[2] The symmoriiform fossils record begins during the late Devonian, and most had become extinct by the start of the Permian, with the genus Dwykaselachus from the Artinskian-Kungurian of South Africa being the latest known uncontroversial occurrence.

[6] Fossil evidence of Symmoriida have been found at Bear Gulch, Fergus County, Montana,[7] Bethel Quarry, Pike County, Indiana, Kinshozan quarry, Alaska, Gifu Prefecture, Japan, Bashkortostan, Russian Federation[8] and possibly also France.

[4] Symmoriiformes were previously regarded as members of the elasmobranchii, making them distant relatives of living sharks and rays.

Zangerl (1981), who erected the order, considered them to be elasmobranchs,[1] and this classification has been followed by some subsequent authors.

[9] The uncrushed braincase of Dwykaselachus indicates that symmoriiforms may be members of holocephali, as much of the internal anatomy, including the otic labyrinth and brain space configuration are similar to those of living chimaeras.