Tiktaalik: Fossil Fish Shows Complexity of Transition to Land

The New York Times’s science pages discuss a research study of Tiktaalik. The study in question has been published in the Journal Nature (1)

It was Neil Shubin’s team that found the Tiktaalik, as they had predicted.

Dr. Shubin said Tiktaalik was “still on the fish end of things, but it neatly fills a morphological gap and helps to resolve the relative timing of this complex transition.”

For example, fish have no neck but “we see a mobile neck developing for the first time in Tiktaalik,” Dr. Shubin said.

Carl Zimmer has an interesting article on The Loom The Shoulder Bone’s Connected to the Ear Bone…

Carl Zimmer wrote:

In some respects, Tiktaalik’s skull was still fish-like. It had a hinge down the middle that allowed the front and back ends to bend. Later in the evolution of tetrapods, that hinge hardened, making the skull stiffer. But other parts of the skull had already evolved to be more like our own. For example, in our close aquatic relatives, there’s no neck to separate the head and shoulders. A boomerang-shaped bone called the hyomandibula was located there instead. Some muscles attaching to it controlled the opening and closing of the gill flaps, while it also buttressed the bones of the skull as they opened and closed when the fish fed. In Tiktaalik, the hyomandibula has become shorter and no longer makes contact with the gill flaps.

This change suggests that by the time Tiktaalik evolved, the ancestors of tetrapods had already begun to rely less on gills for breathing. Living in shallow coastal waters, Tiktaalik may have used its powerful front limbs to do push-ups in order to breathe air instead. And once the hyomandibula was liberated from the shoulders, it could evolve to specialize for new roles. As other fossils reveal, the hyomandibula would evolve in a small bone that came into contact with the skull, able to transmit vibrations from the air. In other words, it became the stapes. A diagram below shows some stages in this transition.

What’s particularly cool about the evolution of our ear is that it was assembled over hundreds of millions of years from other parts. Tiktaalik helps reveal how the stapes evolved some 370 million years ago, but there are other bones in the ear-the incus and malleus-that transmit vibrations in our heads. They only evolved 150 million years later from some bones in the back of the jaw. They too were liberated from old jobs, and free to take on new ones. The ear was not an overnight invention, but the product of an inconceivably long evolutionary tinkering.

Once again, science is slowly unraveling the transitions from fish to tetrapod in exquisite detail. Don’t you wish ID had something similar to contribute to our scientific understanding? I agree, that may be wishful thinking. All Intelligent Design proponents seem to be able to contribute is quote mining research and claim that the Tiktaalik was a poor fossil, when in fact, the fossil shows extensive and important details of the transition.

(1) Jason P. Downs, Edward B. Daeschler, Farish A. Jenkins & Neil H. Shubin, The cranial endoskeleton of Tiktaalik roseae Nature 455, 925-929 (16 October 2008)

Abstract Among the morphological changes that occurred during the ‘fish-to-tetrapod’ transition was a marked reorganization of the cranial endoskeleton. Details of this transition, including the sequence of character acquisition, have not been evident from the fossil record. Here we describe the braincase, palatoquadrate and branchial skeleton of Tiktaalik roseae, the Late Devonian sarcopterygian fish most closely related to tetrapods. Although retaining a primitive configuration in many respects, the cranial endoskeleton of T. roseae shares derived features with tetrapods such as a large basal articulation and a flat, horizontally oriented entopterygoid. Other features in T. roseae, like the short, straight hyomandibula, show morphology intermediate between the condition observed in more primitive fish and that observed in tetrapods. The combination of characters in T. roseae helps to resolve the relative timing of modifications in the cranial endoskeleton. The sequence of modifications suggests changes in head mobility and intracranial kinesis that have ramifications for the origin of vertebrate terrestriality.

(2) Susan Milius, How Tiktaalik got its neck , ScienceNews, Oct 15, 2008.

The new view shows gills in transition too. They’ve lost the fishlike bone flap, the operculum, which enhances the flow of water over gill tissues. “Tiktaalik was already specializing in breathing air,” Shubin says. And that shift to air was playing an important role in gradually changing the old fish head.