FORK STRUCTURE ALLOMETRY IN THE TRILOBITE WALLISEROPS TRIFURCATUS CORRELATES WITH A SEXUALLY SELECTED FUNCTION

The prevalence of sexual selection is evident by the ubiquity of display structures and weapons that appears over a great diversity of extant animal clades. The same concept can likely be applied to extinct organisms that bear similar structures, although, they are not necessarily analogous to those of living forms. The most extreme expression of ornamentation in trilobites occurred in an Asteropygine group. Walliserops (Phacopida: Acastidae: Asteropyginae) possesses a pre-cephalic projection divided into a three-tined fork. In W. trifurcatus, the fork can be as long as the body of the trilobite itself. There have been a number of theories for the function of the fork including defense, predation and display/intraspecific combat. When testing function of structures in fossils, it is necessary to take into account indirect assessments such as allometry. Positive allometry is a greater proportion of growth in the trait than the body size and is strongly correlated to traits influenced by sexual selection. To investigate this, we created three-dimensional morphological datasets of multiple examples of W. trifurcatus representing a range of sizes. These were measured along two axes: from the central tine of the fork to the base of the shaft that connects to the glabellar field and the edge of the doublure to the end of the terminal pygidial spine. The log length against body size was calculated and had a slope greater than 1. The growth of the fork of W. trifurcatus is shown to be positively allometric suggesting the fork structure could be influenced by sexual selection.