Transitions: The Evolution of Life

The above is a picture of a new fossil, called Tiktaalik roseae, that was recently discovered on Canada. The fossil is important because it fills in a gap in the transition from fish to amphibians and provides clues as to how the transition took place. The picture below is of a Coelacanth

Coelacanths are often called living fossils. They are generally considered to be somehow related to the groups that gave rise to tetrapods. In reality this is somewhat inaccurate. Coelacanths are actually highly specialized derivatives of groups common in the Paleozoic and Mesozoic eras. In particular, they are sarcopterygians. Another name for them are lobe finned fishes (as opposed to ray finned). The lobe fins seem to be an adaptation to life on or near the bottom where they can be used to push against the bottom. Because of this research has focused on their relationship to early tetrapods. Over the years a number of important fossils have been discovered that are important to the issue.

Panderichthys - the fish second from the bottom in the above picture - dates to about 385 million years ago. Prior to the discovery of Tiktaalik roseae the earliest tetrapods dated to about 376 million years ago. Tiktaalik roseae dates to somewhere around 382 million years ago. Why is this important? Ahlberg and Clack, in thier recent Nature commentary, explain it the best:

The gap was bounded at the top by primitive Devonian tetrapods such as Ichthyostega and Acanthostega from Greenland, and at the bottom by Panderichthys, a tetrapod-like predatory fish from the latest Middle Devonian of Latvia … Ichthyostega…and Acanthostega5 retain true fish tails with fin rays but are nevertheless unambiguous
tetrapods with limbs that bear digits… Panderichthys… is vaguely crocodile-shaped and, unlike the rather conventional osteolepiform fishes farther down the tree, looks like afish–tetrapod transitional form. The shape of the pectoral fin skeleton and shoulder girdle are intermediate between those of osteolepiforms and tetrapods, suggesting that Panderichthys was beginning to ‘walk’, but perhaps in shallow water rather than on land…

Into this drops Tiktaalik roseae. Like Panderichthys, Tiktaalik has pelvic fin rays, retain fin rays in paired appendages and has well developed gill arches. On the other hand, Tiktaalik is more tetrapodlike in its feeding and breathing apparatus:

These changes probably relate to breathing and feeding, which are linked in fishes because the movements used for gill ventilation can also be used to suck food into the mouth. A longer snout suggests a shift from sucking towards snapping up prey, whereas the loss of the gill cover bones (which turned the gill cover into a soft flap) probably correlates with reduced water flow through the gill chamber.

Tiktaalik also has some interesting features of its postcranial anatomy which link it to later tetrapods. Especially in its fins. The fins are adapted to flex gently upwards - as if the fin were being used to support the body. One of the interesting differences between fins and Tiktaalik limbs is that the later contain bones that comprise mobile wrist and ankles:

Another interesting feature is the central axis formed by the some of the long bones (red arrow in the picture below):

As Shubin et al point out in their article on the pectoral fin of Tiktaalik:

A fin axis that extends distal to the ulnare has been unknown in any tetrapodomorph… until the discovery of Tiktaalik. As in porolepiforms and dipnoans, the axis of Tiktaalik lies in the centre of the fin. If the five radials of Tiktaalik are homologous to digital rays, then the axis of the tetrapod limb would extend from the humerus through digit three. Unfortunately, the absence of a well-defined axis in other tetrapodomorphs leaves uncertain whether a central axis is primitive for tetrapods or if it evolved separately in Tiktaalik. Testing these competing hypotheses awaits the discovery of other tetrapodomorph fins with axes that project into the distal
fin.
The pectoral skeleton of Tiktaalik is transitional between fish
fin and tetrapod limb. Comparison of the fin with those of related
fish reveals that the manus is not a de novo novelty of tetrapods;
rather, it was assembled in fishes over evolutionary time to meet
the diverse challenges of life in the margins of Devonian aquatic
ecosystems.