We looked previously at a partial skull, collected in northern Africa. Apart from the odd outing when it’s been used in teaching, it’s been sat in a box on my desk for a couple of years now, forlornly hoping that it might one day earn a place in the peer-reviewed literature. However, that would only apply if it were a fossil, and as we’ll see that’s contentious.
Anyway, so… what is it?
To begin with, the elongate, pointed rostrum (with stretched premaxillae and maxillae that extend for the entire length of the preserved portion), evidence for a polydont, homodont dentition, enlarged dorsal foramina at the base of the rostrum, laterally extensive maxilla that overhangs the orbit, elevated and retracted nasal region, antorbital notch, and large ventral concavity medial to the orbit (for a massive air sinus) demonstrate that this is a cetacean, an odontocete, and specifically a delphinid. So, yup, it’s definitely a dolphin.
To try and identify it more precisely we need to look at a few details. The premaxillary-maxillary suture line is relatively straight for most of the length of the rostrum, only curving laterally as the rostrum flares outwards as it approaches the antorbital notch. The left premaxilla exhibits only a single foramen. The maxilla tapers at its anterior end but is otherwise of roughly constant width for the length of the rostrum. The small antorbital notch is approximately semi-circular and a small convexity projects laterally from the maxilla at the notch’s anterior margin. Two large anterior foramina and one smaller posterior one are present on the posteromedial part of the maxilla. The dorsal margin of the orbit is approximately semi-circular and the ventral end of the postorbital process is blunt. There are definitely 18 alveoli, and probably 20 (the rostrum’s tip is abraded, making it difficult to be sure). I’ve labelled some of the key bits and pieces in the photos above: the three large maxillary foramina are respectively marked f1, f2 and f3.
Orcininae and Globicephalinae: the presence of a long rostrum rules out further comparison with any of the orcinines or globicephalines (killer whales, pilot whales, false killer whales and kin, all of which have a short, broad rostrum [Orcinus orca shown here]). Peponocephala (the melon-headed whale: it may or may not be a globicephaline (Buchholtz & Schur 2004, May-Collado & Angnarsson 2006)) has particularly large antorbital notches and tooth rows that don’t extend as far along the rostrum as they do in this specimen (Mikkelsen & Sheldrick 1992, Jefferson & Barros 1997).
Steninae: stenine dolphins (humpbacked dolphins, rough-toothed dolphins and tucuxis) can be immediately discounted from comparison as they all possess a more gracile rostrum that has concave margins at the anterior end. The antorbital notches of these dolphins are either V-shaped or very shallow. While I’ve grouped humpbacked dolphins (Sousa) and rough-toothed dolphins (Steno) together here, as per Muizon (1988), some studies (May-Collado & Angnarsson 2006) find Sousa to be a delphinine.
Lissodelphinae: the elongate rostrum of the specimen rules out comparison with little Cephalorhynchus (which also differs in having a pointed postorbital process and a more extensive, much flatter posterolateral expanse of the maxilla) [image here shows skull of Heaviside's dolphin C. heavisidii. Image © Natural History Museum (London), courtesy C. McHenry]. Lissodelphis has a very slender rostrum, small, V-shaped antorbital notches, and possesses over 37-52 teeth in each maxilla (in total, there are over 200 teeth in the jaws), so can also be excluded. Sagmatius (the old name now used for the Hourglass dolphin Lagenorhynchus cruciger and Peale’s dolphin L. australis: Lagenorhynchus as traditionally conceived is not monophyletic) also has a high tooth count, with 28 teeth in each maxilla. It also differs in having a proportionally shorter rostrum that is triangular in dorsal view. Sagmatius is actually rather Cephalorhynchus-like, and some workers think the two genera should be combined (Cephalorhynchus Gray, 1846 is older than Sagmatius Cope, 1866).
Delphininae: it seems that this is where the affinities of the specimen lie. Palatal grooves (present in Lagenodelphis and Delphinus) are absent from the specimen, and Lagenodelphis and Delphinus also differ in having a much higher tooth count (36-44 per maxilla in Lagenodelphis and 40-57 per maxilla in Delphinus) (Jefferson & Leatherwood 1994). Stenella also has a much higher tooth count than the specimen, a far shallower antorbital notch and other differences [image here shows skull of Delphinus (at top) and Stenella. Images © United States National Museum, courtesy C. McHenry]. The remaining ‘lags’ (which are delphinines according to some studies (Harlin-Cognato & Honeycutt 2006) and either lissodelphines or incertae sedis according to others (May-Collado & Angnarsson 2006)) differ from the African specimen in higher tooth counts (22-40 teeth per maxilla), and proportionally shorter rostra. We are left with Tursiops, the bottlenose dolphins. Tursiops possesses 18-26 teeth per maxilla (the specimen probably had 20), its antorbital notch is deep and rounded, and a small convexity is present at the anterior end of the notch. The rostrum is neither particularly robust nor particularly gracile and the position of the premaxillary-maxillary suture generally matches that of the African specimen. In lateral view, the premaxillae of Tursiops are deeper mid-way along the length of the rostrum than they are at the tip and near the orbits, and this seems to be the case in the specimen. The pattern of foramina on the premaxilla and maxilla is pretty much the same as what’s present in the specimen. And with a rostrum about 30 cm long, the specimen is comparable in size to a normal Tursiops. This, then, is where the affinities of the specimen lie. I am very confident that it’s a Tursiops, and I see from the comments that quite a few of you agree with this.
But I’m still not entirely happy. The question I’ve been trying to answer is: which Tursiops species does it belong to, and does it represent a new one? Most people interested in animals know of ‘the Bottlenose dolphin’ Tursiops truncatus. But it’s been argued for a while now that ‘Tursiops truncatus‘ of tradition deserves to be split up again, with offshore and nearshore populations in the Indopacific and Atlantic oceans clearly grouping apart on morphology and genetics (Hoelzel et al. 1998, LeDuc et al. 1999, Wang et al. 1999, Yang et al. 2005 and references therein).
The large, offshore dolphins are now called T. truncatus Montagu, 1821, and the smaller, nearshore ones are T. aduncus Ehrenberg, 1832. T. truncatus is longer-bodied (with 62-67 vertebrae, as opposed to 59-62 in T. aduncus) and has a proportionally shorter rostrum. T. aduncus further differs from T. truncatus in having elongated dark spots on its ventral surface. However, T. aduncus populations from South Africa are highly distinct from so-called T. aduncus populations from Chinese waters, and because the modern use of the name T. aduncus is associated with South African dolphins, Natoli et al. (2004) suggested that the Chinese ‘T. aduncus‘ should warrant recognition as a third species. Given that there are about 20 old Tursiops species names (Hershkovitz 1966), working out which would be the right one to use is not going to be easy [adjacent T. aduncus image from wikipedia].
Furthermore, it now seems that T. aduncus and T. truncatus aren’t close relatives anyway, with T. aduncus being outside a clade that includes T. truncatus, Stenella and Delphinus (LeDuc et al. 1999, Yang et al. 2005). I really hope that this is supported by further studies, as it means that T. aduncus will deserve its own genus: this’ll be the first new extant cetacean genus named since Australophocaena was coined in 1985.
Anyway, with the literature and lots of specimen photos to hand I had a bash at identifying the African Tursiops skull. Wang et al. (2000) included a key on how to differentiate the skulls of T. aduncus and T. truncatus. Unfortunately, you need to know the shape and position of the premaxillary convexity, and also the complete skull length, to get the measurements you need, and the specimen isn’t complete enough for that (frustratingly, a chunk of the premaxilla is missing just where the premaxillary convexity should be… the convexity is usually particularly prominent in T. aduncus). However, the fact that the premaxillary-maxillary suture is not laterally concave (‘pinched in’) suggests that the skull is T. truncatus.
Fossil bottlenose dolphins that aren’t
But there are fossil species to take account of too if, that is, the African specimen is a fossil. I was told that it was (I believe it comes from a place where the Pleistocene fossils are white) and have assumed this to be correct, though I do know that worn and battered cetacean bones can look like fossils even when they aren’t. This has now been challenged, and I’ll need to chase things up to find out either way [UPDATE: it was literally dug out of the ground, not picked up from the surface as I said previously]. Anyway, whatever, quite a few fossil Tursiops have been named, including T. cortesii (Fischer, 1829), T. brochii Capellini, 1863, T. capellinii Del Prato, 1897, T. astensis Sacco, 1891, T. ossennae Simonelli, 1911 and T. oligodon Pilleri & Siber, 1989.
Pilleri (1987) regarded T. capellinii, T. brochii and T. cortesii, all from the Pliocene of Italy, as ‘representatives of the same chronospecies’ (in which case T. cortesii is the correct name), and it now seems that T. cortesii and also T. oligodon from the Pliocene Pisco Formation of Peru aren’t Tursiops at all, but should in fact be recognised as Hemisyntrachelus (T. cortesii is the type species of this genus: it’s an unusual delphinid and might be an orcinine) (Bianucci 1996, Post & Bosselaers 2005). T. ossennae from the Pleistocene seems to be the only valid extinct Tursiops still recognised (Bianucci 1996), but I haven’t seen any figures of it and don’t know what it looks like.
For now, this is where it ends. And this might be another of those ‘must convert this into a technical paper at some stage’ blog posts. You know, like what all the other bloggers do…
Coming soon: a whole week of seriously frickin’ weird cetacean skulls.
Refs – -
Bianucci, G. 1996. The Odontoceti (Mammalia, Cetacea) from Italian Pliocene. Systematics and phylogenesis of Delphinidae. Palaeontographia Italica 83, 73-167.
Buchholtz, E. A. & Schur, S. A. 2004. Vertebral osteology in Delphinidae (Cetacea). Zoological Journal of the Linnean Society 140, 383-401.
Harlin-Cognato, A. D. & Honeycutt, R. L. 2006. Multi-locus phylogeny of dolphins in the subfamily Lissodelphininae: character synergy improves phylogenetic resolution. BMC Evolutionary Biology 2006, 8:87.
Hershkovitz, P. 1966. Catalog of living whales. Smithsonian Insitution United States National Museum, Bulletin 246, 1-259.
Hoelzel, A. R., Potter, C. W. & Best, P. B. 1998. Genetic differentiation between parapatric ‘nearshore’ and ‘offshore’ populations of the bottlenose dolphin. Proceedings of the Royal Society of London B 265, 1177-1183.
Jefferson, T. A. & Barros, N. B. 1997. Peponocephala electra. Mammalian Species 553, 1-6.
- . & Leatherwood, S. 1994. Lagenodelphis hosei. Mammalian Species 470, 1-5.
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.
May-Collado, L. & Angnarsson, I. 2006. Cytochrome b and Bayesian inferences of whale phylogeny. Molecular Phylogenetics and Evolution 38, 344-354.
Mikkelsen, A. M. H. & Sheldrick, M. 1992. The first recorded stranding of a melon-headed whale (Peponocephala electra) on the European coast. Journal of Zoology 227, 326-329.
Muizon, C. 1988. Les relations phylogénétiques des Delphinida (Cetacea, Mammalia). Annales de Paléontologie (Vert.-Invert.) 74, 159-227.
Natoli, A., Peddemors, V. M. & Hoelzel, A. R. 2004. Population structure and speciation in the genus Tursiops based on microsatellite and mitochondrial DNA analyses. Journal of Evolutionary Biology 17, 363-375.
Pilleri, G. 1987. The Cetacea of the Italian Pliocene. Brain Anatomy Institute, Bern, Switzerland.
Post, K. & Bosselaers, M. 2005. Late Pliocene occurrence of Hemisyntrachelus (Odontoceti, Delphinidae) in the southern North Sea. Deinsea 11, 29-45.
Wang, J. Y., Chou, L.-S. & White, B. N. 2000. Osteological differences between two sympatric forms of bottlenose dolphins (genus Tursiops) in Chinese waters. Journal of Zoology 252, 147-162.
Yang, G., Ji, G., Ren, W., Zhou, K. & Wei, F. 2005. Pattern of genetic variation of bottlenose dophins in Chinese waters. The Raffles Bulletin of Zoology 53, 157-164.