Hipposaurus: close to the ancestry of man, but off a wee bit

Figure 1. Therapsida includes the pangolin, Manis, which nests here with Notharctus. one of only a few mammals tested so far.

Figure 1. Therapsida includes the pangolin, Manis, which nests here with Notharctus. one of only a few mammals tested so far.

At the very base of the Therapsida
(Fig. 1) we have a split between the plant-eating Anomodontia (dicynodonts, dromasaurs and kin) and the meat-eating Kynodontia (new name for a new clade that encompasses all other therapsids, including cynodonts and mammals). At the base of the Kynodontia is the rarely discussed, but obviously important taxon, Hipposaurus boonstrai (Fig. 2, Haughton 1929, 21 cm skull. SAM 8950). Biarmosuchus is a sister.

Figure 1. Published material on Hipposaurus permits one to create a reconstruction like this. Not far removed from its ophiacodont / haptodine / pelycosaur precursors, Hipposaurus had longer, more gracile limbs and a distinct sabertooth canine, like Haptodus or Cutleria on steroids!

Figure 2. Published material on Hipposaurus permits one to create a reconstruction like this. Not far removed from its ophiacodont / haptodine / pelycosaur precursors, Hipposaurus had longer, more gracile limbs and a distinct sabertooth canine, like Haptodus or Cutleria on steroids!

Long-legged, saber-toothed Hipposaurus
was originally thought to be a gorgonopsian, but in a note from Dr. Jim Hopson  (U Chicago) who xeroxed Boonstra 1965 for me, Hipposaurus (“horse lizard”) has been considered a biarmosuchian within the Ictidorhinidae since the 1980s.

Figure 2. The skull of Hpposaurus was larger than that of its sisters and predecessors among the basal Therapsida.

Figure 3. The skull of Hpposaurus was larger than that of its sisters and predecessors among the basal Therapsida, including Stenocybus and Cutleria. The fangs were longer too.

There are some odd details
in the manus and pes of this mid-sized carnivore that indicate this is a derived late survivor of an earlier radiation.

  1. Hipposaurus has a large pisiform (post axial carpal, Fig. 1)
  2. The first centrale is quadrant shaped
  3. The second centrale is shaped like a squat chevron
  4. The radiale is twice as long as wide
  5. The fourth and fifth carpals are fused
  6. A small circular sternum present (none in sister taxa)
  7. The posterior calcaneum has a hook like tuber
  8. Two wedge-shaped centralia extend the width of the tarsus
  9. The first distal tarsal is the size of a metatarsal and shifts the proximal metatarsal distally, almost to the mid length of metatarsal 2.
  10. Two mid phalanges are fused on pedal digit 4

Speaking of oddities at clade bases…
as we’ve seen before, clade bases are, by definition, when novelties arise. In the case of Hipposaurus, these novel carpal and tarsal oddities went nowhere. A sister taxon without such novelties, Biarmosuchus, produced all the descendants we all know and love. Hipposaurus became a mere footnote and a short Wikipedia page.

References
Boonstra LD 1952. Die Gorgonospier-geslag Hipposaurus en die familie Ictidorhinidae: Tydskr. Wet. Kuns., v. 12, p. 142-149.
Boonstra LD 1965.
The girdles and limbs of the Gorgonopsia of the Taphinocephalus Zone. Annals of the South African Museum 48:237-249.
Haughton SH 1929. On some new therapsid genera: Annals of the South African Museum, v. 28, n. 1, p. 55-78.

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2 thoughts on “Hipposaurus: close to the ancestry of man, but off a wee bit

  1. Kynodontia are all therapsids sans Anomodontia. Provisionally, that’s a basal split in the LRT. So that clade includes Biarmosuchia and more primitive therapsids, like Cutlieria, wtaxa not included within Theriodontia. Good question, though…

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