Gigarton meyeri, 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 : 890-894
treatment provided by
vp 1964 Hyopsodontidé?; Russell, pp. 247–248, pl. 16,
fig. 5a, c–e. vp 1967 Hyopsodontidé, genre indéterminé; Russell,
Louis & Poirier, p. 853, pl. 21, fig. 4 (left).
Etymology: After Monsieur J.-Cl. Meyer for making available key specimens of louisinids, including this species.
Holotype: LM 2, MNHN.F.BRL-10-G; bed 4, Berru.
Paratypes: Two LP 4 s, MNHN.F.CR-11909, CR-6-Bn; LM 1, MNHN.F.CR-318; RM 1 , CR-971-Pn; RDP 4 , MNHN .F.CR-340-Ph; LP 3 , MNHN .F.CR-1211; LP 4 , MNHN .F.CR-287-L; three RP 4 s, MNHN.F.CR-946, CR-1159, CR-1696-Pn; LM 1, MNHN.F.CR-4331; RM 1 , MNHN .F.CR-13773; all Cernay.
LM 1, MNHN.F.I-251, bed 5, Berru.
Doubtfully referred specimens: LP 3?, MNHN.F.CR- 284-L; RP 3?, CR-80-MD; both Cernay. LP 3?, MNHN.F.BRL-143-P; bed 4, Berru.
Age and distribution: Sables de Châlons-sur-Vesle supérieurs, late Thanetian, Late Palaeocene, Cernay and Berru (beds 4, 5), France.
Diagnosis: Large species of Gigarton , length of M 2 3.37 mm. Cheek teeth bulbous with thickened enamel (shared with G. sigogneauae ), relatively highcrowned, and large main cusps, causing the basins to be shallow. M 1–2 with: weak cresting; large hypocone and occlusally convergent buccal and lingual main cusps (shared with G. sigogneauae ).
P 3?: Three teeth are quite similar in outline shape and cusp pattern to a nonmolariform variant of P 4 of D. petri , but larger ( Fig. 17A, B). They also have a basally basined distal cingulum and an ectocingulum restricted to the distal half like D. petri . They differ in having less cresting, especially on the larger protocone, a paracone with a straight, not bowed mesial wall, a larger parastyle, and a narrow longitudinal valley between paracone and protocone, constricted by the bulbous cusps as on M 1 of G. meyeri . There is a tiny metacone fused to the paracone. The distal cingulum ends lingually without a hypocone and does not ascend the back of the protocone. These teeth occlude reasonably well with the G. meyeri P 3. They are tentatively identified as P 3 of G. meyeri .
P 4: MNHN.F.CR-11909 has basal parts of the mesiobuccal area broken away, but is otherwise well preserved ( Fig. 17C). MNHN.F.CR-6-Bn has a complete crown, but is somewhat rolled, polished, and patchily corroded ( Fig. 17K). Both show slight tip wear. The parastyle is low and blunt. In MNHN.F.CR- 6-Bn, the metacone is fractionally larger than the paracone, but in MNHN.F.CR-11909 the two cusps are the same size. In both teeth the paracone and metacone are connate, but slightly better separated in MNHN.F.CR-11909. There is a hypocone that is slightly smaller than the protocone and situated close to it distolingually. In MNHN.F.CR-11909, the hypocone makes a bulge in the outline, enhanced by a postflexus. In MNHN.F.CR-6-Bn, there is no bulge and the distal margin is straight. Buccal and lingual cusps are fairly close together and a shallow central valley forms a longitudinal groove between them. Crests are virtually absent. There are very faint postmetacrista and centrocrista and there is a weak postcingulum–metacingulum. The postprotocrista is only represented by the change in slope between the central valley and that between the protocone and hypocone; it dies out after a short distance. The preprotocrista is stronger and dips steeply to meet the lingual end of a weak paracingulum. There is no precingulum. The buccal walls of the paracone and metacone are tilted lingually more than the lingual walls of the protocone and hypocone are tilted buccally. The overall appearance is of extreme bunodonty.
M 1: Bunodonty resembles that of P 4, but the outline is trapezoidal, tapering mesially ( Fig. 17D, L, M). The paracone and metacone are further apart than on P 4 and in all three teeth the metacone is larger than the paracone. Paucity of cresting and orientation of those that exist are similar to P 4. M 1 differs, however, in
892 J. J. HOOKER and D. E. RUSSELL having a mesially projecting parastyle and a partial ectocingulum that is restricted to the distal side of the metacone. MNHN.F.I-251 has considerable tip wear and a tiny bump in the central valley between the metacone and the hypocone probably represents a metaconule ( Fig. 17L). The nearly unworn MNHN.F.CR-318 differs in having a better developed trigon with distinct postprotocrista ending in a metaconule that is nearly as large as the hypocone ( Fig. 17D 2 View Figure 2 ). Its buccal cusps are slightly taller than its lingual cusps (Fig. 19K). A third specimen has a short, strong postprotocrista that stops short of the central valley and lacks a metaconule ( Fig. 17M).
M 2: The single tooth so identified is similar to P 4 in outline and structure, but larger; the paracone and the distinctly larger metacone are more widely spaced; there is a thin, sharp, distobuccally directed postprotocrista, but no metaconule; there is a shallow postflexus; and the hypocone is larger ( Fig. 17E). The lingual tilt of the paracone and metacone is greater than on P 4 or M 1.
DP 4: The outline is like M 1, but with a more exaggerated mesial taper ( Fig. 17N). Crown height is lower, the parastyle is large and mesially projecting and the enamel where exposed is slightly thinner. There is a relatively large metaconule at the end of a short, strong postprotocrista. There is a distinct ectoflexus, which interrupts what would otherwise be a continuous ectocingulum. There is also a precingulum, which joins a paracingulum, and the postcingulum– metacingulum is strong.
P 3: The single specimen is somewhat abraded basally and the size of its paraconid is difficult to ascertain ( Fig. 17F). 3.22 mm is a slight underestimate of length and it is 1.76 mm wide. The tooth is bulletshaped in outline, tapering mesially. There is a large area of exposed dentine on the protoconid, which is
› Figure 19. Scanning electron micrographs of gold-palladium coated epoxy casts of teeth of Gigarton sigogneauae gen. et sp. nov. (A–J), Gigarton louisi gen. et sp. nov. (L–N, P), Gigarton meyeri gen. et sp. nov. (K), and Dipavali petri (O), late Thanetian, France. A, RDP 4 (reversed), Cernay (MNHN.F.CR-1218). B, LM 1, Cernay (MNHN.F.CR-78-MD). C, holotype RM 2 (reversed), bed 4, Berru (MNHN.F.BRL-99-P). D, RM 3 (reversed), Cernay (MNHN.F.CR-88-Ph). E, RM 1 (reversed), Cernay (MNHN.F.CR-284-Bn). F, LM 2, bed 5, Berru (MNHN.F.I-682). G, LP 4, Cernay (MNHN.F.CR-15937- Collier). H, LM 1, Cernay (MNHN.F.CR-288-L). I, LM 3, Cernay (MNHN.F.CR-321-L). J, LDP 4, Cernay (MNHN.F.CR-156- Ph). K, LM 1, Cernay (MNHN.F.CR-318). L, RM 1 (reversed), Cernay (MNHN.F.CR-289-L). M, holotype LM 2, Cernay (MNHN.F.CR-93-L). N, LM 3, Cernay (MNHN.F.CR-129-Pn). O, paratype LM 1, Cernay (MNHN.F.CR-1186). P, LDP 4, Cernay (MNHN.F.CR-290-L). Views are: buccal (A1, B1, C1, D1, G2, H2, I2, L1, M1, N1), occlusal (A2, B2, C2, D2, E, F, G1, H1, I1, J, L2, M2, N2, P), lingual (G3, H3, I3), and distal (C3, K, L3, O). Short scale bar = 2 mm for A–K, L3, O. Long scale bar = 2 mm for L1, L2, M, N, P.
almost the only cusp of the trigonid, there being no metaconid. The protoconid is bulbous and has two faint distal crests that die out before reaching the talonid basin. The talonid is slightly wider than the trigonid and bears a small hypoconid, entoconid, and even smaller hypoconulid, but no cristid obliqua.
P 4: Five specimens give a good idea of variation. Shape and general structure are similar to P 3, but there is a metaconid that is substantially smaller than the protoconid ( Fig. 17G, I, J). There is a faint, blunt paracristid that curves around the front of the trigonid to meet a tiny paraconid at the mesiolingual corner, terminating a faint, short precingulid. The paraconid is lacking in MNHN.F.CR-288-L. The protocristid is deeply notched and in MNHN.F.CR-946 is blocked by a supernumerary cusp ( Fig. 17J). The cristid obliqua is variably developed. In the least worn tooth (MNHN.F.CR-1159) it dips steeply mesially then rises sharply a short distance up the back of the trigonid in a buccal position ( Fig. 17G). The talonid cusps are larger than on P 3 and more than half the height of the protoconid in MNHN.F.CR- 1159. This specimen and MNHN.F.CR-1696-Pn have a small entoconulid on a weak pre-entocristid ( Fig. 17G, I). The metaconid is fused with the protoconid high on the crown mesially, so that there is no trigonid basin as a swollen protoconid bulges right to the lingual edge, forming a blunt cusp in front of the metaconid in MNHN.F.CR-946 ( Fig. 17J). There is considerable size variation ( Fig. 18B View Figure 18 ), but the smallest, MNHN.F.CR-15937 is referred to G. sigogneauae .
M 1: The talonid is shorter and narrower than the trigonid and its basin is shallow, matching the pattern of the trigon cusps and valleys of M 1 ( Fig. 17H). The cristid obliqua is weak and steeply dipping as on P 4. The distal slope of the hypoconid is longer than its mesial slope, allowing space behind for occlusion of the large M 1 metacone. MNHN.F.CR-4331 is abraded mesially, but the metaconid appears to have been fused to the protoconid high on the crown as on P 4 ( Fig. 17H 1 View Figure 1 ). Unlike P 4, however, the mesial surface is transverse, not mesially tapering. MNHN.F.CR-4331 is somewhat worn. In contrast, MNHN.F.CR-13773 is unworn but broken mesially and basally. It shows generally taller cusps, a shallower protocristid notch, and a longer distal but shorter mesial slope to the hypoconid than in the P 4 s.
Discussion: There are too few specimens of any upper tooth type belonging to this species to obtain a reliable coefficient of variation for size measurements. However, combining the respective length and width dimensions of the five larger Gigarton M 1 s produces coefficients of variation well above that expected of a single species ( Table 6), so G. meyeri is restricted to the grouping of larger teeth, which are also highercrowned ( Figs 18A View Figure 18 , 19K).
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