New data on the taxonomy of the endemic Myomiminae (Gliridae, Rodentia) from the Late Miocene-Early Pliocene of Gargano (southern Italy) with the description of the new species Stertomys degiulii

The Late Miocene-Early Pliocene “Terre Rosse” vertebrate assemblages, found in the palaeokarst fissure fillings of the Apricena-Poggio Imperiale limestone quarries (North-Western side of the Gargano promontory, southern Italy), document a very long and complex history of endemic populations of a palaeoarchipelago. That history developed in at least four populating phases, which are characterised by changes in faunal diversity and include taxa with different degree of endemism. They have been subject of several studies since the beginning of the 1970s and deserve a particular attention for their evolutionary and palaeogeographical implications. The sampling of the “Terre Rosse” fissure fillings was carried on during successive field surveys from the Leiden Museum (1969-74) and from the Florence University (1980s). Until now, however, the two resulting collections have always been studied separately by different authors: this work is the first partial attempt to integrate the information derived from the two collections. A biochronological framework is proposed integrating the chronological succession of samples by Freudenthal in the scheme elaborated by De Giuli et al. However, some uncertainty remains in the details of the position of some fissure, in particular Rinascita 1. After the pioneering description of the gigantic dormouse Stertomys laticrestatus Daams & Freudenthal 1985, the interest in the systematic of the Neogene Gargano glirids renewed just in the very last years. Four species have been recently described: Stertomys daunius Freudenthal & Martín-Suárez 2006, Stertomys daamsi Freudenthal & Martín-Suárez 2006, Stertomys lyrifer Martín-Suárez & Freudenthal 2007, and Stertomys simplex Martín-Suárez & Freudenthal 2007. In the present work the description of Stertomys from eleven further fissure fillings, documenting most of the whole succession of endemic population phases, is presented and discussed. The samples, for a total amount of 1696 dental elements, have been described by basic statistical parameter of measurements and frequency distribution of Daams’ and Rinaldi’s morphotypes. Where necessary, the differences among samples have been statistically tested (t-test and chi-square test). Seven taxa have been identified: S. daunius, S. laticrestatus, S. aff. laticrestatus, S. ex gr. daamsi, Stertomys degiulii nov. sp., S. cf. degiulii and S. aff. degiulii. In the fissure F15 S. ex gr. daamsi has the same size as S. daamsi from its type-locality Biancone 1, but a slightly more complex morphology in the upper molars. In Trefossi 1, F21b, Cantatore 3A and Fina D, S. ex gr. daamsi shows a rather similar morphology as in F15, but it is clearly smaller in size. S. degiulii, from F1, F9 (type-locality) and San Giovannino is distinctly smaller than S. gr. daamsi and presents a higher frequency of the connection of the trigon crests with the metaloph in the upper molars. S. aff. degiulii from F32 is slightly larger in size and shows a higher frequency of connection of the anterolophid with the protoconid in the lower molars. S. cf. degiulii from Pirro 11A is intermediate in morphology between S. degiulii and S. aff. degiulii. The results of the analysis of the Stertomys species occurring in each fissure filling confirm the outline of the proposed biochronological framework and permit some improvement of the definition of the populating phases of the Gargano palaeoisland. Besides, they confirm the presence of some problems in arranging the details of the fissure succession of the oldest phase. Such uncertainties are possibly due to the frequent faunal exchanges among the islands of the palaeoarchipelago and with the mainland during the time documented by this early phase. The results of the present contribution also suggest that an early radiation occurred in the palaeoarchipelago, producing at least five lineages. These lineages can be arranged in two main branches characterised by different size and morphology. Large-sized Stertomys species, although closely related, cannot be considered as belonging to the same phyletic lineage. Among the small-sized taxa, S. daamsi and S. degiulii might be arranged in a single phyletic lineage where a trend in reduction in size and an increase of the complexity of the pattern of connection among crests occur. S. simplex may be considered as a species with primitive morphology, very close to the ancestor of S. daamsi. Most of the evolutionary divergence is observed among the different lineages, while phyletic gradualistic changes are documented to a lesser extent.