Rarely will I cover scrappy specimens, known from just a few parts (Fig. 1). With so few traits, it’s difficult to nest them on the large reptile tree while maintaining complete resolution. So, generally, they are to be avoided.
However,
I recently came across the Early Permian Stereophallodon (Brinkman and Eberth 1986), described as a basal ophiacodontid with double canines. That made me think about Raranimus, described as a basal therapsid with double canines. Here (Fig. 1) they are together for the first time.
Figure 1. Stereophallodon and Raranimus, two distinct synapsids with double canines at the ophiacodontid-therapsid transition. Both had tiny teeth posterior to the canines, like Biarmosuchus.
Stereophallodon (Earliest Permian) was described by Brinkman and Eberth (1986) as an ophiacodontid more primitive than Ophiacodon (Fig. 2), a taxon nesting at the base of the Therapsida in the large reptile tree. Raranimus (Early Middle Permian) was described by Liu, Rubidge and Li (2009) as a basal therapsid. Funny, they both had double canines, a trait not present in other basal therapsids, or ophiacodontids.
That kind of messes things up, doesn’t it? We may have to dismiss our linear thinking and remember that evolution produces bushes, not ladders.
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.” Unfortunately, Liu et al. (2009) did not recognize the basal nesting of Stenocybus, but did nest Tetraceratops in their tree, not realizing it nests more parsimoniously with the diadectomorph, Tseajaia. Liu et al. (2009) did not include Ophiacodon (Fig. 2) or Stereophallodon in their taxon list. Taxon exclusion is the problem once again.
Figure 2. Ophiacodon skull with double canines highlighted in green.
Not sure what else can be said of these two or their phylogenetic nesting at present, until more is known, but I thought it was worth noting.
References
Brinkman D and Eberth DA 1986. The anatomy and relationships of Stereophallodon and Baldwinonous (Reptilia, Pelycosauria). Breviora 485: 1-34.
Liu J, Rubidge B and Li J-L 2009. New basal synapsid supports Laurasian origin for therapsids. Acta Palaeontologica Polonica 54(3): 393-400.
