Members of the family Bufonidae are commonly referred to as toads. They have a worldwide distribution with about 52 genera and 600 species. The family is absent from Australia and the islands of the South Pacific, except for the introduced Neotropical Cane Toad, Rhinella marina.
The phylogenetic analysis of Pramuk et al. (2008) supported a South American origin for the toads. Their divergence estimates indicate that the family originated 78–99 MYA. The age of the enigmatic Caribbean clade was dated to the late Paleocene–early Eocene. A return of bufonids to the New World in the Eocene was followed by rapid diversification and secondary expansion into South America by the early Oligocene. The South American origin of Bufonidae in the Upper Cretaceous was followed by relatively rapid expansion and radiation around the globe and ended with a return to the Americas via a Eurasian/North American land bridge in the Eocene. Though the exact route of this dispersal (Beringia or North Atlantic) remains unclear, an argument is made for the less frequently invoked North Atlantic connection. The origin of the enigmatic Caribbean lineage was found to be consistent with colonization following the meteor impact at the K/T boundary.
Toads usually have short limbs for walking or hopping and dry, warty skin. Most have enlarged poison-producing parotoid glands behind the eyes. A variety of reproductive strategies are found in this clade. Some lay eggs in water and produce aquatic larvae (all Arizona species). Others have terrestrial eggs with direct development, and yet others are viviparous. Bufonids range in size from small (20 mm) to large (200 mm) anurans in total length, and the Sonoran Desert Toad reaches an impressive size. They lack teeth; many have a Bidder’s organ, a mass of tissue in the male’s body cavity capable of differentiating into a functional ovary; they also share inguinal fat bodies and a highly ossified skull with the skin fused to the bone. Treefrogs (Hylidae) and toads are sister taxa, and they last shared an ancestor about 73.5 MYA. Arizona has seven species of bufonids.
Rodriguez et al. (2017) note that for centuries, toads have been used as traditional folk remedies to treat allergies, inflammation, cancer, infections and other ailments, suggesting they are a source for novel drugs and therapies. Toads have bioactive secretions may have antimicrobial, protease inhibitor and anticancer properties, as well as being active at the neuromuscular level.
Until relatively recently, all North American toads were in the genus Bufo, but significant molecular work suggests relationships between groups that were previously unknown. The Sonoran Desert Toad is more closely related to a group of toads from the Neotropics and was moved from the genus Bufo to Incilius. The basal lineage of Incilius is composed of the Sonoran Desert Toad and the Sierra Madrean Toad, Incilius occidentalis, and the Volcanic Toad, Incilius tacanensis that occur respectively, in the Sonoran Desert, the Sierra Madrean and the Trans-Mexican Volcanic cordilleras, and a small section of the Pacific volcanic chain straddling the Mexico–Guatemala border. This unusual biogeographic association, coupled with the unique morphology of I. alvarius (e.g., smooth skin, tibial glands, etc.) suggests it represents an old lineage that has experienced substantial extinction over time (Mendelson et al. 2011).[/vc_column_text][/vc_column][/vc_row]