''Trilophosaurus'' ( Greek for "

lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...

with three ridges") is a lizard-like

trilophosaurid Trilophosaurs are lizard-like Triassic allokotosaur reptiles related to the archosaurs. The best known genus is ''Trilophosaurus'', a herbivore up to long. It had a short, unusually heavily built skull, equipped with massive, broad flattened che ...

allokotosaur Allokotosauria is a clade of early archosauromorph reptiles from the Middle Triassic, Middle to Late Triassic known from Asia, Africa, North America and Europe. Allokotosauria was first described and named when a new monophyletic grouping of spec ...

known from the Late Triassic of North America. It was a

herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...

up to 2.5 m long. It had a short, unusually heavily built skull, equipped with massive, broad flattened cheek teeth with sharp shearing surfaces for cutting up tough plant material. Teeth are absent from the premaxilla and front of the lower jaw, which in life were probably equipped with a horny beak. Trilophosaurus buettneri - AMNH - DSC06265

Trilophosaurus buettneri - AMNH - DSC06265

The skull is also unusual in that the lower

temporal opening 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, the ...

is missing, giving the appearance of a euryapsid skull. Because of this, the trilophosaurs were once classified with placodonts within Sauropterygia. Carroll (1988) suggested that the lower opening may have been lost to strengthen the skull. Trilophosaurus NT small

''Trilophosaurus'' is traditionally thought to include two valid species: the typical ''T. buettneri'' and the more robust ''T. jacobsi''. In 1993, paleontologists

Hans-Dieter Sues Hans-Dieter Sues (born January 13, 1956) is a German-born American paleontologist who is Senior Scientist and Curator of Vertebrate Paleontology at the National Museum of Natural History of the Smithsonian Institution in Washington, DC. He receiv ...

and

Paul E. Olsen Paul E. Olsen (born August 4, 1953) is an American paleontologist and author and co-author of a large number of technical papers. Growing up as a teenager in Livingston, New Jersey, he was instrumental in Riker Hill Fossil Site being named a Na ...

reassigned ''T. jacobsi'', as well as two additional trilophosaurids (''

Tricuspisaurus ''Tricuspisaurus'' is an extinct genus of reptile originally described as a trilophosaurid; it was later considered likely to be a procolophonid, but recent analyses have affirmed the original classification. Fossils are known from the Ruthin Q ...

'' and ''

Variodens ''Variodens'' is an extinct genus of trilophosaur. Fossils have been found from the Emborough Quarries in the Mendip Hills of Somerset, England. These fossils have been uncovered from a Late Triassic fissure fill within Carboniferous-age limeston ...

''), to Procolophonidae based on similarities between its tricuspid teeth and those of the newly described procolophonid '' Xenodiphyodon''. This view persisted in subsequent publications until the cranial material of ''T. jacobsi'' was described by Heckert ''et al.'' (2006). The new material confirmed the originally classification that ''T. jacobsi'', as well as ''

'' and ''

'' based on similarities to it, are indeed

s. Meanwhile, a third species of ''Trilophosaurus'', ''T. dornorum'', was named by Mueller & Parker (2006) based on teeth of a robust individual.Müller, B. D. & Parker, W. G., 2006
A new species of ''Trilophosaurus'' (Diapsida: Archosauromorpha) from the Sonsela Member (Chinle Formation) of Petrified Forest National Park, Arizona
''Museum of Northern Arizona,'' Bulletin 62:119-125. However, Spielmann ''et al.'' (2009) argued that the robustness of the new species is not sufficient to differentiate it from other Trilophosaurus species, especially in light of new robust specimens of ''T. jacobsi''. Therefore, they considered ''T. dornorum'' to be a

junior synonym The Botanical and Zoological Codes of nomenclature treat the concept of synonymy differently. * In botanical nomenclature, a synonym is a scientific name that applies to a taxon that (now) goes by a different scientific name. For example, Linna ...

of ''T. jacobsi'',Spielmann, J.A., Lucas, S.G., Heckert, A.B., Rinehart, L.F., and Richards, H.R., III. 2009
Redescription of ''Spinosuchus caseanus'' (Archosauromorpha: Trilophosauridae) from the Upper Triassic of North America
''Palaeodiversity'' 2:283-313. a view that was maintained since in other publications. Spielmann ''et al.'' (2006) redescribed the type material of ''

Malerisaurus ''Malerisaurus'' is an extinct genus of archosauromorph known from Andhra Pradesh of India and Texas of the USA. Description ''Malerisaurus'' was a medium-sized archosauromorph which averaged 1.2 meters in length. ''Malerisaurus'' is known from ...

langstoni'' and concluded that it's indistinguishable from ''T. buettneri'', and thus ''M. langstoni'' represents its synonym.Spielmann, J.A., Lucas, S.G., Hunt, A.P., and Heckert. 2006
Reinterpretation of the holotype of ''Malerisaurus langstoni'', a diapsid reptile from the Upper Triassic Chinle Group of West Texas
''The Triassic-Jurassic Terrestrial Transition.'' New Mexico Museum of Natural History and Science, Bulletin 37:543-547. Nesbitt ''et al.'' (2015) performed a phylogenetic analysis focusing on relations within Allokotosauria and recovered ''T. jacobsi'' to be more closely related to ''

Spinosuchus caseanus ''Spinosuchus'' (meaning "spined crocodile") is an extinct genus of trilophosaurid allokotosaur from the Late Triassic of Texas, southern United States. It has been assigned to a variety of groups over its history, from coelophysid dinosaur to p ...

'' than to the type species of ''Trilophosaurus''. To further test this possibility, the types of ''S. caseanus'' and ''T. jacobsi'' were scored separately from the referred the Kahle ''Trilophosaurus'' Quarry elements (referred to ''T. jacobsi'' by Spielmann ''et al.'' (2008) or to ''S. caseanus'' by Spielmann ''et al.'' (2009)). A phylogenetic analysis recovered the three in a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

clade to the exclusion of ''T. buettneri'' based on a single autapomorphy. Furthermore, the types of ''S. caseanus'' and ''T. jacobsi'' as well as the Kahle Quarry material all scored identically, suggesting that ''T. jacobsi'' not only should be reassigned to ''Spinosuchus'', but in fact represents the junior synonym of its type and only species (''S. caseanus''). Nesbitt ''et al.'' (2015) refrained from officially synonymizing the two taxa pending further study of other advanced trilophosaurids.

Sources

* Benton, M. J. (2000), ''Vertebrate Paleontology'', 2nd ed. Blackwell Science Ltd, p. 144 * Carroll, R. L. (1988), ''Vertebrate Paleontology and Evolution'', W.H. Freeman & Co. p. 266 *

External links

Archosauromorpha: Rhynchosaurs and ''Trilophosaurus''
at Palaeos
Trilophosauria. Beaked, lizard-like reptiles
UC Museum of Paleontology
Archive copy
from 6 February 2019. {{Taxonbar, from=Q15041841 Allokotosaurs Prehistoric reptile genera Late Triassic reptiles of North America Fossil taxa described in 1928 Taxa named by Ermine Cowles Case