A horseshoe crab on a Delaware beach.
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Photo by Brian Switek.
A horseshoe crab on a Delaware beach.

Evolution in the Slow Lane

One late spring weekend a few years back, my wife and I drove out to Delaware to see an amazingly old tradition.

Knowing that both time and tide were critical, we had asked around for the best spot and right hour. Prime Hook National Wildlife Refuge at sunset was the most popular answer, and so, after a day of reading in camp, we pulled up to a beach shaded orange by the evening light. Tracey and I strolled down the beach for a while, watching sanderlings and ghost crabs go about their respective business, but we weren’t greeted by the natural spectacle we had hoped to see. The only sign of the ancient players were dried, gull-pecked husks scattered on the sand.

We were about to give up for the night when a receding wave briefly revealed what we had driven so far to see. There, in the dark water, were two horseshoe crabs, the male clasped onto the back of the larger female. They barely looked alive, more like olive-shaded helmets than animals, yet there they were, doing their part to perpetuate the species. On previous nights the tideline had been covered with similar pairs, but, even though I had missed the peak of horseshoe crab mating season, I was happy to get even a glimpse of nuptials that have been going on much the same way since the Jurassic.

But it would be a mistake to call horseshoe crabs “living fossils.” The term is catchy, and was coined by none other than Charles Darwin himself, but it’s only of those sneaky turns of phrase that quickly breaks down under close examination.

The pop definition of living fossil, as handed down by nature documentaries, is “a species that has gone unchanged for millions of years.” But this doesn’t work for even the most famous examples of supposedly static species.

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The evolution of coelacanths (top) versus tetrapods (middle) and ray-finned fish (bottom). From Cavin and Guinot, 2014.

Species are always changing genetically, if not anatomically. That’s why classic “living fossil” species aren’t found in the fossil record. Consider the coelacanth. There are two living species of this fleshy-finned fish, Latimeria chalumnae and Latimeria menadoensis, neither of which is found in the fossil record. Granted, there’s a 66 million year span in which the only coelacanth fossils are questionable fish bits, but it’s still worth noting that today’s coelacanths are readily distinguishable from their Cretaceous counterparts. Furthermore, if “existence of a species in the fossil record” is what makes a living fossil, then the fact that paleontologists have found Homo sapiens remains dating back to 200,000 years ago would place us in that category while exempting the coelacanth.

Instead, today’s coelacanths are representatives of a lineage that has evolved relatively slowly and spun off fewer body plan variations than others. That’s why they seem so ancient. In a study of coelacanth evolution published earlier this year, paleontologists Lionel Cavin and Guillaume Guinot compared the rate at which coelacanths evolved new, distinct evolutionary features with those in tetrapods (four-limbed vertebrates that came up on land and proliferated) and ray-finned fish. Coelacanths, Cavin and Guinot estimated, have evolved at a rate six times slower than tetrapods, and three times slower than ray-finned fish. After a brief spurt of wild evolution around 400 million years ago, coelacanths haven’t evolved dramatically-different body types.

So coelacanths aren’t living time capsules, but are part of a 400 million year old lineage with a conserved body plan. Horseshoe crabs tell a similar tale.

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Fossils of Limulus darwini. From Kin and Błażejowski, 2014.

Earlier this week, paleontologist Błażej Błażejowski published a new paper coauthored with the late Adrian Kin on a 148 million year old horseshoe crab that belongs to the same genus as the copulating arthropods I saw on the Delaware beach. While the living species is Limulus polyphemus, Błażejowski and Kin elected to name the fossil form, found in the limestone of Poland’s Kcynia Formation, Limulus darwini. The resemblance is striking. While there are a handful of characteristics that distinguish them, juveniles of today’s horseshoe crabs are the spitting image of the Jurassic species. This is the oldest representative of the Limulus lineage yet found, supplanting a Cretaceous horseshoe crab that had been discovered in Colorado.

If we took a condescending view, we could say that horseshoe crabs have been stuck since the Jurassic. While other forms of life flourished and were modified into fantastic new shapes, the horseshoe crabs kept grubbing in the sand for worms and clams. But the truth is that they’ve maintained their shape for so long because they are a great evolutionary success story. Modern horseshoe crabs are generalist feeders capable of living in waters cold and warm, deep and shallow. If the same was true of their prehistoric predecessors, it could explain why horseshoe crabs have gotten along just fine without major anatomical overhauls.

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A comparison of modern and fossil Limulus. From Kin and Błażejowski, 2014.

But what should we call such creatures? Living fossil doesn’t work as it obscures the nuances of evolution. And other descriptors coined for slow-evolving lineages, such as George Gaylord Simpson’s concept of “bradytelic” groups, are too technical. Błażejowski and Kin instead suggest a new term – stabilomorphs.

The  concept is a bit more refined than the notion of living fossils. Stabilomorphism, the researchers write, is “relative morphological stability of organisms in time and spatial distribution, the taxonomic status of which does not exceed genus level.” And there’s an additional corollary. Stabilomorphs must have survived at least one major mass extinction. This would mean that today’s Limulus would count as stabilomorphs, but crocodiles, coelacanths, and pearly nautilus would not.

Unfortunately, though, stabilomorph doesn’t have the cultural cachet of living fossil, and I can’t see the term showing up in science headlines anytime soon. If we’re going to sink the term living fossil, we need something that’s a little more accessible. I’m not enough of a wordsmith to coin a new one, but I have always liked Thomas Henry Huxley’s approach to the problem.

While widely known as “Darwin’s Bulldog”, Huxley wasn’t initially enamored with his friend’s formulation of evolution. When he looked into the deep past, Huxley didn’t see transcendent change, but rather minor variations on themes. In Huxley’s estimation, for example, the crocodiles of the Mesozoic looked little different from those of today. He called these examples “persistent types”, and suggested that most of evolution’s great transformations happened during a much earlier, “non-geologic” time, with the products maintaining their general form to the present.

By 1870, though, Huxley had largely given up this view of life. In the “higher Vertebrata”, at least, there were fantastic examples of evolutionary change, among the most spectacular being the evolution of horses from tiny, multi-toed ancestors to their big, single-toed modern forms. And while persistent types still existed, Darwin had squared them with evolution by natural selection. The winnowing edge of natural selection explained change as well as lack of change, meaning that the platypus and the horseshoe crab are just as important to understand as the quickly-diversifying beetles that enchanted Darwin early on.

While Huxley’s term is more qualitative, referring to forms “which have remained with but very little apparent change from their first appearance to the present time”, I think it has a far better chance than stabilomorph at cracking the public consciousness. And while not quite as evocative as living fossil, it still has a tinge of poetry to it. They are forms that truly have persisted, withstanding mass extinctions that have wiped out so many other varieties of life. By seeming to go against the grain, they remind us of the power of natural selection to preserve as well as modify and eliminate. Simply put, persistent types have withstood the test of time.

References:

Cavin, L., Guinot, G. 2014. Coelacanths as “almost living fossils.” Frontiers in Ecology and Evolution. doi: 10.3389/fevo.2014.00049