Protophiomys durattalahensis, Jaeger & Marivaux & Salem & Bilal & Benammi & Chaimanee & Duringer & Marandat & Métais & Schuster & Valentin & Brunet, 2010

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 : 205-207

PROTOPHIOMYS AFF. DURATTALAHENSIS

Referred material: DT2-001, right P 4; DT2-002, left M 1; DT2-003, left M 2; DT2-004, left M 3; DT2-005, left DP 4; DT2-006, right M 3; DT2-007, right DP 4; DT2-008, left M 3; DT2-009, right M 1; DT2-010, right P 4 ( Fig. 6A–J).

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

Description and comparisons: This new species of Protophiomys found in two different localities within the Idam Unit of the Dur At-Talah escarpment represents an advanced morphological evolutionary stage with respect to Pr. algeriensis from the late middle Eocene of Algeria. Protophiomys durattalahensis exhibits a more lophate dental pattern, notably on the upper molars, which show a fully expressed pentalophodonty with stronger development of the antero-, proto-, meso-, meta-, and posteroloph. From M 1 to M 3 (Figs 5M–O, 6B–D), the mesoloph (-ule) is low but complete in reaching a mesostyle on the buccal margin of the teeth, a condition that is reminiscent of that observed in Phiomys , Metaphiomys (see Wood, 1968) and in certain baluchimyines such as Hodsahibia , Bugtimys , Lophibaluchia (see Flynn et al., 1986; Marivaux et al., 2002; Marivaux & Welcomme, 2003) and Baluchimys krabiense (see Marivaux et al., 2000). In contrast, in Pr. algeriensis , this central crest is much shorter, rather ‘incipient’ from the metaconule (sensu Jaeger et al., 1985), and never reaches the buccal margin of the teeth as it does in Baluchimys barryi (see Flynn et al., 1986) and in Phiomys paraphiomyoides (see Wood, 1968). As in baluchimyines (except Lophibaluchia ), the metaloph is directed toward the protocone and not toward the posteroloph, as it is usual in Metaphiomys and often in Phiomys . This transverse crest is occasionally interrupted in its medial part, thereby remaining free of connection (Figs 5M, 6B). In most cases, the metaloph connects with the mesial extremity of a strong anterior arm of the hypocone (Figs 5N–O, 6C–D), which can be slightly inflated mesially in some

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specimens, thereby marking the position of a minute, nearly indistinct metaconule (= mesocone). This position is virtually central on the teeth, and represents a meeting point of the metaloph, mesoloph, anterior arm of the hypocone, and in some cases of a short mure. As in upper molars of Pr. algeriensis , in Pr. durattalahensis the mesostyle, although high, is barely distinguishable, being entirely embedded in a massive buccal enamel wall formed by distal and mesial extensions of the paracone and metacone, respectively. In both species, this buccal wall continues mesially and distally to the paracone and metacone, respectively, by way of strong and elevated connections between the anteroloph and posteroloph with these two buccal cusps. The crown of the upper molars is actually characterized by a high and continuous peripheral wall, which is interrupted only at the level of the lingual internal sinus. Lingually, the protocone displays a strong, long posterior arm, which is directed toward the hypocone. As a result, the internal sinus is very narrow and appears as a lingual notch on the peripheral wall of the crowns. This condition differs from that observed in Phiomys , Metaphiomys , Hodsahibia , Bugtimys , and Lophibaluchia , in which the lingual sinus is particularly wide mesiodistally.

The P 4 ( Fig. 6A) are as in Ph. hammudai , i.e. simple, trilophodont with well-developed proto-, meta-, and posteroloph. On one specimen, the metaloph is limited in its buccal part. The anterior cingulum is low and not connected to the protocone. The protocone and hypocone are twinned, and there is no lingual sinus. In contrast, the DP 4 (Figs 5L, 6E) are molariform, and resemble M 1 in several details, except for the anterior cingulum, which is low and not connected to the protocone. The central metaconule is much more distinct, and there is neither mesoloph nor mure, and the metaloph is short, not connected to the metaconule but to the posteroloph.

As observed in upper molars, the crown of lower molars exhibits a similar peripheral wall, which is also interrupted only at the level of the wide external sinusid. Lingually, the metaconid displays a strong, elevated posterior arm, which connects with the entoconid. A small swelling is sometimes visible in this lingual enamel wall, which corresponds to the mesostylid. On all lower molars, the posterolophid strongly joins the entoconid, thereby extending the lingual wall distally, and therefore closing the metafossettid lingually. A hypoconulid is present, large, but entirely incorporated in the massive posterolophid. The posterior arm of the protoconid is rather small to moderately developed, and is sometimes connected to some enamel accessory crenulations. As in Metaphiomys , Paraphiomys , Talahphiomys , and baluchimyines (except Hodsahibia azrae ), there is no indication of a true mesolophid. Only one tooth (Fig. 5 R) exhibits a low, short enamel crenulation in the position of the mesolophid, which originates from the lingual wall and ends near the lingual extremity of the posterior arm of the protoconid. The ectolophid is complete but less oblique than in Phiomys or Metaphiomys . In a few specimens, a small swelling occurs on its medial part, thus indicating the presence of a minute mesoconid. Mesially, there is no trace of an anterocingulid at the base of the molar crowns.

The lower P 4 ( Fig. 6J) is simple with a vast talonid basin without strong internal transverse crest. Only a short, slender hypolophid occurs in the distal part of the tooth. This crest is connected to the posterolophid rather than with both the ectolophid and anterior arm of the hypoconid. There is also a short and very thin posterior arm of the protoconid. The entoconid is the main cuspid of the tooth. The talonid of this tooth is as wide as that of the molars, with the entoconid widely separated from the hypoconid. In contrast, the trigonid is narrower, but the metaconid and protoconid remain widely spaced, and linked together by a strong metalophulid I. As on molars, the metaconid displays a long, strong posterior arm, which extends backward to connect to the entoconid. As in Pr. algeriensis , the DP 4 (Figs 5P, 6G) are oval shaped, with a narrow anterior part and a larger posterior part. The main cuspids are well individualized, and the crests connecting them are low. The metaconid is situated more mesially than the protoconid. Both cuspids are about the same height and are linked together by the posterior arm of the protoconid, which forms a complete metalophulid II. This condition differs substantially from that observed in DP 4 of Metaphiomys , in which the protoconid is buccolingually opposed to the metaconid, and does not develop a posterior arm. Mesially, there is neither metalophulid I nor an anteroconid as is usual in DP 4 of Phiomys . The metaconid does not display a strong posterior arm and the talonid remains open lingually. A small and isolated mesostylid occurs between the metaconid and entoconid, and there is no indication of mesolophid. The ectolophid is oblique and connects with the hypolophid, and weakly with the protoconid. There is no distinct mesoconid on it. The hypoconulid is well marked on the posterolophid, and this latter does not reach the entoconid.