Therapsid phylogeny revisited – svp abstracts 2013

From the abstract
Kammerer et al 2013 wrote: “Therapsida is comprised of five well-characterized major subclades, all of which appear simultaneously in the middle Permian fossil record: Biarmosuchia, Dinocephalia, Anomodontia, Gorgonopsia, and Eutheriodontia (containing Therocephalia and Cynodontia). Although these subclades have generally been recovered as reciprocal monophyla, the relationships between them has been subject to little consensus. In the past few decades the best-supported topology for therapsids has been a pectinate tree composed of the higher-level clades Theriodontia (Gorgonopsia + Eutheriodontia), Neotherapsida (Anomodontia + Theriodontia), and Eutherapsida (Dinocephalia + Neotherapsida). Recently, numerous advances have been made in our knowledge of the diversity and anatomy of the earliest therapsids, including new discoveries (e.g., Raranimus, Tiarajudens) and redescriptions (e.g., of the earliest known anomodont, Biseridens, and the earliest known gorgonopsian, Eriphostoma). Utilizing this new information, we have produced the most comprehensive phylogenetic analysis of early therapsid relationships yet, including almost every biarmosuchian, dinocephalian, and basal anomodont as well as representative basal dicynodonts, gorgonopsians, therocephalians, and cynodonts. The results of this analysis indicate that therapsid phylogeny is split into two major subclades (Dinocephalia+Anomodontia and Biarmosuchia + Theriodontia), with only Raranimus falling outside of this dichotomy. “Biarmosuchia” is found to be paraphyletic with regards to Theriodontia, with the South African “ictidorhinids” more closely related to theriodonts than Biarmosuchus. Dinocephalian monophyly is poorly supported, although its component subclasses Anteosauria and Tapinocephalia are recovered with strong support. “Neotherapsida” is found to be an artifact of long branch attraction; with the exception of the freestanding dentary coronoid process, all the characters traditionally used to support this clade are absent in early anomodonts like Biseridens. Intriguingly, this topology conforms with prominent pre-cladistic classifications of Therapsida, albeit with different characters supporting these relationships. Characters related to simplification of the palate and expansion of the jaw muscles are reconstructed as particularly homoplastic, with parallel trends in multiple therapsid clades. New work in the middle Permian is of vital importance towards documenting character acquisition during the rapid initial radiation of therapsids.”

Notes
Well, this doesn’t match the results of the large reptile tree, (synapsid subdivision) in which the Therapsida is essentially diphyletic (anomodonts + the rest) and all derived from Ophiacodon + Archaeothyris, not sphenacodontids. The problem may be at the very base. Despite listing “almost every’ therapsid taxon known, every good phylogenetic tree has to go deeply into its ancestral taxa to be sure the correct ancestors are found and to set patterns for the evolution of the basal therapsids. I only hope this was done.

A new tree of basal therapsids illustrated by skulls.

Figure 1. Click to enlarge and see the latest version. This small image of the family tree of the Therapsida including the Dicynodontia is not up to date.

I fear that they also included Tetraceratops, a taxon often associated with the base of the Therapsida, but actually is a limnoscelid close to Tseajaia. Kammerer considered the basal therapsid, Stenocybus, a juvenile dinocephalian. So that’s a problem in that this key taxon at the base of the Anomodontia might not have been included because of its purported juvenile status.

Biseridens was not recovered as an anomodont, but as a dinocephalian in the large reptile tree. We need to look at this together.

Perhaps not a problem here, but…
There’s a disturbing habit emerging of professional paleontologists employing highly derived taxa, like Mesosuchus (Nesbitt 2011) and Tetraceratops, as basal (plesiomorphic) taxa. Remember, it’s the plain brown sparrows of the world that ultimately give rise to the wild bizarre forms, not the other way around.In any case, I look forward to the publication of this work.

References
Kammerer C, Jansen M and Frobisch J 2013. Therapsid phylogeny revisited. Journal of Vertebrate Paleontology abstracts 2013.

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