Millsodon superstes, Butler & Hooker, 2005
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publication ID |
https://doi.org/ 10.5281/zenodo.13618797 |
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persistent identifier |
https://treatment.plazi.org/id/03E187B9-FFD6-0265-FFF5-2F15FCD5FEE5 |
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treatment provided by |
Felipe |
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scientific name |
Millsodon superstes |
| status |
sp. nov. |
Millsodon superstes sp. nov.
Figs. 1D, E View Fig , 3A, B, G, H View Fig , 4A.
pars Eleutherodon oxfordensis sp. nov.; Kermack et al. 1998: 593–594, figs. 15B, 17 [non figs. 1–14, 15A, 16, 18, 22, 23 = Eleutherodon oxfordensis K.A. Kermack, D.M. Kermack, Lees, and Mills ; non figs. 19–21 = Hahnotherium antiquum sp. nov.]. Holotype: BMNH M46645, a first right lower molariform (m1) from Kirtlington Old Quarry, England, bed 3w(i).
Paratype: BMNH M46183 , a last left lower molariform (m3), from Watton Cliff , England. Described by Kermack et al. (1998) as a right lower molar of type Υ .
Referred specimen: BDUC J 3, a right upper molariform from Watton Cliff.
Derivation of name: Latin, superstes , a survivor.
Distribution.—Late Bathonian of England.
Diagnosis.—As for the genus, by monotypy.
Description
Holotype ( Figs. 1D View Fig , 3A View Fig ).—BMNH M46645 is interpreted as a right lower tooth. In crown view it is oval in outline, longer than wide (2.4 × 1.4 mm), rounded distally and truncate mesially. There is a slight indentation on the buccal side, near the mesial end, and a smaller indentation on the mesial border, defining a small mesiobuccal lobe of the tooth base. The crown is dominated by a high, blunt mesial cusp (a1), situated towards the lingual side; it leans slightly forward near the base, and curves backwards near the tip. Minor features of the crown have been largely removed by rolling and abrasion, but there are two longitudinal ridges separated by a median basin. The lingual ridge is continuous with the mesial cusp; it falls in height distally, and shows traces of two cuspal elevations. The buccal ridge, which is lower, curves lingually at its mesial end, where it runs up the buccal side of the mesial cusp. Buccal to the mesial cusp, on the mesiobuccal lobe, is a short ledge, rounded by wear. The top of the buccal ridge is occupied by an elongated strip of exposed dentine, probably representing a wear facet which faces somewhat buccally. The wear extends to the mesial end of the ridge, on the buccal surface of the mesial cusp. The central basin shows a longitudinal wear groove, which inclines to its greatest depth about midway along the tooth, and distally it is nearly horizontal. Enamel has been removed from the distal part by wear. The root has broken off about 0.7 mm below the crown. It is undivided at this level, but grooves on the mesial and buccal sides suggest that a mesiobuccal root, at least, separated at a deeper level.
Comparison ( Fig. 3 View Fig ).—This tooth is similar in size to the largest lower molar of Eleutherodon (BMNH M46461). Both have a mesial main cusp followed by a basin between longitudinal ridges, However, the main cusp in BMNH M46461 is buccal, and a smaller lingual cusp leads the lingual ridge, while in Millsodon both ridges join the main lingual cusp. The main cusp is proportionately larger and more erect in Millsodon , occupying about 2/5 of the length of the tooth. Details of the basin and cusp rows have been removed by wear, but minor elevations indicate two cusps on the lingual ridge; there is no trace of the numerous cusps on the ridges and fluting in the basin that characterise Eleutherodon .
BMNH M46645 more closely resembles the supposed m1 of Thomasia ( Thomasia II of Sigogneau−Russell 1989: fig. 18) ( Fig. 3E View Fig ). There is a large mesial cusp (a1), with a wear facet (labelled 1 by Sigogneau−Russell) and a short cingulum on the buccal side; both ridges join the main cusp; the lingual ridge is a1 the higher and it has two elevations. However, in Thomasia the main cusp is more pointed, its mesial surface is ridged, and it is less predominant over the more distal cusps. The holotype m1 of Staffia aenigmatica ( Heinrich 1999) ( Fig. 3C View Fig ) has a blunt main cusp, less high than in Millsodon and more curved distally. The tooth is proportionately longer and narrower, and not narrowed towards the mesial end, but it shows the mesial and buccal slight embayments of outline seen in Millsodon . It differs in that the lingual cusps are separated by deep valleys (synclines), whereas in Millsodon and Thomasia they are elevations on a ridge.
Paratype ( Figs. 1E View Fig , 3B View Fig ).— BMNH M46183 is a small and proportionately shorter tooth of similar type (1.6 × 1.15 mm) .
BUTLER AND HOOKER—NEW TEETH OF BATHONIAN ALLOTHERIANS 191
1 mm
Kermack et al. (1998) interpreted it as a right lower molar of type Υ, but it is regarded here as a left lower molar. It is heavily worn and abraded. The outline is rounded distally and obliquely transverse mesially; with a small mesial embayment. The large mesiolingual cusp is similar in shape to that of the holotype. It is continuous with the lingual ridge, which has two elevations. The buccal ridge is lower and worn on the top, and the number of buccal cusps is uncertain. There was evidently no large mesiobuccal cusp as in Thomasia (compare Figs. 3B and 3F View Fig ). Unlike in the holotype, the buccal ridge extends to the mesial border and is separated from the main lingual cusp by a valley; a trace of the connection may be represented by a faint ridge on the distobuccal slope of the main cusp, pointing towards the buccal ridge. There is a single root, flattened buccolingually.
Comparison.—The resemblances and differences between BMNH M46183 and BMNH M46645 are consistent with their occupying different dental positions in the same species. BMNH M46183 has a superficial resemblance to the smaller type of Eleutherodon lower molar such as BMNH M46851, but the larger mesial cusp is lingual, not buccal as in Eleutherodon , there are fewer cusps, no indications of fluting, and the main mesial cusp is larger. On the more distal molars of Haramiyavia and Thomasia the buccal row extends forward past a1 to the mesial end of the tooth. This feature is represented by a ridge in BMNH M46183, which because of its small size and undivided root is probably a last molar. However, the normally large cusp (b2), buccal to a1, is not developed ( Fig. 1E 1 View Fig ).
A posterior lower molar of Staffia has recently been described by Heinrich (2001) and it may be compared with BMNH M46183 ( Fig. 3B, D View Fig ). In Staffia the last molar is much smaller in comparison to m1 than in Millsodon . Heinrich interpreted the tooth as from the right side, but comparison with the English specimen indicates that it is a left tooth: the cusps labelled b1 and b2 by Heinrich are lingual and should be called a2 and a3 ( Fig. 3D View Fig 1 View Fig ). If this orientation is accepted, the following differences may be noted: Staffia is shorter relatively to width; the mesial margin is more oblique and lacks the indentation; the mesial cusp (a1) is lower and more convex in lateral profile; it is widely separated from a2 (“b1”) by a valley; a3 (“b2”) is much smaller. Most of these differences may be ascribed to the greater reduction of the Staffia tooth, but the separation of a2 from a1 occurs also on m1. In Staffia the buccal row contains two distinct cusps; it is connected to the buccal side of a1 by a ridge that follows the mesial margin and forms the mesial edge of the basin. This is not present in BMNH M46183. It might be compared with a ridge in the buccal side of a1 on the first molariform of Staffia .
Referred specimen BDUC J 3 ( Figs. 3G View Fig , 4A).—This tooth from Watton Cliff is now missing, and it is described on the basis of scanning electron micrographs and camera lucida drawings. It is included in this paper because of a possible identity with Millsodon .
BDUC J 3 is interpreted as a right upper molariform (?M1), on the assumption that the highest cusp is distal as in other “haramiyidans”. The presence of a third row of cusps indicates an upper tooth, and wear facets show that the additional row is lingual. The tooth measures 2.55 × 1.7 mm, comparable in length with upper molars of Eleutherodon and m1 of Millsodon . The cusps are blunt, rounded and separated by sulci; there are no enamel ridges on their slopes. The highest cusp (a) is distobuccal; it is followed mesially by a buccal cusp b, about half its height, and a still smaller cusp c near the mesial end of the tooth. A distolingual cusp d, is about half as high as a, beyond which it projects distally. Mesial to and slightly more lingual than a is cusp e, similar in height to d and b; it is preceded by a lower cusp f, now worn off. The groove between b and c buccally and e and f lingually represents the basin of other “haramiyidans”. On the lingual margin, mesially to d, is a row of low rounded cuspules, forming a cingulum. The distal root has completely broken off; the mesial root is about 1.7 mm long. If the root is considered to be vertical, the crown basin would be inclined at about 30 degrees to the horizontal, facing forwards.
The tooth is only moderately worn and exhibits clear wear facets ( Fig. 4A). Two principal groups of these are distinguishable ( Fig. 3G View Fig ): facets on b, c, e, and f, sloping into the basin, are presumably due to contact with buccal cusps on the lower molar; lingually inclined facets on a and d imply occlusion against the buccal side of a large lingual cusp of the more posterior lower molar. By superposition of drawings it was possible to fit m1 of Millsodon with two adjacent copies of BDUC J 3 ( Fig. 3H View Fig ). The lower tooth moved backwards and upwards, its mesial main cusp passing lingually to the upper cusps a and d. At the end of the movement the ridges on the distal part of the lower tooth would come into contact with the ridges on the mesial part of the following upper tooth: the buccal lower ridge would bite in the basin of the upper tooth, and the lingual lower ridge would pass to the lingual side of cusps e and f; the tip of upper cusp a would meet the distal end of the basin of the lower tooth. The effect would be a crushing action, directed backward and upward, together with some shear between the main cusps. There is no indication of the horizontal sliding action found in Thomasia , but some resemblance to that postulated for Theroteinus ( Butler 2000) , except that in that case the highest cusps are more centrally placed and the movement was probably vertical.
Compared with Theroteinus (see Butler 2000: fig. 4), the high cusp a appears to correspond to A2, with d, a, b, c representing the buccal (A) row. The B row however is short, with only two cusps, e and f. B1, which in Theroteinus stands lingually to A2, is missing, thus providing space for the large lower cusp a1.
Because BDUC J 3 can be fitted occlusally with BMNH M46645 we refer it to Millsodon superstes as a probable upper molar. It represents a hitherto unknown type of “haramiyidan”: possibly it is a derivative of the Theroteinidae , or a specialised relative of the Haramiyidae . Until better material is found we prefer to classify it as “ Haramiyida ” incertae sedis.
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