''Palaeoxonodon'' is an extinct genus of

cladotheria Cladotheria is a clade (sometimes ranked as a legion) of mammals. It contains modern therian mammals (marsupials and placentals) and several extinct groups, such as the "dryolestoids", amphitheriids and peramurids. The clade was named in 1975 ...

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

from the Middle

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

of England and Scotland.

Discovery

The first fossils of ''Palaeoxonodon ooliticus'' were found in the "Mammal Bed" in Kirtlington Quarry,

Oxfordshire Oxfordshire ( ; abbreviated ''Oxon'') is a ceremonial county in South East England. The county is bordered by Northamptonshire and Warwickshire to the north, Buckinghamshire to the east, Berkshire to the south, and Wiltshire and Glouceste ...

, England. This site was rich in

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

mammal remains from the

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 ...

Forest Marble Formation The Forest Marble is a geological formation in England. Part of the Great Oolite Group, it dates to the late Bathonian stage of the Middle Jurassic.Weishampel, David B; et al. (2004). "Dinosaur distribution (Middle Jurassic, Europe)." In: Weisha ...

. Later, two more species of ''Palaeoxonodon'' were named from the same site, ''P. leesi'' and ''P. freemani''.Sigogneau-Russell D. 2003. Holotherian mammals from the Forest Marble(Middle Jurassic of England). ''Geodiversitas'', 25, 501–537. All of these fossils were individual teeth. However, a recent fossil recovered from the Kilmaluag Formation of Skye, Scotland comprised a lower jaw with five

molar teeth The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone toot ...

, four

premolars The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mout ...

, a canine and one

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

present. This more complete fossil suggests that the separate species previously named from England were in fact all the same species, ''P. ooliticus'', and only appeared different due to their differing position along the tooth row. This was also the case for '' Kennetheridium leesi''. Two dentaries have recently been described.

Description

''Palaeoxonodon ooliticus'', like most mammals in the

, was relatively small. The most complete fossil jaw - from Scotland - measures 11.6mm, but does not include the very front or rear portions of the jaw. The living animal was probably around the size of a mouse and would have been insectivorous. The teeth of ''Palaeoxondon'' have an elongate lower molar

talonid The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone tooth ...

, with hypoconid placed buccal to the midline of the crown, well-developed upper molar metacone and wing-like parastylar region. Compared to its closest relatives, ''Palaeoxonodon'' has rather slender jaws.

Taxonomy

''Palaeoxondon'' belongs in the order Amphitheriida, closely related to Dryolestida. They are among the earliest cladotherian mammals, which have their origins in the Middle

and consist of the descendants of the

last common ancestor A most recent common ancestor (MRCA), also known as a last common ancestor (LCA), is the most recent individual from which all organisms of a set are inferred to have descended. The most recent common ancestor of a higher taxon is generally assu ...

of Amphitheriida, Dryolestida, Peramurida and

Zatheria Cladotheria is a clade (sometimes ranked as a Legion (taxonomy), legion) of mammals. It contains modern therian mammals (marsupials and Placentalia, placentals) and several extinct groups, such as the "dryolestoids", amphitheriids and peramurids. ...

(which includes modern

Theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

n mammals). Recent phylogenetic studies indicate that ''Palaeoxonodon'' was a sister taxon of ''

Amphitherium ''Amphitherium'' is an extinct genus of stem cladotherian mammal that lived during the Middle Jurassic of England. It was one of the first Mesozoic mammals ever described. A recent phylogenetic study found it to be the sister taxon of '' Palae ...

'' itself. Cladogram after Panciroli et al. 2018: However, some studies have not found a close relationship with ''Amphitherium''. Cladogram after Lasseron et al. 2022:

References

{{Taxonbar, from=Q30693356 Cladotheria Bathonian genera Jurassic mammals of Europe Jurassic England Jurassic Scotland Fossils of Great Britain Fossil taxa described in 1976 Prehistoric mammal genera