Systematics of the latest Oligocene to the earliest Miocene mysticeti from New Zealand

Ichishima, Hiroto, 1965-

Oligocene is a critical period in the early history of mysticetes (Cetacea: Mysticeti), but details of evolutionary pattern and process are known poorly. Fossil mysticetes from Zealand are an important source of information; five specimens from latest Oligocene to '.c.ru.u"'"' Miocene strata are described here (Chapter VII). These fossils are not the oldest records mysticetes from New Zealand nor in the southern hemisphere, but they do provide some information on the morphological and taxonomic diversity of early mysticetes. Two of fossils represent a new genus, Pakehocetus, which contains P. hakataramea n. sp. Another two representMauicetus (sensu stricto), species not determinable. The fifth specimen is a · young and thus indeterminable archaic baleen whale. All are from the Kokoamu Greensand or · Otekaike Limestone of the ?upper Whaingaroan-Duntroonian-Waitakian Stages (roughly later · Oligocene to earliest Miocene; -30 Ma to -23 Ma) of the Waitaki V alley region of North Otago and South Canterbury. Sediment character indicates a depositional setting of mid-outer shelf · mostly below the storm wave base (Chapter Ill). Many aspects of the biology of recent mysticetes are not fully understood. However, by : considering features of recent and extinct species through comparative and functional .· morphology, some tantalising insights may be gained into evolutionary change of some structures in mysticetes (Chapter IV). Contrary to the predictions of recent molecular studies, there is no evidence of morphological similarity between mysticetes and sperm whales (Physeteridae) (Chapter V). The classification of those fossil mysticetes called "cetotheres" (the family Cetotheriidae) is a persistent problem which must be addressed. In previous literature, the genus Mauicetus (which is close to Pakehocetus of this work) has been identified as one of the most primitive and oldest "cetotheres". Cladistically, however, Pakehocetus andMauicetus are grouped as sister taxa, and Cetotherium, which is the type genus of the family Cetotheriidae, can be put apart from those New Zealand specimens (Chapter VIII). This reinforces suggestions that Cetotheriidae is a nonmonophyletic group. The concept (diagnosis) of Cetotheriidae should be rethought, using Cetotherium as a core-group of the family (Chapter VI). In the five study specimens the skull has (or is inferred to have had) a relatively flat and toothless rostrum, a fairly anteriorly-lying nostril, a gradually rather than abruptly depressed frontal, a relatively long sagittal crest on the mid-dorsal line, a triangular supraoccipital, a canal formed by the squamosal and periotic (probably associated with the cranial vascular system), a primitive double posterior pedicle of the bulla, and a relatively large mandibular foramen. - These features are probably primitive, but in the periotic, the anteriorly-directed internal acoustic . ·-,,•, ' . I ;\ ,· ,' meatus of the study specimens is peculiar and probably' derived. ·This feature could unite all the specimens described here as a clade of as-yet unresolved rank. Other features of note in some specimens are as follows: the nasal is proportionately long; minor palatal foramina are present; the cervical vertebrae are compressed anteroposteriorly' but are unfused; and the humems is IV long relative to the radius or ulna. Some functional interpretations are possible. The skull architecture of Pakehocetus best fits the feeding model of "swallower" as seen in living rorquals. There is no evidence of feeding styles ("skimmer", "plower") seen respectively in modern right whales and gray whales. Using the seam-like structure of the bulla as a bench mark of the presumed ventral limit of the air sinus system of the cranium, the process of bullar rotation is demonstrated clearly. The New Zealand fossils are interpreted as intermediate, in terms of the degree of the bullar rotation, between archaeocetes or Late Oligocene' archaic mysticetes and modern forms (Chapter VII). Three clusters ( Caperea-Eubalaena, Pakehocetus-Mauicetus, and CetotheriumBalaenoptera- Eschrichtius) are cladistically recognised within mysticetes. However, incompleteness of specimens and/or a morphological gap between living and extinct species and also between living species prevents firm conclusions about their phylogenetic interrelationships (Chapter VIII).

xvii, 352 p., [20] leaves of plates : ill. (some col.), maps ; 30 cm.

1998Ichishima

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