The transition from fish to land animal is regarded by many as well documented: it is number two in Nature's presentation of "15 evolutionary gems". Some of the names given to members of the tetrapod lineage are quite well known: Panderichthys and Tiktaalik, Ichthyostega and Acanthostega, for example.
"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.""
The first animals deemed to be tetrapods (rather than fish) are called Ichthyostega and Acanthostega. They both come from rocks from Greenland. Until recently, Acanthostega was considered to be the most primitive tetrapod, and Ichthyostega was considered to have more derived characters. New research has placed Ichthyostega before Acanthostega in evolutionary development. Whilst this appears a very minor tweaking of the story, there are more important issues raised by the change. Before discussing these, it is worth reminding ourselves that the tetrapod evolution story has morphed dramatically with time. The quotation above represents the 'conquest of the land' model, with fish-like animals crawling out of the water. However, more recent work has exploded this hypothesis. All the Devonian tetrapods have characteristics that suggest they were aquatic and that fin-limbs evolved for negotiating their way through underwater obstacles. Somehow, the hypothesis goes, these structures were co-opted for terrestrial use.
Painting of Acanthostaga at home in the water (Credit Janice McCafferty, Source here)
The researchers examined the humeri of Ichthyostega and Acanthostega. They could not do this by extracting the fossil bones and examining them visually. To avoid damaging them, they utilised computed tomography (CT) scanning to sense the shape of the fossils remotely. Then the animals were "reconstructed" using imaging software, The first significant discovery was that some of the rocks contained juvenile forms of Ichthyostega - previously known only as adults. Interest then focussed on the upper arm bones, or humeri. In his commentary article, Friedman writes:
"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."
The researchers obtained four sets of data: juvenile and adult Ichthyostega humeri, and juvenile and adult Acanthostega humeri. This allowed them to assess developmental differences between the two animals as they matured. An overview of the findings is provided in the press release (which cites Callier as first author):
"Anatomies can morph as animals move towards adulthood, Callier said. And such shifts can help scientists deduce when in development the animal acquired the terrestrial habit. The fossils suggest that Ichthyostega juveniles were aquatically adapted, and that the terrestrial habit was acquired relatively late in development. The fossils bore evidence that the muscle arrangement in adults was better suited to weight-bearing, terrestrial locomotion than the juvenile morphology. It is possible that Ichthyostega came out of the water only as a fully mature adult.
In contrast, in Acanthostega "there is less change from the juvenile to the adult. Although Acanthostega appears to be aquatically adapted throughout the recorded developmental span, its humerus exhibits subtle traits that make it more similar to the later, fully terrestrial tetrapods," Callier said."
This creates an unexpected situation: subtle traits in the humerus of Acanthostega place it nearer the fully terrestrial tetrapods, but other characters point to an aquatic existence. Resolving this tension results in some surprising speculations:
"Ironically, the shape of Acanthostega's limb's, in both adult and the newly-discovered juvenile forms, is more "paddle-like" than Ichthyostega's, Callier said. "They would have been really good swimmers. So, although Acanthostega had limbs with digits, we don't think it was really terrestrial. We think even the adults were aquatic."
"If Ichthyostega is actually more primitive than Acanthostega, then maybe animals evolved towards a terrestrial existence a lot earlier than originally believed," she said. "Maybe Acanthostega was actually derived from a terrestrial ancestor, and then, went back to an aquatic lifestyle.""
Some may consider this situation unconvincing, with rather too much speculation about subtle morphological traits. Nevertheless, since there is a presumption of evolutionary transformation, these data have to be fitted into a coherent framework. Earlier in the fossil record, there are fish and no tetrapods; later in the fossil record, there are clear signs of terrestrial tetrapods - so, it is inferred, there has to be a transition between them. The more we now of the fossil record, however, the more difficult it is to identify an evolutionary branch. Instead of a tree, we see a bush. But this means that the quest for missing links will be elusive. We cannot identify ancestors and descendants in the data accessible to us. This changes the nature of the debate. As Friedman says:
"After 150 years of paleontological research dedicated to filling the gap between fishes and tetrapods, it is time to move past the simple formula of "the next missing link" and confront broader questions about tetrapod origins. The field has come far from its pre-Darwinian roots and ancestor-descendant daisy-chains, and now yields a profusely branching tree and correspondingly nuanced scenarios of terrestrialization, including that proposed by Callier et al: in a very real sense, a tangled bank."
According to Viviane Callier: "If there is one take-home message, it is that the evolutionary relationship between these early tetrapods is not well resolved". To that we can add the need to abandon talk of "ancestor-descendant daisy-chains" and to recognise the role of evolutionary presuppositions in the analysis of data.
Contrasting Developmental Trajectories in the Earliest Known Tetrapod Forelimbs
Viviane Callier, Jennifer A. Clack, and Per E. Ahlberg
Science, 324 17 April 2009: 364-367 | DOI: 10.1126/science.1167542
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 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.
Friedman, M. Emerging onto a Tangled Bank, Science 324, 17 April 2009: 341-342.
From Fish To Landlubber: Fossils Suggest Earlier Land-water Transition Of Tetrapod, ScienceDaily (Apr. 19, 2009)
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