Arthropod pattern theory and Cambrian trilobites

Abstract

An analysis of duplomere ( = segment) distribution within the cephalon, thorax, and pygidium of Cambrian trilobites was undertaken to determine if the Arthropod Pattern Theory (APT) proposed by Schram & Emerson (1991) applies to Cambrian trilobites. The boundary of the cephalon/thorax occurs within the predicted duplomere node 1 (duplomeres 4 or 6). The boundary between the thorax and pygidium generally occurs within node 2 (duplomeres 11–13) and node 3 (duplomeres 18–20) for corynexochids and ptychopariids, respectively. This boundary occurs within field 4 (duplomeres 21–n) for olenellids and redlichiids. The termination of the body generally occurs within node 3 for corynexochids and within field 4 for olenellids, redlichiids, and ptychopariids. In addition, the location of macropleural spines, which may indicate the location of the gonopores or anus, generally falls at the predicted duplomeres. The boundary between the prothorax and opisthothorax of olenellids occurs within or near node 3. These results indicate that the number and distribution of duplomeres within Cambrian trilobites were somewhat constrained by some genetic patterning program. However, the common distribution of boundaries outside of the predicted locations and the possible shifting of nodes suggest that other factors were also controlling the number of duplomeres within the body parts. This variation supports the idea that Cambrian arthropods, unlike modern arthropods, had a simpler genetic program, which easily allowed for changes in the Bauplan.