Panderichthys
Panderichthys is a 90–130 cm long fish from the Devonian period 380 million years ago, (Frasnian epoch) of Latvia. It is named after the German-Baltic palaeontologist Christian Heinrich Pander. It has a large tetrapod-like head.
Panderichthys which was recovered from Frasnian (early Late Devonian) deposits in Latvia, is represented by two species. Panderichthys stolbovi is known only from some snout fragments and an incomplete lower jaw. Panderichthys 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.
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, but they have only recently been examined in full.[3] 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 90–130 cm long fish with a large tetrapod-like head that's 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.[1] 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 exhibits several tetrapod-like features, while the humerus is longer than those found in other lobe-fins. 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). The vertebral column is ossified throughout its length and the vertebrae are comparable to those of early tetrapods.[1]
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.[4] 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 closer together. The external naris is close to the margin of the upper jaw. Paired frontals. Lack of external intracranial joint. Parietal located between the orbits and main portion posterior to orbits. In P. rhombolepis, the squamosal touches the maxilla (varies among specimens though). Teeth have complex polyplocodont structure. Lack of median fins. In panderichthyids and Ichthyostega 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.[6] 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.[7] 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.[7] 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[]
Another key feature of Panderichthys is its intermediate form during the evolution of digits. In the past it was believed that digits and fingers had no analogous part in sarcopterygian fish and were evolutionary novelties. However, recent reexamination[9] of existing Panderichthys fossils using a CT scanner shows four very clearly differentiated distal radial bones at the end of the fin skeletal structure. The study performed by Boisvert et al. in 2008 examined the pectoral fins of Panderichthys and found that the fins of Panderichthys are oriented anteroposteriorly, which is different from the limbs of tetrapods that project at an angle from the body. The humerus, radius (bone), and ulna all are analogous to the parts in tetrapods, but the CT scan uncovered the distal fin endoskeleton for the first time. The CT scan performed displayed an ulnare that articulates with the ulna and two terminal radials as well as an intermedium articulating in line to the lateral ridge of the ulna. The CT scan also uncovered radials that can be interpreted as digits, disproving the hypothesis that digits are new structures in tetrapods. These finger-like bones do not show joints and they are quite short, but nonetheless show an intermediate form between fully fish-like fins and tetrapods. Similar to the humerus, Panderichthys also has a more derived feature than Tiktaalik because it has an ulna that is longer than its ulnare, which is a feature of tetrapods.
Pelvic girdle[]
Additionally, the pelvic girdle and pelvic fins of Panderichthys represents an intermediate in the fish-tetrapod evolution. During the fish-tetrapod evolution the pelvic girdle became a weight bearing structure when the ilium, meso-ventral contact of the sides of the girdle, an ilium, and a sacral rib developed. The femur and humerus became longer and the radius/ulna and tibia/fibula became more equal in length. In general, the pelvic girdle in Panderichthys is more primitive than the pectoral girdle. This is due to the humerus of Panderichthys being a shape that is more of an intermediate, while the femur is more primitive because of the length ratio to the fibula and that it lacks an adductor blade and crest. This implies that Panderichthys was not capable of tetrapod-like hindlimb propelled locomotion because of its small pelvic fins, non-weight bearing pelvic girdle, acetebelum oriented posteriorly, and limited knee and elbow flexion.[11] Boisvert describes the locomotion of Panderichthys as possibly using one of its pectoral fins to anchor itself while side to side undulation propels the body forward.
Skull[]
Lastly, the braincase of Panderichthys demonstrates a key intermediate within the fish-evolution sequence. From the outside, Panderichthys has a tetrapod-like head, but actually retains an intracranial joint that is a characteristic of fish. Panderichthys shares many features with the osteolepiform Eusthenopteron such as similar hyomandibular and basipterygoid processes. Even though its head is shaped similar to that of a tetrapod, tetrapod craniums lack a lateral commissure, jugular groove, basicranial fenestra, arcual plate, and intracranial joint, all of which are present in Panderichthys. What this means is that there was no major change of the braincase construction since the first sarcopterygiian, but instead there had been only changes in skull shape. This implies that the evolution of the braincase during the transition from fish-tetrapod was very rapid and seems to display the same timing as the evolution of the pelvic girdle. In general, Panderichthys demonstrates that the braincase structure evolved much more slowly than the external skull morphology that created the tetrapod-like appearance of the head.[12]
As for the lower jaw and dentition, the lower jaw is similar to Rhipidistians and is composed of a tooth-bearing dentary, four intradentaries, a lingual prearticular, three coronoids, and an adsymphsial plate dorsally. In addition, the teeth are of polyplocodont structure.