Lamellar surface area calculations support respiratory function of trilobite exopodites

Trilobites had biramous appendages with an inner endopodite (walking leg) and outer exopodite (gill) connected to the body through the protopodite (limb base). Whereas both endopodite and protopodite were involved in locomotion and feeding, the exopodite has been subject to various functional interpretations including respiration, ventilation and swimming. Evidence from sites with exceptional fossil preservation indicates that trilobite exopodites show substantial variability in terms of the number and size of their articles, lamellae and setae, but the implications of this morphological diversity have never been investigated. Here, we created anatomically correct three-dimensional models of exopodites in Olenoides serratus and Triarthrus eatoni to calculate the surface area (SA) of the lamellae and explore its relationship with body size. Lamellar SA of O. serratus was 16 589 mm2 (for a specimen 67.8 mm sagittal length) compared with the smaller T. eatoni 2159 mm2 (36.3 mm sagittal length). We also calculated SA for nine additional trilobite species with well-preserved appendages based on lamellar measurements. The results indicate that lamellae SA of trilobites increased exponentially with overall body size. Trilobite data follows the same trendline of gill SA/biomass observed in extant species and thus supports the interpretation of their exopodites as respiratory structures despite substantial variation in morphology.