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| A spectacular 600 foot tall cliff of Madison Limestone extends for miles along the Jones Creek trail. Somewhere near the base of such a cliff lies is an important fossil fish locality awaiting rediscovery. |
GOING FISHING WITH HAMMERS AND CHISELS
Recently
my colleague, Dr. George Engelmann, and I hiked into the deep river canyon
country of Dinosaur to relocate a fossil fish locality discovered over half a
century ago. Information was sparse ---
the site had received only the briefest of mentions in the scientific
literature. We had good information
about where the site is, somewhere along a 100 meter or so stretch of the base
of a great cliff system. However, we
knew almost nothing about the kind of rock the fossils occurred in or the nature
of the fossils. Were they just scales, teeth, fragments of bones, large bones
or small? Nor did we know what kind of
fish they were.
Our
interest in the site was multifold. It is
the only Paleozoic fish fossil locality in Dinosaur. The site occurs in what is mapped as the
Lodore Formation. This rock unit is
Cambrian in age, about 515,000,000 years old.
While primitive fish are known from the Cambrian their remains are small,
fragmentary and incomplete. So any site with Cambrian fish is certainly worth
investigating.
Of
additional interest are the few cryptic statements in the scientific literature
suggesting that the rocks the fossils occurred in are in fact not Cambrian in
age but Devonian. Dinosaur National
Monument has the most complete geological record of any NPS unit. It’s 23 different formations preserve the
pageant of the physical and biological evolution of our planet over the last
1.2 billion years. Yet some time periods
are missing from Dinosaur, removed by erosion in some distant past. One of those missing periods is the
Devonian. So if Devonian age rocks are
hidden in Dinosaur, their discovery would fill in one of the gaps in our
geological record. The fish fossils, if
age diagnostic, might well be the crucial bit of evidence for solving this
puzzle.
During
the Devonian the evolution of fish was explosive and the seas were populated
with a vast array of fish. Some, such as
sharks, would be familiar to you. But many would look otherworldly. Some were small, some were the size of small
whales. Some were flat, slow-moving
bottom dwellers, others were streamlined, fast-swimming, open water
predators. Some were naked, others were
encased in heavy plates of thick boney armor.
Some evolved blunt teeth adapted for crushing shells, others had teeth
for catching and slicing other fish, and some had no jaws at all.
Some were blind, others had excellent vision.
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| Some of the possible Devonian fish from Utah whose remains we were fishing for. A. Acanthodian B. Osteolepid sarcopterygian C. Actinolepid arthrodire D. Cyathaspidid heterostracan E. Pteraspidid heterostracan. F. Antiarch placoderm. G. Cardipeltid heterostracan. H. Cephalaspid osteostracan. All to same scale; cyathaspidid is 20 cm long (from Elliott et al. 1999) |
Given
this diversity and abundance, the right fish fossil could turn out to be
diagnostic for the Devonian and give us the needed age for the rocks in
question. To be age diagnostic the fossil need not be complete. Even small bones, if they can be identified
as belonging to a group of fish known only from the Devonian, would provide the
crucial age information. As preparation
for the trip we read the literature on Devonian fish elsewhere in Utah and felt
fairly confident about what kinds of fossils we would be looking for.
INTO
THE CANYONS
Dinosaur
National Monument is cut into three sections by the spectacular canyons of the
Green and Yampa River. However with
cliff walls sometimes well over 1,500 feet tall, getting to the canyon floor is very difficult
and there are only a few places where you can do it on foot. The Jones Creek trail is one of those
rare access points.
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| Jones Creek is a testimony to the erosional and life giving powers of water in the desert. |
The
trail is eight miles round trip and runs along Jones Hole Creek, ending where
the creek flows into the Green River 1000 feet below the canyon rim. This year-round flowing stream supports an
oasis of riparian vegetation that makes for a wonderful hike amidst trees and
along canyon walls, set to the music of flowing water spilling over rocks and
cobbles. If you ever visit Dinosaur and have a day for a hike, this is the
place to do it! You will never forget
it. It is also an excellent trout
fishing stream and we passed several anglers trying to coax a trout to take
their fly. Little did they realize that
the two encumbered hikers passing them were also on a fishing trip, but for
much older piscines.
SLIP
SLIDING AWAY
Once
at river level we traversed the canyon to the rocks in question. Google Earth had given us a somewhat
erroneous image of the terrain. What had appeared to be a ledge that we wanted
to examine was actually a cliff about 15 meters tall and not possible to
negotiate. Above that loomed the
towering face of the Madison Limestone nearly 600 feet high. To get to the lower, smaller cliff that might
contain the fossil site we had to climb up a high angle slope nearly 100 feet
high. It was covered by a mixture of soil and loose rocks ranging in size from
gravel to blocks the size of kitchen tables. Everything on the slope threatened
to move under our feet foot and it was two steps up and one step back all the
way up.
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| No Country For Old Legs. Only my orange pack prevents me from disappearing into the vastness of Dinosaur's river canyon country. |
Our
search strategy was two fold. First
traverse the lower slope, keeping an eye out for fossils in the loose blocks
covering it. The innumerable loose
blocks and rocks which have fallen onto the slope from above presented us with
an immense random sample from the cliffs that we could not climb over. Hopefully some might contains bits of what we
came here for.
Second,
once we got get up to the lower cliff we could examine at the least the lower
several meters thickness of rock. Maybe,
just maybe, we would find some fishy tidbits in-place.
Although
it was June, this has fortunately been a relatively mild summer so it was not one
the blistering, demoralizingly hot days we have so often encountered in the
past. Nevertheless, the searching proved
arduous. Most vexing was the slope, as
the covering of loose rocks kept wanting to slide with each step. When an
interesting object was seen in a rock, it was a challenge to balance while
holding a rock near one’s face and a 10x hand lens to one’s eye. Occasional gusts of wind didn’t help either.
Walking
long the base of the lower cliff was not much better as we were still walking
on the loose slope, and the exposure of the bottom of the cliff undulated up
and down. Each step was a combination of balance, bracing, and trying not to
slip while simultaneously looking closely for the remains of ancient fish
remains in alternating beds of sandstones and mudstone.
It
took three hours of this struggling to cover the slope and lower cliff.
SO
HOW DID WE DO?
We
didn’t find a single fish fossil.
Not
a scale --- Not
a tooth --- Not
a rib
We
were absolutely in the right place, but the fossils we came for eluded us.
Were
we disappointed? Yes.
Were
we frustrated? Yes.
Have
we given up? Maybe not.
We’ll
need to see if there is any way to find out something about the kind of rock
the fossils had been found in. Was it sandstone, mudstone, limestone? What color was the rock? Green, yellow-brown, purple? If we could find that out and recognize it as
one of the layers we had seen while examining the area, then a trip back would
be worth the effort.
IT WAS NOT ALL FOR NAUGHT
While
we didn’t find any fish fossils, the rocks were full of fossils of a different
kind. They teemed with trace fossils,
the preserved burrows and trails made by creatures as the crawled on and plowed
through, lived and died on, the sand and mud deposited 515,000,000 years ago on
the sea floor of a long extinct ocean.
The
trace fossils were of many different sizes, shapes, and constructions. One of the most striking is known
scientifically as Rusophychus. These
traces are in the form of two elongated lobes and were made by trilobites as
they excavated a depression into seafloor sediments to form a resting place.
Some were so wonderfully preserved that one could see the obliquely oriented
scratches made by the trilobite’s legs as it dug into the sediments. Of interest is that the Rusophychus we saw were much larger than the trilobites known from
body fossils in this formation in our area. So some giants remain to be
discovered.
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| The paired lobes of Rusophycus, the resting trace fossil made by a trilobite. |
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| This well preserved Rusophycus shows ridges running across each lobe. Those ridges were made by the trilobite's legs as it dug into the seafloor. |
The
makers of the many other trace fossils we found is less clear, as similar
simple burrows and trails can be made by many different kinds of marine
animals. Some are long and slender, some
are sort and wide, some are unbrached, others split along their path, some
cross one another, others stay separate, some are trails made on the surface,
some are burrows in the sediment. Nevertheless,
these ancient hieroglyphs, formed by long gone animals, are silent testimony to
an ocean that teamed with life. Virtually none of these trace fossil makers are
known from body fossils in this formation.
Here
is a selection of photos shot in the field showing some of the trace fossil
diversity we discovered.
REFLECTIONS ON THE DAY
In
spite of what you might see on TV, not every fossil excursion is successful.
Our ancient fishing trip was hampered by a lack of detail in some areas but we had
enough good information to justify the effort. If we had found the fish
we might have been able to solve one of the paleontological mysteries of
Dinosaur National Monument. We did
unexpectedly find a diverse deposit of trace fossils that had not previously
been reported in detail.
In
the midst of slogging around on the slippery, rubble laden slope, leaning on our picks to stop,
catch our breath, and rest our legs, we took a moment to look up from the ground
and see the grandeur that was around us. Sometimes your office has no walls and
that can be a really great thing. Those
are days you never forget, even if you don’t find the blasted fish bones.
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| No room with a view. If you are going to not find fossils this is a magnificent place to not find them. |
PHOTOS
/ ILLUSTRATIONS
Restorations of Devonian fish from Utah is from: Elliott, D.K., Reed, R.C., and Heidemarie, G.J. 1999. The Devonian vertebrates of Utah. in: Gillette, D.D. (ed). Vertebrate Paleontology in Utah. Miscellaneous Publication 99-1 Utah Geological Survey: pages 1-12.
Cliffs above Jones Hole, Jones Hole Creek, No Country for Old Legs: George Engelmann
All others: NPS/Dan Chure
Cliffs above Jones Hole, Jones Hole Creek, No Country for Old Legs: George Engelmann
All others: NPS/Dan Chure
