There is a city called Seymour in Texas, USA. A large area of early Permian sediments is exposed in the north of the city. The famous fossil representative of salamanders between reptiles and amphibians was found here.
On the one hand, salamanders resemble a labyrinthine amphibian called a carnivorous lizard. For example, the top of its skull is completely covered, and all the bones of the labyrinth skull are still preserved; there are also sharp teeth like labyrinths on the edges of the upper and lower jaws, especially on the palatine bones. It has the typical large teeth of toothed animals; moreover, it has only one occipital condyle that connects the skull and cervical vertebrae like the carboniferous lizard.
Salamander
But on the other hand, the skeleton of the body of the salamander (anatomically called the postcranial skeleton) shows a series of advanced features similar to those of early reptiles. For example, the composition and shape of its vertebrae, the interclavicular bones in the shoulder girdle that connect the forelimbs to the spine, and the humerus are all similar to those of reptiles; the intestinal bones are much enlarged than those of amphibians; there are two sacral vertebrae, which are only A difference. Although its ankles are still of the amphibian type, the arrangement of the toe bones is similar to that of early reptiles: the thumb and big toe have two digits (toes), and the second digit (toe) has three digits (toes). ) joints, the third finger (toe) has four joints, the fourth finger (toe) has five joints, the little finger has three joints, and the little toe has four joints Bones. This arrangement of finger (toe) joint bones is the typical finger (toe) pattern of primitive reptiles, which can be expressed numerically as 2-3-4-5-3 (4).
So, is a salamander an amphibian or a reptile? The best answer to this question obviously depends on whether the salamander laid amniotic eggs on land like modern reptiles or returned to the water to lay eggs like modern amphibians. Unfortunately, paleontology has so far given us no clues on this issue. However, paleontologist Dr. White has conducted detailed research on salamanders and found that many fossil materials show that salamanders have sexually dimorphic characteristics - the first vein arch protruding under the tail vertebrae of some individuals is far behind the belt. , while the corresponding vein arches of other individuals are closer to the back of the belt. Based on this, it is speculated that those individuals whose vein arch is far away from the posterior edge of the belt indicate that there must be a considerable gap between the vein arch and the posterior edge of the belt. Such individuals represent females, and the large gap reflects the need for large amniotic eggs to pass through. Adaptations of the cloacal passage. If this speculation is combined with the various reptilian characteristics of the bones behind the head and analyzed, the axolotl should undoubtedly belong to a reptile.
However, judging from the characteristics of the skull and teeth, it is true that salamanders cannot be completely separated from amphibians. Moreover, some genera similar to salamanders also retain some more obvious amphibian characteristics. For example, the larvae of the disc lizards found in Europe still use gills to breathe.
Yet it is this seemingly conflicting evidence that illustrates the true meaning of animal evolution. Even the evolution of a species does not develop evenly and uniformly in all aspects. An animal may be progressive in some characteristics but primitive in others, a situation known as "mosaic evolution." The mosaic evolution of salamanders shows that they are a wonderful intermediate type between amphibians and reptiles. Therefore, we can more confidently speculate that reptiles originated from salamanders or amphibians similar to salamanders.
animal tags： salamander reptile amphibian
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