Chapter 23: Question 12

Explain how zoologists think the vertebrate jaw evolved.

Zoologist think the vertebrate jaws evolved from the first or second gill slits or whats called the gill arches. The gnathastons are the earliest vertebtes with a so called "jaw". The agnathans do not have a jaw.

Zoologists also believe that the mandibal acrh may have become enlarged to assist gill ventilation (increased air inflow may have been needed to compensate for the increasing metabolic demands of larger animals) and later the anterior gill arches became hinged and bent forward which is characteristic of today's fishes. These modifications represent one hypothesis of how the jaw developed. Evidence for these modifications are: 1. Both gill arches and jaws form from upper and lower bars that bend forward and are hinged in the middle. 2. Both gill arches and jaws are derived from the neural crest cells. 3.The jaw muscle structure is homologous to the original support muscle structure supported by the distribution of cranial nerves.

Condodonts are microscopic toothlike fossils

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