Hollow-cheeked Euchambersia nests alongside puffy-cheeked Charassognathus

Unique among synapsids, Euchambersia
(Broom 1931, Benoit et al. 2017; Fig. 1) had an antorbital fenestra (= maxillary fenestra and fossa, Fig. 1) that may have housed a venom gland posterior to the canine root.

Reported by Brian Switek in Scientific American online,
“Because of the uniqueness of its skull anatomy,” Benoit and coauthors conclude, “Euchambersia mirabilis is and will remain a puzzling species.”

The ability to be unique in a world of gradual accumulations of derived traits 
made this taxon interesting. I wondered, which taxon did Euchambersia nest alongside? And did that taxon have anything like the antorbital fenestra found in Euchambersia?

The two answers are 1) Charassognathus and 2) yes.

Figure 1. Euchambersia skull with colors and shifting bones added.

Figure 1. Euchambersia skull with colors and shifting bones added.

Turns out Euchambersia was not unique among synapsids
for reasons stated above because its sister in the Therapsid Skull Tree (TST, 75 taxa) Charassognathus (Fig. 2) has a skull bulge posterior to the canine root.

Figure 4. Charassognathus does not share more traits with Abdalodon than other taxa, like Bauria and Promoschorhynchops in the TT.

Figure 4. Charassognathus (SAM-PK-K10369) does not share more traits with Abdalodon than other taxa, like Bauria and Promoschorhynchops in the TST. Note the bulge posterior to the canine root.

According to Wikipedia, citing Botha, Abdala and Smith 2007
Charassognathus is a basal cynodont.

By contrast, in the TST, Charassognathus is a cynodont-mimic nesting with therocephalians. Given the state of taphonomy documented in Euchambersia, the possibility that the unique maxillary fenestra was in life covered by a thin bulge of bone, as in Charassognathus, should be considered a possibility.

Wikipedia notes,
“Charassognathus has a snout that makes up slightly less than half of the total length of its skull and a long facial process on its septomaxilla. Other than these two features its skull is that of a typical cynodont. The odd shape of its septomaxilla is more typical of therocephalians than other cynodonts indicating that it may be close to a common ancestor between the two groups.”

The same is true of Euchambersia.

Figure 4. Therapid Skull Tree with the addition of Euchambersia and Charassognathus apart from cynodonts.

Figure 4. Therapid Skull Tree with the addition of Euchambersia and Charassognathus apart from cynodonts.

Nomenclature tidbit.
According to Wikipedia, “Broom named the genus Euchambersia, which he considered “the most remarkable therocephalian ever discovered”, after the eminent Scottish publisher and evolutionary thinker Robert Chambers, whose Vestiges of the Natural History of Creation was considered by Broom to be “a very remarkable work” though “sneered at by many.”

Chambers was probably happy to get the honor and compliment from Dr. Broom, while others sneered.


References|
Benoit J, Norton LA, Manger PR and Rubidge BS 2017. Reappraisal of the envenoming capacity of Euchambersia mirabilis (Therapsida, Therocephalia) using μCT-scanning techniques. PLoS ONE 12(2): e0172047. doi:10.1371/journal.pone.0172047
Botha J, Abdala F and Smith R 2007. The oldest cynodont: new clues on the origin and diversification of the Cynodontia. Zoological Journal of the Linnean Society. 149: 477–492.
Broom R 1931. Notices of some new Genera and species of Karroo Fossil Reptiles. Rec Albany Mus. 1931; 41: 161–166.

It’s not often that all the references fall within the range of one letter. The odds against that are approximately one in 26 cubed or 17.576.

https://blogs.scientificamerican.com/laelaps/did-this-protomammal-have-a-venomous-bite/

wiki/Euchambersia
wiki/Charassognathus
wiki/Akidnognathidae

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