Ticks are parasitic arachnids that are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external parasites, living by feeding on the blood of mammals, birds, and sometimes reptiles and amphibians. The timing of the origin of ticks is uncertain, though the oldest known tick fossils are from the Cretaceous period, around 100 million years old. Ticks are widely distributed around the world, especially in warm, humid climates. Ticks belong to two major families, the Ixodidae or hard ticks, and the Argasidae, or soft ticks. Nuttalliella, a genus of tick from southern Africa is the only member of the family Nuttalliellidae, and represents the most primitive living lineage of ticks. Adults have ovoid/pear-shaped bodies which become engorged with blood when they feed, and eight legs. Their cephalothorax and abdomen are completely fused.