Placoderms, Dentary Bones, and the Origin of the Jaw – #UREES270 – 2018

Zhu, Ahlberg, Pan, Zhu, Qiao, Zhao, Jia, and Lu, 2016, A Silurian maxillate placoderm illuminates jaw evolution: Science, v. 354, p. 334-336.

What’s it about?

Placoderms are among the earliest vertebrates to have full-blown jaws. These jaws are develop from cartilagenous precursors that were once gill arches (or may have supported gills – that’s a whole ‘nuther discussion there). In more advanced bony fishes, like the common fishes we keep as pets, there are dermal bones that overlay these cartilagenous precursors: the dentary on the lower jaw and the maxillae over the upper jaw.

Until recently, it was thought that placoderms lacked these dermal bones. This paper is a description of the second species of placoderm that appears to have dentaries and maxillae.

Why does it matter?

These dermal bones were thought to be absent from all fish other than osteichthyians. Not only does this species show that this is not the case, the fact that dermal bones exist on these placoderms provides insight into where the osteichthyian jaw and dermal skull came from.

Why did I read this?

This paper is required reading for week 5 in my vertebrate paleontology course. This unit is on the origin of the jaw.

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