''Angonisaurus'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

of kannemeyeriiform

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typica ...

from the

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma ...

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

between 247 and 242 million years ago.Hancox, P. John, Kenneth D. Angielczyk, and Bruce S. Rubidge. "Angonisaurus and Shansiodon, Dicynodonts (Therapsida, Anomodontia) from Subzone C of the Cynognathus Assemblage Zone (Middle Triassic) of South Africa." ''Journal of Vertebrate Paleontology'' 33.3 (2013): 655-76. Only one species, ''Angonisaurus cruickshanki'' has been assigned to this genus. This genus is thought to have been widely spread but rare in southern

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final st ...

. Though few in number, the fossil record of ''Angonisaurus cruickshanki'' contains multiple specimens giving it a measurable stratigraphic range. Sexually dimorphic features are found in ''Angonisaurus'' which include presence or absence of tusks and difference is size and robustness of the temporal arch and the rostral.

Discovery and naming

''Angonisaurus'' was discovered in the African Karoo Basin in 1983. Because of the lack of good lithostratigraphic marker beds, biostratigraphy has proven to be the most reliable aid for stratigraphic subdivision of this Lower–Middle Triassic succession. ''Angonisaurus'' was found in the present day locations of the towns of Sterkstroom and the

Molteno Molteno (; lmo, label= Brianzöö, Mültée) is a ''comune'' (municipality) and a hill-top town in the Province of Lecco in the Italian region Lombardy, located about northeast of Milan and about southwest of Lecco. As of 31 December 2004, it ...

. Within the area, there are three subzones, ''Angonisaurus'' are biostratigraphically constrained to the uppermost ''Cynognathus'' Assemblage Zone (subzone C). Laurasia-Gondwana

''Angonisaurus'' was named by its discoverers Cox and Li in 1983. In the name ''Angonisaurus'', "an" means without, and "gon" means angle(d) or knee, which refers to the erect hind limbs with forelimbs that bend at the elbow.

Description

Specimens

Autapomorphies of ''Angonisaurus'' have not been identified, but the taxon can be differentiated from all other Triassic dicynodonts by the combination of characters displayed by the intertemporal bar (no strong break in slope between intertemporal bar and frontals; postorbitals do not extend the full length of the intertemporal bar to reach the squamosals; parietals widely exposed in dorsal view with well-developed midline groove; interparietal makes a moderate contribution to skull roof and meets the parietals along an interdigitated suture). There are 7 localities in South Africa in which ''Angonisaurus'' fossils were uncovered in the ''Cynognathus'' assemblage zone.

Characteristics

Kannemeyeriiform dicynodont are characterized by the following combination of characters: skull tall;

caniniform In mammalian oral anatomy, the canine teeth, also called cuspids, dog teeth, or (in the context of the upper jaw) fangs, eye teeth, vampire teeth, or vampire fangs, are the relatively long, pointed teeth. They can appear more flattened however ...

process triangular in lateral view; tusks absent; interorbital skull roof wide; preparietal absent; pineal foramen located at the base of a deep, conical pit; no strong break in slope between the interorbital skull roof and the intertemporal bar; postorbitals do not extend the entire length of the intertemporal bar and do not contact the squamosals posteriorly; parietals have broad exposure on the dorsal surface of the intertemporal bar; midline groove with raised edges present on dorsal surface of parietals for at least part of the length of the intertemporal bar; interparietal makes a moderate contribution to the posterior end of the intertemporal bar and meets the parietal along an interdigitated suture; occiput broad; temporal fenestra roughly rectangular in shape; margins of squamosal robust and thickened; fossa for the origin of the M. adductor mandibulae externus lateralis relatively vertically oriented; median wall of posterior dentary sulcus raised and thickened. Autapomorphies of ''Angonisaurus'' have not been identified, but the taxon can be differentiated from all other Triassic dicynodonts by the combination of characters displayed by the intertemporal bar (no strong break in slope between intertemporal bar and frontals; postorbitals do not extend the full length of the intertemporal bar to reach the squamosals; parietals widely exposed in dorsal view with well-developed midline groove; interparietal makes a moderate contribution to skull roof and meets the parietals along an interdigitated suture); raised and thickened.

Classification

Identification was based on a number of characters shared by the South African specimens and the holotype of ''Angonisaurus cruickshanki,'' including the broad

occipital bone The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the c ...

; robust

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...

;

interparietal An interparietal bone (os interparietale or Inca bone or ''os inca var.'') is a dermal bone situated between the parietal and supraoccipital. It is homologous to the postparietal bones of other animals. In humans, it corresponds to the upper ...

contribution to the skull roof;

postorbital The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some v ...

s that do not contact the squamosals on the skull roof, such that the parietals form a significant portion of the temporal bar; pineal foramen located within a deep conical depression; wide interorbital bar; and the triangular, tuskless caniniform process. However, there are marked differences between the South African specimens and the Tanzanian

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...

, including the absence of a midline groove that extends along the entire length of the temporal bar, the absence of a dorsal margin of the occiput that overhangs the remainder of the occipital plate, and the more gracile caniniform processes in the South African specimens. Because the type of ''A. crucikshanki'' was the only known specimen of ''Angonisaurus'' at the time, Hancox and Rubidge expressed uncertainty as to whether these differences reflected a species-level distinction between South African and Tanzanian ''Angonisaurus'', or if they stemmed from intraspecific variation, and they did not offer a species-level identification for BP/1/5530 and BP/1/5531.

Paleobiology

Not much is known about ''Angonisaurus'' that distinguishes it functionally different from other herbivorous dicynodont relatives. The dicynodont skull is highly specialised, light but strong, with the

synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes rep ...

temporal openings at the rear of the skull greatly enlarged to accommodate larger jaw muscles. The front of the skull and the lower jaw are generally narrow and, in all but a number of primitive forms, toothless. Instead, the front of the mouth is equipped with a horny beak, as in

turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked ...

s and

ceratopsia Ceratopsia or Ceratopia ( or ; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassi ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

s. Food was processed by the retraction of the lower jaw when the mouth closed, producing a powerful shearing action, which would have enabled dicynodonts to cope with tough plant material.Crompton, A. W, and Hotton, N. 1967. Functional morphology of the masticatory apparatus of two dicynodonts (Reptilia, Therapsida). ''Postilla'', 109:1–51

References

{{Taxonbar, from=Q3337047 Kannemeyeriiformes Anisian life Middle Triassic synapsids of Africa Triassic Tanzania Fossils of Tanzania Fossil taxa described in 1983 Anomodont genera