SAT Text Structure and Purpose Question

With a shell that measured 1.7 meters, the extinct mollusk Parapuzosia seppenradensis is the largest ammonite in the fossil record. Ever since fossilized Lmains of this species were discovered in 1895, scientists have been perplexed by its size. After examining 154 ammonite fossils including some of P. seppenradensis from Montana and Parapuzosia leptophylla (a smaller ammonite species) from England, paleontologist Christina Ifrim, Andrew S. Gale, and colleagues concluded that P. seppenradensis may have evolved from P. leptophylla and gradually increased in size as larger ammonites were better able to escape being preyed on by mosasars. Which choice best describes the overall structure of the text?