Independent testing of a paleobiological hypothesis: the optical design of two Ordovician pelagic trilobites reveals their relative paleobathymetry

Evidence that can be used to interpret the life habits of extinct organisms usually takes three forms: functional analysis, analogy with living organisms, and geological evidence. Independent quantitative tests for habit are rarely available. A theory of optimum eye design originally derived for living aquatic arthropods provides quantitative data that are used to test previous suppositions about the life habits of two Ordovician pelagic trilobites: that the telephinid trilobite Carolinites was epipelagic while the cyclopygid Pricyclopyge was mesopelagic. Optimum compound eye design theory uses measured lens diameters and interommatidial angles to determine the “eye parameter” (p), which can be used to gauge approximate optimum level of illumination for the eyes of these trilobites. The eye parameter provides an independent test for their relative paleobathymetry. Values of the eye parameter measured in the dorso-ventral direction across two eyes of Carolinites killaryensis utahensis were found to have medians of 2.13 and 3.24. Values measured in the antero-posterior direction have medians of 3.17 and 4.86 for the two eyes. Values measured in the dorso-ventral direction across two eyes of Pricyclopyge binodosa have medians of 4.23 and 4.98, while values measured in the antero-posterior direction have medians of 7.06 and 8.31. Eye parameters are higher in Pricyclopyge than in Carolinites, the difference statistically significant at p = 0.05. The eyes of Pricyclopyge are optimally designed for lower levels of illumination than are those of Carolinites. This accords with the previous interpretations of the former trilobite as mesopelagic and the latter as epipelagic.