Salientia includes frogs and toads, and their close fossil relatives. The closest living relatives of the Salientia are the other amphibians, salamanders and caecilians. Frogs outnumber both of these groups substantially; as of May 2007, there are 172 living species of caecilians, 557 living salamanders, and 5424 living species of frogs.
The earliest known salientian is †Triadobatrachus massinoti, from the Early Triassic of Madagascar. This "proto-frog" is about 250 million years old. "Proto-frog" refers to the fact that it had not yet quite evolved the combination of features that are typically associated with frogs. For more information see †Triadobatrachus massinoti.
The earliest "true" frogs include †Prosalirus bitis and †Vieraella herbsti, from the Early Jurassic era. Thus, perfectly respectable frogs were around just before most of the major groups of dinosaurs had appeared. †Notobatrachus degiustoi from the Middle Jurassic is just a bit younger, about 155-170 million years old.
There are many distinctive features of living frogs. Frogs have at most nine vertebrae in front of the sacrum, and the three or four posterior to the sacrum are fused into a rod called the urostyle. In contrast, caecilians and salamanders have many more vertebrae and they do not have a urostyle. Frogs do not possess tails in the adult stage ("Anura" means without tail), as caecilians and salamanders do. Frogs also have a radioulna, which represents a fused radius and ulna (bones of the forearm), and a tibiofibula, the fused tibia and fibula (bones of the shank). The tibiale and fibulare (ankle bones; also called astragalus and calcaneum) of frogs are greatly elongate. Thus there is effectively an additional lever system that frogs can utilize in jumping. Indeed, the origin of saltation and its morphological correlates (lack of a tail, reduction in vertebrae, elongation of propulsive segments of the body) seems to be one of the features that clearly sets frogs apart from other major vertebrate groups (Gans and Parsons, 1966).
In addition to morphological distinctions, frogs also have a distinctive life phase known as the tadpole, which is a highly specialized "eating machine." Salamanders and caecilians have a larval form, but in neither does the larva possess the many specializations (such as the ceratohyal pump) that frog tadpoles have (Wassersug, 1974). Even the most basal living frogs have the beginnings of a unique mechanism of tongue projection (Nishikawa and Cannatella, 1991; Nishikawa and Roth, 1991) that is associated with extreme modification of the gill arches into a fused hyobranchial plate.
Although there is no scientific distinction between "frogs" and "toads", frogs are typically smooth-skinned, have long hind limbs for leaping, and live in water, while toads have warty, drier skin, with shorter hind limbs for hopping, and live on land (Halliday and Adler, 1986).
With so many species of frogs and toads, it is not surprising that they inhabit a wide variety of habitats. Habitat types range from arid desert regions to mountainous regions to swamps to tropical rainforests.
Temperature and water regulation are critical to frogs and toads, and amphibians in general. Being ectotherms, frogs and toads are reliant on the ambient temperature for body temperature regulation. In the winter months, frogs in temperate zones cannot remain active and must enter into a state of torpor, or extremely reduced activity. In the contrasting summer months, frogs can avoid the extreme heat by remaining underground in daylight, and being active at night (Halliday and Adler, 1986).
Salientians are also susceptible to the loss of body water due to extremely hot or dry conditions. Moisture regulation in frogs varies with their habitat. Those in temperate climates maintain moist skin to aid in evaporative cooling. As external air passes over the moist skin, the frog's body temperature is lowered. Additionally, permeable skin allows the frog the ability to absorb water simply by jumping into a pond or sitting in a puddle. Frogs in arid regions, on the other hand, have different ways of regulating body water. Their skin is often impermeable to water to prevent rapid evaporation and dehydration. Instead, they may cover their bodies with a thick mucus, or burrow to avoid the heat altogether.
Breeding in frogs is triggered by environmental cues such as temperature change and rainfall. During the breeding season (which varies with each species) hundreds or thousands of frogs may be seen in a congregation. Male frogs attract mates by calling; often many males call in chorus. Calling usually occurs near a body of water, such as a pond, where the eggs can be laid and fertilized. Egg masses may be laid in long chains or in large clumps. Parental care in frogs and toads is variable; some species lay many smaller eggs and have little parental care, while others lay a few larger eggs and remain with them until tadpoles or froglets develop.
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