"You Make Several Good Points" Rhinopteraspis/Walliserops Devonian, 403 million years ago, Rheic Ocean I like Walliserops primarily for having a trident on its nose. The fact that it varied in shape and could even have four prongs indicates that the genus likely wasn't using the trident for sexual display, and might've used them to fight over mates like modern stag beetles do! This would normally be a cue to search for sexual dimorphism, but there aren't two distinct morphs of Walliserops that could be attributed to a difference in the sexes. One option is that both sexes fought over mates. Another is that dimorphism was purely cosmetic, and that there's no visible difference. I've chosen a different interpretation - the idea that males and females may have lived in different places and only congregated to breed. These ones are coming together for their once-a-year combat-and-mating session in a natural arena (made of Favosites and Pachyphyllum corals). They're interrupted by a Rhinopteraspis swimming overhead. While the Walliserops are treating it as a threat (and who wouldn't), Rhinopteraspis was a filter-feeder, using its mouthparts to expand its jawless gape and take in that sweet, sweet plankton. Although the genus has primarily been found in Europe, I wouldn't be surprised if they occasionally swam to Morocco, where the Walliserops live - the two were (and are) fairly close by in the Devonian. This is an entry in my Perspectives series! Check out #perspectives on my blog for more.

As with many agnathan groups, heterostracans had no fins besides the tail or caudal fin. In some pteraspids, especially in the psammosteids, the ends of the branchial plates (the plates that covered the gills) is drawn out to form wing-like extensions.