The Silurian is a geological period between the Ordovician and the Devonian. Its rocks preserve evidence relevant to the diversification of jawed vertebrates and early terrestrial ecosystems.
A Silurian placoderm with marginal jaw bones resembling those of Osteichthyes illustrates its evolutionary importance: a fossil's combination of characters can challenge a simple sequence in which each major group acquires a wholly separate body plan.[ref]
A Silurian placoderm with osteichthyan-like marginal jaw bones by Min Zhu and others. 2013, character combination in Entelognathus
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