Palaeoxonodon Freeman, 1976b

Sigogneau-Russell, Denise, 2003, Holotherian mammals from the Forest Marble (Middle Jurassic of England), Geodiversitas 25 (3), pp. 501-537 : 504-507

publication ID	https://doi.org/ 10.5281/zenodo.5374561
persistent identifier	https://treatment.plazi.org/id/8C6F152B-C62F-7757-FB92-2E42DEA25F88
treatment provided by	Marcus
scientific name	Palaeoxonodon Freeman, 1976b
status

Treatment

Genus Palaeoxonodon Freeman, 1976b

TYPE SPECIES. — Palaeoxonodon ooliticus Freeman, 1976b .

REVISED DIAGNOSIS. — Holotherian genus with lower molars having a relatively sharp trigonid, an elongated and incipiently basined talonid with one cusp, a distal metacristid and a more or less concave postero-labial face of the metaconid (“groove that received the paracone”, Clemens & Mills 1971: 105). Cusp f well developed; usually no e cusp. Wear facet A present . Roots equal or subequal. Labial alveolar border of lower jaw notably lower than lingual border. Dental formula unknown. Differs from Amphitherium Blainville, 1838 by a slightly narrower trigonid with sharper cusps, by the labial face of the metaconid being concave (as in Peramus Owen, 1871 ) whereas it is flat or even convex in Amphitherium ; also talonid?more clearly basined and?not covering distally the adjacent paraconid; cuspule f better individualized. Differs from Nanolestes Martin, 2002 by the incipiently basined talonid (see comments on Nanolestes below). Differs from Peramus by the presence of only one cusp on the talonid rim, the absence of an antero-lingual and forwardly inclined cusp e and the equally lingual position of the para- and metaconid. Differs from Minimus Sigogneau-Russell, 1999 , with which it may share an anterior lingual cingulum, by the greater extension of the talonid and the absence of an acute antero-lingual cusp (e).

Palaeoxonodon ooliticus Freeman, 1976b HOLOTYPE. — BMNH M 36508, right lower molar ( Fig. 3A View FIG ; also figured by Freeman 1976b: fig. 1; 1979: pl. 16, 3-7).

ATTRIBUTED SPECIMENS. — BMNH J.59, J.196,?J.242, M 36507 (figured by Freeman 1979: pl. 17, 1-4; now lost), right lower molars; BMNH?J.200, J.236, J.619, J.639, J.849, left lower molars.

DISTRIBUTION. — Clay Bands of the Kirtlington Bone Bed, Kirtlington Quarry, Oxfordshire. Forest Marble (upper Bathonian). Survey Grid reference SP 495200.

REVISED DIAGNOSIS. — No lingual cingulum on the lower molars trigonid; paraconid equal to or higher than the metaconid.

DESCRIPTION (Annexe, Table 2; Figs 3-6 View FIG View FIG View FIG View FIG )

In the following description, we are aware that the terms “rather”, “slightly”, “usually” do not correspond to an objective appreciation.

Nearly all holotherian lower molars present at Kirtlington are characterized by a more or less elongated talonid, but only nine more teeth cor-

a b c d

respond to the morphology of the holotype lower molar of Palaeoxonodon ooliticus . The long talonid curves up distally and ends in one sharp cusp; a very narrow basin is enclosed between the metacristid and the entocristid, the latter defined as the lingual crest or ridge between the talonid cusp and the lingual base of the metaconid, limiting lingually the talonid basin. The metaconid is usually hollowed postero-labially, and is situated almost lingually relative to the protoconid (except BMNH J.849), while the paraconid is anterior to the protoconid (except J.639). Cuspule f is present in all cases. Roots are equal, or slightly unequal with a larger anterior root, at least in lingual view where the latter is more extruding.

Even in this small sample of teeth, the variation appears to be considerable: already the previously attributed lower molar BMNH M 36507 ( Fig. 3B View FIG ; FMK/ 7 in Freeman 1979: pl. 17, 1-3; now lost) differed from the holotype by a much more stocky trigonid, and the talonid surface more inclined lingually. Among the newly attributed specimens are only two complete teeth (BMNH J.236, J.242, Fig. 3C, D View FIG ), which again appear quite different one from the other, J.236 having a short trigonid with a high protoconid like the holotype, J.242 and other attributed teeth (J.639 excepted) being more like BMNH M 36507. The paraconid (not preserved on the holotype) is equal to or larger than the metaconid. It is straight or slightly forwardly inclined (the same variation occurs in Amphitherium or Peramus ); in two cases (BMNH J.200, J.619), it is straight but situated anteriorly relative to the anterior root. The protoconid can also be straight or posteriorly inclined. The f cusp is more or less strong and is rarely prolonged in an antero-labial cingulum. The talonid basin is oriented either occlusally, linguo-occlusally or completely lingually, this orientation not being directly related to the development of the entocristid. Such variation can also be observed along the dental series in Peramus (especially BMNH M 48404 View Materials , let alone the differences between the various specimens of this genus). The metacristid is slightly irregular in J.849, but no other than the holotype has a step (mesoconid; also present in some specimens of Peramus and Nanolestes ). The entocristid bears a bump in two cases (J.849, J.242). Finally, a slight indication of an e bump is present on only two teeth (J.236, J.639).

Very few specimens are sufficiently well preserved to exhibit wear facets. On some specimens however, facet A ( Crompton 1971) is clearly discernible on the posterior face of the paraconid ( Fig. 7 View FIG ) (such a facet is still present on the M1 of Peramus BMNH M 47339 and of Amphitherium M 36822). A frequent ovale facet is the result of wear on cuspule f by the metacone. The anterior and posterior faces of the protoconid can be touched by wear and the latter facet is oriented in a different plane than that which hollows the posterolabial face of the metaconid (this is also the case at least on M3 of Peramus BMNH M 47339 and M 48404 View Materials , though this distinction is not mentioned by Crompton [1971], where both are included in facet 1). The clearest wear facet excavates the paraconal sulcus – almost vertically oriented – between talonid and trigonid; another distinct one can be seen more distally on the labial face of the talonid (BMNH J.200, J.236, facet 3). Finally, there is in some cases (BMNH J.236, J.849) a suspected facet on the lingual distal face of the talonid; this could be interpreted as having been produced by the overlapping trigonid of the posterior tooth, as in Amphitherium ?