Gigarton sigogneauae, Hooker & Russell, 2012

Hooker, Jerry J. & Russell, Donald E., 2012, Early Palaeogene Louisinidae (Macroscelidea, Mammalia), their relationships and north European diversity, Zoological Journal of the Linnean Society 164 (4), pp. 856-936 : 894-896

GIGARTON SIGOGNEAUAE SP. NOV.

( FIGS 18 , 19A–J)

Etymology: After Dr Denise Sigogneau-Russell, invaluable collaborator in the collection of vertebrate fossil material from the French early Tertiary.

Holotype: RM 2 , MNHN.F.BRL-99-P; bed 4, Berru.

Paratypes: LM 1, MNHN.F.CR-78- MD; right M 1, MNHN.F.CR-284-Bn; two RM 3 s, MNHN.F.CR-88-Ph, CR-335-L; LDP 4 , MNHN.F.CR-156-Ph; RDP 4 , MNHN.F. R1218 ; LP 4 , MNHN.F.CR-15937(Collier 836); LM 1, CR-288-L; LM 3, CR-321-L; all Cernay .

LM 2, MNHN.F.I-682; bed 5, Berru.

Referred material: LDP 4, MNHN.F.CR-1206; Cernay.

Age and distribution: Sables de Châlons-sur-Vesle supérieurs, late Thanetian, Late Palaeocene, Cernay and Berru (beds 4, 5), France.

Diagnosis: Medium-sized species of Gigarton , mean length of M 2 2.89 mm. Cheek teeth bulbous and with thickened enamel and weak cresting (shared with G. meyeri ); relatively low-crowned. M 1–2 with: large hypocone and occlusally convergent buccal and lingual main cusps (shared with G. meyeri ).

Description

M 1: Two specimens have nearly trapeziform shape, with shallow postflexi and ectoflexi (Fig. 19B, E). The central valley is longitudinally grooved as in G. meyeri , but in MNHN.F.CR-78-MD it is interrupted by a premetaconule crista (Fig. 19B2). Crown height is much lower than in G. meyeri and trigon cresting and metaconule are stronger. There is also a strong precingulum that joins the paracingulum and a complete ectocingulum, which in MNHN.F.CR-78-MD forms an incipient cingular mesostyle (Fig. 19B1). This specimen also had a small paraconule now largely obliterated by wear. The centrocrista has a v-shaped buccal concavity like Dipavali .

M 2: The two teeth are relatively shorter than M 1, although their exact proportions vary somewhat (Fig. 19C, F). As on M 1, the crown is much lower than in G. meyeri (Fig. 19C3). The metacone is only slightly larger than the paracone. There is a deep buccal notch between the two cusps. MNHN.F.I-682 has a weakly mesially projecting low parastyle (Fig. 19F), OR, observed range; SD, standard deviation; V, coefficient of variation (SD as percentage of the mean).

whereas in the almost unworn holotype there is nothing to qualify as a parastyle on the long extent of strong cingulum that extends from the precingulum buccally right round the tooth as far as the hypocone (Fig. 19C2). MNHN.F.I-682 is fairly heavily worn, exposing thick enamel on the main cusp tips. Its cingular development is weaker than on the holotype, but it is uncertain how much of this is because of greater wear. The central valley is uninterrupted in I-682 as the metaconule is only a small bump at the end of the postprotocrista, whereas in the holotype it is interrupted by a premetaconule crista that is confluent with the postprotocrista, with virtually no expression of a metaconule .

M 3: It is 1.91 mm long by 2.80 mm wide. It has no precingulum and only a weak postcingulum with no hypocone (Fig. 19D). There is a strong ectocingulum that dies out round the metacone. The paracone is bulbous and much larger than the metacone and there is a postprotocrista that stops short of a longitudinal valley. The strength of the ectocingulum matches the other upper molars of this species.

P 4: The single tooth is very similar morphologically to that of G. meyeri , but smaller ( Fig. 18B ). Although worn, it appears to have a slightly taller entoconid and more prominent paraconid than P 4 s of that species (Fig. 19G).

M 1: Most of the talonid is broken away but the mesial edge of the slightly tip-worn hypoconid is preserved (Fig. 19H). This shows a mesially positioned hypoconid with a very steep mesial wall and a barely recognizable cristid obliqua. The hypoconid is relatively tall, probably being more than half the original height of the protoconid, which is now quite worn. The height and mesial position of the hypoconid are like M 1 of G. meyeri and unlike P 4 of either species. Size (trigonid is 2.23 mm wide) is clearly smaller than M 1 of G. meyeri , although the reliability of the difference cannot be statistically tested. Unlike in G. meyeri the trigonid is undamaged. It shows a convex, mesial protoconid wall and no obvious sign of a paracristid. There is a lingually situated, tiny, crestiform paraconid, but no precingulid. The mesial margin in crown view is rounded.

M 3: The single specimen is bunodont with inflated cusps, the trigonid resembling P 4 and M 1 in having the metaconid much smaller than the protoconid, the latter being very blunt (Fig. 19I). The talonid is more conventional with approximately equally spaced hypoconid, hypoconulid, and entoconid, although the last is slightly more distal than the hypoconid. There is an ectocingulid that reaches the mesial edge of the hypoconid. There is a tiny, low, lingually positioned paraconid and an equally tiny accessory cusp just mesial of the metaconid. The tooth is attributed to G. sigogneauae rather than to G. meyeri on size grounds ( Table 6).

DP 4: Three teeth are similar to the M 1 s, but slightly smaller, relatively narrower, with more gracile cusps, more projecting parastyle, and, except in one, thinner enamel. As on M 1 the metacone is larger than the paracone. Cresting is better marked and two specimens have a distinct metaconule with premetaconule crista (Fig. 19A2, J). There is a small cingular mesostyle (Fig. 19A1) in the two better preserved ones and the strong ectocingulum is interrupted at the paracone (Fig. 19A, J).

Figures

Figure 18. Scatter diagrams of Ln length vs. Ln width of species of Gigarton. For explanation of symbols see Figure 8. A, M1–2; B, P. 4