A Giant Step in Evolution: Discovering the Link
Between Fish and Limbed Animals
Ted Daeschler
(Academy of Natural Sciences, Philadelphia, Pennsylvania, USA)
Denison University, Granville, Ohio, USA
12 April 2007
All
of vertebrate history has occurred since the Cambrian (from ~500 m.y. to now).
Devonian - the time of
the highest diversity for jawless fish, placoderms, acanthodians, and
lobe-finned fish. The Devonian had a different assemblage of vertebrates
compared to now. Now, we’ve got sharks & their relatives, ray-finned
fish, and tetrapods. Tetrapods had just started in the Devonian.
Lobe-finned
fish are represented today by lungfish
& coelacanths.
The precursors to limbed animals are found in the lobe-finned fish.
Tetrapods
were not evolving in a vacuum. Consider vascular plants - many
modern plant groups were just getting started in the Late Devonian. The
Early Devonian was the time of the ~first invasion of land by plants.
Look
at the Catskill Formation (Devonian) of the Pocono Plateau & Allegheny
Plateau of Pennsylvania - there are many old & new roadcuts. Ex:
just north of Williamsport, Pennsylvania.
Sauripterus
taylori - a right pectoral
fin with long dermal rods from the Catskill Formation of Lycoming County,
Pennsylvania. The fossil has internal bony structure - gave the fin
strength & provided much more mobility than we’d expect in a lobe-finned
fish.
Red Hill - a
large Upper Devonian roadcut in Clinton County, Pennsylvania that has produced
1000s of fossils from ancient stream deposits. The site also has
paleosols (floodplain soils). These rocks represent an ancient
well-watered floodplain. Lots of plant fossils and freshwater fish
fossils (placoderms, acanthodians, lobe-finned fish, ray-finned fish) plus
early tetrapods. Also millipedes, arachnids, plants.
See
May 1999 National Geographic for a Red Hill reconstruction.
Freshwater
ecosystems are where the fish-tetrapod transition occurred.
See
2 April 2004 issue of Science - article about an important find from Red
Hill, even though it’s only one element.
Looking
at the lineage to tetrapods (lobe-finned fish to tetrapods):
Tulerpeton
Ichthyostega
Acanthostega
Panderichthys
Eusthenopteron
Rhizodont fish
There’s
still a morphological gap between Panderichthys and Acanthostega.
Saw
Dott & Batten’s historical geology book Evolution of the Earth, 2nd
Edition & noticed a map of North America showing the Devonian redbeds
of the Catskill of Pennsylvania & Devonian redbeds in Greenland &
Devonian redbeds in the Canadian Arctic Islands.
Well,
Pennsylvania and Greenland were already well studied. But, the Canadian
Arctic islands hadn’t been explored for fossils. Daeschler went to the
Canadian Arctic in 1999 (then just established as Nunavut). Went to Ellesmere
Island & islands west of there (Mellville Island, Bathurst
Island). Returned in 2000, 2002, 2004, and 2006.
The
rocks there are part of a clastic wedge - there were mountains to the east -
meandering stream, alluvial plains - similar environments then as in
Pennsylvania & Ohio, but the Canadian Arctic sediments are ~5 million years
older.
So,
looked at the Middle-Upper Devonian of Arctic Canada.
Visited
areas on Ellesmere Island (in the east) to areas on Banks
Island (to the west).
The
sections have 8000’ worth of clastic sediments (Frasnian).
Southeastern
Melville
Island, Nunavut - good exposures in general; no vegetation, but some places
have deeply weathered rocks. There are polar bears in the area. The
Sun is up 24 hours a day.
Fram
Formation (Frasnian) - 1800 meters thick, 2-3 m.y. worth of time.
Alluvial, channels, floodplain sediments. Found good material in the Fram
Formation in an unnamed valley near Bird Fiord, southern Ellesmere Island,
including lungfish plate fragments. Did excavation and found more
material. Found skulls, fish bodies, and bodies with fins.
In
2002, got lots of Laccognathus, a large lobe-finned fish. Laccognathus
is not close to the origin of tetrapods, though. Also got an Elpistostege
palette. Elpistostege
is a flat-headed fish with orbits placed high. It is high on the
lobe-finned fish tree, close to tetrapods.
The
elpistostegians are known from the Gaspe and from Latvia - Elpistostege
+ Panderichthys
(close to early tetrapods, but not really well preserved). So, they are
from the Euramerican continent. Canadian Arctic samples are consistent
with this distribution.
Revisited
in 2004 - found more Laccognathus & Elpistostege. Also
got Eusthenopteron
and placoderms and lobe-finned fish and lungfish. Also found a new
elpistostegian that we named Tiktaalik
roseae.
Found
an articulated specimen - saw snout and a lower jaw in the field of what would
become Tiktaalik. Collected a block of rock with some of the Tiktaalik
animal in it & another block with Tiktaalik material. Did
excavation back in the lab - saw orbits raised up. Saw the back end
of the skull. Got a ~complete elpistostegian skull - different from known
elpistostegians. Got a shoulder girdle, fins, scales. Fred Mullison
is the fossil prepator of this material - he saw this stuff first during
excavation in the lab.
Got
a skull articulated with a post-cranial skeleton. Got three skulls in
total, each associated with shoulder girdles and fins. This is now called
Tiktaalik.
Tiktaalik is quite
different from Panderichthys. Tiktaalik has a shortened
back of skull and deep notches at the back of the skull (like early
tetrapods). The genus Tiktaalik was named by Inuit residents -
it’s their word for a freshwater fish.
One
skull shows gill supports (brachial supports). Got pectoral fin material
with humerus + ulna/radius. Also got complete fins. Are they fins or
limbs?
The
pectoral fin of Tiktaalik is like that of lobe-finned fish, but it has
features of tetrapods as well.
The
animal could extend its wrist region, but couldn’t flex it far. Fish
can’t do this (bending at the wrist zone).
Muscle
attachments allow us to determine that it could flex at the wrist area.
Trunk
of Tiktaalik - under the scales are flattened ribs - this is seen in
early tetrapods. Flattened ribs provide support for respiration.
Tiktaalik had two
respiration systems - had lungs & gills (like other lobe-finned fish).
Fish:
no neck
scales
fins
versus:
Tetrapods:
neck - can turn head without turning whole body, as fish have to
no scales
limbs
Tiktaalik has a
mixture of fish & tetrapod features. Under the fish scales are tetrapod
ribs. It has a neck, has scales, has fins, a flat head, and dorsal
eyes. It has a primitive jaw.
Tiktaalik is a mosaic
form. This is a good transitional form. Couldn’t predict this exact
mosaic beforehand, though. Tiktaalik fits in the gap we mentioned
before (the Panderichthys to Acanthostega gap).
Every fossil is a
transitional fossil. But Tiktaalik was a transition between major
groups.
