'' Adelospondyls share a variety of traits with other lepospondyls, although whether these traits are an example of convergent evolution is a controversial topic. Like the aïstopods and
lysorophian "microsaurs", they had very elongated bodies similar to that of
snakes and
eels. In addition, they lacked limbs (similarly to the aïstopods), although forelimbs were supposedly found in various adelogyrinids in the late 1960s. Andrews & Carroll (1991) found that all cases of forelimb bones in adelogyrinids were actually misinterpretations. For example, putative forelimbs discovered in
Adelogyrinus and
Palaeomolgophis by Brough & Brough (1967) were re-identified as
hyoid bones and ribs, respectively. Carroll (1967) also claimed that forelimbs were present in
Adelospondylus, but these later considered to be hyoids as in
Adelogyrinus. As is the case in other lepospondyls, the teeth of adelospondyls did not have a maze-like internal structure like those of "
labyrinthodonts", nor did adelospondyls possess enlarged fang-like teeth on the roof of the mouth. and a few "nectrideans" such as
Scincosaurus and
diplocaulids. Adelospondyls take this one step further. They possess only a single bone between the jaw joint and the skull roof. This bone is often believed to be a fusion between the tabular and the squamosal (termed a "tabular-squamosal" or "squamosotabular"),
Postcranial bones Adelospondyls can also be characterized by their vertebrae compared to other lepospondyls. They were spool-shaped and high in number, with
Acherontiscus having an estimated 64 vertebrae In fact, the name "adelospondyl" is
Greek for "obscure vertebra", referencing both the rarity of adelospondyls and this trait. However, Watson also included
Lysorophus as an adelospondyl rather than a lysorophian "microsaur", and other studies have shown that the absence of neurocentral fusion is very common among tetrapods, and therefore useless as a distinguishing feature. However, adelospondyls do have an additional quality of the vertebrae which is unique compared to other lepospondyls. In most lepospondyls, the left and right halves of each vertebra's neural arch are separate, rather than fused as in other tetrapods. Adelospondyls retain the primitive condition of the two halves being completely fused, making their vertebrae unique among lepospondyls.
Acherontiscus deviates from the norm of the subclass even more than other adelospondyls, as it possessed two separate bony components of the centra, rather than a single centrum (which is the case in adelogyrinids and other lepospondyls). These two different components (the intercentrum at the front and pleurocentrum at the back) were nearly equal in size, similar to the vertebrae of
embolomeres. Although adelospondyls lost all trace of their fore- and hind-limbs, they did retain a notable remnant of their presumably limbed ancestors. This remnant is a large bony dermal shoulder girdle, comprising the plate-like
interclavicle which was positioned on the midline of the chest, two
clavicle bones on either side of it, and two boomerang-shaped
cleithrum bones at the base of the neck. On the other hand, the bones of the endochondral shoulder girdle (i.e. the
scapulae and
coracoids), which supported the forelimbs, were lost along with the limbs. Adelospondyls possessed a large bony
hyoid apparatus, including
gill-supporting bones such as hypobranchials and ceratobranchials. Long, thin
scales have been preserved in most adelospondyl specimens, and they were more abundant on the ventral (belly) side of the body rather than the dorsal (back) side. ==References==