An immature toothed mysticete from the Oligocene of Australia and insights into mammalodontid (Cetacea: Mysticeti) morphology, systematics, and ontogeny

Abstract

Mammalodontids are a clade of toothed mysticetes known only from the Chattian of south-eastern Australia and New Zealand. Despite three named species (Janjucetus hunderi, Mammalodon colliveri, and Mammalodon hakataramea), the majority of mammalodontid material, largely represented by isolated teeth and ear bones found within the Torquay Basin (Victoria, Australia), remains undescribed. Here we describe a new species of mammalodontid, Janjucetus dullardi sp. nov., from the Jan Juc Marl. The holotype was found stratigraphically lower than M. colliveri and has morphology consistent with an immature stage of growth, plus near-unworn dental morphology. We identify a correlation between occipital condyle breadth and bizygomatic width and propose equations that permit us to estimate bizygomatic width and, subsequently, a body size estimate similar to the smallest known toothed mysticetes of ∼2.05-2.18 m. Phylogenetic analysis recovers J. dullardi as sister to J. hunderi within a monophyletic mammalodontid clade in all analyses. It also provides support for Mammalodontidae within Mysticeti, sister to an Aetiocetidae + Chaeomysticeti clade. Finally, revised diagnoses of Mammalodon and Janjucetus incorporating newly recogniszed tympanic and periotic characters permit the identification of isolated ear bones and inference of ontogenetic variation within Janjucetus.