👉 Hydrachnidiae are a group of organisms that belong to the class of microfossils, which include the fossilized remains of extinct animals and plants. These microfossils were found in sedimentary rocks, including those from the Mesozoic era (around 250 million years ago) and early Paleocene-Neogene periods. The term "hydrachnidiae" refers to organisms that are characterized by their ability to retain moisture through a process called dehydration.