Stensioella heintzi ("Heintz's Little Stensio") is an enigmatic placoderm of arcane affinity. It is only known from the Lower Devonian Hunsrück slates of Germany, where the only specimens have been found.
Temporal range: Emsian
Stensioella heintzi has an elongated body, a whip-like tail, and long, wing-like pectoral fins. In life, the animal would have looked vaguely like an elongated ratfish. Like the sympatric Gemuendina, S. heintzi had armor made up of a complex mosaic of small, scale-like tubercles.
Stensioella is tentatively placed within Placodermi as being among the most basal of all placoderms, as from what can be discerned from the only whole specimen found, the shoulder joints of its armor appear to be very similar to other placoderms. Despite this detail, coupled with superficial similarities in skull plates, and gross, superficial similarities between its tubercles, and the tubercles of the rhenanids, some paleontologists believe that 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 Devonian radiation.