standard model assumes one convergence: Bird and mammal endothermy (I will ignore crocodiles here, as they are assumed to have evolved secondary ectothermy in either scenario). The model in the paper also assumed ones convergence: Testudine and lepidosaur ectothermy.

So, it doesn't seem like one is more or less parsimonous than the other. I think part of why people might consider the paper's claim extreme is that it challenges deeply held notions (even among people on a dedicated paleontology subreddit) that mammals are supposed to be above reptiles in the Great Chain of Being and thus, any traits reptiles have that we don't have must be ancestral. That, and there's the fact that the hypothesis is new.

I can understand that one might see the idea of modern reptile ectothermy being derived as a rather "extraordinary" trait, but what you point out about the number of convergences in both scenarios is a good point. The "Great Chain of Being" and mammalian chauvinism is something I felt like I've seen before. I remember arguing with a guy who thought the crocodilians hunting in packs thing was sus and eventually said the "extraordinary claims require extraordinary evidence" thing. As if it was so extraordinary that an animal that's smart enough to use sticks to bait nesting birds could also think of scaring a prey item to another watering hole into the jaws of a conspecific.

[creature386]

I'm in a lecture about dinosaur endothermy at the moment (FYI, the suffocating college work and the exam preparations are why I can't post so much right now). Our professor showed this graph, based on bone histology of extant and extinct taxa:

www.researchgate.net/publication/301279899_Palaeohistological_Evidence_for_Ancestral_High_Metabolic_Rate_in_Archosaurs
Note that this paper has also been cited in Grigg et al. (2021) a few times.

It seems like the notion of endothermy being basal to amniotes (or even tetrapods as a whole) is older than I thought.

[Infinity Blade]

Indeed, I could have sworn seeing that figure being borrowed by the recent paper before. Or maybe I'm just imagining things.