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Comprehensive Description

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Characteristics

When only living taxa are considered, a list of synapomorphies of Anura would include many of the obvious distinctive features of frogs:

  • Shortened vertebral column (nine or fewer presacral vertebrae)
  • Presence of a urostyle formed from developing tail vertebrae
  • Absence of tail in adults
  • Hindlimb longer than forelimb
  • Fusion of radius and ulna into a single element
  • Fusion of tibia and fibula into a single element
  • Elongate ankle bones (tibiale and fibulare = astragalus and calcaneum)
  • Absence of a prefrontal bone
  • Fusion of separate hyobranchial elements into a hyobranchial (=hyoid) plate
  • A tongue that lacks intrinsic skeletal support from the hyobranchial plate
  • A tadpole, with keratinous beaks and denticles as larval mouthparts
  • A single median spiracle in the larva (characteristic of Orton's Type 3 tadpole)
  • Large subcutaneous lymph spaces between the skin and muscle layer
  • Two protractor lentis muscles attached to lens

(Milner, 1988, 1993; Saint-Aubain, 1981; Trueb and Cloutier, 1991).

However, the existence of several early frog fossils, such as †Prosalirus and †Notobatrachus, clouds this picture, because many of these characters cannot be assessed in these fossils, either because of incompleteness of fossils or because the features are soft-tissue characters identifiable only in living taxa.

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