238 Evolution? The Fossils Say YES!
which are more like those of Eusthenopteron than of tetrapods, indicating that its limbs were
not very good for land locomotion. Acanthostega also had the robust vertebrae in the spine
that could support its weight out of water. It’s a perfect fishibian: gills, fins, ears like a fish,
but spine and limbs like a tetrapod. Clack and Coates have convincingly shown that it prob-
ably spent most of its time in the water, even though it has the limbs capable of some land
locomotion. Like the teleost fish mentioned earlier, it only needed to crawl up on land once
in a while, but most of its life was probably aquatic. For that matter, most living salamanders
and newts are almost entirely aquatic and use their limbs primarily to swim and push along
through the vegetation underwater.
The clinching piece of evidence was announced just over a decade ago (Daeschler et al.
2006; Shubin et al. 2006). Nicknamed the “Fishapod” but formally named Tiktaalik, this Late
Devonian fossil from Ellesmere Island in the Canadian Arctic (fig. 10.9) was more fishlike
than Ichthyostega or Acanthostega, yet its limbs show the perfect transition between fins and
feet (fig. 10.7). It had fishlike scales, lower jaw, fin rays, and palate, but unlike any fish, it had
a shortened skull roof and mobile neck (for whipping the head sideways to catch prey), an
ear region capable of hearing in both land and water, and a wrist joint that anticipates the
condition seen in terrestrial tetrapods. Thanks to this discovery, we now have a beautiful
transitional sequence from fully aquatic lobe-finned fish like Eusthenopteron to more amphib-
ian-like forms such as Panderichthys and Tiktaalik to fully four-legged forms like Acanthostega
and Ichthyostega (which still retain fishlike gills, tail fins, and lateral line systems on the face).
This sequence is now so smoothly gradational that it’s hard to tell where the fishes end and
the amphibians begin—yet is it clear even to a creationist that Eusthenopteron is a fish and
Ichthyostega is an amphibian.
The discovery that early tetrapods had seven or eight fingers came as shock at first.
We are so accustomed to the fact that all living tetrapods have only five fingers and toes
(or fewer) that we assumed that they always had that number. Early reconstructions of the
poorly known hand of Ichthyostega often drew in a five-fingered hand, even though there
was no evidence one way or another. But in other ways, this large number of toes is not so
mysterious. My fellow Columbia student, good friend, and coauthor Neil Shubin at the Uni-
versity of Chicago worked on this problem for his dissertation while he was a graduate stu-
dent at Harvard. People had always assumed that fingers formed from the branching of the
fin rays in lobe fins, symmetrically down the central axis. But Neil studied the embryological
development of salamander limbs (Shubin and Alberch 1986) and found that fingers bud off
in an arch, from one side of the hand to the other (not symmetrically down the middle axis).
This beautifully matches the hand in Acanthostega because it apparently had extra fingers
(just like extra fin rays) that budded off from this embryonic development pattern. To get the
modern pattern of five fingers, the development has to shut down just a bit earlier. Since this
discovery, the Hox genes that control this developmental sequence have been identified, so
it is clear that changing a limb from seven or eight fingers to just five is not a big problem.
It just requires a small adjustment in the developmental timing of limb budding.
Finally, we come to Ichthyostega (figs. 10.6, 10.7, and 10.8), the classic transitional form,
found in Upper Devonian rocks of Greenland in the 1930s by Danish and Swedish expedi-
tions. It was described briefly by Säve-Söderbergh in 1932, but not fully monographed in
detail until 1996 by Erik Jarvik. This creature is very similar to Acanthostega, only not as
completely preserved. It is also slightly more advanced than Acanthostega in the direction
of tetrapods, with a smaller tail fin and slightly longer more tetrapod-like limbs. However,