139-9 Quantification of trilobite exopodite surface area through 3D modelling supports a respiratory function

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part II

Presenting Author:

Sarah Losso

Authors:

Losso, Sarah R¹, Vallefuoco, Federica², Foglia, Igino³, Laborieux, Léo⁴, Belén Muñoz-García, Ana⁵, Ortega-Hernández, Javier⁶

(1) Organismic and Evolutionary Biology, Harvard University, Cambridge, MMA, USA, (2) 2Department of Chemical, Materials and Manufacturing Engineering, University of Naples Federico II,, Naples, Italy, (3) 2Department of Chemical, Materials and Manufacturing Engineering, University of Naples Federico II, Naples, Italy, (4) Centre de Biologie pour la Gestion des Populations, University of Montpellier, Montpellier, France, (5) 4Department of Physics, University of Naples Federico II, University of Naples Federico II, Italy, (6) Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA,

Abstract:

Early euarthropods had biramous appendages with an inner endopodite (walking leg) and outer exopodite (gill) connected to the body through the protopodite (limb base). Although trilobites are common Paleozoic fossils thanks to their calcitic dorsal exoskeleton, their non-biomineralized limbs are comparatively much rarer. However, trilobite exopodites show a broad range of morphological variation expressed in the number and size of articles, lamellae and setae. Several functions have been suggested for the exopodite including respiration, ventilation and swimming. Previous estimates of the surface area (SA) of the lamellae in the mid-Cambrian Burgess Shale trilobite Olenoides serratus were used to argue against a respiratory function, on the ground that the SA was substantially lower than that observed in the modern horseshoe crab Limulus polyphemus. More recent work has argued that dumbbell-shaped cross section of lamellae in Triarthrus eatoni provides evidence that the exopodite functioned as a gill for respiration. Previous work has estimated the lamellae SA of T. eatoni at 2067mm² for an animal weighing between 6.887 – 9.183g, which is comparable to modern crabs and fish gills. However, T. eatoni exopodites have an unusual morphology among trilobites, leaving questions about the surface area and potential as a gill for other species. We created morphologically accurate 3D models of exopodites in Olenoides serratus and T. eatoni to previously calculate the SA. Our results indicate a large SA for O. serratus at 16589mm² compared to the 2159mm² for the much smaller T. eatoni. Although other trilobite taxa are less well preserved, lamellar SA was calculated for 10 species. For three species lamellae were measured in cross section to find the perimeter of the dumbbell shape, multiplied by the average lamellae length, the lamellae number, and number of limbs scaled to decrease in size along the body. Estimates for the seven other species followed the same method, but lamellae height was calculated as a ratio of length to width as cross sections were not available. SA and biomass data from modern euarthropods, including marine crabs, shrimps, and horseshoe crabs follow the same trend of exponential increase in gills compared to body size. Trilobites fall along the same trendline of gill SA/biomass, supporting the interpretation of exopodites as serving a respiratory function despite variation in morphology.

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

doi: 10.1130/abs/2025AM-8028

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