Sunday, June 22, 2014

THE ONES THAT GOT AWAY: A DAY AT THE OLD FISH HOLE

 

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.

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. 

 
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.

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.

The paired lobes of Rusophycus, the resting trace fossil made by a trilobite.

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.
 
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

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