Re: Fossil Fish With "Limbs" Is Missing Link, Study Says

Aidan says : I had to pull the network connection and head off to
work before this got posted. Well, the abstracts may still be of interest
when I get back.


The news item from Nature (where the papers were published) says :
Published online: 5 April 2006; | doi:10.1038/news060403-7
The fish that crawled out of the water
A newly found fossil links fish to land-lubbers.

Rex Dalton


A crucial fossil that shows how animals crawled out from the water, evolving
from fish into land-loving animals, has been found in Canada.

The creature, described today in Nature1,2, lived some 375 million years ago.
Palaeontologists are calling the specimen from the Devonian a true 'missing
link', as it helps to fill in a gap in our understanding of how fish
developed legs for land mobility, before eventually evolving into modern
animals including mankind.

Several samples of the fish-like tetrapod, named Tiktaalik roseae, were
discovered by Edward Daeschler of the Academy of Natural Sciences in
Philadelphia, Pennsylvania, Neil Shubin of the University of Chicago in
Illinois, Farish Jenkins of Harvard University in Cambridge, Massachusetts
and colleagues.

The crew found the samples in a river delta on Ellesmere Island in Arctic
Canada; these included a near-complete front half of a fossilized skeleton
of a crocodile-like creature, whose skull is some 20 centimetres long.

The beast has bony scales and fins, but the front fins are on their way to
becoming limbs; they have the internal skeletal structure of an arm,
including elbows and wrists, but with fins instead of clear fingers. The
team is still looking for more complete specimens to get a better picture
of hind part of the animal.

Plugging the gap
Creatures with features of both fish and land-living animals have been
found before. Fish that may have been beginning to 'walk' in shallow
water have been found from about 385 million years ago, and fish with
limbs that bear digits have been seen from more than 365 million years
ago.

Specimens that fall into the gap, such as Tiktaalik, help researchers
to work out the details of this transition. The newly found animal has
a structure on its head that looks like a small gill slit that is on
its way to becoming an ear, for example, and a long snout that would
have been suited to catching prey on land.

"Tiktaalik substantially narrows the gap in the fossil record of the
fish-tetrapod transition," says Per Ahlberg of Uppsala University in
Sweden.

"Tiktaalik was probably an unwieldy swimmer," says John Maisey, a
palaeontologist at the American Museum of Natural History in New York.
It probably lived in shallow waters, says Maisey, only hauling itself
on to land temporarily to escape predators. "Tetrapods did not so much
conquer the land, as escape from the water," he says.

Treasured find
Daeschler and Shubin set off to find this missing link in the
evolutionary chain back in 1999. The pair targeted Ellesmere Island
after noticing that it was listed in an undergraduate textbook as
exposed Devonian rock that had not previously been explored for
vertebrate fossils.

The desolate area was reachable only by plane, and the weather was
so bad that field work could only be done for about two months each
summer. The team first walked around the rocky outcrops looking for
fossils of plant life that indicated stream or delta sediments, in
order to target areas that had once hosted shallow waters. "That is
where the action is on the fish-to-tetrapod transition," says Daeschler.

By 2000 they had found fossils with intriguing fins in the eroding
rocks. "In 2004, we really scored, finding three partial skulls and
numerous jaws," recalls Daeschler.

Shubin remembers finding one simply by wandering off to sit on a
rock for his lunch break. "I looked over at a wall; there was a
Tiktaalik snout looking out of the cliff at me. I couldn't
believe my eyes. I knew the rest of the skeleton was behind it.
We were high fiving right and left."

References

1. Daeschler E. B., Shubin N. H., Jenkins F. A. Jr, Nature, 440.
757 - 763 (2006).
A Devonian tetrapod-like fish and the evolution of the tetrapod body plan
Edward B. Daeschler1, Neil H. Shubin2 and Farish A. Jenkins, Jr3
Abstract -
The relationship of limbed vertebrates (tetrapods) to
lobe-finned fish (sarcopterygians) is well established, but the
origin of major tetrapod features has remained obscure for lack
of fossils that document the sequence of evolutionary changes.
Here we report the discovery of a well-preserved species of
fossil sarcopterygian fish from the Late Devonian of Arctic
Canada that represents an intermediate between fish with fins
and tetrapods with limbs, and provides unique insights into how
and in what order important tetrapod characters arose. Although
the body scales, fin rays, lower jaw and palate are comparable
to those in more primitive sarcopterygians, the new species
also has a shortened skull roof, a modified ear region, a
mobile neck, a functional wrist joint, and other features that
presage tetrapod conditions. The morphological features and
geological setting of this new animal are suggestive of life
in shallow-water, marginal and subaerial habitats.

2. Shubin N. H. Daeschler E. B., , Jenkins F. A. Jr, Nature, 440.
764 - 771 (2006).

The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb
Neil H. Shubin1, Edward B. Daeschler2 and Farish A. Jenkins, Jr3
Abstract -
Wrists, ankles and digits distinguish tetrapod limbs from
fins, but direct evidence on the origin of these features has been
unavailable. Here we describe the pectoral appendage of a member
of the sister group of tetrapods, Tiktaalik roseae, which is
morphologically and functionally transitional between a fin and
a limb. The expanded array of distal endochondral bones and
synovial joints in the fin of Tiktaalik is similar to the distal
limb pattern of basal tetrapods. The fin of Tiktaalik was capable
of a range of postures, including a limb-like substrate-supported
stance in which the shoulder and elbow were flexed and the distal
skeleton extended. The origin of limbs probably involved the
elaboration and proliferation of features already present in the
fins of fish such as Tiktaalik.

Aidan says : Hmmm, no mention of the finger-count. Anyone with
a copy of the paper willing to tell us if it's pentadactyl, hexadactyl,
heptadactyl, or not-preserved-dactyl?
--
Aidan Karley, FGS
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233

.

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