Talahphiomys lavocati, (WOOD, 1968)

Jaeger, Jean-Jacques, Marivaux, Laurent, Salem, Mustapha, Bilal, Awad Abolhassan, Benammi, Mouloud, Chaimanee, Yaowalak, Duringer, Philippe, Marandat, Bernard, Métais, Eddy, Schuster, Mathieu, Valentin, Xavier & Brunet, Michel, 2010, New rodent assemblages from the Eocene Dur At-Talah escarpment (Sahara of central Libya): systematic, biochronological, and palaeobiogeographical implications, Zoological Journal of the Linnean Society 160 (1), pp. 195-213 : 208-209

TALAHPHIOMYS LAVOCATI ( WOOD, 1968)

Remarks: In his revision of the rodents from the Fayum, Wood (1968: 45) described the species ‘ lavocati ’ as a new species of the genus Phiomys on the basis of partial lower jaws and isolated lower teeth. The discovery of both upper and lower teeth attributable to this species, notably the upper molars, allows a revision of the generic attribution, inasmuch as the diagnosis for the upper molars of Phiomys ( Wood, 1968: 32) does not correspond to the morphology of this species. As such, we describe here a new genus, in which we include the species lavocati : Talahphiomys lavocati ( Wood, 1968) .

Referred material: DT2-011, left M 3; DT2-012, right M 3; DT2-013, right M 2; DT2-014, right M 1; DT2-015, left DP 4; DT2-016, left DP 4; DT2-017, left M 1; DT2- 018, left M 2; DT2-019, left M 3; DT2-020, left DP 4; DT2-021, right M 2; DT2-022, right M 2.

Locality and horizon: DT-Loc.2 (DT2), Idam Unit (‘Bioturbated Unit’) of the Dur At-Talah escarpment in central Libya.

Emended diagnosis: Species of the size of Talahphiomys libycus sp. nov. Differs from this in having more bunodont cheek teeth, lower transverse crests, and in showing upper molars being functionally two-crested rather than three-crested, a condition characterized by the absence of mesoloph (-ule). M 2 displays an additional mesiolingual cingulum.

Description and comparisons: The upper molars are quadrangular and the four main cusps (proto-, meta-, para, and hypocone) are bulbous and equal in size. Another cusp, which is slightly smaller, occurs in the centre of M 1 and M 2 ( Fig. 6M, N). This central cusp is an enlarged, displaced metaconule, as also observed on upper molars of baluchimyines (e.g. Baluchimys , Lindsaya , Hodsahibia , and Bugtimys ) from the Palaeogene of South Asia ( Pakistan, Thailand). As in baluchimyines and in early phiomyids (i.e. Protophiomys , Phiomys ), this central cusp is connected to the hypocone by the anterior arm of this latter. However, in T. lavocati this lingual connection is the only one for the metaconule. In this species, this central cusp does not represent a meeting point with the metaloph, mesoloph (-ule) and mure as it does in several baluchimyines and phiomyids. This condition is reminiscent of that observed in upper molars of Lindsaya , a very bunodont baluchimyine ( Flynn et al., 1986), in which the central metaconule is very large and free of connection. As in Lindsaya , there is no mesoloph (-ule) on M 1 and M 2, but on M 3 a short, very thin crenulation can be noticed. As in Lindsaya again, the metacone displays a very small crest on its lingual side, slightly backwardly directed, that could correspond to a very short metaloph. The protoloph is better developed and moderately high, and connects the paracone with the protocone. The posteroloph is also relatively high, long, and ends at the base of the metacone. Mesially, unlike Lindsaya , there is no anteroloph on the upper molars, only a low, long mesial cingulum at the base of the crowns. This cingulum extends more mesiolingually on M 2 than on M 1. On M 2 ( Fig. 6M), the protocone has a short anterior arm, which is not connected to the anterior cingulum. Buccally, the paracone and metacone are widely spaced; there is only a simple mesostyle between both cusps. Lingually, the protocone and hypocone are also widely spaced. There is neither endoloph nor mure, and thus the internal sinus remains buccolingually open (transverse). In this character, T. lavocati differs substantially from Lindsaya and also from Baluchimys , which both have upper molars with a very shallow sinus because of the presence of a strong endoloph.

The sole upper premolar referred to T. lavocati is here identified as a deciduous P 4 ( Fig. 6O). This tooth is slightly smaller than M 1, but structurally identical, except for the metaconule, which is indistinct.

The morphology of the lower molars ( Fig. 6Q–S, U, V) is simple, characterized by bulbous, equally sized cuspids, and by the development of only three transverse crests, which are the posterolophid, the hypolophid, and the metalophulid I. There is neither mesolophid nor posterior arm of the protoconid. In the absence of a posterior arm of the protoconid, T. lavocati differs from all baluchimyines and some phiomyids, which show a moderately (e.g. Baluchimys , Hodsahibia , Protophiomys , Phiomys ) to strong (= metalophulid II; e.g. Lophibaluchia , Hodsahibia , Bugtimys , Metaphiomys ) development of this character. The lack of a posterior arm of the protoconid is also observed in Gaudeamus , Paraphiomys , and Phiocricetomys , which are considered as evolutionarily advanced and specialized phiomyids ( Wood, 1968). The ectolophid is nearly horizontal and there is no indication of a mesoconid. The posterolophid is well developed and ends just at the base of the entoconid, without connecting to it. The metafossettid remains slightly open lingually. The posterolophid bears a distinct hypoconulid in its median part. There is no mesostylid and the posterior arm of the metaconid is short and not very elevated, maintaining a talonid basin that is open lingually.

We identified as DP 4 two oval-shaped teeth ( Fig. 6P, T), having their talonid larger than their trigonid. The organization of both cuspids and crests is similar to that observed for molars, except for the trigonid part, which lacks metalophulid I and bears a small, isolated anteroconid. This additional cuspid is mesially located, and situated between the protoconid and the metaconid. The morphology of these two teeth is strongly reminiscent of Ph. paraphiomyoides and Pr. algeriensis , two taxa that were formerly considered as the most primitive phiomyid species from the Palaeogene of North Africa (see Wood, 1968; Jaeger et al., 1985).