Melaneremia schrevei, Hooker, 2012

Hooker, Jerry J., 2012, A new omomyid primate from the earliest Eocene of southern England: First phase of microchoerine evolution, Acta Palaeontologica Polonica 57 (3), pp. 449-462 : 451-453

Melaneremia schrevei sp. nov.

Figs. 1–3 .

? 1998 Omomyid primate; Godinot et al. 1998: pl. 11.1: k, l.

v. 2009 Melaneremia sp. ; Hooker et al. 2009: 79.

Etymology: Named after Pierre Schreve, who processed the large sample of Woolwich Formation clay and helped in numerous ways during the excavation at Croydon.

Type material: Holotype: Left dentary with p3–m3, NHMUK. M85501 . Paratypes: Right m1, NHMUK. M85504 ; left m2 with broken entoconid, NHMUK. M85503 ; right m2, NHMUK. M85502 .

Type locality: Sandilands cutting, Croydon, Greater London; National Grid Reference TQ339655.

Type horizon: Central channel, upper shelly clays, Woolwich Formation; mammal zone PE II, Neustrian European Land Mammal Age; Early Ypresian, Eocene ( Klaassen 1883; Hooker et al. 2009). NHMUK. M85504 is from the middle to upper levels of the channel fill; the rest are from the base of the channel fill.

Diagnosis.—p4 talonid with small, near vertical contact with m1. Lower molars high−crowned (within the generic range), with paraconids relatively erect. m1–2 with entoconid with steep distal wall. m2–3 with paraconids relatively close to protocristids. m2–3 metaconids with smooth mesial faces. m3 with angle of buccal crown base below protoconid and with postmetacristid straight. Differential diagnosis.— M. bryanti differs in the following

http://dx.doi.org/10.4202/app.2011.0017

ways: p4 talonid with distal wall sloping, and with extensive contact with m1. Lower molars lower−crowned and with procumbent paraconids. m1–2 with entoconid with gently sloping distal wall. m2–3 with paraconids distant from protocristids. m2–3 metaconid mesial faces with accessory cuspules in some individuals. m3 with angle of buccal crown base behind protoconid and with postmetacristid concave.

Description.—The holotype dentary preserves p3–m3 complete and with light wear ( Fig. 1 ). p3 was found isolated in the same sample bag as the dentary. It clearly fits the front of p4 and has been attached. Compared to other omomyids, except M. bryanti , all the cheek teeth have relatively low main cusps. This is particularly noticeable when compared to e.g., Teilhardina belgica ( Teilhard, 1927) , whose cusps are taller and more acute, especially those of p3–4. The same point has already been made about M. bryanti ( Hooker 2007) , although wear and slight corrosion of the p4 of that species have made the differences less clear. Although crown height is low in the genus, it is higher in M. schrevei than in M. bryanti . This difference shows most clearly in lingual view, especially in the shallower distal slope of the entoconid in M. bryanti (cf. Figs. 1 , 2 herein with Hooker 2007: text−figs. 1C–F, 2C–F, 3C–F).

The low p3–4 protoconids create a distinctly backward curving paracristid in M. schrevei ( Fig. 1B ). The contact between p4 and m1 is small, resulting in the absence of the extensive, sloping distal talonid wall characterising M. bryanti , best seen in buccal view ( Fig. 1B ; Hooker 2007: text−fig. 2D). p3 lacks a metaconid, but a short distal crest from the protoconid, slightly lingual of the cristid obliqua, is the homologue of the metaconid developed in p4 ( Fig. 1A–C ). Both p3 and p4 have large, tall, cuspate paraconids, but these do not project mesially from the outline in buccal view ( Fig. 1B ). The p4 paraconid of M. bryanti is probably comparable in terms of its development, but abrasion or corrosion has reduced its size and extent ( Hooker 2007: text−fig. 3E).

The two m1s show individual variation. That of the holotype shows a distinct step and notch in the protocristid, as well as a buccal metaconid buttress ( Fig. 1A ). This buttress does not join the cristid obliqua as it does in Altanius , Washakius , or Trogolemur ( Szalay 1976; Dashzeveg and McKenna 1977). The structure is similar to that in m1s of M. bryanti and Nannopithex zuccolae Godinot, Russell, and Louis, 1992 ( Hooker 1996b: pl. 4: 1, 2007: text−fig. 1E). The other M. schrevei m1 does not show this step in the protocristid ( Fig. 2A 1 ), indicating that the species may be at an intermediate stage in the acquisition of the derived stepped state. However, its presence in both of only two m1s of M. bryanti does not conclusively demonstrate that it is necessarily constantly present in that species either. NHMUK.M85504 differs from the m 1 in the holotype also in having a much wider talonid. There is a complete ectocingulid, which is mainly strong, but which weakens around the hypoconid in both specimens. It is thus stronger than in M. bryanti m1s. The more erect orientation of the m1 paraconid in M. schrevei than in M. bryanti is consistent in both specimens ( Figs. 1C , 2A 2 ).

M 2 also shows this different orientation of the paraconid. The difference from M. bryanti is also more obvious in m2 because the paraconid is essentially the same height as the metaconid and much closer to it in M. schrevei . This difference is consistent across all specimens of each species. The ectocingulid is complete and strong in all three specimens, stronger than in M. bryanti , where it is also incomplete in one specimen around the hypoconid. There is slight variation in the position of the paraconid with respect to its distance from the protocristid. NHMUK.M85503 has the shortest distance, NHMUK.M85502 the longest, in this respect more closely resembling M. bryanti , whilst the holotype is intermediate ( Figs. 1A , 2B 1, C 1 ). However, the similarity between NHMUK.M 85502 and M. bryanti is less in lingual view (cf. Fig. 2B 3 with Hooker 2007: text−fig. 3C).

The single m3 (of the holotype) is similar to those of M. bryanti in having a broadly open hypoconulid lobe and similar development of the ectocingulid, whose weakness around the hypoconulid is most like the holotype. It differs in having a straight, not concave, postmetacristid. It also differs in the position of angulation of the buccal crown base outline. In M. schrevei and in many other primitive omomyids, the buccal crown base descends steeply from the paraconid, then recurves below the protoconid and rises gradually to the hypoconulid lobe. By contrast, the shift in orientation, or angle, in M. bryanti is located distal of the protoconid. N. zuccolae and Vectipithex raabi are also like M. bryanti .

According to lower molar size, M. schrevei is slightly larger than M. bryanti ( Fig. 3 ). However, coefficients of variation of the combined measurements (millimetres) are mainly low: m1 (4) length, 2.94; m1 (4) width, 4.35; m2 (7) length, 3.16; m2 (7) width, 6.58; m3 (6) length, 3.68; m3 (6) width, 5.46. This indicates that the size difference between the species is not statistically significant.

Anterior breakage of the dentary occurs below the front of p4. No mental foramen is preserved and therefore the posterior foramen of the two normally present must have existed more anteriorly, either below the junction of p3 and p4 as in T. belgica , or below p3 as in Omomys ( Szalay 1976) . It is thus unlike N. zuccolae , where the posterior mental foramen is located below p4 ( Hooker 1996b). The most likely position in M. schrevei is below the junction of p3 and p4, the primitive state.

Medially, the dentary is 2.54 mm deep below m2, and thus within range of the two dentaries of M. bryanti (2.45 and 2.68 mm) at this locus. More anteriorly, the M. schrevei dentary deepens to 2.86 mm below p4. It is thus still deepening anteriorly at this point, which is close to the posterior margin of the symphysis, heralded by a sharpening of the ventral edge 1.5 mm before the anterior truncation. The most complete dentary of M. bryanti , which extends anteriorly to the position of the mesial edge of m1 (tooth missing) is also deepening anteriorly at this point ( Hooker 2010: text−fig. 21g, h). In N. zuccolae , the dentary is 3.54 mm deep medially below p4 (still proportionally deeper than any of the Melaneremia dentaries despite its slightly larger tooth size) and probably deepens forwards no further as anterior dental and mandibular contraction has shifted the alveolus for the tip of a greatly enlarged i1 root to below p4 ( Hooker 1996b: pl. 3). In Teilhardina belgica , the dentary begins shallowing anteriorly from just below the front of m1 ( Teilhard 1927: pl. 4: 1, 2). As T. belgica has small incisors, the implication is that M. schrevei and probably M. bryanti had a somewhat enlarged i1, albeit smaller than in N. zuccolae , given its overall shallower dentary depth.

Geographic and stratigraphic range.—Later part of the PETM, earliest Eocene, London Basin and possibly Paris Basin.

Figures

Fig. 1. Scanning electron micrographs of gold–palladium coated epoxy casts of the holotype left dentary of the omomyid primate Melaneremia schrevei sp. nov., NHMUK.M85501, central channel, upper shelly clays, Woolwich Formation, Sandilands cutting, Park Hill, Croydon, in occlusal (A), buccal/lateral (B), and lingual/medial (C) views.

Fig. 2. Scanning electron micrographs of gold–palladium coated epoxy casts of lower molars of the omomyid primate Melaneremia schrevei sp. nov., central channel, upper shelly clays, Woolwich Formation, Sandilands cutting, Park Hill, Croydon. A. Right m1 (reversed), NHMUK.M85504. B. Right m2 (reversed), NHMUK.M85502. C. Left m2, NHMUK.M85503. Views are occlusal (A1, B1, C1), buccal (A3, B2, C2), and lingual (A2, B3, C3).

Fig. 3. Scatter diagram of length vs width (in mm) of lower cheek teeth of Melaneremia schrevei and M. bryanti. Lines join teeth of single individuals. The length dimension is measured lingually from the bulge of the distal crown base to the tip of the paraconid.