The Morphology and Systematics of Neogene Mysticeti from Australia and New Zealand

Bearlin, Robert Kingston

Until recently, Neogene mysticete fossils from Australia and New Zealand had received scant attention as only one species "Aglaocetus ? n.sp." Glaessner, was described formally. There now appears to be a small, but taxonomically significant fauna preserved in these countries.
Seven skulls - consisting of three cetotheres and four balaenopterids - form the nucleus of this fauna. The cetothere, Aglaocetus ? n.sp. (Early Miocene, South Australia), is probably not congeneric with Aglaocetus Kellogg, but rather, has close affinities with Parietobalaena Kellogg, a genus known previously only from the Early-Middle Miocene of the western North Atlantic and possibly from the eastern North Pacific. Another cetothere (Early Miocene, Victoria) is identified as a new species of Pelocetus Kellogg, a genus known previously only from the North Atlantic Early-Middle Miocene. The third cetothere (Middle Miocene, Victoria) is insufficiently complete to enable detailed comparisons, but appears to differ from all previously described taxa. Two balaenopterids are represented by incomplete skulls. The first (Middle Miocene, Victoria) represents an early global record of the Balaenopteridae. This specimen is apparently the most primitive balaenopterid documented to date, but it is not interpreted as a precursor of all balaenopterid species, nor is the western South Pacific necessarily interpreted as the centre of origin for the Balaenopteridae. The other specimen is of uncertain age (probably Neogene, South Island, . New Zealand) and represents a species of Balaenoptera Lacepede, but is too incomplete to identify its taxonomic relationships in detail. The other balaenopterid skulls are fairly complete. One skull (latest Miocene, Victoria) is a new species of Megaptera Gray and is interpreted as the most primitive megapterine known to date, while the other skull (Early Pliocene, North Island, New Zealand) is identified as a new species of Balaenoptera Lacepede and is interpreted as a structural intermediate between primitive balaenopterines such as Balaenoptera cuvierii (Fischer) and extant species of Balaenoptera Lacepede.
Isolated skull fragments and earbones represent at least four balaenid, seven balaenopterid and one cetothere species. These fossils are known from the Early-Middle Miocene Murray Basin, South Australia, as probable remanie from phosphatic nodule beds of Late Miocene age in Victoria and from stata of mid Pliocene age in Tasmania, as well as scattered occurrences throughout the Neogene of New Zealand. Most of these specimens are identifiable to family, although possible conspecifics have been identified between the two countries.
Mysticetes form rare, but conspicuous fossils; indeed most fossil species are represented by single incomplete specimens. The paucity of this record restricts interpretations of patterns or rates of evolution, but some broad trends are apparent. Mysticetes probably arose in the Early Oligocene, although the only established toothed mysticete fossils are relicts of Late Oligocene age and post-date cetotheres which arose by the earliest Late Oligocene. Cetotheres are edentulous and probably possessed baleen. Similarities with extant balaenopterids suggest that cetotheres fed by engulfment. Balaenids arose by the earliest Miocene. Extant balaenids and presumably all fossil balaenids possessed a narrow, arched rostrum and elongate baleen which are interpreted as adaptations for browsing. The feeding mechanism of neobalaenids is not documented, but synapomorphies shared with balaenids suggest functional similarities. Indeed, the Neobalaenidae and Balaenidae are interpreted as sister groups, so this implies that neobalaenids also arose by the Early Miocene, although there are no confirmed neobalaenid fossils. The Balaenopteridae and Eschrichtiidae are interpreted as sister groups which presumably arose by the Middle Miocene based on early records of balenopterids, although eschrichtiid fossils are known confidently only from the Late Pleistocene. By the Middle Miocene the Mysticeti possessed an ecological diversity comparable with that present today. The Megapterinae and Balaenopterinae differentiated by the Late Miocene. Balaenopterids differ from cetotheres by possessing abruptly depressed supraorbital processes of the frontals, transversely wider lateral processes of the maxillae, a more elongate rostrum and more widely-bowed mandibles. These differences suggest relative and absolute enlargement of the surface area of the baleen, expansion of the temporal musculature and expansion of the ventral pouch in response to an increase in absolute body size. Comparable details of structural changes in eschrichtiids, neobalaenids and early balaenids are restricted by a poor fossil record. Eschrichtiids are convergent with balaenids and rieobalaenids in the presence of a gently arched rostrum and transversely flattened-mandibles, but eschrichtiids possess the widest range of feeding strategies seen in all mysticetes - engulfing, browsing and grazing.
Presumably evolution within the Mysticeti was influenced by external (e.g. paleoceanic or pilleoecological) factors, but these cannot be identified for the Neogene. Similarly, zoogeographic trends below genus level remain uncertain as no species have been identified in either Australia or New Zealand which are known from elsewhere, but genera such as Balaena Linnaeus, Balaenoptera Lacepede, Parietobalaena Kellogg and Peloceteus Kellogg probably possessed transequatorial sister species.

xvi, 212 pages, b+w plates

1987Bearlin