The quest for a plausible scenario for tetrapod evolution continues. Contrary to much popular (and some technical) literature, we have not yet arrived. A new fossil find promises to stimulate a fresh debate about the contribution the Coelacanth makes to our thinking.
The new fossil reveals, for the first time, the pectoral fin endoskeleton. Significantly, it is not like the modern form. "The most conspicuous feature of Shoshonia is its broad, fanshaped pectoral fin supported by a central lobe [. . . This] differs from the near-symmetrical finweb common to the living coelacanth Latimeria and lungfishes and their closest extinct relatives." One of the co-authors said that the fossil's pattern is similar to the branching arrangement still embedded in the fins of paddlefishes, sturgeons and sharks. "To understand the developmental evolution of the limbs of tetrapods, we shouldn't be looking at the fins of our nearest living fish relatives - lungfishes and coelacanths - because they're far too specialized."
The find is significant for consigning an extensive discussion of coelacanth and lungfish fins to the filing cabinet of history. The main author is quoted as saying: "Our fossil shows that what we've been using to define a primitive state is actually very specialized, which means it might give a deceptive view of what evolution was like for these fins skeletons."
Previous discussion drew attention to significant similarities of coelacanth and lungfish fins, and this was considered to give confidence in the validity of the argument. The new fossil turns that upside down also. "The discovery suggests that the two living groups of close fish relatives of tetrapods (lungfish and coelacanth) are both highly specialized [. . .]. Both groups acquired many of the same specializations, but independently of one another." Convergent evolution of derived characters replaces shared primitive traits. Such is the vulnerability of the account of tetrapod evolution that many have regarded as plausible!
What can be said about the similarities with paddlefins, sturgeons and sharks? "With this fossil, we have a conservative pattern in a close relative of tetrapods that is actually conserved in other fish groups outside of this immediate group." Where this will lead is not at all clear. The words in the title of the paper, that the discovery "fills a major gap" might suggest to some that the evolutionary trajectory has been clarified. But the content of the paper says: 'back to the drawing board'!
First discovery of a primitive coelacanth fin fills a major gap in the evolution of lobed fins and limbs
Matt Friedman, Michael I. Coates, Philip Anderson
Evolution & Development, 9(4), 329-337 | doi:10.1111/j.1525-142X.2007.00169.x
SUMMARY: The fossil record provides unique clues about the primitive pattern of lobed fins, the precursors of digit-bearing limbs. Such information is vital for understanding the evolutionary transition from fish fins to tetrapod limbs, and it guides the choice of model systems for investigating the developmental changes underpinning this event. However, the evolutionary preconditions for tetrapod limbs remain unclear. This uncertainty arises from an outstanding gap in our knowledge of early lobed fins: there are no fossil data that record primitive pectoral fin conditions in coelacanths, one of the three major groups of sarcopterygian (lobe-finned) fishes. A new fossil from the Middle-Late Devonian of Wyoming preserves the first and only example of a primitive coelacanth pectoral fin endoskeleton. The strongly asymmetrical skeleton of this fin corroborates the hypothesis that this is the primitive sarcopterygian pattern, and that this pattern persisted in the closest fish-like relatives of land vertebrates. The new material reveals the specializations of paired fins in the modern coelacanth, as well as in living lungfishes. Consequently, the context in which these might be used to investigate evolutionary and developmental relationships between vertebrate fins and limbs is changed. Our data suggest that primitive actinopterygians, rather than living sarcopterygian fishes and their derived appendages, are the most informative comparators for developmental studies seeking to understand the origin of tetrapod limbs.
Coelacanth Fossil Sheds Light On Fin-to-limb Evolution, Science Daily, August 1, 2007
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