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

Kiowa Kiowa ( ) or Cáuigú () people are a Native Americans in the United States, 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 Colora ...

language) is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

thalattosuchia Thalattosuchia is a clade of mostly 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 member ...

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. Extinct crocodylomorphs were considerably mor ...

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

Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...

Snowshoe Formation of

Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...

Discovery and naming

The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

was collected by Lupher and Packard. Buffetaut. (1979). Jour.Paleontology. 53,pt.1: 210, tf.2f,g. ''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 Weberg Member of the Snowshoe Formation near the town of Izee. It is a member of Metriorhynchoidea, a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of marine-adapted thalattosuchians that existed until the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

Description and classification

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 ...

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, 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 ...

of Metriorhynchidae, which suggests that it should have been more extensively adapted to marine life than '' Teleidosaurus'' and '' Eoneustes'' were, but less adapted than true metriorhynchids like '' Metriorhynchus'' and '' Cricosaurus'', which were fully marine. The skull of ''Zoneait'' has expanded prefrontals, which support laterally-facing eyes. In combination with a streamlined skull, which is notably more derived than other non-metriorhynchid members of Metriorhynchoidea.

Paleoecology

The lateral-facing eyes of ''Zoneait'', which are similar to those of metriorhynchids, differentiate it from non-marine aquatic crocodylomorphs, which have more upward-facing eyes. 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 or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

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

humerus The humerus (; : humeri) 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 (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

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.

Notes

References

{{Taxonbar, from1=Q20080503, from2=Q124462827 Jurassic animals of North America Monotypic prehistoric reptile genera Prehistoric marine crocodylomorphs Middle Jurassic crocodylomorphs Fossil taxa described in 2015 Thalattosuchia Paleontology in Oregon Prehistoric pseudosuchian genera