Friday, July 4, 2014

Tales from the Field: Return (Almost) to Dinosaur Gorge

Hello, Dear Readers!

We've hit July in the Peace Region, and after a month of meetings, paper revisions, paper writing, meetings, delegations, meetings...did I mention we had a lot of meetings in June?...I am pleased to be able to start the 2014 field season. This year we are not focusing on the hadrosaur excavation. Instead, 2014 is going to be the Year of Ichnology. Our focus is on an unnamed creek which we unofficially call "Dinosaur Gorge": a steep-walled canyon with at least two vertical (really it is a 60 degree slope, but anything that requires ropes to document it is vertical enough for me) Early Cretaceous (Valanginian - between 139 and 134 million years old) track surface on which large ornithopod, large-sized theropod, and medium-sized theropod trackways are preserved.

Image of the main track surface of Dinosaur Gorge, taken August 2013. Can you see the trackways? I admit it is difficult, given the "Lonesome Death of Jordy Verrill" theme of the surface flora. Photo: L. Buckley.
Our goal for the Year of Ichnology is to document this surface using both direct measurements and 3-D photogrammetry, as well as taking latex molds of sections of the trackways. We set a test rope last year and found that we could access most of the track surface safely, so the plan this year is to set at least two ropes: physics starts to interfere with stability when your rope is craned over at 50 degrees.

No matter what your primitive instincts tell you, moss is not a load-bearing surface. Photo: L. Buckley.
The mission for this afternoon was to check out both the access to the site, and to see if the water in the creek was low enough for us to safely walk. We were just about a kilometer from our destination when we encountered this in the road:

Photo: L. Buckley.
We left the truck and walked the rest of the way, scouting out potential campsites along the route. We found that the creek levels were up, but not so much as to bar our access to the track sites. We considered this mission accomplished and turned back to check out a few more vertical surfaces we had spotted on our way to the mouth of the Gorge.

One massive surface was a complete disappointment: coarse, well-sorted sand, massive channel deposits, wood impressions, centimeter diameter silt clasts, but no traces were visible, not even invertebrate traces. Following a drainage channel down to a culvert, we saw material that was more promising: fine, organic rich sand with thin silty beds. The material was out of context, but it was similar in sedimentology to what we see in the Gorge, so we knew that it was local. Sure enough, there were traces!

Paired burrow openings, likely Arenicolites, are common in the sandy shore ichnofacies (the Skolithos Ichnofacies). Photo: L. Buckley.
The coolest traces we saw today were invertebrate traces. I am drawn to the fine-grained surfaces because I am a self-admitted bird track fanatic. When you start looking for small vertebrate traces (bird, amphibian, reptile) you are entering the size range of many invertebrate traces. Not everything that looks like a bird trace is a bird trace. Take this image, for example:

Photo: L. Buckley.
Depending on how you look at it, the structure I am pointing to does superficially resemble a poorly preserved avian trace. However, I have developed a set of criteria that a trace must meet before I will squeal with glee and triumphantly eat the Chocolate Bar of Victory:

1. Is the structure tubular? Some bird prints, depending on the consistency of the sediment, can appear to be ridges, grooves, or even shallow indents, but one thing they rarely preserve as are cylinders

2. Is the structure alone? Until someone discovers the Amazing Cretaceous Unipod Bird (although I did once see a duck with a missing foot - it walked around like a pirate on a peg leg), bird tracks are not going to usually occur alone, or if they do, they are large enough to not be mistaken as an invertebrate trace. Small, Cretaceous-aged avian track-makers behaved in a similar fashion to our extant shorebirds, with one or more individuals foraging back and forth on a wet surface. If that trace is an avian trace, the structure should be repeated somewhere else on the surface, even if it is not part of a trackway.

3. Is the structure connected to an obvious burrow? You would think we shouldn't need to ask this differential, but we do, especially when we have trained our eyes to look at traces with a vertebrate filter. For example, McCrea et al. (2014) addresses an earlier report of Sarjeant and Thulborn (1986) of Duquettichnus kooli, a purported marsupial print from the Early Cretaceous Peace River Canyon. On one surface is preserved what appeared to be a marsupial foot (pes) print. However, when we examined the specimen in 2006, we turned the specimen over: the pes was actually part of an invertebrate burrow that continued on to the other side of the sample. Doctors have the "Zebra Diagnosis", and ichnology isn't immune to supposed zebras trotting around in the Cretaceous when good ol' invertebrates are far more likely. I don't think we've seen the last of invertebrate traces being misinterpreted as the traces of small vertebrates.

I did not see anything today that I would confidently say is a bird track, and although one feature did get me excited for a moment, it was time to get skeptical when you are dealing with an infill surface and the structure is an impression:

Slowly puts away the Chocolate Bar of Victory, and eats the Stale Rice Cake of Defeat. Photo: L. Buckley.
Photo: L. Buckley.
However, my ichnology spirits were refreshed with some of the best Aulichnites-like traces I have seen in this region, along with larger repichnia (crawling traces):


Photo: L. Buckley.
I highly recommend taking a course in invertebrate ichnology. Not only is it fascinating to see how different burrowing, crawling, and feeding invertebrates, for example, alter their behavior based on changing environmental conditions or are restricted to certain environments, invertebrate traces provide vertebrate paleontologists with paleoenvironmental information.

I dedicate this post to our invertebrate track-makers. Proudly line your burrows with fecal pellets, Ophiomorpha trace-maker. Your poopy home is telling us a story!

SAS


References:

McCrea, R. T., L. G. Buckley, A. G. Plint, P. J. Currie, J. W. Haggart, C. W. Helm, S. G. Pemberton. 2014. A review of vertebrate track-bearing formations from the Mesozoic and earliest Cenozoic of western Canada with a description of a new theropod ichnospecies and reassignment of an avian ichnogenus, p. 5-93 in Lockley, M. G. & Lucas, S. G. (eds.), Fossil footprints of western North America. New Mexico Museum of Natural History and Science Bulletin 62.

Sarjeant, W.A.S., and Thulborn, R.A., 1986, Probable marsupial footprints from the Cretaceous sediments of British Columbia: Canadian Journal of Earth Sciences, v. 23, p. 1223-1227.

2 comments:

  1. The Chocolate Bar of Victory and the Stale Rice Cake of defeat almost made me blow coffee out my nose! I'm definitely stealing those for future use!

    I'm sure you already have it, but if not, Trace Fossils: Biology and Taphonomy by Richard Bromley is an invaluable addition to one's invertebrate ichnology library.

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  2. Steal away! I like it when my odd humor hits the right note!

    This is a great reference recommendation! Bromley, Frey, Seilacher...any of these references should be staples in the vertebrate and invertebrate ichnologist's library. Short courses by Pemberton and Gingras are also great resources! I'm sure there are others, but those are the ones with which I have direct experience.

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