Trilobites – and other arthropods like spiders and crabs, for example- are characterized by their long segmented bodies draped in an armor of hard cuticle — think suits of biological chain mail. Hence the 300-million-year-old trilobite species from upstate New York was thought to have just three sets of legs on its thorax, for a total of seven pairs, which it used to walk around a marine seafloor searching for food. The findings provide fresh insight on the evolution and segmentation of trilobite bodies.
Trilobita with a Twist
A study, published on April 17 in the journal Scientific Reports by researchers from the American Museum of Natural History and Nanjing University in China, has discovered an extraordinary example of this involving a species of trilobite that lived in what today is upstate New York. Fossils of the charismatic species Triarthrus eatoni are famous for their golden sparkle, created by pyrite lacing their shells — and may have even highlighted another mystery: beneath the trilobite’s head with its pair of legs there is also an odd instar that probably performed a completely different role.
This discovery, in turn, reassesses how the segments of a trilobite head were believed to be configured. Historically, there are two ways researchers have counted segments: by counting the furrows on one side of the exoskeleton or by tallying preserved antennae and legs on another (the latter approach gives lower estimates). Nevertheless, these two ways of doing the same thing frequently yielded different results, making it frustrating for investigators when asked: Where exactly is the head of a trilobite segmented?
This high-fidelity preservation of the Triarthrus eatoni, permits observation of their 3D appendages from the bottom (or ventral) side in much same way that one would examine living limbs of a horseshoe crab. This extraordinary detail has allowed the researchers to now suggest a new model for how these appendages were attached to the trilobite head, resolving the long-standing conflict.
Decoding Trilobite Evolution
This new leg provides many new pieces of evidence supporting the debate about how odd autarthropod legs are and what it implies for trilobite relationships, a discovery having profound significance in terms both of our understanding of these arthropods and their evolutionary higher level position.
The organisation of these segments and their number are important in understanding relations among arthropods—how they shared a common ancestor and diversified. — Melanie Hopkins, American Museum Of Natural History This is especially true for the head of the trilobite, which consists of a series of coalesced segments; because head appendages fulfilled different sensory, feeding and locomotory roles from those on the trunk.
By comparing the Triarthrus eatoni fossils with those of another remarkably preserved trilobite species, Olenoides serratus from the Burgess Shale in British Columbia, a new model for how the head of a trilobite is segmented has been suggested by Robert Sansom and Sarah Rowland from University College London. According to this model, the head of a trilobite consists of six segments; though some argue that there are only five with a much reduced pair of antennae on one segment.
In addition to clearing up several confusions related to the head segmentation of trilobites overall, this new interpretation matches both of the two traditional modes of criteria used in counting head segments so far and helps clarify evolutionary relationships and adaptations among these enigmatic arthropods. As more work is done to study these amazing exoskeletons, we can only hope that even greater secrets are revealed about this richly diverse life of the ancient trilobite.
Conclusion
The new pair of two-legged appendages beneath the head of Triarthrus eatoni amounts to a major advance in our understanding of these long-extinct arthropods. These new insights into trilobite head segmentation and organization have the potential to inform hypotheses about trilobite evolutionary relationships and more generally, provide a larger framework for deciphering aspects of this well-known group’s diverse morphology in an ecological and – relatively speaking- geological vacuum.
