The beginning of the adaptive radiation of Theridomorpha (Rodentia) in Western Europe: morphological and phylogenetic analyses of early and middle Eocene taxa; implications for systematics

This paper provides a revision of the early and middle Eocene European rodents previously referred to as Ischyromyoidea, including taxa considered to be at the origin of the Theridomorpha. The use of an accurate dental terminology and a better understanding of the size and shape of their infra-orbital foramen (i.o.f.) led us to a substantial revision of this group, which allowed to better characterize them and to appreciate their variability. On these bases, phylogenetic analyses (cladistic and standard Bayesian approaches) of early Ypresian to late Priabonian European rodent species were undertaken in order to highlight the root of the early Theridomorpha and its content. The phylogeny was established based on 343 characters (338 dental) through 45 early Paleogene taxa. The ingroup included on one hand a few North American genera (Reithroparamys, Microparamys, and Acritoparamys) and European ones (Eogliravus, Ailuravus, Corbarimys, Meldimys, Euromys, Plesiarctomys, and Pseudoparamys) considered until now as being related with the North American superfamily Ischyromyoidea. On the other hand, it included genera close to the root of the Theridomorpha (Sparnacomys, Pantrogna, and Hartenbergeromys) and early Theridomyoidea (Masillamys, Protadelomys, and some Pseudosciuridae). The phylogenetic results obtained via the two distinct reconstruction approaches are consistent in virtually all relationships. The proposed systematics here derives from these phylogenetic results. This phylogenetic context led us to change the suprafamilial, familial, subfamilial or generic attribution of several species. Characters of Theridomorpha, like the obliquely developed postprotocristid allied with the occurrence of a metalophulid I, have been found in genera previously considered as Ischyromyidae (Pseudoparamys, Euromys, Sparnacomys, Meldimys, Pantrogna, and Hartenbergeromys) as well as the large i.o.f., when preserved (Pseudoparamys, Hartenbergeromys, and Masillamys). Based on these morphological observations and new phylogenetic considerations, the content of the Theridomorpha clade is here enlarged, thereby extending back the first theridomorph radiations to the early Eocene. Aside, a new taxon (Reinomys rhomboides gen. and sp. nov.) is described from Avenay. In addition, a new genus, Auroremys, is created for the species subita (Comte et al., 2012) from Chery-Chartreuve.