The median eyes of trilobites.

Arthropods typically possess two types of eyes-compound eyes, and the ocellar, so called 'median eyes'. Only trilobites, an important group of arthropods during the Palaeozoic, seem not to possess median eyes. While compound eyes are in focus of many investigations, median eyes are not as well considered.

An overview on trilobite eyes and their functioning.

Great progress has been made during the last decades in understanding visual systems of arthropods living today. Thus it seems worthwhile to review what is known about structure and function of the eyes of trilobites, the most important group of marine arthropods during the Paleozoic. There are three types of compound eyes in trilobites.

Insights into a 429-million-year-old compound eye.

In all arthropods the plesiomorphic (ancestral character state) kind of visual system commonly is considered to be the compound eye. Here we are able to show the excellently preserved internal structures of the compound eye of a 429 Mya old Silurian trilobite, Aulacopleura koninckii (Barrande, 1846). It shows the characteristic elements of a modern apposition eye, consisting of 8 (visible) receptor cells, a rhabdom, a thick lens, screening pigment (cells), and in contrast to a modern type, putatively just a very thin crystalline cone.

Insights into the 400 million-year-old eyes of giant sea scorpions (Eurypterida) suggest the structure of Palaeozoic compound eyes.

Sea scorpions (Eurypterida, Chelicerata) of the Lower Devonian (~400 Mya) lived as large, aquatic predators. The structure of modern chelicerate eyes is very different from that of mandibulate compound eyes [Mandibulata: Crustacea and Tracheata (Hexapoda, such as insects, and Myriapoda)]. Here we show that the visual system of Lower Devonian (~400 Mya) eurypterids closely matches that of xiphosurans (Xiphosura, Chelicerata).

Structure and function of a compound eye, more than half a billion years old.

Until now, the fossil record has not been capable of revealing any details of the mechanisms of complex vision at the beginning of metazoan evolution. Here, we describe functional units, at a cellular level, of a compound eye from the base of the Cambrian, more than half a billion years old. Remains of early Cambrian arthropods showed the external lattices of enormous compound eyes, but not the internal structures or anything about how those compound eyes may have functioned.

Traces of an ancient immune system - how an injured arthropod survived 465 million years ago.

This report of a severely injured trilobite from the Middle Ordovician (~465 Ma) accords with a number of similar observations of healed lesions observed in trilobites. The uniqueness of the specimen described here is that the character of the repair-mechanisms is reflected by the secondarily built structures, which form the new surface of the ruptured compound eye. Smooth, repaired areas inside the visual surface advert to a clotting principle, rather similar to those of today, and the way in which broken parts of the exoskeleton fused during restoration seem to simulate modern samples.

The Hunsrück biota: A unique window into the ecology of Lower Devonian arthropods.

The approximately 400-million-year old Hunsrück biota provides a unique window into Devonian marine life. Fossil evidence suggests that this biota was dominated by echinoderms and various classes of arthropods, including Trilobita, stem lineage representatives of Euarthropoda, Chelicerata and Eucrustacea, as well as several crown group Chelicerata and Eucrustacea. The Hunsrück biota's exceptional preservation allows detailed reconstructions and description of key-aspects of its fauna's functional morphologies thereby revealing modes of locomotion, sensory perception, and feeding strategies.

Exceptional preservation of eye structure in arthropod visual predators from the Middle Jurassic.

Vision has revolutionized the way animals explore their environment and interact with each other and rapidly became a major driving force in animal evolution. However, direct evidence of how ancient animals could perceive their environment is extremely difficult to obtain because internal eye structures are almost never fossilized. Here, we reconstruct with unprecedented resolution the three-dimensional structure of the huge compound eye of a 160-million-year-old thylacocephalan arthropod from the La Voulte exceptional fossil biota in SE France.

Why did the UV-A-induced photoluminescent blue-green glow in trilobite eyes and exoskeletons not cause problems for trilobites?

The calcitic lenses in the eyes of Palaeozoic trilobites are unique in the animal kingdom, although the use of calcite would have conveyed great advantages for vision in aquatic systems. Calcite lenses are transparent, and due to their high refractive index they would facilitate the focusing of light. In some respects, however, calcite lenses bear evident disadvantages.

Discovery of some 400 million year-old sensory structures in the compound eyes of trilobites.

Fossilised arthropod compound eyes have frequently been described. Among the oldest known are those from the lower Cambrian of the Chengjiang Lagerstätte (China, c 525 Ma). All these compound eyes, though often excellently preserved, however, represent just the outer shells, because soft tissues, or even individual cells, usually do not fossilise.

The eyes of a tiny 'Orsten' crustacean - a compound eye at receptor level?

Among the oldest fossil crustaceans are those of the Late Cambrian (Furongian 499 ± 0.3-488.3 ± 1.

The clever strategy of a tiny crustacean eye early in the evolution of vision.

Henningsmoenicaris scutula (Walossek and Müller, 1990) (Fig. 1) is a tiny representative of Crustacea, systematically standing close to the stemline. It is found in stinkstone ('Orsten') nodules from the Alum Shale, where a rich fauna of small organisms is excellently preserved.

The sophisticated visual system of a tiny Cambrian crustacean: analysis of a stalked fossil compound eye.

Fossilized compound eyes from the Cambrian, isolated and three-dimensionally preserved, provide remarkable insights into the lifestyle and habitat of their owners. The tiny stalked compound eyes described here probably possessed too few facets to form a proper image, but they represent a sophisticated system for detecting moving objects. The eyes are preserved as almost solid, mace-shaped blocks of phosphate, in which the original positions of the rhabdoms in one specimen are retained as deep cavities.