The Copan Lagerstätte: Exceptional Crinoid Diversity and Preservation in the Pennsylvanian of Midcontinental North America
James Thomka (Department of Geology, University of Cincinnati, Cincinnati, Ohio, USA)
Dry Dredgers meeting (Cincinnati, Ohio, USA)
28 January 2011
The Pennsylvanian is a unique time for crinoids. The Pennsylvanian doesn’t get much attention in terms of crinoids. There are many crinoids in the Pennsylvanian, diversity-wise.
Looking at the history of crinoids, the Pennsylvanian falls between two significant crinoid events:
1) Mississippian - when crinoids hit their peak, abundance and diversity-wise; get entire rock types from crinoid remains at this time.
2) end-Permian mass extinction - the worst ever; crinoids got decimated. Four genera of crinoids cross the Permian-Triassic boundary. All modern crinoids evolved from them.
The Pennsylvanian seems bland in comparison.
Pennsylvanian crinoids generally don’t occur as articulated specimens - don’t see many crowns.
The only significant exception is the LaSalle Limestone of Illinois.
Now, a second Pennsylvanian crinoid deposit with lots of articulated crowns has been found - the Upper Pennsylvanian Copan Lagerstätte of Oklahoma. It’s named after the small town of Copan in northern Washington County, northeastern Oklahoma. This unique deposit is not regionally extensive - <12 m2 - very restricted geographic distribution. It’s also very thin - 0.5 meters thick - very restricted, stratigraphically.
The Copan Lagerstätte is in a concretionary mudstone interval in the lower Barnsdall Formation.
This crinoid deposit is late Missourian (Upper Pennsylvanian) in age.
The site was discovered in the 1940s by Strimple, who is one of the few Pennsylvanian crinoid specialists. In the 1940s, the site was called the “Tankdike locality”, in reference to it being a petroleum storage tank cut. There was an abundance of crinoids in float at the cut - rich in new crinoids.
Eight new crinoid taxa were found and described from here in the 1950s to 1970s.
The fossils consist of fairly articulated individuals - cups and crowns.
The crinoids were never found in-situ, only in float.
The site was rediscovered in 1984 by Dan Mosher, an amateur. He found lots of Pennsylvanian articulated crinoids. He wanted to find the source horizon. He used pennies on the outcrop for every float crown he found. He used the distribution of the pennies to locate the source horizon. He sieved and collected lots of crinoid crowns. Mosher took the crinoids to Roger Pabian.
Heavy equipment was brought in later - the overburden was removed and they benched off the source horizon. Collecting resulted in an impressive yield of crinoids. Some were preserved in siderite concretions. Some long lengths of stems were found, plus some intact crinoid holdfasts.
1250 articulated or partially articulated specimens were recovered from an only-inches-thick horizon.
The Copan Lagerstätte produced 44 genera and 50 species of crinoids. This is a higher crinoid diversity than the Crawfordsville, Indiana crinoid fauna (Mississippian).
Copan is the most diverse Pennsylvanian crinoid lagerstätte in the world. The next most diverse Pennsylvanian crinoid lagerstätte has 32 genera.
What’s leading to this diversity and preservation?
In the Pennsylvanian, midcontinental North America was under shallow seas. The Copan, Oklahoma site was close to the Ouachita Mountains.
Iowa had extensive shallow-water carbonates. Nebraska was a more open-ocean, shallow marine shelf with algal mound environments in places.
Cyclothems are important in the Pennsylvanian Midcontinent. Each cyclothem represents one relative rise and fall in sea level. Oklahoma has Kansas-type cyclothems - the most important unit in KS-type cyclothems is the core shale - black to gray shale representing dysoxic to anoxic conditions and having low-diversity fossils, if any.
Core shales actually consist of an upper core shale, which is light-colored (often greenish) with diverse fossils, and a lower core shale, which is dark gray to black and horizontally laminated.
During deposition of the upper core shale, the seafloor was hospitable to life. The upper core shale represents the first shallowing from the deepest part of the cycle. Benthic invertebrates fluorished during upper core shale conditions.
The Copan Lagerstätte site has concretionary shale in the upper core shale and the crinoid lagerstätte in the lower core shale - black shale.
Most of the Barnsdall Formation is blocky, blah-gray shale.
The 0.5 meter interval of interest is not just monotonous mudstone - it has 3 discrete crinoid horizons.
The lowest one has 44 genera and 50 species of crinoids - this is what Mosher found - >1200 specimens.
The paleoenvironment is interpreted as an oxygenated, low-energy, low sedimentation, distal shelf setting.
The shale intervals between crinoid horizons have productid brachiopods, clams, and scaphopods.
The crinoids occur in concretionary shales. Concretions are indicators of low-sedimentation rates - they form in the shallow subsurface.
Very large siderite concretions indicate long periods of sediment starvation on the distal shelf. Thousands of years of sediment starvation, apparently. Very sediment-starved conditions.
Sedimentation was very slow - these conditions are associated with the well-preserved crinoids.
Kope Formation concretions are smaller than Barnsdall Formation siderite concretions.
Encrustation rates on hard biogenic substrates are high in crinoid-rich, concretion-rich intervals.
There’s also evidence for rapid deposition - how else does one get beautifully-preserved crinoid crowns?
Mobile organisms were there too - their skeletal remains are found in the horizons of interest, but they are not well preserved. Occasionally a productid brachiopod is found in living position. Occasionally a bryozoan sheet is found. Complete regular echinoids are never found here.
Burial events were thin enough to only affect immobile organisms, like crinoids.
One specimen of a crown of a flexible crinoid has one side well preserved/intact and the other side a plate hash. This indicates partial burial (thin sediment cover) - the crown was partially exposed on the seafloor.
Other evidence for thin burial - infaunal scavenging is inferred from partially disturbed crowns.
Burrowing bivalves and scaphopods in life position are seen in the intervening thick shale/mudstone intervals.
There are multiple horizons (3) of thin, crinoid-bearing, siderite concretionary horizons. These horizons have no winnowing, no graded bedding, and no piles of skeletal material.
There’s no evidence that the rapid burial was high-energy - no big blast. In-situ holdfasts are present - crinoids were toppled over in-place and buried. No violent burial.
Thicker layers have abundant, sharp-based lags - rapid deposition of considerable material under much higher energy conditions. These thicker mudstones also have large, siderite-filled burrows. The burrows have sharp-walled boundaries - indicates firmground conditions. These firmground burrows were temporary substrates - they indicate minor erosive episodes - the soupy upper layer got removed.
So, seeing a proximality difference.
Crinoid horizons experienced sediment starvation and episodic, very thin, rapid burial events.
Multidirectional currents associated with rapid burial is diagnostic of storm sedimentation in distal shelf settings. These are tempestites - not single event surges.
Some crinoids were buried alive or prior to any major degradation.
Shaving brush posture in crinoid crowns - the animal was trying to protect critical parts/anatomy from stressful environmental conditions.
Can also see brief periods of incipient decay prior to final burial.
All of this is expected/associated with distal storm events.
Taphonomy - can see the loss of arm plate tips before the first losses of pinnules!
So, there was a brief period of decay.
Gonads are the most nutrient-rich parts of the crinoids for scavenging organisms.
Some partially (inferred) scavenge-disrupted crinoid crowns were growth sites for siderite concretions. The scavenged portion of a crown will have more water movement, resulting in more concretion mineralization potential.
The Copan Lagerstätte consists of thin, distal tempestites. Sediment starvation allowed for stacking of obrution layers with very little dilution by background sedimentation.
This motif is probably seen/present in many other areas. We’ll see.
It’s possible that original seafloor topographic control explains the limited geographic distribution of the Copan Lagerstätte. The Copan was deposited in a seafloor depression.
Many different arm types are present in the recovered crinoids, indicating many different feeding styles.
Sediment channeling (funneling) may have been playing a role.
Inferred water depth - there are no algae present to indicate photic zone depths - probably ~100 m deep?
Crinoids got asphyxiated by a little fine-grained sediments - they died and fell over. The crinoids were not buried to the height of the upright animal. Death preceded burial. Burial was by a ~few cm of uncompacted mud, but it varied, apparently.