Complex sentence: Its chemical composition makes it difficult to match the amber to its producers--it is most similar to the resins produced by flowering plants; however, there are no flowering plant fossils until the Cretaceous, and they were not common until the Upper Cretaceous.
Split into simpler sentences: Its chemical composition makes it difficult to match the amber to its producers--it is most similar to the resins produced by flowering plants. There are no flowering plant fossils until the Cretaceous, and they were not common until the Upper Cretaceous.