Summary of the Article

Summary and Analysis

The article A Silurian placoderm with osteichthyan-like marginal jaw bones is written by Min Zhu, Xiaobo Yu, Per Erik Ahlberg, Brian Choo, Jing Lu, Tuo Qiao, Qingming Qu, Wenjin Zhao, Liantao Jia, Henning Blom & You'an Zhu. According to this article gnathostome (vertebrae having jaws) crown group consists of two extant clades having contrasting complements of character. The Chondrichthyes (cartilaginous fish) do not have dermal bones that are the specialty and identifying feature of Osteichthyes (tetrapods and bony fish). The polarization of the morphology of the ancestors of crown gnathostomes and the differences make this topic debatable. In this study, they have discussed the preserved three-dimensional, four hundred and nineteen million year old placoderm fish belonging from the Silurian of China that shows the first stem gnathostome along with dermal marginal jaw bones (premaxilla, dentary and maxilla). These are the features that were previously limited to Osteichthyes (Zhu et al., 2012). These researchers conducted the phylogenetic analysis, which put the new form near the top of the group of gnathostome stem, but this does not help in finding association or relationship with other placoderms. All acanthodians were also assigned to the chondrichthyan stem group by the analysis. The results showed that the last ancestors of both the groups Osteichthyes and Chondrichthyes comprised of macromeric dermal skeleton and provided a novel framework for researching on the divergence of gnathostome divergence (Zhu et al., 2012). In the article, the researchers presented the new fish from the Silurian of China that combines the braincase, dermal skull roof-like placoderm, osteichthyan-like dermal bones of the hyoid arches mandibular with shoulder girdle and a palatoquadrate derived characteristics with crown gnathostomes.

For understanding the reason that why this new species is interesting, we have to understand some background information. First of all there is ...