Dinwoody and Lingula
Tectonics meets Paleontology
The photo above shows the Triassic Dinwoody formation west of Melrose, Montana. These rocks were deposited about 251 million years ago, along the marine margin of North America.
The Dinwoody is a thin-bedded shaly (mud-bearing) limestone. The thickest, chunkiest pieces in this photo are parts of the thickest beds, at perhaps 4 inches or so thick. Most of the rock is much thinner, broken apart on bedding planes in the shaliest parts.
The thin, shaly beds of the Dinwoody make this package of rocks incompetent, meaning it is relatively weak in terms of resistance to deformation. Although there is plenty of obvious brittle deformation in this photo, revealed by the broken, brick-like segments of all the pieces but especially in the thicker (more competent) zones, some of the most thin-bedded parts have practically flowed as they deformed. This “flow” is not in the same sense as a liquid (or even a mud slurry), since these rocks were in the solid state, but in places the broken thin beds really do more or less flow, like spilling a piggy bank full of coins onto a surface, or letting a deck of cards slip from your hands to the table.
That flow, together with the brittle breaking, makes this outcrop chaotic. The lines of possible faults and fold axes in the lower photo are just arm-waving approximations of some of the most evident features. To an extent, it would be fair to think of many of the breaks in the rock as little faults with their own small displacement or offset from their neighbors, though many are just cracks or joints.
The geologic map above of the Dillon Quad (USGS Map I-1803-H) shows this location (the red star) on the west flank of one of two small anticlines (up-arched folds) that formed about 70 to 50 million years ago, 180 to 200 million years after the Dinwoody was deposited. The anticlines (red lines) look like they were formed by the northwest-trending faults on the map, and the light blue cores of those folds are relatively competent, massive, resistant rocks including sandstone, chert, and thick-bedded limestone. The strength or competence of that package contributes to the chaotic “reaction” of the Dinwoody, which bends and breaks into all these pieces rather than folding as a unit like the other formations probably do.
Much of this deformation is probably tectonic and related to the larger folding that this outcrop is part of. But some of it is undoubtedly recent, related to the rocks creeping, breaking, as the whole works slides slowly toward the left following the slope of the ground surface. Some of the breakage and folding is probably even related to the creation of the road cut, which allows the rocks to slide and break under the pull of gravity on the face of the outcrop.
The Dinwoody formation was named for exposures in Dinwoody Canyon on the flank of the Wind River Mountains near Dubois, Wyoming. The canyon was named for William Dinwiddie, a calvary officer at nearby Ft. Washakie. The English surname Dinwiddie appears to derive from Gaelic “dun,” a hill, or from Welsh “din,” forest + “gwydd,” bushes.
If you look carefully at the Dinwoody rocks, you’ll see survivors of the greatest extinction in earth history. Well, not these specific individuals – they didn’t even survive a storm surge that broke their shells up on a muddy carbonate shelf during early Triassic time, as the Dinwoody shaly limestone was deposited.
But the Lingula group of brachiopods, represented here by these little blue-black flakes (broken shells), has survived for about 520 million years to this day with little change in outward appearance, structure, or behavior.
The rock holding the Lingula shell fragments is the Triassic Dinwoody formation, which represents a rapid transgression of the sea eastward onto the western edge of the North American Craton during Early Triassic time. In fact, the very earliest Triassic time, immediately after the “great dying” extinction at the end of the Permian Period, now dated quite accurately to 251.91 million years ago, ± about 30,000 years, which likely represents the span of the “event,” 60,000 years or so — it was not an instant in time, and factors including volcanism may have contributed to the “event” for several hundred thousand years, at least.
Hoffman and others (2013, A new Paleoecological look at the Dinwoody Formation: Journal of Paleontology 87:854–880) refer these rocks to the Griesbachian sub-stage of the Induan Stage of the Triassic, dated to 251.9 to 251.2 million years ago, the first 700,000 years right after the Permian-Triassic extinction.
Lingula normally lives in low-oxygen near-shore environments, unfavorable to many organisms, but in the Dinwoody it dominates multiple environments, from near-shore muds to more open carbonate seas. For the first few hundred thousand years after the end-Permian extinction, this opportunistic survivor apparently invaded ecological niches that were abandoned by animals decimated or destroyed by the extinction. Lingula declined to its normal ecological obscurity later in the early Triassic and continued to survive quietly to the present day.
Although Lingula has barely changed in overall appearance and structure over its 520-million-year evolutionary span, leading Darwin and others to call it a “living fossil,” recent genome analysis (the first decoding of a brachiopod genome; Luo and others, 2015, Nature Communications 6:8301) indicates remarkably rapid change at the gene level.
As one of the first animals to begin using biomineralization to make protective shells 520 million years ago, Lingula took a different evolutionary path from most invertebrates. Lingula shells are slightly flexible combinations of around 50% calcium phosphate (the mineral apatite, which vertebrates have in bones and teeth) and 50% organic collagen, to make something like a hard, phosphatic fingernail. Chemical alteration of the organic-phosphatic material over time is what makes the shell fragments in my specimen look blue-black. Most other invertebrates (including many lineages of brachiopods other than Lingula) use calcium carbonate (either of the minerals calcite or aragonite) to make their shells and skeletons. Sponges and some other animals make silica, quartz, for supporting structures.
Luo and others (2015, cited above, using genetic evidence that’s far beyond my understanding) do not find a relationship between Lingula’s phosphatic shells and phosphate manufacture in vertebrate bones; they believe it to likely be a case of parallel evolution.
I collected these Lingulas in the Dinwoody on 5/16/2020. The shells have a distinctive sheen making them (and the Dinwoody) easy to recognize. Collected along the Trapper Creek Road about 3.5 miles west of Melrose, Montana. Cat. No. 1716.
The type species, Lingula anatina, was described by Lamarck in 1801. “Lingula” probably comes from Latin for “small tongue,” but it might be from the word for “spoon.” “Anatina” probably comes from Latin for “belonging to a duck,” for its similarity to a duck bill, but it might be from a French word for the goose barnacle, which Lingula also resembles.
And now, the weather report for Novosibirsk, Russia, 252 million years ago, near the end of the Permian Period. Generally cool conditions will be ameliorated by warm summer monsoonal flow off the Paleo-Tethys Ocean. Downslope winds from the Siberian Platform will bring hot acidic smoke from the ongoing volcanic conditions there, and conditions are expected to remain unpleasant (and fatal in many places) for at least 80,000 years.
Great weather report!