Dactyly

In biology, dactyly is the arrangement of digits (fingers and toes) on the hands, feet, or sometimes wings of a tetrapod animal.

It is traditionally believed that all living tetrapods are descended from an ancestor with a pentadactyl limb, although many species have now lost or transformed some or all of their digits by the process of evolution.

However, this viewpoint was challenged by Stephen Jay Gould in his 1991 essay "Eight (or Fewer) Little Piggies," where he pointed out polydactyly in early tetrapods and described the specializations of digit reduction.

Tetradactyly (from Greek τετρα tetra 'four') is the condition of having four digits on a limb, as in many birds, amphibians, and theropod dinosaurs.

Tridactyly (from Greek τρία tría 'three') is the condition of having three digits on a limb, as in the rhinoceros and ancestors of the horse such as Protohippus and Hipparion.

Didactyly (from Greek δι- di- 'two') or bidactyly is the condition of having two digits on each limb, as in the Hypertragulidae and two-toed sloth, Choloepus didactylus.

In humans this name is used for an abnormality in which the middle digits are missing, leaving only the thumb and fifth finger, or big and little toes.

Monodactyly (from Greek μόνος monos- 'one') is the condition of having a single digit on a limb, as in modern horses and other equidae (though one study suggests that the frog might be composed of remnants of digits II and IV, rendering horses as not truly monodactyl[2]) as well as sthenurine kangaroos.

Functional monodactyly, where the weight is supported on only one of multiple toes, can also occur, as in the theropod dinosaur Vespersaurus.

It is sometimes incorrectly called hypodactyly or confused with aphalangia, the absence of the phalanx bone on one or (commonly) more digits.

[4] Ectrodactyly, also known as split-hand malformation, is the congenital absence of one or more central digits of the hands and feet.

In many secondarily aquatic vertebrates, the non-bony tissues of the forelimbs and/or hindlimbs are fused into a single flipper.