Treasure hunt ends with a stunning fossil of a flying insect

Richard Knecht was lost in a swamp. His hunt for fossils was not going according to plan.

One of Knecht’s students had found an unpublished masters thesis from 1929, which described a nearby area that was supposedly rich in fossils. Knecht set out to find the site, a quest that led to a thick wooded field behind a Massachusetts strip mall. That was when he got lost. When he finally emerged from the swamp, he found an outcrop of shale and sandstone that matched the student’s description. He took out his hammer, and immediately uncovered an exceptional treasure.

Nestled within two sandstone slabs, Knecht found an imprint that had been made by a small flying insect, much like a modern mayfly, around 310 million years ago. It might have been thirsty. It might have been blown off course. Either way, it landed in a shallow pool of water, settling on the muddy bottom before taking off again.

Its brief moments in the sediment left an incredibly detailed imprint of its body, one that survives to this day, embedded in red sandstone. This was the fossil that greeted Knecht at the end of his misadventure. “Sometimes, in life you think you are lost and it turns out you are not so far from your destination after all!” he muses.

The imprint is the oldest full-body impression of a flying insect. It is so clear that Knecht could tell it was made by a mayfly, one of the earliest groups of flying insects to evolve. The main tube is the insect’s body. You can see the first segments of its six legs protruding outwards, and you can even make out individual segments of its abdomen.

There are no clear impressions of the head or wings, as the insect was probably holding those above the water. However, some of the imprints to the side of the main patch could have been made by the downbeat of flapping wings. That’s not clear, but it’s telling that there aren’t any other imprints around the main one. The animal clearly didn’t walk into place; it landed there.

“The fact that this fossil exists at all is amazing in itself,” says Knecht. “It is nearly impossible to recreate the conditions in a lab and try to get a modern insect to leave such a trace.” For the sediment to have retained the fine details of the insect’s imprint without filling in again, it must have had a very specific combination of moisture content and grain size. “It really was an intersection of perfect conditions and behaviour that led to the creation of this fossil.”

Because the insect’s body hasn’t fossilised, Knecht’s specimen is a “trace fossil”, a fossilised piece of behaviour, like a track or a burrow. Classifying such fossils is a complicated business, because it’s very rare that they preserve enough information to accurately identify their makers. However, they often detailed enough for scientists to group them according to their characteristics, suggesting that different tracks were probably (but not necessarily) made by the same animal. In such cases, the tracks are given genus and species names just as normal body fossils are – these are known as “ichnotaxa”. It’s a fascinating practice, like classifying a shadow or categorising negative space.

Ichnotaxa can also be defined by the groups according to the behaviour their show. For example, Praedichinia is the group of trace fossils that represent hunting behaviour, like drill holes left by killer snails or bite marks left on bones. Repichnia refers to tracks. And Volichnia includes imprints made by flying animals landing on sediment, as with Knecht’s mayfly.

In this case, the mayfly imprint is detailed enough for Knecht to identify the animal that made it, but he finds himself in the strange position of being unable to name it. The International Code of Zoological Nomenclature (ICZN) – the group that governs the naming of life – forbids scientists from assigning a standard two-part species name unless there are body fossils to hand.

The only other option is to give the animal an ichnotaxon, based on its imprint. But Knecht doesn’t see the point. He argues that the purpose of creating ichnotaxa is to allow scientists to talk to discuss their finds effectively. It’s easier to compare different sets of tracks if you know they were made by the same animal, and you can convey that by giving them a shared name.

In this case, there’s no second imprint of a similar mayfly-like animal, it’s very unlikely that people will find another any time soon. Assigning a name to this beautiful rarity would, in Knecht’s view, only create “clutter”. Unless someone finds a second specimen, or the ICZN changes its rules, Knecht’s wondrous find will simply be known by the dreary name of specimen SEMC-F79.

Reference: Knecht, Engel & Benner. 2011. Late Carboniferous paleoichnology reveals the oldest full-body impression of a flying insect. PNAS http://dx.doi.org/10.1073/pnas.1015948108

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