Australia: The Land Where Time Began

A biography of the Australian continent 

The earliest known fish don't have paired fins, though some early jawless fish had simple fin folds along the sides of the body. The vital step in fin structure that eventually led to the evolution of limbs in all higher vertebrates took place partly in some of the later agnathans, but particularly in the next step in fish evolution, the gnathostomes (jawed-fish). The pectoral and pelvic fins, with their associated internal support structures, shoulder and hip girdles, were the forerunners of arms and legs of the higher vertebrates. This step in vertebrate evolution occurred partly in the agnathans, the final steps being taken by the gnathostomes. The internal fin structure of these early fish already had the same main bones that have carried through to all other higher vertebrates, including humans. Such bones as the humerus, ulna and radius can all be recognised in internal structure of the pectoral fin of fish such as Barameda decipens, from Mansfield, Victoria, Australia. The most common remains of the rhizodontiform fish are these bones that were robust even at this early stage of fish evolution.

Osteostracans, armoured fish, were the first to have an internal shoulder girdle supporting the pectoral fins. They had a primitive scapulocoracoid bone that had 2-3 perforations that passed a blood supply and nerves to the fins. A cut-water was developed in many species that took the form of a well-developed part of the head shield for the shoulder girdle. It is believed there were no ossified fin bones, indicating a lack of significant muscular control of the fin, that is believed to have probably consisted of a few cartilaginous fin rays, possibly functioning in a simple up and down motion, to aid in the steering of the head end of the fish. No agnathans have any sign of pelvic fins or girdles. these occurred soon after the development of jaws.

Development of pelvic fins or girdles did not develop in the agnathans, arising following the development of jaws with the appearance of the gnathostomes in the fossil record. The placoderms and acanthodians were the earliest gnathostomes to be found as complete fossils, had well developed pectoral and pelvic girdles.

By this stage of evolution there are well-developed pelvic and pectoral girdles, as seen in well preserved 3D fossils. Arthrodires had a pelvic girdle of perichondrally ossified bone, nerves and blood vessels, arteries and veins, passed through numerous foramina. Simple perichondral tubes were present around the rays of the pelvic fins in some species.

The earliest development of sexual dimorphism is found in the placoderms. One group has pelvic girdles with male intromittent organs (claspers), as seen in 1 group, the ptyctodontids.

A series of stages from chondrichthyans - simple ray-finned fish - to lobe-finned crossopterygians, demonstrates the evolution of the pectoral fin into the standard vertebrate arm pattern. In its most primitive form the pectoral fin has many rays supporting it. These rays are grouped into 3 main regions, a leading propterygium, the mesopterygium, a middle division, and the metapterygium, a branching 3rd region. Even in some very primitive gnathostomes, such as sharks, the metapterygium can branch is such a way that a leading solid bone articulates with 2 strong elements, which then branch further down the fin.

The robust crossopterygian lobe fin resulted from the greater development of the metapterygium and the loss of the first 2 parts of the fin. Heterochrony was the mechanism for this to occur, paedomorphic loss of the pro- and metapterygium, and the peramorphic development of the metapterygium, eventually displacing the first 2 divisions of the fin entirely.

The first apterygial element is the humerus in the advanced crossopterygians such as osteolepiforms, it is the same bone supporting the upper arm in all later tetrapods. It is a robust, complex bone with a caput humeri and well developed entepicondylar process and foramen, features otherwise seen only in the tetrapods. In these fish the humerus articulates with the ulna and radius, after that there is no 1-to-1 correspondence with the wrist bones of primitive tetrapods.

Sources & Further reading

1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995

Links

New Onychodontiform (Oeteichthyes; Sarcopterygii) from the lower Devonian of Victoria, Australia