The origin of endothermy triggered by the P-Tr extinction event?

Benton 2020 reports,
“the emergence of endothermy in a stepwise manner began in the Late Permian but accelerated in the Early Triassic. The trigger was the profound destruction wrought by the Permian-Triassic mass extinction (PTME).”

Two more abbreviations found in Benton 2020 that will be making the rounds:

  1. Mesozoic Marine Revolution (MMR)
  2. Triassic Terrestrial Revolution (TTR).

According to Benton 2020:
“Among tetrapods, both synapsids and archosaurs survived into the Triassic, survivors were marked by the acquisition of endothermy, as shown by bone histology, isotopic analyses, and the acquisition of insulating pelage. Both groups before the PTME had been sprawlers; after the event they adopted parasagittal (erect) gait.”

Actually lots of other clades lacking endothermy, a pelage or a parasagittal gait also survived into the Triassic, as everyone knows.

Actually, none of these traits appeared in any of the above-named groups until the Middle Triassic and then tentatively. Even so, that was just a few million years after the extinction event.

Actually, one group of lepidosaurs also produced endotherms with insulating pelage: fenestrasaurs (including pterosaurs; Fig. 1). This has been a thorn in Benton’s side since Peters 2000–2009. Benton and his colleagues have ignored and omitted these peer-reviewed contributions to the literature for the last twenty years to preserved some sort of status quo.

As everyone knows., crocs returned to a sprawling gait and ectothermy. Hmmm…

Figure 3. The origin of pterosaurs now includes Kyrgyzsaurus, nesting between Cosesaurus and Sharovipteryx.

Figure 1. The origin of pterosaurs now includes Kyrgyzsaurus, nesting between Cosesaurus and Sharovipteryx.

Benton continues making classic mistakes by

  1. including pterosaurs with dinosaurs.
  2. omitting the bipedal basal crocodylomorphs that are essential to the dinosaur origins story.
  3. omitting the basal bipedal fenestrasaurs that are essential to the pterosaur origins story.
  4. omitting Repenomamus and other mammal mimics.

Benton claimed
“there is now substantial evidence that dinosaurs originated in the Early Triassic following several discoveries in 2010 and 2011. Asilisaurus, from the Manda Formation (Anisian, Middle Triassic, 247–242 Ma) of Tanzania, and postulated that this was a representative of a new group called the silesaurids, close to dinosaurs.”

Unfortunately, adding taxa reveals
the poposaur Silesaurus and Asiliisaurus are poposaurs, dinosaur mimics not close to dinosaurs.

Another Middle Triassic poposaur with erect limbs,
Lotosaurus
, would have supported Benton’s claims, but was not mentioned. The origin of the poposaurs (Turfanosuchus (Fig. 2) certainly could go back to the Early Triassic.

Figure 1. Poposauridae revised for 2014. Here they are derived from Turfanosuchus at the base of the Archosauria, just before crocs split from dinos.

Figure 2. Poposauridae revised for 2014. Here they are derived from Turfanosuchus at the base of the Archosauria, just before crocs split from dinos.

Benton perpetuates the myth
of the Pseudosuchia, an invalid clade in the LRT.

Benton employs too few taxa
to realize the basal dichotomy between Euarchosauriformes and Pararchosauriformes that preceded the extinction event.

Likewise, Benton employs too few taxa
to realized the basal reptile split between lepidosauromorphs (including pterosaurs and rhynchosaurs) and archosauromorphs (including synapsids, non-lepidosaur diapsids and archosauriformes) goes back to the Early Carboniferous).

Key taxa,
like Euparkeria, Youngosuchus, Decuriasuchus, Turfanosuchus, Pseudhesperosuchus, Gracilisuchus and PVL 4597 (Fig. 2), are not mentioned in the Benton text.

The Middle Triassic is when archosauriformes radiated widely,
perhaps with Early Triassic and Late Permian roots, but everyone knew this already.

In short,
there is little to nothing new here and lots of mythology. I think everyone knew animals on planet Earth essentially picked themselves up and started all over again after the end Permian extinction event which wiped out 95% of all species.

Figure 1. The origin of dinosaurs in the LRT to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.  Note the phylogenetic miniaturization at the origin of Archosauria (Crocs + Dinos).

Figure 3. The origin of dinosaurs in the LRT to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.  Note the phylogenetic miniaturization at the origin of Archosauria (Crocs + Dinos).

More taxon inclusion would have saved this paper.
You may remember Benton and Hone 2007, 2009 also deleted taxa and citations to achieve the end they sought, based on Benton’s 1999 paper on pterosaur origins, featuring the tiny bipedal croc with tiny hands, Scleromochlus. Sadly, Benton’s latest cherry-picking and machinations are destroying any good reputation earlier work may have earned him.


References
Benton MJ 2020. The origin of endothermy in synapsids and archosaurs and arms races in the Triassic, Gondwana Research (2020), https://doi.org/10.1016/j.gr.2020.08.003
Hone DWE and Benton MJ 2007. An evaluation of the phylogenetic relationships of the pterosaurs to the archosauromorph reptiles. Journal of Systematic Palaeontology 5:465–469.
Hone DWE and Benton MJ 2008. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330

http://reptileevolution.com/pterosaur-wings.htm

https://pterosaurheresies.wordpress.com/2012/04/13/a-supertree-of-pterosaur-origins-hone-and-benton-2007-2009/

Shrinking dinosaurs and the evolution of endothermy in birds

A new paper by Rezende et al. 2020
correlate small size with endothermy at the genesis of birds from larger theropod precursors.

A problems arises
due to taxon exclusion at the origin of dinosaurs (Fig. 1) when small size, bipedalism and the genesis of proto-feathers already correlates with endothermy… tens of millions of years before the advent of birds. Rezende et al. apparently decided not to include the genesis of dinosaurs in their study… but should have done so.

Figure 1. The origin of dinosaurs in the LRT to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.  Note the phylogenetic miniaturization at the origin of Archosauria (Crocs + Dinos).

Figure 1. The origin of dinosaurs in the LRT to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.  Note the phylogenetic miniaturization at the origin of Archosauria (Crocs + Dinos).

From the abstract:
“The evolution of endothermy represents a major transition in vertebrate history, yet how and why endothermy evolved in birds and mammals remains controversial.”

Controversial? No. Everyone knows the warm-blooded, high-energy tetrapods all had their genesis after phylogenetic miniaturization. We covered that earlier with mammals, dinosaurs and pterosaurs.

“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.”

This has been known for decades.

“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.”

The authors are unaware that phylogenetic miniaturization preceded the origin of dinosaurs in the tiny Middle Triassic taxon PVL 4597 (Figs. 1, 4).

“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.”

Seems logical, but as mentioned above, these authors are a few nodes too late. Small endothermic dinosaurs were present in the Late Triassic following PVL 4597.

The Rezende et al. cladogram
(Fig. 2) includes many large to giant theropod dinosaurs and it does not match the large reptile tree (LRT, 1631+ taxa, Fig. 3), which includes more smaller theropods.

Figure 2. Cladogram from Rezende et al. with colors added to show three size classes, under a meter, about a meter, and more than a meter in length.

Figure 2. Cladogram from Rezende et al. with colors added to show three size classes, under a meter, about a meter, and more than a meter in length. Note the transition from large (purple) to medium (green) to little (small). Compare to figure 3 from the LRT.

Figure 4 in Rezende et al.
shows the evolution of ectothermy (240-220mya) to inertial homeothermy (giant taxa, 370kg, 215-190mya) to feathers (190-160mya) to endothermy (180-160mya) to flight (170-160mya).

Taxon inclusion sets can be biased
to present the story you want to tell. In the Rezende et al. cladogram (Fig. 2) a large number of Middle and Late Jurassic giants are included. In the LRT (Fig. 3) small taxa are present throughout the lineage of theropods. Scipionyx (at the base of Jurassic large theropods) is also a small taxon, but workers consider it a juvenile of a medium-sized taxon.

Figure 3. Subset of the LRT focusing on theropods and basal birds. Colors added for large (greater than a meter), medium (about a meter), and small (less than a meter) in length. Compare to figure 2 from Rezende et al. Note the depth of small taxa, some of which give rise to large taxa.

Figure 3. Subset of the LRT focusing on theropods and basal birds. Colors added for large (greater than a meter), medium (about a meter), and small (less than a meter) in length. Compare to figure 2 from Rezende et al. Note the depth of small taxa, some of which give rise to large taxa. Scipionyx at the base of the giant Jurassic theropods, is also a tiny taxon, but is considered a juvenile.

From the Rezende et al. discussion section:
“Two exceptional phenomena are observed during the evolution of birds: a sustained (but not necessarily gradual) miniaturization lasting millions of years and the emergence of endothermy. We argue that these phenomena are mechanistically linked.”

Figure 4. The genesis of the Archosauria embodied in PVL 4597 to scale with a modern archosaur, Cyanocitta, the blue jay.

Figure 4. The genesis of the Archosauria embodied in PVL 4597 to scale with a modern archosaur, Cyanocitta, the blue jay.

Unfortunately,
taxon exclusion invalidates this entire paper. The origin of the clade Archosauria (crocs + dinosaurs) had its genesis in a tiny taxon, PVL 4597 (Figs. 1, 4). That’s where endothermy first evolved. That’s where extradermal membranes (proto-feathers on naked skin) first appeared (more or less retained in both theropods and phytodinosaurs) and later turned into scales on larger dinos.

Birds likely have a higher endothermy
than non-avian theropods, and giant dinosaurs might have had a lower endothermy than smaller dinosaurs, but small theropods with a high endothermy and a bipedal configuration were present throughout the Triassic and Jurassic.

Many times tiny dinosaurs gave rise to
medium, large and giant clades in the LRT. The origin of birds was not the first time dinosaurs became small and endothermic. It was the second time.

If the Rezende et al.  paper sounds familiar, it is.
…and we looked at it earlier here. 


References
Rezende EL, Bacigalupe LD, Nespolo RF and Bozinovic F 2020. Shrinking dinosaurs and the evolution of endothermy in birds. Science Advances 2020:6 eaaw4486 1 January 2020
Lee MSY, Cau A, Naish D and Dyke GJ 2014. Sustained miniaturization and anatomicial innovation in the dinosaurian ancestors of birds. Science 345(6196):562–566.

 

Several traits indicate pterosaurs were aerobic and endothermic (warm-blooded)

Pterosaurs, by all accounts, were not your ordinary saurians.
Pterosaurs arose from a previously unreported clade of extinct lepidosaurs, the Tritosauria, not from any living squamates. They could fly and some were fantastically adorned with crests and soft tissues that enabled flight. Moreover, many, if not all, had hair/fibers/fur. The origin of these fibers appears to be in non-volant Middle Triassic fenestrasaurs at the level of Sharovipteryx and Cosesaurus, long before dinosaurs and birds developed protofeathers.

Living lizards are ectothermic (cold-blooded). Pterosaurs are widely considered to be endothermic (warm-blooded) due to their fur-covering, but that’s not the complete story. There’s more:

Jeholopterus2013-588

Figure 1. Jeholopterus in lateral view. Note the extreme length of the dermal fibers, unmatched by other pterosaurs, likely to keep biting insects away from its sensitive skin as it exploited and made wounds on dinosaurs.

1. Ptero fur — aka: pycnofibers, covered pterosaur bodies according to several well-preserved fossils of small pterosaurs. Most of our evidence for a pelage comes from small German and Chinese pterosaurs, but at least some specimens of Zhejiangopterus, a large azhdarchid, had ptero-fur. Preservation of hair apparently depends on subtle differences in substrate geochemistry. Like feathers, ptero-fur could have had uses other than trapping body heat, like keeping flying insects (mosquitoes and flies) from biting sensitive skin, as in Jeholopterus (Fig. 1) and, or course, could be considered decoration or camouflage if striped, spotted or colored.

2. Tiny adult size and even tinier hatchlings — We’ve seen in phylogenetic analysis that tiny pterosaurs succeeded fading larger clades and preceded expanding larger clades. Thus reducing adult size was a survival mechanism for the gene pool. Since moisture loss and heat loss would have been more stressful for tiny pterosaurs and especially the hatchlings of tiny pterosaurs, a pelage might have been useful to keep the wee ones warm, but mostly moist. “Endothermy originated in smaller, active eurythermal ectotherms living in a cool but variable thermal environment,” according to Clarke and Pörtner 2010. Desiccation is the main problem facing today’s tiniest reptiles, all of whom are restricted to moist leaf litter environs (Hedges and Thomas 2001). Unfortunately, we have no examples of tiny pterosaurs with ptero-fur.

Pterodactylus with hair in life pose, preparing to take off.

Figure 1. Pterodactylus with hair in life pose, preparing to take off.

Conversely large pterosaurs with soft and hard crests and extremely long necks and wings increased their surface-to-volume ratios, expanding these natural passive heat radiators when deploying their wings, evidently reducing the need for insulation and fur. We don’t see the body diameter length ptero-hair on large pterosaurs, like we do on Jeholopterus. Rather large pterosaurs, like Zhejiangopterus, appear to have had a short pelage.

3. Flying — Active muscle rapidly gets warm and steady activity due to flying gets a boost from an endothermic aerobic metabolism. The most widely accepted explanation for the evolution of endothermy has been selection for enhanced aerobic capacity.

On the flip side, flying by its very nature, requires a constant airstream and with it, heat loss by convection — if the ambient air is cooler than the body. This is emphasized in pterosaurs with their long wings laced with blood vessels, perhaps acting like giant gills, if not in oxygenation, then in heat exchange.

4. Short, laterally stiff torso — Most lizards cannot breathe while running quadrupedally. Undulating lizards experience Carrier’s constraint because their lungs cannot fill with air while laterally undulating (one lung compresses as the other expands then vice versa beneath the expanding and contracting ribcage). Short torso pterosaurs (and Sharovipteryx) did not undulate. Like birds, they don’t use their tail muscles to retract their hind limbs. Femora retractors have shifted to the enlarged hips. Pterosaurs breathed like we do and like birds do, by expanding both sides of their ribcage at once. (Not but rotating their prepubes back and forth! Gaak!)

5. Hollow bones –- Like warm-blooded birds, many pterosaurs had hollow bones that probably contained air sacs that inflated and cooled the bones with air from their advanced lungs.

6. Erect hind limbs —  Like warm-blooded birds, pterosaurs walked with more or less erect hind limbs that elevated their bellies far above the substrate. Maintaining this configuration required more energy than belly-floppers typically muster.

Clarke and Pörtner (2010) declared the metabolic status of pterosaurs remains unresolved. They reported, “Endothermy has evolved at least twice, in the therapsid-mammal and theropod-bird lineages. The benefits of endothermy are clear: a high and relatively constant internal body temperature allows a fine tuning of metabolism, high muscular power output, fast growth, and a significant degree of independence from environmental temperature. The costs are also well understood: the high rate of metabolism needed to sustain endothermy requires a great deal of food. Undoubtedly the most successful hypothesis, however, has been the suggestion of Bennett & Ruben (1979) that the key factor in the evolution of endothermy was selection for an enhanced aerobic capacity to allow increasingly sustained locomotor activity. The evolution of a higher body temperature and endothermy followed as secondary events. This proposal, the aerobic scope hypothesis, has withstood two decades of further research, and it remains the most widely accepted theory for the evolution of endothermy.”

Benefits of a warmer body

  1. Processing of food proceeds more rapidly
  2. Speed of nervous conduction is temperature dependent
  3. Higher growth rates in the young
  4. Improved food-gathering capability by adults for provisioning developing young. [This is likely not important for pterosaurs, who were likely independent from the moment of hatching because they could fly.]

Embryo development
Like other lizards, pterosaur mothers held eggs within their bodies until just before hatching. This warmth likely decreased the in-utero period by accelerating the embryo’s development, enabling flying shortly after hatching. Since pterosaurs likely laid only one egg at a time, (none have been found in clutches), accelerating embryo development would have increased the reproductive rate, especially among tiny pterosaur adults, which reached adulthood more rapidly.

References
Clarke A and Pörtner H-O 2010. Temperature, metabolic power and the evolution of endothermy. Biological Reviews online.
Hedges SB and Thomas R 2001. At the Lower Size Limit in Amniote Vertebrates: A New Diminutive Lizard from the West Indies. Caribbean Journal of Science 37:168–173.