Therocephalians evolved to smaller size? Large Carnivora did not?

Brocklehurst 2019 reports,
“If these results are reliable, they support the traditional paradigm that therocephalians originated as large predators, and only later evolved small body sizes. The patterns observed in mammals do not appear to apply to therocephalians. Mammalian carnivores, once they have reached large size and a specialized bauplan, are apparently unable to leave this adaptive peak. Therocephalians, on the other hand, retreated from the hypercarnivore niche and evolved small sizes later in the Permian.”

Figure 1. Cladogram from Brocklehurt 2019, colors added. Lycosuchus, listed as a basal therocephalian by Brocklehurst, also nests close to cynodonts in the TST. No gorgonopsids are shown here. Biarmosuchus is the outgroup taxon here, a more distant outgroup taxon in the TST.

Figure 1. Cladogram from Brocklehurt 2019, colors added. Lycosuchus, listed as a basal therocephalian by Brocklehurst, also nests close to cynodonts in the TST. No gorgonopsids are shown here. Biarmosuchus is the outgroup taxon here, a more distant outgroup taxon in the TST.

Brocklehurst’s cladogram
posits that Therocephalia and Cynodontia arose as sisters from a last common ancestor: Biarmosuchus. In the therapsid skull tree (TST, 67 taxa, Fig. 4), Therocephalia (including Cynodontia) arises from Gorgonopsia (Fig. 2).

Figure 2. Gorgonopsids, therocephalians and cynodonts to scale.

Figure 2. Gorgonopsids, therocephalians and cynodonts to scale.

The question arises,
what is a ‘large size’ member of the Carnivora? Certainly big cats and walruses (Fig. 3) fall into this definition and do not give rise to smaller ancestors, as Brocklehurst notes. However, if the basalmost member of the Carnivora, Vulpavus, is considered ‘large’ then it breaks the ‘rule’ because it has smaller descendants in the LRT: Mustela and Procyon (Fig. 3). Talpa, the mole, is the smallest member of the Carnivora in the LRT. Talpa has been traditionally omitted from Carnivora studies while being wrongly lumped with the unrelated shrew, Scutisorex, instead.

Figure 3. Carnivora to scale. Note: one branch does increase in size over time (ignoring toy poodles for the moment), while another branch, the one leading to Talpa the mole, shrinks in size.

Figure 3. Carnivora to scale. Note: one branch does increase in size over time (ignoring toy poodles for the moment), while another branch, the one leading to Talpa the mole, shrinks in size. Brocklehurst is correct: once carnivores achieved large size, few to no examples of phylogenetic miniaturization appear in the fossil record.

I wish Brocklehurst 2019 had added
a few sample reconstructions to scale to help readers visualize the size ranges that he found in his cladogram. After all, the subject was ‘size’. I was unfamiliar with the vast majority of therocephalian taxa in his cladogram (Fig. 1).

Figure 4. TST revised with new data on Patranomodon and sister taxa.

Figure 4. TST revised with new data on Patranomodon and sister taxa. Here the therocephalian, Bauria, nests closer to cynodonts than in Brocklehurst 2019 (Fig. 1).

Brocklehurst is correct:
once carnivores achieved large size (Fig. 3), no examples of phylogenetic miniaturization subsequently appear. Brocklehurst contrasted this with therocephalians, presuming that Lycosuchus (Fig. 2) was a basal therocephalian, rather than a basal cynodont by definition.

Remember:
Hopson and Kitching 2001 defined  Cynodontia as the most inclusive group containing Mammalia, but excluding Bauria. In the TST (Fig. 4) Abdalodon and Lycosuchus nest on the cynodont side of Bauria.

In the TST
(Fig. 4), cynodonts show no strong size trends until mammals, like Megazostrodon (Fig. 2), evolved tiny sizes. Therocephalians likewise show no strong size trends either (but then, I have not measured every taxon in the Brocklehurt cladogram, Fig. 1). Those that also appear in the TST are in white boxes, and they appear in several clades within Therocephalia.


References
Brocklehurst N 2019. Morphological evolution in therocephalians breaks the hyper carnivore ratchet. Proceedings of the Royal Society B 286: 20190590. http://dx.doi.org/10.1098/rspb.2019.0590

5 thoughts on “Therocephalians evolved to smaller size? Large Carnivora did not?

  1. Obviously, the post-Paleozoic evolution of the mammal-like reptiles, as opposed to the archosaurs, was driven by selection forces that favored archosaurs over the mammal-like reptiles. In the former, body size tended to become smaller and smaller, whereas, in the latter, body size tended to become bigger and bigger, and ultimately gigantic.

    Consider this possibility. The mammal-like reptiles in the Permo-Triassic were very advanced physiologically and were approaching mammal-like endothermy, which means, in accordance to the Whiggish thinking of mainstream paleontology, that they should have triumphed, since they were obviously “superior,” right? In contrast, the archosaurs (typified, then as now, by crocodiles, but more famously by dinosaurs), were rather primitive physiologically, and remained ectothermic, i.e. cold-blooded, and, thus, being “inferior,” they should have been supplanted by the superior mammal-like reptiles, right? However, given that the therapsid lineages were already being suppressed by the archosaurs in the Mid-Triassic (for reasons that are not obvious to everyone), the only recourse for the advanced therapsids and their descendants was to become smaller because of their high-metabolism requirements, i.e. thanks to scaling, whereas the more primitive (low-metabolism) archosaurs continued to grow bigger and bigger.

    This is a paleontological problem of ecology and adaptation to the environment, which cannot be solved by cladistics.

    Bill Erickson

    • The external factors you mention are well-documented and form the consensus, but that story can only be told after including size as a factor within a phylogenetic framework, which is also part of the tradition and consensus. Curious what are those “for reasons that are not obvious to everyone”?

      • To what do you attribute the complete suppression of the therapsid lineages during the Middle and Late, which had dominated the Permo-Triassic fauna, “practically to the exclusion of all else” (quoting Bakker here) by the formerly obscure archosaurs?

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