Stensioella heintzi

The Early Devonian fish Stensioella heintzi was an enigmatic fish of arcane affinity. It is only known from the Lower Devonian Hunsrück slates of Germany, where the only specimens have been found.
In life, it was a blocky-looking fish that resembled either a squat, pug-nosed combination chimaera-stargazer, or an uncompressed Gemuendina (Gemuendina also happened to be its contempary in Hunsrück). Like Gemuendina, it had armor made up of a complex mosaic of tubercles.
It is tentatively placed within Placodermi, as from what can be discerned from the few whole specimens found, the shoulder joints of its armor appear to be very similar to other placoderms. However, aside from this, and gross, superficial similarities between its tubercles, and the tubercles of the rhenanids, there are very few concrete reasons for S. heintzi's placement in Placodermi. The paleontologist, Philippe Janvier, suggests that it was actually a holocephalid, and not a placoderm at all. However, if this is true, then the holocephalids (chimaeras, iniopterygians, petalodonts, et al) diverged from sharks before the Chondrichthyan Carboniferous radiation.

Here, one of the beasts is settling in with a starfish, Helianthaster rhenanus, while a Phacops ferdinandi trilobite explores a forgotten orthocone nautiloid shell.