''Zoneait'' (pronounced "zone-eight" and meaning "large tooth" in the

Kiowa Kiowa () people are a Native American tribe and an indigenous people of the Great Plains of the United States. They migrated southward from western Montana into the Rocky Mountains in Colorado in the 17th and 18th centuries,Pritzker 326 and eve ...

language) 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 ...

thalattosuchia Thalattosuchia is a clade of marine crocodylomorphs from the Early Jurassic to the Early Cretaceous that had a cosmopolitan distribution. They are colloquially referred to as marine crocodiles or sea crocodiles, though they are not members of Cro ...

crocodylomorph Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. During Mesozoic and early Cenozoic times, cro ...

known from a single species, ''Zoneait nargorum'', from the

Middle Jurassic The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations co ...

of Oregon. ''Z. nargorum'' was named in 2015 by paleontologist Eric Wilberg on the basis of several partial skulls, vertebrae, and forelimb bones that were found in an outcrop of the Snowshoe Formation near the town of Izee. It is a member of

Metriorhynchoidea Metriorhynchoidea is an extinct superfamily of thalattosuchian crocodyliforms from the Early Jurassic to the Early Cretaceous (Toarcian - Valanginian, possibly as late as early Aptian) of Europe, North America and South America. Metriorhynchids a ...

, a

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, ...

of marine-adapted thalattosuchians that existed until the

Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pro ...

. The skeleton of ''Zoneait'' possesses several adaptations for offshore marine life but retains features characteristic of its land-living ancestors, indicating that it is a

transitional form A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross a ...

between the fully marine

metriorhynchid Metriorhynchidae is an extinct family of specialized, aquatic metriorhynchoid crocodyliforms from the Middle Jurassic to the Early Cretaceous period (Bajocian to early Aptian) of Europe, North America and South America. The name Metriorhynchidae ...

s of the late Middle Jurassic to Early Cretaceous, and earlier non-marine crocodylomorphs. The Snowshoe Formation was deposited in a shallow marine environment within a tropical

forearc basin Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...

, suggesting that ''Zoneait'' was a marine predator. Wilberg found that ''Zoneait'' is the

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

of Metriorhynchidae, which suggests that it should have been more extensively adapted to marine life than ''

Teleidosaurus ''Teleidosaurus'' is an extinct genus of carnivorous metriorhynchoid crocodyliform from Middle Jurassic (late Bajocian to early Bathonian stage) deposits of Normandy, France.Wilkinson LE, Young MT, Benton MJ. 2008. A new metriorhynchid crocodilia ...

'' and ''

Eoneustes ''Eoneustes'' (meaning "dawn swimmer") is an extinct genus of metriorhynchoid crocodyliform from Middle Jurassic (late Bajocian to early Bathonian stage) deposits of France. ''Eoneustes'' was a carnivore that lived in the oceans and spent much, ...

'' were, but less adapted than true metriorhynchids like ''

Metriorhynchus ''Metriorhynchus'' is an extinct genus of marine crocodyliform that lived in the oceans during the Late Jurassic. The type species, ''M. brevirostris'' was named in 1829 as a species of ''Steneosaurus'' before being named as a separate genus by ...

'' and ''

Cricosaurus ''Cricosaurus'' is an extinct genus of marine crocodyliforms of the Late Jurassic. belonging to the family Metriorhynchidae. The genus was established by Johann Andreas Wagner in 1858 for three skulls from the Tithonian (Late Jurassic) of German ...

'', which were fully marine. ''Zoneait'' has a streamlined skull with eyes that faced laterally like those of metriorhynchids, unlike the more upward-facing eyes of other non-marine aquatic crocodylomorphs. The shift in eye orientation is thought to reflect changes in feeding ecology; upward-facing eyes would have been adaptive for aquatic crocodylomorphs ambushing land-living prey from beneath the surface of the water, whereas side-facing eyes would have been adaptive for marine crocodylomorphs hunting in open marine environments. The forelimbs are not flattened into paddles as in metriorhynchids, but the

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

(lower arm bone) is reduced in length, indicating that forelimb reduction began at the lower limb and progressed upward (the

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

or upper arm bone of ''Zoneait'' not reduced). Taken together, the transitional features of ''Zoneait'' indicate that metriorhynchoids' adaptation of a marine lifestyle began with a shift in feeding ecology and only later involved changes in swimming locomotion.

References

Jurassic animals of North America Monotypic prehistoric reptile genera Prehistoric marine crocodylomorphs Middle Jurassic crocodylomorphs Fossil taxa described in 2015 Thalattosuchians Paleontology in Oregon Prehistoric pseudosuchian genera {{jurassic-reptile-stub