An Examination on Primitive characteristics of the Trilobite group
Saloni Shyama
Zoology
September 2021
In a sample of 60 species that covers a large portion of the group's temporal, phylogenetic, and ecological range and includes quantitative information on segmentation and growth increments between putative consecutive instars, we conducted a tree-based study of postembryonic trilobite development. The development of trunk segmentation, average per-molt growth rate, and compliance to a constant per-molt growth rate, for which a novel measure was developed, were three developmental features that were examined. Growth rates are consistent with other arthropods' usual ranges and generally follow Dyar's rule. In early juveniles but not in later stages, randomization experiments reveal a statistically significant phylogenetic signal for growth. The strongest supported evolutionary model throughout all ontogenetic stages is one in which growth rates fluctuate independently across species, akin to Brownian motion on a star phylogeny. However, a model in which growth rates are drawn to a single stationary peak also receives substantial support. These findings point to the effect of an adaptive zone rather than unconstrained, Brownian-motion-like evolutionary processes. Our findings imply that trilobite developmental features were very malleable throughout evolutionary history.
© Airo Journals 2021
