New specimens and species of the Oligocene toothed baleen whale Coronodon from South Carolina and the origin of Neoceti

Abstract

Baleen whales (Mysticeti) are gigantic filter-feeding cetaceans possessing the unique soft tissue structure baleen and lacking adult teeth; Oligocene fossils have revealed a wealth of early diverging tooth-bearing mysticetes highlighting the transition from archaeocete ancestors to early toothless baleen-bearing eomysticetid whales. The archaeocete-like, toothed mysticete Coronodon havensteini from the lower Oligocene Ashley Formation of South Carolina possesses a number of peculiar aspects of feeding morphology suggesting dental filter-feeding in the earliest diverging mysticete lineage. New fossils of Coronodon are described in detail, including (1) supplementary description of the holotype skull and skeleton of Coronodon havensteini; (2) description of two new juvenile skulls of C. havensteini and a partial skull and postcranial skeleton of an adult; (3) description of the new species Coronodon planifrons n.sp.; and (4) description of the new species Coronodon newtonorum. New specimens of Coronodon havensteini include a partial adult skeleton preserving new elements for the species including incisors, numerous upper premolars and molars, lower m4, scapula, lumbar, and caudal vertebrae, and two juvenile skulls with tympanoperiotics and teeth. Fossils from the overlying unit, the Chandler Bridge Formation, represent two new species: Coronodon newtonorum n. sp. and Coronodon planifrons n. sp. Coronodon newtonorum possesses a concave-up alveolar profile, a mandibular condyle elevated far above the toothrow, and a gracile periotic resembling those of juvenile C. havensteini. Coronodon planifrons n. sp. possesses a horizontal supraorbital process, successively smaller upper molars, massively inflated periotic, and longer intertemporal region. Coronodon planifrons n. sp. preserves one of the most complete vertebral columns among toothed mysticetes, indicating nine thoracic vertebrae, ten lumbar vertebrae, and at least 20 caudal vertebrae. The column exhibits a somewhat stabilized caudal peduncle with enlarged lumbocaudal vertebrae, and rectangular terminal caudals indicate the presence of tail flukes. Juvenile skulls reveal several ontogenetic trends in Coronodon havensteini, including the anterior migration of the orbitotemporal crest, anteroposterior elongation of the intertemporal region, inflation of the body of the periotic, enlargement of the tympanic bulla, and continued postnatal emergence of the premolars and molars from their alveoli. Disarticulated skulls suggest a degree of rostral kinesis in this genus. Phylogenetic analysis of the largest assembled supermatrix of Mysticeti (n =138 OTUs; four archaeocetes, 10 odontocetes, 124 mysticetes; 391 morphological and 27,225 molecular characters) confirms placement of Coronodon as the earliest diverging lineage of Mysticeti under equally weighted analyses whereas implied weighting places Coronodon and similar taxa outside Neoceti, prompting a review of character transformations at the base of Neoceti.