436-Million-Year-Old Scorpion Was Among the Planet's First Air Breathers

By George Dvorsky on at

A newly described scorpion dating back to the early Silurian period is answering important questions about the first arachnids and the adaptations that enabled some of the earliest animals on Earth to migrate from aquatic to terrestrial habitats.

New research published in Scientific Reports describes Parioscorpio venator – now the oldest scorpion in the fossil record. Its name means “progenitor scorpion hunter”, and it lived during the early Silurian period sometime between 437.5 and 436.5 million years ago. The previous record for the world’s oldest scorpion fossil belonged to Scotland’s Dolichophonus loudonensis, which at 434 million years old is around 1 million to 3 million years younger than Parioscorpio venator.

Two fossils of this newly described species were uncovered at the former site of a shallow, tropical sea. It lived alongside other marine animals such as trilobites, cephalopods, worms and other arthropods. Parioscorpio venator was an aquatic creature but, as the new research shows, it was also capable of spending some time on land, as evidenced by its unique anatomy. Fascinatingly, it’s now one of the earliest air breathers known to science in addition to being the oldest scorpion in the fossil record.

This discovery is important because scorpions are among the first animals to fully transition to a terrestrial mode of life. Accordingly, Parioscorpio venator shows which adaptations likely enabled scorpions and other animals to make the paradigmatic leap from water to land.

A zoomed-in section of the fossil (left), a view of its internal organs (centre), and a reconstructed view of the newly described scorpion. Image: A. J. Wendruff et al., 2020.

The fossils of Parioscorpio venator were found in Wisconsin back in 1985, after which they sat unstudied for nearly 35 years at the University of Wisconsin. Recognising the potential significance of these fossils, palaeontologists Loren Babcock from Ohio State University and Andrew Wendruff from Otterbein University decided to take a closer look.

The Parioscorpio fossils were way too old for carbon dating, so Babcock and Wendruff turned to a technique called biostratigraphy.

“We examined specific microfossils which occur in small time ranges,” explained Wendruff in an email to Gizmodo. “This allowed us to confidently compare the age of this scorpion with the previous oldest one.”

To do this, the scientists searched for conodont microfossils – extinct eel-like creatures whose remains can be used to constrain and identify geological time periods. In this case, a particularly helpful conodont belonged to a species known as Pterospathodus eopennatus, which lived at the same time as Parioscorpio. Its remains were found in the same stratigraphic layer as Parioscorpio, which “gives us a rather precise position corresponding to an age of 437.5 to 436.5 million years ago,” said Babcock in an email to Gizmodo.

Using microscopes and high-resolution imagery, the palaeontologist studied the fossils in detail. Parioscorpio measured around 2.5 centimetres (0.98 inches) in length and exhibited characteristics seen in other early animals, such as compound eyes. But it also had features seen in living scorpions, such as a stinger on the tip of its tail. Incredibly, the scientists could see some internal organs on the fossils, including a small chamber where the scorpion’s venom was stored.

Importantly, the authors uncovered a possible anatomical feature that may have allowed this scorpion – and possibly other early animals (including vertebrates) – to transition from marine to terrestrial habitats. Parioscorpio didn’t have lungs or gills, but it did feature a narrow, hourglass-shaped structure similar to the circulatory and respiratory systems seen in modern scorpions and also horseshoe crabs, according to the research. The authors speculate that early scorpions were able to stay on land for extended periods of time, breathing air in a manner reminiscent of modern horseshoe crabs.

Wendruff said this scorpion “was found in what was an ancient nearshore environment with other organisms that lived in the ocean”, but “the preserved respiratory and cardiovascular systems in the fossil were just like modern scorpions which live on land and breathe air”. Together, this evidence points to a hybrid existence in which the creature could transition between land and water. Parioscorpio, therefore, is an exceptionally early air breather, whose adaptations represented “a major step in colonising land”, Wendruff told Gizmodo .

Another remarkable aspect of this discovery is how this supremely ancient scorpion resembles those living today. Obviously there are some important differences, but it’s clear that scorpions stumbled upon a highly successful evolutionary strategy very early on. We asked Babcock about this, and why the scorpion form is so successful.

“Good question,” he said. “Sometimes evolutionary success is dictated by what creature first breaks the adaptive barrier, such as being among the first to become land-capable. The biological model may have been tested and ‘perfected’ early in their evolutionary history, and didn’t need much tweaking afterward,” said Babcock, adding that this is “just a guess”. Today, some diversity does exist among scorpions, “but the basic body plan has changed remarkably little since the Devonian Period”, he said.

Keep this in mind the next time you cross paths with a scorpion. Their ancestors were early innovators, representing some of our planet’s first terrestrial, air-breathing creatures. This latest study further solidifies their place as evolutionary legends.

All images: A. J. Wendruff et al., 2020.