A new Early Cretaceous eutherian mammal from the Sasayama Group, Hyogo, Japan

Abstract

We here describe a new Early Cretaceous (early Albian) eutherian mammal, Sasayamamylos kawaii gen. et sp. nov., from the 'Lower Formation' of the Sasayama Group, Hyogo Prefecture, Japan. Sasayamamylos kawaii is characterized by a robust dentary, a distinct angle on the ventral margin of the dentary at the posterior end of the mandibular symphysis, a lower dental formula of 3 -4 : 1 : 4 : 3, a robust lower canine, a non-molariform lower ultimate premolar, and a secondarily reduced entoconid on the molars. To date, S. kawaii is the earliest known eutherian mammal possessing only four premolars, which demonstrates that the reduction in the premolar count in eutherians started in the late Early Cretaceous. The occurrence of S. kawaii implies that the relatively rapid diversification of eutherians in the mid-Cretaceous had already started by the early Albian.

A new fossil lizard from the Lower Cretaceous Sasayama Group of Hyogo Prefecture, western Honshu, Japan

“…the Aptian-Cenomanian. Recently, Kusuhashi et al (2013) obtained high-precision zircon-derived U-Pb ages using a sensitive high-resolution ion microprobe, estimating the age of the stratum from which the eutherian mammal fossils (Sasayamamylos kawaii) and the lizard mandibles were excavated to be 112 Ma. We, therefore, adopt this last estimate as the age of our material.…”

“…The lower formation is composed mainly of conglomerate, sandstone, and mudstone, with several intercalated tuff beds, of which one yields U-Pb ages of ca. 112.1 § 0.4 Ma (Kusuhashi et al, 2013). In 2006, a new terrestrial vertebrate fossil assemblage was found in a red mudstone of this formation, exposed in the bank of the Sasayama River at Kamitaki, Tamba City (Saegusa et al, 2008a(Saegusa et al, , 2008b.…”

“…1). The assemblage is dominated by fragmentary cranial and postcranial elements of lizards, besides four mammalian lower jaws (Sasayamamylos kawaii; Kusuhashi et al, 2013) and a few cranial elements of ceratopsians. Of the lizard fossils, one mandibular element was referred to the genus Pachygenys on the basis of tooth arrangement.…”

Dinosaur eggshell assemblage from Japan reveals unknown diversity of small theropods

“…1a, b), which is composed of conglomerates, sandstones, mudstones, and contains several intercalated tuff beds (Yoshikawa, 1993). Radiometrically-dated tuff beds in the lower parts of the 'Lower Formation' and 'Upper Formation' constrain the age of the locality between 112.1 ± 0.4 Ma and 106.4 ± 0.4 Ma, indicating an early Albian age (Kusuhashi et al, 2013).…”

An assemblage of lizards from the Early Cretaceous of Japan

“…Pachygenys was based on two mandibles from Early Cretaceous deposits in Shandong Province, China. Recently, however, closely similar jaws were recovered from the Sasayama Group of Hyogo Prefecture, Japan (Ikeda et al, 2015), dated as early Albian (112 Ma, Kusuhashi et al, 2013) and named Pachygenys adachii. As in Hakuseps imberis, the dentary of Pachygenys thlastesa is curved and has small number of teeth (eight) concentrated at the anterior end of the dentary.…”