2016-11-11 11:48:25
Just over 160 million years ago, in the middle of the Jurassic, the Ardèche department in south-eastern France was covered by a sea a few hundred meters deep. A nearby fault with hydrothermal activity of black fumaroles periodically released toxic sulphurous gases that caused the death of many animals, which fell to the muddy bottom and were very quickly covered by sediments, in an environment poor in oxygen and very calm, at a depth sufficient to that the effect of the waves was not felt, so its state of conservation is exceptional, in three dimensions and with a very fine level of detail, in some cases down to the cellular level.
In the century XIX there was in the same place, already on the mainland, an iron mine from which up to 60,000 tons of mineral were extracted per year. There the first fossils of crustaceans and fish appeared from what would later be known as the La Voulte-sur-Rhône site. Later they have also found bivalves, cephalopods, sea spiders, sponges, sea lilies, brittle stars…
Among the inhabitants of that Mesozoic sea, Dollocaris stands out, a predator between 5 and 20 centimeters in length, with an enormous head and the body enclosed in a shell from which three pairs of long legs emerge with pincers that allow it to catch its prey. among which are small crustaceans, fish… Dollocaris’s eyes are huge, occupying the entire face of the animal and a quarter of its total length. Dollocaris is unique to the La Voulte-sur-Rhône site, it has not been found anywhere else. It belongs to the group of thylacocephali.
The thylacocephalians are a group of arthropods between 1.5 and 25 centimeters in length, with large bulbous eyes in a frontal position, with a voluminous and laterally flattened carapace that protects the entire body, three pairs of long prehensile legs in front of the trunk and a series of eight to twenty small filamentous swimming legs at the rear, tapering towards the rear. They are reminiscent of a bathyscaphe due to their shape, with its rigid body, its globose cabin and its manipulating pincers. The lateral surface of the carapace of thylacocephalians may be ornamented with grooves and ridges. Many species have a rostrum, a beak-like structure that extends forward between the eyes, and also a similar extension at the back. They also generally have eight pairs of highly developed gills on the underside of the trunk, similar to those of decapod crustaceans.
Little else is known of its anatomy; the shell prevents knowing its internal structure: mouth, digestive system, segmentation, exact identity of the appendages… The latter, according to different interpretations, could derive from antennae, jaws, thoracic legs… All this makes their classification very difficult; They are often associated with crustaceans, but their parentage is unclear.
The oldest known thylacocephalians date from the Lower Cambrian, with the group surviving until the Late Cretaceous, some four hundred million years later; They went extinct with the dinosaurs. Their fossils have been found all over the world.
In 2016, a team of researchers from various centers in France, Germany and the United Kingdom carried out a detailed study of Dollocaris using X-ray microtomography, which has greatly increased our knowledge of this species. The compound eyes of Dollocaris are made up of regular hexagonal facets with a density of about five hundred per square millimeter, meaning that each eye was made up of about 18,000 hexagonal facets, more than any other known fossil, and second only to living animals. for the dragonflies. The fossils studied are so well preserved that it is possible to distinguish the internal structure of each facet: the external corneal lens, the crystalline lens, and the photosensitive cells, called retinulae. Dollocaris had excellent eyesight, which allowed him to detect very small objects.
The circulatory and respiratory systems indicate that Dollocaris was a fairly active animal. The mouth, located in the lower part of the head, between the eyes, communicates with a digestive system similar to that of modern crustaceans. Dollocaris was a visual hunter. It was not a fast swimmer, and probably hunted by ambush, concealed on rocky reefs. However, its excellent vision, adapted to fairly bright environments, does not match the depth at which the deposit was formed, confirmed by other fossils typical of deep and dark waters. Either Dollocaris swam better than we think and hunted in open water, and fell to the bottom to die; or made frequent vertical displacements; or else it had, like certain species of bees and night wasps of today, neural circuits capable of processing visual signals and increasing its sensitivity in the dark. But fossils, for now, can’t tell us that much.
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reticular infiltrate
Reticular Infiltrator is the first novel in the trilogy The Saga of the Borelians. Do you want to see how it starts? Here you can read the first two chapters.
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