Panderichthys is a genus of extinct sarcopterygian (lobe-finned fish) from the late Devonian period, about 380 Mya. Panderichthys, which was recovered from Frasnian (early Late Devonian) deposits in Latvia, is represented by two species. P. stolbovi is known only from some snout fragments and an incomplete lower jaw. P. rhombolepis is known from several more complete specimens. Although it probably belongs to a sister group of the earliest tetrapods, Panderichthys exhibits a range of features transitional between tristichopterid lobe-fin fishes (e.g., Eusthenopteron) and early tetrapods. It is named after the German-Baltic paleontologist Christian Heinrich Pander. Possible tetrapod tracks dating back to before the appearance of Panderichthys in the fossil record were reported in 2010, which suggests that Panderichthys is not a direct ancestor of tetrapods, but nonetheless shows the traits that evolved during the fish-tetrapod evolution.
Discovery and history Panderichthys is represented by two different species: Panderichthys rhombolepis and Panderichthys stobolvi. P. rhombolepis was discovered by Gross in 1930 and P. stobolvi was discovered and figured by Emilia Vorobyeva in 1960. P. rhombolepis was discovered in Lode, Latvia within Frasnian deposits and according to P.E. Ahlberg can definitely be found in other Frasnian deposits in Latvia. Although fossils of Panderichthys have been known for a long time, they have only recently been examined in full. The first time they were recognized as being phylogenetically closer to tetrapods than fish was by Shultze and Arsenault in 1985.
Description
Panderichthys is a 1.5–2 m (4 ft 11 in – 6 ft 7 in) long fish with a large tetrapod-like head that is flattened, narrow at the snout and wide in the back. The intracranial joint, which is characteristic of most lobe-fin fishes, has been lost from the external elements of the skull, but is still present in the braincase. The patterns of external bones in the skull roof and cheeks are more similar to those of early tetrapods than those of other lobe-fins. The transitional qualities of Panderichthys are also evident in the rest of the body. It lacks the dorsal and anal fins (fish fin) and its tail is more like those of early tetrapods than the caudal fins of other lobe-fins. The shoulders exhibit several tetrapod-like features, while the humerus is longer than those found in other lobe-fins. The vertebral column is ossified throughout its length and the vertebrae are comparable to those of early tetrapods. On the other hand, the distal parts of the front fins are unlike those of tetrapods. As would be expected from a fin, there are numerous lepidotrichia (long and thin fin rays). Panderichthys has many features that can be considered an intermediate form during the fish-tetrapod evolution and displays some features that are more derived than its phylogenetic position indicates, while others that are more basal. The body form of Panderichthys and Tiktaalik represents a major step in the transition from fish to tetrapods and they were even able to haul out on land. According to Shultze and Trueb, Panderichthys shares ten features with tetrapods:
The skull roof is flat compared to fish skulls. The orbits are more dorsal and lie closer together. The external naris is close to the margin of the upper jaw. Frontals are paired. There is no external intracranial joint. The parietal bone is located between the orbits and main portion posterior to orbits. In P. rhombolepis, the squamosal touches the maxilla (varies among specimens though). The teeth have a complex polyplocodont structure. A lack of median fins. In panderichthyids and Ichthyostega, the ribs are attached to the neural arch and there is an intercentrum.
Humerus One of the key transitional features of Panderichthys is its humerus. During the transition from fish to tetrapods the limbs began to move and became located at a right angle to the body rather than being oriented toward the posterior end. As a result, the muscles became perpendicular to the body and caused the limbs to move in a more anteroposterior and dorsoventral pattern. This in turn affected the shape of the humerus and as a result early tetrapods have an L-shaped humerus. Due to a recent discovery of a humerus of Panderichthys that was not flattened, the specimen could be analyzed in much greater detail. The humerus of Panderichthys displays a variety of features including ones that are both primitive and derived. Despite being placed as basal to Tiktaalik, the humerus of Panderichthys has features that are more derived, but overall is very similar. Both Panderichthys and Tiktaalik have humeri that are dorsoventrally flattened with a blade like entepicondyle curving ventrally, separated epipodial facets, a latissimus dorsi process and ectepicondule process that is parallel to the preaxial margin. The humeri of both species are considered transitional forms because they are almost L-shaped, have a low latissimus dorsi process, a low entepicondyle, and an intermediate entepicondylar canal. The humerus of Panderichthys is more derived than that of Tiktaalik because of the presence of a more preaxially oriented radial facet as well as a more slender shaft. One feature that is unique to Panderichthys is that the entepicondyle does not project as far as the epipodial facets and the humeral ridge does not go into the entepicondyle. The result of the analysis of the humerus of Panderichthys is that the transition of the humerus from the fish-like organisms to tetrapods occurred much slower than previously thought and Panderichthys now provides a base to determine many autapomorphies. Due to the orientation of the fin towards the posterior end, the attitude of the limb is more horizontal than vertical and the operational space in which it acts is level to the shoulder joint, which causes the muscles to pull at a right angle to the body. This resulted in the ability of Panderichthys to prop up its large head most likely to breathe.
Fins and digits
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