Tiktaalik is a monospecific genus of extinct sarcopterygian from the late
Devonian period, about 375 Mya , having many features akin to those of
tetrapods. Tiktaalik has a possibility of being a representative of the
evolutionary transition from fish to amphibians. It is an example from
several lines of ancient sarcopterygian fish developing adaptations to the
oxygen-poor shallow-water habitats of its time, environmental conditions which
led to the evolution of tetrapods. It and similar animals may possibly be the
common ancestors of the broad swath of all terrestrial fauna: amphibians,
reptiles, birds, and mammals. The first well-preserved Tiktaalik fossils were
found in 2004 on Ellesmere Island in Nunavut, Canada.
Etymology The name Tiktaalik is an Inuktitut word
meaning "burbot", a freshwater fish related to true cod. The "fishapod"
genus received this name after a suggestion by Inuit elders of Canada's
Nunavut Territory, where the fossil was discovered. The specific name roseae
cryptically honours an anonymous donor. Taking a detailed look at the internal
head skeleton of Tiktaalik roseae, in the October 16, 2008, issue of Nature,
researchers show how Tiktaalik was gaining structures that could allow it
to support itself on solid ground and breathe air, a key intermediate step in
the transformation of the skull that accompanied the shift to life on land by
our distant ancestors. Description
Tiktaalik provides insights on the features of the extinct closest
relatives of the tetrapods. Unlike many previous, more fishlike transitional
fossils, the "fins" of Tiktaalik have basic wrist bones and simple rays
reminiscent of fingers. The homology of distal elements is uncertain; there have
been suggestions that they are homologous to digits, although this is
incompatible with the digital arch developmental model because digits are
supposed to be postaxial structures, and only three of the eight rays of
Tiktaalik are postaxial. However, the proximal series can be directly compared
to the ulnare and intermedium of tetrapods. The fin was clearly weight
bearing, being attached to a massive shoulder with expanded scapular and
coracoid elements and attached to the body armor, large muscular scars on the
ventral surface of the humerus, and highly mobile distal joints. The bones
of the forefins show large muscle facets, suggesting that the fin was both
muscular and had the ability to flex like a wrist joint. These wrist-like
features would have helped anchor the creature to the bottom in fast moving
current. Also notable are the spiracles on the
top of the head, which suggest the creature had primitive lungs as well as
gills. This attribute would have been useful in shallow water, where higher
water temperature would lower oxygen content. This development may have led
to the evolution of a more robust ribcage, a key evolutionary trait of
land-living creatures. The more robust ribcage of Tiktaalik would have helped
support the animal’s body any time it ventured outside a fully aquatic
habitat. Tiktaalik also lacked a characteristic that most fishes
have—bony plates in the gill area that restrict lateral head movement. This
makes Tiktaalik the earliest known fish to have a neck, with the pectoral girdle
separate from the skull. This would give the creature more freedom in hunting
prey either on land or in the shallows. Tiktaalik is sometimes compared to gars
of the Lepisosteidae family, with whom it shares a number of characteristics:
diamond-shaped scale patterns common to the Crossopterygii class;
teeth structured in two rows; both internal and external nostrils;
tubular and streamlined body; absence of anterior dorsal fin;
broad, dorsoventrally compressed skull; dorsally placed eyes;
paired frontal bones; marginal nares;
subterminal mouth; lung-like organ.
Paleobiology Tiktaalik generally had the
characteristics of a lobe-finned fish, but with front fins featuring arm-like
skeletal structures more akin to those of a crocodile, including a shoulder,
elbow, and wrist. The fossil discovered in 2004 did not include the rear fins
and tail. It had rows of sharp teeth of a predator fish, and its neck could move
independently of its body, which is not common in other fish. The animal had a
flat skull resembling a crocodile's; eyes on top of its head, suggesting that
it spent a lot of time looking up; a neck and ribs similar to those of
tetrapods, with the ribs being used to support its body and aid in breathing
via lungs; well developed jaws suitable for catching prey; and a small gill slit
called a spiracle that, in more derived animals, became an ear.
The fossils were found in the "Fram Formation", deposits of meandering
stream systems near the Devonian equator, suggesting a benthic animal
that lived on the bottom of shallow waters and perhaps even out of the water
for short periods, with a skeleton indicating that it could support its
body under the force of gravity whether in very shallow water or on land. At
that period, for the first time, deciduous plants were flourishing and
annually shedding leaves into the water, attracting small prey into warm
oxygen-poor shallows that were difficult for larger fish to swim in. The
discoverers said that in all likelihood, Tiktaalik flexed its proto-limbs
primarily on the floor of streams and may have pulled itself onto the shore
for brief periods. In 2014, the discovery of the animal's pelvic girdle
was announced; it was strongly built, indicating the animal could have used
them for moving in shallow water and across mudflats. Neil Shubin and Ted
Daeschler, the leaders of the team, have been searching Ellesmere Island for
fossils since 2000 We're making the hypothesis that this
animal was specialized for living in shallow stream systems, perhaps swampy
habitats, perhaps even to some of the ponds. And maybe occasionally, using its
very specialized fins, for moving up overland. And that's what is
particularly important here. The animal is developing features which will
eventually allow animals to exploit land.
Classification and evolution Tiktaalik roseae is the only species
classified under the genus. Tiktaalik lived approximately 375 million years
ago. Paleontologists suggest that it is representative of the transition between
non-tetrapod vertebrates such as Panderichthys, known from fossils 380
million years old, and early tetrapods such as Acanthostega and Ichthyostega,
known from fossils about 365 million years old. Its mixture of primitive fish
and derived tetrapod characteristics led one of its discoverers, Neil Shubin, to
characterize Tiktaalik as a "fishapod". Tiktaalik is a transitional fossil; it
is to tetrapods what Aurornis is to birds, troodonts and dromaeosaurids.
While it may be that neither is ancestor to any living animal, they serve as
evidence that intermediates between very different types of vertebrates did once
exist. The mixture of both fish and tetrapod characteristics found in
Tiktaalik include these traits: Fish
fish gills fish scales
fish fins "Fishapod"
half-fish, half-tetrapod limb bones and joints, including a functional wrist
joint and radiating, fish-like fins instead of toes
half-fish, half-tetrapod ear region Tetrapod
tetrapod rib bones tetrapod mobile neck with separate
pectoral girdle tetrapod lungs
Phylogenetic position = 2006 - 2010 =
The phylogenetic analysis by Daeschler et al. placed Tiktaalik as a sister
taxon to Elpistostege and directly below Panderichthys preceded by
Eusthenopteron. Tiktaalik was thus inserted above Acanthostega and
Ichthyostega as a transitional form and a true "missing link".
Such order of the phylogenetic tree was initially adopted by other experts, most
notably by Per Ahlberg and Jennifer Clack. However, it was questioned in a
2008 paper by Boisvert at al. who noted that Panderichthys, due to its more
derived distal portion, might be closer to tetrapods than Tiktaalik or even that
it was convergent with tetrapods. Ahlberg, co-author of the study,
considered the possibility of Tiktaalik's fin having been "an
evolutionary return to a more primitive form."
= 2010 - now = In January 2010, a group of
paleontologists published a paper accompanied by extensive supplementary
material which showed that first tetrapods appeared long before Tiktaalik
and other elpistostegids. Their conclusions were based on numerous
trackways and individual footprints discovered at the Zachełmie quarry in
the Holy Cross Mountains. A tetrapod origin of those tracks was established
based on: distinct digits and limb morphology;
trackways reflecting quadrupedal gait and diagonal walk;
no body or tail drag marks; very wide stride in relation to body
length; various size footprints with some
unusually big indicating body lengths of over 2.5 m.
Track-bearing layers were assigned to the lower-middle Eifelian based on
conodont index fossil samples and "previous biostratigraphic data obtained
from the underlying and overlying strata" with subsequent studies
confirming this dating. Both Titaalik's discoverers were
skeptical about the Zachelmie trackways. Edward Daeschler said that trace
evidence was not enough for him to modify the theory of tetrapod evolution,
while Neil Shubin argued that Tiktaalik could have produced very similar
footprints. However, Ahlberg insisted that those tracks could not have
possibly been formed either by natural processes or by transitional species
such as Tiktaalik or Panderichthys. Instead, the authors of the publication
suggested ichthyostegalians as trackmakers, based on available pes
morphology of those animals. However, a 2012 study indicated that Zachelmie
trackmakers were even more advanced than Ichthyostega in terms of quadrupedalism.
Grzegorz Niedźwiedzki's reconstruction of one of the trackmakers was identical
to that of Tulerpeton. Prof. Narkiewicz, co-author of the
article on the Zachelmie trackways, claimed that the Polish "discovery has
disproved the theory that elpistostegids were the ancestors of tetrapods", a
notion partially shared by Philippe Janvier. There have been a number of new
hypotheses suggested as to a possible origin and phylogenetic position of the
elpistostegids: their phylogenetic position remains
unchanged and the footprints found in the Holy Cross Mountains are attributed
to tetrapods but as a result there are at least six long ghost lineages
separating Zachelmie trackmakers from various elpistostegalian and
ichthyostegalian species; they were "late-surviving relics rather
than direct transitional forms"; they were "an evolutionary dead-end";
they were a result of convergent or parallel evolution so that apomorphies
and striking anatomical similarities found in both digited tetrapods and
elpistostegalians evolved at least twice.
It should be noted that convergency is considered responsible for uniquely
tetrapod features found also in other non-elpistostegalian fish from the
period like Sauripterus or Tarrasius. Estimates published after the discovery
of Zachelmie tracks suggested that digited tetrapods may have appeared as
early as 427.4 Myr ago and questioned attempts to read absolute timing of
evolutionary events in early tetrapod evolution from stratigraphy.
Until more data becomes available, the phylogenetic position of Tiktaalik and
other elpistostegids remains uncertain. Discovery
In 2004, three fossilized Tiktaalik skeletons were discovered in rock formed
from late Devonian river sediments on Ellesmere Island, Nunavut, in northern
Canada. Estimated ages reported at 375 Ma, 379 Ma, and 383 Ma. At the time of
the species' existence, Ellesmere Island was part of the continent Laurentia,
which was centered on the equator and had a warm climate. When discovered, one
of the skulls was found sticking out of a cliff. Upon further inspection, the
fossil was found to be in excellent condition for a 375-million-year-old
specimen. The discovery, made by Edward B.
Daeschler of the Academy of Natural Sciences, Neil H. Shubin from the
University of Chicago, and Harvard University Professor Farish A. Jenkins,
Jr, was published in the April 6, 2006, issue of Nature and quickly recognized
as a transitional form. Jennifer A. Clack, a Cambridge University expert on
tetrapod evolution, said of Tiktaalik, "It's one of those things you can point
to and say, 'I told you this would exist,' and there it is."
After five years of digging on Ellesmere Island, in the far north of Nunavut,
they hit pay dirt: a collection of several fish so beautifully preserved
that their skeletons were still intact. As Shubin's team studied the species
they saw to their excitement that it was exactly the missing intermediate they
were looking for. 'We found something that really split the difference right
down the middle,' says Daeschler. See also
Walking fish Crocodilefish
Amphibious fish Spotted handfish
Other lobe-finned fish found in fossils from the Devonian period:
Coelacanth Eusthenopteron
Gogonasus Ichthyostega
Panderichthys References
External links University of Chicago website dedicated
to the discovery Interview with Neil Shubin on The
Inoculated Mind, February 12, 2008. Interview with Neil Shubin on Tech
Nation where he discusses the discovery of the Tiktaalik, February 14, 2008.
Lecture by Neil Shubin about the discovery of Tiktaalik on YouTube
Fishapod stars in music video, YouTube. Accessed on December 27, 2008.
Finding Tiktaalik: Interview with Neil Shubin, Royal Institution video,
February 2013