Morphology and evolution of caudal fin in lamniform sharks

Sharks have a distinct asymmetrical caudal fin referred to as heterocercal tail that is a key characteristic of the group and has diversified within sharks in ways that are correlated with lifestyle. However, practically no study examining the evolutionary trend and history of the caudal fin morphology within a specific shark group exists. Here, I examined the caudal fin morphology and evolution of the shark order Lamniformes that consists of 15 extant species with diverse behaviors and lifestyles. The goals of this study are to describe the skeletal morphology of the caudal fin in each lamniform species based primarily on radiographic analysis, to examine the evolutionary pattern and history of the caudal fin through phylogenetic mapping, and to relate different caudal fin types observed in lamniforms to their known behaviors and life styles. This study suggests that caudal fins with a more horizontally directed curvature of the vertebral column are plesiomorphic, whereas those with a large dorsally directed curvature of the vertebral column are apomorphic within Lamniformes. It also shows that caudal fins with posteriorly directed hypochordal rays are plesiomorphic, and that those with ventrally directed hypochordal rays are apomorphic within Lamniformes. Three basic caudal fin types are recognized in extant lamniforms on the basis of these skeletal variables. One important discovery form the recognition of the three fin types is that the evolution of external morphology of caudal fin does not necessarily correspond to the evolution of its internal (skeletal) anatomy in lamniform sharks. Certain behaviors and lifestyles seen in different lamniforms are correlative with the different caudal fin types. A less asymmetrical tail is a derived feature in lamniforms that evolved for fast swimming to capture fast swimming prey