Goniatite fossils are commonly found in northern Arkansas. They are the remains of creatures that were cephalopods, like a modern squid or octopus, but with a hard shell. They lived for around 170 million years, and would have swum in the shallow, warm lakes that covered the area around contemporary Arkansas at that time. One variety of goniatite, Arkanites relictus, was named after Arkansas, leading some scientists to suggest that it is the fossil most commonly associated with the state.
While goniatites are an example of marine invertebrates that lived in Arkansas during the early Devonian period (roughly 435 million years ago), there are many other varieties to be found. They are mainly clustered around the inner highland parts of the state, especially the Ouachita Mountains. There, it is possible to find the remains of bryozoans, crinoids and other invertebrates that would have lived in the shallow water that covered much of the state.
Mosasaurs were varieties of aquatic reptile that lived during the late Cretaceous period, roughly 85 million years ago. They were carnivorous, and fed on flightless birds called Hesperornis. It is possible to find their remains in the lowlands towards the south of Arkansas, as this is where water coverage would have been the deepest. They provide some of the largest fossil remains in the state, with large pieces of bone available for fossil hunters to find.
Brachiopods were filter feeders that would have lived on the ocean floor. They had short, muscular tethers called pedicles, which they used to anchor themselves to the sea bed. This is one of the reasons why their fossil remains are so plentiful. They look similar to clams and lived during the Paleozoic period. Brachiopods are one of the most common fossils on earth, and as such can be found in many locations throughout Arkansas.
