| Australothyris | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | †Parareptilia |
| Order: | †Procolophonomorpha |
| Genus: | †Australothyris Modestoet al.,2009[1] |
| Type species | |
| †Australothyris smithi Modestoet al., 2009 | |
Australothyris is anextinctgenus of basalprocolophonomorphparareptile known from theMiddle Permian (middleCapitanian stage) ofTapinocephalus Assemblage Zone,South Africa. The type and only known species isAustralothyris smithi. As the most basal member of Procolophonomorpha,Australothyris helped to contextualize the origin of this major parareptile subgroup. It has been used to support the hypotheses that procolophonomorphs originated inGondwana and ancestrally possesstemporal 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, longpostfrontal, small interpterygoid vacuity, and a specialized interaction between thestapes andquadrate.[1]
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 MiddlePermianTapinocephalus Assemblage zone of the upperAbrahamskraal Formation. This specimen, SAM-PK-K8302, included most of a skull and portions of the rest of the skeleton, which had mostly been eroded away prior to its discovery. It was initially referred toOwenetta based on the numerous teeth and long postfrontal, until a reexamination revealed a temporal fenestra, which was absent inowenettids. In the wake of this restudy, the specimen was recognized as a new taxon, which was namedAustralohyris smithi bySean P. Modesto,Diane M. Scott, andRobert R. Reisz in 2009. Thegeneric name translates to "southern opening" in recognition that it supports the hypothesis that parareptiles originated inGondwana and went through a phase of evolution where they possessed a temporal fenestra, an opening in the skull behind the eyes. Thespecific name honors Robert Smith.[1]
Portions of the snout and upper skull have been weathered away, but many notable features are preserved. Themaxilla contains 31 teeth, an unusually high number which is only surpassed byMicroleter[2] andLanthanosuchus among parareptiles. The teeth are small, slender, and conical, retaining roughly the same size and shape except for a subtle decrease in size towards the rear of the maxilla. Theprefrontal is simple, hosting a small buttress in front of theorbits (eye holes) and being dissimilar in shape to that ofprocolophonids. Although thefrontal does contact the upper edge of the orbit as in other amniotes, it lacks the distinct lappet observed inlanthanosuchoids. A distinctlateral temporal fenestra is present behind the orbit, completely surrounded by thejugal,quadratojugal,postorbital andsquamosal. Other parareptiles with lateral temporal fenestrae (apart fromlanthanosuchids) 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. Thepostfrontal is uniquely elongated, as its rear branch contacts the boxysupratemporal bone and separates the postorbital from theparietal. Thepineal foramen is large, similar in size to that of procolophonids andbolosaurids. Thequadrates are massive, being quite broad but also not very tall as inAcleistorhinus. 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]
Thevomers possess an array of ridges, the largest being at the edge of thechoanae. There are also teeth in small rows or solitary positions on the vomers. Thepalatines are characteristically large, possessing several low ridges covered with small teeth. Thepterygoids were notably broad, owing to extensive contact with each other along the midline of thepalate. As a consequence, the interpterygoid vacuity (gap between the pterygoids) is short, restricted to a triangular opening in front of theparabasisphenoid. 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 ofLanthanosuchus. Uniquely,Australothyris even possesses patches of teeth on the basipterygoid processes of the parabasisphenoid.[1]
The rest of the braincase was fairly typical. Thebasioccipital was broad, with poorly-developed basitubera and kidney-shapedoccipital condyle, and theexoccipitals do not meet at the midline of theforamen magnum. Thesupraoccipital was also broad and fused with theopisthotics in its lower portion, while the upper portion of the bone was overlapped by the smallpostparietal andtabular bones of the skull roof. The opisthotics were thickest at the base and generally similar to those ofMilleretta. They each connected to a thick yet complexstapes which possessed a conspicuous footplate, stapedial foramen, and a dorsal process. A knob on the outer edge of the stapes likely connected to a characteristic spur on the quadrate. What can be seen of the lower jaw indicates that it was primarily formed by the dentary in its front half, and the low, elongatedsurangular andangular in its rear half. Thecoronoid had a low peak and the tallarticular 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 thecervical vertebrae andinterclavicle 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 (likeankyramorph, 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 forMilleretta.[1]
The original describers ofAustralothyris used aphylogenetic analysis designed by Muller & Tsuji (2007) to investigate its relations to other parareptiles. The analysis found that it had an optimal position as a relativelybasal parareptile, specifically the sister taxon toAnkyramorpha (the group containinglanthanosuchoids and morederived parareptiles). This was nevertheless more derived thanmesosaurs andmillerettids, and the paper's authors assigned the nameProcolophonomorpha 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 ofMicroleter, 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 ofAustralothyris also supports another hypothesis which argues that procolophonomorphs evolved inGondwana (southernPangea) before spreading to and diversifying in more northern regions,[1] althoughMicroleter, known fromOklahoma, once more casts doubt on this hypothesis.[2]
Cladogram after Modesto, Scott, & Reisz (2009).[1]
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