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How did early Jurassic mammals differ from the large mammals of later times?

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Jurassic paleogeography — Jurassic Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Jurassic Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

Early Jurassic mammals differed significantly from the large mammals of later times primarily in their size and ecological prominence. During the Jurassic Period, mammals were generally small, often no larger than squirrel-sized, and occupied relatively minor roles in terrestrial ecosystems dominated by archosaurs such as dinosaurs.4

While most early mammals were small insectivores or generalists, the fossil record reveals a surprising degree of diversity even early in their history. For instance, the Middle Jurassic genus Castorocauda represents an exception to the typical small-bodied norm; weighing 500 to 800 grams, it was a semiaquatic, beaver-like mammal adapted for swimming and digging, with a carnivorous, fish-eating diet.1

In contrast to the dominant, large-bodied mammals of the Cenozoic Era, Jurassic mammals were "insignificant" components of their environments. Their anatomy, including specialized teeth and skeletal structures, indicates they were primarily insectivorous or omnivorous, lacking the massive body sizes and diverse ecological dominance that characterize mammalian groups in later geological eras.2341