Panderichthys

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.[7] 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.[8] 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.[8] 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.[9]

Comparisons between tetrapodomorphs during the transition from pectoral fins to forelimbs. Panderichthys is second from the left.

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, a reexamination[10] of existing Panderichthys fossils using a CT scanner shows at least four very clearly differentiated distal radial bones at the end of the fin skeletal structure. This 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, and ulna all are recognizable as analogous to the parts in tetrapods.[11]

CT scans allowed the authors to see below the scales and lepidotrichia (fin rays), uncovering the distal fin endoskeleton for the first time. The CT scan displayed an ulnare, a blocky carpal (wrist bone) that articulates with the ulna and two terminal radials. The wrist also included a more slender intermedium, articulating in line to the lateral ridge of the ulna. The CT scan additionally uncovered radials that can be interpreted as digits, disputing the hypothesis that digits are entirely new structures in tetrapods. These finger-like bones show neither muscle development nor joints and they are extremely small, but nonetheless show an intermediate form between fully fish-like fins and tetrapods. Similar to the humerus, Panderichthys also has a more derived feature similar to tetrapods and unlike Tiktaalik: the ulna is significantly longer than the ulnare.[11]

Restoration showing side to side undulation

The pelvic girdle (hip) 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.[12] 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.[12]

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 sarcopterygian, 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.[13]

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.[6]

As an intermediate in the fish-tetrapod evolution, Panderichthys had the capacity to breathe air. The trend from the early sarcopterygians to the first tetrapods was an increase in the size of the spiracular chamber and its opening to the outside. Compared to Eusthenopteron, the spiracular chamber of Panderichthys is greatly expanded and the hyomandibula is shorter compared to those in fish. The opercular series was also shorter compared to other osteolepiforms.[3] Panderichthys also has a single external nasal opening and a palatal choana. In contrast to earlier osteolepiforms, the palatal choana is elongated and the nariochoanal lamina is narrow. Along with the spiracular chamber, this feature in Panderichthys can be considered transitional during the evolution from fish to tetrapods.[6] Sarcopterygians such as Panderichthys can be considered at least facultative air breathers and demonstrate an intermediate form as air breathing was becoming more abundant.[3]