Vestigiality in the context of "Perissodactyla"

Moa (order Dinornithiformes) are an extinct group of flightless birds formerly endemic to New Zealand. During the Late Pleistocene-Holocene, there were nine species (in six genera). The two largest species, Dinornis robustus and Dinornis novaezelandiae, reached about 3.6 metres (12 ft) in height with neck outstretched, and weighed about 230 kilograms (510 lb) while the smallest, the bush moa (Anomalopteryx didiformis), was around the size of a turkey. Estimates of the moa population when Polynesians settled New Zealand circa 1300 vary between 58,000 and approximately 2.5 million.

Moa are traditionally placed in the ratite group. Genetic studies have found that their closest relatives are the flighted South American tinamous, once considered a sister group to ratites. The nine species of moa were the only wingless birds, lacking even the vestigial wings that all other ratites have. They were the largest terrestrial animals and dominant herbivores in New Zealand's forest, shrubland, and subalpine ecosystems until the arrival of the Māori, and were hunted only by Haast's eagle. Moa extinction occurred within 100 years of human settlement of New Zealand, primarily due to overhunting.

Perissodactyla (/pəˌrɪsoʊˈdæktɪlə/, from Ancient Greek περισσός, perissós 'odd' and δάκτυλος, dáktylos 'finger, toe'), or odd-toed ungulates, is an order of ungulates. The order includes about 17 living species divided into three families: Equidae (horses, asses, and zebras), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs). They typically have reduced the weight-bearing toes to three or one of the five original toes, though tapirs retain four toes on their front feet. The nonweight-bearing toes are either present, absent, vestigial, or positioned posteriorly. By contrast, artiodactyls (even-toed ungulates) bear most of their weight equally on four or two (an even number) of the five toes: their third and fourth toes. Another difference between the two is that perissodactyls digest plant cellulose in their intestines, rather than in one or more stomach chambers as artiodactyls, with the exception of Suina, do.

The order was considerably more diverse in the past, with notable extinct groups including the brontotheres, palaeotheres, chalicotheres, and the paraceratheres, with the paraceratheres including the largest known land mammals to have ever existed.

A psychological adaptation is a functional, cognitive or behavioral trait that benefits an organism in its environment. Psychological adaptations fall under the scope of evolved psychological mechanisms (EPMs), however, EPMs refer to a less restricted set. Psychological adaptations include only the functional traits that increase the fitness of an organism, while EPMs refer to any psychological mechanism that developed through the processes of evolution. These additional EPMs are the by-product traits of a species’ evolutionary development (see spandrels), as well as the vestigial traits that no longer benefit the species’ fitness. It can be difficult to tell whether a trait is vestigial or not, so some literature is more lenient and refers to vestigial traits as adaptations, even though they may no longer have adaptive functionality. For example, xenophobic attitudes and behaviors, some have claimed, appear to have certain EPM influences relating to disease aversion, however, in many environments these behaviors will have a detrimental effect on a person's fitness. The principles of psychological adaptation rely on Darwin's theory of evolution and are important to the fields of evolutionary psychology, biology, and cognitive science.

Semi-slugs, also spelled semislugs, are land gastropods whose shells are too small for them to retract into, but not quite vestigial. The shell of some semi-slugs may not be easily visible on casual inspection, because the shell may be covered over with the mantle.

This is a type of gastropod that is intermediate between a slug (without an external shell) and a land snail (with a large enough shell to retract completely into).

The gladius (pl.: gladii), or pen, is a hard internal bodypart found in many cephalopods of the superorder Decapodiformes (particularly squids) and in a single extant member of the Octopodiformes, the vampire squid (Vampyroteuthis infernalis). It is so named for its superficial resemblance to the Roman short sword of the same name, and is a vestige of the ancestral mollusc shell, which was external. The gladius is located dorsally within the mantle and usually extends for its entire length. Composed primarily of chitin, it lies within the shell sac, which is responsible for its secretion. Some species, like the bigfin reef squid, still has a gladius with some degree of mineralization.

Gladii are known from a number of extinct cephalopod groups, including teudopseids (e.g. Actinosepia, Glyphiteuthis, Muensterella, Palaeololigo, Teudopsinia, Teudopsis, and Trachyteuthis), loligosepiids (e.g. Geopeltis, Jeletzkyteuthis, and Loligosepia), and prototeuthids (e.g. Dorateuthis, Paraplesioteuthis, and Plesioteuthis).

Pelvic spurs (also known as vestigial legs) are external protrusions found around the cloaca in certain superfamilies of snakes belonging to the greater infraorder Alethinophidia. These spurs are made up of the remnants of the femur bone, which is then covered by a corneal spur, or claw-like structure. This femur derives from ancestral hind limbs found in the most recent common ancestor of modern snakes and the other reptiles of the clade Toxicofera, many of which have fully functional front and hind limbs. Due to the fact that the spurs derive from the ancestral state of functional legs, but are no longer functional for locomotion specifically, these structures meet the criteria for being considered vestigial. Nonetheless, uses for the structures have been thoroughly documented. Species that have external spurs have corresponding muscles, neurological structures, and vascularization to allow for independent movement. The spurs are more pronounced and visible in male specimens and have been observed in use during courtship behavior. The spurs are specifically used in the clasping and stimulation of females by males during courtship and mating. In certain species, males will also use their spurs to engage in combat with one another.

A vestigial response or vestigial reflex in a species is a response that has lost its original function. In humans, vestigial responses include ear perking, goose bumps and the hypnic jerk.