First vertebrate faunal record from submerged Sundaland: The late Middle Pleistocene, hominin-bearing fauna of the Madura Strait

The Sunda Shelf currently holds the world’s largest shelf sea. During Pleistocene sea-level lowstands the area emerged, forming a vast landmass called Sundaland. Intermittently exposed Sundaland played an important role in the regional vertebrate biogeography and evolution of species including Homo erectus and, in the Late Pleistocene, Homo sapiens. So far, our models of the Pleistocene biogeography of Sundaland were based on the distribution of extant species and on island fossil assemblages, notably from Java (Indonesia). Here we present the first vertebrate fossil record from subsea Sundaland. The fossils derive from the sandy fill of a lowstand valley of the Solo River in the Madura Strait, which was OSL-dated to 162 ± 31 and 119 ± 27 ka. The cemented sand was recently extracted and used for a landfill in the port of Surabaya. The assemblage consists of at least 36 species and provides a unique insight into the lowland fauna of emerged Sundaland, during the late Middle Pleistocene. The lowland river had a rich reptile community, consisting of various turtle species, crocodilians, varanids, and pythons. The nearby estuary was frequented by several shark species, including the extinct Hemipristis serra, as well as several species of stingrays. Some of these also ascended the river. The lowland plains surrounding the river were populated by herbivores such as Hippopotamidae and several species of Bovidae, Cervidae, and Proboscidea. The terrestrial community represents a grass-dominated environment and is similar to the contemporaneous fossil assemblage of Ngandong (Java). However, on a detailed level, there are conspicuous differences. For example, Duboisia santeng, Axis lydekkeri, and Epileptobos groeneveldtii are absent in Ngandong and were generally believed to have become extinct by the end of the Middle Pleistocene. Their presence in the Madura Strait assemblage shows that the species persisted on the lowland plains. Interesting is also the presence of more rarely encountered species such as Varanus komodoensis, Macaca sp. and Homo erectus on the lowland plains. Metrically, Elephas sp. molars from the Madura Strait site are relatively high-crowned, whereas Bos palaeosondaicus is relatively small-statured compared to its Middle Pleistocene relatives from Java. The fossil fauna from the Madura Strait reflects a thus far unknown vertebrate coastal lowland community that lived in and around the lower reaches of the Middle Pleistocene Solo River.