The extremely well preserved fossils of over 200 shrimp killed in a "mass mortality" event around 95 million years ago has been discovered by scientists. The shrimp were killed while searching for females to mate with in a water vent in what is now Columbia. The level of preservation means researchers were able to see the finest anatomical details of these ancient creatures, including their eyes, mouths and hairs.
The comma shrimp fossils were found in a rock from the mid-Cretaceous period. They are estimated to be between 90 and 95 million years ago—when the dinosaurs still walked the Earth.
Comma shrimp—also known as cumacea—are small crustaceans that today are found in marine environments across the globe, with well over 1,000 species within the order.
However, despite their abundance they have one of the poorest fossil records of any marine arthropods, so our understanding of their origins has been limited.
In a study published in the Proceedings of the Royal Society B: Biological Sciences, Yale's Javier Luque and Sarah Gerken from the University of Alaska Anchorage have announced the discovery of a swarm of ancient comma shrimp ancestors with "exceptional preservation" that helps fill in over 160 million years of the shrimp fossil record.
Luque first found the fossils in 2005 during a field expedition to the Andes. "It was already late in the afternoon, and I just decided to swing my hammer one more time to check the rocks we were sitting on," he told Newsweek. "It was there when I discovered a layer in the rock with hundreds to thousands of fossils of marine creatures, most of them crustaceans of different shapes and sizes, and among them the comma shrimp that is the focus of our study today."
The rock was found to contain over 200 individuals measuring between 0.2 and 03 inches in length. Most were found to have "large and conspicuous antennal flagella"—a feature characteristic of males. There were just a few females and no juveniles at all, something the researchers say is "remarkable."
"It is common for adult males to swarm at night in search of mates in shallow water species, and cumaceans found in night time plankton tows and light traps tend to be exclusively or strongly biased towards adult males," Gerken said.
They believe these ancient shrimp were in search of females when something happened that caused them all to die. They then gently drifted down to their watery grave below, where they would be preserved in the soft sediment. "As for the trigger of the mortality event, we are still unsure," Luque said. "Perhaps something in the water column, like an abrupt change in water temperature that shocked them down, an episode of low oxygenation, or heavy metals, etc. We are working on it."
The researchers say these shrimp, which have been named Eobodotria muisca, represent "the first and oldest unambiguous fossil crown cumacean reported to date."
Concluding, they said the discovery shows this specific family of comma shrimp were present by the mid-Cretaceous period, and potentially even earlier. "Our findings bridge a gap of more than 160 million years in the evolution of true comma shrimp," the study says.
"Although our understanding of the origins and relationships of comma shrimp are far from settled, these findings provide hypotheses about the early evolution of crown Cumacea and the timing of divergence of the cumacean clade."
Luque added: "This not only allowed us to compare Eobodotria in great detail with every known group of comma shrimp, but to recognize it as a long time extinct member of one of the most species-rich families of comma shrimp today, and how little has their body form changed in nearly a 100 million years."
"I hope that this research spurs recognition of more cumacean fossils, so we can add to their evolutionary history," Gerken said.
This article has been updated to include comments from Javier Luque and Sarah Gerken. Images of the fossils have also been added.
