Even more recently extinct, island dwelling crocodilians
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
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
* 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
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
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
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.
Refs - -
Arnold, E. N. 1976. Fossil reptiles from Aldabra Atoll,
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
- . &
Burness, G. P., Diamond, J. & Flannery, T. 2001. Dinosaurs, dragons, and dwarfs: the evolution of maximal body size. Proceedings of the
Langston, W. 1965. Fossil crocodilians from
- . & 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 (
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),