On identifying a dolphin skull

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.

i-66254935df605d223cae0eb880a70c79-Tursiops composite labelled resized.jpg

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.

i-3fc2ddf6e54e8af7296544f0dba48a1b-Orcinus oblique 24-7-2008.jpg

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.

i-472fd8e1ef864860655b251c96955669-Cephalorhynchus heavisidii BMNH 1972.4.jpg

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).

i-bacd90ec4cb407fc08c8164d6622385c-Delphinus and Stenella USNM skulls McHenry.jpg

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).

i-b7e0e24cd897d434990e60e328aff6e2-Tursiops aduncus wikipedia.jpg

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.

i-36c8bd04a2d098b3caf18db292747537-Tursiops wikipedia 24-7-2008.jpg

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.


More like this

Let's face it, all the frickin' weird cetacean skulls we've looked at so far have belonged to frickin' weird cetaceans: sperm whales and river dolphins. Time for something less frickin' weird, though still frickin' weird, if you get my meaning. It's a boring old dolphin. But is it just a boring old…
Welcome to day 2 of seriously frickin' weird cetacean skull week, and here we look at one of my favourites: Platanista, the Asian river dolphins or susus. Susu is a Hindi onomatopoetic name based on the exhalation noise these dolphins make, and other local names include susuk, sishuk, shushuk and…
Welcome to another of those week-long series of themed posts, produced (ostensibly) to save me from spending time on blogging (other jobs require priority). Previous series have been ankylosaur week and sea monster week. This time round we're looking at seriously frickin' weird cetacean skulls…
More on weird odontocete skulls. This time, another river dolphin: this is the skull of the Amazon river dolphin or Boto Inia geoffrensis, also known as the tonina, bufeo or pink dolphin. Three generally recognised Inia taxa exist, and views differ as to whether these are subspecies or species. I…

Are the bottlenoses you see in delphinaria T. truncatus or aduncus?

By Andreas Johansson (not verified) on 25 Jul 2008 #permalink

Awww nooo! This is the first time I've ever got one of your id quiz's right and I didn't bother commenting as I thought it was probably some obscure fossil I knew nothing about and not the bottlenose it looked just like! Gah.

Mildly interesting bit of trivia on bottlenose skulls is that there was a bit of a kerfuffle in lincolnshire near my previous abode when someone turned one up about 20 miles inland and various cryptozoological suggestions were offered before the truth was established. Then the mystery became how it got there (I just assumed a tribe of marshdwelling fenlanders had brought a stranding back to the village to tuck into).

Wow... I never quite expected an old dolphin skull to prove to be so fascinating to me. Cetacean diversity is one aspect that always gets me really excited, considering how we seem to be finding 'new' species hidden among supposedly familiar species every now and then. There were the short-beaked and long-beaked common dolphins (Delphinus), the tucuxi and costero (Sotalia), the Irrawaddy and Australian snubfin (Orcaella), not to mention that Bryde's whale and minke whale comprise of at least 2 distinct species each, and don't get me started on Omura's whale...

Ah yes... the old problem of how to differentiate the two bottlenose dolphin species. I've pretty much given up trying to tell them apart based on photographs alone.

I was under the impression that T. aduncus is an inshore Indo-Pacific species, and so includes among them the famous habituated dolphins of Shark Bay, Australia. I am led to understand that elsewhere, such as in the Atlantic and East Pacific, both the inshore and offshore bottlenose dolphins are T. truncatus.

And like Andreas Johansson, I am seriously confused as to the identity of captive dolphins, given that a large number of them are caught in the wild in the Indo-Pacific and then shipped to North American oceanariums. It would be interesting to find out if some of these captives are T. aduncus, which would raise interesting questions on the provenance of our captive dolphins, and to see if hybridisation has taken place between captive T. truncatus and T. aduncus. (Maybe that's a possible contributing factor as to why they don't seem to breed so well in captivity!)

Well this is the first time Ive even been close to getting it right! Interesting stuff.

While on the subject of identifing bones, I found the ones shown here in barn owl pellets. CAn anyone help with the mystery bone and the bottom (and I'm not 100% sure on the others. Im afraid Im used to seeing small mamals with fur and flesh still present!)

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...

What all the other bloggers do is put a "Blogging on Peer-Reviewed Research" icon up whenever they do just that. You'd have to put up that icon almost every time.

What YOU need is a "Blogging Into Peer-Reviewed Research" icon, which you could put on your azhdarchid paleobiology post, this one (eventually), and others as they happen along.

Thanks for the tour of odontocete cranial osteology--another great post.

I'm actually working on Miocene and Pliocene cetaceans right now - this week's posts should be interesting.

Incidentally... I'm working on Mio-Pliocene cetaceans as well... Bob K, where/what are you studying in particular?

Humph. was it a fossil or not? And would something as recent as the Pleistocene be fossilised (ie mineralised...though I dont know the exact definition of what we popularly regard as a fossil??)

Remind me...did the Chinese River dolphin have an asymetic skull? I seem to remember reading something somewhere.

If so, why?

(Male Narwhals have assymetic skulls, but thats cus they are male narwhals...)

"seriously frickin' weird cetacean skulls"

Isn't that kinda redundant?
Looking forward to 'em.

"Remind me...did the Chinese River dolphin have an asymetic skull? I seem to remember reading something somewhere."

Lipotes has a slightly asymmetrical skull. There aren't very many odontocetes with symmetrical skulls - some examples are extant phocoenids, the Fransiscana (Pontoporia) and... I can't think of any other exant examples (there may be a couple).

In any event, here's the short answer: the right side of the facial region is much larger because the right nasal passage is adapted for sound production, while the left nasal passage is more or less unmodified (with regards to sound production) and retained for breathing.

I don't really know whether or not its a fossil (would want to see it in person) - and the original discoverer is certainly going to be your best source of info. I would just say that depending on where it was found, it doesn't have to be a fossil. Could have been dug out of Recent sediments on the beach for example (there are plenty plenty of places on the coast with Recent sediments and cliffs of Pleistocene exposure all around the world). It certainly does look like a weather-worn modern specimen though.

Hi all, thanks for comments. Assorted responses...

-- Hai-Ren says 'I am led to understand that elsewhere, such as in the Atlantic and East Pacific, both the inshore and offshore bottlenose dolphins are T. truncatus'. Yes, some studies report that both nearshore and offshore Tursiops populations in the Atlantic are T. truncatus (e.g., Kingston & Rosel 2004, Segura et al. 2006). Kingston & Rosel (2004) found these two morphotypes to be more distinct genetically than two Delphinus species, and both were distinct from T. aduncus. Coastal Tursiops from the UK (including the extinct Humbar River estuary population) have been identified as T. truncatus (Nichols et al. 2007). However, Hoelzel et al. (1998) had indicated that the nearshore Tursiops from the Atlantic were T. aduncus, which is what I had in mind when saying that the presence of T. aduncus had been demonstrated in the Atlantic. I may change the wording.

-- Re: blogging on peer-reviewed research, I decided when I first learnt about this not to participate. Not because I dont think its an excellent idea, but because all I do is blog on peer-reviewed research: Tet Zoo is, pretty much, not like other blogs. Your insight serves you well. Bury your feelings deep down, Matt. They do you credit, but they could be made to serve the Emperor.

-- Finally, is the specimen a fossil or not? Its discoverer (Dave Martill) extracted it from clay sediment and regards it as a fossil. However, I learnt in discussion with him today that he holds a particularly controversial view of what a fossil is, and essentially regards anything naturally entombed in sediment as a fossil. I can see that its difficult to provide a clear definition of a fossil (given that mineralization, or loss of organic content, is not a criterion, and that the fossilisation process is a continuum), but I dont think I can agree. At most, the specimen is thousands of years old.

Refs - -

Kingston, S. E. & Rosel, P. E. 2004. Genetic differentiation among recently diverged delphinid taxa determined using AFLP markers. Journal of Heredity 95, 1-10.

Nichols, C., Herman, J., Gaggiotti, O. E., Dobney, K. M., Parsons, K. & Hoelzel, A. R. 2007. Genetic isolation of a now extinct population of bottlenose dolphins (Tursiops truncatus). Proceedings of the Royal Society, London B 274, 1611-1616.

Segura, I., Rocha-Olivares, A., Flores-Ramírez, S. & Rajos-Bracho, 2006. Conservation implications of the genetic and ecological distinction of Tursiops truncatus ecotypes in the Gulf of California. Biological Conservation 133, 336-346.

@Boesse: I'm at the Calvert Marinue Museum in Maryland, USA. I'm presently wrapping project on a cetacean rib with shark predation trauma and starting one on an unidentified pontoporiid rostrum. Where/what are you working?

Ah, another Tet Zoo special week to look forward to. I would suggest that all cetacean skulls are a bit weird. Are you only covering extant forms, or also some of the fossil ones? Among the fossil forms are 'dolphins' pretneding to be walruses and others pretending to be sharks - and we have of course our extant 'dolphin' pretending to be an unicorn.

You mention turning this into a technical paper - I see you also turn some of your blog material into popular articles - I just read (well last night actually) your Fortean Times article on Intelligent Dinosaurs. Nicely written - though I would take issue with two points:
1. You state "people who regard the humanoid shape as somehow inevitable often hold religious convictions." I have to say that has not been my experience. I guess that your not talking about Creationists (who would tend to disagree that dinosaurs would evolve into anything!)- but rather persons who have a teleological view of evolution, like Teilhard de Chardin. Still even then I'm not sure I could agree that there is such a pattern.
2. You refer at the end to the 'Avisapiens' as having an "element of biological plausibility". It is certainly less contrived than the other 'reconstructions', but is it plausible, Im afraid I must disagree.

Notwithstanding these 2 quibbles (that's a lovely word - defined as 'a petty distinction or an irrelevant objection') - it was a thoroughly enjoyable read which obviously drew on your blog on the same subject.

As an aside, what do you think of the Irish 'nail-tailed' wildcat referred to in another article in the same issue?

By the way Darren, thanks for your blog, a new Tet Zoo article always makes my day!

By Mark Lees (not verified) on 26 Jul 2008 #permalink

Ah, so it really could be as young as it looks... that confused me...

The official definition of "fossil" is "more than 10,000 years old". Anything younger is a subfossil. Of course this doesn't always make sense in terms of the state of preservation.

By David Marjanović (not verified) on 26 Jul 2008 #permalink

Bobk, I'm working with pinnipeds and cetaceans from the Purisima Formation of central California. As far as cetaceans go, right now I'm primarily working with some phocoenid fossils, and the bizarre mysticete Herpetocetus. I take it you've already read Godfrey and Barnes 2008, on the new St. Marys Fm. pontoporiid?

Yes, I have read it; actually Godfrey is my boss/advisor at the museum. It would be great if the rostrum I'm working with goes with that new pontoporiid, but I don't think one could conclusively say. By the way, the rib is from the Yorktown Formation of North Carolina, and the rostrum is from the St. Mary's Formation of Maryland.

I thought I'd add something as far as Dave Martill's definition of fossil goes -

I, for one, couldn't agree more. It *unfortunately* means that fossils so defined could only be a few hundred years old... However, this I believe is the definition that most agrees with the principle of uniformitarianism. I do like the use of the term subfossil as a good compromise. However, being familiar with much of Dave Martill's taphonomic work, I understand using geologic/taphonomic/uniformitarian logic rather than invoking some magical cutoff age.

Mark: am covering extant taxa, stay tuned.

Am pleased to see that my Fortean Times article is out, though haven't seen it yet. Yes, it evolved out of stuff you've previously read on Tet Zoo. Incidentally, there is also a kid's TV series on big-brained dinosaurs being broadcast here in the UK (infuriating title = Dinosapien): I would love to know if it also owes its origins to Tet Zoo, but have yet to find this out.

1. You state "people who regard the humanoid shape as somehow inevitable often hold religious convictions." I have to say that has not been my experience. I guess that your not talking about Creationists (who would tend to disagree that dinosaurs would evolve into anything!)- but rather persons who have a teleological view of evolution, like Teilhard de Chardin. Still even then I'm not sure I could agree that there is such a pattern.

Hmm, maybe I should have been more specific: those scientists who have expressed the opinion that the humanoid body shape is somehow inevitable do indeed seem to hold religious convictions.

2. You refer at the end to the 'Avisapiens' as having an "element of biological plausibility". It is certainly less contrived than the other 'reconstructions', but is it plausible, Im afraid I must disagree.

Well, that's your opinion. I would argue that it does indeed have an 'element of biological plausibility' (which is not the same as saying 'it is plausible'!).

As an aside, what do you think of the Irish 'nail-tailed' wildcat referred to in another article in the same issue?

Again, I haven't seen the article yet, but I already know quite a lot about the subject as the author (Gary Cunningham) is a good friend of mine and we often talk about Irish wildcats. My take on this has been that the 'Irish wildcat' reports were too vague to be informative and did not convincingly refer to a small cat. That's still the case so far as I know but we now know that wildcats were indeed present in Ireland in the Holocene (mentioned here), so it's conceivable that they hung on there until modern times.

Returning to dolphin skulls, David wrote...

The official definition of "fossil" is "more than 10,000 years old". Anything younger is a subfossil. Of course this doesn't always make sense in terms of the state of preservation.

Dave Martill tells me that this is completely arbitrary and not helpful, given that there isn't one magical cutoff point at which 'fossilisation' begins. I have no strong opinion on this either way and want to learn more.

It's funny how a lot of recent interesting articles make me think of old Tetrapod Zoology posts.

It appears that a 'new' species of bottlenose dolphin has been identified.


And if the results are any indication, it shows that Stenella is polyphyletic, T. aduncus really deserves a new genus, and so does this 'new' southern Australian bottlenose dolphin.

Möller, L.M., Bilgmann, K., Charlton-Robb, K. & Beheregaray, L. 2008. Multi-gene evidence for a new bottlenose dolphin species in southern Australia. Molecular Phylogenetics and Evolution 49(2): 674-681.

You can access the paper (made accessible via the Molecular Ecology Lab of Macquarie University) by clicking on my name.