Temporal range: Early Triassic (?) – Recent (Gerobatrachus is Early Permian)
The Lissamphibia are a subclass of animals that includes all recent amphibians. For several decades, this name has been used for a group that includes all extant amphibians, but excludes all the main groups of Paleozoic tetrapods, such as Temnospondyli, Lepospondyli, Embolomeri, and Seymouriamorpha. Some authors hold that Lissamphibia is a clade, that the subclass consists of all the descendants of an ancestral lissamphibian, but others hold that frogs and salamanders derive from temnospondyls, whereas caecilians derive from lepospondyls, so Lissamphibia is polyphyletic.
Living amphibians fall into one of three orders: the Anura (frogs and toads), the Caudata or Urodela (salamanders and newts), and the Gymnophiona or Apoda (the limbless caecilians). An extinct group, the family Albanerpetontidae in the order Allocaudata, was moderately successful, spanning 160 million years from the Middle Jurassic to the Early Pliocene, an interval that ended 3.6 million years ago.
Some writers have argued that the Early Permian dissorophoid Gerobatrachus hottoni is a lissamphibian. If it is not, the earliest known lissamphibians are Triadobatrachus and Czatkobatrachus from the Early Triassic.
Some, if not all, lissamphibians share the following characteristics. Some of these apply to the soft body parts, hence do not appear in fossils. However, the skeletal characteristics also appear in several types of Palaeozoic amphibians:
- Double or paired occipital condyles
- Two types of skin glands (mucous and granular)
- Fat bodies associated with gonads
- Double-channeled sensory papillae in the inner ear
- Green rods (a special type of visual cell, unknown in caecilians)
- Ribs do not encircle body
- Ability to elevate the eyes (with the levator bulbi muscle)
- Forced-pump respiratory mechanism
- Cylindrical centra (the main body of the vertebrae; cylindrical centra are also found in several groups of early tetrapods)
- Pedicellate teeth (the crowns of the teeth are separated from the roots by a zone of fibrous tissue; also found in some Dissorophoidea; the teeth of some fossil salamanders are not pedicellate)
- Bicuspid teeth (two cusps per tooth, also found in juvenile dissorophoids)
- Operculum (small bone in the skull, linked to shoulder girdle by the opercularis muscle; perhaps involved in hearing and balance; absent in caecilians and some salamanders, fused to the stapes (ear bones) in most anurans)
- Loss of posterior skull bones (also in Microsauria and Dissorophoidea)
- Small, widely separated pterygoid bones (also found in Temnospondyli and Nectridea)
- Wide cultriform process of the parasphenoid (also found in some Microsauria (Rhynchonchos) and Lysorophia)
Relationships and definition
The features uniting the Lissamphibia were first noted by Ernst Haeckel, even though in Haeckel's work, Lissamphibia excluded the caecilians. Nevertheless, Haeckel considered the caecilians to be closely related to what he called Lissamphibia (which is now called Batrachia and includes frogs and salamanders). In the early to mid 20th century, a biphyletic origin of amphibians (and thus of tetrapods in general) was favoured. In the late 20th century, a flood of new fossil evidence mapped out in some detail the nature of the transition between the elpistostegalid fish and the early amphibians, most paleontologists no longer accept the view that amphibians have arisen twice, from two related but separate groups of fish. With the single origin of amphibians being accepted by most herpetologists and paleontologists, it was assumed that Lissamphibia was monophyletic as well. However, the origin and relationships of the various lissamphibian groups both with each other and among other early tetrapods remains controversial. Not all paleontologists today are convinced that Lissamphibia is indeed a natural group, as there are important characteristics shared with some non-lissamphibian Palaeozoic amphibians.
Currently, the three prevailing theories of lissamphibian origin are:
- Monophyletic within the temnospondyli
- Monophyletic within lepospondyli
- Diphyletic (two separate ancestries) with apodans within the lepospondyls and salamanders and frogs within the temnospondyli.
One of the hypotheses regarding their ancestors is that they evolved evolved by paedomorphosis and miniaturization from early tetrapods.
Recent molecular studies of extant amphibians based on multiple-locus data favor one or the other of the monophyletic alternatives and indicate a Late Carboniferous date for the divergence of the lineage leading to caecilians from the one leading to frogs and salamanders, and an Early Permian date for the separation of the frog and salamander groups.
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