Teleocrater

Extinct genus of reptiles

Teleocrater (meaning "completed basin", in reference to its closed acetabulum) is a genus of avemetatarsalian archosaur from the Middle Triassic Manda Formation of Tanzania. The name was coined by English paleontologist Alan Charig in his 1956 doctoral dissertation, but was only formally published in 2017 by Sterling Nesbitt and colleagues. The genus contains the type and only species T. rhadinus. Uncertainty over the affinities of Teleocrater have persisted since Charig's initial publication; they were not resolved until Nesbitt et al. performed a phylogenetic analysis. They found that Teleocrater is most closely related to the similarly enigmatic Yarasuchus, Dongusuchus, and Spondylosoma in a group that was named the Aphanosauria. Aphanosauria was found to be the sister group of the Ornithodira, the group containing dinosaurs and pterosaurs. A carnivorous quadruped measuring long, Teleocrater is notable for its unusually long neck vertebrae. The neural canals in its neck vertebrae gradually become taller towards the back of the neck, which may be a distinguishing trait. Unlike the Lagerpetidae or Ornithodira, the hindlimbs of Teleocrater are not adapted for running; the metatarsal bones are not particularly elongated. Also unlike lagerpetids and ornithodirans, Teleocrater inherited the more flexible ankle configuration present ancestrally among archosaurs, suggesting that the same configuration was also ancestral to Avemetatarsalia but was lost independently by several lineages. Histology of the long bones of Teleocrater indicates that it had moderately fast growth rates, closer to ornithodirans than crocodilians and other pseudosuchians.

In life, Teleocrater would have been a long necked and carnivorous quadruped that measured some in length.

Carnivory can be inferred for Teleocrater from the single tooth that was preserved, which is compressed, recurved, and bears serrations on both edges. Like other members of the Archosauria, the recess in the maxilla in front of the antorbital fenestra (the antorbital fossa) extends onto the backward projecting process of the bone, and the palatal projection of the two maxillae contacted each other. Additionally, like early dinosaurs, there is a depression on the frontal bone in front of the supratemporal fenestra (the supratemporal fossa).

The cervical vertebrae of Teleocrater from the front half of the neck are quite long, up to 3.5 times as long as they are high; they are among the longest of Triassic avemetatarsalians. Proportionally, they are longer than either the rest of the cervical vertebrae or any of the vertebrae from the front of the trunk. On the cervical vertebrae, the tops of the neural spines are blade like, but are accompanied by rounded and roughened projections; the front portions of the neural spines strongly overhang the preceding vertebrae; and the cervical vertebrae from the back of the neck have an additional projection above the parapophysis, previously identified by Nesbitt as part of a "divided parapophysis". These are shared characteristics of the Aphanosauria. In contrast to most other archosauriforms, the openings of the cervical neural canals in Teleocrater are large, subelliptical, and transition from being wider than they are tall at the front of the neck to being taller than they are wide at the back of the neck; this may be unique to the genus. The epipophyses from the front and middle cervical vertebrae project backwards, and, as in Yarasuchus and some pseudosuchians, the back cervical vertebrae appear to have supported three headed ribs. On the dorsal vertebrae, the accessory articulations known as the hyposphene hypantrum articulations are well developed. Like other aphanosaurians, there are pits located on the side of the base of the dorsal vertebrae. Two vertebrae are associated with the sacrum in Teleocrater; there are three such vertebrae in Nyasasaurus. The ribs associated with the latter sacral vertebra bear processes that project backward and outward, which is only otherwise seen in Yarasuchus, Spondylosoma, and members of the dinosauriforms. There were no bony osteoderms preserved in association with the specimen, which indicates that Teleocrater probably lacked osteoderms, unlike pseudosuchians. but not pterosaurs or silesaurids. Another aphanosaurian characteristic is the wide bottom end of the humerus, which is about 30% of the bone's length. The hand was apparently quite small. and Silesaurus both also possess it. The inner surface of the ilium in front of the acetabulum curves inwards, forming a pocket. Like both Asilisaurus the front portion of the ilium is separated from the rest of the bone by a ridge that rises vertically from the top rim of the acetabulum. As in other aphanosaurians, the ischia contact each other extensively along the midline, but less so near the tops of the bones; the bottom back portion of each ischium is rounded, and the top of the shaft of each ischium bears a longitudinal groove. The femur of Teleocrater shows a combination of diverse characteristics. Like other aphanosaurians, the top end of the femur bears a transverse groove, and also bears a scar for the attachment of the iliofemoralis externus muscle that is connected to the intermuscular line; the same condition is seen with the anterior trochanter in dinosaurmorphs, yet the scar is clearly separated from that of the iliotrochantericus caudalis as it is in Dongusuchus, Yarasuchus, and early archosaurs. These specimens were stored at the Natural History Museum, London. Alan J. Charig described the remains of Teleocrater in his 1956 PhD thesis for the University of Cambridge. In 2015, a bonebed designated as Z183 was discovered within of the approximate location described by Parrington. This bonebed contained at least three individuals of different sizes, represented by 27 bones, all of which were mixed in with the remains of an allokotosaurian; new elements not known previously included the maxilla, quadrate, braincase, axis, sacral vertebrae, humeri, ischia, and calcaneum. They were stored at the National Museum of Tanzania. It is quite possible, given the proximity, that this bonebed represents the same site that the original specimens were recovered from. In 2017, these remains, along with the holotype, were described by a study published in Nature, co authored by Sterling Nesbitt and others. They formally named the genus Teleocrater, and the type and only species T. rhadinus. The late Charig was honoured as a co author on this study. most of the vertebrate remains are concentrated within a section of this overlap. Discontinuous veins, or stringers, of brown claystone are also present. This layer has been biostratigraphically correlated to Subzone B of the South African Cynognathus Assemblage Zone, which is situated in the Anisian epoch of the Triassic period. This makes Teleocrater the oldest known bird line archosaur, preceding the previous record holder Asilisaurus. Teleocrater was variously considered as a rauisuchian, an ornithosuchian (Ornithosuchia being in fact synonymous with Avemetatarsalia), or a thecodont. The position of Teleocrater remained enigmatic due to the absence of additional remains

Nesbitt et al. utilized two phylogenetic datasets to analyze the affinities of Teleocrater: one published by Nesbitt himself in 2011, and poor quality of skeletal casts; these characteristics play a substantial role in the topology of basal avemetatarsalians. Traditionally, the "crocodile normal" and "advanced mesotarsal" ankle arrangements have been considered as a dichotomy among archosaurs: early archosaurs and pseudosuchians possess the more mobile "crocodile normal" configuration, while pterosaurs and dinosauromorphs (including birds) possess the stiffer "advanced mesotarsal" configuration. The presence of the "crocodile normal" ankle in Teleocrater (convex joint with the astragalus, presence of a tuber, and the convexity of the fibular facet on the calcaneum) indicates that this configuration was probably plesiomorphic for archosaurs, including avemetatarsalians, supported by reconstructions of character state evolution using the two datasets. At the same time, features associated with the "advanced mesotarsal" ankle (lack of a tuber and the concavity of the fibular facet on the calcaneum) were reconstructed as having appeared at least two different times among ornithodirans, with basal dinosaurs also possessing a mixture of "crocodile normal" and "advanced mesotarsal" characteristics. This demonstrates that the evolution of ankle morphology in avemetatarsalians is more complex than previously thought, and led Nesbitt et al. to conclude that the strict "crocodile normal"/"advanced mesotarsal" dichotomy is reductionist. and comparable to silesaurids, but less than that of Nyasasaurus,

In Bonebed Z183, from where the newer specimens of Teleocrater (and possibly the type specimen) are known, the fauna can generally divided into two types. Larger bones originate from the dicynodont Dolichuranus sp. and the cynodont Cynognathus sp. ; they tend to be closely associated and semi articulated, suggesting minimal transportation by water after death. Smaller bones originate from Teleocrater rhadinus, the temnospondyl "Stanocephalosaurus" pronus, an unnamed allokotosaurian, and another unnamed small reptile; they tend to be more fragmented, suggesting that they were worn and transported by several floods before they were finally deposited. Overall, the preservational environment is consistent with the crevasse splay of a floodplain, where the animals were killed and transported by sheetfloods before being buried by the crevasse splay complex. Elsewhere in the assemblage of the lower Lifua Member, the ctenosauriscid Hypselorhachis mirabilis is also present.