''Trilophosaurus'' (

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

for "

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

with three

ridges A ridge is a long, narrow, elevated geomorphologic landform, structural feature, or a combination of both separated from the surrounding terrain by steep sides. The sides of a ridge slope away from a narrow top, the crest or ridgecrest, wi ...

") 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 ...

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

known from the

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...

of North America. It was a

herbivore A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...

up to 2.5 m long.

Description

''Trilophosaurus'' 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 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 mammals h ...

and front of the lower jaw, which in life were probably equipped with a horny beak. Based on evidence derived from tooth wear patterns, ''Trilophosaurus'' was able to masticate labiolingually. The skull is also unusual in that the lower

temporal opening The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

is missing, giving the appearance of a

euryapsid __NOTOC__ Euryapsida is a polyphyletic (unnatural, as the various members are not closely related) group of sauropsids that are distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal bon ...

skull. Because of this, the trilophosaurs were once classified with

placodont Placodonts (" tablet teeth") are an extinct order of marine reptiles that lived during the Triassic period, becoming extinct at the end of the period. They were part of Sauropterygia, the group that includes plesiosaurs. Placodonts were generall ...

s within

Sauropterygia Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic diapsid reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosau ...

. Carroll (1988) suggested that the lower opening may have been lost to strengthen the skull.

Taxonomy

''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 1956) is a German-born American palaeontologist who is a Senior Research Geologist and Curator of Vertebrate Paleontology at the National Museum of Natural History of the Smithsonian Institution in Washington, DC. Career ...

and Paul E. Olsen reassigned ''T. jacobsi'', as well as two additional trilophosaurids ('' Tricuspisaurus'' and '' Variodens''), to

Procolophonidae Procolophonidae is an extinct family (biology), family of small, lizard-like parareptiles known from the Late Permian to Late Triassic that were distributed across Pangaea, having been reported from Europe, North America, China, South Africa, Sou ...

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 '' Tricuspisaurus'' and '' Variodens'' 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 In taxonomy, the scientific classification of living organisms, a synonym is an alternative scientific name for the accepted scientific name of a taxon. The botanical and zoological codes of nomenclature treat the concept of synonymy differently. ...

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 Telangana 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 the h ...

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 In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...

focusing on relations within

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

and recovered ''T. jacobsi'' to be more closely related to '' Spinosuchus caseanus'' 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 biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

clade to the exclusion of ''T. buettneri'' based on a single

autapomorphy In phylogenetics, an autapomorphy is a distinctive feature, known as a Synapomorphy, derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or Outgroup (cladistics), outgroup taxa, not ...

. 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 Trilophosauridae Prehistoric reptile genera Late Triassic reptiles of North America Fossil taxa described in 1928 Taxa named by Ermine Cowles Case