| Salientia | |
|---|---|
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| Hourglass TreefrogsHyla ebraccata | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Amphibia |
| Superorder: | Batrachia |
| Clade: | Salientia Laurenti, 1768 |
| Subgroups | |
TheSalientia (Latinsalire,salio meaning "to jump") are atotal group ofamphibians that includes theorderAnura, the frogs and toads, and variousextinct proto-frogs that are more closely related to the frogs than they are to theUrodela, thesalamanders andnewts.[1] Theoldest fossil "proto-frog" appeared in theearly Triassic ofMadagascar, butmolecular clock dating suggests their origins may extend further back to thePermian, 265 million years ago.
Very few fossils of early salientians have been found, which makes defining the characteristics of the group and their taxonomic relationships difficult. The arrangement of pectoral elements and the number of vertebrae are some guides, but the degree of vertebral articulation and the arrangement of the bones in the leg have not been found to be reliable indicators. The early proto-frogs developed fromtemnospondyl ancestors in which some of the elements of their vertebrae remained separate. The structure of the salientian pelvis and hind limb was probably developed for swimming rather than jumping. From the structure of the vertebrae, the group appears not to bemonophyletic. The evolution of salientians seems to have been rapid and radiative. The essential features of recent groupings seem to have been established during theMesozoic or earlyTertiary. The familiesAlytidae,Pipidae, andPelobatidae are ecologically isolated, theharlequin frogs, restricted to a neotropical range in Central and South America, and theRanidae andBufonidae probably radiated from tropical regions of Africa and Asia.[2]
The origins and evolutionary relationships between the three main groups of amphibians are hotly debated. Amolecular phylogeny based onrDNA analysis dating from 2005 suggests thatsalamanders andcaecilians are more closely related to each other than they are to frogs, and thedivergence of the three groups took place in thePaleozoic or earlyMesozoic before the breakup of the supercontinentPangaea and soon after their divergence from thelobe-finned fishes. This would help account for the relative scarcity of amphibian fossils from the period before the groups split.[3] Another molecular phylogenetic analysis conducted about the same time concluded thelissamphibians first appeared about 330 million years ago and that thetemnospondyl-origin hypothesis is more credible than other theories. Theneobatrachians seemed to have originated in Africa/India, the salamanders in East Asia and the caecilians in tropical Pangaea.[4] Other researchers, while agreeing with the main thrust of this study, questioned the choice of calibration points used to synchronise the data. They proposed that the date of lissamphibian diversification be put in thePermian, rather less than 300 million years ago, a date in better agreement with the palaeontological data.[5] A further study in 2011 using both extinct and living taxa sampled for morphological, as well as molecular data, came to the conclusion that the Lissamphibia aremonophyletic and should be nested within theLepospondyli rather than within theTemnospondyli. The study postulated the Lissamphibia originated no earlier than the lateCarboniferous, some 290 to 305 million years ago. The split between Anura andCaudata was estimated as taking place 292 million years ago, rather later than most molecular studies suggest, with the caecilians splitting off 239 million years ago.[6]
In 2008,Gerobatrachus hottoni, a temnospondyl with many frog- and salamander-like characteristics, was discovered inTexas. It dated back 290 million years and was hailed as amissing link, astem batrachian close to thecommon ancestor of frogs and salamanders, consistent with the widely accepted hypothesis that frogs and salamanders are more closely related to each other (forming aclade called the Batrachia) than they are to caecilians.[7][8] However, others have suggested thatGerobatrachus hottoni was only adissorophoid temnospondyl unrelated to extant amphibians.[9] The earliest known salientians (see below), closer to the extant frogs than to the extant salamanders, areTriadobatrachus massinoti, from the EarlyTriassic ofMadagascar (about 250 million years ago), and the fragmentaryCzatkobatrachus polonicus from the Early Triassic ofPoland (about the same age asTriadobatrachus).[10] The skull ofTriadobatrachus is frog-like, being broad with large eye sockets, but the fossil has features diverging from modern frogs. These include a longer body with more vertebrae. The tail has separate vertebrae, unlike the fused urostyle or coccyx found in modern frogs. The tibia and fibula bones are also separate, making it probable thatTriadobatrachus was not an efficient leaper.[10]The Salientia (Latinsalere (salio), "to jump") are astem group including modern frogs in the order Anura and their close fossil relatives the "proto-frogs" (e.g.,Triadobatrachus andCzatkobatrachus). The common features possessed by the "proto-frogs" in the Salientia group include 14presacral vertebrae (modern frogs have eight or nine), a long and forward-slopingilium in thepelvis, the presence of afrontoparietal bone, and alower jaw without teeth.
The earliest salientian yet discovered isTriadobatrachus massinoti, known from a single fossil specimen found in Madagascar. It dates back to theEarly Triassic, about 250 million years ago. It had many frog-like features, but had 14 presacral vertebrae, while modern frogs have nine or 10. Previous fossil amphibians had many more presacral vertebrae than this andT. massinoti provides a missing link between salamanders and frogs. Other characteristics that distinguish it from modern frogs include the possession of a short tail with unfused vertebrae, a separate radius and ulna in the fore limb, and separate tibia and fibula in the hind limb. The features it shares with modern frogs include a forward-sloping ilium, the fusion of the frontal and parietal bones into a single structure known as the frontoparietal, and a lower jaw bone with no teeth.[11]
Czatkobatrachus is another proto-frog with some characteristics similar toTriadobatrachus. It is from the early Triassic in Poland and has a shortened vertebral column, reduced tail, and elongated ilium.[12]
Another early proto-frog wasProsalirus bitis, several fossil specimens of which have been found in Arizona. It dates back to theEarly Jurassic, 190 million years ago. It has primitive features, but has a urostyle and an elongated, forward-directed ilium in its pelvis. These adaptations made it better able to absorb the impact of landing after a jump.[13]
Dating back to a similar date isVieraella herbsti, a single specimen of which has been found in Santa Cruz Province, Argentina. It had 10 presacral vertebrae, but is considered to be more basal thanNotobatrachus and living frogs.
Several specimens ofNotobatrachus degiustoi have been found in Patagonia, Argentina. They date back to theMiddle Jurassic, 160 million years ago. Whether it should be considered the first modern frog or be placed in a sister group to Anura is uncertain.[14]
| Amphibia |
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Cladogram from Tree of Life Web Project.[15]
