''Stereognathus'' is an extinct genus of

tritylodontid Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, bearing several mammalian traits like erect limbs, endothermy and details ...

cynodont The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...

s 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 the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...

.Fossilworks. "Stereognathus." Retrieved from
/ref> There is a single named species: ''S. ooliticus'', named after the

Great Oolite The Great Oolite Group is a Middle Jurassic stratigraphic unit that outcrops in southern England. It consists of a complex set of marine deposits primarily mudstone and bioclastic ooidal and fine grained limestone, deposited in nearshore to she ...

deposits of England. A second species, ''S. hebridicus'', was named after the

Hebrides The Hebrides (; gd, Innse Gall, ; non, Suðreyjar, "southern isles") are an archipelago off the west coast of the Scottish mainland. The islands fall into two main groups, based on their proximity to the mainland: the Inner and Outer Hebrid ...

in Scotland, where it was found; it was synonymized with ''S. ooliticus'' in 2017.

Discovery and naming

''S. ooliticus'' was the first tritylodontid species ever found, being described by Charlesworth in 1855 and later by Sir Richard Owen. In 1972, ''S. hebridicus'' was named from several postcanine teeth recovered in

Bathonian In the geologic timescale the Bathonian is an age and stage of the Middle Jurassic. It lasted from approximately 168.3 Ma to around 166.1 Ma (million years ago). The Bathonian Age succeeds the Bajocian Age and precedes the Callovian Age. Strat ...

age deposits on the

Isle of Skye The Isle of Skye, or simply Skye (; gd, An t-Eilean Sgitheanach or ; sco, Isle o Skye), is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated ...

in Scotland by palaeontologists R. J. G. Savage and Michael Waldman.Waldman, M and Savage, R.J.G 197
The first Jurassic mammal from Scotland
Journal of the Geological Society of London 128:119-125 Despite being the first tritylodontid genus found and named, ''Stereognathus'' remains poorly represented, being known mainly from isolated molar teeth. There is, however, one holotype fragment of maxilla with three damaged molars, and a second fragment of maxilla with four sets of molar roots. In 2017, Elsa Panciroli and colleagues found no points of variance between the anatomy of ''S. ooliticus'' and ''S. hebridicus'', accounting for changes with growth, which makes ''S. hebridicus'' a

junior synonym The Botanical and Zoological Codes of nomenclature treat the concept of synonymy differently. * In botanical nomenclature, a synonym is a scientific name that applies to a taxon that (now) goes by a different scientific name. For example, Linna ...

of ''S. ooliticus''. Also in 2017, Alexander Averianov and colleagues considered species of '' Polistodon'', '' Xenocretosuchus'', and ''

Montirictus ''Montirictus'' is an extinct genus of Tritylodontidae, tritylodonts known from the Early Cretaceous Kuwajima Formation of Japan. It is currently the latest surviving tritylodontid species, and is closely related to the earlier ''Xenocretosuchus ...

'' to be within the genus ''Stereognathus''; they also retained ''S. hebridicus'' as a separate species. However, this would mean ''Stereognathus'' persisted almost unchanged for over 60 million years across the whole of Eurasia, which would be very unusual. The issue remains unresolved.

References

Prehistoric cynodont genera Jurassic synapsids of Europe Tritylodontids Fossil taxa described in 1854 {{paleo-therapsid-stub