Evolution of the World Fauna of Aquatic Free-Living Arthropods

From the Cambrian to the present, the evolution of the world fauna of aquatic free-living arthropods has followed patterns of logistic growth in specialization and diversity among its constituent orders. Specialization in limb mechanisms, which has generally proceeded progressively by specialization and tagmatization of specific limb pairs for specific functions, is measured in terms of the Brillouin diversity of limb types. This measure of morphological complexity in limb tagmosis, narrowly variable within orders, provides a rough measure of ecological specialization for arthropod types. Over intervals of geologic time from the Early Cambrian to the present, the average value of this measure has increased along a logistic curve. Over the same time intervals, these average values are highly correlated with the number of orders known from each interval, suggesting that, within the adaptive zone of aquatic free-living arthropods, groups have become more specialized as they have become more diverse. This relationship provides an indirect means of roughly calculating the completeness of the fossil record for arthropod orders. In general, completeness increased from around 60% in the Cambrian to around 80% for the Cenozoic as measured against the Recent fauna. The record for taxa up through the level of family, however, is probably so incomplete that sampling biases probably determine in large part the apparent trends of diversity through time. Logistic increase of order diversity probably reflects in a gross way diversity trends at lower levels while smoothing out their fluctuations. Despite the catastrophes of geologic history, the diversification of orders has proceeded in a continuous and seemingly predictable pattern, with periods of extensive radiations and extinctions falling near critical points of the curve. Logistic growth of specialization and diversity among orders, a pattern characteristic of growth of systems in general, is explained as a reflection of the evolutionary pattern for the many ecological systems of which aquatic free-living arthropods have been parts. Up to a point, increase in complexity of the taxonomic and ecological structure of their world fauna has resulted in stabilization of that structure. Yet the major features of the modern world fauna have remained stable for some 200 million years since the Permo-Triassic in the face of tremendous potential for continued diversification, this potential revealed in surprisingly high intrin