The exoskeleton of a Devonian period trilobite was made of hard parts, the hardness was the material chitin, Chitin C8H13O5N, a long-chained polymer of an N-acetylglucosamine. Chitin in Devonian trilobites is made up of calcite and calcium phosphate, this proteinaceous matrix is sclerotin which formed into a protein latticework, this process is sclerotization. The carapace of an extant trilobite was very tough when formed in this way, strong and durable, quite rigid without the effect of being brittle. The exoskeleton covered the dorsal area of the arthropod, the upper part. The dorsal exoskeleton or carapace wrapped around the edges back onto itself, this wrapped under the edge of the carapace and this edge is termed the doublure.
The carapace covering the entire upper dorsal region of the trilobite had three main parts, the cephalon which is the head shield part, the thorax the midsection or body and the pygidium which is the tail section. The naming of the trilobite comes from three other features of the carapace, three distinct lobes which run longitudinally from cephalon to pygidium. The central lobe is termed the axial lobe, this lobe is usually raised above the side lobes which are termed the pleural lobes. Ok, so that's the basic anatomy, the interesting piece of the trilobite puzzle we're featuring today is the hypostome, often noted as the chin, it is in the forefront of the trilobites ventral cephalon or underside of the cephalon. The hypostome is the mouth cover, a very interesting and important anatomical feature. Palaeontologists can deduce a great deal from the shape and size of this hypostome.
If the hypostome was fixed by muscle tissue which was not fixed rigidly, this morphology can aid in the determination of the feeding habits of the individual, if the hypostome is stronger and more securely fixed then another deduction. The strong hypostome meant the trilobite could be using its mouth cover to macerate its food, in the fossil record this more robust mouth cover is often found on larger fossil trilobites.
It is deduced these were more likely to be predators and from that one could make another step that these larger predators were more likely to be pelagic, free to swim and search the seas for prey, such as other trilobites or arthropods. these pelagic giants would need a different type of eye and they did, one which would be able to pick out prey at distance and above them, to the side and below as they swam around, so they developed different, perhaps one could say more sophisticated eyes than the sea bottom trilobite sifters. As many types of trilobite evolved so did the hypostome, becoming specialised to each genus. Some had rasps and edges which would have aided macerations and dissection of prey and other exotic features. There are three main types which briefly are named natant, impendent and conterminant. The natant is thought to be a less evolved hypostome or one that changed little and therefore had a singular use as a sifter, the conterminant is a general predator type tool and the impendent a more specialised hypostome, for macerating specific prey.
The hypostome is the bottom dweller was less of a tool for maceration, as the bottom dwellers we see had eyes in different positions, atop the cephalon looking sideways, forwards and possibly slightly raised, some like tall skyscrapers of the trilobite world had cylindrical convex eyes with thousands of lenses which could search or look out for those top dog predators ready to munch through the softer underparts of a trilobites anatomy. These bottom dwellers had hypostome that was used to shovel the silts into their mouths, filtering the silts or sands for an organism to devour, a much more sedate way of dredging for one's supper. These hypostomes were bulkier in design, less streamlined, stout was the need for dredging.
So the science of palaeontology can learn a lot from these very small elements of the ancient arthropod. Very often the hypostome is not recovered with the fossil carapace, as the muscle tissue rots away and the hypostome becomes detached. the most successful recovery rate of hypostome is when the trilobite like some woodlouse has a catastrophe of turning proportions and is entombed legs flailing towards the surface of the sea, then the hypostome drops back down onto the ventral side of the carapace as the animal dies and decomposes, as it is petrified and finally fossilised, so some palaeontologist may come along crack it on the head to split a nodule open to gain and wonder again at the processes of evolution.