Saturday, March 15, 2014

[PaleoMammalogy • 2014] Semirostrum ceruttii • Unique Feeding Morphology in a New Prognathous Extinct Porpoise from the Pliocene of California

Semirostrum ceruttii 
Racicot, Deméré, Beatty & Boessenecker 2014 
extinct porpoise (Phocoenidae) from the Pliocene San Diego Formation, California.
PaleoArt: Bobby Boessenecker [i]

- A new extinct porpoise species possesses a prognathous fused mandibular symphysis
- This is the first described mammal with an elongate prognathous mandibular symphysis
- Tooth wear, postcrania, and innervation of mandible imply benthic probing function

 Skull and mandible of SDNHM 65276, holotype specimen of Semirostrum ceruttii, a strange porpoise (Phocoenidae) from the Pliocene San Diego Formation, California.

Modern porpoises (Odontoceti: Phocoenidae) are some of the smallest cetaceans and usually feed near the seafloor on small fish and cephalopods. Within both extinct and extant phocoenids, no evidence for specialized mandibular morphology has been documented. Here we describe a new species of extinct porpoise, Semirostrum ceruttii, from the marine Pliocene San Diego (4.2–1.6 mega-annum, Ma) and Purisima (5–2.5 Ma) formations of California. The mandibles comprise a long, fused, and nearly edentulous prognathous symphysis, extending farther beyond the rostrum than in any known mammal. Phylogenetic analyses based on morphology reconstruct Semirostrum ceruttii as sister to extant (crown) porpoise species with moderate support. We describe the spectacularly preserved holotype specimen based on computed tomography (CT) scans, which allowed visualization of the elongate mental and accessory canals within the symphysis. The elongate canals are similar to those found in Rynchops birds and were likely involved in sensory function. Oblique labial wear facets present on numerous small conical mandibular teeth posterior to the symphysis suggest regular contact with benthic substrate. The unique mandibular and dental characteristics, along with robust scapulae, sternum, and unfused cervical vertebrae, support the interpretation that this species employed a form of benthic skim feeding by using its mandible to probe for and obtain prey.

Figure 1. Skull, Mandible, Periotic, and Postcranial Morphology of Semirostrum ceruttii Holotype Specimen SDSNH 65276
(A) Schematic reconstruction based on composite postcrania and skull material. (B) Labeled outlined 3D isosurface reconstructions based on CT scans. From top to bottom: right lateral view of skull, right lateral view of mandible, dorsal view of skull, dorsal view of mandible. (C) Left lateral view of black skimmer bird Rynchops niger with analogous mandibular morphology. (Photo by Don Faulkner, Wikimedia Commons; image cropped and converted to grayscale.) (D) Labeled 3D isosurface reconstruction of holotype skull in posterior view. (E) Labeled 3D isosurface reconstructions of right periotic based on CT scans in dorsal (top) and ventral (bottom) views.

Rachel A. Racicot, Thomas A. Deméré, Brian L. Beatty, Robert W. Boessenecker. 2014. Unique Feeding Morphology in a New Prognathous Extinct Porpoise from the Pliocene of California. Current Biology. DOI: 10.1016/j.cub.2014.02.031
Semirostrum ceruttii: Scientists Discover Extinct Porpoise with Unique Underbite
Extinct Big-Jawed Porpoise Fossil Discovered off California Coast