Uh-oh. Evolutionists discover two more gaps in the fossil record!
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Shubin et al. (2004) have found an interesting new fragmentary fossil of a late Devonian tetrapod, one that they suggest represents a new transitional form between the distinctly fishy Panderichthys and the significantly more amphibian-like Acanthostega.
Tetrapods exist in a range of different forms (tetrapodomorphs). Here they are arranged in a cladistic sequence, each with its left forelimb or forefin in dorsal view. The new humerus (ANSP 23150) from a Devonian tetrapod discovered in Pennsylvania by Shubin et al. is shown at the node of the cladogram (question mark) suggested by the authors.
It is ‘only’ the humerus, or upper arm bone, but this is a significant part of the animal, since it is these limbs that were undergoing a transformation as the lineage evolved away from the water and towards a more terrestrial lifestyle. The experts suggest that the structure of this particular limb was not appropriate for crawling on land, but was a step away from the paddles of a fish and was part of a stout limb that could have propped up the heavy, bony head of this predator as it lurked on the bottom.
Comparisons between the humeri of primitive fish and basal tetrapods reveal major functional stages in the origin of tetrapod limbs. The first important transformation happened in the common ancestor of panderichthyids and tetrapods: The humerus became a relatively immobile platform to support the body. The significance of this change is seen in correlated changes across the shoulder and humerus: in the expanded coracoid plate in the shoulder, the new flattened architecture of the humerus, the enhanced and reoriented muscular attachments, and the restricted rotatory movements possible at the articular surfaces of the shoulder. The new function of the appendage is not necessarily associated with adaptation to walking on land: Many of the changes seen in these Devonian taxa are also seen in modern fish, in which pectoral girdle enlargement, basal radial extension, and appendicular muscle elaboration are linked to trunk lifting and station holding in water. We argue that this function represents the intermediate condition between primitive steering and braking functions in fins and the derived aquatic or terrestrial walking gait. In this regard, the origin of tetrapods involved the evolution of increased mobility in distal parts of paired appendages but the retention of restricted mobility proximally. Recent discoveries reveal an increasing functional diversity within these earliest stages of tetrapod evolution, implying that there likely will be a corresponding variety of trackway patterns attributable to tetrapods within Devonian rocks.
The tetrapod transition is one of the best documented evolutionary sequences known, and this discovery adds one more step in the series.
Clack JA (2004) Enhanced: From Fins to Fingers. Science 304:57-58.
Shubin NH, Daeschler EB, Coates MI (2004) The Early Evolution of the Tetrapod Humerus. Science 304:90-93.
It's strange thing how the more of these fossils we find, the more "gaps" there are for the creationists to point at! I just don't understand how one can look at, say, Neoceratodus, Acanthostega, Elginerpeton and any common salamander and fail to see that there's a pattern there.