Three young shrimp went house seeking around 100 million years ago. They picked a huge clam, not the largest on the block, but comfortable at approximately 10 inches across, maybe to avoid predators far from protecting coral reefs.

They made their home, only to be rapidly engulfed in sludge and mud. Their ideal refuge had turned into a grave. They sat there until 2016, when an Australian farmer discovered them. The three shrimp, measuring around 1.2 inches long, are now on display at Australia's Kronosaurus Korner museum.

The fossil, which was just published in Palaeogeography, Palaeoclimatology, and Palaeoecology, is the oldest example of a shrimp using another species (or creature's house) for protection, something that lives shrimp still do today. Many animals, both on land and in the waters, exhibit this behavior, known as inquilinism.

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Ancient Shrimps

Because the fossilized shrimp were kept entire, they were most likely living in the shell when they were quickly engulfed in mud, maybe during an earthquake or a severe storm. They would not have been intact if they had washed into the shell after they perished. "Shrimp are rather fragile," says René Fraaije, director of the Netherlands' Oertijdmuseum natural history museum, who was not involved in the research. "It had to have been a live animal if you discover entire specimens with the carapace, tail, and legs linked to one other."

The shrimp may have rushed into the clam to lay eggs or molt, but no evidence of either action has been found. They could have been seeking shelter from the storm that eventually buried them, but nailing down a short sequence of events like that "without a time machine" is nearly impossible, according to Russell Bicknell, a paleontologist at the University of New England in Australia and the study's lead author.

How Ancient Shrimps Survived

According to Bicknell, the shrimp were likely acting on a basic survival instinct: sheltering from predators. "Shrimp wasn't at the top of the food chain," he explains. "Almost anything, except for filter-feeding bivalves, could have gotten a hold of these little creatures."

A Great Find

The find is the latest in a long line of fossilized invertebrates who took sanctuary from other species. It shows researchers that inquilinism has been practiced by certain shrimp for at least 100 million years.

"This is a huge find," says Ninon Robin, a paleontologist at the Royal Belgian Institute of Natural Sciences who was not involved in the research. "It's unusual to come across examples like these that are associated." It's a stroke of good fortune."

The fossil retains early evidence of the animals learning to survive on the bottom of the shrimp did indeed go within the clam to hide from predators or environmental upheaval. "They've been adjusting to this very specialized environment from the beginning," Robin explains. "It was the only way they could survive."

Bicknell is ecstatic by the discovery. "I adore that these accidental fossils, these needle-in-a-haystack objects, have been preserved," he adds. "They're almost time capsules... that provide us a pretty wonderful glimpse into how individuals of an extinct ecosystem interacted with one another," says the researcher.

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