With the discovery of Schinderhannes bartelsi, we now know that the anomalocarids (formerly only found in the Cambrian Period) were around at least until the Devonian. Unfortunately, anomalocarids’ relatively soft bodies mean their fossils are rarely found, meaning we have yet to fully appreciate their true diversity and abundance.
Here I’ve imagined a hypothetical group of filter-feeding anomalocarids called Ceticarids (since they fill an ecological niche similar to that of modern day baleen whales). In this taxon are two subgroups:
Lagganiamorphs have retained all their lateral fins and closely resemble their namesake, Laggania cambria.
The example shown here is Dolichopterus pelagicus, an open-ocean species similar to the modern-day manta ray.
Schinderhannimorphs, on the other hand, have lost most of the lateral fins save for the first and last pair, giving them a rather fish-like appearance.
The example here is Cetimimus heliophilus, which swims near the surface like a basking shark. With Cetimimus, the tail spine of its Schinderhannes-like ancestor has been modified into a strong tail fluke similar to that of a humpback whale. The head and front fins of C. heliophilus are covered with callosities, rough patches of skin infested with “sea lice” (in this case, highly-modified trilobites). Here’s a more detailed description of some of the critters living on the Whale-Mimic: [link]