Trilobite! by Richard Fortey, HarperCollins, £15.99, ISBN 0002570122
THEY may resemble oversized baroque woodlice, but trilobites seem to fascinate all who love fossils. These distant marine relatives of horseshoe crabs are more than just attractive shells, however. Richard Fortey, who earlier gave us the bestseller Life: An unauthorised biography, shows in Trilobite! how their humble remains help us to date the ancient world and unravel the intricacies of evolution.
Fortey’s new book is an “unabashedly trilobito-centric view of the world”. It’s suffused with the expertise and affection of a lifetime spent with these common and attractive fossils: from a teenage find on the Welsh coast all the way to his post at the Natural History Museum in London. Rather than a photograph album (though there are plenty of good shots) or a technical treatise, this is a tour of trilobite science. Its charm lies in the fact that throughout, the personal is interwoven with the scientific.
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Fortey begins his trilobite tales with a gripping walk up a steep headland in the footsteps of a literary hero. Thomas Hardy’s novel A Pair of Blue Eyes has a key scene featuring a trilobite, but artistic licence means that the author could toss the fossil on any old cliff he chose.
Trilobitologists themselves are as gloriously mixed as Hardy’s fossil plot, says Fortey, from the Swede who learned English from P. G. Wodehouse novels and the bohemian Joachim Barrande, who inspired Prague’s Trilobite Bar, to C. D. Walcott, discoverer of the Burgess Shale, who found trilobite legs while distracting himself from grief over his wife’s death.
The first scientific sighting of trilobites took place 300 years ago, when Edward Lhwyd reported a “sceleton of some Flat Fish” from Llandeilo in Wales to the Royal Society in 1699. Trilobites were not recognised as a distinct group of animals, however, until 1771. Since then, we have derived a fantastic amount of information from their shells – some smooth, others rough or improbably spiky – that give us clues to their ways of life, whether mud-grubbing, walking on the bottom or free-swimming.
My favourite section has to be the one on eyes. Trilobites had staring arrays of little eyelets pointing in different directions. These compound eyes show very plainly what interested trilobites. The benthic ones looked at the seafloor, while the pelagic kept an eye, or rather eyes, all round. No other animals use crystalline calcite for lenses, which is hardly surprising given its image-splitting property of birefringence. The cunning trilobites grew the calcite lens for each eyelet in such a way that the light rays passed along the one crystal axis where birefringence does not occur.
One trilobite, Phacops,had spherical lenses through which an enterprising palaeontologist took photographs. How on earth could this creature see? Theoretically, it was mission impossible, because spherical lenses would warp the creature’s view of the world. But it emerged that Phacops differentially doped the calcite of the lens with magnesium to create doublet lenses that overcame the aberration.
Invertebrates rarely star in fossil books. Mostly it’s dinosaurs, or perhaps mammals, that grab the spotlight. This is a shame. Invertebrates are not only different and interesting in their own right, but often so much more numerous that you can do things with them that you can’t with dinosaurs. Trilobites thus help to trace the history of the Palaeozoic world – but only if one keeps up the taxonomists and museum collections necessary to identify them accurately.
Being marine and widespread, but also evolving reasonably rapidly, trilobite species prove to be excellent marker fossils for rocks of specific ages. It was partly the trilobite collections of Victorian geologists that helped us to trace the Cambrian, Ordovician and Silurian periods, allowing even amateurs to make a real contribution to science.
There’s more. Long series of strata rich in trilobites supply good statistical samples for microevolutionary research. That’s no guarantee that interpretations will agree, of course. One famous study supported punctuated equilibrium, while another detected phyletic gradualism, fuelling rather than resolving the debate on whether small-scale evolution proceeds gradually or jerkily.
Trilobites also play key roles in the wider issues of evolution. They are part of the puzzling Cambrian explosion – the apparently sudden appearance of shelled animals. The trilobites were already so diverse that we know that they were around and evolving long before, but this period remains hidden from us. They’re in the fauna of the Burgess Shale, that weird and wonderful mixture of animals that – on the face of it – represents a range of evolutionary experiments, only some of which survived. Fortey comments interestingly on the dispute over whether this diversity is real, as Stephen Jay Gould argued in Wonderful Life, or illusory, as Cambridge’s Simon Conway Morris holds.
Trilobites track changing geographies driven by plate tectonics. This enables researchers, including Fortey, to follow land masses independently of the palaeomagnetic evidence of geophysicists. Imagine two continents moving closer. The shallow-water trilobite faunas of the opposing coasts would originally be different, but as the separating ocean shrank, species would begin dispersing from one coast to the other, gradually becoming indistinguishable.
Trilobites ended a very respectable record of survival in the Permian. Fortey touchingly confesses to having once hoped that some survive, coelacanth-style, in the deep sea.
He ends his splendid book by commenting feelingly on the beauties of trilobite research (something that he has fully demonstrated) and historical science. It is, he says, neither the stamp collecting that hard physicists have been known to call it, nor barmy boffinry. He reflects on the nature of science as an interconnected web where minor details, such as rural fossil finds, can become central. And if you want true immortality, then forget books, theories, displays and buildings: get a trilobite named after yourself.