kw: animals, fossils
This is the trilobite Erbenochile, surely the creature with the weirdest eyes ever. These fossils are found in Morocco, Algeria and points between, though only in Morocco have complete, well-preserved specimens been found. This genus is Devonian in age, perhaps 400 million years ago. By this time, Trilobites had been around for more than 100 million years, long enough for all sorts of fantastic types to arise.
Those eyes! They are half-columns of calcite lenses. Their arrangement makes them more sensitive, and they are focused on the surroundings rather than above. These critters must have lived in dimmer surroundings, perhaps muddier or deeper water than most of their kin.
There is much more on this taxonomic family to be found at the Phacopida page of the Virtual Fossil Museum.
When I was a child I wondered how closely the Horseshoe Crab is related to Trilobites. Once I learned that "trilobite" means "having three lobes", and that these "lobes" run from head to the tip of the tail, the difference became obvious. You can see on the fossil image that the lobes, right, left and middle, are fully segmented all the way.
On this living Horseshoe Crab (Limulus), what look like lobes are ornaments on the shell, which is only partly separated by spiracles for bringing water from above to the gills, which lie within the shell. Trilobite lobes were articulated, and the side lobes could move relative to the central lobe. This animal's shell articulates only between the large front and smaller back sections, plus the tail spike.
Trilobites belong to their own taxonomic Class, Trilobita, while Limulus belongs to Class Merostomata, which means "legs around the mouth". The legs radiate from the middle of the Horseshoe Crab's forebody, while a Trilobite's legs arise all along the body. Ancestral horseshoe crabs, and their Class-mates the Eurypterids, lived alongside Trilobites in Carboniferous and Permian times. Both these Classes are members of the Subphylum Chelicerata, which includes spiders and scorpions.
None of these animals is that closely related to modern crabs, which are in the Subphylum Crustacea. Strangely, the little pill bugs and sowbugs, which look a little like tiny trilobites, are crustaceans. Is this starting to sound like "Who's on First?" The last half-billion years has seen plenty of surprising animals come and go. The trilobites are a favorite of mine, and I'm sorry they are found only as fossils.