Kenyasaurus not a tangasaur… not a diapsid… It’s a very basal dromasaur!

Earlier we looked at marine younginiformes. Perhaps conspicuous by its absence was Kenyasaurus, which was originally considered related to tangasaurid younginiformes. Last night I found data, plugged it into the large reptile tree and was surprised at where Kenyasaurus nested.

Kenyasaurus mariakaniensis
(Harris and Carroll 1977; Early Triassic; KNM-MA1, National Museum of Kenya) is represented by a headless skeleton with only a partial forelimb and pectoral girdle (Fig. 1).

Figure 1. Kenyasaurus in situ. Click to enlarge. This rather plain specimen nests not with tangasaurids, but with dromasaurids according to the large reptile tree. Boxed area: the primitive dromasaur, Galechirus and its foot to scale for comparison. Haptodus foot for comparison, not to scale. Pink and green tarsals are absent in Kenyasaurus and dromasaurs.

Figure 1. Kenyasaurus in situ. Click to enlarge. This rather plain specimen nests not with tangasaurids, but with dromasaurids according to the large reptile tree. Boxed area: the primitive dromasaur, Galechirus and its foot to scale for comparison. Haptodus foot for comparison, not to scale. Pink and green tarsals are absent in Kenyasaurus and dromasaurs. Note the similarity of the pes of Kenyasaurus and Haptodus, sharing the same number and proportion of pedal elements, less the two tarsals.

Originally considered
a relative of Tangasaurus and Hovasaurus, the large reptile tree nested Kenyasaurus with the arboreal herbivorous dromasaurid synapsids. If so, the purported ‘well-developed sternum’ (Fig. 1, lavender) must instead be the posterior coracoid because synapsids do not have a sternum*. Harris and Carroll (1977) noted the long tail was unlike those of the Tangasaurus and Hovasaurus and that the tarsus lacked a fifth distal tarsal, as in dromasaurs. The caudal transverse processes gradually diminished over 30 vertebrae creating a cylindrical, muscular tail similar to those found in dromasaurs, only longer.

Figure 2. Two other dromasaurs, Suminia and Galechirus.

Figure 2. Two other dromasaurs, Suminia and Galechirus. Note the similar ilium shapes.

Currie 1982
also examined Kenyasaurus. At that time Currie did not have a computer or software to test traditional nestings. And he had just been studying Tangasaurus and Hovasaurus. So he considered Kenyasaurus a tangasaurid.

Currie (1982) diagnosed Kenyasaurus on the basis of five autapomorphies:

  1. low but anteroposteriorly elongate neural spines in the dorsal region
  2. 56 caudal vertebrae and
  3. 28 pairs of caudal ribs and transverse processes.
  4. Astragalus almost triangular rather than primitive L-shape
  5. Pronounced process on fifth metatarsal for insertion of peroneus brevis

How do these compare to dromasaurs?

  1. Neural spines in known dromasaurs and outgroups are taller than long
  2. About 45 caudal vertebrae are present in Galechirus, but they get very tiny at the tip, 52 are present in Suminia
  3. 22 pairs of caudal ribs and transverse processes are present in Suminia
  4. Astragalus triangular present in Suminia, square present in Galechirus
  5. No pronounced process on fifth metatarsal

So, no wonder Kenyasaurus was not considered a dromasaur.

Bickelmann, Müller and Reisz 2009
did have a computer and software to test traditional nestings. They found support for two distinct families within “Younginiformes”: the aquatic Tangasauridae, and the terrestrial Younginidae. However, they found no support for the inclusion of Kenyasaurus within any of those families. Unfortunately that study also included the unrelated Lanthanolania, Palaegama, Saurosternon and Coelurosauravus (all basal lepidosaurifomes related to Triassic rib gliders) within the same clade that also included Claudiosaurus and the Younginiformes. Very odd.

Shift Kenyasaurus closer to Tangasaurus
and you’ll add 10 steps to the most parsimonious tree. Whether a sternum was present or not makes little difference.

Delete the two dromasaurs
from the large reptile tree and Kenyasaurus creates a large polytomy (loss of resolution) among basal synapsids.

Post-pectoral characters shared by Kenyasaurus and dromasaurs
to the exclusion of basal synapsids include:

  1. Gastralia present and rodlike (otherwise last seen in Ophiacodon)
  2. Ventral pelvis: separate plates, small medial opening
  3. Pubis orientation: medial
  4. Overall size: < 30 cm tall, 60 cm long

So, not a lot to work with.

Other dromasaurids
are known from the Late Permian, so Kenyasaurus would have been a late-survivor in the Early Triassic, despite its more basal nesting. That’s another black mark against Kenyasaurus being a dromasaur. Nevertheless, among the 542 taxa in the inclusion set, Kenyasaurus is most attracted to the dromasaurs with the present data set and scores.

Some final thoughts
Kenyasaurus displays no reduction of the middle phalanges of digits 3 and 4 of the manus and pes, so it resembles more primitive pelycosaur-grade synapsids in this regard. Based on this fact, the reduction of the three middle pedal phalanges may have occurred by convergence  within Therapsida, once in anomodonts and again in the main line beginning with biarmosuchids.

Basal anomodonts likely split from basal therapsids, like Stenocybus and Cutleria in the Early Permian. So Kenyasaurus was a very late (Early Triassic) remnant of that earlier radiation. So the autapomorphies that Currie listed (above) could have evolved during those tens of millions of years. And yes, I am making excuses for this taxon because it does not exactly match the ideal we might imagine. But those excuses could be true.

References
Bickelmann C, Müller J and Reisz RR 2009. The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of “younginiform” reptiles. Canadian Journal of Earth Sciences 46:651-661.
Currie P 1982. 
The osteology and relationships of Tangasaurus mennelli Haughton. Annals of The South African Museum 86:247-265. http://biostor.org/reference/111508
Harris JM and Carroll RL 1977. Kenyasaurus, a New Eosuchian Reptile from the Early Triassic of Kenya. Journal of Paleontology 51:139–149.

* We’ll look at the mammals sternum/manubrium issue later….

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