Allotheria

Butler, Percy M. & Hooker, Jerry J., 2005, New teeth of allotherian mammals from the English Bathonian, including the earliest multituberculates, Acta Palaeontologica Polonica 50 (2), pp. 185-207 : 202-203

Allotheria incertae sedis

Material.—Upper second incisors. BMNH M46234, left I2 from the Kirtlington Mammal Bed; BMNH M46056, left I2, worn, from Watton Cliff. Both are of late Bathonian age.

Description ( Figs. 6D , 13 ).— BMNH M46234 is a singlerooted tooth with a procumbent main mesial cusp and two lower distal cusps. It resembles I2 of “plagiaulacidan” multituberculates and incisors referred to Thomasia by Sigogneau−Russell (1989). The main cusp is tilted forward in relation to the root. Its tip, which is unworn, is backwardly curved, with distobuccal and mesiolingual ridges. The buccal surface is convex, the lingual surface flatter with a weak median ridge. The conical distobuccal cusp is about half as high as the main cusp, from which it is separated by a V−shaped valley. The distolingual cusp, which is also conical, is lower than the distobuccal cusp and placed somewhat more distally. The two distal cusps are separated by a valley, and a number of enamel ridges run down their slopes. The mesiolingual ridge curves distally towards the base, where it has been removed by wear. The wear facet extends along the lingual margin of the tooth as far as the base of the distolingual cusp. It faces lingually (i.e. medially), indicating that the lower incisor closed medially to the upper.

The tooth measures 1.1 mm in length (perpendicularly to the root) and 0.65 mm in width; the height of the main cusp, from the buccal enamel border is 1.2 mm. Only the basal part of the root is preserved. This measures 0.75 mm mesiodistally and 0.6 mm buccolingually.

BMNH M46056 is probably a second example of the same tooth. It is somewhat larger: length 1.3 mm, width 0.7 mm, height of cusp 1.4 mm. It is heavily worn, especially from the lingual side where the distolingual crest has disappeared, but the remains of the two distal cusps can be recognised.

Comparison.—In Paulchoffatiidae ( Hahn 1969) the main cusp is stouter and more prismatic, with a stronger median lingual ridge. Some specimens have two or more distal cusps, but these are smaller in proportion to the main cusp than on the Kirtlington tooth. Wear involves first the tip of the main cusp and spreads down its lingual surface, whereas the Kirtlington tooth wears at the lingual margin. A strong median lingual ridge is present again in the eobaatarid Loxaulax ( Clemens and Lees 1971; Butler and Ford 1977), and in the allodontid Psalodon . These have a single distal cusp, proportionately smaller than the distobuccal cusp of BMNH M46234.

Upper incisors referred to Thomasia ( Sigogneau−Russell 1989) agree with the Kirtlington tooth in the weak development of the median lingual ridge. Some specimens have two distal cusps, though they are proportionately smaller than in BMNH M46234. Wear appears to be from the lingual side: a specimen ( SNP 363) shows a large facet involving the base of the mesiolingual ridge and extending to the distolingual cusp, as in BMNH M46234.

In Paulchoffatiidae I2 is large, comparable in length with the molars: the length ratio I2/ M 1 in Kuehneodon simpsoni is 0.95, in K. dryas 0.97, and in Henkelodon naias 1.03 ( Hahn 1969, 1977). Based on teeth from different individuals, I2/ M2 is about 0.85 in the plagiaulacid Bolodon osborni ( Kielan−Jaworowska and Ensom 1992) and in Loxaulax valdensis ( Clemens and Lees 1971) . BMNH M46234 is smaller in proportion to upper molars from Kirtlington: the ratio of lengths is in Eleutherodon (mean) 0.45, in Kirtlingtonia 0.50, in Kermackodon 0.39, and in Hahnotherium 0.54. The ratios for the larger incisor BMNH M46056 range from 0.45 to 0.63. Thus the Bathonian incisors are proportionately smaller than in later multituberculates. In Thomasia from Saint−Nicolas−de−Port, Butler and MacIntyre (1994) estimated that the ratio I2/ M was about 0.6. The Bathonian incisors may therefore be “haramiyidan”. Possibly they belong to Eleutherodon oxfordensis , which is the species most frequently represented by molars.

Figures

Fig. 6. Scanning electron micrographs of epoxy resin casts. A–C. Kermackodon multicuspis gen. et sp. nov. A. BMNH M46822, right M2 (holotype) in occlusal (A1), oblique lingual (A2), and oblique buccal (A3) views. B. BMNH M46684, right p4 (referred) in distal (B1), lingual (B2), buccal (B3), and occlusal (B4) views. C. BMNH M46640, right upper premolar (referred) in lingual (C1) and occlusal (C2) views. D. Allotheria incertae sedis, BMNH M46234, left I2, lingual view. E, F. Hahnotherium antiquum gen. et sp. nov. E. BMNH M46717, left M2 (holotype) in occlusal (E1) and mesio−lingual (E2) views. F. BMNH M46773, right m2 (referred) in mesio−occlusal (F1), occlusal (F2), and mesio−lingual (F3) views. Mesial end upwards in A1, B4, C2, E1, F2, crown upwards in A2, A3, B1, B2, B3, C1, D, E2, F3.

Fig. 13. Allotheria incertae sedis, BMNH M46234, left I2 in lingual (A), distal (B), occlusal (C), and buccal (D) views. Mesial end upwards in C, crown upwards in the rest.