During the Devonian period, the earliest lobe-finned fish appeared and differentiated into two major categories: lungfish (order) and lobe-finned fish (order).
Unlike ray-finned fishes, early lobe-finned fishes had a suprachordate lobe located above the body axis on their heterocercal tails. Their fins had a central skeleton and smaller bones radiating outwards from both sides of the central skeleton, with bony fin rays extending from the ends of these bones. Such fins are called protofins. Primitive ray-finned fishes had only one dorsal fin; early lobe-finned fishes had two. The scales of primitive lobe-finned fishes were dentate, with a thick layer of dentate scales above the bony base of the scales; while the scales of primitive ray-finned fishes had very limited dentate scales, but were covered by a thick layer of enamel.
In the evolutionary path of fish themselves, lobe-finned fish can be considered a side branch of evolution. However, from the perspective of the evolution of vertebrates as a whole, lobe-finned fish are a very important group, because the tetrapod vertebrates that appeared later evolved from lobe-finned fish.
The earliest lobe-finned fish - Diplodocus
