What Is The Turtle - Pareiasaur Connection Based On?How solid is the turtle - pareiasaur connection? Are turtles descendants of or at least related to those giants of the Permian? They certainly share an outwardly similar shape, but as we can see from convergent evolution, outward appearance is no indicator of close family ties.
There are two possibilities. Turtles are descendants of either the diapsid line that gave rise to all other living reptiles, or they came from the otherwise now extinct group called the anapsid reptiles, as shown to the right.These two groups separated at the very beginning of reptile history. Members of these groups can be identified by differences in the skulls and in the structure of the foot bones, in particular the hind feet of turtles. Looking at these two features lets us understand the cause for debate. The SkullThe turtle skull, shown on the left in the image below, is a solid shield with no openings behind the eye sockets. This is the primitive condition that can be seen in all ancient amphibians such as Eryops. This is the defining feature of the anapsid group. Based on the skull alone, we would have to say that turtles come from the anapsid line.
By comparison, diapsid reptile skulls are much reduced with large openings in the outer bone shield. The vast difference in skull structures lead many to believe that turtles cannot possibly descended from the same stock as diapsid reptiles. The Hind Foot
The hand and foot bones of early reptiles are very different from those of amphibians. You can see that many of the bones are reduced in number; this is due to two or more bones fusing, or joining, to make one bone.TarsalsOf some interest here are the light blue colored bones which are called tarsals. While the earliest reptiles had 5 tarsals, both the diapsid and anapsid reptiles very quickly reduced this to 4. It is not yet certain whether the move from 5 to 4 tarsals happened before the two families separated or after. MetatarsalsMetatarsals are the long bones that come after the ankle but before the toes. Turtles have a strange looking hooked fifth (outside) metatarsal. The purpose of the hooked metatarsal is to provide a heel-like structure to make walking more efficient. This is a feature not seen in any anapsid reptiles, but it is clearly evident in several groups of diapsid reptiles. Have a look at the comparison of metatarsal arrangements below:
Note here that the arrangement of the ankle bones may not be accurate; I have left them the same in the three images to avoid distraction. Embryology has confirmed that when turtles and modern diapsid reptiles are developing in the egg, the hooked foot bone grows from the same source in both and not from the fusion of any other parts of the foot.
GeneticsRecent work on the mitochondrial DNA of turtles and other reptile species has suggested that turtles are more closely related to crocodiles and birds than previously thought. Both of these groups belong to the diapsid line of reptiles. However, while the genetic resemblances in certain regions of the genome are remarkable, the actual nature of the relationship is still not clear. Horizontal Gene Transfer has been shown to be a major force in bacterial evolution and its importance or rate of occurrence in vertebrates is far from clear. It is possible therefore that some sequences of mitochondrial DNA have migrated between early turtles and the ancestor of crocodiles and birds (called archosaurs). If this did happen, the genetic similarities between the two are no indication of an evolutionary relationship. In ConclusionPerhaps the only thing that is clear here is the source of confusion. Until more fossil remains clearly showing intermediate forms are found, we will never be certain of the origin of turtles. Return from Turtle-Pareiasaur to Turtle Evolution or return to the Reptiles Home Page. |
