Other methods to date fossils and rocks include radiometric dating and paleomagnetism , a method in which magnetic polarization, acquired by the minerals in a rock at the time the rock was deposited or solidified, is analyzed. At right, the gar Lepisosteus , a large predatory fish also common in the Formation. Complete and articulated fossils such as these suggest that the lakes present at that time were fairly deep, to allow the anoxic conditions required for preservation. Approximately 60 vertebrate taxa have been found at Green River, including fish, reptiles, birds and mammals. Eleven species of reptiles have been found, including one species of snake, Boavus idelmani .
Invertebrate fossils are abundant, with remnants of snails and insects being common.
The plant fossils, including many reeds, leaves and wood specimens, are also very prevalent at Green River. A large majority of known fossils are fragmentary but some complete skeletons exist of fish, birds, reptiles and one mammal, Brachianodon westorum . There are many reasons that explain why the fossils are found in fragments. These include scavenging, fossil disturbance, lake currents, decomposition of the body prior to fossilization and changing physical conditions. A specific example is the process of fossilization for fish in the Green River lakes.