Hitching a Ride on a Trilobite

Fossil skeletons are static things. They no longer grow or respond to the stresses and strains of life. Tucked away on shelves or propped up on intricate armatures, fossil bones and other mineralized hard parts show us the shape of ancient organisms, but we can only perceive whispers of the lives those remains represent.

The way we present long-extinct creatures in museum halls underscores this bare-bones view of prehistoric life. Skeletons often stand frozen on isolated little islands of rock and plaster, as if waiting to be clothed in flesh again. We know that these osteological vestiges were once the natural architecture of living creatures, but, looking at what remains of these animals as a whole, it is easy to think of these skeletons as little more than naturally-occurring sculptures.

Detecting the signs of ancient life requires much closer inspection. A sharp-eyed observer can pick out broken ribs, worn-down teeth, and inflamed bone from runaway infections among the skeletons of many fossil halls, but the word “skeleton” does not just apply to vertebrates. Many invertebrates also had skeletons, albeit of different sorts, and these mineralized remnants can also tell us something about these animals lived.

Trilobites are among the great “also-rans” of prehistory. Familiar to many, yet rarely appreciated for their diversity and evolutionary success, these archaic arthropods persisted from the riot of animal evolution triggered during the Early Cambrian 526 million years ago through their ultimate demise in the worst mass extinction of all time 250 million years ago. There were scores of genera and species represented today by an innumerable assortment of specimens, and given such a rich fossil record it is not all that surprising that some trilobites have been preserved in cloaks made of other organisms.

About 450 million years ago, during the latter part of the Ordovician period, much of the area around southern Ohio and northern Kentucky was underwater. The fossils from this time and place represent deepwater, hard-bottom ecologies frequented by trilobites such as Flexicalymene and a variety of other organisms (including corals, crinoids, bivalves, and other marine animals) which began life by colonizing whatever hard surfaces they were fortunate enough to land on. The shells of trilobites were among the bits of real estate prized by colonizing organisms, and a few instances of these interactions have just been surveyed in a paper by a team of researchers led by Dickinson College scientist Marcus Key.

Trilobites that played host to other organisms had been reported before, but Key and his colleagues focused on a large sample of specimens from the trilobite-rich shales around southern Ohio. Finding appropriate trilobites to study took quite a while.  Out of  the nearly 15,000 Flexicalymene specimens the scientists examined, only a very few – 0.1 percent – had been fouled by other organisms. The creatures that lived on the bodies of the trilobites were generally referred to as episkeletozoans in this study since they colonized the exoskeletons of living and dead Flexicalymene.

Of the 16 Flexicalymene specimens that were encrusted by other organisms, most (13) were true carcasses – meaning that they died in that state – while three were shed carapaces which had become preserved. This was significant, as a trilobite could be rid of its uninvited guests when it molted its old carapace. There was no single species of colonizer. Taken together, the Flexicalymene trilobites were hosts to multiple species of brachiopods (archaic bivalves), bryozoans (filter-feeding invertebrates), and tube worms, and in general the trilobite hosts were mature animals which ranged from early adults to very old individuals.

The lack of smaller encrusted Flexicalymene made sense based upon what is known of the trilobite’s life cycle. Not only would adult trilobites have provided larger surfaces for encrusting organisms to land on, but they molted less frequently and therefore colonizers had more of a chance to accumulate on the living animal. And, when colonizers were fortunate enough to land on a trilobite, they typically colonized the highest parts of the exoskeleton, preferring areas such as the eye ridges and the middle portion of the head (called the glabella). Why they did this is difficult to ascertain, but, on the basis of previous studies that used model trilobites, the scientists behind the new report hypothesize that the colonizers may have picked areas of the skeleton which would have brought them food through minor eddies and turbulence in the water caused by the anatomy of the trilobite.

In general, though, the colonization of trilobites by episkeletozoans was rare and represented only a tiny portion of the encrusting organisms in these types of environments. Sessile organisms were colonized more often and by more species, while highly mobile species – such as cephalopods – were only colonized rarely. The trilobites from the Ohio shales appear to fall towards the less-colonized end of the spectrum, and this may be attributable to the burrowing habits of Flexicalymene. If these trilobites were truly digging in under the surface as paleontologists have reconstructed, then there would have been relatively little opportunity for larvae of other organisms to become established on them. Furthermore, when larvae did land on the exoskeleton, the burrowing activity of the trilobite would have scrubbed the colonizers right off.

How did this close association affect the trilobites? That is difficult to answer. Given the difficulty starfish have finding clams encrusted with other organisms it has been proposed that fouled trilobites might have benefited from a similar type of camouflage. The problem with this hypothesis is that many trilobites were only covered over a small part of their body and were unlikely to fool a hungry predator. Still, the fact that many of the encrusted trilobites were old individuals may suggest that the relationship was an example of commensalism – the colonizing animals got a surface to attach to and the trilobites were not adversely affected. In other words, the trilobites were playing host to hitchhikers, not life-sucking parasites.

A bryozoan-sheathed trilobite isn’t exactly the type of specimen which gets top-billing at museums. Even though paleontology has matured beyond the stage of simply collecting the best, display-quality specimens, a fouled trilobite just can’t compete with a Tyrannosaurus rex skeleton when it comes to public attention. That is unavoidable. Nevertheless, there is something intricately beautiful about a Flexicalymene covered in other organisms. Encrusted with the traces of other ancient life, these specimens are rare vestiges of ancient interactions that we would otherwise be blind to.

Top image: Three Flexicalymene trilobites encrusted by other organisms. 1) Trilobite encrusted by bryozoan Heterotrypa. 2) The same specimen as seen from the side. 3) Another trilobite colonized by the bryozoan Amplexopora and an unknown tube-dwelling organism. Image created from illustrations in Key et al., 2010.

References:

Key, M., Schumacher, G., Babcock, L., Frey, R., Heimbrock, W., Felton, S., Cooper, D., Gibson, W., Scheid, D., & Schumacher, S. (2010). Paleoecology of Commensal Epizoans Fouling Flexicalymene (Trilobita) from the Upper Ordovician, Cincinnati Arch Region, USA Journal of Paleontology, 84 (6), 1121-1134 DOI: 10.1666/10-018.1

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