416-359 million years ago

acanthostega armacanthostega leg

Acanthostega's arm (left) and leg (right) possessed fingers and toes although the limbs were polydactyl (with 8 digits) and the legs could not support the body's weight on land.

In Acanthostega's shoulder, the glenoid cavity was shallow. The radius was twice the length of the ulna and the ulna lacked an olecranon process(Clack, 2002). The femur was longer than the humerus. Although Acanthostega had limbs and digits, it couldn't have supported its weight and walked on these limbs since there were almost no modifications of the sacral vertebrae to interact with pelvic bones. Interestingly, it seems that the number of digits in a tetrapod limb wasn't fixed at five in the first amphibians: Acanthostega had 8 fingers, Ichthyostega had 7 toes, and Tulerpeton had six fingers (Lebedev, 1997; Clack, 2002).