Data for: Long-term clade-wide shifts in trilobite segment number and allocation during the Palaeozoic

Arthropods are characterized by having an exoskeleton, paired jointed
appendages, and segmented body. The number and shape of those segments
varies dramatically and unravelling the evolution of segmentation is
fundamental to our understanding of arthropod diversification. Because
trilobites added segments to the body post-hatching which were expressed
and preserved in biomineralized exoskeletal sclerites, their fossil record
provides an excellent system for understanding the early evolution of
segmentation in arthropods. Over the last 200 years, palaeontologists have
hypothesized trends in segment number and allocation in the trilobite
body, but they have never been rigorously tested. We tabulated the number
of segments in the post-cephalic body for over 1500 species, selected to
maximize taxonomic, geographic, and temporal representation. Analysis
reveals long-term shifts in segment number and allocation over the
250-million-year evolutionary history of the clade. For most of the
Palaeozoic, the median number of segments in the body did not change
significantly. Instead, the total range decreased over time and there was
long-term increase in the proportion of segments allocated to the fused
terminal sclerite relative to the articulated thoracic region. There was
also increased conservation of thoracic segment number within families.
Neither taxonomic turnover nor trends in functionally-relevant defensive
behaviour sufficiently explain these patterns.