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Living baleen whales (suborder Mysticeti, in the mammalian
order Cetacea) include the biggest animals to have lived. They are well
adapted in terms of hydrodynamic, thermal and biological strategies to
life in water. But, they retain some typically mammalian attributes which
indicate a distant origin on land. These features include air-breathing,
live birth, a 4-chambered heart, warm blood, and middle ear ossicles. Specialised
feeding adaptations are particularly distinctive in mysticetes: they lack
teeth, but use baleen to filter food (mainly zooplankton) from the water.
Baleen plates hang from the upper jaw in a uniquely constructed skull.
Amongst living mysticetes, rorquals (Balaenopteridae) are fast-moving "gulp-feeding"
predators, while right whales (Balaenidae) are slow-moving "skimmers"
and gray whales (Eschrichtiidae) are "bottom-grubbing" filter-feeders.
Many mysticetes feed in polar waters, and migrate annually to the tropics
to breed.
For years, it was not clear how baleen whales evolved. Recently, the fossil
record has provided many insights. Fossils from New Zealand and the margins
of the Southern (Circum-Antarctic) Ocean, and from localities around the
North Pacific, are particularly important. The oldest southern filter-feeding
whales are dated at around 34 Ma (million years old), that is, from the
Eocene/ Oligocene boundary. These animals are transitional between extinct
archaic whales (archaeocetes) and modern baleen whales. They probably filter-fed
using teeth, perhaps assisted by "proto-baleen". At this time,
the Southern Ocean was starting to open as a result of continental drift,
with changes to climate, water circulation, and oceanic ecosystems. Perhaps
mysticetes evolved in response to such oceanic changes.
Mysticetes were quite diverse during Late Oligocene times (23-30 Ma). Toothed
mysticetes are known from scattered localities (e.g. Aetiocetus,
North Pacific; Mammaldon, Southwest Pacific). Also, by about 30
Ma, the first toothless baleen whales ("cetotheres") had evolved.
Like modern species, these had baleen in the thin flat upper jaw and a
cylindrical toothless lower jaw, but otherwise they were quite primitive
in structure. For example, they had a blowhole nearer the skull tip than
in modern whales, the ear region was less advanced, and the neck vertebrae
were not particularly compressed. These early baleen whales probably included
the ancestors of modern rorquals (gulpers). Other groups of baleen whales,
now extinct, indicate early "ecological experiments" which disappeared
for uncertain reasons.
During the Miocene (5-23 Ma), "modern" mysticetes diversified.
Right whales appeared before about 20 Ma, and rorqual-like animals evolved
perhaps by 15 Ma. By the end of Miocene time, and well before our own human-like
ancestors walked upright, baleen whales were structurally similar to modern
species. Within the last 5 million years, the problematic Cetotherium-group
of baleen whales finally disappeared, possibly linked with global cooling.
There are no good fossils for two distinctive living baleen whales, each
of which is placed in its own family: the gray whale (Eschrichtiidae),
and the pygmy right whale (Neobalaenidae).
Some generalisations can be suggested for mysticete evolution. Filter-feeding
is a key feature for mysticetes. (Odontocetes don't filter-feed, but are
echolocation assisted predators, while archaeocetes show no evidence of
filter-feeding or echolocation.) Different families of mysticetes are separated
from one another largely in the structure of the feeding apparatus, although
it is hard to understand the detailed functional significance of such differences.
Most mysticetes migrate into polar waters in summer, to feed on zooplankton.
Perhaps migration is a very ancient behaviour which appeared amongst the
earliest mysticetes. Because the nature, abundance and distribution of
food is governed by global geography and oceanic circulation (e.g. zooplankton
is abundant at convergences and the melting ice edge), changes in geography
and circulation over geological time have probably played a major role
in the evolution and extinction of baleen whales. Fossil hint that rorquals
are more diverse than right whales, perhaps because their feeding methods
allowed them to exploit more niches. In contrast to odontocetes, mysticetes
do not seem to have invaded fresh waters at any stage in their history.
Have they had the oceans to themselves? - perhaps, for no other marine
mammal group has taken up the lifestyle of filter-feeding.
Dr．Robert Ewan Fordyce
Associate Professor, Department of Geology，University of Otago, PO Box
56, Dunedin, NZ
fax 64-3-479-7527, tel 64-3-479-7510
web: http://www.otago.ac.nz/Geology/
Biography
Ewan Fordyce was appointed to the Geology staff
at the University of Otago in 1982. He is currently an associate professor.
(His other affiliation is as a research associate, Department of Vertebrate
Zoology, Museum of Natural History, Smithsonian Institution.) He teaches
paleontology, stratigraphy and related topics, such as microfossils, paleoecology,
origins of the New Zealand fauna and flora, evolution, and fossil vertebrates.
Originally he trained as a zoologist at the University of Canterbury. His
Ph.D. work was followed by a postdoctoral fellowship (to carry out research
on fossil whales) in the Department of Paleobiology at the Smithsonian
Institution in Washington D.C., and another postdoctoral fellowship in
the Department of Earth Sciences, Monash University, Melbourne. This work
led to ongoing research on the history and diversity of whales and dolphins,
based on living and fossil species from around the world. It also led to
study on other New Zealand fossil vertebrates, particularly penguins and
reptiles. Fordyce's publications deal with whales and dolphins (stranding,
anatomy, paleontology, evolution etc), fossil penguins, and other aspects
of the natural history of southern New Zealand.
Some articles
Fordyce, R. E., and Barnes, L. G. 1994. The evolutionary history of whales
and dolphins. Annual review of earth and planetary science 22: 419-455.
Carwardine, Mark, Hoyt, Erich, Fordyce, R. Ewan, and Gill, Peter. 1998.
Whales, dolphins & porpoises. (Nature Company Guides.). Time-Life Books.
288 p.
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