Worm Salad


Remarkable Reptile #6
December 7, 2008, 9:48 pm
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Okay, so fair warning — this post might get very long, since it’s all about the group that I am currently studying. And while I’m passionate to ramble about reptiles for thousand-word posts, when it comes to today’s subject, I’m good to ramble for, well, years probably. But, well, it wouldn’t be fair to list off a group of remarkable reptiles without including at least one crocodilian, and, of course, I picked the world’s most unique crocodile as my subject of study.

Crocodiles really are my favorite animals. People will tell you that they’re living fossils and that they have remained unchanged since the Jurassic, and while that’s sort-of true in that there’s a generalized ‘croc-shape’ that crocodiles tend to fall back on (and there certainly aren’t as many around as there used to be), it ignores the incredible fossil diversity of crocodiles through time. There are crocodiles that became terrestrial herbivores and crocodiles that took to an exclusively marine life and crocodiles that evolved mammal-life teeth and crocodiles that grew to forty feet. The ancestors of crocodiles were lithe, upright little predators with long legs who lived only on land, while the more typical crocodilian niche was occupied by their close-relatives: phytosaurs … which looked pretty much like a modern croc, except that they had their nostrils between their eyes.

See, look, I knew I would digress. The point is that crocodilian diversity is amazing, and even if they aren’t as diverse as they once were, their current diversity is amazing: from the five-foot long dwarf caiman of South America to Nile Crocodiles who live in marginal environments on the edge of the Sahara and their fossil diversity is beyond incredible. Their closest living relatives are birds, and, depending on who you ask, they might be very closely related to turtles. They differ from birds, though, in the morphology of their ankle — while bird ankle joints allow for a very limited front-back flexion, crocodile ankles, like yours, can rotate. This has some pretty interesting implications for their gait: basically it allows them to adopt a variety of different walking stances — they can walk upright like a mammal, sprawl like a lizard and can even gallop. The crocodilian biology database has a really fantastic page with videos of all the exciting forms of croc locomotion.

Nile Crocodile by padiyan on Flickr, used under CC license

Nile Crocodile, Crocodylus niloticus

Look, I’m digressing again. At this rate, we’ll never get to my remarkable crocodilian. Why don’t you just read on to find out what he is, before I get completely distracted.The most remarkable crocodilian in the world, hands-down, no-holds barred, is, of course, the Indian Gharial, Gavialis gangeticus. At a first glace, that might be hard to see. After all, one crocodilian is pretty much like another, and even the most remarkable feature of the gharial, its extraordinarily long snout, has been duplicated (although to not quite the enormous degree seen in Gavialis) in other members of the crocodilian group, like in these beautiful African Sharp-Snouted crocodiles (Mecistops cataphractus)

Mecistops, from pandiyan on flickr, used under CC license

Mecistops cataphractus

But gharials actually rearrange the bones in their skull to allow for their thin-snouted morphology, and their nasal bones never touch their external nostrils. This is just one of hundreds of unique features found in their skull and braincase; and all of these features have traditionally been held up as solid evidence that gharials are the most primitive of all of the living crocodiles: closer to fossil forms than to anything living today.

Image by sherseydc on flickr, used under CC license

Who're you callin' primitive?

Of course, the real story turns out to be much more complex than that. Because looking at anatomy isn’t the only way that we can trace an animal’s ancestry: we can also look at it’s DNA, to determine how closely it matches the DNA of other species. The more similar your DNA is to another species, the more closely you’re related to it …

That’s the theory, at least. There are all sorts of practical problems with applying the theory, of course, because DNA mutates, and random mutations can start to build up awfully similar patterns, without any degree of relationship at all. There are problems in groups that diverged long ago, especially if you’re also including groups with relatively recent divergence dates in the mix. No analysis is perfect, and, it’s here that the story of Gavialis gets complicated. Because, looking at the bones of the animal, no self-respecting morphologist would tell you that the gharial is closely related to another long-snouted crocodilian: the false gharial, Tomistoma schlegelii, from Southeast Asia. Tomistoma has a pretty standard crocodile’s skull, and is most closely related to all of the other true crocodiles: things like nile crocs, and sharp-snouted crocs and dwarf crocodiles. This isn’t surprising, since the group occurs around the world, and has often evolved a long-snouted form. Its bones certainly look nothing like those of the primitive-appearing gharial.

Tomistoma by batbob (flickr), used under CC license

Tomistoma, the false gharial

So what do the molecules say? Well, against the wildest expectations of … anyone on the planet, really, Gavialis turns out to be most closely related to the false gahrial. The closest cousin of the true gharial, is, in fact, the false. Or, at least, that’s what the molecules say. As time has progressed, the case for the DNA signal being better than the morphological signal has definitely strengthened, and the chances of simple convergence between the genes of these two species of crocodiles are next to zero. But the debate is far from over, and the question of why gavials are so primitive in appearance is still  completely unresolved.

My current work is attempting to look at the growth and development of gharials, compared to other crocodiles, to determine whether or not they have a unique developmental path that might explain some of their weirdness. Since it’s unpublished, I probably can’t let you in on my secrets, but let’s just say that I’m finding some incredibly interesting results…

Of course, you wouldn’t expect that gharials would evolve such a remarkable anatomy if they were just doing the same thing as their crocodilian counterparts. And, of course, they aren’t. They are extremely specialized fish-eaters, and even though they are the second-largest crocodilian in the world (Saltwater crocodiles, unsurprisingly, take a terrifying first place), they pose almost no danger to humans (the exception, of course, as in all crocodiles, involves females guarding their nests or young). Unlike all other crocodiles, gharials cannot raise their body to high walk — they must slide on their belly when they are on land, but, in the water, they are extremely graceful swimmers. Even the snapping jaw motion that they use to catch fish is unique in the crocodilian world. The males are sexually dimorphic from females, and breeding males will form a large, bulbous structure on the ends of their souts in order to show dominance and court the ladies. It might be that this unique structure is the reason that these reptiles are associated with Indian fertility gods…

By pirateparrot on flickr, used under CC license.

The ghara of a breeding male

Unfortunately, though, as with all crocodilians, the gharials have a sad story to tell. Both the true and false gharials are highly endangered, threatened by habitat loss, especially, but with lesser threats from poaching and water pollution. There are only about two hundred breeding pairs of gharial left in the wild: that’s a pretty scary number; the ICUN lists gharials as critically endangered. Crocodiles have survived for millions of years on earth, and have always come out as survivors — even at their current levels of diversity, they show remarkable adaptations to a huge variety of habitats and ways of life, from the sea-faring salt-water crocodile to the unique gem that is Gavialis. Crocodilian conservation, unsurprisingly, is one of my big passions.

Please visit the Tomistoma Task Force and Gharial Conservation Alliance to find out more. If conservation  isn’t enough to motivate you, I will point out that the GCA has the world’s most adorable gallery of little kids’ pictures of crocodiles.

And I know that after a two-day hiatus, you must be itching for some butt-kicking reptilian action. That’s good, because I have not one but two surprises planned for you … tomorrow’s post, and a new blog page, which might be up as early as tonight!

Relevant Papers:

The rapid accumulation of consistent molecular support for intergeneric crocodylian relationships
John Gatesey and George Amato, Molecular Phylogenetics and Evolution, February 2008

(Want more? Just ask … I just figured I’d put up a recent review and leave it at that, since I have every paper ever published on gharials … or so it seems.)

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