Some scientists have suggested that mammals in the Mesozoic era were not a very ________ group, but paleontologist. Zhe-Xi Luo's research suggests that early mammals living in the shadow of dinosaurs weren't all ground-dwelling insectivores. Fossils of various plant-eating mammals have been found in China, including species like Vilevolodon diplomylos, which Luo says could glide like a flying squirrel.

Which choice completes the text with the most logical and precise word or phrase?