This strange fish is Euphanerops longaevus, which is one of two species of 370 million year old jawless fishes (the other is Endeiolepis aneri, and the paper suggests that they may actually represent differently preserved members of the same species). These are soft-bodied animals that are usually poorly preserved, and are of interest because they seem to have some properties in common with both the lampreys and the gnathostomes, or jawed fishes. Their exact position in the vertebrate family tree is problematic, and the experts go back and forth on it; sometimes they are grouped with the lampreys, sometimes as cousins more closely related to the gnathostomes.
That ‘branchial basket’ in the figure above is a curious feature, and in some specimens a ‘paired ventrolateral scale’ has been identified in about the same place. Some recent well preserved specimens suggest that they are all the same thing: they are lamprey-like pouches that contain gill filaments.
Apparently, this is the first time the organization of the gills has been preserved in a fossil jawless vertebrate specimen, and they are surprisingly lamprey-like, enclosed in a pouch. Another surprise is the size of those things; they extend from just behind the head all the way back to the anus. That’s a long, long battery of gills, and suggests two things: that ventilation of the gills may have been less effective than we find in modern fish, which actively pump water over the gills; and that this animal lived in an oxygen-poor enviroment.
You may be wondering who cares about a few impressions of gills from an obscure group of extinct fish, but these do illustrate an important property of transitional forms (even transitional forms in which we aren’t entirely sure what they are in transition between!), as the authors explain.
Fossil gnathostome-related jawless vertebrates now demonstrate that the assembly of the gnathostome body plan was not a burst of anatomical innovations, and that lamprey-like gills could persist in fishes that already possess some gnathostome features, long before the rise of jaws.
As we saw in the creationist response to Tiktaalik, there is a common misconception that all the evolutionary changes in a transition have to be in lockstep, with everything shifting in synchrony over short periods of time. What we see instead is much more mosaic, with species containing a mixture of archaic and advanced features, each one changing independently at its own pace and on its own unique schedule. These fossils are showing a similar mixture of the advanced and the primitive.
Janvier P, Desbiens S, Willett JA, Arsenault M (2006) Lamprey-like gills in a gnathostome-related Devonian jawless vertebrate. Nature 440:1183-1185.