Posted by: scrubmuncher | May 16, 2010

Who needs limbs?

Limbs are such endlessly useful appendages that it seems odd that so many terrestrial vertebrates should have lost them. More than any other vertebrates it seems the reptiles have gone out of their way to eschew limbs – just consider the diversity of snakes as a testament to how successful this legless way of life is. These limbless reptiles, indeed all limbless vertebrates have one thing in common: they are or they have evolved from fossorial animals. A fossorial existence is one way of life where limbs are nothing more than a hindrance and in many of the vertebrates that turned their back on the sky for a life underground evolution has gradually reduced these structures to mere vestiges of their former, multi-functional glory.

As well as being quite common, the loss of limbs has also spawned some of the most bizarre terrestrial vertebrates, with my personal favourites being the amphisbaenids. Commonly known as worm lizards these very peculiar animals are, unsurprisingly, best described as worm-like lizards, but their exact evolutionary relationships to the other lizards is something of a mystery.

Fossorial and seldom seen, precious little is known about the biology of the 160 or so described species of worm lizard. Almost all of them live in the tropics or sub-tropics and it is their admirable adaptations for a subterranean existence that really single them out. Along with their limbs they have all but lost their eyes and they retain just the vestiges of these organs – tiny, beady and covered with a thin scale.

Although this worm lizard must have been miffed it didn't get away it still looks decidely cheerful. Note the tiny eyes beneath a transparent scale. Iberian worm lizard (Blanus cinereus) found near Trujillo, Spain (Ross Piper)

A subterranean way of life has freed up the constraints on skin colour in many species of worm-lizard. There's virtually no selection pressure for cryptic colouration in an animal that spends its life underground. When worm-lizards are seen on the surface it almost always because flooding has forced them out of their burrows or they've been dug out. The species shown here is Amphisbaena fuliginosa (johann97313.skyrock.com)

Worm lizards make their tunnels by using their head like a battering ram, a rather brutal, yet effective technique necessitating a heavily reinforced skull where there individual bones of skull are all fused. Some species have a spade-shaped head, which is used to compress the soil into the top of the tunnel, while other species have a keel shaped head they use to press the soil in to the sides of the tunnel by vigorously moving their head from side to side. Another adaption to a fossorial way of life peculiar to these animals is their skin, which is very loosely attached to their body, enabling them to ‘slide’ through their integument backwards just as well as forwards, an important attribute in the confines of a tunnel.

Worm lizards have among the most specialized skulls of any vertebrate. They're extremely strong to withstand the rigours of burrowing. Also, take a look at the teeth. It's clear these animals are nowhere near as innocuous as they look (digimorph.org)

Worm lizards may look pretty innocuous, but they are actually voracious predators with a mean set of teeth. Their dentition is unique, as a single tooth in the middle of the upper jaw meshes perfectly with a pair of teeth in the lower jaw forming an effective pair of pincers. Using these pincers and their powerful jaw muscles the worm lizards make short work of any small animal they find in their tunnels and some species are known to invade the nests of ants and termites to feed on the nutritious goodies within. They will even make daring forays to the surface to forage for suitable prey.

In Latin, Amphisbaena means to ‘move in both ways’ reflecting the animal’s ability to move forwards and backwards equally well. Also, in myth and legend the amphisbaena was a fabulous beast with a head at either end of its body, perhaps the brainchild of a medieval chronicler who saw a worm lizard in its defensive posture. Worm lizards on the surface are at their more vulnerable and some species raise both ends of their body towards the sky to confuse the predator as to which end is its head. Any predator is wise to steer clear of the head as the powerful jaws with their sharp teeth are capable of inflicting a painful bite. As well as a nasty nip many of the worm lizards also defend themselves by wrapping their strong bodies around the body of an attacker in the hope the predator will become perturbed and lose interest.

Worm lizards can confuse a potential predator by pretending its has two heads (an illustration by Mike Shanahan taken from Extraordinary Animals).

Although characterised by a lack of limbs, a really odd group of worm lizards from the Baja peninsula have retained their forelimbs making them look like something that has been put together for a prank and to be honest they look more than a little unsettling (see below). The forelimbs are akin to those you might find on the front end of a mole and they use them to great effect to tunnel in loose soil.

If you’re perturbed by odd looking beasts it’s probably best to look away now:

Now, if there's a more sorry looking reptile than the Baja worm lizard (Bipes biporus) I'd like to see it.

This is how the Bipes biporus uses its incongruous looking forelimbs:

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