This group includes spiders, scorpions, trilobites, insects, crabs, shrimp, crayfish, etc., who all have the basic feature of the group: arthro-poda, for "jointed"-"appendages". Arthropods don't have mineralized skeletons of calcite or silica, but are made of a hard organic compound called chitin. You know, probably, from seeing how solid crab shells can be, that chitin can be quite hard. But, it isn't strictly mineral material, so it decays more easily after death. We do get fossils of crab claws and such, but they are less common than, say, fossils of clam and snail shells, which are calcite. There is one thing about arthropods though, that results in more abundant fossils when they do preserve. That is the form of growth, called molting, that involves the periodic shedding of the exoskeleton, that involves a temporary softening of the exoskeleton, so that the soft animal within it can squirm out of a breach and "re-grow" a new, larger exoskeleton. The reason for molting is that the skeleton is an exo -skeleton, which is more like a container than the internal skeletons of other organisms, who can grow from the inside-out more easily. The result, for the arthropod fossil record, especially for the trilobites, is that often we are looking at the fossilized molted exoskeleton, and not the skeleton that was part of an organism when it died.
We can start our look at fossil arthropods by examining an Early Paleozoic predatory group called eurypterids. These "sea scorpions" were probably some of the first top predators, as they were around during part of their existence before there were large fishes that might have eaten them. In this specimen, you see the rather front-heavy shape of a eurypterid, and the large claws, and the long straight tail structure called a telson:
Eurypterids got to be over one meter long, and were certainly formidable predators, searching for smaller animals of the day. We have a cousin to the eurypterids still around today: the horseshoe crabs, also having that telson on the tail:
This specimen of a modern horseshoe crab is broken on the left side, but you can see some similarities.
The most famous fossil arthropods are the trilobites, perhaps the favorite fossil of many people. Trilobites crawled around the sea bottom, eating organic matter, working as part of the clean-up crew of the marine community. They, like other arthropods, molted during growth, and many specimens are molted exoskeletons. They ranged in size from tiny, smaller than your pinkie fingernail, up to some larger forms, but most just a few centimeters long.
Note the old label on this specimen -- this rock has been rattling around in a museum cabinet for some time. Also, see the chisel marks around the trilobite, just above and to the right of the quarter. This is typical preservation of trilobites in fine mud (if the sediment has larger grain size, that would indicate more water current, which is not so good for preservation of delicate trilobite exoskeletons). To find fossil trilobites, you can take a chisel and aim it parallel to the sedimentary bedding to cleave (split) along the layering. This is the approach taken by the preparator (= a person who "prepares" fossils for study, for museum storage, or for display), who aimed the head of the chisel at a low angle to split off little pieces of the rock, which is thinly laminated. If you are patient and have an eye for detail (did I mention, if you are patient), then this could be a line of work for you! Most people don't have what it takes for this delicate work though.
Here's a mostly-complete trilobite (fossils, nine times out of ten, are missing some parts), that shows the segmentation ("ribbed") appearance of the abdominal area:
The head area, called the cephalon, forms about the top third of the specimen. There are large compound eyes, faceted, with the many lenses, which is typical, generally, of arthropods. If you look laterally (left - to - right) at the specimen, you notice three body divisions, with that central area usually ridged up a bit. This is the reference for the name tri-lobe-ite.
Trilobites could roll up:
You may have played with an insect called a pill bug on your backyard patio, letting them crawl over you -- after somebody told you they don't bite. A pill bug is not a trilobite, but it does roll up like this, when you bother them.
Trilobites are Cambrian - Permian. They were most abundant during the early and middle Paleozoic, and had started to dribble down before the great Permian extinction finished them off.
For other arthropod fossils, we could look at fossil crustaceans, which have been important, more so since the Mesozoic, or insects and spiders, which don't fossilize very well, except in special circumstances. Crustaceans , such as lobsters and crabs, have massive parts of their exoskeletons, plus they are reinforced with calcite, making them preserve better as fossils.
You've probably heard of the "Jurassic Park" mosquitoes-stuck-in-amber plot line. Many other amber fossils show exquisite detail of various bugs and spiders. Amber is fossilized sap (also called resin), the sticky stuff that oozes out of trees where there has been an injury to the bark. Insects get stuck in the sap, and covered over. Then the sap dries, hardens, falls to the ground, and gets washed into a stream where it is buried along with sediment, later hardening more to become amber after long burial.
Another way to get preservation of delicate animals like insects is through burial under volcanic ash. The ash is fine-grained, so there is potential for detailed preservation. The ash, blown from an erupting volcano, suddenly knocks insects down from flight or covers them (and anything else around), and sometimes will get buried more, instead of washing away before preservation.
The oldest fossil insects come from the 400 million year-old Rhynie Chert (Devonian), of Scotland.
For an example of how we can see good insect fossil records locally, under special conditions of preservation, read about the Miocene insect fauna (fauna is a group of animal fossils from a locality, or a close grouping of fossil localities) of Stewart Valley, Nevada . This web page showcases research done by paleontologists at the California Academy of Sciences, and is a nice discussion of the geology and sedimentary environment, explaining the preservation. Make sure you click the next link at the bottom of the California Academy of Sciences web page, to see examples of fossil insects from the Stewart Valley, alongside photographs of modern day specimens (or click here ).