Rattus rattus

Rattus rattus View in CoL species complex

Two partial dentaries ( Fig. 6 View Figure 6 ) are identified as a member of the “ Rattus rattus Species Complex” (see Aplin et al. [2003, 2011] for a discussion of taxonomic issues in this group). Both specimens were recovered from with the upper 20 cm of the Daeo Cave no. 2 deposit. They appear less mineralized than the other subfossil remains and may represent a more recent addition to the archaeological deposit. It is not possible at present to identity the subfossil taxon any more precisely, pending clarification of species boundaries within the Rattus rattus Species Complex ( Aplin et al., 2003, 2011). In recent decades, many authors have referred to populations previously identified as “ Rattus rattus ” in the southeast Asian region as a separate species, Rattus tanezumi , following Musser and Carleton (2005). However, the true taxonomic situation is much more complex, involving various evolutionary lineages, with differential humanmediated dispersal histories, that are closely related to Rattus rattus in the strict sense (e.g., Aplin et al., 2011; Louys et al., 2020). The widespread commensal Rattus tiomanicus , usually considered to be restricted to the continental shelf of Sundaland (e.g., Musser & Newcomb, 1985; Corbet & Hill, 1992; Musser & Carleton, 2005) is also a member of the Rattus rattus Species Complex ( Aplin et al., 2011) and has recently been identified living in Wallacea, on the island of Halmahera ( Fabre et al., 2023). This points to a need to more firmly resolve the taxonomy of all commensal mediumsized Rattus populations, both modern and Holocene, that have been referred to Rattus rattus , Rattus tanezumi , and Rattus tiomanicus in recent publications. In any case, the subfossil specimens under discussion would traditionally be identifiable as “ Rattus rattus ” (and more recently as “ R. tanezumi ”) and pending further clarifying work, we refer to these specimens from Daeo Cave no. 2 as “ Rattus sp. cf. rattus .”

These subfossil specimens referred to “ Rattus sp. cf. rattus ” are immediately distinguished from R. morotaiensis by their smaller and higher-crowed molars ( Table 1 View Table 1 ; Fig. 6 View Figure 6 ), less crenulated enamel, more elongate and unevenly bilobed anterior lamina on M 1, shallower lower incisor and various details of dentary morphology including the lower placement of the mandibular foramen. They differ from R. nitidus , another commensal species in the region, in the form of the angular process of the dentary, which is narrower and projects further posteriorly in R. nitidus . Rattus nitidus is native to mainland Southeast and EastAsia but occurs as a consequence of human introduction in several parts of island SoutheastAsia including, relative to Morotai, the island of Seram to the south, Sulawesi to the west, and the Vogelkop Peninsula of New Guinea to the east, as well as from Luzon in the Philippines and Palau in Micronesia ( Musser & Newcomb, 1985; Helgen, 2003). The timing and pattern of spread of R. nitidus , which usually occurs as an introduced species in montane contexts, has received less attention than the dispersal of other murine commensals in the region and remains a fascinating area of study for archaeologists, geneticists, and mammalogists in the future, as it may illuminate important aspects of human history across the archipelagos of the Asia-Pacific.