Morphometric analysis of ontogeny and allometry of the Middle Ordovician trilobite Triarthrus becki

摘要

Traditionally, the distinction between meraspis and holaspis among trilobites has been based on the achievement of the full adult complement of thoracic segments. Using a large sample (over 700 specimens collected from a single bed) we explore the utility of employing the ontogenetic trajectory of the cranidium as an alternative means to differentiate trilobite growth stages. This method is particularly useful for species represented solely by exuviae and disarticulated individuals. We use geometric morphometrics to examine shape change among cranidia ranging in size from 0.9 mm to 11.6 mm in cephalic length. The 114 measured specimens exhibit a rather continuous gradation in size in which no distinct instars are evident.