''Tetraceratops insignis'' ("four-horned face emblem") is an extinct

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 reptil ...

from the Early Permian that was formerly considered the earliest known representative of Therapsida, a group that includes mammals and their close extinct relatives. It is known from a single skull, discovered in Texas in 1908. According to a 2020 study, it should be classified as a primitive non-therapsid sphenacodont rather than a genuine basal therapsid.

Description

''Tetraceratops'' is known from a single

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

discovered in Texas in the early 1900s. Contrary to its

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 nom ...

name, ''Tetraceratops'' actually has six horns, one pair being on the

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

bones, one pair on the prefrontal bones, and one pair on the angular processes of the

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

. When it was discovered and described in 1908, the skull was still embedded in a matrix, and only the premaxilla and prefrontal pairs were visible. In life, thus, it would have resembled a large lizard with four horns on its snout, and a pair of large spines emanating from the corners of its jaw. In addition to horns, ''Tetraceratops'' also had an impressive set of

teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, t ...

. The second pair of teeth on the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...

ry bones were large and fang-like. Likewise the first teeth in the upper jaw were long and dagger-like. Tetraceratops skull

Classification

''Tetraceratops'' was originally identified as a member of a group called Pelycosauria, an

evolutionary grade A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. Definition An evolutionary grade is a group of sp ...

of synapsids more basal than therapsids. It has been variously grouped in the family

Sphenacodontidae Sphenacodontidae (Greek: "wedge point tooth family") is an extinct family of small to large, advanced, carnivorous, Late Pennsylvanian to middle Permian pelycosaurs. The most recent one, ''Dimetrodon angelensis'', is from the late Kungurian or ...

, which is closely related to Therapsida, and Eothyrididae, which is more distantly related. Recent

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

studies classify it as either a pelycosaur-grade synapsid or the basalmost therapsid, rendering its exact phylogenetic placement unclear. However, a new study by Spindler (in press) concluded that no convincing morphological evidence could be made for a therapsid placement of ''Tetraceratops'' and that this genus was better placed as a basal sphenacodontian.

References

External links

Palaeos.com's article on therapsids
{{Taxonbar, from=Q60565 Prehistoric sphenacodonts Prehistoric synapsid genera Cisuralian synapsids of North America Transitional fossils Fossil taxa described in 1908 Taxa named by William Diller Matthew Kungurian genus first appearances Kungurian genus extinctions

Description

Classification

See also

References

External links