VARIOEVODEVO: INTRASPECIFIC VARIATION IN PALEOZOIC AMMONOIDS

Variation of phenotypic traits can be quantified in extinct and living organisms. It occurs through ontogeny and is usually not constant. Changes in variation of phenotypic traits through ontogeny have rarely been quantified although this knowledge is important primarily to judge the reliability of traits to introduce new taxa and to determine existing ones and, secondarily, to carry out phylogenetic analyses as well as for estimates of paleobiodiversity. Ammonoids (extinct cephalopods with chambered external conchs) are nearly perfect paleontological model organisms to study evolutionary processes because they evolved rapidly, produced a high diversity, some taxa are abundant, and they preserve information from their entire ontogeny in their coiled conchs. Here, we present quantitative analyses of measurements and ratios of the conchs of eleven ammonoid genera of Devonian and Carboniferous age (Paleozoic).

We measured basic conch parameters such as diameter (dm), whorl height (wh), apertural height (ah), umbilical width (uw) and imprint zone width (iz). Then, we calculated the following ratios: whorl expansion rate (WER), umbilical width index (UWI) and imprint zone rate (IZR) throughout ontogeny. Accordingly, their average values and standard deviation were calculated in order to assess intraspecific variability through ontogeny. Based on these parameters, we discuss the following questions: 1) How do important ratios of conch parameters vary through ontogeny? 2) How does this ontogenetic variation differ between species or genera? 3) Which growth stages show the highest and smallest variability?