Searching Wikipedia for "red nucleus evolution" found 5,520,000 claimed hits.
Cross references: Red Nucleus Myoglobin Muscle InnervationAs noted in Shark Muscles , the "Review of the Origin ..." says " ... the red muscle is innervated by small motoneurons in the lateral portion of the motor column, while the white muscle is innervated by larger motoneurons in its medial part." and Wikipedia says "The rubrospinal tract descends with the lateral corticospinal tract, and the remaining three descend with the anterior corticospinal tract." This suggests the possibility that the lower motor neurons which innervate the red muscle fibers originate in the red nucleus. So the red nucleus is now my new focus.
Motor Neuron Evolution Motor Nerve Organization
Medial Motor Column Fast vs. Slow Twitch Muscles
Evolution of the red nucleus and rubospinal tract. (Goog) - 1988
Only abstract available online.
"A red nucleus, defined by its relative position in the tegmentum mesencephali, its contralateral rubrospinal or rubrobulbar projections and by crossed cerebellar afferents, is found in terrestrial vertebrates and certain rays. A crossed rubrospinal tract occurs in anurans, limbed urodeles and reptiles, birds and mammals, but is apparently absent in boid snakes, caecilians and sharks."
"A crossed tegmentospinal tract, possibly a rubrospinal tract, is found in lungfishes. Although evidence was presented for a rubrospinal tract in more advanced snakes, the available experimental data in lower vertebrates suggest that the presence of a rubrospinal tract is related to the presence of limbs or limb-like structures. "
This implies that the amphioxus, lampreys and sharks had slow twitch muscles and nerves without an identifiable red nucleus. However it may be that the apparent lack of a red nucleus is due to the lack of red Myoglobin . I may need to look at the evolution of Myoglobin .