Unless They’re Zombies, Fossils Don’t Live

I hate the phrase “living fossil.” The term should be eradicated from the vocabulary of science writers, and anyone who employs it should be promptly encased in Carbonite. “Missing link” is the only slogan that pisses me off more.

My acute allergic reaction to the idiom may be a little overwrought, I admit. But, to me, “living fossil” is nonsense that obscures more than it elucidates. Take the coelacanth, for example.

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. Horseshoe crabs are much the same. The Limulus polyphemus I used to watch on the tidelines of Delaware beaches are similar to horseshoe crabs from the deep past, but the species itself evolved geologically recently. No one has ever found a petrified example.

Ever since Darwin, however, such archaic creatures have been treated as the evolutionary equivalent of hipsters. Coelacanths, horseshoe crabs, the duck-billed platypus, and their ilk could not survive in places of high competition, so they stagnated in a static evolutionary adolescence far longer than is socially acceptable. Darwin, in his pivotal 1859 book On the Origin of Species, characterized such creatures this way:

As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus [duck-billed platypus] or Lepidosiren [lungfish], which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station.

Darwin believed that such primeval forms had some utility in filling evolutionary gaps, but their primary importance was illustrating that natural selection accounted for lack of change as well as dramatic transformation. Should an organism settle into a cozy niche somewhere, and be afforded protection from more vigorous competitors, then there would be little reason for change. If evolution keeps creating descendant species that are fitter than their ancestors, the argument goes, such hold-outs could only survive by finding some quiet pocket where they would not be bothered by all that “nature red in tooth and claw” business.

I think Darwin’s principle is essentially correct. Natural selection accounts for the fact that some evolutionary stragglers persist alongside highly-derived, specialized forms of life that have changed considerably since they split from their last common ancestor with the more archaic forms. But I still think “living fossil” is an appellation that ultimately masks the diversity of organisms that Darwin celebrated.

Think of all the classic examples of evolutionary sluggards. What do they have in common? Typically, waders in nature’s backwaters are easy to pick out because they are among the last of their kind – there may only be a single species, or a handful of very similar species which closely resemble a prehistoric relative. But it’s easy to connect one modern example to a similar species in the fossil record. This totally obscures the fact that seemingly primitive creatures are just the remaining examples of groups that were once more diverse and disparate. 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.

By the typical criteria, our species could fit the description of a “living fossil.” We’re a lone species, we have a fossil record going back about 200,000 years or so, and, really, we’re not exceptionally different from the first hominins. Even though the emergence of the first humans is one of the most-studied, debated, and popularized transitions in evolutionary history, there is a much deeper difference between the first land-dwelling whales and their later, entirely-aquatic counterparts than between us and early humans such as Ardipithecus. (Provided that Ardipithecus is a human; a debate for another day.)

Of course, we don’t see ourselves as living fossils. We look at our past and say “Wow. Look at how far we’ve come!” But imagine if duck-billed platypus or crocs had the same perceptive powers. They’d probably write books about the glorious rise of their kind and the dramatic transformation from eo-platypus or the ur-croc to the modern forms (only their academics would have the advantage of poison spurs and snapping jaws, respectively, in their scientific debates). The designation of “living fossils” is entirely subjective – a title bestowed by those who back into the chasm of prehistory and say “Meh, looks all the same to me.”

I intentionally wrote that last bit to be provocative. Hominins have undergone significant changes during the past six million years, from the tops of our heads to the soles of our feet. Still, the change isn’t as overwhelmingly spectacular as the evolution of the first tetrapods or the first avian dinosaurs, to pick just two. We have a habit of finding significance in every jot and tittle of hominin history because we’re talking about us. So it’s not really surprising that many of the organisms we think of as “living fossils” are far removed from us on the evolutionary tree. Their anatomy is so unfamiliar that it’s easy for us to brush off similar tweaks with a shrug. To us, a croc looks like a croc. To crocs, on the other hand, humans have looked like food for millions of years.

This has all been background for two papers that have sent me into this snit. Don’t get the wrong idea, though. The papers didn’t disrespect persistent forms of life with the title I abhor , but, instead, rightly undermined the hackneyed concept.

The first was published by Andrew Wendruff and Mark Wilson in the Journal of Vertebrate Paleontology this past May. While the modern image of the coelacanths is of a slow piscine dullard awkwardly paddling in the depths of the Indian Ocean, Wendruff and Wilson described a Triassic coelacanth of a very different sort. Named Rebellatrix divaricerca, this lobe-finned fish had a forked tail and a much more streamlined shape than seen among other coelacanths – signs that this was a fast-moving predator.

Regardless of how Rebellatrix made a living, though, Wendruff and Wilson ascertained that the 240 million year old fish was distinct enough to challenge that idea that coelacanths have been evolutionarily static for the past 300 million years. Previously, paleontologists thought that coelacanths went through an initial explosion of disparity – with many different body forms – only to settle in to the typical body plan we see today. Rebellatrix shows that this wasn’t the rule, and that there may be other unusual coelacanths waiting to be found. A combination of preservational quirks and expectations about evolutionary patterns may have hidden additional strange coelacanths from us. While it’s certainly true that coelacanths similar to the modern species have been around for a long time, today’s coelacanths don’t entirely represent the history of these peculiar fish. By thinking of still-swimming coelacanths as living fossils, totally stuck in a prehistoric guise, it’s easy to overlook the prehistoric fish that departed from what we think of as the standard coelacanth body type.

Researchers C. Meloro and M.E.H. Jones attack the living fossil label more directly in a new Journal of Evolutionary Biology paper about Sphenodon, better known as 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 his latest book, paleontologist Richard Fortey cast the tuatara, coelacanth, and similar organisms as “survivors.” This is a more positive spin on their identity. Rather than being left behind by evolution, and therefore aberrations unworthy of our attention, these primitive-looking forms of life can tell us much about the history of life on earth and the mechanics of evolution. And there certainly is a phenomenon worth investigating here. “Living fossil” is awful because it’s an inaccurate term that doesn’t do justice to the lineages the label is applied to, but why certain forms have persisted for so long is something worth puzzling about.

Even though they were talking about species-level change in their controversial punctuated equilibria paper, rather than higher-level trends, Niles Eldredge and Stephen Jay Gould reminded us that stasis is just as important to understand as rapid transmutation. What can “persistent types”, as Thomas Henry Huxley once called them, tell us about the history of life on earth? That is something that will undoubtedly be debated for some time. If anything, though, the confusion over living fossils illustrates that we truly do need the fossil record. I’m exasperated with writers like Richard Dawkins who claim that we don’t really need fossils to illustrate evolution. True, evolution is continuing all around us, but the fossil record provides the essential context we require to perceive evolution as fact, theory, and path. Without a deep knowledge of prehistoric life, we only end up insulting some of the hardiest, most wonderful forms to have ever evolved. Long live the platypus!

References:

Meloro C, & Jones ME (2012). Tooth and cranial disparity in the fossil relatives of Sphenodon (Rhynchocephalia) dispute the persistent ‘living fossil’ label. Journal of evolutionary biology PMID: 22905810

Wendruff, A., Wilson, M. (2012). A fork-tailed coelacanth, Rebellatrix divaricerca, gen. et sp. nov. (Actinistia, Rebellatricidae, fam. nov.), from the Lower Triassic of Western Canada Journal of Vertebrate Paleontology, 32 (3) DOI: 10.1080/02724634.2012.657317