Snakes and Lizards
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As part of my work on the K-T mass extinction, I've recently been studying the lizards and snakes that lived alongside the last dinosaurs, focusing on fossils from the late Maastrichtian Lance Formation of Wyoming and Hell Creek Formation of Montana, 65 MYA. These fossils are challenging to work with, because they consist exclusively of isolated elements- jaw bones, bits of the skull roof, vertebrae, and soforth. However, by using a phylogenetic approach building off of Jacques Gauthier's Amniote Tree of Life matrix, it's been possible to understand the relationships and diversity of these forms.
We ended up creating a new matrix to deal with this problem and the relationships of snakes, which involved endless hours of staring at CT movies of snake skulls, and playing with a snake skeleton, to try to figure out how they were put together, how they worked, and how they were related. The result is a new phylogeny of snakes that, we feel, represents a major advance.
Snakes are extraordinarily rare, so we were interested in figuring out what on earth Coniophis really was. Attempts to run this material in the AToL matrix produce some puzzling results. Coniophis kept coming out near Najash, the most primitive known snake. The initial assumption was that something was wrong with the analysis, but after further study of the material by myself and Anjan-Bhart Bhullar, we came to the conclusion that the animal was, in fact, some kind of weird proto-snake.
One of the more surprising results was the "discovery" of the most primitive known snake, Coniophis precedens (Longrich et al. 2012, Nature). Coniophis is not a new animal, in fact it was named in the 19th century by O.C. Marsh, the first head of the Peabody Museum. But all Marsh had to work with were vertebrae. It was generally assumed that Coniophis was related to the anilioids, or pipesnakes, on the basis of the overall resemblance of their vertebrae. In in 1960s, Richard Estes (Jacque's former advisor) identified pieces of Coniophis from new material collected from the Lance. However, the material was never described, and ended up neglected in collections for half a century. Nobody seemed to know that it even existed. Estes mentions the material in his classic 1964 paper on Lance vertebrates, but only in a footnote listing the fossil specimens. The main body of the text makes no mention of the jaws, so people who only read the text (such as myself) never realized there were any skull bones.
Jacques, Anjan and I have also recently completed a project looking at lizard diversity at the end of the Cretaceous (Longrich et al., 2012, PNAS). We show that the K-T mass extinction had a major effect on the evolution of lizards and snakes.
Coniophis has vertebrae of the sort associated with burrowing, which led us to conclude that snakes are not (as sometimes argued) descended from marine forms; instead theyare descended from burrowing lizards. The oldest good snakes are in fact marine- but these are already highly advanced, boa-like snakes, deeply nested in the snake tree. This seems sort of bizarre- your oldest snake are also among your most advanced- but the anatomy strongly supports this arrangement. The implication is that there was a huge explosion in snake diversity between 150-100 million years ago. The reason the marine snakes show up first is that the fossil record is just far better for marine stuff. The poor snake fossil record is probably also a function of their initial evolution occurring (I suspect) in South America/Africa, where the fossil record is absurdly poor.
We made the argument that snake diversity really takes off once cranial kinesis is in place. Coniophis has a relatively inflexible skull. Advanced snakes have kinetic skulls- they can literally dislocate their entire face, moving their left and right maxillae independently and splaying their jaws open to swallow prey as large as themselves. Once this system is in place, snakes have perfected the triple threat posed by their design: (1) the ability to hunt in darkness (using the tongue to scent prey, and the modified ears to detect vibrations; pythons and pit-vipers are also able to sense infrared with pit organs), (2) the ability to engulf huge prey (allowed by the ability to expand the jaws, as well as the rib cage, and (3) the limbless body, which allows them to burrow, to maneuver through densely vegetated environments, to climb, to crawl, and to swim. Together, these adaptations allowed snakes to act as cryptic, nocturnal predators of vertebrates. They are exclusively predators, but oh, what subtle predators they are... snakes eat pretty much anything. Insects, worms, frogs, lizards, other snakes, birds, bird eggs, little mammals, big mammals. In the Miocene, the colubroids (garters, cobras, racers, and soforth) stage a major radiation. Colubroids tend to be more day-active; the rise of diurnal snakes may be associated with cooling. When the global climate cooled, it may have been difficult for snakes to be active at night outside of the tropics.
In combining all of these adaptations, snakes were able to undergo an adaptive radiation that led to the evolution of almost 3,000 species.
In the Paleocene, constrictors suddenly exploded in diversity, taking advantage of an abundance of mammals and a rarity of predators after the K-T extinction. Among their prey were early mammals. And for the next 65 million years, the ancestors of humans evolved living alongside snakes large enough to constrict, kill, and eat them. Any primate that didn't have an instinctive fear of snakes tended to end up inside of one; that's why snakes have such a fascination for us, and so many villains (Voldemort, Satan, the bad guy from Conan the Barbarian) tend to be associated with snakes.
