Testicle, glands, digestive tract intact in 50-million-year-old insect fossil

The "Arethaea solterae" specimen died millions of years ago and in a remarkable turn of events, many of its "soft" body parts are still preserved in its fossil.

Present-day katydids of the genus Arethaea, like the one pictured here, have an internal structure similar to that of the fossils. (Salvador Vitanza, University of Illinois)

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When scientists find fossils, it is usually the hard parts of the animals’ bodies that are preserved. But 50 million years ago, a katydid—relative of modern grasshoppers—sank to the bottom of a lake in what is now northwestern Colorado and was buried in fine sediments. It remained there and its compressed fossil remains were discovered recently, with some surprise parts included.

“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. 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.” Sam Heads, lead author of new research published in the journal Palaeoentomology, in a press statement.

The katydid falls into the genus Arethaea, a group that today contains “thread-legged katydids” that are called that due to their very slender legs, according to Heads. The fossil specimen belongs to an extinct but newly-discovered species, according to the University of Illinois at Urbana-Champaign, where Heads leads the Centre for Palaeontology. It has been named Arethaea solterae.

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What makes the fossil find especially remarkable is the rare glimpse into the soft internal organs of a 50-million-year-old fossil.

“Part of the digestive tract is preserved, a part of the midgut we call the ventriculus. That’s not so unusual; we have other specimens from this location that have gut traces, so I wasn’t particularly struck by that,” added Heads.

But when the scientists looked at the specimen under a microscope, he saw evidence of other internal structures that he was not expecting to be preserved. The specimen contained traces of fibres that make up muscles associated with the insect’s wings or flanks. There was also some undifferentiated tissue known as “fat body,” which is an organ that aids in insect metabolism, according to the university.

The surprises just began there. “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. 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,” explained Heads.

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The researchers then dissected many katyidid species that belonged to the same genus to match what they were seeing in the fossil. They found that they all look the ame, the accessory glands, the “ventriculus” and all other organs.