MORPHOLOGICAL VARIATION IN THE AGNOSTID TRILOBITE PERONOPSIS INTERSTRICTA

Morphologically simple organisms present a particular challenge to taxonomists. The relative lack of distinct characters means that the classification of these groups often rests on subtle variations in morphology and the judgment of individual paleontologists. Here we present a case in which a more detailed analysis of the morphological variability in a relatively simple species shows a surprisingly complex pattern. Peronopsis interstricta is a common agnostid trilobite from the Wheeler Shale Formation in Millard County, Utah. Peronopsis interstricta is a typical species of the Order Agnostida, in its relative lack of skeletal ornamentation and low number of thoracic segments. In this study, 167 specimens of Peronopsis interstricta were digitally landmarked using 10 homologous points. Principal component analysis of these landmarks reveals that this single species contains three distinct morphotypes. These morphotypes each occupy a discrete region of a two-dimensional trilobite morphospace the axes of which can be correlated to a few simple anatomical features.