Trilobite! Eyewitness to Evolution

In preparation for Andy Secher's new book Travels with Trilobites I decided to first reach back in time to read Richard Fortey's 1999 book Trilobite! as a warm-up exercise. Why? For no other reason than that Fortey's autobiography A Curious Boy impressed me so much that I bought several of his earlier books and I need an excuse to read them. This, then, is the first of a two-part dive into the world of that most enigmatic extinct creature: the trilobite.

Fortey's first encounter with a trilobite fossil at the age of 14 led to a life-long career as a palaeontologist and trilobite specialist at the London Natural History Museum, from which he retired in 2006. As a group, trilobites were tremendously successful, surviving for almost 270 million years from the early Cambrian (521 million years ago) to their extinction at the end of the Permian (252 million years ago), spanning nearly all of the Palaeozoic era. Looking through their eyes (which is something you can literally do), there are numerous fascinating facets to their story, and Fortey expertly reveals some of these here.

Take their curious biology. Their segmented forms are instantly recognisable, but look closer and you will see a "mixture of strangeness and familiarity [....] The trilobites are lodged in this betwixt and between category, familiar as arthropods, yet strange in all their particularities" (p. 207). The fossils we find are frequently only (fragments of) cast-off shells. Trilobites went through numerous moults as they grew, just as crustaceans and insects do today, and thus were "veritable fossil factories" (p. 34). Their soft undersides were preserved only very rarely so we initially had no idea what their legs looked like. That answer came at the beginning of the 20th century from pyrite fossils and x-ray photos.

Consider, next, their growth. Starting as a disc-shaped larva, individuals would develop a split between the head and tail, with the tail-end budding off thoracic segments as the larva grew. When the species-specific number of thoracic segments was reached, the individual would simply grow larger with each subsequent moult without adding further segments. Finally, consider the eyes. Compound eyes like those of insects, yes, but uniquely made out of pure, transparent calcite. Fortey details how they worked and how the orientation of the individual hexagonal lenses has been used to reconstruct a trilobite's field of vision. Remarkably, some species with larger lenses even solved the problem of chromatic aberration by incorporating a thin, bowl-shaped layer of magnesium in the lens, nature pre-empting the very solution that scholars such as Huygens and Descartes would propose in the 16th century. For Fortey, they are a clear example of the tempting but mistaken belief of life's story as one of linear progress. How would you compare the trilobite eye against that of the dragonfly or the wasp? "Who is to calibrate progress, who to legislate on the unit of improvement?" (p. 103).

But trilobites also enlighten other aspects of science. They were studied by Victorian geologists who defined and named the geological periods we still use today. Part of Fortey's work has been to use the distribution of species through time and space—their palaeobiogeography—to reconstruct maps of our planet through time. As an example, the trilobite faunas found on the eastern and western sides of Newfoundland differ radically (Newfoundland was once split in two), while those of western Newfoundland match those recovered in Scotland (the two were once connected). The picture he restores shows how "around each ancient plate the continental shelves were stacked in order, carrying tier after tier of different trilobites, each minding their own particular business. [....] trilobites dissected their world into throngs of niches: this is how they became the 'beetles of the Palaeozoic'" (p. 205–206).

Other big-ticket evolutionary topics trilobites have shone a light on is that of punctuated equilibrium, the idea that evolution proceeds in fits and starts. Niles Eldredge's work on trilobites provided the first spark. He saw periods of rapid species turnover followed by long periods of stasis which contrasted with the then-dominant idea of slow and steady evolutionary change (as Fortey shows, there are examples of this as well in the trilobite fossil record). And then there is the matter of the Cambrian Explosion, which saw the sudden appearance of a wild diversity of arthropod forms. "In popular accounts it became an ancient moment of madness, a magnificent evolutionary Mardi Gras, when a parade as bizarre as could have been devised by a surrealist on speed would be permitted for a geological day" (p. 125). But how much of an explosion was this really? Fortey has drawn up family trees of the Burgess Shale invertebrates, showing high degrees of relatedness; we may have over-egged the peculiarities. Where are their Precambrian ancestors? One interesting idea he floats here is that trilobites appear suddenly in the fossil record thanks to an increase in size; "the 'explosion' was a dramatic appearance of characters that had been rehearsing out of sight for more than a hundred million years" (p. 133).

Finally, the sometimes remarkable lives of individual scientists are wrapped up in the story of trilobites. Fortey mixes in some autobiographical elements but mostly highlights the lives of others. There is the tragic story of Rudolf Kaufmann, executed in WWII, who came to the same insights on punctuated equilibrium 40 years before Eldredge. There is the intriguing story of 19th-century French geologist and palaeontologist Jacques Deprat who build up a formidable reputation up to World War I, to then fall from grace when doubts arose about the identity of some supposed Chinese trilobite specimens. After being evicted from the Société Géologique de France in 1920, he took on a new identity and resurfaced as a successful novelist, a fact not revealed until his death in 1935. And Fortey shared offices with the Cambridge scholars that studied the Cambrian Explosion. He analyses in some detail the fall-out between Simon Conway Morris and his erstwhile mentor Gould over what exactly the invertebrates of the Burgess Shale tell us about the evolutionary history of life.

This multifaceted story is driven along by Fortey's writing, which I found to be as captivating when he was writing in 1999 as when writing A Curious Boy in 2021. The book is complemented by black-and-white photography of notable species. One or two of the inline photos are poor-quality monochromes, but especially the plate sections contain some particularly fine photography. However, if it is visual splendour you want, we need to turn to Travels with Trilobites, which I will review next.