Modeling the Size Frequency Distributions of the Trilobite Aulacopleura koninckii and its Implications for Understanding Trilobite Biology and Preservation Potential
- Author(s): Kolenko, Rachel Lynn
- Advisor(s): Hughes, Nigel C.
- et al.
Trilobites were a diverse group of Paleozoic marine arthropods, whose growth is characterized by exoskeletal ecdysis; molting of the outer cuticle. If all sclerites from all growth stages were preserved in a fossil assemblage, the distribution would be expected to be strongly right skewed because individuals must have passed through smaller molt stages, but not all small individuals would have survived to large size. However, the overwhelming majority of observed trilobite size frequency distributions have normal size distributions. This thesis investigates this discrepancy using a modified version of the method used by Hartnoll and Bryant (1990) to model crab size frequency distributions. Here I apply this method to the trilobite Aulacopleura koninckii, a Silurian species whose growth is among the best known for any fossil, and for which size-specific assemblages of articulated individuals are observed. This required the use of parameters known for A. koninckii, in combination with other estimates based on living crab biology. Observed growth parameters from A. koninckii suggest that this trilobite underwent up to 33 post-protaspid instars separated by 32 post-protaspid molts. A mortality rate of 10% per instar was assumed based on evidence that this taxon occupied a low predation environment. Given the assumption of constant recruitment and crab-based estimates of X and Y, derived from the observation that intermolt period increased systematically as size increased, I was able to model predicted distributions of A. koninckii sizes that matched the largest individuals observed. Estimated life spans of A. koninckii according to these parameters lie between 1 and 10 years. As expected these distributions predicted far larger numbers of smaller specimens than larger ones, which is not what is observed. To explain the dearth of smaller individuals I accounted for the probability of preservation in order to explore how the size frequency distributions behaved. Simulations suggest that the size frequency distributions observed for A. koninckii could be the outcome of either a preservation bias against smaller trilobites, an event that killed off a live population, or the result of the fact that size frequency distributions have many variables that cannot be determined from the fossil record.