December 6, 2011 | 44
It’s apparently a good idea in scientific blogging to produce ‘clearing house’ blog articles every now and again: that is, articles that include links to all of your other articles on a given subject. I suppose anything that gets people looking anew at old articles and reminding them what you have ‘in the archives’ is a good idea.
With all of this in mind, I decided for no particular reason to create a CETACEA clearing house. Well, that’s not entirely honest – I suppose there are two reasons why I might benefit from publishing this. One being that I want links to all of my cetacean content in the same place; the other being that articles about cetaceans are perennially popular, and reminding people that they exist is a good move as I strive desperately to get more hits*. And this is not a bad time to talk about whales in a phylogenetic context, since a large number of studies published in recent years have aimed to piece together the different section of the whale family tree.
* Tet Zoo ver 3 is still getting less hits than ver 2. I’m sure (transl. = I hope) things will change eventually.
The aim of this article (aaaand the following one), then, is to whiz briefly through the whole of cetacean diversity, all the while providing links to previous Tet Zoo articles that guide the reader where to go should they want to read more about the specific groups we meet on the way. As we’ll see, coverage of the different cetacean clades has actually been fairly even, and they makes me happy. Off we go. We start in the Eocene…
Whales of the Eocene: from pakicetids to Basilosaurus, Dorudon and more
Stem-cetaceans – the archaic, sometimes amphibious or semi-terrestrial Eocene forms – are all the rage these days, with exciting new discoveries appearing fairly regularly in top tier journals and hence on the newswires. It turns out that the very oldest cetaceans – the Asian pakicetids – weren’t seal-like protowhales as once thought, but morphologically more like slim-snouted artiodactyls (Thewissen et al. 2001). Exactly where these slender-limbed, at least partly terrestrial little hoofed mammals fit with respect to other early artiodactyls is mildly controversial. Indeed, it’s now generally agreed that Cetacea is deeply nested within Artiodactyla, and I see no point at all in using the name ‘Cetartiodactyla’ for the version of Artiodactyla that’s understood to include Cetacea. Thewissen et al. (2007) argued that cetaceans are closest to the raoellids (see also Spaulding et al. (2009)), and that even raoellids were aquatic waders that spent time swimming in and under the water. However, other authors still find mesonychians to be closest to cetaceans (O’Leary & Gatesy 2008), though with the mesonychian + whale clade being deep within Artiodactyla.
Pakicetids seemingly gave rise to the larger, longer-snouted ambulocetids [Pakicetus skeleton above photographed by Ghegoghedo]. The precise relationships among the other Eocene cetacean groups haven’t yet been resolved with confidence, but the particularly odd, very long-snouted remingtonocetids and a paraphyletic assemblage of robust-bodied forms conventionally termed protocetids seem successively closer to Pelagiceti, the clade that includes all those fully aquatic cetaceans with reduced or absent hindlimbs (Uhen 2008). ‘Protocetids’ include reasonable diversity: the most familiar forms, like Protocetus and Rodhocetus, might have looked something like a cross between a big seal and a deep-bodied artiodactyl, albeit it with large, webbed feet and a stout tail (Gingerich et al. 2001, 2009). Some were badass predators: the ‘baby’ whale inside Maiacetus inuus might actually be the remains of a consumed prey item.
Pelagicetans include the famous long-bodied Basilosaurus and its possible close relatives (Cynthiacetus, Basiloterus) and Dorudon and other ‘dorudontids’ (Chrysocetus, Zygorhiza, Saghacetus, Stromerius etc.). Dorudontid-grade pelagicetans were highly similar to modern whales in general proportions, but still had hindlimbs and an archaic, heterodont dentition. We now know that archaic pelagicetans survived beyond the end of the Eocene and into the Oligocene and perhaps the Miocene. The group concerned – the kekenodontids – were apparently similar to Dorudon and kin in being fusiform and pelagic. Anyway, I haven’t much covered any of these archaic cetaceans at Tet Zoo…
With these archaic forms out of the way, we turn to crown-group cetaceans, termed Neoceti (so far as I know, no-one has suggested that the term Cetacea be restricted to the crown). Early on in its history, Neoceti diverged into Mysticeti and Odontoceti.
Baleen whales: rorquals, grey whales, right whales and their relatives
Mysticetes or baleen whales are familiar enough animals, but the fossil record shows that the oldest stem forms (the Oligocene mammalodontids) were relatively small with relatively small skulls, a heterodont dentition and a relatively short, deep rostrum. Some of these animals may have been beluga-like generalist feeders that used suction to capture benthic animals (Fitzgerald 2009) while others had more robust rostra and were able to kill and handle reasonably big vertebrate prey. From mammalodontid-like ancestors, the longer-skulled, flatter-snouted (but still toothed) aetiocetids evolved. Aetiocetids (Aetiocetus, Chonecetus, Ashorocetus and Morawanocetus) may have indulged in some filter-feeding, but their small, conical teeth and occlusal wear facets suggest that they were still catching small prey with their teeth (Deméré et al. 2008, Fitzgerald 2009).
Toothless mysticetes – grouped together as Chaeomysticeti – possess relatively enormous skulls and laterally bowed mandibles. All fossil forms have specialisations related to bulk filter-feeding, and crown-mysticetes like rorquals are of course able to engulf literally tons of prey and water in a single mouthful (or, at least, the giant species are).
An assemblage of fossil mysticetes that (grotesque generalisation) look superficially like prototype versions of the living species were previously lumped together as ‘cetotheres’ (and sometimes regarded as close relatives, or ancestors, of the living Grey whale – read on). The term cetothere in the strict sense (as in, Cetotheriidae) is now restricted to the clade that comprises Piscobalaena, Herpetocetus, Metopocetus, Cetotherium, Nannocetus and Mixocetus (Bouetel & Muizon 2006, Steeman 2007, Geisler et al. 2011). Cetotheriids proper are united by distinctly wedge-shaped nasal bones (look at Metopocetus below) and a list of other skull bone characters.
Additional fossil mysticetes that were classified in the new ‘families’ Pelocetidae, Aglaocetidae and Diorocetidae by Steeman (2007) have been regarded as especially close relatives of balaenopterids by some authors (Steeman 2007), but as stem-mysticetes by others (Bouetel & Muizon 2006, Geisler et al. 2011). Geisler et al. (2011) used the name Balaenomorpha for Diorocetus, Pelocetus and crown-mysticetes. Cetotheriids definitely deserve coverage on Tet Zoo some time, as do the also extinct eomysticetoids (eomysticetids and cetotheriopsids).
Crown-mysticetes belong to one of two lineages: the right whale lineage, and the rorqual + grey whale lineage (though read on for complications). Right whales are well known for their massive arched upper jaws, incredibly long baleen and remarkable rotund proportions. Their fossil record perhaps extends back to the Upper Oligocene, and the Lower Miocene form Morenocetus shows that the ‘modern’ right whale skull form has been around for more than 20 million years. There’s nothing of substance about right whales on Tet Zoo, only…
The other living mysticetes have been written about a fair bit at Tet Zoo, largely because rorquals and humpbacks (classified together as Balaenopteridae) amaze me in having crazy distensible jaws, pleated, expandable throat skin, and an amazing ability to morph from a streamlined torpedo shape into a fat, bloated tadpole. The largest cetacean, largest mammal and largest tetrapod is, of course, a member of this group. Balaenopterids grow quickly, with even Blue, Fin and Sei whales reaching sexual maturity in a decade or less: Minke and Humpback whales may be sexually mature at just two or three years of age (Sigurjónsson 1995). A review of living and fossil balaenopterid diversity was provided by Deméré et al. (2005).
Both right whales and balaenopterids have suffered tremendously from the whaling industry: the populations of species that are no longer hunted are now a tiny fraction of what they used to be, but there are indications that some of these (e.g., the Antarctic Blue whale Balaenoptera musculus population) are now stable or perhaps increasing. There are of course concerns about the future of these animals given climate change, increases in global shipping, and damage to their hearing caused by low-frequency sonar.
The Grey whale/Gray whale Eschrichtius robustus is a peculiar coast-hugging mysticete with coarse baleen and a distinctive series of low bumps running along the top of its back and tailstock [adjacent photo from here]. The scientific name we presently used was originally given to fossils from Sweden, and only later was it realised that this was the same animal as the living whale of the north Pacific coasts. One of the most remarkable recent stories concerning Grey whales is the discovery of a live individual in the eastern Mediterranean Sea. That’s a bizarre and fascinating thing, but I can’t help but feel that it might be bad news – are things really so bad for these whales that they’re having to move well away from their feeding grounds? The Grey whale was once regarded as a close relative or descendant of the cetotheres but is most likely a close relative of the balaenopterids (though see Steeman (2007) where there is a Cetotherioidea clade that includes grey whales and cetotheriids).
Caperea – the Pygmy right whale – is an unusual and little-known mysticete that, while once regarded as a close relative of the right whales, may in fact be closer to balaenopterids. Geisler et al. (2011) proposed the new name Plicogulae for the clade that includes Caperea, balaenopterids and grey whales. I’m pleased to say that Caperea has been written about quite a few times on Tet Zoo.
Part II: odontocetes!
Refs – -
Bouetel, V. & de Muizon, C. 2006. The anatomy and relationships of Piscobalaena nana (Cetacea, Mysticeti), a Cetotheriidae s.s. from the early Pliocene of Peru. Geodiversitas 28, 319-395.
Deméré, T. A., Berta, A. & McGowen, M. R. 2005. The taxonomic and evolutionary history of fossil and modern balaenopterid mysticetes. Journal of Mammalian Evolution 12, 99-143.
- ., McGowen, M. R., Berta, A. & Gatesy, J. 2008. Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales. Systematic Biology 57, 15-37.
Fitzgerald, E. M. G. 2009. The morphology and systematics of Mammalodon colliveri (Cetacea: Mysticeti), a toothed mysticete from the Oligocene of Australia. Zoological Journal of the Linnean Society 158, 367-476.
Geisler, J. H., McGowen, M. R., Yang, G. & Gatesy, J. 2011. A supermatrix analysis of genomic, morphological, and paleontological data from crown Cetacea. BMC Evolutionary Biology 2011, 11:112 http://www.biomedcentral.com/1471-2148/11/112
Gingerich, P. D., Haq, Mu, Zalmout, I. S., Khan, I. H., & Malkani, M. S. (2001). Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan. Science, 293 (5538), 2239-42 PMID: 11567134
- ., ul-Haq, M., von Koenigswald, W., Sanders, W. J., Smith, B. H. & Zalmout, I. S. 2009. New protocetid whale from the Middle Eocene of Pakistan: birth on land, precocial development, and sexual dimorphism. PLoS ONE 4(2): e4366. doi:10.1371/journal.pone.0004366
O’Leary, M. A. & Gatesy, J. 2008. Impact of increased character sampling on the phylogeny of Cetartiodactyla (Mammalia): combined analysis including fossils. Cladistics 24, 397-442.
Pilleri, G. & Pilleri, O. 1989. Bartenwhale aus der Pisco-Formation Perus. In Pilleri, G. (ed) Beiträge zur Paläontologie der Cetacean Perus. Hirnanatomisches Institut (Bern, Switzerland), pp. 11-38.
Sigurjónsson, J. 1995. On the life history and autecology of North Atlantic rorquals. In Blix, A. S., Walløe, L. & Ulltang, Ø. (eds) Whales, Seals, Fish and Man. Elsevier Science, pp. 425-441.
Spaulding, M., O’Leary, M. A. & Gatesy, J. 2009. Relationships of Cetacea (Artiodactyla) among mammals: increased taxon sampling alters interpretations of key fossils and character evolution. PLoS ONE 4(9): e7062. doi: 10.1371/journal.pone.00070672
Steeman, M. E. 2007. Cladistic analysis and a revised classification of fossil and recent mysticetes. Zoological Journal of the Linnean Society 150, 875-894.
Thewissen, J. G. M., Cooper, L. N., Clementz, M. T., Bajpai, S. & Tiwari, B. N. 2007. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450, 1190-1195.
- ., Williams, E. M., Roe, L. J. & Hussain, S. T. 2001. Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature 413, 277-281.
Uhen, M. D. 2008. New protocetid whales from Alabama and Mississippi, and a new cetacean clade, Pelagiceti. Journal of Vertebrate Paleontology 28, 589-593.