Teleocrater

Teleocrater Temporal range:
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	†Aphanosauria
Genus:	†Teleocrater Nesbitt et al., 2017
Type species
Teleocrater rhadinus Nesbitt et al., 2017

Teleocrater (meaning "completed basin") is a genus of avemetatarsalian belonging to the basal clade Aphanosauria 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.

Description

In life, Teleocrater would have been a long-necked and carnivorous quadruped that measured some 7–10 feet (2.1–3.0 m) in length.

Skull

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).

Axial skeleton

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 are wider than they are tall; 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.

Appendicular skeleton

Like other archosaurs as well as the proterosuchids, Teleocrater has a distinct acromion process on the scapula, and like silesaurids there is a thin ridge on the back of the bone. The socket of the scapula is oriented downwards and backwards, more so than that of Yarasuchus. On the humerus, there is a long deltopectoral crest that stretches for about 30% of the bone's length, as with other aphanosaurians; such a long crest is also seen in Nyasasaurus and dinosaurs, 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.

Teleocrater is named after its mostly-closed acetabulum, or hip socket (the eponymous "basin"). There is a small and concave notch on the bottom edge of the part of the ilium that extends to meet the ischium, which suggests a small perforation within the acetabulum. This is not a unique characteristic; Asilisaurus 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 and Marasuchus, 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.

Hindlimb

In terms of hindlimb proportions, Teleocrater is more similar to silesaurids, pseudosuchians, and early archosaurs than lagerpetids or ornithodirans, in that the metatarsus is not particularly lengthened with respect to the femur and tibia. The lengthening of the metatarsus in the latter groups probably represents adaptations to running. It is not entirely clear how many times the metatarsus independently lengthened among avemetatarsalians.

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. An additional aphanosaurian trait is that the bottom articulating surface of the femur is concave. On this articulating surface, the back of the medial condyle bears a vertical scar, also seen in dinosauromorphs. The femur is overall quite similar to that of Dongusuchus; however, in Teleocrater, the sides of the top end are more rounded and the inner surface is concave, the posteromedial tuber on the top end is convex instead of flat, and the length relative to midshaft width is shorter.

Unlike either proterochampsids or dinosauromorphs, the tibia of Teleocrater does not bear a cnemial crest. The fibula bears a long, twisted crest for the attachment of the iliofibularis, and the front edge of the top of the bone is expanded outwards. Additional features shared by aphanosaurians, silesaurids (namely Asilisaurus and Lewisuchus), and pseudosuchians occurs in the calcaneum. It has a convex-concave joint with the astragalus that allows for free movement, a tuber on its surface that is tall, broad, and directed backwards, and its articulation with the fibula is distinctly rounded. This "crocodile-normal" configuration was probably plesiomorphic for archosaurs, including avemetatarsalians. Lagerpetids and pterosaurs both lack the tuber (lagerpetids also lack the rounded fibular articulation), and dinosaurs lack the convex-concave joint. Like the lengthening of metatarsals, the development of this more specialized "advanced mesotarsal" configuration probably occurred convergently.

Discovery and naming

The holotype specimen of Teleocrater, NHMUK PV R6795, was found by Francis Rex Parrington in 1933. It consists of a partial, disarticulated skeleton that includes four vertebrae from the neck, seven from the trunk, and seventeen from the tail; parts of one neck and one trunk rib; part of a scapula and coracoid; the radius and ulna from the right forelimb; part of the left ilium; both femora and tibiae, as well as the left fibula; and isolated fragments from metatarsals and phalanges. Parts of the trunk vertebrae and humerus, likely originating from another individual, were referred to the same animal under the specimen number NHMUK PV R6796. Although the exact locality is unknown, Parrington recorded the specimen as originating from near the village of Mkongoleko, "south of river Mkongoleko", in the Ruhuhu Basin of southern Tanzania. 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. He was the first to apply the name Teleocrater, derived from Greek teleos ("finished", "complete") and krater ("bowl", "basin"), in reference to the closed acetabulum of the animal. His initial thesis listed tanyura as the specific name of Teleocrater; later, in a 1967 overview of reptiles, he revised it to rhadinus, from Greek rhadinos ("slender", in reference to the bodyplan of the animal). However, given that it was never formally published, it remained an invalid nomen nudum.

In 2015, a bonebed designated as Z183 was discovered within 1 kilometre (0.62 mi) 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.

Bonebed Z183 belongs to the lower portion of the Lifua Member of the Manda Formation. The bonebed is located in a gully, and is surrounded by pinkish-grey cross-bedded sandstone containing well-rounded quartz pebbles. The sandstone is overlain near the top by reddish-brown and olive-grey siltstone in a digit-like pattern characteristic of point bars; most of the vertebrate remains are concentrated within a 45 centimetres (18 in) 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.

Classification

Prior to the formalization of the definitions of these groups by Jacques Gauthier in 1984 and 1986, Teleocrater was variously considered as a rauisuchian (a group now considered non-monophyletic), 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 and the lack of a phylogenetic analysis incorporating the taxon. A 2008 histological study of early archosauriforms tentatively identified Teleocrater as an archosauriform of uncertain phylogenetic placement, but possibly closely related to Eucrocopoda.

Nesbitt et al. found that it is closely related to the previously problematic Dongusuchus and Yarasuchus, which are also aphanosaurians.