TRILOBITES AS ARTHROPODS: THE IMPLICATIONS OF THE ARTHRODIAL MEMBRANE FOR TRILOBITE TAPHONOMY

It is trivial to point out that trilobites are arthropods. However the functional and taphonomic consequences of that phylogenetic statement are rarely considered. Though the thoracic segments may invite comparisons to plate armor from medieval times, such a comparison breaks down when the arthropod nature of the exoskeleton is considered. Arthropod exoskeletons are fundamentally made out of a continuous covering of chitin—even at the articulating joints of the exoskeleton. Arthropods can finely control the spatial robustness of the exoskeleton in several ways: by adding mineral salts to the chitin framework (creating a stronger composite material), by sclerotizing or by tanning it (both means of cross-linking proteins to increase rigidity).

There are several implications of this. First, the discrete trilobite sclerites we observe in the fossil record were actually linked by the unmineralized bit of exoskeleton called the arthrodial membrane in life. Disarticulation of the mineralized exoskeletal sclerites, after death or molting, necessarily requires degradation of the chitinous arthrodial membrane first. Second, as observed by Seilacher (2007), the ventral chitinous membrane of trilobites would have also required molting in order to grow. How that would have been accomplished in light of proposed trilobite ‘molt configurations’ is not completely clear, though it may have involved a biphasic molting process.

Articulated trilobite remains are used as indicators of calm sedimentary environments. However, the chitinous exoskeleton of freshly dead/molted remains is surprisingly resistant to transport. As such, they should be interpreted in light of other sedimentological evidence.