Dvinia enters the TST

Not to be confused with Dvinosaurus (a basal tetrapod)…
Dvinia (Fig. 1) is a rabbit-sized chiniquodontid cynodont with a fang-pierced rostrum and a high cranial crest.

Figure 1. From Amalitskii 1922, Dvinia skull and mandible from various views.

Figure 1. From Amalitskii 1922, Dvinia skull and mandible from various views slightly larger than actual size.

Dvinia prima (Amalitskii 1922; Late Permian, 254mya; 7 cm skull) nests between Chiniquodon and Pachygenelus in the Therapsid Skull Tree (TST, 69 taxa). The lower canine fit into a maxillary opening. The molars had a circle of cusps around a single large cusp. The postorbital is very tiny, a vestige that would be lost in derived taxa, like basal mammals and Pachygenelus. The lateral temporal fenestrae were huge housing strong jaw muscles, divided by a narrow crest in which a smal brain was located.

Ivakhenko 2013 reported:
“The study of the skull of the Late Permian cynodont Dvinia prima Amalitzky, 1922 shows a combination of the general primitive skull design (many incisors, preservation of the precanine and large interpterygoid fenestra, etc) with the development of a number of “advanced” features (expansion of the temporal fenestra, development of the parietal crest, and closed pineal foramen, unusual structure of the premaxilla, complicated postcanines, and reduction of the angular wing). Dvinia prima is treated as a specialized omnivore and assigned to the family Dviniidae Sushkin, 1928 of the superfamily Thrinaxodontoidea Seeley, 1894.”

Double canines
sometimes appear in theriodonts (gorgonpopsids, therocephalians, cynodonts) and other synapsids. The second is a replacement canine, so it is not a trait one can score in phylogenetic analysis.


Sidenote:
My computer was in the shop for about 48 hours Friday and Saturday after downloading a virus originating from .tk (Tokelau, a territory of New Zealand located in the South Pacific famous for free domain registry and malicious web masters) that I downloaded when I clicked on a Facebook video that was supposed to show a sperm whale rotating underwater along with a diver. Do not click on that video.


References
Amalitskii VP 1922. Diagnoses of the new forms of vertebrates and plants from the upper Permian of North Dvia: Bulletin de l’Académie des Sciences de l’URSS, Math and Natural Sciences, 1922, p. 329-340. and in Izv. Ross. Akad. Nauk, Ser. 6 25 (1), 1–12.
Ivakhnenko MF 2013. Cranial Morphology of Dvinia prima Amalitzky (Cynodontia, Theromorpha). Paleontological Journal 47( 2): 210–222. © Pleiades Publishing, Ltd., 2013. Original Russian Text published in Paleontologicheskii Zhurnal, 2013, No. 2:81–93.

wiki/Dvinia

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