Tongues untied

The early embryos of mammals and other vertebrates typically have six pharyngeal arches, tissue bands under the early brain that develop into structures of the head and neck. The first arch gives rise to the mammalian malleus and incus (middle ear bones), mandible (part of the lower jaw), and tympanic bone (which supports the ear drum); the second, the stapes (middle ear bone) and part of the hyoid bone (a horseshoe-shaped structure in the neck); and the third, the remainder of the hyoid. Although the evolutionary transition of the first pharyngeal arch is well documented by fossil evidence, those of the second and third arches have received little attention in the developmental and paleontological literature. On page 276 of this issue, Zhou et al. (1) report on a newly discovered 165-million-year-old fossil from China in which the bones of the first three pharyngeal arches are preserved. The fossil defines a new taxon named Microdocodon.