The Aristonectidae is a

taxonomic family Family ( la, familia, plural ') is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between order (biology), order and genus. A family may be divided into subfamily, subfamilies, which are intermediate r ...

of poorly known

plesiosaurs The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared ...

from the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

and

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

periods. They are closely related to

polycotylid Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widespread during the early Late Cretaceous. ...

plesiosaurs. The family is made up of ''

Tatenectes ''Tatenectes'' is a genus of cryptoclidid plesiosaur known from the Upper Jurassic of Wyoming. Its remains were recovered from the Redwater Shale Member of the Sundance Formation, and initially described as a new species of '' Cimoliosaurus ...

'', ''

Kimmerosaurus ''Kimmerosaurus'' ("lizard from Kimmeridge") is an extinct genus of plesiosaur from the family Cryptoclididae. ''Kimmerosaurus'' is most closely related to ''Tatenectes''. Discovery There are very few fossil remains of ''Kimmerosaurus'' known. ...

'', ''

Aristonectes ''Aristonectes'' (meaning 'best swimmer') is an extinct genus of plesiosaur from the Late Cretaceous Paso del Sapo Formation of what is now Argentina, the Quiriquina Formation of Chile and the Lopez de Bertodano Formation of Antarctica. The ...

'', and ''

Kaiwhekea ''Kaiwhekea'' () is an extinct genus of plesiosaur from the Late Cretaceous (Maastrichtian age) of what is now New Zealand. History of discovery The type species, ''Kaiwhekea katiki'', was first described by Arthur Cruickshank and Ewan Fordyce ...

''. This group was formerly known as the Cimoliasauridae, but since ''Cimoliasaurus'' is indeterminate and quite possibly elasmosaurid, this replacement name was erected. ''

'' and ''

'' represent an earlier Oxfordian Jurassic radiation from

Laurasia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...

, while ''

'' and ''

'' represent a later

radiation from Gondwana.

Appearance

Aristonectidae were characterized by a relatively larger head and shorter neck than the

Plesiosauridae The Plesiosauridae are a monophyletic group of plesiosaurs.Ketchum, H. F., and Benson, R. B. J., 2010. "Global interrelationships of Plesiosauria (Reptilia, Sauropterygia) and the pivotal role of taxon sampling in determining the outcome of phylo ...

and

Elasmosauridae Elasmosauridae is an extinct family of plesiosaurs, often called elasmosaurs. They had the longest necks of the plesiosaurs and existed from the Hauterivian to the Maastrichtian stages of the Cretaceous, and represented one of the two groups of p ...

. Teeth resemble those of the

. The group is known only from scanty and fragmentary remains. The formal diagnosis of the

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

is that they possess: * A snout that is relatively long but unconstricted and wide anteriorly * A paroccipital process of the opisthotic that articulates only with the squamosal * Teeth that are small and relatively narrow (crown length < 1 cm) * Number of premaxillary teeth is 7 or more * Number of maxillary teeth is greater than 30 * Palate with a ventrally expanded boss projecting out of the plane of the palate between the posterior and anterior interpterygoid vacuity * Number of cervical vertebrae greater than 32 * Cervical vertebrae much wider than long * Cervical vertebrae with poorly defined rims of articular surfaces * Marked dorso-ventral constriction of cervical centra on ventral midline (binocular-shaped) * Neural arch and canal very small relative to centrum diameter

References

* O'Keefe, F. R. and Street, H. P. 2009. Osteology of the Cryptocleidoid Plesiosaur ''Tatenectes laramiensis'', with Comments on the Taxonomic Status of the Cimoliasauridae. ''Journal of Vertebrate Paleontology'', 29(1): 48–57
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{{Taxonbar, from=Q4791058 Jurassic plesiosaurs Cretaceous plesiosaurs Oxfordian first appearances Late Cretaceous extinctions