Taxonomy
There are three families of ticks, one of which – Nuttalliellidae – comprises a single species, Nuttalliella namaqua. The remaining two families contain the hard ticks (Ixodidae) and the soft ticks (Argasidae).
Ixodidae (>700 species) are distinguished from the Argasidae by the presence of a scutum or hard shield. Ixodidae nymphs and adults both have a prominent capitulum (head) which projects forwards from the body; in the Argasidae, conversely, the capitulum is concealed beneath the body.
Argasidae contains 193 species, although the composition of the genera is less certain, and more study is needed before the genera can become stable. The currently accepted genera are Antricola, Argas, Nothaspis, Ornithodoros and Otobius. Though common in North America, they feed rapidly, primarily on birds, and are very rarely found to parasitize land animals or humans.
Nuttalliellidae contains only a single species, Nuttalliella namaqua, a tick found in southern Africa from Tanzania to Namibia and South Africa,. It can be distinguished from ixodid ticks and argasid ticks by a combination of characters including the position of the stigmata, lack of setae, strongly corrugated integument, and form of the fenestrated plates.
Fossilized ticks are common. Recent hypotheses based on total-evidence approach analysis place the origin of ticks in the Cretaceous (65 to 146 million years ago) with most of the evolution and dispersal occurring during the Tertiary (5 to 65 million years ago). The oldest example is an argasid (bird) tick from Cretaceous New Jersey amber. The younger Baltic and Dominican ambers have also yielded examples, all of which can be placed in living genera.
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