Final answer:
Eusthenopteron evolved into Acanthostega over a period of approximately 20 million years, with Eusthenopteron dating back to about 385 million years ago and Acanthostega to about 365 million years ago.
Step-by-step explanation:
The student has asked about the evolutionary timeline from Eusthenopteron to Acanthostega. To provide context, Eusthenopteron was a lobe-finned fish from the Late Devonian period, roughly 385 million years ago, that is often cited as a transitional fossil between fish and tetrapods. Acanthostega is one of the earliest known tetrapods, with more developed limbs, and lived around 365 million years ago. This suggests that the evolution from Eusthenopteron-like fish to early tetrapods like Acanthostega took about 20 million years.
Aquatic tetrapods of the Devonian period, such as Ichthyostega and Acanthostega, likely had both gills and lungs, and due to their limb structure, would have had difficulty supporting their weight on land. These species represent intermediate forms in the evolution of vertebrates from an aquatic to a terrestrial lifestyle.