Passage: The discovery of Castorocauda lutrasimilis is the first sign that a close relative of mammals adapted to water before dinosaurs lost dominance 66 million years ago, pushing back the estimated date for mammal relatives adapted to a semi-aquatic lifestyle by 110 million years. Based on fossils known at present, the mammal line would not see another semi-aquatic form evolve until the Eocene. Because few fossilized remains had been found, it was previously thought that, until the Cretaceous–Paleogene boundary (K–Pg boundary), all mammals were tiny, ground-dwelling or tree-dwelling, nocturnal animals akin to shrews, hedgehogs, treeshrews, or tenrecs. This notion has now been falsified by the armadillo-like Fruitafossor, the dinosaur-eating Repenomamus, the flying squirrel-like Volaticotherium, the small and spiny tenrec-like creature called Spinolestes and now the beaver-like Castorocauda.

Question: Why was it once thought that mammals were originally tiny ground dwelling or tree dwelling animals?
few fossilized remains had been found