INSTAR DETERMINATION OF AN EARLY CAMBRIAN TRILOBITE, ELRATHIA KINGII

Modern arthropods and many extinct invertebrates have rigid exoskeletons that must be molted to allow continued growth. In most cases the timing of the molt may be variable depending on many conditions, but exoskeleton sizes occur in clumps at pre-determined intervals. If plurimodality of instar size is found in Elrathia kingii, it could provide a glimpse at one of the earliest examples of instar determination.

Complete and nearly complete E. kingii fossils throughout the known size range were measured. The measurements were then analyzed graphically to show instar groups. The groups were studied with regression analysis to determine the accuracy of the graphical associations.

Using measurements of glabellar area, twelve instar groups were found. Regression analysis showed that the groups were not as closely associated as those documented for later organisms, indicating a possible early state of instar size determination.