Thursday, April 19, 2018

[PaleoMammalogy • 2018] Macrosqualodelphis ukupachai • A New Large Squalodelphinid (Cetacea, Odontoceti) from Peru Sheds Light on the Early Miocene Platanistoid Disparity and Ecology

Macrosqualodelphis ukupachai  
Bianucci, Bosio, Malinverno, de Muizon, Villa, Urbina & Lambert, 2018

   DOI: 10.1098/rsos.172302 

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.

KEYWORDS: Odontoceti, Squalodelphinidae, Early Miocene, Peru, phylogeny, palaeoecology

Systematic palaeontology
Cetacea Brisson, 1762
Neoceti Fordyce and Muizon, 2001

Odontoceti Flower, 1867
Platanistoidea Gray, 1863
Squalodelphinidae Dal Piaz, 1917

Type genus. Squalodelphis Dal Piaz, 1917

Other genera included. Huaridelphis, Medocinia, Notocetus, Phocageneus.

Macrosqualodelphis, gen. nov.

Etymology. From ‘Macro’, large, and ‘Squalodelphis’ the type genus of the family. Gender masculine.

Macrosqualodelphis ukupachai, sp. nov.

Holotype and only referred specimen. MUSM 2545 consists of a skull lacking the anterior portion of the rostrum, the ear bones, both mandibles and the hyoid bones. The ventralmost portion of the rostrum and of the basicranium is worn along a plane slightly anterodorsally sloping with respect to the horizontal plane of the skull (erupted portion of maxillary teeth, basioccipital crests, ventral part of exoccipitals and postglenoid processes of squamosals missing). MUSM 2545 also preserves three detached anterior teeth; the atlas, two thoracic, two lumbar and eight caudal vertebrae; the left humerus, radius and incomplete ulna; one phalanx and one metacarpal; and two small fragments of ribs.

Type locality. About 3 km south of the fossiliferous Cerro Colorado locality, Western Ica Valley, Ica Region, southern Peru. 710 m above sea level. The holotype was discovered and collected by one of the authors (M.U.).

Etymology. From ‘Uku Pacha’ (Uku = withininsidePacha = Earth), the Inca lower world, located below the Earth's surface, in reference to the discovery of the specimen buried in sediment.

Figure 16. Skeletal remains and inferred body outline of the squalodelphinids from the early Burdigalian of the Chilcatay Fm (Pisco Basin, Peru) and skeletal and body outline of the extant P. gangetica. Body lengths based on the Pyenson & Sponberg [2011] equation for the fossils and on Jefferson et al. [2008] for the extant P. gangetica.

Cranium of the holotype (MUSM 2545) of Macrosqualodelphis ukupachai, from the early Burdigalian of the Chilcatay Fm (Pisco Basin, Peru).

Figure 3. (a) Dorsal view; (b) corresponding explanatory line drawing;   Linear hatching indicates major breaks and cross-hatching areas covered by the sediment.
Figure 4. (a) Ventral view; (b) corresponding explanatory line drawing;  Linear hatching indicates major breaks, cross-hatching areas covered by the sediment and dark shading worn surface.
Figure 5. (a) Right lateral view; (b) corresponding explanatory line drawing; (c) left lateral view. Cross-hatching indicates supporting frame.

Macrosqualodelphis ukupachai is a new species of the extinct platanistoid family Squalodelphinidae based on a well-preserved partial skeleton collected from the Early Miocene (ca 19–18 Ma) fossiliferous beds of the Chilcatay Fm outcropping in the Western Ica Valley (southern coast of Peru). The age of this skeleton is further constrained via 40Ar/39Ar dating of a local volcanic ash layer to 18.78 ± 0.08 Ma (early Burdigalian).

Our phylogenetic analysis supports the referral of M. ukupachai to the monophyletic family Squalodelphinidae, of which it constitutes the earliest diverging lineage.

The main distinctive character of M. ukupachai is its large size: its estimated TBL is approximately 3.5 m, significantly larger than all other known squalodelphinids, including N. vanbenedeni (2.5 m) and H. raimondii (2.0 m), both also found in the Chilcatay Fm. Combined with cranial and dental features (robust rostrum less tapered than in other squalodelphinids, large temporal fossa, prominent nuchal and temporal crests, and more robust teeth), the large body size of M. ukupachai suggests that this squalodelphinid was able to prey upon larger prey items. Consequently, M. ukupachai would have been positioned higher along the local trophic chain than the roughly contemporaneous N. vanbenedeni and H. raimondii. Therefore, it is suggested that the squalodelphinid diversity, both locally and worldwide, could be related to their partition into different dietary niches, as is observed in the extant delphinids.

This new record further illustrates the first, Early Miocene, broad radiation of crown odontocetes in marine environments, with a major contribution of homodont platanistoids. This Early Miocene morphological and ecological diversification of platanistoids (including squalodelphinids) was followed by the radiation of delphinidans (porpoises, true dolphins and relatives) during the Middle–Late Miocene. The only extant survivor of the platanistoid ‘golden age’ is the endangered South Asian river dolphin P. gangetica, confined in freshwater ecosystems of the Ganges, Indus and Brahmaputra river basins.

Giovanni Bianucci, Giulia Bosio, Elisa Malinverno, Christian de Muizon, Igor M. Villa, Mario Urbina and Olivier Lambert. 2018. A New Large Squalodelphinid (Cetacea, Odontoceti) from Peru Sheds Light on the Early Miocene Platanistoid Disparity and Ecology. Royal Society Open Science. 5(4)  DOI: 10.1098/rsos.172302

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