binomial name was erected for this specimen in 2014. Aenigmastropheus was probably fully terrestrial.[1]
Discovery
Proximal end of the right ulna
Fossils of Aenigmastropheus were first described by the British paleontologist Dr.
dorsal vertebrae, the distal half of the right humerus, a fragment of probable left humeral shaft, the proximal end of the right ulna, and three indeterminate fragments of bone, one of which may represent a partial radius.[1]
In his article, Parrington (1956) highlighted the apparent contrast between the primitive appearance of the forelimb bones and the more derived appearance of the vertebrae, resembling those of
generic name is derived from aenigma, "enigmatic" in Latin, and stropheus, "vertebra" in Greek, in allusion to the problematic taxonomic history of the holotype and only known specimen. The specific name, parringtoni, honors Dr. Francis Rex Parrington for the discovery and initial description of UMZC T836, and for his contribution to the understanding of Permo-Triassic amniotes.[1]
Restoration of Aenigmastropheus (lower right) and other animals from locality B35 of the Usili Formation
UMZC T836 was collected in fossil-bearing levels that correspond to locality B35 of Stockley (1932), which is located close to the road near the town of Ruanda in the Songea District, part of the “Lower Bone Bed” corresponding to his K6 horizon of the Songea Series. This outcrop is currently assigned to the upper part of the
dicynodonts. Based on UMZC catalogue and unpublished field notes of Parrington in UMZC collections, an isolated maxilla of a dicynodont listed as cf. "Esoterodon" uniseries (UMZC T969, now Endothiodon), as well as other dicynodont (UMZC T779, T1170) and gorgonopsid (UMZC T882, T883) remains, were collected at locality B35, along with UMZC T836.[1]
vertebrae, as well as the right humerus and ulna. The ratio between certain measurements of the axial and appendicular elements also supports this interpretation.[1]
Aenigmastropheus was a medium-sized
Helveticosaurus zollingeri. The humerus of Aenigmastropheus has a thick posteroventral, diagonal ridge on the anterior surface of the shaft, an autapomorphy that is unique to this taxon among basal diapsids. The humerus also lacks entepicondylar (unlike non-archosauromorph reptiles) and ectepicondylar foramina, and shows strongly developed capitellum and trochlea (unlike Helveticosaurus). Finally, its olecranon process is strongly developed, forming a single ossification with the rest of the ulna, unlike other stem archosaurs and Helveticosaurus.[1]
Paleobiology
Histological slices
Ezcurra et al. (2014) performed bone
Trilophosaurus buettneri and rhynchosaurs exhibit a slow overall growth pattern. This suggests that even among basal members of Archosauromorpha, diverse growth strategies were already present.[1]
UMZC T836 was likely not a
neurocentral sutures, but possibly also not fully-grown, as in at least some elements like the fifth vertebra, the notochordal canal is still connected to the neural canal through an hourglass-shaped opening. Histology suggests that UMZC T836 may have reached skeletal maturity, as closely spaced growth marks are present in the lamellar-zonal bone of the outer part of the cortex. It is not known why the outermost thin layer of bone directly below the external bone surface does not show these marks, interpreted as an outer circumferential layer. If they are truly absent, not simply obscured, this would argue against such interpretation indicating that this individual had a prolonged period in life of several years as an adult in which it experienced unfavorable growth conditions, like nutritional shortage, drought or disease, leading to limited growth.[1]
Ezcurra et al. (2014) suggested a fully terrestrial mode of life for Aenigmastropheus, as was already supported by gut content for its relative Protorosaurus speneri. The ratio between the inner diameter and outer diameter of the humerus of Aenigmastropheus was found to be consistent with that of some bones of terrestrial basal archosauriforms, such as the humerus and
Proterosuchus fergusi, while considerably smaller ratio is observed in aquatic and semi-aquatic animals. Furthermore, the strongly developed distal humeral condyles and olecranon process of the ulna also suggest a fully terrestrial mode of life.[1]
Phylogeny
Cervico-dorsal vertebraeVarious bones
The
synapomorphies, with two possible synapomorphies supporting the position within Protorosauria. However, due to the fragmentary nature of the holotype of Aenigmastropheus which can be scored for less than 10% of the characters, under constrained topologies only one additional step is required to position Aenigmastropheus outside Archosauromorpha. A more robust version of the same analysis, published in 2016, found it to be one step away from Protorosaurus on the archosauromorph family tree, rather than forming a clade with it. The cladogram below shows the phylogenetic position of Aenigmastropheus among the Sauria following this analysis.[2]
