Microposaurus

During 1923, the first species of Microposaurus were found by Dr. E. C. Case on a venture into the redbed exposures of Wonderboom Bridge.[3][4] These sites, just south of Burgersdrop Formation in the Eastern Cape of South Africa, were from the Cynognathus zone.[4] When found, the skull was described as an “embedded palate upwards, in a fairly soft dark-green shaly mudstone” which is characteristic of the amphibious behavior of Microposaurus.[4] Relating to the name of the discoverer, the given name was Microposaurus casei. Unfortunately, the skull had a ferruginous material coating that could not be removed without damaging the already crushed skull.[4] Also, from the coating, the sutures typical of skulls were unable to be determined decisively.[4] From these drawbacks, many features stated by Haughton in his first examination of the specimen were later determined to be incorrect. As a result, when Damiani wrote his paper on M. casei he noted many features that Haughton had named or described incorrectly.[1]

Several decades later, a new Microposaurus species was uncovered. During an expedition in the Anisian Rouse Hill Siltstones, Steven Averyi found and named the holotype and single specimen of Microposaurus averyi.[2] Unlike M. casei, this specimen was found in the Ashfield Shale Formation of New South Wales, Australia.[5] Quite a distance from South Africa, the environment during the life period of Microposaurus would have been similar in both cases. The marine landscape, with black/gray, silty claystone was indicative that M. averyi also lived an amphibious lifestyle.[5]

The classification of the first discovered skull of Microposaurus were to the lineage of trematosaurines. Common trematosaurines features observed from the specimen were their orbits were within the anterior half of their skull, the postorbital-prepineal growth zone was present, the anterior palatal vacuities were paired, the transvomerine tooth row was reduced or absent, the parasphenoid was antero-posteriorly elongated, the exoccipitals were underplated by the parasphenoid being posteriorly expanded, and in adults the orbits were small and located near the lateral margins of the skull.[1] Based on these characteristics many authors agreed on the evolutionary placement of these amphibians.[1] Furthermore, one main aspect of M. casei provided evidence for branching off from their sister family Lonchorhynchinae. The physically striking contrast between Microposaurus and Lonchorhynchinae was their seemingly short snouts compared to the elongated counterpart of the latter.[1]

While Microposaurus lived during the Anisian period of the Triassic, the transition from the Permian-Triassic created more aquatic environments for these species to survive. In the Karoo Basin of South Africa, after the extinction, multiple new rivers were created that ran through areas of the continent.[6]

Due to the aquatic lifestyle of Microposaurus, their diet was determined to be piscivorous.[7] For all trematosaurids, most were marine at one point in their life cycles.[7] The normal size of these carnivorous predators was about one to two meters in total body length.[8] Inferred to be eating smaller vertebrates found in their wide range of environments.

At the time of diverging of trematosaurids (between the short and long-snouted (Lonchorhynchinae)), these aquatic swimmers had a nearly global distribution.[8][2] The contribution to this wide distribution is from their possible euryhaline ability and preference to “nearshore marine to distal deltaic habitats”.[9][10] These observations were “based on the associated invertebrate faunal elements such as ammonoids and bivalves”.[9] Alternatively, marine temnospondyls were interpreted as “‘crisis progenitors’” who were “‘initially adapted to perturbed environmental conditions of the mass extinction interval, readily survive this interval, and are among the first groups to seed subsequent radiation into unoccupied ecospace during the survival and recovery intervals’”.[8] Further supporting their radiation to other parts of Pangea that lead to the high diversification during the Early Triassic.[11] Also resulting in the spreading from South Africa to South America (Brazil) where more fossils of Stereospondyls were found.[11] Even a mandible from the Lower Anisian Mukheiris Formation in Jordan was discovered, and shared many similarities with the genus, including a similarly sized Meckelian fenestra on the inner surface of the lower jaw, teeth with sharp edges, and large fangs in the upper jaw.[12]