|Ruler is in inches|
It was a good fish, as far as 210-million year old Late Triassic fossil fish go. I had done all the prep work that needed doing for now on this little guy at the St. George Dinosaur Discovery Site at Johnson Farm and so for one last time I tucked it carefully and lovingly back into its museum drawer. Goodbye, old friend! Maybe we’ll meet again someday at the Utah Museum of Natural History! It’s time to move on to a new fish.
From the drawer I chose a broken sample that looked intriguing. There were many small scattered patches of fish scales visible on one large flattish surface of the reddish brown mudstone; also visible from the side and parallel to this surface was a thin line of scales about an inch long.
What lies hidden underneath those mudstone layers? It may be one fish in there, or it may be many; it might be one relatively intact specimen, or it might be a jumbled mush of small fish carcasses all piled on top of one another. Time will tell as I meticulously start to pick away at the matrix, layer by layer.
|Needle tip at fish scales|
This new sample (along with many others, including the above-mentioned “old friend”) was brought back from an on-going study area in San Juan County near Moab, Utah. According to Andrew Milner, paleontologist with the Dinosaur Discovery Site, it had originally been thought that these fish came from the 210-million year old Owl Rock member of the Chinle Formation, but is now believed to be from the oh-so-slightly younger Church Rock member.
The Owl Rock represents deposits in a large, not very fossilliferous lacustrine (lake) system, whereas the Church Rock member represents a unique area of a huge fluvial (river) system. Here, massive amounts of material were transported into the system from highlands to the east and west.
|Image courtesy Utah Geologic Survey|
According to the Utah Geological Survey, a Late Triassic Chinle fluvial (river) system covered much of the Western Interior of North America. Sources of these Chinle rivers were in remnant highlands of the Appalachian Mountain system.
Another interesting specimen being prepped (not by me) is shown in the images below. Notice at the lower left of the photo how its fin has folded underneath the fish. Also, notice the fine detail of the fins, bones, and scales.
What really impressive prep work!
This is also a semionotid and is very likely a new species.
However, this semionotid is not from eastern Utah but was found in an area of the Dinosaur Discovery Site @ Johnson Farm known as Freeman Quarry. It was found in the upper part of the Whitmore Point member of the Early Jurassic Moenave Formation, about 200-198 million years old.
For an interesting article on the “fishing dinosaurs” at the edge of southwest Utah’s prehistoric Lake Dixie, click here.
Thanks to Andrew Milner for taking the time to explain the nuances of the Owl Rock and Church Rock members of the Late Triassic Chinle Formation and also of the Whitmore Point member of the Early Jurassic Moenave Formation.
For stratigraphic and lithologic columns see Biek, R.F., Willis, G.C., Hylland, M.D., and Doelling, H.H., 2000, Geology of Zion National Park, Utah, in Sprinkel, D.A., Chidsey, T.C., and Anderson. P.A., editors, Geology of Utah’s Parks and Monuments, Utah Geological Association Millennium Guidebook Publication 28, p. 110-111.