Palaeoxonodon freemani, Sigogneau-Russell, 2003

Sigogneau-Russell, Denise, 2003, Holotherian mammals from the Forest Marble (Middle Jurassic of England), Geodiversitas 25 (3), pp. 501-537 : 507-508

Palaeoxonodon freemani n. sp.

HOLOTYPE. — BMNH J.745, left lower molar ( Fig. 8A ).

ETYMOLOGY. — freemani , after Mr. E. Freeman, who initiated the paleontological work in the Kirtlingon Bone Bed.

ATTRIBUTED SPECIMENS. — BMNH J.197, J.213, J.388, J.530, J.569, J.593, J.715,? M 44303 , right lower molars; BMNH?J.220, J.?350, J.?618, J.626, J.628, J.657 (in a jaw fragment), J.701, J.727, J.825, J.827, J.837,? M 51823 , left lower molars.

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

DIAGNOSIS. — Differs from P. ooliticus by the presence of an anterior lingual cingulum. Trigonid slightly wider; paraconid equal to or smaller than the metaconid. Talonid cusp slightly more upright. Cusp f tends to be more acute.

DESCRIPTION (Annexe, Table 2; Figs 4-6 ; 8 ; 9A, B ) A lingual cingulum occurs below the paraconid; it can be slight, moderate or forming a ledge. In spite of the presence of this cingulum, a tiny cusp e emerges on two teeth only. The paraconid is thus reduced in height though it is equal to, or even larger than, the metaconid in two cases (BMNH J.220, M 51823 ). Cusp f can be followed by a faint antero-labial cingulum. The protoconid is again straight or, more rarely, posteriorly inclined (three cases); this cusp is generally more slender than in P. ooliticus except for the holotype of this species and also J.236 (the shape of the trigonid is also variable in the hypodigm of Nanolestes [ Martin 2002: fig. 2]). The hollowing of the labial part of the metaconid occurs in only six cases. This cusp is entirely lingual or slightly posterior (BMNH J.350 [ Fig. 8B ], J.745, J.837) relative to the protoconid. Finally, the holotype J.745 is noteworthy by the presence of a step at the base of the metacristid (mesoconid) and of a bump on the entocristid (such a bump is also visible on J.530 and J.618); this gives the appearance of a threecusped talonid. J.220 is larger than most. The talonid surface is again variously oriented. The roots are slightly unequal, the anterior one being usually more salient labially (this is not the case for J.745 and J.618, which may be an m1: the same variation occurs along the jaw in Amphitherium ). J.350 (short and fused stubby roots) and J.618 (long and thin roots) have a blunt trigonid and the lingual cingulum barely reaches the anterior border; their attribution remains uncertain. Moreover, on J.350 and J.220, f is followed by an anterolabial cingulum fading at mid-length of the protoconid.

The same wear facets as those described above for P. ooliticus are identifiable on the rare specimens sufficiently well preserved. However, that affecting the metaconid (paracone wear) is more frequently situated on the same plane as that affecting the posterior face of the protoconid ( Fig. 7 , facet 1); in consequence it is more posterior than labial. That affecting the sulcus between trigonid and talonid is possibly less vertically oriented. On two teeth only (J.388, J.593) could one detect an incipient oval wear facet on f (metacone wear). When the paraconid is sufficiently well preserved, an A facet (anterior side of metacone wear) is discernible.

Figures

FIG. 8. — Lower molars of Palaeoxonodon freemani n. sp.; A, holotype BMNH J.745, × 34; B, BMNH J.350, × 40; C, BMNH J.657, × 17; a, labial view; b, lingual view; c, posterior view; d, anterior view; e, occlusal view.

FIG. 4. — Diagram of the width (W) versus length (L) of the lower molars of Palaeoxonodon Freeman, 1976b. Measurements units in 1/100 of mm.

FIG. 5. — Diagram of the length of the talonid (T1) versus total antero-posterior length (L) of the lower molars of Palaeoxonodon Freeman, 1976b. Measurements units in 1/100 of mm.

FIG. 6. — Diagram of the length of the talonid (T2) versus trigonid width (W) of the lower molars of Palaeoxonodon Freeman, 1976b. The use of tooth width instead of length is aimed at reducing the effect of variation in the number of molars present in the jaw (suggestion P. M. Butler). Measurements units in 1/100 of mm.

FIG. 9. — SEM photographs; A, B, Palaeoxonodon freemani n. sp., BMNH M 44303 lower molar, lingual and occlusal views; C, D,?Palaeoxonodon sp., BMNH M 34994 upper molar, posterior and occlusal views. Scale bar: 1 mm.

FIG. 7. — Occlusal relationships of upper with lower molars in amphitheriids (A, A’) and peramurids (B). Arrow points anteriorly.