PREDATORY TRILOBITES: COMBINING MORPHOLOGICAL AND ICHNOLOGICAL DATA

Interpretations of trilobite feeding habit are typically made using either morphological or ichnological data. Morphological data focus on functional interpretations of the hypostome and proximal limb segments; trilobites with spinose gnathobases and conterminant hypostomes are thought to have been predators. Ichnological interpretations for trilobite predator behavior are based on the co-occurrence of “worm” burrows with the trilobite trace fossil Rusophycus. In this study we combine morphological and ichnological data to test whether interpretations of trilobite predator behavior based on morphologic characters are congruent with the interpretations based on ichnofossils.

A small number of Rusophycus ichnospecies is associated with “worm” (prey) burrows; we binned them as two form ichnospecies: R. carleyi-type and R. pudicum-type. The probable trilobite trace-makers for R. carleyi-type traces associated with putative prey burrows are members of the Asaphida. Asaphid trilobites possessed spinose gnathobases and conterminant hypostomes so the morphological and ichnological data are consistent and support the interpretation that these trilobites were predatory. The stratigraphic range of R. carleyi-type traces associated with worm burrows coincides with the range of asaphid trilobites, supporting this association between specific trilobite taxa and their prey. R. pudicum-type burrows are attributed to trace makers from at least four different trilobite families. The majority of R. pudicum-type traces associated with a prey burrow are attributable to trilobites that possessed spinose gnathobases and conterminant hypostomes, compatible with interpretations based separately on morphological and ichnological data that these trilobites were predatory. R. pudicum-type Rusophycus are known from the Cambrian to Triassic (post-Permian Rusophycus are attributable to non-trilobite arthropods), but R. pudicum-type specimens associated with prey burrows are known only through the Early Devonian. The absence of R. pudicum-type trilobite hunting burrows after the Early Devonian may reflect the extinction of trilobites that pursued infaunal prey. Most R. pudicum-type traces are not associated with prey burrows, suggesting that these trilobites may have been opportunistic hunters.