This fossil has a testicle

Some things are easier to fossilise than others. Bones, shells and teeth? Easy. 

Insects’ tiny exoskeletons? Despite this hard outer shell, they are still small and fragile. It’s rarer, but it occasionally does happen.

The internal organs like the muscles, digestive tract, and a testicle? Pretty much never.

A new research paper has therefore found a bit of a white whale – a 50-million-year-old bush cricket (katydid) fossil with internal organs and even a testicle.

The research has been published in the journal Palaeoentomology.

“Katydids are very rare in the fossil record, so any new katydid fossil you find represents a new data point in the evolutionary history of katydids,” said University Illinois palaeoentomologist and first author Sam Heads.

“But perhaps the most striking feature of this fossil is the really exceptional, remarkable preservation of internal organs – organs that you just don’t see in fossils.”

The stunning fossil was discovered in the Rio Blanco County in the state of Colorado in the Green River Formation. This is a site that’s well known for species of Orthoptera – grasshoppers, crickets and katydids.

The fossil represents a new species that’s long extinct, which the team has named Arethaea solterae.

Sam Heads, right, who named the specimen after Leellen Solter, left, an insect pathologist and volunteer curator of the insect fossil collection of the PRI Center for Paleontology. Credit: Michelle Hassel

“Obviously, having a fossil species of a modern genus is really significant because it confirms the antiquity of this lineage,” Heads said.

“Now we know that about 50 million years ago, this genus had already evolved and already had a morphology that mimics the grass in which it lives and hides from predators.”

But the most surprising find is the internal organs. The researchers found the digestive tract, as well as an organ that aids in digestion called the ‘fat body’, and a testicle and associated glands.

Although the digestive tract has been found before in other fossils, this is the first time that researchers have found the fat body or the testis in a katydid fossil.

Three views of the fossil insect include impressions of its wings, top; internal structures, middle; and parts of the rear legs, bottom. Credit: Jared Thomas

“There are these little tubules that all seem to connect to a round structure – and that can only be a testis and accessory glands that are associated with the testis,” Heads said.

“That’s just phenomenal. I was not expecting to see that kind of structure preserved in a rock compression. I’ve never seen that before.”

The team even had to dissect several modern-day specimens to double check.

“They look exactly the same,” he added.

“The testis, the accessory glands and the ventriculus were all the same in the present-day katydids. I was just blown away by it. To my knowledge, this is the first example of this level of preservation.”

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