These Spectacularly Preserved Trilobite Fossils Come Complete With Guts, Gills and Legs

Over 300 million years ago, insect-like creatures about the size of a football plowed across seafloors. These Paleozoic sea creatures known as trilobites disappeared during a mass extinction roughly 250 million years ago—leaving behind their armored skeletons in the fossil record. And until now, that was all scientists thought remained of the ancient marine arthropods, reports Michelle Z. Donahue for National Geographic.

Paleontologists discovered 478-million-year-old fossilized trilobites with preserved soft tissues, allowing them to examine the creatures’ guts, gills and legs. Their findings, recently published in the journal Scientific Reports, not only provide insight on trilobite behavior and anatomy, but could explain a mysterious series of trace fossils that have long stumped researchers, reports Laura Geggel for Livescience.com.

There are more than 20,000 species of trilobite previously identified, Donahue reports. But scientists knew little about their patterns of movement or eating habits since soft tissues, such as muscle and skin, usually decay and disappear rapidly over time. So when paleontologists Diego García-Bellido and Juan Carlos Gutiérrez-Marco came across three nearly complete trilobites (Megistaspis hammondi) they were amazed.

The creatures hail from the Fezouata formation in Morocco. They were collected by the Ben Moula family, who have previously provided an impressive array of preserved ancient creatures, Geggel notes. The fossils ended up in the Museo Geominero, a museum of minerals, rocks and fossils in Spain.

Analysis of the spectacularly preserved creatures suggests that the the trilobites had a digestive system with what’s known as midgut gland, which secretes enzymes to help dissolve and digest food. They also have a crop, or a kind of pouch found in modern sediment feeders. Based on this anatomy, the researchers believe that the trilobites foraged for food by sifting through the upper layers of sea sediment, Donahue writes.

“Like modern arthropods, trilobites were very versatile, and to survive they had to exploit their ecological environment to the best of their advantage,” García-Bellido, of the University of Adelaide in Australia, tells Donahue.

One particular anatomical feature that roused attention, however, was the first three pairs of legs located in the head of the Megistaspis hammondi, García-Bellido explains in a press release. The legs have short, strong spines, which differed from the smooth legs at the thorax and tail.

“I said, ‘Where have I seen these marks before?’” García-Bellido tells Donahue. His colleague, Gutiérrez-Marco, versed in trace fossils, realized that the leg spines could be to blame for a set of trace fossils known as Cruziana rugosa.

These fossilized footprints are commonly found in the rocks of supercontinent Gondwana—which was a mashup of modern Africa, South America, India, Australia, the Arabian Peninsula and Antarctica. These particular trace fossils consist of up to 12 parallel scratches, and had been thought to be connected to a trilobite but the evidence was never strong enough, Geggel reports.