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Taxonomy and phylogeny

Ticks belong to the Parasitiformes, a distinctive group of mites that are separate from the main group of mites, the Acariformes. Whether the two groups are more closely related to each other than to other arachnids is uncertain, and studies often recover them as not closely related. Within the Parasitiformes, ticks are most closely related to the Holothyrida, a small group of free living scavengers with 32 described species confined to the landmasses that formed the supercontinent Gondwana.

Relationships among members of the Parasitiformes, after Klompen, 2010:

Fossilized ticks have been discovered from the end of the Early Cretaceous onwards, most commonly in amber. The oldest discovered tick fossils are an argasid bird tick from Late Cretaceous (Turonian ~94-90 million years ago) aged New Jersey amber, and various ticks found in Burmese amber, including Khimaira which does not belong to any living family of tick, the living genus Nuttalliella and the possible nuttalliellid genera Deinocroton and Legionaris, as well as the members of the living ixodid genera Amblyomma, Ixodes, Haemaphysalis, Bothriocroton and Archaeocroton dating the earliest Cenomanian stage of the Late Cretaceous, around 99 million years ago. An undescribed juvenile tick is known from late Albian amber, dating to 105 million years ago. The younger Baltic and Dominican ambers have also yielded examples that can be placed in living genera. A phylogenetic analysis suggests that the last common ancestor of all living ticks likely lived around 195 million years ago in the Southern Hemisphere, in what was then Gondwana.

Ticks belong to three different families. The majority of tick species belong to the two families: Ixodidae (hard ticks) and Argasidae (soft ticks). The third living family is Nuttalliellidae, named for the bacteriologist George Nuttall. It comprises a single species, Nuttalliella namaqua, and as such is a monotypic taxon. Nuttalliella namaqua is found in southern Africa ranging from Tanzania to Namibia and South Africa.

Relationships of living and extinct tick families, after Chitimia-Dobler et al. 2022:

The Ixodidae contain over 700 species of hard ticks with a scutum or hard shield, which the Argasidae lack. The Argasidae contain about 200 species; the genera accepted as of 2010 are Antricola, Argas, Nothoaspis, Ornithodoros, and Otobius. They have no scutum, and the capitulum (mouth and feeding parts) is concealed beneath the body. The phylogeny of the Ixodida within the Acari is shown in the cladogram, based on a 2014 maximum parsimony study of amino acid sequences of 12 mitochondrial proteins. The Argasidae appear monophyletic in this study.