Evolution and extinction It was thought for a time that placoderms became extinct due to competition from the first bony fish and early sharks, given a combination of the supposed inherent superiority of bony fish and the presumed sluggishness of placoderms. With more accurate summaries of prehistoric organisms, it is now thought that they systematically died out as marine and freshwater ecologies suffered from the environmental catastrophes of the Late Devonian and end-Devonian extinctions. Fossil record The earliest identifiable placoderm fossils are of Chinese origin and date to the early Silurian. At that time, they were already differentiated into antiarchs and arthrodires, as well as other, more primitive, groups. Earlier fossils of basal Placodermi have not yet been discovered. The Silurian fossil record of the placoderms is both literally and figuratively fragmented. Until the discovery of Silurolepis (and then, the discoveries of Entelognathus and Qilinyu), Silurian-aged placoderm specimens consisted of fragments. Some of them have been tentatively identified as antiarch or arthrodire due to histological similarities; and many of them have not yet been formally described or even named. The most commonly cited example of a Silurian placoderm, Wangolepis of Silurian China and possibly Vietnam, is known only from a few fragments that currently defy attempts to place them in any of the recognized placoderm orders. So far, only three officially described Silurian placoderms are known from more than scraps: the basal antiarch Silurolepis, from the Ludlow epoch of Yunnan, China, known from an almost complete thoracic armor Entelognathus, a placoderm incertae sedis that combines features of primitive arthrodires with jaw anatomy otherwise only seen in bony fish and tetrapods. Qilinyu, a close relative of Entelognathus that further links Entelognathus as a transitional form between placoderms and other stem-gnathostomes and crown-group gnathostomes. The first officially described and oldest Silurian placoderm is an antiarch, Shimenolepis, which is known from distinctively ornamented plates from the late Llandovery of Hunan, China. Shimenolepis plates are very similar to the early Devonian yunnanolepid Zhanjilepis, also known from distinctively ornamented plates. Paleontologists and placoderm specialists suspect that the scarcity of placoderms in the Silurian fossil record is due to placoderms' living in environments unconducive to fossil preservation, rather than a genuine scarcity. This hypothesis helps to explain the placoderms' seemingly instantaneous appearance and diversity at the very beginning of the Devonian. During the Devonian, placoderms went on to inhabit and dominate almost all known aquatic ecosystems,