In the large reptile tree (LRT, 1306 taxa) Hipposaurus (Fig. 1; Haughton 1929, skull length 21cm, length 1.2m) nests at the base of the carnivorous therapsids, not far from finless pelycosaurs, like Haptodus and basal amonodonts like Stenocybus. For all that time the vertebral column has remained buried and unstudied.
Figure 1. The skull of Hpposaurus was larger than that of its sisters and predecessors among the basal Therapsida.
We looked at Hipposaurus
earlier as a more likely trackmaker for Dimetropus ichnites.
Peecock et al. 2018
describe µCT scans of the previously undescribed vertebral column of Hipposaurus. With this data they propose new relationships with other hipposaurids known only from vertebrae.
FIgure 2. Hipposaurus compared Dimetropus. The overall and leg length is right, as are many of the digits. Unfortunately the medial digits are too short in Hipposaurus. Hipposaurus has a narrower gauge and lifted its belly of the ground, as did the Dimetropus trackmaker.
The Peecock team concludes with caution,
“Phylogenetic analysis underscores the startling homoplasy between biarmosuchians and
archosauromorphs: when biarmosuchian vertebrae are coded into an archosauromorph data matrix, they form a monophyletic clade within Avemetatarsalia. Extreme caution is needed when interpreting Permian vertebrae as archosauromorphs.”
As we’ve seen before,
convergence is rampant in the LRT.
References
Boonstra LD 1952. Die Gorgonospier-geslag Hipposaurus en die familie Ictidorhinidae: Tydskr. Wet. Kuns 12:142-149.
|Haughton SH 1929. On some new therapsid genera: Annals of the South African Museum 28(1):55-78.
Peecock et al. (5 co-authors) 2018. Vertebral osteology of Hipposaurus boonstrai (Therapsida, Biarmosuchia) from the Middle Permian of South Africa, with implications for the evolution of Archosauromorpha. SVP abstracts.
The Pterosaur Heresies 