Australia: The Land Where Time Began

A biography of the Australian continent 

Comparative Anatomy

All modern vertebrates are variations on themes that had evolved before the fish at the base of the lines leading to the present-day vertebrates, including humans, left the water. Bilateral symmetry was a basic step that led to later vertebrates. The presence of a head end, leading eventually to a head, is first seen in flatworm-type organisms.

The development of the ear drum in early amphibians was a major step towards permanent life on land. The crossopterygian jaw joint is braced by the hyomandibular bone, in the early tetrapods the same bone was modified to brace the otic membrane, or skin above the inner ear. The stapes bone evolved to transmit the vibrations from the air to the inner ear. The incus and malleus didn't evolve until the arrival of mammals and reptiles. There was virtually no change in the nostrils and eyes of the fish in the transition to amphibians.

The reptiles were the first tetrapods to become waterproofed enough to move further from water.

Sexual dimorphism first appears in the placoderms.

Skull bones

The earliest of the fish were agnathans (jawless fish) found living mainly in freshwater and apparently on the bottom of water bodies. They were small, probably all or nearly all, filter feeders, having no jaws to allow them to become predators. For some reason they developed thick, bony body armour. Suggestions as to the function of this armour include the storage of minerals such as calcium. Romer (1959) has suggested that the armour was to protect them against the only known predator large enough to threaten hem at the time, the water scorpions, eurypterids, which grew much larger than the vertebrates of the time, reaching as much as 12 ft (nearly 4 m) long, and their biting mouth parts indicate they were predatory.  There were no vertebrate predators so the eurypterids seem the only plausible threat to them, in fact agnathans and eurypterids are often found in the same deposits. The other invertebrates that could possibly have preyed on the agnathans were marine forms.

As the vertebrates moved to the ocean and became stronger swimmers they tended to lose their armoured coverings, though even the most advanced forms retained some bony armour covering the head. As the fish became amphibians and moved onto land they retained the bones of the head, as have the later mammals, including humans, where most of the bones of the head are of dermal origin, but are now covered with muscles and skin.

Jaws

In primitive vertebrates there are bony bars on either side of the throat lining the gill openings, with 2 main parts, upper and lower. The jaws are in line with the bars and, like them,  have a joint joining the upper and lower parts. According to Romer, the jaws probably arose from the enlargement of the front pair of gill bars that rotated to have their free ends facing forward, taking on the new function of obtaining food as jaws. The upper jaw is usually still a separate element in sharks, being loosely attached to the skull, the main upper joint (hyomandibular) of the first arch just behind the jaws has been adapted to prop the jaw joint against the side of the braincase . The Devonian shark, Cladoselache, the jaws appear to be obviously in series with the gill bars. In humans and most higher forms the upper jaw is fused to the skull.

Limbs

The earliest agnathans appear to have been restricted to the bottom, so all the movement they needed to undertake was in a horizontal plane. As with other fish, propulsion is provided by the sinuous movement of the body, and the muscles on either side of the body alternate in their contractions, with additional propulsion being provided by the caudal fin, pushing the fish forward, the only fins needed for such a horizontal movement are vertical rudders for steering. Once jaws had evolved and the new gnathostomes left the bottom, changed their body shape to a cylindrical form and moved in 3 dimensions, the paired pectoral and pelvic fins developed to keep them stable and steer in 3D movements. In Cladoselache and other early fish, the paired fins appear to have developed from longitudinal skin folds, being used as stabilisers for free swimming fish.

Some of the early agnathans (jawless fish) had developed simple fin folds. 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 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.

In the Crossopterygians, the shoulder girdle and the internal shoulder girdles (scapulocoracoid) are well-ossified. The pectoral and pelvic fin skeletons are well ossified in all crossopterygians, but especially so in the osteolepiforms and rhizodontiforms. These forms have a powerful humerus articulating with the ulna and radius, as in all higher vertebrates, except the limbless forms. The shaft of the humerus of Panderichthyids was longer than that of any other crossopterygian. This is one of the features that make it a candidate for the form that led to the amphibians.

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. These limb bones probably became robust enough to allow the fish, and then the amphibian, to function as an ambush predator, where there was a requirement for a sudden powerful thrust to grasp passing prey. Limbs developed for the purpose of a powerful thrust would be at least partially pre-adapted from walking on land where they would be required to support the full weight of the animal.

Amphibians

In Ichthyostegalids, the earliest tetrapods, the front and hind limbs had 7-8 digits, and on each hand or foot the digits could be divided into a series of large digits and a series of much smaller elements.

An amphibian, Tulerpeton from the Russian Devonian, had 6 digits on its front limbs. The cleithrum is a large, high bone, and external shoulder girdle bones are not seen on the outside of the animal.

Links

1. Vertebrate dentitions at the origin of jaws: when and how pattern evolved
2. Scientists Discover the Evolutionary Origin of Fins, Limbs
3. The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb
4. The origin of vertebrate limbs

Sources & Further reading

1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
2. Alfred S. Romer, The vertebrate Story, University of Chicago Press, 1959