Oolon Colluphid
27 Apr 2009, 01:44 PM
Science Vol. 324. no. 5925, pp. 341 - 342 (17 April 2009)
Perspectives
PALEONTOLOGY: Emerging onto a Tangled Bank
Matt Friedman
Open any paleontology text or children's book on prehistoric animals, and you will find something between fish and tetrapod, forelimbs or fins planted on the land, tail receding into the water, eyes cast hopefully forward. These images encapsulate an episode of vertebrate history spanning the latter half (390 to 360 million years ago) of the Devonian, the waning days of the "Age of Fishes." Research on the fish-tetrapod transition overwhelmingly targets changes across the evolutionary tree, but the phylogenetic shift from water to land must have been mirrored by transitions within individual life spans as species began to explore shores and banks. On page 364 of this issue, Callier et al. (1) report the earliest evidence for just such a life-history transformation.
The authors examined the humeri of two iconic tetrapods from East Greenland: Ichthyostega (see the first figure) and Acanthostega. Found in both the forelimbs of tetrapods and the lobed fins of their "fish" relatives, the humerus is the single bone that links the appendage to the body. It is a complicated, festooned with bumps and ridges marking muscle origins and insertions. Because humeri are integral to the pectoral appendages, they record the biomechanical signature of the shift from fins to weight-bearing limbs.
Callier et al. examined nine humeri of Ichthyostega from six individual animals, and four humeri from three Acanthostega specimens. By arranging these bones based on size and other probable proxies of maturity, they identified divergent developmental trajectories in Acanthostega and Ichthyostega. [...]
http://www.sciencemag.org/content/vol324/issue5925/images/medium/341-2-med.gif
Remodeling of the humerus in Ichthyostega. In small Ichthyostega (left), the pectoral process (pink) lies near the center of the lower surface of the humerus, whereas, it rests near the anterior margin of the bone in larger specimens (right). Callier et al. (1) argue that this pattern reflects increasingly terrestrial habits during the life spans of Ichthyostega individuals.
In Ichthyostega, Acanthostega, and other tetrapods, the ventral (lower) surface of the humerus bears the pectoral process, a raised area marking the insertion of pectoral muscle. This feature occupies the conventional tetrapod position near the leading edge of the humerus in all Acanthostega and the most mature Ichthyostega, but in smaller Ichthyostega, the process lies close to the center of the bone (see the second figure).
On the basis of their new interpretations of humeri in close "fish" relatives of tetrapods, Callier et al. argue that the change within Ichthyostega development mirrors evolutionary patterns across phylogeny.
Science Vol. 324. no. 5925, pp. 364 - 367 (17 April 2009)
Contrasting Developmental Trajectories in the Earliest Known Tetrapod Forelimbs
Viviane Callier, Jennifer A. Clack, Per E. Ahlberg
Ichthyostega and Acanthostega are the earliest tetrapods known from multiple near-complete skeletons, with Acanthostega generally considered the more primitive.
New material indicates differing ontogenetic [developmental] trajectories for their forelimbs: In Ichthyostega, the pattern of muscle attachment processes on small humeri (upper arm bones) resembles that in "fish" members of the tetrapod stem group such as Tiktaalik, whereas large humeri approach (but fail to attain) the tetrapod crown-group condition; in Acanthostega, both small and large humeri exhibit the crown-group pattern. We infer that Ichthyostega underwent greater locomotory terrestrialization during ontogeny.
The newly recognized primitive characteristics also suggest that Ichthyostega could be phylogenetically more basal than Acanthostega.
I'm particularly intrigued by the last lines of the report:
Overall, the relative phylogenetic positions of Acanthostega and Ichthyostega are best regarded as uncertain; Ichthyostega may well be the more basal of the two. Such a placement of the more terrestrially adapted Ichthyostega (16), taken together with the features indicating weight-bearing ability (ventrally facing radial and ulnar facets) of the very primitive ANSP 21350 (9), would suggest a scenario of rapid early terrestrialization rather different from the currently predominant "aquatic Devonian tetrapods" model.
If I'm understanding this correctly, that model is that these early tetrapods shuffled around in among aquatic vegetation, using their fin-limbs to push their way about, as it were, and so were pre-adapted for the move to land. This seems to be saying that -- while that may be the case too -- the main thing these limbs were used for really was moving on land.
Perspectives
PALEONTOLOGY: Emerging onto a Tangled Bank
Matt Friedman
Open any paleontology text or children's book on prehistoric animals, and you will find something between fish and tetrapod, forelimbs or fins planted on the land, tail receding into the water, eyes cast hopefully forward. These images encapsulate an episode of vertebrate history spanning the latter half (390 to 360 million years ago) of the Devonian, the waning days of the "Age of Fishes." Research on the fish-tetrapod transition overwhelmingly targets changes across the evolutionary tree, but the phylogenetic shift from water to land must have been mirrored by transitions within individual life spans as species began to explore shores and banks. On page 364 of this issue, Callier et al. (1) report the earliest evidence for just such a life-history transformation.
The authors examined the humeri of two iconic tetrapods from East Greenland: Ichthyostega (see the first figure) and Acanthostega. Found in both the forelimbs of tetrapods and the lobed fins of their "fish" relatives, the humerus is the single bone that links the appendage to the body. It is a complicated, festooned with bumps and ridges marking muscle origins and insertions. Because humeri are integral to the pectoral appendages, they record the biomechanical signature of the shift from fins to weight-bearing limbs.
Callier et al. examined nine humeri of Ichthyostega from six individual animals, and four humeri from three Acanthostega specimens. By arranging these bones based on size and other probable proxies of maturity, they identified divergent developmental trajectories in Acanthostega and Ichthyostega. [...]
http://www.sciencemag.org/content/vol324/issue5925/images/medium/341-2-med.gif
Remodeling of the humerus in Ichthyostega. In small Ichthyostega (left), the pectoral process (pink) lies near the center of the lower surface of the humerus, whereas, it rests near the anterior margin of the bone in larger specimens (right). Callier et al. (1) argue that this pattern reflects increasingly terrestrial habits during the life spans of Ichthyostega individuals.
In Ichthyostega, Acanthostega, and other tetrapods, the ventral (lower) surface of the humerus bears the pectoral process, a raised area marking the insertion of pectoral muscle. This feature occupies the conventional tetrapod position near the leading edge of the humerus in all Acanthostega and the most mature Ichthyostega, but in smaller Ichthyostega, the process lies close to the center of the bone (see the second figure).
On the basis of their new interpretations of humeri in close "fish" relatives of tetrapods, Callier et al. argue that the change within Ichthyostega development mirrors evolutionary patterns across phylogeny.
Science Vol. 324. no. 5925, pp. 364 - 367 (17 April 2009)
Contrasting Developmental Trajectories in the Earliest Known Tetrapod Forelimbs
Viviane Callier, Jennifer A. Clack, Per E. Ahlberg
Ichthyostega and Acanthostega are the earliest tetrapods known from multiple near-complete skeletons, with Acanthostega generally considered the more primitive.
New material indicates differing ontogenetic [developmental] trajectories for their forelimbs: In Ichthyostega, the pattern of muscle attachment processes on small humeri (upper arm bones) resembles that in "fish" members of the tetrapod stem group such as Tiktaalik, whereas large humeri approach (but fail to attain) the tetrapod crown-group condition; in Acanthostega, both small and large humeri exhibit the crown-group pattern. We infer that Ichthyostega underwent greater locomotory terrestrialization during ontogeny.
The newly recognized primitive characteristics also suggest that Ichthyostega could be phylogenetically more basal than Acanthostega.
I'm particularly intrigued by the last lines of the report:
Overall, the relative phylogenetic positions of Acanthostega and Ichthyostega are best regarded as uncertain; Ichthyostega may well be the more basal of the two. Such a placement of the more terrestrially adapted Ichthyostega (16), taken together with the features indicating weight-bearing ability (ventrally facing radial and ulnar facets) of the very primitive ANSP 21350 (9), would suggest a scenario of rapid early terrestrialization rather different from the currently predominant "aquatic Devonian tetrapods" model.
If I'm understanding this correctly, that model is that these early tetrapods shuffled around in among aquatic vegetation, using their fin-limbs to push their way about, as it were, and so were pre-adapted for the move to land. This seems to be saying that -- while that may be the case too -- the main thing these limbs were used for really was moving on land.