Extinct genus of reptiles
Australothyris is an
temporal fenestrae, due to its large and fully enclosed temporal fenestra and South African heritage. It also possessed several unique features, including a high tooth number, long
postfrontal, small interpterygoid vacuity, and a specialized interaction between the
stapes and
quadrate.
[1]Discovery
Australothyris is known from a single specimen discovered at the Beukesplaas farm by Robert Smith in 1995. The fossil site at the Beukesplaas farm contains a diverse parareptile and synapsid fauna positioned in the Middle
generic name translates to "southern opening" in recognition that it supports the hypothesis that parareptiles originated in
Gondwana and went through a phase of evolution where they possessed a temporal fenestra, an opening in the skull behind the eyes. The
specific name honors Robert Smith.
[1]Description
Portions of the snout and upper skull have been weathered away, but many notable features are preserved. The
lateral temporal fenestra is present behind the orbit, completely surrounded by the
jugal,
quadratojugal,
postorbital and
squamosal. Other parareptiles with lateral temporal fenestrae (apart from
lanthanosuchids) typically exclude the postorbital from its edge through contact between the jugal and squamosal, or have an open lower edge due to a loss of contact between the jugal and quadratojugal. The
postfrontal is uniquely elongated, as its rear branch contacts the boxy
supratemporal bone and separates the postorbital from the
parietal. The
pineal foramen is large, similar in size to that of procolophonids and
bolosaurids. The
quadrates are massive, being quite broad but also not very tall as in
Acleistorhinus. Minor ornamentation is present on several bones, including broad grooves (on the nasal), shallow pits (on the jugal), clusters of knobs and furrows (on the postorbital), and low mounds (on the squamosal).
[1]The
parabasisphenoid. Tooth rows occur along the inner edge of the pterygoids, on the main underside of the bones, and at the transverse flanges at their rear. The branches of the pterygoids leading to the quadrates are offset from the transverse flanges by a distinct notch. Overall the palate most closely resembles that of
Lanthanosuchus. Uniquely,
Australothyris even possesses patches of teeth on the basipterygoid processes of the parabasisphenoid.
[1]The rest of the braincase was fairly typical. The
surangular and
angular in its rear half. The
coronoid had a low peak and the tall
articular had a small retroarticular process. Only one tooth was exposed, and it was similar to those of the maxilla, albeit smaller.
[1]The articulated postcranial skeleton is weathered to the point that only portions of the cervical vertebrae and interclavicle are in good enough condition to describe. The cervicals had slight excavations on their outer surface, robust neural arches, and low neural spines, with that of the axis overhanging its predecessor. The interclavicle is anchor-shaped (like ankyramorph, or "anchor-form" parareptiles), but in contrast to ankyramorphs, the center of the interclavicle is thicker than the front edge. Overall the postcranium is congruent with that known for Milleretta.[1]
Classification
The original describers of Australothyris used a
derived parareptiles). This was nevertheless more derived than
mesosaurs and
millerettids, and the paper's authors assigned the name
Procolophonomorpha to parareptiles more derived than millerettids.
Australothyris was recovered as the first branch of Procolophonomorpha, suggesting that the group as a whole originated simultaneously with the evolution of a large, fully enclosed temporal fenestra in parareptiles.
[1] However, the subsequent discovery of
Microleter, which had a roughly equivalent phylogenetic position and a much more restricted temporal emargination, casts doubts on this hypothesis for the origin of temporal fenestration. Certain millerettids have also been observed to possess temporal fenestrae.
[2] The position of
Australothyris also supports another hypothesis which argues that procolophonomorphs evolved in
Gondwana (southern
Pangea) before spreading to and diversifying in more northern regions,
[1] although
Microleter, known from
Oklahoma, once more casts doubt on this hypothesis.
[2]Cladogram after Modesto, Scott, & Reisz (2009).[1]
References
