Biology / Evolution / Palaeontology / Science

Trilobite shakeup

Paradoxides sp

The extinct trilobite Paradoxides. From Wikimedia Commons, image by Dwergenpaartje.

There are over 20,000 known species of trilobite, and their fossils are found all over the world. They existed in Earth’s oceans for around 300 million years in total before being finally wiped out at the end of the Permian period, and study of their fossils has contributed greatly to our understanding of evolutionary biology.

Trilobites, for example, were one of the first complex creatures to develop eyes, rather than simple light-sensitive patches. The American Museum of Natural History gives us an idea of the importance of trilobite eyes in the fossil record: “Few morphological features in the entire fossil record are as singularly significant as trilobite eyes.”

So it’s a bit of a shake-up to hear someone suggesting, as a team including Johan Lindgren have, that we’ve misunderstood some fundamental facts about trilobite eyes. Normally, we take it as read that trilobite eyes have lenses formed of calcite (calcium carbonate), because that’s how they’re found in the fossil record. There’s been research into how they could work, confirming (perhaps) that trilobites could have seen light and colour and had, all in all, a very useful set of lenses.

However, Lindgren’s team have found similar calcite crystals in fossils of crane flies. This doesn’t make much sense, they argue, because eye lenses made of calcium carbonate are not found in the surviving crane fly lineages we know of. They’re led to the conclusion that the calcite lenses are actually a trick of preservation, and that this has serious implications for the “well known” fact of trilobites and their calcite lenses.

It would be quite a shake-up if true, but others have been quick to point out that calcite lenses have been found in many different species of trilobites, fossilised under all kinds of different conditions. It would be surprising if the same accident of fossilisation happened in exactly the same way to every trilobite fossil. One accident of fossilisation is one thing, and the calcite in the crane flies’ eyes is at the moment a singular event — and thus much more likely to have been a one-off freak of preservation.

So, it’s probably not time to revise the textbooks, but it is still fascinating to think about.

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