Mineralogy meets Paleontology
Pyritized brachiopods
A quarry west of Toledo, Ohio, USA is famous for well-preserved trilobites and many other fossils dating to the Givetian age, Middle Devonian, about 385 million years ago. The Silica Formation near Sylvania, Ohio, is a limy mud rock, so fine-grained that it is a great medium for preserving fossils, especially the trilobite Phacops rana milleri, now renamed Eldredgeops milleri.
When I was there in 1969, I only found fragments of trilobites, but I also found the pyritized brachiopods at the top of this post.
Brachiopods survive to this day, but they proliferated in the Ordovician to Devonian periods of the Paleozoic Era. During the Devonian, early stages of the Appalachian Mountains were rising in what is now New York northward into the Canadian Maritimes and Scotland and Norway, which were adjacent to North America at that time.
Ohio and the Midwestern USA were about 30° south of the equator in mid-Devonian time, and covered by warm, shallow seas adjacent to lowlands that shed the limy mud into the adjacent waters. Although muddy, the environment supported abundant life; at the quarry at Sylvania, Ohio, parts of the deposit are practically fossil hash.
Pyrite in sediments is usually related to anoxic environments, so it’s a little challenging to associate it with the abundant life. Possibly the iron and sulfur percolated through the rock after (or during) lithification, with the pyrite (iron sulfide) selectively replacing the calcite in the fossil shells. Whatever the exact process, it must have been on almost a molecular scale, because not just shells but also structures within them are faithfully preserved.
The brachidium in brachiopods is a coiled structure that supports the lophophore, the filamentous, hairy organ the animal used for suspension feeding. The typically spiral brachidium was calcareous, but delicate and fragile, so its preservation in fossils almost 400 million years old is remarkable. The replacement must have been comparable to silica replacement in wood, resulting in petrified wood in which detailed features at the cellular level are preserved.
A possibility for an anoxic environment whose iron and sulfur might have percolated into the Givetian (Middle Devonian) rocks at Sylvania is the anoxic period during the Late Devonian (Frasnian-Famennian ages), about 372 million years ago, only 10-15 million years after the Silica Formation rocks were deposited with their abundant fossils. The Frasnian-Famennian “event” was one of the “big five” mass extinctions in earth history, in which about 19% of all families, 50% of genera, and 75% of species became extinct. Brachiopods were especially hard hit, with many large groups disappearing entirely. They did recover, but never to their level of success in the Ordovician to Devonian.
The ultimate causes of the extinctions are complex and debated, but there is little doubt that the nearly world-wide anoxia in the marine environment was one major proximal cause of the extinctions. You can read (or listen to) more about the end-Devonian extinctions at The History of the Earth Calendar .
“Brachiopod” means “arm-foot” from Greek, for the brachidium (“little arm”) that extended from the body out of the shell when the animal was feeding. “Pyrite” is from Greek puritēs ‘of fire,’ because it generates sparks when struck against something hard.
We have some Permian spirifids in Tasmania that have quartz crystals inside, retaining the structure. Some are said to include fluorite crystals too, but I haven't seen any of those.
Excellent photo of brachiopid brachidium.
Beautiful specimen.
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