29-3 Evidence for convergent evolution of large body size in Early Paleozoic artiopodans

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts (Posters)

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Abstract:

Artiopodans are a disparate Paleozoic arthropods that include the megadiverse trilobites and several groups of non-biomineralized taxa primarily known from sites with exceptional preservation. Although most artiopods share a generalized flattened dorsal exoskeletal morphology, they display a broad range of sizes, from the miniscule early Cambrian Sinoburius lunaris (9mm in length) to comparatively gigantic forms such as the mid-Cambrian Tegopelte gigas (270 mm) and Ordovician Isotelus rex (720 mm). Artiopodans thrived over several million years and lived in a variety of different marine habitats and latitudes, but the processes responsible for the variability in their body size remains almost entirely unexplored. Cope’s rule states that lineages tend to get larger over time, suggesting that the longest surviving groups would reach the greatest sizes. Evolution of giant body sizes can be driven by several factors including competition with other taxa, oxygen availability, temperature, latitude and shifts in environments. Conversely, some clades such as eurypterids, crustaceans and fish do not exhibit trends in size based on such external factors. Recent work has shown trilobites display a pattern of body size reduction in response to decreased availability of oxygen. Here, we use a dataset of 74 taxa selected from 25 formations for a broad sampling of Artiopoda. Maximum sizes were sourced from the literature or measured on specimens. Lithology and depositional environments were extracted from the Paleobiology Database for each taxon. We created a new tip-dated tree of artiopodans and used phylogenetic comparative methods to examine ancestral states of body size and environment for each node. Our findings indicate that artiopodans were typically small, with a median size of ca. 50 mm in length, but with a select few reaching substantially larger sizes.  The largest body sizes evolved convergently within Trilobita and Conciliterga. In the latter context, large body sizes appeared in  Tegopelte and its sister taxon Saperion, Helmetia and Arthroaspis with multiple instances of small sizes evolving within the clade. Both Tegopelte and Helmetia are known from the Burgess Shale (Cambrian, Wuluian, Canada) whereas Saperion is known from the Chengjiang biota (Cambrian, Stage 2, China), and Arthroaspis from Sirius Passet (Cambrian, Stage 2, Greenland) showing evolution of large body sizes in different marine environments.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8034

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