Fossil finder
Corwin Sullivan, a paleontology graduate student at Harvard University
was one of the few Canadians on the field crew that discovered the
Tiktaalik fossil on Ellesmere Island in Nunavut. The Kitchener-born
Sullivan gives Canadian Geographic a first-person glimpse into one
of Canada's greatest fossil finds.
CG: This is a significant fossil find. What's
it like to be part of the team that made this discovery?
CS: In a word, exciting. Field palaeontology
is scientific treasure hunting — there's never any guarantee
of finding anything important, or indeed of finding much of anything
at all. Mounting an expedition, especially to a place like Nunavut,
requires a significant investment of money and time, and it may
all end up being wasted. So when things go well, and you discover
a specimen that's informative and interesting, the scientific satisfaction
is flavoured with a more visceral sense of having beaten the odds.
I seem to recall that we went through a lot of whisky towards the
end of the 2004 field season, when we knew that we had something
good in our plaster jackets.
CG: What exactly did the team set out to find?
CS: By 2002, we all saw finding an early tetrapod
- a full land vertebrate, with limbs rather than fins - as the holy
grail. What we eventually found, of course, was a creature that's
a very close cousin to the tetrapods, but not quite a member of
the club. In many ways, that's even more exciting.
CG: Was there ever any doubt that the team would
be able to find what it was looking for?
CS: At the end of the 2002 field season, we weren't
entirely sure whether we'd be going back to Nunavut. We'd found
a number of fish specimens, but nothing that we considered exceptional.
However, one of the things that makes field work interesting is
that you never know exactly what you've found until you've had a
chance to get the specimens back to the lab and clean them up. It
turned out that among the material we'd collected in 2002 were a
few bits and pieces that were reminiscent of "elpistostegalian" fish
- the fish that were, prior to the discovery of Tiktaalik,
considered to be most closely related to tetrapods. Those were the
specimens that got our hopes up, and made the idea of going back
in 2004 seem definitely worthwhile. But we still couldn't be sure
what we would find!
CG: Your team began its search several years
ago. Was there a eureka moment when you knew you had found what
you were looking for?
CS: It was more of a succession of small eureka
moments. The first one, which I wasn't present for, was the discovery
of the quarry site back in 1999. A large number of fossil fragments
had eroded out of the rock and were sitting right on the surface,
so it was clearly a productive site. The second eureka was probably
the realization that there were some elpistostegalian-like bones
in the samples we'd collected in 2002. Then in 2004, finally with
a clear and very specific idea of what we were looking for, we resumed
digging at the site and began finding bits of Tiktaalik almost
immediately. After that, it was a minor eureka every few days as
more and more bones came to light, continuing almost until the very
end of the season.
CG: What does this find mean for evolutionary
science?
CS: Tiktaalik sheds a lot of light
on the evolutionary transition from fish to tetrapods. Tetrapods
are different from fish in a number of fundamental ways, the most
obvious that they have legs instead of fins.
Another major one is that tetrapods have a flexible neck — if you
were a fish, your shoulder blades would be attached to the back
of your skull, which would be pretty strange. Before Tiktaalik was
discovered, the closest known relatives of tetrapods were "good" fish
in that they had no necks, and had inflexible limbs that ended in
fin-rays rather than fingers. In Tiktaalik, the shoulder
blades are separate from the skull, and the front limbs have flexible
joints that suggest they could have been used to support the body
on land. However, the limb bones also look suspiciously fish-like
in their detailed structure, and there's still a fringe of fin-rays.
The limbs are at least as much like bendable fins as they are like
tetrapod front legs, so their weight-bearing function apparently
evolved before any major change in structure. Some evolutionary
change involves small anatomical modifications with no obvious functional
consequences, but Tiktaalik is almost the opposite - a
fish that was crudely jerry-rigged for dragging itself around in
defiance of gravity, despite lacking many of the detailed features
that are characteristic of tetrapods. It's a good example of the
major role of functional change in evolution. With that said, it
would be really nice to have more material from the back half of
the animal, to go along with the beautiful skulls, ribs, and forelimbs.
CG: It's a thrill to have such an important fossil
turn up in Canada. Should
Canadians be excited about this?
CS: I hope Canadians can spare a bit of excitement
for Tiktaalik! The main importance of the find, from the
point of view of Canadian palaeontology, is that it helps add breadth
to the country's diversity of fossil finds. When most foreign
palaeontologists and informed laymen think of Canadian fossils,
they probably think of Cretaceous dinosaurs from Alberta. As spectacular
as the Alberta dinosaurs are, this fish from Nunavut is a good reminder
that other parts of the country, and rocks of other ages, are well
worth looking at. You never know what you might find.
CG: Isn't finding a fossil like this a bit like
looking for a needle in a haystack? How exactly did the team come
across the find?
CS: I've never tried looking for a needle in
a haystack, but I would guess that looking for fossils is somewhat
easier, for three major reasons. First, the proverbial haystack
contains only one needle, but any productive area is likely to have
many worthwhile fossil sites. Second, a haystack is unstructured,
but in palaeontology you can use geological maps to make sure you're
searching rocks of the right age, and topographic maps or air photos
to concentrate on areas where the rocks are likely to be well exposed.
Third, the needle is small and self-contained, but good fossil sites
are usually quite incontinent. As the surrounding rock erodes, fragments
of bone begin to weather out of the rock, and you can spot this "float" at
the surface. Tiktaalik came from a quarry that was found
by a field assistant walking along a slope with nicely exposed rocks
of the right age - and what caught his eye was the huge amount of
float that was scattered around. He deserves a lot of credit for
making the discovery, but it happened because he was looking in
the right place, and there were visual clues available.
CG: Is this the new hot spot for fossil hunting?
CS: I doubt the Devonian rocks of Nunavut will
ever be too much of a hot spot, partly because "elpistostegalian"
fish and early tetrapods are of minor interest even among palaeontologists
and partly because Nunavut is just so far north. It's expensive
to get people and supplies up there, and the field season is short
because the snow seems to melt in June and come back in August — maybe
that's an exaggeration, but not much of one. We had snow on Canada
Day in 2002. As an alternative to sending field crews up there on
expensive prospecting trips, it would be wonderful if more people
from the local communities became involved in fossil hunting, like
the famous "local farmers" who are always finding things
in China. But of course, that interest would have to develop naturally
within the communities themselves. In the shorter term, some members
of the team will definitely be heading back to Nunavut to look for
more complete specimens of Tiktaalik, and any other surprises
the gods may have in store.
CG: Is this the missing link?
CS: It's definitely a missing link.
The fact of the matter is that there are interesting gaps and areas
of uncertainty all over the vertebrate family tree. In a sense, Tiktaalik fills
a gap between a fish called Panderichthys and the early
tetrapods. But now there are new gaps (admittedly smaller ones)
between Panderichthys and Tiktaalik, and between Tiktaalik and
tetrapods. There are also good-sized gaps near the evolutionary
origins of several other animal groups, including turtles, frogs,
bats, snakes, and - some would argue, at least - people. The finer
the level of detail you consider, the more gaps appear. So the discovery
of Tiktaalik is a major step forward in palaeontological
knowledge, but there's plenty more to do.
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