DAVIS, M.C.*; SHUBIN, N.H.; DAESCHLER, E.B.: The evolution of the endochondral and dermal skeleton in fins and limbs

The paired appendages of the two major clades of bony fish, actinopterygians and sarcopterygians, are composed of two separate skeletal units: A proximal endochondral skeleton and a more distal dermal skeleton. The relative contribution of these skeletal components to the fin is one of the key morphological differences between ray-finned and lobe-finned designs. Shifts in the pattern and development of each of these skeletal units can affect the function of the appendage as a whole. However, studies of the development of living fish cannot tell the whole story. Fossils reveal intermediate developmental and functional conditions that would not be predicted from a knowledge of recent taxa alone. Newly discovered fossils of the Devonian sarcopterygian Sauripterus show an extremely limb-like endochondral skeleton that is sandwiched between massive unjointed dermal rays. The endochondral elements of Sauripterus correspond in position to both the digits and mesopodium of tetrapods. The biomechanical function of the digit-like elements in Sauripterus is linked with the dermal skeleton; most movements of the fin would rely on the function of both skeletal components. This situation differs greatly from that seen in the earliest tetrapods. Tetrapods lack the dermal rays and, consequently, the digits interact directly with the substrate. Indeed, one of the main shifts in the origin of tetrapods may not be the origin of the autopodial bones, per se, but the loss of the dermal radials. This suggests that the loss of the dermal skeleton in appendage development is correlated with the evolution of mechanisms that patterned preexisting endochondral elements as an independently varying unit.