Newly found species fills evolutionary gap between fish and land animals

Paleontologists have discovered fossils of a species that provides the missing evolutionary link between fish and the first animals that walked out of water onto land about 375 million years ago. The newly found species, Tiktaalik roseae, has a skull, a neck, ribs and parts of the limbs that are similar to four-legged animals known as tetrapods, as well as fish-like features such as a primitive jaw, fins and scales.

These fossils, found on Ellesmere Island in Arctic Canada, are the most compelling examples yet of an animal that was at the cusp of the fish-tetrapod transition. The new find is described in two related research articles highlighted on the cover of the April 6, 2006, issue of Nature.

"Tiktaalik blurs the boundary between fish and land-living animal both in terms of its anatomy and its way of life," said Neil Shubin, professor and chairman of organismal biology at the University of Chicago and co-leader of the project.

Tiktaalik was a predator with sharp teeth, a crocodile-like head and a flattened body. The well-preserved skeletal material from several specimens, ranging from 4 to 9 feet long, enabled the researchers to study the mosaic pattern of evolutionary change in different parts of the skeleton as fish evolved into land animals.

The high quality of the fossils also allowed the team to examine the joint surfaces on many of the fin bones, concluding that the shoulder, elbow and wrist joints were capable of supporting the body-like limbed animals.

"Human comprehension of the history of life on Earth is taking a major leap forward," said H. Richard Lane, director of sedimentary geology and paleobiology at the National Science Foundation. "These exciting discoveries are providing fossil 'Rosetta Stones' for a deeper understanding of this evolutionary milestone--fish to land-roaming tetrapods."

One of the most important aspects of this discovery is the illumination of the fin-to-limb transition. In a second paper in the journal, the scientists describe in depth how the pectoral fin of the fish serves as the origin of the tetrapod limb.

Embedded in the fin of Tiktaalik are bones that compare to the upper arm, forearm and primitive parts of the hand of land-living animals.

"Most of the major joints of the fin are functional in this fish," Shubin said. "The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals."

At the time that Tiktaalik lived, what is now the Canadian Arctic region was part of a landmass that straddled the equator. It had a subtropical climate, much like the Amazon basin today. The species lived in the small streams of this delta system. According to Shubin, the ecological setting in which these animals evolved provided an environment conducive to the transition to life on land.

"We knew that the rocks on Ellesmere Island offered a glimpse into the right time period and the right ancient environments to provide the potential for finding fossils documenting this important evolutionary transition," said Ted Daeschler of the Academy of Natural Sciences in Philadelphia, a co-leader of the project. "Finding the fossils within this remote, rugged terrain, however, required a lot of time and effort."

The nature of the deposits where the fossils were found and the skeletal structure of Tiktaalik suggests the animal lived in shallow water and perhaps even out of the water for short periods.

"The skeleton of Tiktaalik indicates that it could support its body under the force of gravity whether in very shallow water or on land," said Farish Jenkins, professor of organismic and evolutionary biology at Harvard University and co-author of the papers. "This represents a critical early phase in the evolution of all limbed animals, including humans--albeit a very ancient step."

The new fossils were collected during four summers of exploration in Canada's Nunavut Territory, 600 miles from the North Pole, by paleontologists from the Academy of Natural Sciences in Philadelphia, the University of Chicago and Harvard University. Although the team has amassed a diverse assemblage of fossil fish, Shubin said, the discovery of these transitional fossils in 2004 was a vindication of their persistence.

The scientists asked the Nunavut people to propose a formal scientific name for the new species. The Elders Council of Nunavut, the Inuit Qaujimajatuqangit, suggested "Tiktaalik" (tic-TAH-lick)--the word in the Inuktikuk language for "a large, shallow water fish."

The scientists worked through the Department of Culture, Language, Elders and Youth in Nunavut to collaborate with the local Inuit communities. All fossils are the property of the people of Nunavut and will be returned to Canada after they are studied.

The team depended on the maps of the Geological Survey of Canada. The researchers received permits from the Department of Culture, Language, Elders and Youth of the Government of Nunavut, and logistical support in the form of helicopters and bush planes from Polar Continental Shelf Project of Natural Resources Canada. The National Science Foundation and the National Geographic Society, along with an anonymous donor, also helped fund the project.

Here's Creation-Evolution Headlines' commentary on this 'discovery':

Fish-o-pod ‘Missing Link’ Discovered: Media [and FRevolutionists] Goes Nuts

Fish-o-pod ‘Missing Link’ Discovered: Media Goes Nuts 04/06/2006
Evolutionists could hardly feel more relieved. Just when anti-evolutionary sentiment is on the rise, a new fossil has been announced that gives pro-evolutionists a missing link to run up the fishpole. Neil Shubin (U of Chicago) and two partners found a “tetrapod-like fish” fossil on a Canadian island. It helps fill one of the most puzzling transitions in the fossil record: the evolution from a fish to a land animal.
To hear the media frenzy resulting from this find, which some supporters are ranking with Archaeopteryx in importance, the war is over and evolution wins. Creationists have been complaining about gaps in the fossil record, and here is a perfect case of a transitional form. Here are just a few of the claims being made about Tiktaalik roseae, a new icon of evolution (emphasis added in all quotes):

EurekAlert: a “key marker in the evolutionary transition of fish to limbed animals.”
News@Nature: this is “the fish that crawled out of water” – a true ‘missing link’ that “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.”
BBC News: “Fossil animals found in Arctic Canada provide a snapshot of fish evolving into land animals... giving researchers a fascinating insight into this key stage in the evolution of life on Earth.”.... Could “could prove to be as much of an ‘evolutionary icon’ as Archaeopteryx – an animal believed to mark the transition from reptiles to birds.”
Scientific American: “Newfound Fossil Is Transitional between Fish and Landlubbers.”
New Scientist: “IT WAS one of the most important events of the last 400 million years: the moment our fishy ancestors began hauling themselves onto dry land. Now a fossil from the very beginning of that crucial transition has been found in the remote Arctic.” Neil Shubin calls it a “a ‘fishopod’: part fish, part tetrapod.”
AP, via MSNBC: “Scientists have caught a fossil fish in the act of adapting toward a life on land, a discovery that sheds new light on one of the greatest transformations in the history of animals. Researchers have long known that fish evolved into the first creatures on land with four legs and backbones more than 365 million years ago, but they’ve had precious little fossil evidence to document how it happened.... ‘It sort of blurs the distinction between fish and land-living animals,’ said one of its discoverers, paleontologist Neil Shubin of the University of Chicago.”
National Geographic: “It’s our closest fish cousin, scientists say. Millions of years ago it apparently hoisted its croc-like head out of the water—and the rest is history.”
LiveScience: “Fishy Land Beast Bridges Evolutionary Gap.”

One gets the distinct impression they think this is an important fossil. Now that the parade has passed by, perhaps it would be a good time to delve into the original scientific papers and see what exactly was said. It was the cover story of Nature, and there were two papers inside by Shubin’s team, and a review article by Jennifer Clack, a leading researcher on tetrapod origins. In journal articles, where scientists talk to themselves, they are expected to be more formal, reserved and cautious about interpretations. Let’s see.
The research was first submitted to Nature in October, but released today. The fact that the mainstream media were all prepared with instant artwork, interviews and sound bites makes it likely they were clued in with plenty of time to make a splash. Though it is clear the authors all believe this is an evolutionary transitional form, the most interesting statements from scientific papers are usually the caveats and disclaimers. Most of all, the observational data must always take precedence over interpretations.
In the first paper by Daeschler, Shubin and Jenkins,¹ they begin, “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.” That is a strange statement. It sounds something like, We know it’s true; we just lack evidence.

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.

Sounds like the popular press so far; now, into the details. They admit that “The evolution of tetrapods from sarcopterygian fish is one of the major transformations in the history of life and involved numerous structural and functional innovations, including new modes of locomotion, respiration and hearing.” In other words, many substantial changes had to come together in one animal to go from breathing through gills to breathing with lungs, developing feet that could support the weight, developing digits and ankles and toes and learning how to use them, and much more:

During the origin of tetrapods in the Late Devonian (385–359 million years ago), the proportions of the skull were remodelled [sic; implies intelligent design], the series of bones connecting the head and shoulder was lost, and the region that was to become the middle ear [sic; implies progress] was modified. At the same time, robust limbs with digits evolved, the shoulder girdle and pelvis were altered, the ribs expanded, and bony connections between vertebrae developed.

Few of these innovations are seen in the closest relatives of tetrapods, they say. They talk about Panderichthys, Acanthostega and Ichthyostega, which have been discussed earlier in these pages (see 04/05/2004 and 08/09/2003, “Evolution of the Darwin Fish.”). Surprisingly, however, they dismiss them as fragmentary and of doubtful utility. This includes the earlier leading candidate:

Panderichthys possesses relatively few tetrapod synapomorphies [convergent features], and provides only partial insight into the origin of major features of the skull, limbs and axial skeleton of early tetrapods[. In view of the morphological gap between elpistostegalian fish and tetrapods, the phylogenetic framework for the immediate sister group of tetrapods has been incomplete and our understanding of major anatomical transformations at the fish–tetrapod transition has remained limited.

The disparagement of previous candidate missing links was the build-up to the new fossil, which “significantly enhances our knowledge of the fish–tetrapod transition.” (This should be taken with a grain of salt, considering that similar claims were made about Panderichthys earlier.) Proceeding on, they place Tiktaalik somewhere between Panderichthys and tetrapods. The paper provides the obligatory data for a new species: location found, taxonomy, nomenclature, description of the fossil, photos, drawings, etc. The head was remarkably well preserved, and three specimens were found. Naming and classifying an extinct species, however, provides the discoverers some leeway in placing it into the presumed evolutionary framework.
A technical description of parts ensues. Compared to the earlier known fossils, Tiktaalik has a larger this and a smaller that, etc. Side-by-side skull comparisons do not look that informative, especially when there are no soft parts and no videos of how the creature actually lived. It must be remembered, for instance, that Coelacanth was long considered a transitional form because of its bony fins, but when discovered alive, did not use them for walking or raising itself up in any way.
Without soft parts such as gills and organs, and without living examples, interpretation of bony parts is at best a subjective exercise in educated guesswork. They turn finally to the phylogenetic position of Tiktaalik. What features make them decide these specimens are transitional?

A phylogenetic analysis of sarcopterygian fishes and early tetrapods (Fig. 7) supports the hypothesis that Tiktaalik is the sister group of tetrapods or shares this position with Elpistostege. Tiktaalik retains primitive tetrapodomorph features such as dorsal scale cover, paired fins with lepidotrichia, a generalized [sic] lower jaw, and separated entopterygoids in the palate, but also possesses a number of derived [sic] features of the skull, pectoral girdle and fin, and ribs that are shared with stem tetrapods such as Acanthostega and Ichthyostega. Tiktaalik is similar to these forms in the possession of a wide spiracular tract and the loss of the opercular, subopercular and extrascapulars. The pectoral girdle is derived [sic] in the degree to which the scapulocoracoid is expanded dorsally and ventrally, and the extent to which the glenoid fossa is oriented laterally. The pectoral fin is apomorphic [i.e., derived, more developed] in the elaboration of the distal endoskeleton, the mobility of segmented regions of the fin, and the reduction of lepidotrichia distally.

In summary, they think that Panderichthys, Elpistostege and Tiktaalik represent a “paraphyletic [partially evolved] assemblage of elpistostegalian fish along the tetrapod stem that lack the anterior dorsal fins and possess broad, dorsoventrally compressed skulls with dorsally placed eyes, paired frontal bones, marginal nares, and a subterminal mouth.” However, “Some tetrapod-like features evolved independently in other sarcopterygian groups,” while other two other fossils seem to have features shared with basal tetrapods by convergent evolution (homoplasy).
That’s basically all that was claimed in the primary announcement. Their second paper² discussed the pectoral fin of Tiktaalik, which they claim is “morphologically and functionally transitional between a fin and a limb.” They think the front fins allowed the creature to hoist itself up and drag its tail behind. The “wrist,” however, lacked five digits (fingers), and represents a “mosaic” of features found in more “basal” taxa. Though additional “wrist” bones extended distally are new features of this fossil, the presence of five digits is inferred on their diagram by dotted lines. Lacking living representatives, they also are unable to tell for certain what the fin bones actually were used for.
While acknowledging that the transition from water to land would require “major shifts in developmental genetics, skeletal structure, and biomechanics,” the most telling aspect of the fin is the angle of the putative “wrist” homolog, though there is no evidence any true digits for locomotion later evolved from the fin bones of this animal. Since they might have, though, reporters were probably more tuned to the confident conclusion:

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 [hand] 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.

OK, now what do other experts think? In the same issue,³ Erik Ahlberg and Jennifer Clack gave their analysis. It is unknown whether Clack, who has been in the forefront of research into tetrapod evolution, was scooped by this discovery, or whether any personal feelings or rivalries were involved. She did, however, with Ahlberg, put a few brakes on the interpretations, though acknowledging the significance of the find. First, a little sermonette on missing links:

The concept of “missing links” has a powerful grasp on the imagination: the rare transitional fossils that apparently capture the origins of major groups of organisms are uniquely evocative. But the concept has become freighted with unfounded notions of evolutionary ‘progress’ and with a mistaken emphasis on the single intermediate fossil as the key to understanding evolutionary transitions. Much of the importance of transitional fossils actually lies in how they resemble and differ from their nearest neighbours in the phylogenetic tree, and in the picture of change that emerges from this pattern.
We raise these points because on pages 757 and 764 of this issue are reports of just such an intermediate: Tiktaalik roseae, a link between fishes and land vertebrates that might in time become as much of an evolutionary icon as the proto-bird [sic[ Archaeopteryx.

Though this fossil goes “a long way” to filling in the gap, it does not go quite all the way, they say. Its closest match is Elpistostege, a fragmentary fossil thought to be closer to tetrapods than Panderichthys. They admit, “the authors demonstrate convincingly that Elpistostege and Tiktaalik fall between Panderichthys and the earliest tetrapods on the phylogenetic tree. End of story?
Though impressed, they raise some issues. Of the fin bones, they say:

Although these small distal bones bear some resemblance to tetrapod digits in terms of their function and range of movement, they are still very much components of a fin. There remains a large morphological gap between them and digits as seen in, for example, Acanthostega: if the digits evolved from these distal bones, the process must have involved considerable developmental repatterning. The implication is that function changed in advance of morphology.

Though each fossil seems to represent a mosaic of characteristics rather than a straight line of evolution, the two are ready to agree that the creature was “evidently an actual step on the way from water to land,” and that “it seems, our remote ancestors [sic] were large, flattish, predatory fishes, with crocodile-like heads and strong limb-like pectoral fins that enabled them to haul themselves out of the water.” Nevertheless, this is just one specimen, and many more are needed. This one creature must be seen in context. Perhaps the most important transitional forms are found in the future:

Of course, there are still major gaps in the fossil record. In particular we have almost no information about the step between Tiktaalik and the earliest tetrapods, when the anatomy underwent the most drastic changes, or about what happened in the following Early Carboniferous period, after the end of the Devonian, when tetrapods became fully terrestrial. But there are still large areas of unexplored Late Devonian and Early Carboniferous deposits in the world – the discovery of Tiktaalik gives hope of equally ground-breaking finds to come.

¹Daeschler et al., “A Devonian tetrapod-like fish and the evolution of the tetrapod body plan,” Nature 440, 757-763 (6 April 2006) | doi:10.1038/nature04639; Received 11 October 2005; ; Accepted 8 February 2006.
²Shubin et al., “The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb,” Nature 440, 764-771 (6 April 2006) | doi:10.1038/nature04637; Received 11 October 2005; ; Accepted 8 February 2006.
³Per Erik Ahlberg and Jennifer A. Clack, “Palaeontology: A firm step from water to land,” Nature 440, 747-749 (6 April 2006) | doi:10.1038/440747a.

You didn’t get this much detail from the major news media. You didn’t hear the discoverers hedge their bets and admit that this fossil is just a tiny piece of a huge puzzle that is mostly not understood. You didn’t hear the AP (Associated Preach) tell the truth that the fossil record is characterized by large and systematic gaps between groups, not isolated and questionable transitional forms. No, you got hype and bluster and far-fetched exaggeration, where the actual bones were incidental to the true goal of making Charlie not look as dead as he is. Meanwhile, an explanation of the origin of all the genetic information required for such a transition was completely glossed over; and, of course, not a single credible non-Darwinian paleontologist got a word in edgewise in the din of the mainstream media’s Charlie pride parade. If you got mad last time (04/05/2004) it’s time to get mad again – for the same reasons.
A reader writes: “Dear Staff... The April 6, 2006 article on the ‘Fish-O-Pod’ found in Canada is great news... Now we know where all the Walking Catfish in the lakes in Orlando, Florida came from... They actually walk up on the interstate and get eliminated by cars! FISH-O-POD is nothing new, we have been squashing them for years!”

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