The first time I can remember seeing a trilobite, it wasn’t in a museum case or a book about prehistoric animals. It was on card 39 of the gratuitously gory Dinosaurs Attack! card series, a horrific vignette depicting four of the invertebrates crawling over the bloodied face of their hapless victim. (No indication was given as to how the “flesh-eating worms”, as the card identified them, had subdued the man.) The card was entirely fiction, of course, but it still fit in with the image of trilobites as mud-grubbing bottom dwellers which fed on whatever detritus they might find. In almost every depiction of trilobites I can remember, the marine arthropods were shown in the same way – doggedly trundling over the sea-bottom on their tiny legs.
These simplified illustrations of prehistoric sea life did not do the trilobites justice. Time and again they were shown as relatively boring denizens of the Cambrian sea – I had no idea that they were a highly diverse group of marine arthropods which persisted for over 270 million years. They were not simply Cambrian bottom-crawlers, and a new discovery presented by Brigitte Schoenemann, Euan Clarkson, Per Ahlberg, and Maria Alvarez in the journal Palaeontology helps illustrate the disparity trilobites evolved during their heyday.
The term “trilobite” is about as specific as “mammal” – it represents a large, diverse group of organisms. One of the many groups of trilobites were the olenids, a group which straddled the time between the end of the Cambrian and the Late Ordovician (or about 499 million years ago to 445 million years ago), and even though most members of this group did skitter over the sea bottom at least one Cambrian species was a highly-specialized form which lived a very different kind of life. First described in 2002 from fossils found in Sweden and called Ctenopyge ceciliae, it was among the smallest of all trilobites (measuring only about a millimeter and a half across the long spines sticking out from either side of its head shield), and the clues as to what it was doing was found in its fossilized eye.
Like other trilobites, Ctenopyge ceciliae had compound eyes which became larger as it aged, but the size of each lens stayed the same throughout its development. The lenses increased in number rather than size as it grew, going from about 10 lenses in a young animal to about 150 in the adult. These lenses were each oriented in a slightly different direction, and while young individuals had just enough lenses to detect the direction of light older specimens had enough lenses to create a rough, mosaic image which probably would have allowed the trilobite to pick out other trilobites and organisms among the floating plankton. (As the authors state, the light coming into each lens would be like a single pixel in an digital photograph – the more a trilobite had, the greater its visual resolution.)
At first glance it appeared that Ctenopyge ceciliae was active in low-light conditions. Its lenses were comparatively large and few, meaning that in the trade-off between increased resolution and taking in more light it was pushed down the latter route. When the overall size of the trilobite was taken into account, however, a different perspective emerged. Perhaps the lenses of Ctenopyge ceciliae were comparatively large for its size because they simply couldn’t get much smaller. The constraints of its evolutionary history and the peculiarities of optics caused it to be a tiny trilobite with big lenses best suited to taking in as much light as possible. Hence the authors of the new paper suggest that, rather than eschewing the light, Ctenopyge ceciliae was dependent upon it. Indeed, the eyes of young individuals would not have been able to do much more than detect the direction of light, and while adults had a bit better resolution they, too, probably would have had difficulty seeing when things got dark.
Given that the eyes of Ctenopyge ceciliae at any life stage would not have performed well under dark or low-light conditions, the authors propose that it remained in the thin slice of sea which could be penetrated by light. It may very well have been a planktonic trilobite, floating near the surface while its relatives plowed about on the seafloor. It did not possess the same adaptations to free-swimming as other strange trliobites, but instead appeared to be well-suited to floating along.
Unfortunately we cannot observe a living Ctenopyge ceciliae to further test these hypotheses about its lifestyle, but given its small size, eyes which could do little more than detect light and shapes, and its array of spines which would have helped keep it from sinking, it is reasonable to restore it as a component of the late Cambrian plankton community. Whereas other trilobites may have had planktonic larval stages, Ctenopyge ceciliae remained among the plankton assemblage for its whole life, and the compact anatomy of the adult stage attest to the evolutionary constraints which shaped its evolution.
SCHOENEMANN, B., CLARKSON, E., AHLBERG, P., & ÁLVAREZ, M. (2010). A tiny eye indicating a planktonic trilobite Palaeontology DOI: 10.1111/j.1475-4983.2010.00966.x