Australodelphis mirus

Morphological description and paleobiological considerations of Australodelphis mirus (Cetacea, Odontoceti) from the Miocene of Antarctica

Australodelphis mirus is an extinct species of toothed whale discovered in middle Miocene deposits (approximately 12 to 14 million years ago) in the Sørsdal Formation of the Antarctic continent, specifically on Seymour Island (Weddell Sea). This taxon represents one of the few fossil marine mammal records from such high southern latitudes and offers key insights into the evolution of toothed whales during the Neogene in polar regions.

The type material of Australodelphis mirus , consisting of a nearly complete skull and associated vertebrae, exhibits an unusually elongated and narrow rostrum, with an extremely prolonged and thin rostral rostrum. This condition is convergent with that observed in modern beaked whales (family Ziphiidae), although Australodelphis does not belong to that clade. The shape of the rostrum, combined with the hydrodynamic design of the skull and jaws, suggests adaptations for suction feeding or rapid capture of small prey, such as cephalopods and fish, possibly in continental shelf or ice-edge environments.

A notable feature of A. mirus is the braincase, which exhibits specialized development of the ear cavities, with dense and compact tympanic bullae, compatible with directional hearing and echolocation capabilities, as is typical in odontocetes. However, unlike true beaked whales, Australodelphis displays a reduced but still present dental structure, with small, uniform teeth, distinguishing it from modern beaked whales, which have vestigial or absent dentition in most adult individuals.

The arrangement and morphology of the cervical vertebrae suggest that Australodelphis had limited cervical mobility, as is common in modern toothed whales adapted to a fully pelagic marine lifestyle. The estimated body size, based on cranial and vertebral proportions, suggests a total length of between 2 and 3 meters, similar to the size of some extant oceanic dolphin species.

Taxonomically, Australodelphis mirus has been placed within Odontoceti, but its precise phylogenetic position remains uncertain, displaying a mixture of traits that suggest affinities with both the Delphinoidea and more basal forms of toothed cetaceans. This combination of derived and primitive features, together with its unusual geographic distribution, supports the hypothesis that Australodelphis represents an endemic lineage adapted to cold marine conditions during a period of relative polar warmth in the middle Miocene.

In paleoenvironmental terms, the presence of Australodelphis in Antarctic latitudes suggests that toothed whales had already colonized cold-water habitats long before the consolidation of modern ice caps. Their anatomy indicates specialization for predation in deep or poorly lit waters, and their morphological convergence with beaked whales illustrates a recurring evolutionary trend in cetaceans toward elongated forms adapted to suction in different ecological contexts.

Approximate measurements of Australodelphis:

• Scale 1:35 Complete
• Length 71 mm
• Height 28 mm
• Width 20 mm
• Snout-tail length 85 mm
  
  
• Scale 1:20 Complete
• Length 124 mm
• Height 48 mm
• Width 34 mm
• Snout-tail length 150 mm