Durlstodon ensomi, Sweetman & Smith & Martill, 2017
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publication ID |
https://doi.org/ 10.4202/app.00408.2017 |
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persistent identifier |
https://treatment.plazi.org/id/03A07060-276F-7210-FCD0-CDD22672FAC9 |
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treatment provided by |
Felipe |
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scientific name |
Durlstodon ensomi |
| status |
sp. nov. |
Durlstodon ensomi sp. nov.
Figs. 3–6 View Fig View Fig View Fig View Fig .
Etymology: In honour of Paul Ensom, recognising his major contribution to Purbeck palaeontology.
Holotype: NHMUK PV M 99992, a well preserved upper right distal-most (M3) tooth crown lacking the parastylar wing ( Figs. 3 View Fig , 4B View Fig , 5B View Fig , 6B View Fig .
Type locality: Durlston Bay, Dorset, southern England, at British National Grid Reference SZ 03673 78377 ( Fig. 1 View Fig ).
Type horizon: A beach-level exposure of bed DB 83 ( Clements 1993) of the Lower Cretaceous, Berriasian ( Ogg et al. 1994; Hunt 2004), Purbeck Group ( Fig. 2 View Fig ).
Diagnosis.—Monotypic tribosphenic mammal with a unique combination of derived characters. Differs from stem therians including Juramaia and Acristotherium, from the Oxfordian and Barremian of China respectively, but resembles Late Cretaceous eutherians, particularly members of Zhelestidae (sensu Archibald and Averianov 2012) in: height and expansion of the protocone relative to labial cusps; possession of conules placed labially immediately adjacent to the paracone and metacone with sharp internal cristae; protocone anteroposteriorly expanded and similar in height to the metacone; conular region width more than 51% of the total molar width (c. 64%); lower molars with a substantial entoconid as evidenced by a wear facet on the distal side of the protocone. Differs from members of Zhelestidae in possession of a paracone that is substantially taller than the protocone and metacone. Differs from Durlstotherium gen. nov. in: the presence of a rectangular trigon basin with internal conular cristae rather than a triangular trigon basin lacking these; in the more expanded protocone lacking any apical constriction; in presence of an angled postprotocrista (exaggerated by wear); in possession of an apparently more robust paraconule (possibly exaggerated by wear); in greater separation and relative sizes of the paracone and metacone; possession of a cuspidate ectocinglum; absence of pre- and postcingulum; pattern of wear (see below); and greater size. Other characters cannot be determined due to loss of the mesiolabial corner of the tooth.
Description.— NHMUK PV M 99992 is an upper molar lacking the labial roots and the parastylar wing. It is otherwise well preserved but bears a number of cracks in the region of the paracone. As with NHMUK PV M 99991 described above, it is substantially worn due to occlusion with the lower dentition and dietary attrition. It is also a distal-most upper right molar (M3) identified on the basis of its general outline in occlusal view and the lack of a metastylar lobe ( Figs. 3 View Fig , 4B View Fig , 5B View Fig , 6B View Fig ). Measurements are shown in Fig. 3 View Fig .
The connate paracone is somewhat labiolingually compressed apically and is the more robust of the labial cusps and descends vertically. It is a little less than twice the height of the metacone but the relative height of the cusps prior to substantial wear cannot be determined. The basal diameter of the paracone is twice that of the metacone. The paracone is truncated by a substantial wear facet (facet 3, Fig. 6B View Fig ). This extends down the distal margin and across the centrocrista which is substantially broader than that of NHMUK PV M 99991. A wear facet (facet 1, Fig. 6B View Fig ), occupies the apex of the paracone on its mesial surface and extends mesially along the preparacrista. The metacone is separated from the paracone by a U-shaped valley and is placed distal and slightly lingual to it. A metastylar lobe is absent but a low ectocingulum extends from the linguobasal extremity of the metacone to the distolabial extremity of the broken surface. It bears minute cuspules, the largest of which is at the base of the metacone at its distolabial corner. There is a substantial but heavily worn paraconule. It is separated from the paracone by a narrow valley the mesial margin of which comprises the preparaconule crista. This extends from the apex of the cusp to the broken surface at the labiomesial corner of the preserved part of the crown. The paraconule is considerably worn mesially, facet 1 ( Figs. 4B View Fig , 6B View Fig ). The metaconule is missing. However, examination of the lingual margin of the metacone and the area immediately lingual to it suggests that breakage has occurred here and that a substantial metaconule with a triangular basal profile was present and located close to the metacone.
The central part of the crown lingual to the labial cusps comprises a valley surrounded by a substantial ridge. This extends from the mesiolingual corner of the base of the metacone following the postprotocrista to a point opposite a sharp change in angle as the latter approaches the protocone. Here the ridge bends at right-angles mesially to approach the preprotocrista but before reaching this it bends again at right-angles to reach the lingual margin of the paraconule. Except as described above in connection with wear on the distolingual surface of the paraconule the valley appears to be unworn, other than over a narrow area adjacent to the mesiolingual corner of the metacone. In contrast, the remainder of the occlusal surface external to the valley is heavily worn ( Figs. 4B View Fig , 6B View Fig ).
The preprotocrista appears to extend from the apex of the protocone to overlie the paracone but substantial wear has separated it from the now apparently overlying preparaconule crista and exposed the intervening dentine. Mesially a residual enamel ridge extends as far as the point of breakage labial to the paracone. The ventral surface is worn over its entire length except where obscured by the postparaconule crista. There is also lateral wear extending from a point above the apex of the paraconule to the labial end of the crest as preserved. Confluent lateral wear is also present on the postparaconule crista along its length. The postprotocrista is also heavily worn. It extends labially to the point of breakage at the lingual extremity of the missing metaconule.
The protocone is a robust cusp slightly taller than the heavily worn metacone. It is placed lingual to the distal margin of the paracone and mesiolingual to the metacone. The apex defined by the cristae is broadly V-shaped and the postprotocrista is sharply inflected midway along its length, this inflection being exaggerated by a substantial wear facet facet 6, Fig. 6B View Fig ). The protocone expands basally and its apicobasal axis is tilted apicomesially at c. 75°, more so than in NHMUK PV M 99991. A precingulum is absent but there is a basal swelling on the mesial surface of the protocone and a small, vertically orientated facet is also present on its mesial surface (facet 5, Fig. 6B View Fig ). It is situated below the preprotocrista midway between its apex and that of the paraconule. There is also a swelling towards the base of the protocone on the distal side. This appears to be more pronounced than that on the mesial side. However, there is a large wear facet facet 6, Fig. 6B View Fig ) which has partially obliterated it. This facet is at a shallower angle than the apicobasal axis of the cusp and must represent wear from a substantial entoconid on the occluding lower molar.
The root overlying the protocone is very robust, mesiodistally narrow and labiolingually broad. It is broken away a short distance from its base. Despite being robust it overlies a smaller area of the crown than the root in a similar position in NHMUK PV M 99991. The root overlying the paracone, and which would evidently also have extended to support the missing parastylar wing, is also laterally compressed as in NHMUK PV M 99991 and is large compared to that overlying the metacone. It appears to be about two thirds the size in basal area of that overlying the protocone but its labiobasal end is missing. The root above the metacone is approximately circular and the smallest of the roots. Its diameter is smaller than the remaining part of the root above the paracone. Proportionately, however, it is of greater diameter relative to the other roots than the comparable but somewhat more mesially placed root in NHMUK PV M 99991.
Stratigraphic and geographic range. —Currently known only from the type horizon and locality.
| NHMUK |
Natural History Museum, London |
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.