Monday, September 28, 2020

[PaleoMammalogy • 2020] Rhaphicetus valenciae • A New Longirostrine Sperm Whale (Cetacea, Physeteroidea) from the lower Miocene of the Pisco Basin (southern coast of Peru)

Rhaphicetus valenciae 
Lambert, Muizon, Urbina & Bianucci, 2020

The modern sperm whales Kogia and Physeter (superfamily Physeteroidea) represent highly disparate, relict members of a group of odontocetes that peaked in diversity during the middle to late Miocene. Based on a highly informative specimen (including the cranium with ear bones, mandibles, teeth and some postcranial elements) from the lower Miocene (early Burdigalian, 19–18 Ma) of the Chilcatay Formation (Pisco Basin, Peru), we describe here a new genus and species of physeteroid, Rhaphicetus valenciae gen. et sp. nov. The latter is one of the geologically oldest physeteroids. This medium-sized species (estimated body length between 4.7 and 5.7 m) differs from all other physeteroids by the following, probably autapomorphic, features: a narrow, cylindrical rostrum comprising nearly 75% of the condylobasal length; the two main dorsal infraorbital foramina located posterior to the antorbital notch; an upper tooth count of at least 36 teeth per quadrant; and anterior-most upper alveoli filled by thick bony pads. Our phylogenetic analysis recovers R. valenciae as one of the earliest branching stem physeteroids. The highly unusual filling of the anterior upper alveoli by bony pads is interpreted as part of a mechanism leading to the loss of apical and subapical upper teeth. By comparison with other odontocetes displaying some degree of anterior reduction of the dentition, this condition may have corresponded to the rostrum being anteriorly longer than the mandible. The elongated rostrum with a circular cross-section, the long temporal fossa, and the high number of slender, pointed upper and lower teeth all suggest that R. valenciae used its dentition to grasp relatively small prey, possibly via rapid movements of the head. On the one hand, this new Peruvian record increases our knowledge of the morphological disparity of sperm whales during the Miocene. On the other hand, it may provide clues to the ancestral morphotype for all physeteroids.
Keywords: Burdigalian, dental reduction, functional morphology, palaeobiology, phylogeny, stem Physeteroidea

Reconstruction of the skull of Rhaphicetus valenciae MUSM 2543 (holotype) in right lateral view. Stippled lines for main reconstructed bony parts; dark grey shading for a hypothetical reconstruction of the soft tissue outline of the head, including an anteriorly short spermaceti organ. The anterior tip of the mandibles being missing, the anterior extent of the lower jaw remains unknown.

Cranium of Rhaphicetus valenciae MUSM 2543 (holotype) in dorsal view and ventral view. 

Cranium of Rhaphicetus valenciae MUSM 2543 (holotype) 
in ventral view and right lateral view.  

Systematic palaeontology
Order Cetacea Brisson, 1762
Pelagiceti Uhen, 2008
Neoceti Fordyce & Muizon, 2001

Suborder Odontoceti Flower, 1867b
Superfamily Physeteroidea Gray, 1821

Genus Rhaphicetus gen. nov.

Type species. Rhaphicetus valenciae sp. nov.

Derivation of name. From the ancient Greek rhaphisneedle, and from the Latin cetuswhale: the whale with a needle-shaped rostrum.

Rhaphicetus valenciae sp. nov. 

Derivation of name. valenciae, honouring Dr Niels Valencia Marciano Chacón, a biologist at the Universidad Nacional Mayor de San Marcos studying plant ecology and the director of the MUSM, for his constant support for the palaeontological activities at that institution, including many fruitful palaeontological expeditions in the Pisco Basin.

Olivier Lambert, Christian de Muizon, Mario Urbina and Giovanni Bianucci. 2020. A New Longirostrine Sperm Whale (Cetacea, Physeteroidea) from the lower Miocene of the Pisco Basin (southern coast of Peru). Journal of Systematic Palaeontology. 18(20); 1707-1742. DOI: 10.1080/14772019.2020.1805520

18 Million Year Old Sperm Whale With 'Needle-Shaped' Snout