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).

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. 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. 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.

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. 

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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.

 

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