Fossils show mayflies swarmed 115 million years ago
Several years ago my daughter and I took a minivacation to Kelleys Island, home of the famous Glacial Grooves. While we were there the island experienced an explosion of mayflies – they were everywhere.
A recent report from a famous fossil site in Brazil indicates they’ve been doing that for a long time.
The mayfly fossils were found in the Crato Formation, which formed in large lakes in what is now northeastern Brazil during the Cretaceous Period, the last period of the non-avian dinosaurs. The lakes may have been 30 miles wide and 60 miles long.
The Crato Formation is an example of what is known as a lagerstatte, a site that has a lot of fossils or fossils that are preserved exceptionally well, sometimes (but rarely) both.
Another lagerstatte you might have heard of is the Burgess Shale, a 500-million-year-old Middle Cambrian site in the Canadian Rockies of British Columbia. Another is the 150-million-year-old Solnhofen Limestone of the Jurassic Period from the Bavarian region of Germany, source of the only known specimens of Archaeopteryx, the first bird.
The Crato Formation certainly qualifies as a lagerstatte. The fossil fish are so well preserved that, once removed from the enclosing rock, they look like the remains of a fish dinner you had the night before rather than something that lived 115 million years ago. Even their muscle fibers are preserved.
Although fish are the most common fossil vertebrates with 25 species, it also has the exceptionally preserved remains of turtles, crocodiles, birds (including their feathers), and pterosaurs (flying reptiles).
Invertebrates include spiders, scorpions, crayfish, and insects, over 350 species, some of which are so well preserved you can even see the color patterns on their wings. The insects include cockroaches, butterflies and moths, dragonflies, flies, wasps, and bees.
The recent research looked at one thin layer on which, in a small area, were the remains of over 40 mayfly larvae, each about 1 centimeter long. They were not close to molting into adults and flying away, and there was no preferential orientation, so they weren’t washed in by currents.
The only other animal fossilized on that layer were 18 skeletons of very small fish. Layers immediately above and below had salt crystals and cubic impressions of salt crystals, suggesting drought and high salinity.
That was supported by the fossils of plants, which were adapted to dry conditions with small leaves and thick cuticles.
The researchers say the mayfly larvae probably died due to low water levels and low oxygen levels, compounded by low oxygen levels produced by the high temperatures.
In rocks above and below the mayfly accumulation were numerous layers with mass deaths of small fish — 9 such episodes in 10 feet of rock.
Similar mayfly mass mortalities have been found in Cretaceous rocks in Mongolia, northern China, Russia, and northwest Africa as well as rocks from later times. I guess for mayflies, the more things change the more things stay the same.
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.
gnidovec.1@osu.edu