Adelospondyli is an order of elongated, presumably aquatic, Carboniferous ( Sensu). They have a robust skull roofed with solid bone, and orbits located towards the front of the skull. The limbs were almost certainly absent,
Most adelospondyls belong to the family Adelogyrinidae, and prior to 2003 the order and family were considered synonymous. In 2003, Ruta et al. assigned Acherontiscus to the order as the only known non-adelogyrinid member.
Description
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
Lysorophia "microsaurs", they had very elongated bodies similar to that of
Snake and
Eel. 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 bone and ribs, respectively. Carroll (1967) also claimed that forelimbs were present in
Adelospondylus, but these later considered to be hyoids as in
Adelogyrinus.
Skull
Adelospondyls had somewhat long and low skulls, with large orbits (eye sockets) shifted towards the front of the skull. As a result, the proportions of the skull bones were affected. For example, the
Nasal bone along the midline of the skull in front of the eyes were much shorter than the
Frontal bone directly behind them, which extended forwards past the level of the eyes. The frontal bones were omitted from the edge of the orbits (eye holes) due to a contact between the
Prefrontal bone- and
Postfrontal bone which typically lie in front of and behind the orbits, respectively. Unlike lysorophians and aïstopods, which had snake-like skulls with large openings and reduced bone material, adelospondyl skulls were strongly built and covered with ridges, pits, and grooves, including
lateral line sulci. They typically possessed many teeth, although such teeth differ in structure between families. Adelogyrinids, for example, had many numerous "chisel-shaped" teeth,
while
Acherontiscus had blunt teeth at the back of the mouth and sharp, thin teeth at the front.
As is the case in other lepospondyls, the teeth of adelospondyls did not have a maze-like internal structure like those of "
Labyrinthodontia", nor did adelospondyls possess enlarged fang-like teeth on the roof of the mouth.
Similar to various other lepospondyls, adelospondyls lost several bones in the temporal region of the skull, which is at the back of the skull between the quadratojugal bone of the jaw joint and the
Parietal bone and postparietal bones at the midline of the skull roof. Stem- and
Crown group-tetrapods typically have three to four bones on each side of the skull in this region (from top to bottom): the
Tabular bone, supratemporal (and sometimes an adjacent intertemporal), and
Squamosal bone. The intertemporal is lost (or fused into other bones) in a variety of unrelated tetrapod groups. The same process additionally occurs to the supratemporal in several lepospondyls, namely "microsaurs"
and a few "nectrideans" such as
Scincosaurus and
Diplocaulidae.
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"),
although it is also conceivable that the tabular disappeared completely leaving only the enlarged squamosal in its place.
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
and an indeterminate juvenile adelogyrinid having at least 70.
Most of these vertebrae lack haemal spines, indicating that the primary elongation took place in the body rather than the tail. The neural spines are not fused to the centra, a characteristic which D.M.S. Watson (1929) used to characterize the adelospondyls as a distinct order.
In fact, the name "adelospondyl" is
Greek language 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.
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
Embolomeri.
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 Scapula and Coracoid), which supported the forelimbs, were lost along with the limbs. Adelospondyls possessed a large bony hyoid apparatus, including Branchial arch 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.
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Carroll, RL (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
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von Zittel, K.A (1932), Textbook of Paleontology, CR Eastman (transl. and ed), 2nd edition, vol.2, p. 225, Macmillan & Co.
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