The Aristonectidae is a taxonomic family of poorly known plesiosaurs from the Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 and Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 periods. They are closely related to polycotylid plesiosaurs. The family is made up of Tatenectes

Tatenectes

Tatenectes is an extinct genus of cryptoclidid plesiosaur known from the Upper Member of the Sundance Formation of Wyoming. It was named by O'Keefe and Wahl in 2003 and the type species is Tatenectes laramiensis....

, Kimmerosaurus

Kimmerosaurus

Kimmerosaurus is an extinct genus of plesiosaur from the family Cryptoclididae, closely related to Tatenectes.-Etymology:...

, Aristonectes

Aristonectes

Aristonectes is an extinct genus of plesiosaur from the Late Cretaceous of what is now South America and Antarctica...

, and Kaiwhekea

Kaiwhekea

Kaiwhekea is an extinct genus of plesiosaur from the Late Cretaceous of what is now New Zealand. The type species, Kaiwhekea katiki, was first described by Arthur Cruickshank and Ewan Fordyce in 2002. Kaiwhekea was approximately 7 meters long and lived around 69-70 million years ago...

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

Tatenectes

Tatenectes

Tatenectes is an extinct genus of cryptoclidid plesiosaur known from the Upper Member of the Sundance Formation of Wyoming. It was named by O'Keefe and Wahl in 2003 and the type species is Tatenectes laramiensis....

and Kimmerosaurus

Kimmerosaurus

Kimmerosaurus is an extinct genus of plesiosaur from the family Cryptoclididae, closely related to Tatenectes.-Etymology:...

represent an earlier Oxfordian Jurassic radiation from Laurasia

Laurasia

In paleogeography, Laurasia was the northernmost of two supercontinents that formed part of the Pangaea supercontinent from approximately...

, while Aristonectes

Aristonectes

Aristonectes is an extinct genus of plesiosaur from the Late Cretaceous of what is now South America and Antarctica...

and Kaiwhekea

Kaiwhekea

Kaiwhekea is an extinct genus of plesiosaur from the Late Cretaceous of what is now New Zealand. The type species, Kaiwhekea katiki, was first described by Arthur Cruickshank and Ewan Fordyce in 2002. Kaiwhekea was approximately 7 meters long and lived around 69-70 million years ago...

represent a later Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 radiation from Gondwana.

Appearance

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

Plesiosauridae

Plesiosauridae was a monophyletic family of plesiosaurs.-References:...

 and Elasmosauridae

Elasmosauridae

Elasmosauridae was the family of plesiosaurs. They had the longest necks of the plesiosaurs and survived from the Late Triassic to the end of the Cretaceous. They had a diet of fish and shelless cephalopods.-Size:...

. Teeth resemble those of the Plesiosauridae

Plesiosauridae

Plesiosauridae was a monophyletic family of plesiosaurs.-References:...

. The group is known only from scanty and fragmentary remains.

The formal diagnosis of the clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

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