Fish to Tetrapod - Body Scales and Fin Rays

Australia: The Land Where Time Began

Fish to Tetrapod - Body Scales and Fin Rays – Changes

There are 2 more structures that Clack¹ used to contrast the tetrapods from their ancestral fishes, the bony scales and the fin rays, both of which are derived from the dermal component of the bony skeleton.

Scales are formed in fish mainly in the dermis, the middle layer of skin, which is part of the skin containing structures such as blood vessels, sensors such as touch and pressure, glands as well as some superficial muscle tissue. The scales were of a bony texture in early lobe-finned fishes and some species had scales covered by cosmine, a shiny enameloid material that matches the dermal bones of their skulls, to which the epidermis, the external layer, also contributed. Polypterus, a modern ray-finned fish shows bony, enameloid scales that are similar to those seen in many forms from the Palaeozoic, though the bony layer has been lost in almost all modern fishes, the scales being left as thin proteinaceous sheets.

The scales were interlocked in diagonal rows that encompassed the body, meeting each other dorsally and ventrally at an acute angle in the early ray-finned and lobe-finned fishes. Each of the scale rows articulated with the one in front of and behind it, and along a line extending along the middle of each side of the body there was a tube containing part of the lateral line organ on each scale.

Much of this scale covering, especially those on the back, was lost in the early tetrapods, and the parts that remained were modified. On the early tetrapods the scales are known as scutes or gastral scales that formed flexible armour covering only the belly and parts of the sides. There was no enameloid layer on these gastral scales, which were of various shapes such as oval, spindle or rhomboid, and there was an internal ridge and groove on each where each element articulated to the ones dorsal and ventral to it, though not with scales on either side of it. Where the rows met ventrally the formed a V that pointed forward, with the exception of around the shoulders and interclavicle, where the V pointed backwards, the orientation being reversed. It is also known that there were dorsal scales on some early tetrapods, though these were usually rounded and there was a tendency for them to be less well ossified, which has resulted in them not fossilising well. The caecilians, some amphibians that belong to this group, still retain vestiges of these primitive dermal scales. Gastral scales are not equivalent to the scales of modern reptiles. The gastral scales on early tetrapods would have been covered by a thin layer of outer skin, the epidermis, as they were of dermal origin. The scales of modern reptiles are formed in the epidermis itself.

In the temnospondyls (Witzmann, 2007), an early group of tetrapods, it has been demonstrated that there was an ontogenetic sequence of scale morphology, according to which the scales begin as relatively thin elements that are rounded, changing to the interlocking spindle-shaped scales allowing flexibility in the scale cover during growth, and then to a shape that is more rhomboid and the interarticulations are more rigid. As a paedomorphic trait this sequence can be modified with some taxa retaining a juvenile morphology. When dorsal scales are present they tend to retain the round morphology present in juveniles. In nontemnospondyl taxa oval or spindle-shaped scales are also found, and similar growth patterns may be involved.

Dermal rays support the fins in bony fishes, which are essentially narrow, elongated scales forming a fringe that suspends a thin web of skin. Narrow fringes of fin web surrounded each paired fin, the tail, an anal fin and 2 dorsal fins in the early lobe-finned fishes. A series of radials additionally supported the dorsal and anal fins, the pattern of these radials varying according to the family. Almost all the fin webbing disappeared in tetrapods, as well as all traces of dorsal and anal fins, the only remnants being known of in the 2 earliest known tetrapods which had retained some tail fin web, and supraneural spines have been found in a single specimen from an embolomere (Suborder embolomere of Reptiliomorpha).

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