UNMINERALIZED EXOSKELETAL MORPHOLOGY OF ISOTELUS (ARTHROPODA, TRILOBITA, ASAPHIDA): POSSIBLE EVIDENCE OF CEPHALIC LIMB DIFFERENTIATION

Trilobites, like all arthropods, possessed an exoskeleton composed mainly of chitin but with a spatially variable mineral content. This exoskeleton included a dorsal mineralized portion (of articulating plates or sclerites) and a ventral unmineralized portion (often referred to as ‘soft-parts’). The much lower fossilization potential of the unmineralized exoskeleton means that we have relatively little information on its morphology or how the two portions functioned as a whole.

The common Ordovician trilobite Isotelus reflects this contrast: its mineralized exoskeleton is well-known, while its soft-parts and unmineralized exoskeleton are rarely preserved. Three examples of Isotelus are known to preserve evidence of the unmineralized ventral exoskeleton in at least two different preservational modes (pyrite and calcite). Two of the three specimens are preserved in three dimensions, revealing how the body parts fit together and interacted.

Contrary to previous reports, no gut tract is preserved. In one specimen, the entire body cavity is filled with sparry calcite, as observed in trilobites from the Ordovician Walcott-Rust Quarry in upstate New York. In the pygidial region, only the axial region accommodated significant amounts of tissue. The broad pleural areas were covered ventrally by a ‘soft’ exoskeleton preserved as a thin layer of transparent spar. Head appendages are evident in only one specimen. There appears to be a differentiated smaller pair of appendages (2^nd pair?) positioned under the inner margin of the hypostome. The presence of these limbs supports an earlier hypothesis of a differentiated feeding mechanism in asaphid trilobites related to their distinctive hypostome morphology. Thus, the ventral morphology of trilobites may not have been as uniform as previously thought.