Oligodyromys libanicus Freudenthal

Lu, Xiaoyu, Costeur, Loïc, Hugueney, Marguerite & Maridet, Olivier, 2021, New data on early Oligocene dormice (Rodentia, Gliridae) from southern Europe: phylogeny and diversification of the family, Journal of Systematic Palaeontology 19 (3), pp. 169-189 : 179-180

Oligodyromys libanicus Freudenthal & Mart́ ın-Súarez, 2007b

( Fig. 4U–W)

1971 Pseudodryomys aff. fugax Hugueney et al. : 2432.

Type locality. Montalb́an 8 (MP22).

Holotype. Departamento de las Ciencias de la Tierra, University of Zaragoza, MLB8 1183, left m1.

Specimens and measurements. UCBL-FSL_218045, left M1, L = 1.38 W = 1.31 ( Fig. 4U); UCBL-FSL_218068.001, left M1, L = 1.15 W = 1.33; UCBL-FSL_218046, right M2, L = 1.09 W = 1.32 ( Fig. 4V); UCBL-FSL_218068.002, right M2, L = 1.25 W = 1.52; UCBL-FSL_218068.003, broken right M2, L = 1.25 W = 1.44; UCBL-FSL_218047, left m2, L = 1.34 W = 1.44 ( Fig. 4W).

Description of the specimens from Montalb́an 1D.

M1/M2. The occlusal surfaces of M1 and M2 are strongly concave. Both M1 and M2 display a rectangular shape, but M1 is slightly longer labially. All cusps are well developed. The anteroloph connects with paracone labially in M1, but remains free in M2. Both anterotrope and posterotrope are absent. The central valley is ‘V’- shaped; on its lingual side the endoloph of M1 is more developed and longer than in M2, which results in the metaloph being slightly bent posteriorly. In contrast it is always straight in M2. Both the precentroloph and the postcentroloph are well developed, with no tropes in the central valley. Sometimes there are some bulges in between the centrolophs and main lophs. Because they are not crest-like, we consider them as the variation of the centrolophs. The posteroloph is free in all specimens.

m2. The only m2 shows irregular tropids. All cuspids are well developed and the metaconid is much higher than other cuspids. Six crests are present in our specimen. The anterotropid is absent. The metalophid is parallel to the anterolophid, it connects with the protoconid and reaches the base of the rising metaconid. The centrolophid is well developed and originates from the posterior arm of the metaconid. It is interrupted in the middle of the tooth, and the labial part extends posteriorly to connect with mesolophid. The mesoconid connects with the entoconid through the mesolophid. The posterotropid is well developed and twisted. There is a weak crest in the valley between the centrolophid and the mesolophid, but this crest is as high as the centrolophid. We consider it as a variation of the centrolophid.

Remarks. Oligodyromys was referred to Glirinae Thomas, 1897 by Bahlo (1975) and was later transferred to the Myomiminae Daams, 1981 because of the concave occlusal surface. Bosma & de Bruijn (1982) synonymized Oligodyromys with Branssatoglis Hugueney, 1967 . However, Freudenthal & Mart́ın-Súarez (2007b) restored the genus because too many species are classified inside the genus Branssatoglis . As a result, Branssatoglis has been used as a wastebasket taxon; however, the different species of Oligodyromys and the type species of Branssatoglis ( Branssatoglis concavidens Hugueney, 1967 ), being distant in our phylogeny, indicate that both genera are clearly differentiated and that Oligodyromys is valid. O. libanicus is described from the early Oligocene of Montalb́an 8 as the largest known Oligodyromys ; only Oligodyromys sjeni Unay ¨, 1989 from Kocayarma ( Turkey) is of similar size (Freudenthal & Mart́ın-Súarez 2007b). The anterotropid of O. libanicus is only well developed on m1. Unfortunately, we do not have any large-sized m1. The anterotrope in M1/2 is infrequently present in the type locality and also absent in all our specimens. Six crests exist on both upper and lower molars. The patterns are also similar to Butseloglis micio and B. itardiensis , the latter species possibly being synonymous with B. micio ( Vianey-Liaud, 1994) . Freudenthal & Mart́ın-Súarez (2007b) considered that the larger specimens of B. itardiensis from Hoogbutsel probably belongs to O. planus ; our specimens differ from it by lacking the anterotropid and having fewer crests in upper molars. Considering that on the one hand our specimens are larger than B. micio and B. itardiensis , and that the patterns are simpler than O. planus and O. sjeni , and on the other hand the size is close to that of O. libanicus from Montalb́an 8 (M1: mean length = 1.25, mean width = 1.34; M2: mean length = 1.24, mean width = 1.42; m2: mean length = 1.36, mean width = 1.38; Freudenthal & Mart́ın-Súarez 2007b), we tentatively refer our specimens to O. libanicus . Oligodyromys is common from MP18 to MP22, but remains rare in Montalb́an 1D (MP23), only represented by a few specimens of O. cf. parvus (Freudenthal & Mart́ın-Súarez 2007b). This is the first occurrence of O. libanicus or such a large species in this locality.