MODELING SIZE FREQUENCY DISTRIBUTIONS OF TRILOBITES AND ITS IMPLICATIONS FOR TRILOBITE BIOLOGY AND PRESERVATION POTENTIAL

Trilobites were a diverse group of marine arthropods that grew through the process of ecdysis; the molting of the outer cuticle. This growth pattern means that if all molts and carcasses of a population were preserved in a fossil bed, the distribution would be strongly right skewed. Curiously, however, the majority of observed trilobite size frequency distributions have normal distributions. In 1987, Hartnoll and Bryant devised a method to model the size frequency distributions of decapod crustaceans using certain biological parameters (duration of each developmental stage, mortality rates, number of stages) and their relation to each other. The result was that standing populations yielded normal size distributions rather than right skewed distributions because of the balance between declining molt frequency and mortality. A similar method can be utilized to produce modeled size frequency distributions of trilobite remains using the same parameters, based on certain assumptions. Some parameters, such as size change per molt, can now be reasonably estimated directly from trilobite data. Several other parameters, such as molt frequency and mortality can not be observed directly in the fossil record therefore data from extant arthropods has been used to make a range of reasoned estimates for trilobites. Published data from extant arthropods such as lobsters, horseshoe crabs, shrimp, and isopods were collected and then computer simulations using these values were performed. The production of normal distributions from standing populations of live individuals requires a decline in molt rate through ontogeny. Where molts and carcasses are also included in the population, selective omission of small specimens through some destructive process is required explain the low numbers of smaller specimens in samples showing normal size distribution. We are currently applying this method to the ontogenies of trilobites with maximal available information on molt size increment, conformity to Dyar's law, and observed size frequency distributions in order to determine whether observed size frequency distributions are more consistent with certain proposed developmental schedules than with others.