''Plesiotylosaurus'', meaning "near ''

Tylosaurus ''Tylosaurus'' (from the ancient Greek (') 'protuberance, knob' + Greek (') 'lizard') is a genus of mosasaur, a large, predatory marine reptile closely related to modern monitor lizards and to snakes, from the Late Cretaceous. Description A ...

''", is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of marine lizard belonging to the

mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on th ...

family. It is classified as part of the

Mosasaurinae The Mosasaurinae are a subfamily of mosasaurs, a diverse group of Late Cretaceous marine squamates. Members of the subfamily are informally and collectively known as "mosasaurines" and their fossils have been recovered from every continent except ...

subfamily, alongside genera like ''

Mosasaurus ''Mosasaurus'' (; "lizard of the Meuse River") is the type genus (defining example) of the mosasaurs, an extinct group of aquatic squamate reptiles. It lived from about 82 to 66 million years ago during the Campanian and Maastrichtian sta ...

'' and ''

Prognathodon ''Prognathodon'' is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Mosasaurinae subfamily, alongside genera like ''Mosasaurus'' and ''Clidastes''. ''Prognathodon'' has been recovered from depo ...

''. The genus contains one species, ''Plesiotylosaurus crassidens'', recovered from deposits of Middle

Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval from ...

age in the

Moreno Formation The Moreno Formation is a Mesozoic geologic formation located in San Joaquin Valley (California).Dinosaur remains diagnostic to the genus level are among the fossils that have been recovered from the formation.Weishampel, et al. (2004). "Dinosaur ...

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

. Though it is classified as a mosasaurine mosasaur, and not closely related to ''

'', the name is not entirely misplaced as a number of cranial features found in the relatively intact holotype skull suggest some degree of

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

with tylosaurine mosasaurs.

Description

''Plesiotylosaurus'' was a relatively large mosasaur. The holotype, LACM 2759, consists of a poorly preserved and partially distorted yet almost complete skull and mandibles collected from the

Panoche Hills The Panoche Hills are a low mountain range in the Southern Inner California Coast Ranges System, in western Fresno County, California. They are east of the Diablo Range, on the west side of the San Joaquin Valley. They define the eastern side of ...

in the

. The lower jaw of the holotype skull measures in length, which would give the whole animal an estimated length of about . The jaws of ''Plesiotylosaurus'' were elongated and its skull was quite robust, similar to ''

'', which suggests a distinct prey preference. Lindgren (2009) did a comprehensive study on the cranial osteology of ''Plesiotylosaurus'' based on the holotype skull, LACM 2759, and another recently described cranial specimen, UCMP 137249. Though obviously closely related to ''

'' and firmly placed within the

, the skull of ''Plesiotylosaurus'' does have certain characteristics otherwise only found in the

Tylosaurinae The Tylosaurinae are a subfamily of mosasaurs,Williston, S. W. 1897. Range and distribution of the mosasaurs with remarks on synonymy. ''Kansas University Quarterly'' 4(4):177-185. a diverse group of Late Cretaceous marine squamates. Members of t ...

. These characteristics include having a solid, bony, rostrum at the tip of the snout and mandibles, a wide and virtually unconstricted internarial bar that arises from a transversely rectangular base on the posterior face of the dentigerous portion of the premaxilla, a maxillo-premaxillary suture that forms a relatively long junction between the tooth-bearing bones of the upper jaw, an anterior postorbitofrontal process that forms a broadly overlapping flange beneath the supraorbital wing of the prefrontal and an anterior mandibular channel that is developed into a long and narrow slit. Lindgren considered these shared characteristics to be an obvious example of

. Lindgren (2009) also provided an emended diagnosis for the genus as follows: LACM 2759 outline

Classification

''Plesiotylosaurus'' is most frequently recovered as a mosasaurine mosasaur sharing close relations with the genus ''

''. Indeed, the morphology of the skull of ''Plesiotylosaurus'' is amazingly similar to that of ''Prognathodon'' and had it not been for the prominent premaxillary rostrum of ''Plesiotylosaurus'', the subrectangular internarial bar and the distinctive predental prow on the dentary, they could be seen as congeneric. The cladogram below is modified from Aaron R. H. Leblanc, Michael W. Caldwell and Nathalie Bardet, 2012 and showcases the relations between ''Plesiotylosaurus'' and the rest of the

References

* ''Dinosaurs and Other Mesozoic Reptiles of California'' by Richard P. Hilton, Kevin Padian, and Ken Kirkland (page 30) * ''Ancient Marine Reptiles'' by Jack M. Callaway and Elizabeth L. Nicholls (page 286) * ''The Osteology of the Reptiles'' by Alfred Sherwood Romer (page 562) {{Taxonbar, from=Q7204696 Mosasaurines Mosasaurs of North America Fossil taxa described in 1942 Taxa named by Charles Lewis Camp