Editing Placoderm (section)

===History of study===
The earliest studies of placoderms were published by [[Louis Agassiz]], in his five volumes on fossil fishes, 1833–1843. In those days, placoderms were thought to be shelled jawless fish akin to [[ostracoderms]]. Some naturalists even suggested that they were shelled invertebrates or even [[turtle]]-like vertebrates.

In the late 1920s, Dr. [[Erik Stensiö]], at the [[Swedish Museum of Natural History]] in [[Stockholm]], established the details of placoderm anatomy and identified them as true jawed fishes related to [[shark]]s. He took fossil specimens with well-preserved skulls and ground them away, one tenth of a millimeter at a time. After each layer had been removed, he made an imprint of the next surface in [[wax]]. Once the specimens had been completely ground away (and so destroyed), he made enlarged, three-dimensional models of the skulls to examine the anatomical details more thoroughly. Many other placoderm specialists thought that Stensiö was trying to shoehorn placoderms into a relationship with [[shark]]s; however, as more fossils were found, placoderms were accepted as a sister group of [[Chondrichthyes|chondrichthyans]].

Much later, the exquisitely preserved placoderm fossils from Gogo reef changed the picture again. They showed that placoderms shared anatomical features not only with chondrichthyans but with other [[gnathostome]] groups as well. For example, Gogo placoderms show separate bones for the nasal capsules as in gnathostomes; in both sharks and bony fish those bones are incorporated into the braincase.<ref>{{cite journal | last1 = Young | first1 = G.C. | last2 = Goujet | first2 = D. | last3 = Lelievre | first3 = H. | year = 2001 | title = Extraocular muscles and cranial segmentation in primitive gnathostomes – fossil evidence | journal = Journal of Morphology | volume = 248 | page = 304 }}</ref><ref>{{cite book |last1=Goujet |first1=Daniel |last2=Young |first2=Gavin |year=2004 |chapter=Placoderm anatomy and phylogeny: new insights |pages= |chapter-url=http://www.pfeil-verlag.de/wp-content/uploads/2015/05/3_52d06.pdf |editor1-first=G. |editor1-last=Arratia |editor2-first=M. V. H. |editor2-last=Wilson |editor3-first=R. |editor3-last=Cloutier |title=Recent Advances in the Origin and Early Radiation of Vertebrates |publisher=Verlag Dr. Friedrich Pfeil |location=Munchen, Germany |isbn=3-89937-052-X }}</ref>

Placoderms also share certain anatomical features only with the jawless [[osteostracan]]s; because of this, the theory that placoderms are the sister group of chondrichthyans has been replaced by the theory that placoderms are a group of basal gnathostomes.