that is half their body weight or greater evolved shorter and more sturdy radii, ulnas, and humeri to decrease the likelihood of the bone breaking or fracturing while hunting. Predators hunting prey less than half their body weight tended to have longer and more slender forelimb long bones to improve energetic efficiency. Polydactyly Tetrapods were initially understood to have first developed five digits as an ancestral characteristic, which were then reduced or specialized into a number of uses. Certain animals retained 'primitive' forelimbs, such as pentadactylous (five-fingered) reptiles and primates. This has mostly held true, but the earliest tetrapod or "fishapod" ancestors may have had more than five digits. This was notably challenged by Stephen Jay Gould in his 1991 essay "Eight (Or Fewer) Little Piggies". Polydactyly in early tetrapods should be understood as having more than five digits to the finger or foot, a condition that was the natural state of affairs in the very first tetrapods. Early groups like Acanthostega had eight digits, while the more derived Ichthyostega had seven digits, the yet-more derived Tulerpeton had six toes. Tetrapods evolved from animals with fins such as found in lobe-finned fishes. From this condition a new pattern of limb formation evolved, where the development axis of the limb rotated to sprout secondary axes along the lower margin, giving rise to a variable number of very stout skeletal supports for a paddle-like foot. Digit specialization Digits may be specialized for different forms of locomotion. A classic example is the horse's development of a single toe (monodactyly). Other hooves, like those of even-toed and odd-toed ungulates, and even the hoof-like foot of extinct hadrosaurs, may be regarded as similar specializations. To bear their immense weight, sauropods, the most derived being titanosaurs, developed a tubular manus (front foot) and gradually lost their digits, standing on their metacarpals. The stegosaurian forelimb has evidence for a sauropod−like metacarpal configuration This was a different evolutionary strategy than megafaunal mammals such as modern elephants. Therapsids started evolving diverse and specialized forelimbs 270 million years ago, during the Permian. Opposable thumbs Modern humans are unique in the musculature of the forearm and hand, though opposable thumbs or structures like them have arisen in a few animals. In dinosaurs, a primitive autonomization of the first carpometacarpal joint (CMC) may have occurred. In primates, a real differentiation appeared perhaps 70 mya, while the shape of the human thumb CMC