Australia: The Land Where Time Began

A biography of the Australian continent 

Origin of Jaws

According to Benton, the traditional view of the origin of jaws may be over simplified. The traditional view holds that jaws evolved by the modification of the anterior arches of an agnathan, a jawless fish. In the agnathans the gill slits are separated by gill arches of bone or cartilage. The author uses a hypothetical ancestral agnathan that has 8 gill slits and 9 gill arches as an example. In this hypothetical case loss of 4 gill slits and the fusion and modification of the anterior 3 gill arches leads to an early gnathostomes. In this hypothetical gnathostomes parts of the braincase floor may be formed by the most anterior gill arches. The palatoquadrate, that is the main portion of the upper jaw,  may have been formed by modification of the second gill arch, and Meckelís Cartilage that is core of the mandible (lower jaw). The next step was the modification of the third gill arch to form a skull bone and a mandible bone that forms part of the joint of the lower jaw, in the skull, the hyomandibular, and in the lower jaw, the ceratohyal.

According to the author, the evidence from anatomical studies suggests the processes involved in the traditional gill-arch theory of the origin of jaws may be more complex than previously suspected.

In lampreys the gill elements the gill lamellae develop medially to the supporting skeleton. The development of the gills of gnathostomes differs in that the gills develop laterally to the skeleton. This indicates a transition must have taken place from the gill arches being internal to being external prior to the evolution of jaws. It has been suggested that jaws originally evolved to assist with breathing (Mallatt, 1999). According to this suggestion the mandibular branchial arch first enlarged in pre-gnathostomes to increase the intake of oxygenated water. The evolution of suction feeding led to the use of jaws for feeding.

Evidence from developmental studies also indicates the classic theory is probably not correct (Kuratani et al., 2001). Jaw development in gnathostomes is from cells initially arising in the neural crest, and in lampreys similar cells are present early in development, where they develop into the upper lip and velum. In gnathostomes they develop into the precursor of the jaw. As a major repatterning during development is indicated by the evidence, jaw development in adult jawless vertebrates would not be expected.

In mice developmental genetic studies (Depew et al., 2002) the first branchial arch has been found to have mandibular bulge, a precursor of the Meckelís cartilage, and maxillary bulge, a precursor of the palatoquadrate. Dlx5 and Dlx6, the homeobox genes code for identity and anterior/posterior orientation of the upper and lower jaws. Dlx gene action: they possess 3 pairs of homeobox genes that activate sequentially, is seen in gnathostomes. Lampreys display Dlx expression in the branchial arches that is not nested, suggested to possibly indicate the condition in pre-gnathostomes.



Email: Author: M.H.Monroe
Last Updated 17/09/2011


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