The base of the Diapsida: bipeds, quadrupeds and swimmers

Earlier we looked at how often bipeds give rise to aquatic taxa. Today we’ll look at the base of the Diapsida, the clade that includes Petrolacosaurus, enaliosaurs, protorosaurs and archosauriforms and does not include lepidosaurs (which are not related and arrive at the diapsid configuration by convergence).

Figure 1. The base of the archosauromorph Diapsida, compressed to delete derived  Enaliosauria. Here Eudibamus, Spinoaequalis, Tangasaurus and Thadeosaurus are basal to several major clades.

Figure 1. The base of the archosauromorph Diapsida, compressed to delete derived Enaliosauria. Here Eudibamus, Spinoaequalis, Tangasaurus and Thadeosaurus are basal to several major clades.

The great diapsid radiation
begins with these few taxa derived at the base of their several clades: Eudibamus, Spinoaequalis, Tangasaurus and Thadeosaurus (Fig. 2). Milleropsis is the proximal outgroup.

Eudibamus and Spinoaequalis nest at the base of the araeoscelids, which soon became extinct. The two Tangasaurus nest at the base of the aquatic enaliosaurs, which became extinct at the close of the Cretaceous. Thadeosaurus nests at the base of the protorosaurs and archosauriforms which are alive today in the form of birds and crocs.

Figure 2. Basal archosaurmorph diapsids, including Eudibamus, Spinoaequalis, two Tangasaurus and Thadeosaurus to scale.

Figure 2. Basal archosaurmorph diapsids, including Eudibamus, Spinoaequalis, two Tangasaurus and Thadeosaurus to scale. These are all sister taxa. The tangasaurs are basal to the aquatic enaliosaurs. Thadeosaurus is basal to protorosaurs and archosauriforms. Eudibamus and Spinoaequalis are basal to araeoscelids.

What do they all have in common?
These four Permian taxa are all about the same size and have a lizardy appearance, but they are not related to lizards.

The hind limb and foot are larger than the forelimb and hand. Pedal digit 1 is quite short and pedal digit 5 is quite long. The ilium is elongated, chiefly posteriorly. The limbs are gracile in most cases, the long-neck Tangasaurus the exception. The humerus is much broader distally in all. Hemal arches are deep in all except Eudibamus. The pubis has a dorsal process in all taxa. The manus is sub equal in length to the ulna. The pes is subequal in length to the tibia. None of these taxa had large teeth.

Figure 2. Tangasaurus specimens in dorsal view. They are both wide, like pancakes, with very wide anterior caudals.

Figure 2. Tangasaurus specimens in dorsal view. They are both wide, like pancakes, with very wide anterior caudal verts. Those are ribs on the holotype (above), gastralia on the long neck specimen (below). Those large caudal ribs (transverse processes) anchor strong hind limb muscles. Those large coracoids and that large sternum anchor strong forelimb muscles.

Differences?
The skull is poorly known in several of these taxa. The cervicals are robust in most cases, Spinoaequalis and the Tangasaurus holotype are the exceptions. The scapulocoracoid is fused only in Thadeosaurus.

Speaking of the skull…
Currie (1982) reports more than 300 specimens of Tangasaurus and the related but more aquatic Hovasaurus are known, but none preserve the entire skull.

These taxa are rarely studied but they are key basal taxa in each of their clades and united by their diapsid ancestry. Probably all were active and speedy insect-eaters, whether terrestrial or aquatic.

Pedal digit 5
The lateral toe is much longer in these taxa, inherited from Milleropsis. It remains long in enaliosaurs, like mesosaurs and thalattosaurs. Pedal digit 5 becomes much shorter in Eudibamus (Fig. 1) and other araeoscelids and, by convergence, following Thadeosaurus and its terrestrial descendants.

Basilisk walking on water.

Figure 1. Basilisk walking on water.

A modern analogy?
The extant basilisk runs bipedally through water, keeping its torso above the surface.  From Wikipedia, “Once a basilisk submerges, it continues swimming. Although this lizard stays close to water to escape terrestrial predators, it swims only when necessary because some other aquatic animals would eat the basilisk given the chance.” The soft tissue crests are sexually dimorphic.

On the the other hand…
Eudibamus was found in an upland paleograben containing no aquatic vertebrates.

On the other-other hand…
Thadeosaurus was found in split nodule in a rapidly filling deep rift valleys in Madagascar, some open to the sea. The presence of oolites replaced with collophane suggests a rich phosphate source, such as deep marine upwellings, similar to the situation of Galapagos marine iguanas. Milleropsis was also found in a split nodule that contained several specimens all living together, something that likewise occurred in Heleosaurus, its phylogenetic ancestor.

References
Berman, DS, Reisz RR, Scott D, Henrici AC, Sumida SS and Martens T 2000. Early Permian bipedal reptile. Science 290: 969-972.
deBraga M and Reisz RR 1995. A new diapsid reptile from the uppermost Carboniferous (Stephanian) of Kansas. Palaeontology 38 (1): 199–212. palass-pub.pdf
Carroll RL 1976. Galesphyrus capensis, a younginid eosuchian from the Cistephalus zone of South Africa. Annals of the South African Museum 72: 59-68.
Carroll RL 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society London B 293: 315-383
Currie PJ 1984. Ontogenetic changes in the eosuchian reptile Thadeosaurus. Journal of Vertebrate Paleontology 4(1 ): 68-84.
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
Haughton SH 1924. On Reptilian Remains from the Karroo Beds of East Africa. Quarterly Journal of the Geological Society 80 (317): 1–11.
Reisz RR, Modesto SP and Scott DM 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society B 278 (1725): 3731–3737.

wiki/Tangasaurus
wiki/Thadeosaurus
wiki/Spinoaequalis
wiki/Eudibamus

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