Bolosaurus

Bolosaurus was a small ground or tree dwelling anapsid reptile that lived in the Permian period, some 290 - 250 million years ago. This was the first described animal of what has now become a whole family of related fossil species, called the Bolosauridae. The divisions of the family are given below:

XXX* Eudibamus
XXXXX- Eudibamus cursoris (Late Carboniferous to lower Permian)

XXX* Bolosaurus (lower Permian)
XXXXX- B. grandis
XXXXX- B. striatus

XXX* Belebey (middle Permian)
XXXXX- Belebey vegrandis
XXXXX- Belebey maximi
XXXXX- Belebey chengi

If we look at the relationships in picture form things become a little clearer.

Bipedal

The ancestor of the group, Eudibamus, is thought to have been able to run on two legs. This 25 centimeter long plant eater is now the earliest known partially bipedal reptile. While its hip bones were not well suited to the task, the length of the back legs and the possible positioning of the rear feet would have allowed this creature to run on two legs in much the same way as many lizards, particularly dragons, do today.

The images below show Eudibamus running on two legs and sitting on all fours. These pictures are based on the current literature regarding Eudibamus with the positioning of the limbs based on how lizards with long back legs run and sit. They're a bit rough at present; I'll update them when I've improved the sketches.

Openings In The Skull

Several Bolosaurus family members showed openings in the skull in the temporal region (behind the eye) such as the reconstruction of the skull shown to the right. These are different to the skull holes called apsids. While they served the same purpose, which as to allow more jaw muscle attachment, the holes were formed between different bones than the apsids used to classify reptile groups. A skull hole that is not an apsid is called a fenestra, and so we can say that some Bolosaurus species had fenestrated skulls.

The difference in the location of the skull holes between Bolosaurids and the other reptile groups is shown below. Note the location of the holes relative to the bones that are surrounding them.

((image to be added std skulls (4) with fenestra shown on bolo. skull))

It doesn't look like much of a difference, but it is details like this that allow scientists to work out the relationships of fossil reptiles.

This hole in the skull makes an interesting point. Bolosaurids were clearly exploring evolutionary possibilities in similar ways to other reptile groups. The skull opening would have allowed the head to weigh less and have greater muscle strength in that region. This would have allowed a greater range of diets to be available to these animals and had they survived they would no doubt have diversified as the other reptile groups did.

It also indicates that reduction of the heavy, muscle-limiting skull was a general trend in reptiles. This has been seen in all reptile groups and can even be seen appearing independently in several of the parareptile (anapsid) groups. Even turtles, which are considered the only living anapsid reptiles, have extensive reduction of the rear of the skull, presumably for the same reasons.

While the skull identifies the Bolosaurids as anapsid reptiles, the teeth suggest at least some members of this group were evolving to resemble synapsid reptiles, the group which later gave rise to mammals. The evidence for this is the presence of different kinds of teeth in the jaw of several Bolosaurids. This is a mammal-like reptile feature; other reptile groups tend to have all their teeth the same.

Where this feature leaves the Bolosaurus species in terms of their evolution is not entirely clear. They are likely to have evolved from an ancestor common to both the synapsid and anapsid reptiles. Whether they can be directly grouped with other anapsids is not clear, and this may further point to anapsids actually evolving from several sources.

Modified Teeth

Almost all reptiles have teeth that are all the same. Having different teeth such as incisors and molars is strictly the realm of mammals and the early synapsid reptiles that gave rise to them. Nevertheless, the Bolosaurid reptiles showed a high degree of specialization in their teeth. This fact alone led early researchers to propose that members of this group were closely related to mammal like reptiles, perhaps even the ancestors of the whole group.

The image below shows a reptilian crocodile with uniform teeth, a mammalian dog with modified teeth, and the teeth of the Bolosaurid Belebey vegrandis.

However the structure of the skull has shown them to be unrelated to mammal-like reptiles. This is another example of convergent evolution. Personally I find examples like this fascinating. Who knows what may have happened if these animals had not become extinct. There may even have been another branch of "mammals" descended from Bolosaurus and its kin.

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