An article published in the journal “Current Biology” reports a study on an almost complete fossil of Adelophthalmus pyrrhae, a species that lived about 340 million years ago, in the Carboniferous period, in today’s France. This species belongs to the eurypterid group, the arthropods commonly known as sea scorpions. A team of researchers led by Professor James Lamsdell of West Virginia University subjected it to a CT-scan that allowed them to create a 3D reconstruction. This made it possible to carry out a detailed examination that brought to light some anatomical characteristics of its respiratory system that indicate that it could breathe air. These conclusions suggest that the ancestors of the arachnids, related to the eurypterids, were also semi-terrestrial in the Cambrian and Ordovician periods.
Eurypterids (Eurypterida) are an extinct group of arthropods whose first fossil traces date back to about 467 million years ago, in the Ordovician period, and may have emerged in the previous period, the Cambrian. The nickname sea scorpions is due to the appearance similar to that of scorpions and to the fact that they were animals that lived at least mainly in water, but as a description it’s incorrect. The eurypterid group is separate from that of the arachnids and paleontological studies indicate that they lived in freshwater.
The genus Adelophthalmus was first described in 1854 with the species Adelophthalmus Granosus. Other species attributed to this genus have been discovered over time in various continents indicating a considerable diffusion and a period of time in which they have existed of over 120 million years. Many fossils from those eras are fragmentary, a typical problem in the field of paleontology, and this makes it difficult to understand the relationships among the various species. At times, this also leads to discussions regarding the attribution of fossils to a certain species and the identification of a new species.
An almost complete fossil was discovered in France and kept in a museum in Glasgow, Scotland for the past 30 years. James Lamsdell read about this fossil 25 years ago, during his doctoral studies. At the time, the exams suggested that it was a marine animal that could occasionally move on land, but there were no clues to suggest it could breathe air. The closest relatives to the eurypterids existing today are horseshoe crabs, which lay their eggs on land but can’t breathe air.
Many years later, James Lamsdell came up with the idea of using modern technology to re-examine a long-known fossil, as has been happening more and more often in recent years. To do this, he talked to his colleague Victoria McCoy of the University of Wisconsin in Milwaukee. Together with Opal Perron-Feller of Oberlin College and Melanie Hopkins of the American Museum of Natural History, they subjected the fossil to a CT-scan that allowed them to create a 3D reconstruction that was examined in detail with very interesting results.
In particular, the researchers’ interest focused on the trabeculae, structures that connect different parts of the Adelophthalmus pyrrhae specimen’s gills. These are structures that still exist today in spiders and scorpions. In this species of eurypterid’s case, they allowed the gills not to collapse when they came out of the water and therefore to breathe air.
The presence of trabeculae in both this sea scorpion and modern arachnids indicates that they may have developed in common ancestors. As horseshoe crabs still do, it’s possible that moving to the mainland was initially advantageous to lay eggs in a quieter place. That’s because that behavior may have started before the mainland was colonized by many animal species.
Once again, the application of modern technologies in the field of paleontology offered new information on a fossil, in this case, known for decades. James Lamsdell intends to continue studying the Adelophthalmus pyrrhae specimen to better understand how it lived, for example by examining the characteristics of its legs to obtain information on their use. An almost complete specimen can be very useful to paleontologists.