This is Mazon Monday post #261. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Who doesn't like a nice cyclid fossil? Well, the Lauers (and friends) are here with a full plate of them... The fossil plate comes from the legendary Bear Gulch locality in Montana, US. The Bear Gulch Beds of the Heath Formation date to the Mississippian Period, some 320 million years ago. It's well known for its diverse assemblage of well preserved offish, arthropods, and soft-bodied animals. For more information about the Bear Gulch fossil locality, see "The geological and biological environment of the Bear Gulch Limestone (Mississippian of Montana, USA) and a model for its deposition" on ResearchGate.
Abstract
2002. — The geological and biological environment of the Bear Gulch Limestone (Mississippian of Montana, USA) and a model for its deposition. Geodiversitas 24 (2) : 295-315. ABSTRACT The Bear Gulch Limestone (Heath Formation, Big Snowy Group, Fergus County, Montana, USA) is a Serpukhovian (upper Mississippian, Namurian E 2 b) Konservat lagerstätte, deposited in the Central Montana Trough, at about 12° North latitude. It contains fossils from a productive Paleozoic marine bay including a diverse biota of fishes, invertebrates, and algae. We describe several new biofacies: an Arborispongia-productid, a filamentous algal and a shallow facies. The previously named central basin facies and upper-most zone are redefined. We address the issue of fossil preservation, superbly detailed for some of the fish and soft-bodied invertebrates, which cannot be accounted for by persistent anoxic bottom conditions. Select features of the fossils implicate environmental conditions causing simultaneous asphyxiation and burial of organisms. The organic-rich sediments throughout the central basin facies are rhythmically alternating microturbidites. Our analyses suggest that these microturbidites were principally generated during summer mon-soonal storms by carrying sheetwash-eroded and/or resuspended sediments over a pycnocline. The cascading organic-charged sediments of the detached turbidity flows would absorb oxygen as they descended, thereby suffocating and burying animals situated below the pycnocline. Seasonal climatic vari-ability would have provided the cycling between vertically mixed to density-stratified water column. These dynamics are likely to have promoted the high biodiversity of the bay, would have produced the rhythmic repetition of microturbidites that characterize the Bear Gulch Limestone, and provide a compelling explanation for the detailed preservation of its fossils.
Details of the crustacean's legs, antennae and gills can be made out in the fossil. Credit: Bicknell et al.
The cyclid paper "Gregarious behaviour in Carboniferous cyclidan crustaceans" was published in The Royal Society's journal Biology Letters. There are 50 articulated cyclid, Schramine montanaensis, specimens on the plate, which show appendages and possible gill preservation.
Abstract
Gregarious behaviours in modern and fossil arthropods are commonly associated with defensive strategies, mass moulting and synchronous reproduction. Such behaviour is scarcely documented in the crustacean fossil record. Identifying clusters in extinct Pancrustacea is, therefore, important for understanding the evolutionary history and origin of crustacean gregariousness. Cyclida, an order of extinct, enigmatic pancrustaceans that have been subject to limited palaeoecological examination, represents an ideal group for testing the presence of gregarious behaviour. Here, we report a cluster of 50 Schramine montanaensis individuals from the Serpukhovian-aged Bear Gulch Limestone of Montana, USA, expanding the exceptionally rare record of cyclidan aggregations. The presence of articulated specimens with appendages and possible gill preservation supports the interpretation of carcasses that were preserved during a rapid burial event. We propose that this cluster records either a mass moulting event or clustering for shelter, representing one of the oldest records of crustacean gregariousness. These findings provide important insights into cyclidan life modes and ecological interactions in Carboniferous marine environments.
Close up of Schramine montanaensis in figure 1 showing anatomical information. (A) Individual showing possible gill structures (grey arrow). (B) Individual
showing antennules (black arrow) and walking legs (white arrows). (C) Individual showing antennules (black arrow) and walking legs (white arrows). All images under
simultaneous UV A, UV B and UV C lighting. Scale bars: all 2.0 mm.
Phys.org has a nice summary by one of the paper's authors.
The slab was found in the Bear Gulch Limestone of Montana, U.S.. These rocks are renowned for preserving fossils in fine detail, including the remains of sharks, worms and even an octopus ancestor named after former US President Joe Biden.
They're the last remnants of a marine ecosystem that existed over 320 million years ago in a monsoon climate of alternating dry seasons and heavy rain. During the rainy season, vast amounts of sediment were swept into the bay and buried anything unfortunate enough to be caught in its path.
This process was so quick that the bodies of the animals were rapidly cut off from oxygen, which stopped their remains from decaying as quickly. As a result, their soft tissues fossilized rather than rotting away.
In the case of the cyclidans, details of their walking legs, antennae and gills can still be made out in the rock. This allowed paleontologists to identify the crustaceans as the species Schramine montanaensis and investigate what they were up to.
"There's no sign that the cyclidans were washed together as they're not arranged with a preferred orientation as you'd expect from a current," Greg says. "As a result, we can be confident that they've clustered together for a behavioral reason. That's not unexpected, as living crustaceans are known to aggregate for many different reasons."
