Finally, a single basal therapsid: Cutleria

Updated May 10, 2016 with a new cladogram. 

Earlier we considered Cutleria (Fig. 1, Lewis and Vaughn 1965, USNM 22099, early Permian, 299-290mya) a basal therapsid based on its morphology, but not tested in phylogenetic analysis. Laurin (1994) redescribed Cutleria wilmarthi as a sister to the Sphenacodontidae + Therapsida at a time when therapsids were thought to descend from sphenacodontids. Wiki considers Cutleria a basal sphenacodontid.

Before today
The large reptile tree recovered a diphyletic Therapsida, with Anomodontia on one branch and the remainder of the therapsids on another.

Figure 3. Basal therapsid tree.

Figure 3. Basal therapsid tree.

Today
a phylogenetic analysis nests Cutleria as THE basal therapsid (among the 50 included taxa, Fig. 2) and it still descends from Ophiacodon. So Cutleria is still just another ophiacodontid. It had simply taken the next evolutionary step toward mammals, dicynodonts, etc. etc. And it probably was not too far from sphenacoodontids even so.  Evidently there was an early Permian radiation that included tailback pelycosaurs and basal therapsids, largely at the same time, both derived from ophiacodontids.

Figure 1. Ophiacodon, Cutleria and Stenocybus, the lineage of basal therapsids.

Figure 1. Ophiacodon, Cutleria and Stenocybus, the lineage of basal therapsids.

Earlier
we looked at the diphyletic nature of the therapsid family tree. One branch led to the Anomodontia (dromasaurs + dicynodonts and kin). The other branch (let’s call them Biarmosuchiformes) led to dinocephalians, theriodontids and by way of extension mammals and us humans.

Today we have a common ancestor for these two therapsid branches, Cutleria, in the therapsid tree at right. Cuteria does not have a single large canine, but two small canines, as in Raranimus and Stereophallodon.

Liu et al. (2009) reported, “While sphenacodontid synapsids are considered the sister-group of therapsids, the place of origin of therapsids is an enigma, largely because of a long standing morphological and temporal gap (Olson’s Gap) in their fossil record.”

Note: Tetraceratops, once thought to be a basal therapsid, is not part of this clade, as it nests with the limnoscelid, Tseajaia.

Benson (2012) recovered several basal synapsid trees, almost all with Cutleria close to the base of the Therapsida, but far from Ophiacodon. Benson erred when he included caseasaurids, which nest with millerettids in the large reptile tree. His outgroups (save Protorothyris) were largely new lepidosauromorphs like Captorhinus and Limnoscelis, which are not recovered by the large reptile tree. Hylonomus and Coelostegus would have been more appropriate and phylogenetically closer. Benson’s tree did recover Haptodus as a basal sphenacodontid, not far removed from Ophiacodon, if we get rid of caseasaurs.

References
Benson RBJ 2012. Interrelationships of basal synapsids: cranial and postcranial morphological partitions suggest different topologies. Journal of Systematic Palaeontology 10(4):601-604.
Brinkman D and Eberth DA 1986. The anatomy and relationships of Stereophallodonand Baldwinonous (Reptilia, Pelycosauria). Breviora 485: 1-34.
Cheng Z and Li J 1997. A new genus of primitive dinocephalian – the third report on Late Permian Dashankou lower tetrapod fauna. Vertebrata PalAsiatica 35 (1): 35-43. [in Chinese with English summary]
Laurin, M 1994. Re-evaluation of Cutleria wilmarthi, an Early Permian synapsid from Colorado. Journal of Vertebrate Paleontology 14(1): 134-138.
Lewis GE and Vaughn PP 1965. Early Permian Vertebrates from the Cutler Formation of the Placerville Area Colorado. United States Geological Survey Professional Papers 503-C:1-50.
Liu J, Rubidge B and Li J-L 2009. New basal synapsid supports Laurasian origin for therapsids. Acta Palaeontologica Polonica 54(3): 393-400.

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