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 coelacanth 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 tetrapods. 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.
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.
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.
The relationships among the crossopterygians are still strongly disputed among vertebrate palaeontologists, so there are a number of suggestions on the matter. One proposal, has the lungfish as the pinnacle of osteichthyan evolution. The implication here is that the lungfish are a subgroup of the crossopterygians. This theory has been criticised because of its reliance on soft tissue characteristics of the coelacanth and modern lungfish to prove that lungfish are more advanced than crossopterygians.
A clearer picture of the anatomical features of the crossopterygians have become available recently in new well-preserved finds in Australia and Russia. The evidence seems to be favouring the traditional theory, held since the 19th century, that the crossopterygians, not the lungfish, gave rise to the amphibians.
One theory recognises the crossopterygians as a natural grouping, with successive stages in their evolution clearly delineated. In this scheme there are 2 main divisions of crossopterygians, one encompasses the primitive orders such as - coelacanths and onychodontids. The other, the Rhipidistia - Porolepiformes, Rhizodontiformes, Osteolepiformes, Panderichythyida, and Tetrapoda.
Rhipidistians have a well developed skull roof with a set pattern of bones, lower jaw with 4 large infradentaries, plicidentine ( a complex pattern of infolded dentine and enamel) in the teeth. A supraorbital sensory line canal that joins the main lateral line. It has been found that Powichthys and Youngolepis, "stem group crossopterygians", have intermediate stages in the intra-cranial joint closure, suggesting they are more closely related to lungfish. In this theory, the lungfish would be the pinnacle of 1 crossopterygian lineage.
Of the crossopterygians the most specialised group has forms with a strong pectoral fin, and with a humerus, ulna and radius. This group includes Rhizodontiformes, Osteolepiformes, Panderichthyida, and Tetrapoda. The penultimate group includes the forms with a choana (palatal nostril) - Osteolepiformes, Panderichthyida, and Tetrapoda. Recent papers describe the close relationship between the panderichthyids and the first amphibians.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|