Hahnotherium antiquum, Butler & Hooker, 2005

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 : 200-202

Hahnotherium antiquum sp. nov.

Figs. 4C, 10 , 11A , 12 .

pars Eleutherodon oxfordensis sp. nov.; Kermack et al. 1998: 595–597, figs. 19–21 [non figs. 1–14, 15A, 16, 18, 22, 23 = Eleutherodon oxfordensis KA. Kermack, D.M. Kermack, Lees, and Mills ; non figs. 15B, 17 = Millsodon superstes sp. nov.].

Holotype: BMNH M46797 , a left second upper molar, from the Old Cement Works Quarry, Kirtlington, bed 3p. Described by Kermack et al. 1998: 596, figs. 19B, 21 as Eleutherodon oxfordensis , group ζ.

Referred specimens: (a) BMNH M46763, a right second lower molar. Figured by Kermack et al. 1998: 595, figs. 19A, 20 as Eleutherodon oxfordensis , group Υ; (b) BDUC J 782, left upper premolar (now missing). Both are from Kirtlington, bed 3w(i).

Derivation of name: antiquum (Latin) , ancient. This is one of the earliest multituberculates.

Distribution.—Late Bathonian of England.

Diagnosis.— As for the genus, by monotypy.

Description of upper molar ( Figs. 6E , 10 ).—BMNH M46797 is identified as multituberculate because of its horizontal, as opposed to basined, longitudinal wear groove between buccal and lingual cusp rows. It was described by Kermack et al. (1998) as a left lower molar, but it has more resemblance to a paulchoffatiid left upper molar. The mesial end is oblique, the lingual cusp row projecting beyond the buccal row, as is usually the case in paulchoffatiid upper molars. The tooth measures 1.9 mm in length and 1.55 mm in breadth, giving a L/B index of 1.23; this is below the range of later paulchoffatiid M1 (1.36–1.82) but within the range of M2 (1.0–1.5) (data from Hahn and Hahn, 1998a). However, there is no anterobuccal ridge, and the tooth is less narrowed distally than is usual in multituberculate M2. Identification as M2 rather than M1 is supported by the absence of wear on the lingual surface of the lingual cusps, by the presence of facets on the buccal side of the buccal cusps, and by a contact facet on the mesial surface but not at the distal end.

There are five buccal cusps, but Kermack et al. (1998) suggested that there may have been only four, the mesial cusp having been cut in two post−mortem by a boring organism. There appear to be three “borings” into the buccal row; the two more distal ones are in line with grooves in the buccal slope of the central basin and they are clearly intercuspal. Careful cleaning of the region showed that there are no borings: the cusps are separated by steep−sided grooves in which the enamel is intact. The grooves are bordered by crests emanating from the mesial and distal sides of the adjacent cusps. Buccally they are terminated by meeting of the mesial and distal crests to form a rim; lingually they extend more shallowly into the central basin where bifurcating crests trend mesiolingually and distolingually from the cusps. The buccal cusps are not connected by a longitudinal crest as in Kermackodon . The two most mesial buccal cusps (B1, B2) are higher than the rest; B3, the smallest, is displaced lingually; B5 is distolingual to B4, on the distal margin of the tooth. In Paulchoffatiidae the buccal cusps distal to B2 are reduced or absent, except in Meketichoffatia . The anterobuccal ridge is absent, unless it is represented by a swelling at cingulum level below B1–B2.

The seven lingual cusps have the appearance of low elevations on a ridge, but their tips have been removed by wear. They form a graded series that diminishes mesially and distally from a maximum at L3~L5. Adjacent cusps are separated by shallow grooves, crossed by a fragment of the longitudinal crest. Each cusp gives rise to a ridge on the lingual slope of the central valley. There are no ridges on the lingual surface of the lingual cusps. L1 is connected to B1 by a ridge that follows the mesial margin of the tooth; it shows two low elevations. The central valley is open distally between B5 and L7. Its sides are richly ornamented with enamel ridges from the cusps, similar to the fluting of Eleutherodon .

Wear is evident on both sides of the central valley, due to the lingual cusps of m2. The lingual surface of the lingual cusps is unworn. The buccal cusps, except B3, are worn at the tip and high on their buccal sides, due to wear in the valley of m2 ( Fig. 10B ). This pattern of wear shows that the tooth was displaced lingually in relation to m2 as in other multituberculates.

The tooth probably had three roots, though only two (mesiolingual and distal) are preserved; a large area of the tooth base having broken off on the buccal side. The probable presence of a third root suggests comparison with the very worn tooth from Watton Cliff described by Freeman (1976). This is not very different in size and proportions (2.0 × 1.5 mm; L/B 1.33). It has two roots at one end, which is presumably mesial. Comparison.— Eleutherodon upper molars resemble Hahnotherium in the fluting of the central valley, but differ in many other respects, for example the third row of cusps, basining of the central valley, and the large distal cusp. Kermackodon agrees with Hahnotherium in lacking the anterobuccal ridge and in having 5 buccal and 7 lingual cusps, but there are many differences: the largest buccal cusps are distal in Kermackodon , mesial in Hahnotherium ; in Kermackodon the tooth is narrowed distally, and the central valley is blocked by the distal lingual cusp; the cusps are sharper, with stronger ridges; the buccal cusps are connected by a longitudinal crest; ornamentation of the central valley is very different.

In Paulchoffatiidae B1 and B2 are enlarged, and the more distal buccal cusps are reduced or lost; the number of lingual cusps is also reduced, most frequently to 4 or 5. The tooth is narrow distally, and the central valley is usually closed by the last lingual cusp. In other “Plagiaulacida” the central valley is open distally as in Hahnotherium , but buccal cusps distal to B2 are absent and there are only three lingual cusps. The lingual cusps of Hahnotherium , which are low elevations on a ridge, differ from those of other multituberculates, which are broader, rounded, and separated by transverse grooves. Though folding of the enamel in the central valley is usual, it does not have the regular pattern shown by Hahnotherium . Absence of the anterobuccal ridge is a character shared only with paulchoffatiid species A and with Ctenacodon , among “plagiaulacidans”.

These differences exclude Hahnotherium from existing families of the “Plagiaulacida”, and a new family Hahnotheriidae is proposed for it.

Description of referred lower molar ( Figs. 6F , 11A ).— BMNH M46763 was interpreted by Kermack et al. (1998) as a left lower molar (group Υ) of Eleutherodon , but it has more resemblance to a multituberculate right second lower molar. There is a row of three discrete cusps, which must be lingual, and a buccal row in which the cusps are minor elevations on a ridge. The tooth is equal in length and width (1.6 × 1.6 mm), and thus proportionately shorter than the holotype upper molar. It is moderately worn, and a small area at the distal end is broken. The buccal margin is convex and somewhat shorter than the lingual margin. The lingual cusp row occupies two thirds of the width of the tooth. The lingual cusps are blunt and pyramidal, and separated by deep transverse grooves. They diminish in size from L1 to L3. Their lingual surfaces are worn flat, presumably against the lingual cusps of M2. Ridges on their buccal surface have been largely removed by wear against the buccal cusps of M2. A stronger buccal ridge on L1 runs in an obliquely distal direction across the central valley towards B3. Wear on the main buccal ridge increases towards the distal end. Four buccal cusps are visible on its mesial half; more distal cusps have probably been removed by wear. Each cusp gives rise to a ridge on the buccal side of the central valley, producing a similar ornamentation to that on the lingual side of the valley of the holotype.

Comparison ( Fig. 11 ).—With a length/breadth ratio of 1.0, this tooth agrees with m2 of Parachoffatia staphylos ( Hahn and Hahn 1998a) , but it is proportionately shorter than m2 of other multituberculates. It is also distinctive in the proportionately short and narrow buccal cusp row, which is longer than the lingual row in other multituberculates.

In Paulchoffatiidae there is a large lingual cusp, labelled L2 or L1+2 by Hahn and Hahn (1998a) because a more mesial cusp occurs in Kuehneodon and Guimarotodon . Except in Paulchoffatia , a ridge arising from L2 crosses the central valley. This ridge might be compared with the more oblique ridge that arises from L 1 in BMNH M46763, in which case L1 of Hahnotherium might be homologous with L2 of paulchoffatiids. Distal to L2 there are two small cusps in Paulchoffatia , one in Meketibolodon and Kuehneodon . In Pinheirodontidae ( Hahn and Hahn 1999) there are three lingual cusps that diminish in size from L1 to L3. A ridge from L2 crosses the valley, but it is inclined mesially, instead of being transverse as in Paulchoffatiidae or inclined distally as in BMNH M46763. In Ctenacodon and Glirodon there are three lingual cusps of equal size. Bolodon and Eobaatar have two equal lingual cusps, which is the usual condition of later multituberculates.

In Paulchoffatiidae and Pinheirodontidae the buccal row is reduced to a ridge, on which numerous minute cusps can be seen on unworn teeth (e.g., Parachoffatia, Hahn and Hahn 1998a : fig. 41). The valley is ornamented by numerous ridges that arise from the cusps. In Bolodon and Eobaatar the buccal row again forms a ridge, from which cusps are rapidly removed by wear. To judge by the number of transverse ridges on the buccal side of the valley, the number of buccal cusps was probably more than three, perhaps six or more. In Ctenacodon and Glirodon the buccal cusps are reduced to three; they are more distinct, but still united by a longitudinal ridge, whereas the lingual cusps are separated by deep grooves.

Wear on the lingual surface shows that the lingual shift in occlusal relations between the last molars had taken place. BMNH M46763 resembles BMNH M 46797 in width, cusp form and enamel ornamentation, and it is possible that they represent m2 and M2 of the same species. However, BMNH M46763 is much shorter mesiodistally, whereas in those later multituberculates in which upper and lower molars of the same species are known m2 is somewhat longer than M2. Reference of BMNH M46763 to Hahnotherium antiquum must be considered provisional.

Note on Hahnodontidae .— Hahnodon taqueti is known by a single tooth from the Early Cretaceous of Morocco, interpreted as m2 by Sigogneau−Russell (1991) because of resemblances to the basined m2 of paulchoffatiids. It has three cusps: a mesial cusp on one side of the central valley, and two cusps on the other side, of which the distal one is the larger. Sigogneau−Russell was uncertain of the orientation: she hesitantly interpreted the single cusp as buccal, and the tooth as from the left side. However, this makes the buccal cusp higher than the lingual cusps, a character otherwise unknown in multituberculate m2. Alternatively, if the tooth is from the right side, the single cusp, from which a ridge obliquely crosses the valley, might be compared with the large lingual cusp (L2) of paulchoffatiids and L1 of Hahnotherium .

Hahn and Hahn (2003) have described as Denisodon moroccensis another molar from the same locality. They followed Sigogneau−Russell’s (1991) orientation, regarding the highest cusp as mesiobuccal (b1). This tooth differs from Hahnodon in that the mesial lingual cusp (l1) is larger and joined at the base to b1. The distal lingual cusp (which they call l3) is smaller, and there is a small distal buccal cusp (b2). l3, b2 and a small distal elevation stand at the edge of the basin that occupies the distal part of the crown. It seems not impossible that Denisodon is the first molar of Hahnodon . Neither tooth shows evidence of horizontal chewing, but wear occurs on the inclined surfaces of cusps around the basin, implying a large orthal component of chewing. The action seems to have been one of pestle−and−mortar crushing rather than of the horizontal grinding that is characteristic of multituberculates. Moreover, there is no evidence of wear on the lateral surface that would indicate a lingual shift in the relative position of M2. These characters in our view remove the Hahnodontidae from the order Multituberculata , and we classify them in the paraphyletic order “ Haramiyida ” sensu Butler (2000).

Description of referred left upper premolar ( Figs. 4C, 12 ).— This specimen is now missing. The following description is based on scanning electron micrographs and drawings.

BDUC J 782 is the crown of a tooth, lacking roots, and with the pulp cavity exposed in basal view. It measures (from the scaled photograph) 1.8 mm long and 1.2 mm wide. The mesial margin and most of the lingual margin have broken off, but the tooth was probably wider mesially than distally. The buccal margin is convex, and the distal end is oblique and most prominent lingually. The tooth resembles P4 and P5 of paulchoffatiids. A longitudinal groove separates a lingual row of four cusps from a middle row, also of four cusps, and there are two additional cusps on the mesial part of the buccal margin. Wear is light except at the mesial end of the lingual row. The cusps are low, but pointed at the tips, with apical angles of about 100°.

The lingual cusps increase in height from L1 to L3; L1 has been almost removed by wear, but L3 has lost only the extreme tip. L4, of similar height to L3, stands on the distolingual prominence of the outline. It is connected to L3 by a low longitudinal crest. The central valley is flat longitudinally and shallowly concave transversely. Its buccal margin is formed by the middle (B) row of cusps, of which the highest is B3. A mesiolingual ridge on B3 joins a distolingual ridge on B2, and a distolingual ridge on B3 joins a mesiolingual ridge on B4. A small elevation on the distal margin partly closes the central valley. The cusps of the third row (BB1, BB2) are joined by rounded ridges with B1 and B2 respectively; BB2 is equal to B 2 in height and BB1 is lower. A transverse valley separates B1+BB1 from B2+BB2, and similar valleys demarcate B2+BB2 from B3, and B3 from B4.

Comparison.—Typically in multituberculates the posterior premolars (P4, P5) differ from M 1 in the reduction or eventual loss of the lingual row, and the development of additional (BB) cusps on the buccal side. As a result the original buccal row is displaced lingually to a median, or in advanced forms a lingual position. Kuehneodon is exceptional in that the lingual cusps are large and the buccal cusps stand on the buccal half of the crown. BDUC J 782 agrees with Kuehneodon in this respect, but it possesses two BB cusps on the mesial part of the buccal margin. These two cusps are a widespread feature of “plagiaulacidan” upper premolars, and their absence in Kuehneodon may be secondary.

The low crown of BDUC J 782, its pointed cusps, and the rounded, rather than furrowed valleys, may be primitive features resulting from a smaller thickness of enamel. Alternatively, the absence of roots may indicate that it is a milk tooth that has been shed. BDUC J 782 is comparable in size to Hahnotherium antiquum , and it might belong to that species. It cannot occlude with the lower premolar referred to Kermackodon . Also, it is unlikely to be adjacent to the upper premolar BMNH M46640, which completely lacks the lingual cusp row.

Figures

Fig. 10. Hahnotherium antiquum gen. et sp. nov., BMNH M46797, left M2 (holotype). A. Occlusal view, mesial end upwards. B. Oblique mesiobuccal view; to show buccally facing facets on buccal cusps; and contact facet at mesial end. C. Lingual view, crown upwards.

Fig. 11. Right second lower molars. A. BMNH M46763, referred to Hahnotherium antiquum gen. et sp. nov. B. Kuehneodon sp. (based on Hahn 1969: fig. 36). C. Pinheirodon vastus (based on Hahn and Hahn 1999: fig. 4b). D. Bolodon osborni (based on Kielan−Jaworowska and Ensom 1992: pl. 3: 5). Mesial end upwards.

Fig. 12. BDUC J 782, left upper premolar, referred to Hahnotherium antiquum gen. et sp. nov. in occlusal (A), lingual (B), distal (C), and buccal (D) views. The three rows of cusps are labelled L, B, BB. Crown upwards in C, mesial end upwards in the rest.

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.