The morphology and systematics of Mammalodon colliveri (Cetacea: Mysticeti), a toothed mysticete from the Oligocene of Australia

Abstract

Mammalodon colliveri is an unusual toothed archaic mysticete (Cetacea) from the Upper Oligocene Jan Juc Formation of south-east Australia. The morphology of the holotype skull and postcrania are described in detail. Superimposed on the generally plesiomorphic archaeocete-like morphology of Mammalodon are subtle mysticete synapomorphies. Derived features of Mammalodon include a short and bluntly rounded rostrum, reduced premaxillae, and anterodorsally directed orbits. Within Mysticeti, this suite of features is unique. The aberrant rostral morphology of Mammalodon suggests specialization for suction feeding. Janjucetus hunderi is placed in an expanded family Mammalodontidae. Phylogenetic analysis corroborates the monophyly of Basilosauridae, Neoceti, Odontoceti, and Mysticeti, and yields a novel hypothesis of toothed mysticete relationships: a basal clade of undescribed toothed mysticetes from South Carolina, a Llanocetidae + Mammalodontidae clade, and monophyletic Aetiocetidae are posited as successive sister taxa to edentulous baleen whales (Chaeomysticeti). Toothed archaic mysticetes clearly employed diverse prey capture strategies, with exaptations for filter feeding evolving sequentially in stem group Mysticeti. A stratigraphically calibrated phylogeny implies that the initial diversification of Mysticeti occurred during the Late Eocene.

Discovery of a sensory organ that coordinates lunge feeding in rorqual whales

“…3). Our analyses showed that the organ receives neurovascular bundles that emerge from vestigial alveolar foramina of the mandibles, a structure that is homologous with the lower first incisor tooth socket of fossil 'toothed' mysticetes [14][15][16] . The presence and identity of these neurovascular bundles, which contain branches of the mandibular division of the trigeminal nerve, were confirmed by subsequent dissection of the same specimen.…”

“…The origin of suspension feeding in mysticetes represents an evolutionary novelty with no parallel in any other mammalian or known synapsid lineage 15 . Moreover, the innovation of baleen preceded the origin of extremely large body sizes in mysticetes 16 , which rank among the largest vertebrates ever.Although mysticetes use several different modes of filter feeding 3 , rorquals exclusively engulf large volumes of prey-laden water in a single, rapid gulp 4 . In fin whales (Balaenoptera physalus), for example, the entire process of a lunge occurs in a short time span (,6 s), during which approximately 60-80 m 3 of water and prey are engulfed, a volume equal to or greater than that of the individual rorqual itself 17 (Fig.…”

“…The origin of suspension feeding in mysticetes represents an evolutionary novelty with no parallel in any other mammalian or known synapsid lineage 15 . Moreover, the innovation of baleen preceded the origin of extremely large body sizes in mysticetes 16 , which rank among the largest vertebrates ever.…”

A new group of late Oligocene mysticetes from México

“…Ventrolaterally, the basilar part extends forming a prominent and thick basioccipital crest with a slightly eroded convex surface. Basioccipital crest is long and bulbous, mediolaterally and dorsoventrally thickened, as in all mysticetes (Fitzgerald, 2010). The crest curves laterally in a right angle at the level of the postglenoid process.…”

“…Ventrally, the paroccipital process in the exoccipital is thick (~30 mm anterposterioly) and is exposed posteroventrally. Apparently, its surface has a broad attachment area for the abductor digastric muscle (see El Adli and Fitzgerald, 2010). It is smooth medially and becomes flat to slightly convex dorsally.…”