A few days ago I got to see Apex, which is thought to be the largest and one of the most complete stegosaurus specimens ever uncovered. I am sure that most museumgoers had the same reaction I had, which was a sense of profound wonder at the fact that the stegosaurus (and indeed nearly all dinosaurs) had a radius and ulna below the elbow of each forelimbs, exactly like humans (and nearly all mammals), and had a tibia and fibula below the knee of each hindlimb, again exactly like humans.
Given that dinosaurs and mammals are separate branches of the evolutionary family tree, how can this possibly have worked out this way?
By way of background, the word “ulna” means “elbow” in Latin. “Tibia” means “shin bone” in Latin. “Fibula” in Latin means “brooch”, somehow evoking the notion that the tibia and fibula together supposedly look like a clasp or brooch.
We now return to the question presented. Given that dinosaurs and mammals are separate branches of the evolutionary family tree, how can this possibly have worked out this way?
People who do this stuff for a living (paleontologists) have arrived at consensus as to how this worked out this way. They have found what is believed to be the common ancestor of all tetrapods (animals having four limbs). This includes all dinosaurs and all mammals (and indeed all reptiles and birds and amphibians). This common ancestor is is a fleshy-finned fish, dating from the time of coelacanths and lungfishes, that lived between about 390 and 360 million years ago during the Devonian Period. This fish developed forelimbs with humerus, radius, and ulna, and hindlimbs with femur, tibia, and fibula (Wikipedia article).
The dinosaurs descended from this fish, as did all mammals and eventually humans. And they all have a radius and ulna and tibia and fibula.
Do paleontologists offer any suggestion of what evolutionary advantage that bone structure provided?