Complex sentence: At some point, humans began to associate the fossils with jewels that some believed to form in the heads of toads due to supernatural causes, and were first recorded by Pliny the Elder in the first century.
Split into simpler sentences: At some point, people began to associate the fossils with jewels that some believed were formed, by supernatural means, in the heads of toads. They were first recorded by Pliny the Elder in the first century.