Zombie Fossils

I was reading Brian Switek at Laelaps who goes on a bit of a rant, albeit a carefully researched and knowledgeable rant, on a favourite topic of mine: the non-existence of living fossils. In his article, he takes on some of the most commonly misrepresented groups: coelacanths, crocodyliforms, and Sphenodon (also known as the tuatara), he also brings in the platypus and even human beings.

First the coelanths:

The coelacanth is the quintessential primeval creature – a form of fish that was thought to have disappeared with the last of the non-avian dinosaurs about 66 million years ago, at least until Marjorie Courtnay-Latimer recognized a modern one at a South African fish market in 1938. To specialists, the coelacanth species Latimer stumbled across is called Latimeria chalumnae. More recently, marine biologists discovered a second species, Latimeria menadoensis. These species have never been found in the fossil record. The modern coelacanth species belong to an ancient lineage of fish that goes back 390 million years, but they are not unchanged hold-outs, either.

A restoration of the Triassic coelacanth Rebellatrix. Art by Michael Skrepnick.

Than the crocodyliforms:

Consider the crocodyliforms. Today, all of these archosaurs – the alligators, caimans, crocodiles, and gharials – are roughly similar-shaped aquatic ambush predators, but their prehistoric kin include everything from swift, terrestrial dinosaur-hunters to fearsome marine predators that were beautifully adapted to life at sea. The common claim that crocs “haven’t changed since the time of the dinosaurs” is bullshit.

Dakosaurus andinensis & Caypullisaurus. by Dmitry Bogdanov

And then the tuatara:

The little reptile looks like a lizard, but is actually the last remaining member of a different evolutionary branch called the Rhynchocephalia. This has made the tuatara another classic example of natural stagnation, with some researchers claiming that the reptile has remained unchanged for 220 million years.

Meloro and Jones demonstrate that today’s tuatara isn’t a Triassic zombie. After using a method called geometric morphometrics to compare the skulls of modern Sphenodon with thirteen of its fossil relatives, the biologists found that the tuatara is actually quite different from its prehistoric forebears. The shape of the eye sockets, snout, back of the skull, and number of teeth, for example, differed among the lizard-like reptiles, most likely reflecting changes in diet. In terms of skull shape, at least, the rhynchocephalia were a disparate group of reptiles that had a variety of lifestyles. Why only one species is left today is unclear, but, as Meloro and Jones note, this makes the conservation of the tuatara all the more critical. The reptile isn’t a Triassic leftover, but the last remaining part of a varied and long-lived lineage. If we lose this single species, an entire branch of the evolutionary tree will die.

In referring to human beings, Switek brings up that we are the sole survivors of the long and varied number of species that have existed since the first hominid species approximately somewhere between 2 and 8 million years ago, or perhaps even longer.

Undated artist’s reconstruction of Pierolapithecus catalaunicus, 13 million-year-old ape that MAY be the common ape-human ancestor.

The suggested alternative ‘survivor species’ captures the concept of these species or groups much better than idea of a ‘living fossil’. It’s definitely time to retire the words and the idea of living fossils.

About these ads
This entry was posted in Biology and tagged , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s