A paper by Spindler (2014) sought the oldest therapsid…
and found no reason to nest the former best candidate, Tetraceratops (Fig. 1), with basal therapsids, confirming what was reported here in 2011.
From the abstract: “Since the successful clade of therapsids occurs rather suddenly in the fossil record of Guadalupian age*, the reconstruction of their origin is questionable and based on little data. Concerning the Artinskian taxon Tetraceratops insignis**, broadly accepted as the oldest and basal-most member, no close relation to therapsids could be found during the re-documentation. Instead, a fragmentarily preserved vertebral sequence from the Desmoinesian assemblage [Late Pennsylvanian, Westphalian D] of Florence, Nova Scotia, is considered to be a new candidate for the oldest therapsid. This pushes back their origin farther than required by phylogenetic results. Moreover, it supports the ghost lineage of unknown Carboniferous and Early Permian therapsids.”
Spindler reports, “The problems when evaluating Tetraceratops are (1) its highly autapomorphic character combination, such as ornamentation, short facial region, and specialized dentition, and (2) the poor preservation of the single holotypic skull. The specimen has been re-studied carefully and is currently under re-evaluation. Anatomical identifications take into account a high degree of compaction, but although a simple mode of deformation. In contrast to previous workers, the therapsid synapomorphies could not be reproduced, resulting in a haptodont-grade classification*** independent from the same result by CONRAD & SIDOR (2001) and supported by LIU et al.(2009).”
*** Spindler did not consider a larger taxon list that included Tseajaia and limnoscelids.
I cannot nest a vertebral sequence in the large reptile tree, so until I can (probably never), I accept Spindler’s observations and interpretations.
Spindler F 2014. Reviewing the question of the oldest therapsid. Paläontologie, Stratigraphie, Fazies (22) Freiberger Forschungshefte C 548: 1–7.