A Miocene relative of the Ganges River dolphin from the amazonian basin

Abstract

Today, only three odontocete (toothed whales) genera are restricted to freshwater habitats: the Amazon River dolphin (Inia geoffrensis), the possibly extinct Yangtze River dolphin (Lipotes vexillifer) (Turvey et al., 2007), and the Ganges and Indus river dolphins (Platanista gangetica). Even if their phylogenetic relationships are still debated, Inia, Lipotes, and Platanista are now recognized as belonging to different families (Iniidae, Lipotidae, and Platanistidae, respectively) on the basis of morphological and molecular characters (Muizon, 1988; Cassens et al., 2000; Hamilton et al., 2001; Nikaido et al., 2001; McGowen et al., 2009; Geisler et al., 2011, 2012). Considering their geographic distribution and the discovery of some fossil relatives in marine deposits, for example, the iniid Meherrinia, the lipotid Parapontoporia, and the platanistids Prepomatodelphis, Pomatodelphis, and Zarachis, the current habitat of freshwater dolphins must be explained by independent episodes of colonization of freshwater environments (Cassens et al., 2000; Geisler et al., 2011, 2012). Several fragmentary fossil specimens, isolated teeth or jaw fragments, have been tentatively attributed in the past to unknown species that are thought to be closely related to extant river dolphin genera (e.g., Zhou et al., 1984; review in Muizon, 1988), but until now there were no diagnostic fossil remains that could provide clues about the early steps of these colonization episodes. More specifically, the fossil record of Platanistinae, the subfamily including the extant Platanista, is scarce, with only one tentative record from early Miocene, coastal deposits of Oregon, north Pacific. This specimen consists of an isolated mandibular symphyseal region that is transversely compressed (Barnes, 2006). Even if we consider this attribution as valid, a long ghost lineage characterizes most of the history of the Platanistinae, the latter having diverged from the extinct subfamily Pomatodelphininae since at least the latest early Miocene (Barnes, 2002, 2006). We present here a new fossil platanistine specimen: a periotic from the middle Miocene of Peruvian Amazonia. This highly diagnostic ear bone partly fills the ghost lineage mentioned above and provides insights on the shifts to freshwater environments by various odontocete clades, a phenomenon probably underestimated due to the lack of fossils from the freshwater sedimentary record, and probably not just limited to extant clades of freshwater odontocetes (Fordyce, 1983).