The discovery of Entelognathus revealed the presence of maxilla (the jaw or jawbone, specifically the upper jaw in most vertebrates), premaxilla (either of two bones that form the front part of the upper jaw in vertebrates), and dentary (one of a pair of membrane bones that in lower vertebrates form the distal part of the lower jaws and in mammals comprise the mandible).
The Entelognathus is a supposedly diagnosable osteichthyan (boned fish), in a Silurian (the third period of the Paleozoic era) placoderm (an extinct fish). However, the relationship between these marginal jaw bones and the gnathal (jaw) plates of conventional placoderms, thought to represent the inner dental arcade, [up to now] remained uncertain.
Researchers now report a second Silurian maxillate placoderm;
Qilinyu rostrata bridges the unknown development of the gnathal (lower jaw only) and later maxillate (upper jaw) conditions for extinct fish.
Researchers propose that the maxilla, premaxilla, and dentary are homologous to the gnathal plates of placoderms and that all belong to the same dental arched group. The gnathal-maxillate transformation occurred concurrently in upper and lower jaws, predating the addition of infradentary (a serially homologous group ventral to the dentary) bones to the lower jaw.
Science continually refines the indistinct granularity of vertebrate development.
“SCIENCE” magazine, 21 Oct 2016,
pages 334 – 336