Terrestrial animals are animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s that live predominantly or entirely on land (e.g., cat

Cat

The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

s, ant

Ant

Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...

s, emu

Emu

The Emu Dromaius novaehollandiae) is the largest bird native to Australia and the only extant member of the genus Dromaius. It is the second-largest extant bird in the world by height, after its ratite relative, the ostrich. There are three subspecies of Emus in Australia...

s), as compared with aquatic animal

Aquatic animal

An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...

s, which live predominantly or entirely in the water (e.g., fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, lobster

Lobster

Clawed lobsters comprise a family of large marine crustaceans. Highly prized as seafood, lobsters are economically important, and are often one of the most profitable commodities in coastal areas they populate.Though several groups of crustaceans are known as lobsters, the clawed lobsters are most...

s, octopus

Octopus

The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...

es), or amphibian

Amphibian

Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s, which rely on a combination of aquatic and terrestrial habitat

Habitat

* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

s (e.g., frog

Frog

Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...

s). The term terrestrial is also frequently used for species that live primarily on the ground, in contrast to arboreal species, which live primarily in trees.

Taxonomy

Terrestrial invasion is one of the most important events in the history of life. Terrestrial lineages evolved in several animal phyla, among which vertebrates, arthropods, and mollusks are representatives of more successful groups of the epifaunal terrestrial life.

Terrestrial animals do not form a unified clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

; rather, they share only the fact that they live on land. The transition from an aquatic to terrestrial life has evolved independently and successfully many times by various groups of animals. Most terrestrial lineages are considered to have originated under mild or tropical climate

Tropical climate

A tropical climate is a climate of the tropics. In the Köppen climate classification it is a non-arid climate in which all twelve months have mean temperatures above...

 during the Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 and Mesozoic

Mesozoic

The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

, whereas few animals became fully terrestrial during the Cenozoic

Cenozoic

The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

.

When excluding internal parasites, free living species in terrestrial environments are represented by the following ten phyla; Flatworm

Flatworm

The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...

s (Planaria), Nemertea

Nemertea

Nemertea is a phylum of invertebrate animals also known as "ribbon worms" or "proboscis worms". Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela. Although most are less than long, one specimen has been estimated at , which would make it the longest animal ever...

 (ribbon worms), Nematoda (roundworms), Rotifer

Rotifer

The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

s, Tardigrada (water bears), Onychophora (velvet worms), Arthropod

Arthropod

An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s, mollusks (gastropods: land snail

Land snail

A land snail is any of the many species of snail that live on land, as opposed to those that live in salt water and fresh water. Land snails are terrestrial gastropod mollusks that have shells, It is not always an easy matter to say which species are terrestrial, because some are more or less...

s and slug

Slug

Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...

s), Annelida and Chordata (tetrapods). Roundworms, tardigrades, and rotifers are microscopic animals that require a film of water to live in, and are not considered truly terrestrial. Flatworms, ribbon worms, velvet worms and annelids all depend on more or less moist habitats, while the three remaining categories, arthropods, gastropods and tetrapods, are the only ones that contain species that have adapted to predominantly dry terrestrial environments.

Difficulties

Labeling an animal species "terrestrial" or "aquatic" is often obscure and becomes a matter of judgment.

Many animals considered terrestrial have a life-cycle that is partly dependent on being in water. Penguins, seals

Pinniped

Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...

, and walruses sleep on land and feed in the ocean, yet they are all considered terrestrial. Many insects and all terrestrial crabs (as well as other clades) have an aquatic life cycle stage: their eggs need to be laid in and to hatch in water; after hatching, there is an early aquatic form, either a nymph

Nymph

A nymph in Greek mythology is a female minor nature deity typically associated with a particular location or landform. Different from gods, nymphs are generally regarded as divine spirits who animate nature, and are usually depicted as beautiful, young nubile maidens who love to dance and sing;...

 or larva

Larva

A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

.

There are crab species that are completely aquatic, crab species that are amphibious, and crab species that are terrestrial. Fiddler crabs are called "semi-terrestrial" since they make burrows in the muddy substrate, to which they retreat during high tides. When the tide is out, fiddler crabs search the beach for food. The same is true in the Mollusca

Mollusca

The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

: many hundreds of gastropod genera and species live in intermediate situations, such as for example, Truncatella. Some gastropods with gills live on land, and others with a lung live in the water.

As well as the purely terrestrial and the purely aquatic animals, there are many borderline species. There are no universally accepted criteria for deciding how to label these species.

Terrestrial gastropods

Gastropod mollusks are one of the most successful animals that have diversified in the fully terrestrial habitat. They have evolved terrestrial taxa in more than nine lineages. They are commonly referred to as land snail

Land snail

A land snail is any of the many species of snail that live on land, as opposed to those that live in salt water and fresh water. Land snails are terrestrial gastropod mollusks that have shells, It is not always an easy matter to say which species are terrestrial, because some are more or less...

s and slug

Slug

Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...

s.

Terrestrial invasion of gastropod mollusks has occurred in Neritopsina, Cyclophoroidea

Cyclophoroidea

Cyclophoroidea is a superfamily of land snails with an operculum, terrestrial gastropods within the informal group Architaenioglossa, that belongs to the clade Caenogastropoda....

, Littorinoidea

Littorinoidea

Littorinoidea is a superfamily of both sea snails and land snails which have a gill and an operculum, terrestrial and marine gastropod mollusks in the clade Littorinimorpha....

, Rissooidea

Rissooidea

Rissooidea, originally named Rissoacea by Grey, 1847, is a large taxonomic superfamily of small and minute saltwater, brackish water, freshwater, and semi-terrestrial snails which have gills and an operculum...

, Ellobioidea, Onchidioidea, Veronicelloidea

Veronicelloidea

Veronicelloidea is a superfamily of air-breathing land slugs. They are terrestrial pulmonate gastropod molluscs.- Taxonomy :The following two families have been recognized in the taxonomy of Bouchet & Rocroi :* family Veronicellidae Gray, 1840...

, Succineoidea, and Stylommatophora

Stylommatophora

Stylommatophora is a taxon of air-breathing land snails and slugs, terrestrial pulmonate gastropod molluscs. This taxon is currently considered to be a clade. It was previously regarded as an infraorder...

, and in particular, each of Neritopsina, Rissooidea and Ellobioidea has likely achieved land invasion more than once.

Most terrestrialization events have occurred during the Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 or Mesozoic

Mesozoic

The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

. Gastropods are especially unique due to several fully terrestrial and epifaunal lineages that evolved during the Cenozoic

Cenozoic

The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

. Some members of rissooidean families Truncatellidae

Truncatellidae

Truncatellidae, common name the "looping snails", is a family of small amphibious snails, with gills and an operculum, semi-marine gastropod mollusks or micromollusks.-Shell description :...

, Assimineidae

Assimineidae

Assimineidae is a family of minute snails with an operculum that live in saltwater, freshwater or on the land; they are mostly aquatic gastropod mollusks or micromollusks, in the clade Littorinimorpha.- Ecology :...

, and Pomatiopsidae

Pomatiopsidae

Pomatiopsidae is a family of small mainly freshwater snails with gills and an operculum, aquatic gastropod mollusks in the superfamily Rissooidea ....

 are considered to have colonized to land during the Cenozoic. Most truncatellid and assimineid snails amphibiously live in intertidal and supratidal zones from brackish water

Brackish water

Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salty"...

 to pelagic areas. Terrestrial lineages likely evolved from such ancestors. The rissooidea

Rissooidea

Rissooidea, originally named Rissoacea by Grey, 1847, is a large taxonomic superfamily of small and minute saltwater, brackish water, freshwater, and semi-terrestrial snails which have gills and an operculum...

n gastropod family Pomatiopsidae

Pomatiopsidae

Pomatiopsidae is a family of small mainly freshwater snails with gills and an operculum, aquatic gastropod mollusks in the superfamily Rissooidea ....

 is one of the few groups that have evolved fully terrestrial taxa during the late Cenozoic in the Japanese Archipelago

Japanese Archipelago

The , which forms the country of Japan, extends roughly from northeast to southwest along the northeastern coast of the Eurasia mainland, washing upon the northwestern shores of the Pacific Ocean...

 only. Shifts from aquatic to terrestrial life occurred at least twice within two Japanese endemic lineages in Japanese Pomatiopsidae and it started in the Late Miocene

Late Miocene

The Late Miocene is a sub-epoch of the Miocene Epoch made up of two stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch....

.

Further reading

• Clark J. A.
  Jennifer A. Clack
  Jennifer Alice Clack, FRS, is an English paleontologist, an expert in the field of evolutionary biology. She studies the "fish to tetrapod" transition— the origin, evolutionary development and radiation of early tetrapods and their relatives among the lobe-finned fishes...
  
   (2002). Gaining ground: the origin and evolution of tetrapods. Indiana University Press
  Indiana University Press
  Indiana University Press, also known as IU Press, is an academic publisher at Indiana University that specializes in the humanities and social sciences. It was founded in 1950. Its headquarters are located in Bloomington, Indiana....
  
  , 369 pp., ISBN 9780253340542.
• Cloudsley-Thompson J. L. (1988). Evolution and adaptation of terrestrial arthropods. Springer
  Springer Science+Business Media
  - Selected publications :* Encyclopaedia of Mathematics* Ergebnisse der Mathematik und ihrer Grenzgebiete * Graduate Texts in Mathematics * Grothendieck's Séminaire de géométrie algébrique...
  
  , 141 pp., ISBN 9783540181880.
• Dejours P. et al. (1987). Comparative physiology: life in water and on land. Liviana Editrice, Italy, 556 pp., ISBN 9780387965154.
• Gordon M. S. & Olson E. C. (1995). Invasions of the land: the transitions of organisms from aquatic to terrestrial life. Columbia University Press
  Columbia University Press
  Columbia University Press is a university press based in New York City, and affiliated with Columbia University. It is currently directed by James D. Jordan and publishes titles in the humanities and sciences, including the fields of literary and cultural studies, history, social work, sociology,...
  
  , 312 pp., ISBN 9780231068765.
• Little C. (1983). The colonisation of land: Origins and adaptations of terrestrial animals. Cambridge University Press
  Cambridge University Press
  Cambridge University Press is the publishing business of the University of Cambridge. Granted letters patent by Henry VIII in 1534, it is the world's oldest publishing house, and the second largest university press in the world...
  
  , Cambridge. 290 pp., ISBN 9780521252188.
• Little C. (1990). The terrestrial invasion. An ecophysiological approach to the origin of land animals. Cambridge University Press, Cambridge. 304 pp. ISBN 9780521336697.