Protojaw tortoise is the common ancestor of living tortoise and tortoise. It is not much different from the modern tortoise except that its head can't be retracted into the shell. Its teeth have disappeared and its jaw has formed a horny beak, so its body is protected by its shell.

The shape of protognathus is the most basic feature of some turtles, so the evolution of turtles after protognathus is based on it. For example, in the later stage, the turtle's teeth disappeared completely and began to evolve into horny beaks.

However, the original jaw turtle still has teeth on its jaw bone. In the later period, the tortoise also developed the ability to retract its head, limbs, tail and other parts into the shell, but the original jaw turtle did not, which made the later turtles better adapt to the environment.

History of Protojaw Turtles

The earliest turtle fossil is Zhengnan turtle found in Permian strata in South Africa. This is a small animal only 10 cm long, and its fossil is very broken. The well-preserved bones show that there are tiny teeth on the edge of the lower jaw and upper jaw of Zhengnan turtle; There are 9 slender vertebrae between the shoulder and the sacrum, but only 8 pairs of ribs.

However, these eight pairs of ribs are very different, each piece becomes very wide and contacts the adjacent ribs back and forth. This situation just reflects that turtles began to specialize in the direction of developing tortoise shells on the road of evolution.

The Triassic turtles have evolved into typical modern turtles to adapt to radiation. For example, the number of glans bones in the original jaw decreased, the teeth disappeared from the edge of the palatal bone, and the body grew a protective hard shell.

These characteristics are the most basic adaptation direction of turtles, and the future evolution of turtles is only the perfection of these characteristics established on the original jaw turtles. For example, in the future, all the teeth of turtles will disappear, while the original jaw turtles still keep their teeth in the palatal bone; More advanced turtles have developed the ability to retract their heads, limbs and tails into their shells, while primitive jaw turtles cannot.