December 8, 2011 | 20
Welcome to part II of the Tet Zoo cetacean clearing house. With stem-cetaceans (‘archaeocetes’) and mysticetes out of the way (go here for part I), we come to odontocetes.
Many key odontocete traits are found in the maxilla and ear region, most of which are related to soft tissue structures involved in noise-making and hearing. For these, of course, are the echolocating cetaceans. The odontocete radiation includes sperm whales, beaked whales, the various ‘river dolphins’, and the true dolphins and their relatives. However, a large number of fossils forms don’t fall neatly into any of these four clusters. Some – including Archaeodelphis, Agorophius, Simocetus and Squaloziphius – might be stem-odontocetes, outside the clade formed by the modern species (Uhen et al. 2008, Geisler et al. 2011). I haven’t covered any of the respective taxa at all yet and really should do, since they’re very neat. Squaloziphius was originally described as an archaic beaked whale (Muizon 1991).
Sperm whales (Physeteroidea) are among the most morphologically aberrant of odontocetes and also seem to be one of the oldest surviving lineages. I prefer a taxonomic arrangement where the ‘big sperm whales’ are treated as a different ‘family’ from the dwarf and pygmy sperm whales: that is, they’re classified in Physeteridae and Kogiidae.
Members of both groups have been covered on Tet Zoo a few times. Physeter – the big sperm whale (aka Great sperm whale, or just Sperm whale) – is one of the most remarkable of all mammals in my opinion and there’s virtually no end to the amount of neat stuff there is to say about it. I’ve written about the peculiar dentition of Physeter, about various fossil sperm whales [that’s Zygophyseter shown above, from Bianucci & Landini (2006)], and about kogiid anatomy. I still plan to talk about the soft-tissue anatomy of Physeter’s head at some stage (you might know, or recall, that this topic was featured in the most recent series of Inside Nature’s Giants).
Beaked whales (and possible kin)
Beaked whales (Ziphiidae) are diverse but famously little known, and an improving fossil record shows that they’ve been substantially diverse since the Miocene at least. Beaked whales are exciting for lots of reasons. We know comparatively little about their biology and behaviour, and we also know next to nothing about how various of the species are doing in terms of their populations and distributions. Several new species have been named in recent decades (making the group of special interest to those who keep tabs on new and recently discovered large animal species), and more are definitely due to come.
It has sometimes been hypothesised that beaked whales formed a clade with sperm whales: Muizon (1988a) named this Physeterida while Fordyce (1994), Geisler & Sanders (2003) and Uhen et al. (2008) used Physeteroidea for this grouping. Other studies reject the monophyly of Physeterida, instead finding beaked whales to be part of a clade that includes ‘river dolphins’ and true dolphins but not sperm whales. Geisler et al. (2011) used the name Synrhina for this clade, defining it as the node-based clade that includes Platanista gangetica, Ziphius cavirostris and Tursiops truncatus. Beaked whales have been covered a few times on Tet Zoo…
A group of long-beaked, mostly Miocene, fossil odontocetes called Eurhinodelphinidae were regarded as close relatives of ziphiids by Lambert (2005). Eurhinodelphis – the exemplar of the group (often depicted as looking something like a cetacean billfish) – has been regarded as a close relative of Platanista by other workers (Geisler & Sanders 2003) and was regarded ‘traditionally’ as a close relative of true dolphins. Possible freshwater eurhinodelphinids are known from Australia (Fordyce 1983). [Photo of Eurhinodelphis (and Squalodon) skull below by Ghedoghedo.]
River dolphins: of Platanista, Baiji, Boto and Franciscana
Within Synrhina, it has been widely acknowledged for years now that the so-called river dolphins are not monophyletic (e.g., Cassens et al. 2000, Nikaido et al. 2001); however, the South American Boto (Inia) and its fossil relatives (the clade Iniidae) and the Franciscana (Pontoporia) and its fossil relatives (the clade Pontoporiidae) are consistently recovered as sister-taxa in just about all studies. The name Inioidea exists for this clade. The Baiji or Yangtze river dolphin Lipotes vexilifer – the sole modern representative of Lipotidae – may be closely related to Inioidea.
The remaining ‘river dolphins’ are the two Platanista species of the Indus and Ganges. Appearance-wise, these are among the most incredible of cetaceans, with their miniscule, lens-less eyes, slim, interlocking teeth, broad flippers and habit of swimming on their sides. Muizon (1987, 1994) suggested that Platanista might be closely related to the long-beaked fossil squalodonts of the Oligocene and Miocene, mostly on the basis of shared features of the scapula. Barnes (2006) included the ‘families’ Allodelphinidae, Squalodontidae, Waipatiidae and Squalodelphinidae alongside Platanistidae within ‘Superfamily Platanistoidea’ (see also Fordyce (1994)). This has been informally dubbed the ‘squalo-susu hypothesis’ (Susu is a local name for Platanista). The idea that Platanista represents the sole, sorry survivor of the once mighty squalodont empire is a really appealing idea, but some recent studies haven’t supported it (Geisler & Sanders 2003, Geisler et al. 2011). However, even these studies do find at least a few fossil long-beaked odontocetes previously regarded as squalodonts to be close relatives of Platanista, including everybody’s favourite – the incredibly long-beaked Zarhachis.
All river dolphins are, as is well known, superficially alike: they’re long-beaked, typically greyish odontocetes with small or very small eyes, slender teeth and broad flippers. All are endangered or critically endangered by pollution, damming, habitat loss, hunting, competition for prey and other factors. Their future is not bright, and indeed it now seems that the Yangtze river dolphin is functionally extinct (Turvey et al. 2007). This means that, while there might still be a handful of individuals in existence (sightings of a live one were reported in 2007), there are almost certainly not enough to sustain a population. For Tet Zoo articles on river dolphins and such, see…
True dolphins and their relatives the kentriodontids, porpoises, monodontids and walrus whales
Finally, we come to Delphinoidea: the clade that includes narwhals and belugas (Monodontidae), porpoises (Phocoenidae) and true dolphins (Delphinidae). A few fossil taxa – including Albireo, the various kentriodontids and the incredible ‘walrus whales’ Odobenocetops – don’t fall within these clades but are probably closely related to them. Geisler et al. (2011) found both Albireo and Kentriodon (the type taxon for Kentriodontidae) and Atocetus (another supposed kentriodontid) to be stem-delphinoids.
Kentriodontids are (mostly) small (2 m long or less), archaic, Oligo-Miocene odontocetes with symmetrical skulls, originally imagined to be early relatives of delphinids. Oddballs include Tagicetus from Portugal with its especially long rostrum (it was originally identified as a eurhinodelphinid) and the relatively huge Macrokentriodon from Maryland (perhaps four times as large as most other members of the group). Convincing evidence for kentriodontid monophyly has yet to be found (though see Muizon 1988b), and so has evidence putting them especially close to delphinids.
Monodontids are among my favourite cetaceans – they truly are very bizarre. I mean to say about a lot more about them, but so far have at least written about the possible function of the male Narwhal’s incredible tusk. The two Odobenocetops species likely represent close relatives of crown-monodontids, but this hypothesis still awaits proper testing. Recent work indicates that monodontids and porpoises are sister-taxa, forming the clade Monodontoidea (Waddell et al. 2000, Geisler et al. 2011). Living porpoises are all small, blunt-headed delphinoids but some fossil species were long-skulled and superficially dolphin-like. One of them (as yet undescribed) is super-weird in having a lower jaw that’s rather longer than the upper one.
True dolphins (Delphinidae) include both short-snouted orca-like and pilot-like forms (sometimes grouped together as globicephalines) and long-beaked forms like bottlenose dolphins (Tursiops) and the Stenella and Delphinus species. There are also some weird fossil forms, like the toothless, ziphiid-mimicking Australodelphis from the Pliocene of Antarctica (Fordyce et al. 2002) and stupid-headed Platalearostrum from the Plio-Pleistocene of the North Sea (Post & Kompanje 2010). How many species several of the classic genera (like Orcinus, Tursiops and Delphinus) actually contain remains the topic of argument and there are various different views on how these animals are related. Orcinus may not be at all close to Globicephala (the pilot whales). The ‘lags’ – the Lagenorhynchus dolphins – prove not to be monophyletic, so recent work has resulted in the use of the new or resurrected generic names Leucopleurus (for the Atlantic white-sided dolphin) and Sagmatius for several of the others (LeDuc et al. 1999). Interestingly, there are indications that some other traditional dophin genera might also not be monophyletic. A new species of bottlenose dolphin was named earlier this year from Australia (the Burrunan dolphin Tursiops australis) and it grouped well away from the rest of Tursiops in a phylogenetic analysis (Charlton-Robb et al. 2011). The authors therefore suggested that it could warrant a new ‘genus’ (they suggested the potential name Tursiodelphis) but retained it in Tursiops for conservative reasons.
So that’s that – a quick tour of Cetacea… though it certainly wouldn’t be all that quick if you followed all the links and read all the linked-to articles. As usual, there is still tons of work left to do in terms of properly covering this diverse, successful and fascinating clade.
Refs – -
Barnes, L. G. 2006. A phylogenetic analysis of the superfamily Platanistoidea (Mammalia, Cetacea, Odontoceti). Beiträge zur Paläontologie 30, 25-42.
Bianucci, G. & Landini, W. 2006. Killer sperm whale: a new basal physeteroid (Mammalia, Cetacea) from the Late Miocene of Italy. Zoological Journal of the Linnean Society 148, 103-131.
Cassens, I., Vicario, S., Waddell, V. G., Balchowsky, H., Van Belle, D., Ding, W., Fan, C., Mohan, R. S., Simões-Lopes, P. C., Bastida, R., Meyer, A., Stanhope, M. J., & Milinkovitch, M. C. (2000). Independent adaptation to riverine habitats allowed survival of ancient cetacean lineages. Proceedings of the National Academy of Sciences of the United States of America, 97 (21), 11343-7 PMID: 11027333
Charlton-Robb, K,, Gershwin, L.-a., Thompson, R., Austin, J., Owen, K. & McKechnie, S. 2011. A new dolphin species, the Burrunan dolphin Tursiops australis sp. nov., endemic to southern Australian coastal waters. PLoS ONE 6(9): e24047. doi:10.1371/journal.pone.0024047
Fordyce, F. E. 1983. Rhabdosteid dolphins (Mammalia: Cetacea) from the Middle Miocene, Lake Frome area, South Australia. Alcheringa 7, 27-40.
- . 1994. Waipatia marewhenua, new genus and new species (Waipatiidae, new family), an archaic Late Oligocene dolphin (Cetacea: Odontoceti: Platanistoidea) from New Zealand. Proceedings of the San Diego Society of Natural History 29, 147-176.
- ., Quilty, P. G. & Daniels, J. 2002. Australodelphis mirus, a bizarre new toothless ziphiid-like fossil dolphin (Cetacea: Delphinidae) from the Pliocene of Vestfold Hills, east Antarctica. Antarctic Science 14, 37-54.
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
Geisler, J. H. & Sanders, A. E. 2003. Morphological evidence for the phylogeny of Cetacea. Journal of Mammalian Evolution 10, 23-129.
Kellogg, R. 1928. The history of whales – their adaptation to life in the water (concluded). Quarterly Review of Biology 3, 174-208.
Lambert, O. 2005. Phylogenetic affinities of the long-snouted dolphin Eurhinodelphis (Cetacea, Odontoceti) from the Miocene of Antwerp, Belgium. Palaeontology 48, 653-679.
LeDuc, R. G., Perrin, W. F. & Dizon, A. E. 1999. Phylogenetic relationships among the delphinid cetaceans based on full cytochrome b sequences. Marine Mammal Science 15, 619-648.
Muizon, C. de 1987. The affinities of Notocetus vanbenedeni, an Early Miocene platanistoid (Cetacea, Mammalia) from Patagonia, southern Argentina. American Museum Novitates 2904, 1-27.
- . 1988a. Les vertebres fossiles de la Formation Pisco (Perou). Troisieme partie: Les Odontocetes (Cetacea, Mammalia) du Miocene. Editions Recherche sur les Civilisations 78, 1-244
- . 1988b. Les relations phylogénétiques des Delphinida (Cetacea, Mammalia). Annales de Paléontologie (Vert.-Invert.) 74, 159-227.
- . 1994. Are the squalodonts related to the platanistoids? Proceedings of the San Diego Society of Natural History 29, 135-146.
- . 1991. A new Ziphiidae (Cetacea) from the Early Miocene of Washington Sate (USA) and phylogenetic analysis of the major groups of odontocetes. Bulletin de la Musee d’Histoire Naturelle de Paris (4e sér.) 12, 279-326.
- . & Domning, D. P. 2002. The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications. Zoological Journal of the Linnean Society 134, 423-452.
Nikaido, M., Matsuno, F., Hamilton, H., Brownell, R. L., Cao, Y., Ding, W., Zuoyan, Z., Sheldock, A. M., Fordyce, R. E., Hasegawa, M. & Okada, N. 2001. Retroposon analysis of major cetacean lineages: the monophyly of toothed whales and the paraphyly of river dolphins. Proceedings of the National Academy of Sciences 98, 7384-7389.
Post, K. & Kompanje, E. J. O. 2010. A new dolphin (Cetacea, Delphinidae) from the Plio-Pleistocene of the North Sea. Deinsea 14, 1-13.
Turvey, S. T., Pitman, R. L., Taylor, B. L., Barlow, J., Akamatsu, T., Barrett, L. A., Zhao, X., Reeves, R. R., Stewert, B. S., Wang, K., Wei, Z., Zhang, X., Pusser, L.T., Richlen, M., Brandon, J. R. & Wang, D. 2007. First human-caused extinction of a cetacean species? Biology Letters 3, 537-540.
Uhen, M. D., Fordyce, R. E. & Barnes, L. G. 2008. Odontoceti. In Janis, C. M., Gunnell, G. F. & Uhen, M. D. (eds) Evolution of Tertiary Mammals of North America Volume 2: Small Mammals, Xenarthrans, and Marine Mammals. Cambridge University Press, pp. 566-606.
Waddell, V. G., Milinkovitch, M. C., Bérube, M. & Stanhope, M. J. 2000. Molecular phylogenetic examination of the Delphinoidea trichotomy: congruent evidence from three nuclear loci indicate that porpoises (Phocoenidae) share a more recent common ancestry with white whales (Monodontidae) than they do with true dolphins (Delphinidae). Molecular Phylogenetics and Evolution 15, 314-318.