258 Evolution? The Fossils Say YES!
crawl on its belly. Most people have bad feelings about snakes for a variety of reasons on
which psychiatrists have long speculated. To a great extent, it is probably because there are
many different species of venomous snakes, so all sorts of animals (including ourselves)
have evolved a natural fear of them. But snakes can be wonderful, too. They are amaz-
ing machines of adaptation, with the ability to completely stretch their skulls and mouths
around a prey item much larger than their head, and their incredible adaptations for living
in every environment, from the desert to the jungle treetops to the open ocean. Only the cold
temperatures of the polar and subpolar regions prevent them from living in those habitats.
They are tremendously diverse, too, with over 20 families and dozens of genera and species
alive today.
The odds of finding “missing links” for the “snake kind” seem astronomical. Snake
skulls and skeletons are made of hundreds of tiny, lightweight bones that break up easily
when the snake dies and have a poor potential for fossilization. Only a handful of fossils of
complete snake skeletons are known. Most fossil snake species are known only from iso-
lated vertebrae. Thus we have a very patchy fossil record for snakes in general, as would be
expected for animals with such poor potential for preservation.
However, occasionally we get lucky. In some extraordinary localities like the Eocene
Messel lake beds in Germany, we get complete articulated snake skeletons of essentially
modern-looking snakes. And for some reason, we also got lucky when snakes were first
branching out from their lizard ancestors in the Cretaceous. A number of fossil snakes
with hind limbs (fig. 11.9) are known from the mid-Cretaceous of Israel, Lebanon, and
Croatia, including Eupodophis, Pachyrhachis, and Haasiophis. In 2006, a slightly older fossil
snake, Najash rionegrina, was reported from the mid-Cretaceous of Argentina. It not only
had hind limbs but fully functional hip bones as well. Then, in 2007, an even better tran-
sitional fossil was reported from rocks in Slovenia that are 95 million years old. Dubbed
Adriosaurus microbrachis, it was an extremely long-bodied marine lizard with fully func-
tional hind limbs but tiny vestigial front limbs (fig.11.10E and F), showing yet another
step between normal lizards and completely legless snakes. Finally, in 2015, the fossil
Tetrapodophis (“four-legged snake”) was found in beds from the Early Cretaceous of Brazil.
It has four tiny limbs, but they are clearly too small to have any real function, and must be
vestigial (Martill et al. 2015).
This should come as no surprise; we know snakes evolved from lizards that have empha-
sized locomotion driven by the sinuous motion of their bodies. In fact, a number of lizards,
such as the skinks, have highly reduced limbs that are almost nonfunctional. And the legless
snakelike body form has evolved several times in the vertebrates, not only in the snakes
but also in another living group of reptiles known as amphisbaenids, in a living group of
amphibians known as apodans, and in an extinct group of lepospondyl amphibians known
as aistopods. Finally, the proof is found in the living snakes as well. A number of groups,
such as the boids, still retain the vestiges of their hind limbs and pelvis (fig. 4.9).
Where did snakes come from? What is the nearest relative of snakes? This is a hotly
debated topic. Some scientists point to evidence that links them with the marine mosasaurs
(Caldwell and Lee 1997), while others cluster them with the amphisbaenid lizards (Rieppel
et al. 2003), and the molecular evidence seems to link them with anguimorph and iguanid
lizards (Harris 2003). Clearly, the jury is still out, and we have a lot of work to do. The biggest
problem is that the snake skeleton is so specialized, with so many elements reduced or lost,
that it is hard to make comparisons with any other group of reptiles. But whether we know