anatomy, fossils and molecular classification of whales

From
Date
Reply-To
>I wonder if you can shed some light on an area where there seems to be some
>disagreement between biochemical studies and comparative anatomy  [cut]
>Similarly, we remember reading of a revised classification of whales based
>on genetic comparisons.  The author(s) believe Sperm whales are more closely
>related to baleen whales than to any other toothed whales.  At least one
>paleo whale expert scoffs at the idea, but we observe some superficial
>similarities between Sperm whales and baleen whales while noting many
>differences between Sperm whales and other toothed whales. As we understand
>it, anatomists are loath to accept the idea that echolocation might have
>evolved 'twice', and teeth are here to stay.

Molecular and anatomical classifications of sperm whales conflict quite
dramatically.  In many ways, the molecular work is very persuasive.
Several different authors have produced similar results which are
technically sound and quite convincing.  The analyses include careful
detailed cladistic studies.  But, the case is not closed; studies of
macroscopic anatomy, particularly that of fossils, strongly support the
traditional view.   Some details follow.

To summarise the molecular work, sperm whales (classically placed in
suborder Odontoceti) cluster with the filter-feeding rorquals (classically,
suborder Mysticeti).  This cuts across traditional classification.
Molecular analyses also suggest that physeterids and balaenopterids had a
geologically more recent common ancestor (10-15 to perhaps 25 million years
ago - Middle Miocene to Late Oligocene) than the fossil record indicates.
These results could imply that the Odontoceti (traditional sense) are
paraphyletic, and that the ability to echolocate either was lost
secondarily in rorquals or evolved independently in different odontocete
groups.  For accounts of molecular work, see Milinkovitch et al. 1993
and1994, Milinkovitch 1995, and Adachi and Hasegawa 1995.

However, there is no doubt whatsoever about the long separate histories of
sperm whales and balaenopterid mysticetes, both of which form clearly
defined clades (Fordyce and Barnes 1994).  Sperm whales have a long fossil
record.  The oldest sperm whale is the inappropriately-named
Ferecetotherium, of Late Oligocene age (>23 Ma).  For this and other early
sperm whales, there is no hint of structures converging towards a
mysticete-like state, which one might expect if molecular concepts of
relationships hold.  Rather, both fossil and modern sperm whales show
features (many synapomorphies) which link them strongly with other
odontocetes.

Critics, such as Novacek, have argued that these "synapomorphies" for sperm
whales + other odontocetes might just reflect convergent adaptation to
echolocation.  Thus, the supposed synapomorphies would have evolved once in
sperm whales and once in other odontocetes.  But, how can we identify
convergence?  Problems of convergence can be tackled in various ways.  For
this issue involving fossils, I would invoke convergence if superficially
similar structures turned out, on close study, to have fundamentally
different detailed form.  This is not the case with sperm whales, which
show bony similarities with other odontocetes at all levels of anatomical
study that I have been able to carry out (observations from the scale of
the skull - metre dimensions - down to millimeter level).  As far as I can
determine from bone morphology, sperm whales are structurally certain
odontocetes.

Rorquals - Balaenopteridae - have a shorter record than sperm whales, back
to the Late or possibly Middle Miocene.  Traditionally, the rorquals are
viewed as descending from a loosely-defined grade family of archaic baleen
whales, the cetotheres (Cetotheriidae).  Cetotheres have a record well back
into the Late Oligocene (to about 30 Ma), and other more archaic mysticetes
have been reported from about the Eocene/Oligocene boundary.  Cetotheres
are paraphyletic and probably polyphyletic, and clearly need more taxonomic
study.  Even so, we can see that some taxa within the cetotheres are
probably close to the rorquals (again, judging by synapomorphies).  Amongst
the oldest cetotheres, there are no specimens at all which converge in
structure with sperm whales.  (Such a trend towards more of a sperm whale
structure might just be expected if sperm whales and mysticetes really are
sister taxa.)  Rather, the early cetotheres, like rorquals, show a range of
bone structures which are good synapomorphies for mysticetes.

The traditional anatomically-based classification is still weakly supported
on paper.  There are still no published computer-generated cladograms based
on macroanatomy which specifically address the relationship of sperm whales
to other odontocetes and to mysticetes.  Heyning (1989), Heyning and Mead
(1990) and Fordyce (1994) considered sperm whales incidentally in analyses
for other purposes, but detailed study of character distribution and
polarity is still needed.  I am carrying out more work at present.

Finally, reflect that anatomical approaches to phylogeny can involve many
years of study of often-problematic structures.  There is complex
anatomical terminology to be learned (especially for mammalian skulls) and
applied to structures of sometimes-uncertain ontogeny and function.
Overall, anatomical study is hard work embedded in tradition and
superficially lacking the charisma of modern molecular/numerical
approaches.  Conversely, automatic gene sequencing, coupled with use of
package programs in analysis, provides a much more seductive - and easier?
-  approach to phylogeny.   But, for this case study, I am not convinced
that it gives a better answer.

*Adachi, J., and Hasegawa, M.  1995.  Phylogeny of whales: dependence of
the inference on species sampling.  Molecular biology and evolution 12 (1):
177-179.
*Fordyce, R. E.  Waipatia maerewhenua, new genus and new species
(Waipatiidae, new family), an archaic Late Oligocene dolphin (Cetacea:
Odontoceti: Platanistoidea) from New Zealand.  Proceedings of the San Diego
Museum of Natural History 29: 147-176.
*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.
*Heyning, J. E.  1989.  Comparative facial anatomy of beaked whales
(Ziphiidae) and a systematic revision among the families of extant
Odontoceti.  Contributions in science, Natural History Museum of Los
Angeles County 405: 1-64.
*Heyning, J. E., and Mead, J. G.  1990.  Evolution of the nasal anatomy of
cetaceans.  Pages 67-79 in J. Thomas and R. Kastelein, (editors), Sensory
abilities of cetaceans.  Plenum, New York.
*Milinkovitch, M. C.  1995.  Molecular phylogeny of cetaceans prompts
revision of morphological transformations.  Trends in ecology and evolution
10 (8): 328-334.
*Milinkovitch, M. C., Ortí, G., and Meyer, A.  1993.  Revised phylogeny of
whales suggested by mitochondrial ribosomal DNA sequences.  Nature 361:
346-348.
*Milinkovitch, M. C., Orti, G., and Meyer, A.  1995.  Novel phylogeny of
whales revisited but not revised.  Mol. Biol. Evol. 12 (3): 518-520.

R. Ewan Fordyce, Associate Professor
Department of Geology, University of Otago, PO Box 56, Dunedin, NZ
fax 64-3-479-7527, ph. 64-3-479-7510