In 1962, Sergius Mamay and Ellis Yochelson analysed North American coal balls.[28] Their discovery of marine organisms led to classification of coal balls were sorted into three types: normal (sometimes known as floral), containing only plant matter; faunal, containing animal fossils only; and mixed, containing both plant and animal material.[29] Mixed coal balls are further divided into heterogeneous, where the plant and animal material was distinctly separated; and homogeneous, lacking that separation.[30]