Archaeology Substance eyes: The visual device these days’s horseshoe crabs goes back 400 million years

The eyes of modern horseshoe crabs include substances, so-called ommatidia. Unlike, for example, insects that have substance eyes with an easy lens, the ommatidia of horseshoe crabs are geared up with a lens cylinder that continually refracts light and transfers it to the sensory cells.

These sensory cells are grouped in the type of a rosette around a central light conductor, the rhabdom, which belongs to the sensory cells and converts light signals into nerve signals to send them to the main nervous system. At the centre of this ‘light transmitter’ in horseshoe crabs is a highly specialized cell end, which can connect the signals of neighbouring substances in such a way that the crab views shapes more clearly. This can be particularly useful in conditions of low exposure under water. In the cross-section of the ommatidium, it is possible to determine the end of this specialized cell as a bright point in the centre of the rhabdom.

Brigitte Schoenemann used electron microscopes to take a look at fossil Jaekelopterus rhenaniae specimens to discover whether the substance eyes of the giant scorpion and the associated horseshoe crabs are comparable or whether they are more similar to insect or crustacean eyes. She discovered the very same structures as in horseshoe crabs. Lens cylinders, sensory cells and even the highly specialized cells were plainly noticeable.

‘ This bright spot comes from an unique cell that only takes place in horseshoe crabs today, but obviously already existed in eurypterida,’ discussed Schoenemann. ‘The structures of the systems are identical. It follows that extremely most likely this sort of contrast enhancement currently developed more than 400 million years earlier,’ she added. Jaekelopterus probably hunted placoderm. Here, its visual apparatus was clearly an advantage in the dirty seawater.

Sea scorpions, which initially appeared 470 million years back, passed away out about 250 million years ago, at the end of the Permian age– in addition to about 95 percent of all marine life. Some specimens were big oceanic predators, such as Jaekelopterus rhenaniae It reached a length of 2.5 meters and came from the household of eurypterida, the extinct relatives of the horseshoe crab. Eurypterida are arthropods, which come from the subphylum Chelicerata, and are for that reason connected to spiders and scorpions.

Among the arthropods there are two large groups: mandibulates (crustaceans, pests, trilobites) and chelicerates (arachnid animals such as sea scorpions). Over the last few years, Schoenemann has actually been able to clarify the eye structures of numerous trilobite species and to make decisive contributions to research study into the development of the substance eye. ‘Till recently, researchers believed that soft tissues do not fossilize. Thus these parts of specimens were not taken a look at until not so long ago’, she concluded.

The new findings on the eye of the sea scorpion are crucial for the development of the compound eyes not just of chelicerates, however also for determining the position of sea scorpions in the pedigree of these animals and for the contrast with the eyes of the related group of mandibulates.

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