Pleistocene murid rodent fossils from Semadong Cave in northern Peninsular Malaysia: taxonomic, zoogeographic, and environmental implications

The new fossil site of Semadong Cave in Perlis State, Malaysia, has yielded a very rich array of small vertebrate fossils that are composed mainly of fragmentary craniodental and postcranial remains of murine rodents plus some fragments of insectivores, large mammals, and amphibians. Here, we describe 383 dental remains of fossil murine rodents recovered from the whitish-grey silty clay, the fossiliferous layer deposited in the cave of Semadong. The fossil rodent assemblage is composed of ten identified murine species within seven genera, including two globally (Prohadromys varavudhi and Saidomys siamensis), and six locally (Hadromys humei, Bandicota savilei, Berylmys berdmorei, Rattus andamanensis, Mus cf. pahari, and Mus cervicolor) extinct taxa, two living species (Rattus argentiventer and Rattus rattus), and numerous remains identified as belonging to Rattus sp. and an indeterminate murine. The most abundant specimens were Bandicota savilei, followed by the indeterminate murine and Rattus rattus. The taxonomic results provide age estimates of the Pleistocene for micromammal fossil deposits. The presence of two primitive murid rodent species demonstrates the widespread distribution of Indochinese elements, extending further south of the Isthmus of Kra, into northern Sundaland. Paleoenvironmental evidence based on the presence of Bandicota savilei, Rattus argentiventer, and Mus cervicolor suggests that the expansion of Pleistocene tropical grasslands might have played a key role in facilitating their southward distribution. Meanwhile, other species, such as Hadromys humei, indicate the existence of forested areas under drier and cooler conditions than those found today.