What Remains in the Rock

Last summer, I drove hours out of my way to visit Archaeopteryx. The gorgeous Thermopolis specimen – the only representative of this feathered dinosaur in the United States – is housed in a small display at the Wyoming Dinosaur Center. Even though the stop added hours to my marathon drive between Ekalaka, Montana and Dinosaur National Monument, Utah, I couldn’t resist rolling halfway across Wyoming to see the exquisite fossil.

For over a century, paleontologists cast Archaeopteryx as the urvogel – an archaic bird that smoothed the transition from reptile to avian. Strange, then, that Charles Darwin didn’t see Archaeopteryx that way. You’d think that this creature, sprinkled with transitional features, would have sent Darwin into fits of joy – Archaeopteryx was first described in 1861, just two years after On the Origin of Species was published.

While he was privately excited about the discovery, Darwin didn’t publicly crow about Archaeopteryx as a confirmation of his ideas. Darwin used the feathery theropod to call attention to a bigger, more important fact of paleontology. No one expected that something quite like Archaeopteryx would be found. In fact, many paleontologists and geologists of Darwin’s time believed that the fossil record had been more or less completely mapped – there were no missing chunks of Deep Time, and the various strata had all been examined. Darwin, following the lead of his friend and colleague Charles Lyell, insisted otherwise. The world’s rocks only recorded a fraction of the full history of life on earth, and naturalists had only just begun to discover prehistory. Archaeopteryx was a gorgeous confirmation of this fact, and Darwin portrayed it as such in the fourth, 1866 edition of On the Origin of Species. “Hardly any recent discovery shows more forcibly than this how little we as yet know of the former inhabitants of the world,” Darwin wrote.

Today, we know much more about prehistoric life than Darwin and his contemporaries did. Even dinosaur colors – one of the mysteries I was repeatedly told we’d never have any clue about – are starting to come into focus. But we’ve barely tapped the well of enigmas and spectacular fossils that remain in stone. Indeed, I was reminded of Darwin’s words this week when Nature News asked me to write a story for them about prehistoric turtle sex. I couldn’t resist. Preserved in 47 million year old slabs from Germany’s Messel fossil site, several pairs of turtles are locked in copulatory death positions. The association of males and females turtles, as well as the position of the tails in two specimens, indicate that these turtles were mating when they sank into a layer of toxic water and perished. The full details are available in the open-access paper at Biology Letters.

As turtle specialist Walter Joyce and colleagues note, their paper is not the first to suggest that Messel’s fossil turtles were mating when they died. Other paleontologists speculated about the same possibility for years. The significance of the new paper is that it provides the delicate evidence that these turtles truly did suffer death in the middle of reproduction, and that the unfortunate turtles can tell us something about the ecology of the ancient Messel Lake. I never thought I’d see prehistoric vertebrates “caught in the act” – as the paper’s title says – but here it is.

Yet there may be an even older example of vertebrates who died at the most inopportune moment. Twitter user Wolfram Schlossmann brought the curious fossil to my attention earlier today.

In 1990, paleontologist Richard Lund wrote a review of the various chondrichthyan fish – sharks, rays, and their closest relatives – found in Montana’s 320 million year old Bear Gulch limestone. Like Messel, this site was a lagerstätten deposit – a site of beautiful, high-definition preservation. Prehistoric sharks, often known only from teeth, can be seen in their entirety here.

Lund featured a special pair of sharks in the illustrations – a duo he had described a few years before in a different paper. The two, roughly six-inch-long sharks were a male and female of Falcatus falcatus. Both had the generalized, streamlined shark body form, but males are easily distinguishable by a long unicorn spike that curves up from behind their eyes. In this particular specimen, the female died with her jaws around the male’s head ornament, such that she died with her belly along his back.

These sharks were not copulating when they died. They were in the wrong position. Male sharks have cylindrical mating organs called claspers which jut out along their pelvic fins, and, in this fossil, the male shark’s genitals were not even close to being in contact with the female’s. All the same, the close association of a mature male and female is tantalizing. Perhaps these sharks were setting the mood before they were killed and rapidly buried. That seems to be a better possibility than the alternative of two sharks just happening to get buried together, especially since a pair of similarly-equipped sharks called Damocles serratus have been found in a similar position. Frustratingly, though, the fossil remains more of an innuendo than a clear demonstration.

The Bear Gulch sharks may or may not represent the earliest evidence of vertebrates mating in the fossil record. The amorous activities of the geologically younger Messel turtles are much clearer, thanks to the quirks of their behavior and preservation. And while these fossils offer only brief glimpses at the lives of long-dead animals, they are a reminder of the wonders that are most assuredly still out there, waiting to be discovered and described. As strange as prehistoric life seems, I am certain the past will seem ever stranger as we learn more. We yet know little about the former inhabitants of the world, as Darwin said; least of all how the myriad of bizarre prehistoric creatures got it on.

[I previously mentioned Darwin’s views on Archaeopteryx on p. 96 of my book Written in Stone.]

References:

Darwin, C. 1866. On the Origin of Species, 4th ed. John Murray: London. p. 367

Joyce, W., Micklich, N., Schaal, S., Scheyer, T. (2012). Caught in the act: the first record of copulating fossil vertebrates Biology Letters DOI: 10.1098/rsbl.2012.0361

Lund, R. (1990). Chondrichthyan life history styles as revealed by the 320 million years old Mississippian of Montana Environmental Biology of Fishes, 27 (1), 1-19 DOI: 10.1007/BF00004900