Fossils often leave much room for very different interpretations of relatively poor evidence.
Consideration of the basic characteristics of nerve impulse propagation suggests that the speed of conduction in mammals is close to the maximum possible.
One area where very fast nerve conduction is vital is vision, more specifically, in keeping the eyes fixed on some object in the field of vision while in motion.
As every medical student comes to learn when first dissecting the human body at medical school, our limbs are almost entirely composed of muscles.
It is likely that no further improvement in muscle power can be achieved by increasing the density of packing of the myosin motors.