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A Real-Life “Darwin Fish”April 11th, 2006 by Mike Keesey :: see related comic |
The “Darwin Fish” is real! Well, we already had animals that were essentially fish with legs, like Acanthostega and Ichthyostega, but Tiktaalik roseae Daeschler et al. 2006, depicted here, sheds light on an earlier stage of the tetrapodomorph transition from water to land, when fish had jointed limbs, but no digits. Like Acanthostega and Icthyostega, Tiktaalik was still aquatic, using its limbs to push itself along or raise its head up when in shallow water. (Yes, our ancestors were bottom feeders—Tiktaalik itself is named after a modern shallow-water fish.)
This transitional form was accurately predicted by evolutionary theory. Flesh-finned, tetrapod-like fish, like Panderichthys, were known from 385 million years ago and the aforementioned fish-like tetrapods from 365 million years ago. Paleontologists started looking in deposits laid down by freshwater 375 million years ago and, lo and behold: a form intermediate between the two. Is it a fish-like tetrapod or a tetrapod-like fish? The authors jokingly call it a “fishapod”.
Apparently it is not that “difficult” for a fish to evolve terrestriality and/or jointed limbs—look at modern-day mudskippers and batfish, which are unrelated to tetrapod-like fish, and yet have convergently acquired semi-limbs. Perhaps these fish could, over time, evolve into diverse terrestrial forms—except that the first fish to evolve limbs already beat them to it. It’s difficult to establish a foothold (pun intended) in terrestrial niches when they are already occupied by reptiles, birds, mammals, and amphibians.
References:
- Daeschler, E. B., N. H. Shubin, and F. A. Jenkins, Jr. 2006 April 6. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440: 757—763.
- Shubin, N. H., E. B. Daeschler, and F. A. Jenkins, Jr. 2006 April 6. The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440: 764–771.
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