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Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

Earth: Crocodile Empire homeworld (crocodiles part I)

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Highly simplified cladogram showing one of the several phylogenetic hypotheses proposed for crocodiles (this topology based on Brochu 2000a, b and McAliley et al. 2006). Photos (top to bottom) by Mo Hassan, Davric, Naish, Thesupermat (see below for licensing on last image).

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Prior to the epic destruction wrought upon their kind by the noble and beautiful Homo sapiens, crocodiles had it pretty good.

One thing that I’ve read several times and found especially annoying is the idea that crocodiles, alligators and gharials are “the last remnants of a once mighty empire”, or something along those lines. No, crocodiles and other living crocodilians are not anachronisms, relics or sorry vestiges of a former glory. It’s certainly true that living crocodilians do have a superficial similarity to the sorts of crocodilians known from fossils to have been around for more than 100 million years. But modern crocodilians don’t, contra all claims to the contrary, have especially long evolutionary histories and they’re no more anachronistic or archaic than numerous other living things that we take for granted and never consider ‘of the past’.

Close to the smile of a Salwater, Estuarine or Indopacific croc (C. porosus). Verily, a thing of such great beauty.

In fact, crocodiles alone (that is, totally ignoring alligators, caimans and gharials) include about 13 species that, historically, occurred across much of Africa, the American tropics, southern Asia and the Indopacific region. This diversity and this global range parallels or exceeds that of many living animals often regarded as the epitome of modernity and success, so “last remnants of a once mighty empire” my arse.

While those of us who care about such things will always remain frustrated by continuing descriptions of crocodiles and kin as “living dinosaurs”, or as “living fossils”, it is at least becoming better appreciated that crocodilians are actually complex creatures with interesting social lives, uncanny sensory abilities and a surprising, weird and sophisticated anatomy.

The crocodile consensus

On to a bit of phylogeny. One thing that's been universally agreed upon for a while is that dwarf crocodiles (Osteolaemus) form the sister-group to Crocodylus [though, read on]. And I talk of dwarf crocodiles in the plural sense since, as you might remember from the previous crocodile article, it’s increasingly agreed that Osteolaemus contains more than one species. Incidentally, please assume in this article and those that follow that I’m using the word crocodile in the specific (correct) sense.

Slender-snouted crocodile (Mecistops cataphractus), photographed in captivity by Thesupermat. I originally thought that this photo actually showed a False gharial (Tomistome schlegelii) but a clearer dorsal view of the same animal reveals the scute pattern of Mecistops; it is definitely not a Tomistoma (SEE BELOW). Image licensed under Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.

An interesting surprise is that one species often imagined to be part of Crocodylus – the highly distinct Slender-snouted crocodile, conventionally Crocodylus cataphractus – is no such thing (that is, it isn't a species of Crocodylus), but is in fact probably closer to Osteolaemus. The data supporting this is from nuclear and mitochondrial genes, and it clearly shows the Slender-snouted crocodile to be as distant genetically from Crocodylus as is Osteolaemus (Schmitz et al. 2003, McAliley et al. 2006). This means that cataphractus can’t be included in Crocodylus anymore; at least, not if we want to keep Osteolaemus separate from Crocodylus, as we do. The sister-group to Crocodylus thus consists entirely of African taxa; an observation that might indicate an African origin for Crocodylus itself (Brochu 2000a) (though see later articles in this series).

Slender-snouted crocodile, from Wermuth (1953). Click to enlarge.

The idea that cataphractus might not be especially close to Crocodylus is not actually novel. John E. Gray gave this species its own genus – Mecistops – back in 1844. More recently, Aoki (1976, 1992) argued that Mecistops should be treated as a distinct genus, and as a close relative of gharials and false gharials. Indeed, the common name African gharial has sometimes been used for this species. That last idea hasn't been supported by other work (both molecular and morphological); anyway, the name Mecistops is now back in business for the Slender-snouted crocodile.

As for the relationships within the Crocodylus species, it’s fair to say both that contrasting phylogenetic hypotheses have been published, and that certain species have been recovered in radically different positions. Despite this, it’s probably most useful to imagine Crocodylus to consist of three main lineages: an Indopacific one, a New World one, and the Nile crocodile C. niloticus (e.g., Brochu 2000a, b, McAliley et al. 2006), as shown in the simplified cladogram above. To reiterate, and as we’ll see, the monophyly and content of these lineages has been challenged by various recent discoveries.

The scute pattern present on the anterior part of the dorsal shield, and in the cervical shield, shows that the crocodile shown above is indeed a Slender-snouted crocodile (Mecistops cataphractus), not a False gharial (Tomistoma schlegelii). Photo by Thesupermat, licensed under Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license. Diagrams of scute patterns from Ross & Mayer (1983).

In this and the following few articles, we’re going to go through all the world’s living 13 or so crocodile species. The article continues the theme initiated with May’s ‘Dissecting a crocodile’ article. Rather than discuss the same sort of information you’ve heard before, my aim is to highlight recent ideas about phylogeny, history and diversity – the sort of material that rarely makes it into the popular and semi-popular literature. We start with the Mugger and other species that appear to be close to (but not necessarily part of) the Indopacific assemblage.

The Mugger or Marsh crocodile: from Iran to Burma

Basking Muggers in captivity. Photo by Kmanoj, licensed under Creative Commons Attribution 3.0 Unported license.

The Mugger C. palustris, also called the Marsh or Swamp crocodile (very occasionally, the Iranian crocodile), is uniformly dark with a relatively short, broad snout (proportionally the broadest of any crocodile, actually, sometimes being only 1.3 times as long as it is wide at its base). You can sometimes understand why a naive person might misidentify a Mugger as an 'alligator'. Adult Muggers generally reach 4 m in total length, with 4.5 m probably being a maximum. Lengths of up to 8 m (Wermuth & Fuchs 1978) are reported in older literature. As with other crocodile species, extraordinary lengths of this sort should be considered highly unlikely: they are either mistakes or the result of exaggeration. The Mugger is mostly associated with freshwater habitats but its presence in coastal marshes is on record too. [Adjacent image by Kmanoj.]

Though usually thought of as an Indian animal, the Mugger also occurs in Pakistan, Nepal, Bangladesh, Bhutan and Burma. Or, at least, it did occur in these countries, since it now seems extinct in Bangladesh, Bhutan and Burma. The last records from Burma come from the 1860s, the last Bhutanese ones were seen in the 1960s, and the Bangladesh population might have lingered on until the 1990s. Occasional, rumoured sightings and the undetermined origins of captive specimens in small collections suggest that a few crocodiles might persist in the wild in these countries; either way, it’s clear that the animal has been eradicated from part of its historical range.

Mugger photographed in India by Herbert Ponting in 1907: the photo was originally labelled 'An Indian alligator'. Image in public domain.

It seems little known that the Mugger’s distribution extends west into southern Afghanistan and Iran. A 2007 survey estimated a population of 200-300 Muggers in Iran while the number present in Afghanistan seems unknown. I've been unable to learn anything about the alleged presence of the species in Afghanistan and am keen to know more - my only source for the presence of crocodiles in that country comes from the minutes of a 2006 "Crocodile Specialist Group Steering Committee Meeting".

Dorsal scute compliment of a typical Mugger, from Ross & Mayer (1983).

I think I can just about identify a Mugger from the proportions of its head, and overall colour and body shape. But the best way to know that you’re dealing with a Mugger is to look at its dorsal osteoderm compliment. Muggers sometimes have a rather low number of scutes in each transverse dorsal row (four), but they can also have six or eight. The most anterior transverse band in the dorsal shield is often very short (that is, consisting of just two or four scutes) and may even be absent altogether. A band of granular skin then separates the dorsal shield from the cervical one; the latter typically consists of six reasonably large scutes (Ross & Mayer 1983). I should use this an opportunity to note that dorsal scute compliment really is the most reliable way of distinguishing crocodile species - other features that you might rely on, including size, colour and the general appearance of the jaws and teeth, are often unreliable.

The Mugger population on Sri Lanka supposedly differs from the typical form just described in having especially broad throat scutes and six longitudinal rows of dorsal scutes (as opposed to four). Accordingly, there have been various suggestions that it warrants taxonomic distinction. The names C. p. brevirostris and C. p. kimbula have both been used for this population.

The Mugger is recovered as the sister-taxon to the Indopacific assemblage in some studies (Brochu 2000b, McAliley et al. 2006), though positions especially close to C. porosus, C. mindorensis and C. siamensis have all been proposed on occasion. In fact, molecular data sets often find C. palustris, C. porosus and C. siamensis to group together (Li et al. 2007, Gatesy & Amato 2008, Feng et al. 2010, Man et al. 2011), and the Mugger and Siamese crocodile C. siamensis form a clade in some of these studies (Li et al. 2007, Man et al. 2011).

More soon. For previous Tet Zoo ver 3 articles on crocodilians, see...

As you'll know if you follow me on twitter or facebook, Tet Zoo ver 2 has now been deleted in entirety. So, goodbye lengthy articles on sebecosuchians, giant fossil caimans, mekosuchines and a ton of other stuff.

Refs - -

Aoki, R. 1976. On the generic status of Mecistops (Crocodylidae), and the origin of Tomistoma and Gavialis. Bulletin of the Atagawa Institute 6/7, 23-30.

- . 1992. Fossil crocodilians from the late tertiary strata in the Sinda Basin, eastern Zaire. African Study Monographs 17, 67-85.

Brochu, C. A. 2000a. Congruence between physiology, phylogenetics and the fossil record on crocodylian historical biogeography. In Grigg, G. C., Seebacher, F. & Franklin, C. E. (eds) Crocodilian Biology and Evolution. Surry Beatty & Sons (Chipping Norton, Aus.), pp. 9-28.

- . 2000b. Phylogenetic relationships and divergence timing of Crocodylus based on morphology and the fossil record. Copeia 2000, 657-673.

Feng, G., Wu, X. B., Yan, P., & Li, X. Q. 2010. Two complete mitochondrial genomes of Crocodylus and implications for crocodilians phylogeny. Amphibia-Reptilia 31, 299-309.

Gatesy, J., & Amato, G. 2008. The rapid accumulation of consistent molecular support for intergeneric crocodilian relationships Molecular Phylogenetics and Evolution, 48, 1232-1237

Li, Y., Wu, X., Ji, X., Yan, P. & Amato, G., 2007. The complete mitochondrial genome of salt-water crocodile (Crocodylus porosus) and phylogeny of crocodilians. Journal of Genetics & Genomics 34, 119-128.

Man, Z., Yishu, W., Peng, Y. & Wu, X. 2011. Crocodilian phylogeny inferred from twelve mitochondrial protein-coding genes, with new complete mitochondrial genomic sequences for Crocodylus acutus and Crocodylus novaeguineae. Molecular Phylogenetic and Evolution 60, 62-67.

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.

Ross, F. D. & Mayer, G. C. 1983. On the dorsal armor of the Crocodilia. In Rhodin, A. G. J. & Miyata, K. (eds) Advances in Herpetology and Evolutionary Biology. Museum of Comparative Zoology (Cambridge, Mass.), pp. 306-331.

Schmitz, A., Mansfeld, P., Hekkala, E., Shine, T., Nickel, H., Amato, G. & Böhme, W. 2003. Molecular evidence for species level divergence in African Nile crocodiles Crocodylus niloticus (Laurenti, 1786). C. R. Palevol 2, 703-712.

Wermuth, H. 1953. Systematik der Rezenten Krokodile. Mitteilungen aus dem Zoologischen Museum im Berlin 29, 375-514.

- . & Fuchs, K. 1978. Bestmmen von krokodilen und ihrer Häute. New York, Gustav Fischer.

The views expressed are those of the author and are not necessarily those of Scientific American.

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