Even more recently extinct, island dwelling crocodilians

In the previous post we looked at the small, island dwelling crocodilians of the south-west Pacific. I personally find it exciting that such animals were (in the case of at least some of the species) alive until just a few thousand years ago, that they were encountered by people, and that their remains have eluded detection until recent decades. The odds are high that further species await discovery.

All of the island dwelling crocodilians I discussed in the previous post were members of the predominantly Australasian mekosuchine radiation. But there is one recently extinct crocodilian of the south-west Pacific that I didn’t mention, and which isn’t a mekosuchine. First reported by Charles De Vis in 1905, it’s a long-snouted form known from Pleistocene remains discovered at Busai on Murua, one of the Solomon Islands. Because of its long, slender jaws, De Vis regarded this animal as a gharial and named it Gavialis papuensis. It then languished in obscurity until 1982 when Ralph Molnar published a redescription.

Molnar (1982) concluded that the Murua crocodilian almost certainly didn’t belong in the genus Gavialis, and that it was more likely closely related to Charactosuchus, Euthecodon or Ikanogavialis, with a relationship with the last named taxon being deemed most likely. That’s good news, because Ikanogavialis (best known for I. gameroi from Upper Miocene* Venezuela [a jaw segment from this taxon is figured at left]) is – while not the same thing as Gavialis – still undoubtedly a member of the gharial family, Gavialidae. South American Miocene Charactosuchus, while gharial-like, has been regarded as a highly unusual crocodylid of uncertain affinities (Langston 1965, Langston & Gasparini 1997), while Euthecodon – a uniquely African taxon, some species of which approached 10 m in length – is also a crocodylid, and perhaps a close relative of the living dwarf crocodiles (and we’ll discuss those more in a moment). Most recently, Rauhe et al. (1999) listed the Murua gharial as belonging to Ikanogavialis, and if this has been accepted then presumably we should refer to it as Ikanogavialis papuensis. The presence of this genus in both Venezuela and the south Pacific might seem odd given that living gharials are freshwater animals, but the fossil record shows that gharials formerly occurred widely in marine environments around the world.

* And, according to Langston & Gasparini (1997), not from the Pliocene as usually stated.

Like the south Pacific mekosuchines, the Murua gharial was again fairly small, at 2-3 m long. Its fossils were associated with those of sea turtles and sirenians, so it was almost certainly marine. To the list of small crocodilians that inhabited the south-west Pacific, we can add gharials then. Whether the Murua gharial became extinct before humans colonised the region, or whether its extinction was caused by people, we again don’t know. Indeed the only known specimen's exact geological age is unknown. Given this, and the many anthropogenic extinctions that occurred in the region, I can’t help but speculate that the Murua gharial survived into the Holocene, and that humans killed it off, but there’s no direct evidence for this. Regardless, it’s surprising that small, marine gharials survived so relatively late [the adjacent photo figures the living gharial species Gavialis gangeticus].

Besides mekosuchines and gharials, we also know of a third crocodilian group that included island dwelling forms, and again the species concerned became extinct geologically recently. To look at the members of this group we need to move over to the Indian Ocean. And it’s here that we find the most recently named of all the crocodilians we’ve looked at: Aldabrachampsus dilophus, from Aldabra (Brochu 2006). Though recently named, Aldabrachampsus was actually first described in 1976, though at this time it was misidentified as representing a dwarf population of Crocodylus niloticus (Arnold 1976). In being of Pleistocene age, Aldabrachampsus is like both Volia from Fiji (see previous post) and the Murua gharial in being too old to have its extinction indisputably linked to the arrival of people. Arnold (1976) discussed environmental changes that occurred on Aldabra that may have caused the extinction of endemic reptiles, among them geckos, iguanas and skinks, with the most notable of them being the breaching of the atoll rim and subsequent habitat degradation that occurred about 4000 years ago.

Various skull features make Aldabrachampsus unusual, including the shape of its premaxillae, and the orientation of its tooth row and external nostrils. However, its most obvious feature would almost certainly have been the convex crests that grew from the dorsolateral edges of the squamosal bones at the back of its skull. Some living crocodiles have crest-like projections in this region, but none have the prominent, elongate structures present in Aldabrachampsus. These crests explain the specific name, ‘dilophus’ meaning ‘with two crests’.

Given that Aldabrachampsus was comparable in size to the smallest living crocodilians – that is, between 2 and 2.5 m long – it is again tempting to assume that, like so many island dwelling tetrapods, it was a dwarf. This would actually be odd for a crocodilian, given that other island dwelling forms are not dwarfed relative to their mainland relatives (and, as we saw in the previous post, this seems to go for island dwelling mekosuchines as well). Indeed the stratigraphic occurrence of Aldabrachampsus doesn’t provide support for the idea that it’s a dwarf: it’s from sediments that were deposited shortly after a period of Aldabran submergence, and the species is therefore unlikely to have evolved on the island. It must therefore have swum in from elsewhere.

What sort of crocodilian was Aldabrachampsus? It was a crocodylid, but there’s no indication that it was anything to do with the mekosuchines: instead, there are reasons for thinking that it was an osteolaemine. That is, a member of the same crocodylid clade as the west African dwarf crocodiles (Osteolaemus*) - see picture at top - and the extinct Madagascan species ‘Crocodylus’ robustus (Brochu 1997, 2006). Some phylogenetic studies find that the osteolaemines also include Euthecodon, the bizarre gharial-like African taxon we met above, as well as Rimasuchus (Brochu 1997), a broad-snouted east African taxon that grew to 7 m or more in length. The African slender-snouted crocodile Crocodylus cataphractus might also be an osteolaemine, a view that would be in agreement with data suggesting that it needs removing from Crocodylus (the old generic name Mecistops Gray, 1844 is available: see McAliley et al. 2006), and additional fossil African crocodylids also seem to belong to this group. If this is all valid, then little Osteolaemus is a sorry remnant of a once diverse group that included several enormous species. Anyway, within this group, an affinity between Aldabrachampsus and ‘Crocodylus’ robustus is particularly plausible given that both taxa share a vaulted palate and large squamosal crests.

* Though conventionally thought to include just a single living species (O. tetraspis), new data has caused some workers to regard a second species as valid (McAliley et al. 2006). This is O. osborni, a taxon from the Congo (first described in 1919 and given its own genus, Osteoblepharon) until recently regarded as a subspecies of O. tetraspis.

Unlike the crests of Aldabrachampsus, those of ‘Crocodylus’ robustus were large horn-like growths (see photo above, my hands for scale), and unlike both Aldabrachampsus and Osteolaemus, this species was large and comparable in size to a Nile crocodile Crocodylus niloticus. Originally described in 1872, ‘Crocodylus’ robustus has been mostly considered synonymous with C. niloticus, but ‘this synonymy results from an inadequate initial description and from subsequent misidentifications of living C. niloticus from Madagascar as C. robustus’ (Brochu & Storrs 1995). The ‘true’ ‘Crocodylus’ robustus was a broad-snouted species sharing a list of skull characters with Osteolaemus, so it doesn’t belong in the genus Crocodylus and needs a new name (hence the use of quotes).

It wasn’t as big as Euthecodon or Rimasuchus, reaching 4-5 m in length (Burness et al. (2001) estimated its weight as 170 kg). This size would have made it the largest predator on Madagascar, and given that prehistoric Madagascar was also home to giant eagles and fossas, the lemurs, elephant birds and other animals of the island would certainly have lived in fear of formidable predators.

Again, what fascinates me most about ‘Crocodylus’ robustus is how recently it was alive. So far as I can tell from the literature, an exact date for its extinction is unknown, and I’d be interested to know if it disappeared as part of the anthropogenic wave of extinctions that occurred on the island. Chris Brochu is due to publish on this species in the near future, so more information will appear soon.

For a previous post on island dwelling non-avian reptiles see Tortoises that drink with their noses. For the latest news on Tetrapod Zoology do go here.

Refs - -

Arnold, E. N. 1976. Fossil reptiles from Aldabra Atoll, Indian Ocean. Bulletin of the British Museum (Natural History). Zoology 29, 85-116.

Brochu, C. A. 1997. Morphology, fossils, divergence timing, and the phylogenetic relationships of Gavialis. Systematic Biology 46, 479-522.

- . 2006. A new miniature horned crocodile from the Quaternay of Aldabra Atoll, western Indian Ocean. Copeia 2006, 149-158.

- . & Storrs, G. W. 1995. The giant dwarf crocodile: a reappraisal of ‘Crocodylus’ robustus from the Quaternary of Madagascar. In Patterson, Goodman & Sedlock (eds) Environmental Change in Madagascar, p. 70.

Burness, G. P., Diamond, J. & Flannery, T. 2001. Dinosaurs, dragons, and dwarfs: the evolution of maximal body size. Proceedings of the National Academy of Sciences 98, 14518-14523.

Langston, W. 1965. Fossil crocodilians from Colombia and the Cenozoic history of the Crocodilia in South America. University of California Publications in Geological Sciences 52, 1-169.

- . & Gasparini, Z. 1997. Crocodilians, Gryposuchus, and the South American gavials. In Kay, R. F., Madden, R. H., Cifelli, R. L. & Flynn, J. J. (eds) Vertebrate Paleontology in the Neotropics: The Miocene fauna of La Venta, Colombia. Smithsonian Institution Press (Washington, D.C.), pp. 113-154.

McAliley, L. R., Willis, R. E., Ray, D. A., White, P. S., Brochu, C. A. & Densmore, L. D. 2006. Are crocodiles really monophyletic? – Evidence for subdivisions from sequence and morphological data. Molecular Phylogenetics and Evolution 39, 16-32.

Molnar, R. E. 1982. A longirostrine crocodilian from Murua (Woodlark), Solomon Sea. Memoirs of the Queensland Museum 20, 675-685.