Terrestrial animal
Terrestrial animals do not form a unified clade; rather, they are a polyphyletic group that share only the fact that they live on land.
If internal parasites are excluded, free living species in terrestrial environments are represented by the following eleven phyla: Roundworms, gastrotrichs, tardigrades, rotifers and some smaller species of arthropods and annelids are microscopic animals that require a film of water to live in, and are therefore considered semi-terrestrial.
The three remaining phyla, arthropods, mollusks, and chordates, all contain species that have adapted totally to dry terrestrial environments, and which have no aquatic phase in their life cycles.
Fossil evidence has shown that sea creatures, likely arthropods, first began to make forays onto land around 530 million years ago, in the Early Cambrian.
A more likely hypothesis is that these early arthropods' motivation for venturing onto dry land was to mate (as modern horseshoe crabs do) or to lay eggs out of the reach of predators.
[7] Among arthropods, many microscopic crustacean groups like copepods and amphipods and seed shrimp can go dormant when dry and live in transient bodies of water.
[citation needed] By approximately 375 million years ago[3] the bony fish best adapted to life in shallow coastal/swampy waters (such as Tiktaalik roseae).
[9] Most truncatellid and assimineid snails amphibiously live in intertidal and supratidal zones from brackish water to pelagic areas.
[9] The rissooidean gastropod family Pomatiopsidae is one of the few groups that have evolved fully terrestrial taxa during the late Cenozoic in the Japanese Archipelago only.
[10] Their success in colonizing different habitats is due to physiological, behavioral, and morphological adaptations to water availability, as well as ionic and thermal balance.
Although eutardigrades only have lifespans of a few months, they famously can enter suspended animation during dry or hostile conditions and survive for decades, which allows them to be ubiquitous in terrestrial environments despite needing water to grow and reproduce.
Many microscopic crustacean groups like copepods and amphipods and seed shrimps are known to go dormant when dry and live in transient bodies of water too.
