Australia: The Land Where Time Began

A biography of the Australian continent 

Panderichthyids - Fish or Finned Amphibian

The panderichthyids have for a long time been classed as a group in the osteolepiform crossopterygians. Recently they have been redescribed and assigned to a new order, the Panderichthyida. This was done because they share more features with the early amphibians than they do with other fish.

There are 5 species in 3 genera in this order. Only 1 of these, Panderichthys (Pander's fish) is known in any detail. The first specimen of Panderichthys was found in the site of Lode, in Latvia. The fossils from this site are very well preserved in 3-D form, in sediments of soft clay. More recent discoveries of the remains of this fish have allowed the description of almost all aspects of its anatomy.

A number of features characterise this fish - in the snout, the median rostral bone doesn't contact the premaxilla, under the head, they have a very large gular bone. The mouth is beneath the snout, and in the nasal capsule there is a lateral recess. A number of their specialised features are also found in the early amphibians. Some of these features are - they are long bodied fish, with a head nearly 1/4 of their total length. They have large pectoral fins and small pelvic fins, with no dorsal or anal fins. The eyes are close together on top of a broad, flat skull, and have distinct brow ridges.

The external nostrils are close to the margin of the mouth on the ventral side. The teeth show strong similarities with those of some amphibians, having a complex form of labyrinthine infolding of the enamel and dentine. In the skull roof bones there are 3 pairs of median bones from the back of the skull to the eyes, other crossopterygians have 2. They had large cheek bones, and the squamosal is separated from the maxilla by the jugal bone, as in amphibians. There is a large spiracular slit along each side of the skull table. So the the cheek bones meet the skull roof only along part of the length of the skull. The intracranial joint has fused, as can be seen by the external pattern of skull roof bones, so that the skull was not kinetic as in many of the crossopterygians. The humerus, ulna and radius are strongly ossified in the pectoral fins, the humerus having a longer shaft than any other crossopterygian. The vertebrae have only the intercentrum (ventral component) present, having large neural arches straddling the notochord. As in the tetrapods, the ribs are attached to neural arch and the intercentrum. The body is covered with rhombic scales but no cosmine.

Elpistostege is the second best known genus, from the Escuminac Formation of Quebec. When an incomplete first specimen was first described in 1938 the palaeontologist was convinced it was a primitive amphibian. When  better preserved specimens were described it was realised it was a panderichthyid crossopterygian. Elpistostege is known from only a few partial skulls and is generally very close to Panderichthys.

Obruchevichthys is the 3 rd genus and is found in Russia, but thought by some probably should not be assigned to the order based only on a lower jaw showing many features in common with tetrapods. Some regard it as an early tetrapod.

When seen only as partial skulls, the panderichthyids are almost impossible to distinguish from early amphibians, a situation that has led to some confusion.

Metaxygnathus denticulus was originally believed to be a rhipidistian crossopterygian, but has since been reassigned to the panderichthyids. It differs from the related fish in that it has a more massive dentary bone, and on the back of the jaw is a retroarticular process, a large bony projection, for the attachment of the muscles involved in opening the jaw. There are also a different set of muscles that open the jaw, the depressor mandibulae in amphibians and hypobranchial musculature in fish. The fish most closely related to these amphibians had a different set of stresses on their jaws, resulting in a difference in shape and orientation of the articulation between the upper and lower jaws. Where the adductor mandibulae, muscles that close the jaw attach, just in front of the jaw articulation and around the articulation, the part of the jaw subject to the greatest stress is the point at which the jaw is thickest. In amphibians, the jaws are thinnest to the front and the back, as there is less stress at these places. The attachment site of the depressor mandibulae is at the rear of the jaw, used to lower it. In some cases there are no muscles attached to this point.

It had a variety of conical teeth, the largest of which were scattered over the middle of the jaws, some being curved towards the inside, apparently for holding onto small prey.

Sources & Further reading

1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
2. Patricia Vickers-Rich, Thomas Hewitt Rich, Wildlife of Gondwana, Reed Australia, 1993

Links

1. Panderichthys
2. Panderichthys images
3. The pectoral fin of Panderichthys and the origin of digits
4. The origin and early diversification of tetrapods
5. Palaeontology: a firm step from water to land
6. Palaeontology: fins made for walking
7. Fish with fingers?
8. The pelvic fin and girdle of Panderichthys and the origin of tetrapod locomotion
9. A primitive amphibian from the Late Devonian of New South Wales

Fish