ptyctodonts. As such, placoderm experts consider Pseudopetalichthyida to be the sister group of the Arthrodires + Phyllolepida + Antiarchi trichotomy and the Acanthothoraci + Rhenanida dichotomy. Stensioellida Stensioellida ("[Heintz's] little Stensio") contains another problematic placoderm of uncertain affinity, known only from the Lower Devonian Hunsrück slates of Germany. Stensioella was a thin fish that, when alive, looked vaguely like an elongated ratfish, or a skinny Gemuendina with thin, strap-like pectoral fins. Similar to those of the Rhenanida, its armour was a complex mosaic of small, scale-like tubercles. The shoulder joints of its armour are similar to other placoderms, and there are superficial similarities in skull plates, and even more superficial similarities between its tubercles and the tubercles of the rhenanids. It is tentatively placed within Placodermi as a primitive placoderm, though some paleontologists believe the rationale for the placement is inadequate. The paleontologist Philippe Janvier has suggested that Stensioella is not a placoderm, but instead is a holocephalian. If this is true, then the holocephalians diverged from sharks before the Chondrichthyan Devonian radiation. Critics of Janvier's position say that aside from a bodyplan superficially similar to primitive holocephalians, the two groups have little else in common anatomically. Cladogram The following cladogram shows the interrelationships of placoderms according to Carr et al. (2009): However, the cladogram had changed significantly over the years, and the placoderms are now thought to be paraphyletic, with some being more closer to the Eugnathostomata than others. The updated cladogram (Zhu et al., 2016): See also Acanthodii List of placoderms Ostracoderm Chondrichthyes Entelognathus Notes References Citations Other references Janvier, P. Early Vertebrates Oxford, New York: Oxford University Press, 1998. Long, J.A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996. External links Annetta Markussen-Brown, "Devonian Armoured Fish" 2000 Introduction to the Placodermi Extinct armored fishes with jaws Placoderms Placodermi: Overview BBC—report on Dunkleosteus terrelli Prehistoric fish classes Devonian fish Silurian fish Llandovery first appearances Devonian extinctions Taxa named by Frederick McCoy Paraphyletic groups