Australia: The Land Where Time Began
Crossopterygii - Lobe-finned fish (tassel-finned fish)
The crossopterygians were large predators in the Devonian, the sole surviving species is the coelocanth Latimeria chalumnae.
By the Middle Devonian several types of crossopterygian had arisen, including osteolepiforms, the group that gave rise to the first 4-legged animals. The crossopterygians were all large predators. Some had evolved highly refined dental tissue, like enamel on the stabbing teeth. Some crossopterygians such as porolepifoms and onychodontiforms achieved their peak of diversity during the Devonian, then suddenly disappeared from the fossil record. Others, coelacanths and rhizodontiforms didn't reach their peak until the Carboniferous. Some of the rhizodontiforms dominated the lakes and rivers of their time, reaching an estimated size of up to 6 m. The coelacanths are the only crossopterygians that survived the Palaeozoic. They reached their modern form by the start of the Mesozoic, remaining relatively unchanged ever since.
Study of this complete skeleton found there were large spiracles on top of the skull with a down-folded cosmine-covered bone lamina on the tabular bone. It's spiracles were almost as large as those of elpistostegalians fish, such as Tiktaalik, and early tetrapods, such as Acanthostega. The structure of the internal limb skeleton shows closer similarity to elpistostegalians than to the more generalised tetrapodomorph fish such as Eusthenopteron. Gogonasus is now used to demonstrate the evolutionary stages leading from lobe-finned fish to tetrapods because it is better preserved and so more detail is known about its structure, and interpretation is not required, than the previously used Eusthenopteron.
The superficial appearance of Gogonasus is similar to that of generalised tetrapodomorph fish such as Osteolepis from Scotland. Its advanced features show that the primitive-looking cosmine-covered forms had evolved significant specialisations in the direction of the tetrapods.
The oldest known crossopterygians have been found in deposits from the Early Devonian. The only crossopterygians that had appeared in the record by this time were the porolepiforms and some primitive forms of uncertain affinity. By the Middle Devonian all the main groups had appeared, suggesting rapid radiation of the group. Among the primitive crossopterygians from the Early Devonian are Youngolepis and the similar form, Powichthys. Both were small predators up to 30 cm. They had thick rhomboid scales and heavily ossified head bones. Like lungfish, they had many small bones around the main paired bones of the skull roof, making them appear more advanced than other crossopterygians, another feature found in lungfish. They had small eyes and a well-developed lateral line.
These 2 species are both known mainly from their cosmine-covered skulls. Their skulls had a relatively long frontal shield and a short parietal shield. Unlike other crossopterygians, the braincase is not completely divided into 2 components in both forms. It appears to be a primitive stage that precedes the division of the braincase into 2 components, as occurs in all other known crossopterygians.
Youngolepis has several fused cheek bones, but has a cheek bone pattern much like that of an osteolepiform. Both forms have large central skull roof bones fringed with small bones. Enameloid tissue dips into flask-shaped cavities in the cosmine layer. This feature is further developed in porolepiforms, other crossopterygians don't have it. Further study has indicated that both forms are primitive members of the porolepiform group. It also suggests that they are closer to lungfish than to other crossopterygians.
Relationships of the Crossopterygian Groups
John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
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