UNDERSTANDING THE BIOLOGY AND PRESERVATION POTENTIAL OF TRILOBITE; AULACOPLEURA KONINCKII BASED ON MODELING ITS SIZE FREQUENCY DISTRIBUTION

Like extant arthropods, trilobite growth was accompanied by repeated molting of the outer cuticle. If all exuviae and corpses belonging to a population were preserved in the fossil record, the distribution should be skewed towards smaller sizes because all large individuals must have pass through the smaller stages, but not all small individuals survived to large size. However, such right skewed distributions are seldom observed in the fossil record, and the majority of trilobite size frequency distributions have normal distributions. This inconsistency between observed and modeled distributions suggests that there may be a discrepancy in the types of trilobite remains or a preservation bias in the fossil record.

In 1990, Harntoll and Bryant created a model aiming to predict the size frequency distributions of live populations, corpses, and exuviae of decapod populations. Using the desktop application platform, Node-Webkit, a program was created that utilized and expanded upon the methods and parameters published by Hartnoll and Bryant. We have applied this model to the trilobite species, Aulacopleura koninckii for which parameters such as size, number of instars, and the per-molt growth constant can be determined empirically from the fossil record. A range of values for other unknown variables, such as mortality rate and stage durations were derived from knowledge of modern arthropods.

Based on extrapolated data from brine shrimp, Artemia salina, 18 instars and reaching maturity within eight days, and horseshoe crab species, Limulus polyphemus, at least 18 instars and achieving maturity in 9-11 years we were able to produce size frequency distributions like those in Hartnoll and Bryant. However normal distributions could only be produced for live populations when sorted by instar, not size. Therefore we can conclude that the overall shape of these distributions is derived from either the selective destruction of early stage exuviae and corpses (and therefore taphonomic in origin) or extenuating circumstances such as mass kill-offs of a single population or cohort.