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| Janjucetus dullardi Duncan, Rule, Park, Evans, Adams & Fitzgerald, 2025 |
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.
Australia, Cetacea, Janjucetus, Mammalodon, Mammalodontidae, Mysticeti, Oligocene, ontogeny, phylogeny
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| Artist’s reconstruction of the complete skull of Janjucetus dullardi. Parts preserved in the fossil are white and light grey. Art by Ruairidh Duncan |
Janjucetus dullardi sp. nov.
Etymology: The species name dullardi honours Mr Ross Dullard, who discovered the holotype.
Type locality, horizon, and age: The holotype of Janjucetus dullardi (NMV P256471) was discovered near the town of Jan Juc in the lowest beds of the exposed onshore Jan Juc Marl at Half Moon Bay, southwest of Bird Rock (Fig. 2) at approximately 38°21′03″S, 144°18′02″E (precise locality details are available on request). The lithology of this cliff face describes a neritic, inner mid-shelf environment, which coarsens upwards, and with relatively low abundance of infauna (Li et al. 1999, McLaren et al. 2009). The type of J. dullardi was found near the base of this outcrop, exposed beneath a wave-eroded overhang, slightly above the level of the tidal beach sands (Fig. 1). The lithology of this horizon [unit 1 and 2 of Reeckmann (1979)] is one of a silty, sandy grey marl, interspersed with bedding planes of coarse silts to fine sandstones and rich in glauconite. A horizon ∼3 m above the fossil-producing layer was aged to 24.67 Mya using strontium isotope ratios (Korasidis et al. 2018). The top of the Bells Headland outcrop, interpreted as ∼2 m stratigraphically lower than the base of the Bird Rock outcrop, was aged to 26.05 Mya at a point correlated to ∼2.5 m below the horizon containing NMV P256471 using the same methodology (Korasidis et al. 2018). We therefore give an estimated age of this specimen of between 24.67 and 26.05 Mya (middle Chattian).
Diagnosis: We interpret J. dullardi as a mammalodontid on the basis of: the presence of an involucral concavity on the medial surface of the tympanic bulla ventral to the involucral ridge; a dorsomedially extended and anteroposteriorly wide crista transversa, which reaches to the level of the rim of the internal acoustic meatus medially (also seen in Salishicetus and Coronodon); and the presence of apicobasal ridges on both the buccal and lingual surfaces of the tooth crowns (also seen in Morawanocetus, Coronodon, and Llanocetus).
This specimen shares the following features with J. hunderi to the exclusion of Mammalodon: an obliquely truncated rather than squared anterior margin of the tympanic bulla; an inner posterior prominence of the tympanic bulla mediolaterally broader than the outer posterior prominence at the widest point of the bulla; periotic body and posterior process greatly exceed the posterior margin of the fenestra rotunda; anteroposterior and dorsoventral diameters of the anterior process of the periotic are subequal to one another; and the mandibular posterior postcanine teeth lack a distal accessory shelf near the base of the crown.
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| Janjucetus dullardi calf and mother swimming through the shallow seas, 25 million years ago. Art by Ruairidh Duncan |
Ruairidh J Duncan, James P Rule, Travis Park, Alistair R Evans, Justin W Adams and Erich M G Fitzgerald. 2025. An Immature Toothed Mysticete from the Oligocene of Australia and insights into mammalodontid (Cetacea: Mysticeti) Morphology, Systematics, and Ontogeny. Zoological Journal of the Linnean Society. 204(4); zlaf090. DOI: doi.org/10.1093/zoolinnean/zlaf090 [12 August 2025]
https://theconversation.com/a-cornucopia-of-tiny-bizarre-whales-used-to-live-in-australian-waters-heres-one-of-them-262806
