''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, with several mammalian traits including erect limbs, endothermy, and some d ...

cynodont Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 Megaannum, mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extin ...

s 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 ...

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 Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

.Fossilworks. "Stereognathus." Retrieved from
/ref> There is a single named species: ''S. ooliticus'', named after the Great Oolite deposits of England. A second species, ''S. hebridicus'', was named after the

Hebrides The Hebrides ( ; , ; ) are the largest archipelago in the United Kingdom, off the west coast of the Scotland, Scottish mainland. The islands fall into two main groups, based on their proximity to the mainland: the Inner Hebrides, Inner and Ou ...

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 (geology), age and stage (stratigraphy), stage of the Middle Jurassic. It lasted from approximately 168.2 ±1.2 annum, Ma to around 165.3 ±1.1 Ma (million years ago). The Bathonian Age succeeds ...

age deposits on the

Isle of Skye The Isle of Skye, or simply 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 by the Cuillin, the rocky slopes of which provide some of ...

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 In taxonomy, the scientific classification of living organisms, a synonym is an alternative scientific name for the accepted scientific name of a taxon. The botanical and zoological codes of nomenclature treat the concept of synonymy differently. ...

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

Tritylodontidae Middle Jurassic synapsids of Europe Fossil taxa described in 1854 {{paleo-cynodont-stub