Palaeoxonodon ooliticus Freeman, 1976b

Panciroli, Elsa, Benson, Roger B. J. & Butler, Richard J., 2018, New partial dentaries of amphitheriid mammal Palaeoxonodon ooliticus from Scotland, and posterior dentary morphology in early cladotherians, Acta Palaeontologica Polonica 63 (2), pp. 197-206 : 198-201

Palaeoxonodon ooliticus Freeman, 1976b

Fig. 1 .

Holotype: NHMUK PV M36508, right lower molar ( Freeman 1976b: pl. 17: 1–4).

Type locality: Kirtlington, Oxfordshire, England, UK.

Type horizon: Forest Marble Formation, Bathonian (Mammal Bed).

Material.—NMS G. 2015.17.10, partial left dentary referred to this species by Close et al. (2016); NMS G.1992.47.123 and NMS G.2017.37.1, two partial left dentaries, described here; from the Kilmaluag Formation (Bathonian), Isle of Skye, Scotland, UK.

Specimens originally referred to Palaeoxonodon freemani , Palaeoxonodon sp. , Kennetheridium leesi , and Kennetheridium sp. ( Close et al. 2016). Right lower molars: NHMUK PV MJ.59, MJ.196,?MJ.242, M36507 ( Freeman 1979: pl. 17: 1–4, but now lost), MJ.197, MJ.213, MJ.388, MJ.530, MJ.569, MJ.593, MJ.715,?M44303, MJ.702, MJ.290, MJ.117, MJ.379,?MJ.430, MJ.514, MJ.428, MJ.515, MJ.532, MJ.801; left lower molars: NHMUK PV?MJ.200, MJ.236, MJ.619, MJ.639, MJ.849,?MJ.220,?MJ.350,?MJ.618, MJ.626, MJ.628, MJ.657, MJ.701, MJ.727, MJ.825, MJ.827, MJ.837,?M51823, MJ.802, MJ.53, MJ.746, MJ.289,?MJ.518, MJ.824, MJ.214, MJ.846); right upper molars: NHMUK PV MJ.146, MJ.524, MJ.749, MJ.754, MJ.792, M36512, MJ.99, MJ.238, MJ.241, MJ.458, MJ.817, M36532. More questionably, Sigogneau-Russell (2003), also referred right upper molars: NHMUK PV MJ.44, MJ.231, MJ.669, M34994, MJ.32, MJ.506, MJ.508, MJ.512bis, MJ.742, MJ.743, MJ.788; and left upper molars NHMUK PV MJ.137, MJ.392, MJ.436, MJ.636, M36504, M36530, M36526, MJ.25, MJ.294, MJ.627. All from the Forest Marble Formation (Bathonian), Kirtlington, Oxfordshire, England, UK.

Emended diagnosis (modified from Close et al.2016).—Tribosphenic cladotherian mammal differing from dryolestoids in possessing an elongate lower molar talonid with hypoconid placed buccal to the midline of the crown, well-developed upper molar metacone, and winglike parastylar region. Differs from all other non-zatherian cladotherians in possession of a mesiolingual lower molar cingulid. Differs from Amphitherium in possession of five molars instead of six or seven, the more lingual position of the hypoconid, and presence of an entocristid partially enclosing an incipiently basined talonid (sensu Sigogneau-Russell 2003). Differs from Nanolestes in possessing less cuspidate upper molars with weaker development of cusp B, and relatively taller and less procumbent lower molar paraconid. Differs from Arguimus in the presence of a more fully triangulated trigonid of m1 with a larger paraconid, more spirelike lower molar cusps, absence of a distinct cusp e, and larger metaconid on ultimate molar. Resembles Amphitherium and Nanolestes but differs from zatherians in the absence of a distinct cusp e on lower molars, absence of hypoconulid, and buccally positioned metacone. Relatively slender dentary unlike dryolestids, with a deep masseteric fossa and coronoid scar as in other amphitheriids. The masseteric fossa intrudes slightly into the body of the dentary below the ultimate molar, unlike all other early cladotherians. The posterior opening of the mandibular canal, the mandibular foramen, is offset from the Meckel’s sulcus, unlike Amphitherium or Peramus . The posteriormost mental foramen is positioned in the canine/incisor region of the dentary.

Description.—NMS G.1992.47.123 is a fragment of left dentary containing m2, m3, and m 4 in position, with roots, and the anterior part of the alveolus for m5 ( Fig. 1 ). The molar crowns were broken and detached from their roots, and the latter remained inside the dentary fragment. The crowns have been restored to their original positions digitally to facilitate description and comparison. The dentary fragment measures 2.5 mm in anteroposterior length, 0.62 mm in width at the widest point, and the dentary dorsoventral depth below m3 is 1.3 mm (for tooth measurements see Table 1).

All three crowns of NMS G.1992.47.123 are well preserved, except for the following damage: the protoconid, metaconid, and paraconid cusp tips of m4 are missing; the hypoconid tip and tip of cusp f of m3 are missing; and the hypoconid and buccal apical portion of the protoconid are missing from m2 ( Fig. 1 ). The surface of the paraconid of m3 is damaged, missing enamel on the lingual surface of the paraconid, metaconid, and cingulid, and some of the anterobuccal enamel surface is also missing apically on the protoconid. The cristid obliqua of m3 is well preserved, revealing a distinct incipient talonid basin in occlusal view ( Fig. 1C ), defined lingually by the entocristid. The incipient talonid basin of m4 is well preserved, with a clear entocristid on the lingual edge.

NMS G.2017.37.1 is a fragment of left dentary 7.2 mm long with an approximate dorsoventral depth of 1.6 mm below m3 ( Fig. 2 ) (for tooth measurements see Table 1). The teeth m3 and m4 are preserved in position. Most of m2 is also preserved in position, but was fragmented prior to discovery. A fragment of the m1 root is also present. The alveolus for m5 is present but empty ( Fig. 2B 1 ). The cusp tips are missing from both m3 and m4, but the protoconid, paraconid, and metaconid positions are clear on both molars, as are the hypoflexid, hypoconid, and incipient talonid basin, defined lingually by the entocristid. None of these features are easily identified in m2, in which the hypoconid and hypoflexid are missing, and only the base of the anterior half of the crown is present.

Multiple fractures are present on the dentary, but the general osteology is evident. The Meckel’s sulcus is preserved on the lingual surface of the dentary, extending anentocristid distal metacristid talonid basin

metaconid paraconid teriorly from a point just ventral to the mandibular foramen, up to the fractured region ventral to m2 ( Fig. 2C 1 ). The mandibular foramen is present posteriorly on the lingual surface of the dentary, posteroventral to the empty alveolus for m5 and offset from the Meckel’s sulcus. The pterygoid shelf is unclear from the preserved morphology.

The masseteric fossa, located posteriorly on the buccal surface of the dentary, is deep and is defined by ridges anterodorsally and ventrally. A masseteric foramen is located anteroventrally within the masseteric fossa. The anterior margin of the masseteric fossa is well-developed and extends anteriorly, ventral to the alveolus for m5. It extends just slightly into the body of the mandible ventral to m5 ( Fig. 2A 1, B 1 ).

Stratigraphic and geographic range.—Middle to Late Bathonian, England and Scotland.

distal metacristid metaconid

paraconid

Figures

Fig. 1. Amphitheriid mammalian Palaeoxonodon ooliticus Freeman, 1976b (NMS G.1992.47.123) from the Kilmaluag Formation, Bathonian, in lingual A), buccal (B), and occlusal (C) views; partial left dentary (A1–C1); dentition only (A2–C2). Arrows indicate anterior direction.

Fig. 2. Amphitheriid mammalian Palaeoxonodon ooliticus Freeman, 1976b (NMS G.2017.37.1) from the Kilmaluag Formation, Bathonian, in lingual (A), buccal (B), and occlusal (C) views; partial left dentary (A1–C1), dentition only (A2–C2). Arrows indicate anterior direction.