A new large squalodelphinid (Cetacea, Odontoceti) from Peru sheds light on the Early Miocene platanistoid disparity and ecology

Abstract

The South Asian river dolphin (Platanista gangetica) is the only extant survivor of the large clade Platanistoidea, having a well-diversified fossil record from the Late Oligocene to the Middle Miocene. Based on a partial skeleton collected from the Chilcatay Formation (Chilcatay Fm; southern coast of Peru), we report here a new squalodelphinid genus and species, Macrosqualodelphis ukupachai. A volcanic ash layer, sampled near the fossil, yielded the 40Ar/39Ar age of 18.78 ± 0.08 Ma (Burdigalian, Early Miocene). The phylogenetic analysis places Macrosqualodelphis as the earliest branching squalodelphinid. Combined with several cranial and dental features, the large body size (estimated body length of 3.5 m) of this odontocete suggests that it consumed larger prey than the other members of its family. Together with Huaridelphis raimondii and Notocetus vanbenedeni, both also found in the Chilcatay Fm, this new squalodelphinid further demonstrates the peculiar local diversity of the family along the southeastern Pacific coast, possibly related to their partition into different dietary niches. At a wider geographical scale, the morphological and ecological diversity of squalodelphinids confirms the major role played by platanistoids during the Early Miocene radiation of crown odontocetes.

Extensive Diversity and Disparity of the Early Miocene Platanistoids (Cetacea, Odontoceti) in the Southeastern Pacific (Chilcatay Formation, Peru)

“…42, state 1). The new phylogeny here presented supports the referral of Furcacetus to the Squalodelphinidae, although the relationships within this family remain poorly resolved, as in previous analyses [24,26,108]. However, the Adams consensus tree shows a more satisfactory result, with Furcacetus in a derived position among squalodelphinids, forming a clade together with Huaridelphis and Medocinia + Squalodelphis.…”

“…The premaxillary sac fossae are moderately transversely concave, they slope medioventrally, and they have the same transverse width. The right ascending process of the premaxilla ends with a posterior point incised by a notch followed anteriorly by a longitudinal wide groove, similar to the premaxillary cleft described in Waipatia [99], also observed in most other platanistoids (e.g., [24,26,112]), and in several other archaic odontocetes (e.g., [105,118]). The left ascending process of the premaxilla has a rounded posterior margin without incision or groove.…”

“…The temporal fossa is significantly elevated (ratio between the vertical height of the fossa, in lateral view, and the vertical distance from the lower margin of the occipital condyles to the vertex of the skull = 0.70), as in Ensidelphis, Macrosqualodelphis, Notocetus, and Platanista. The temporal fossa of Furcacetus is also anteroposteriorly elongated (ratio between the horizontal length and vertical height = 1.42), similarly to Macrosqualodelphis and Notocetus [26]. Premaxilla.…”

“…More specifically, this paper focuses on the fossils of the Ullujaya-Zamaca assemblage belonging to the superfamily Platanistoidea, an odontocete clade that underwent a major radiation during the early Miocene and which today is only represented by the South Asian river dolphin (Platanista gangetica) confined to the freshwaters of the Indus and Ganges river systems [23]. Following other works already published by us on the platanistoids from the Chilcatay Formation [24][25][26], the fossils here described further support the great diversity and morphological disparity of this clade. On the whole the Eocene-Pliocene sedimentary succession of the East Pisco Basin represents one of the most significant marine vertebrate Lagerstätte of the Cenozoic Era due to the exceptional preservation and the elevated concentration of fossils [21,22,[40][41][42][43][44][45][46] referred to cetaceans (archaeocetes [47][48][49]; odontocetes [24,25,[50][51][52][53][54][55][56][57][58][59][60][61][62]; mysticetes [10,[63][64][65][66][67][68]), pinnipeds [69,70], marine birds [71]…”

The shark-toothed dolphin Squalodon (Cetacea: Odontoceti) from the remarkable Montagna della Majella marine vertebrate assemblage (Bolognano Formation, central Italy)

“…Possibly among the major marine predators of their time, squalodontids are often seen as bridging the anatomical gap between the archaic Oligocene odontocetes and their late Miocene to Holocene relatives (GODFREY, 2013;MARX et al, 2016). For a long time, these longirostrine toothed whales have been regarded as members of the crown Odontoceti clade, in the superfamily Platanistoidea, which also includes the South Asian river dolphin Platanista gangetica (e.g., MUIZON, 1991MUIZON, , 1994FORDYCE, 1994;FORDYCE & MUIZON, 2001;TANAKA & FORDYCE, 2016); now however, many phylogenetic analyses recover squalodontids as late-branching stem odontocetes (e.g., GEISLER & SANDERS 2003;TANAKA & FOR-DYCE, 2014;CHURCHILL et al, 2016;BIANUCCI et al, 2018a). The current lack of consensus upon the phylogenetic position of the squalodontids is highlighted by the observation that several recent papers dealing with the affinities of the heterodont long-snouted toothed whales present both the aforementioned solutions as plausible (e.g., TANAKA & FORDYCE, 2015a, 2015bLAMBERT et al, 2018;MUIZON et al, 2018;VIGLINO et al, 2018).…”