DESCRIPTION OF THE CLASS BATRACHOMORPHA (SÄVE-SÖDERBERGH 1934)

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Batrachomorpha (ba-TRAK-o-MORF-a) comes from two Greek roots meaning “frog form” [frog- batrachos (βάτραχος) and form- morphi (μορφή)]. The reference is to the frog-like nature of the animals in this class. The name was coined by Säve-Söderbergh (1934) to include all living and extinct amphibian taxa. Its definition in the narrow sense that we use is from Benton (2004).

INTRODUCTION TO THE BATRACHOMORPHA Benton (2004) uses the term Batrachomorpha to define a monophyletic group (see Figure 1A & B) that includes the extinct Temnospondyls (from two Greek roots meaning “cut vertebrae”) and the living Lissamphibia (from two Greek roots meaning “smooth amphibians”). The batrachomorphs are united by several synapomorphies including four digits on the forefeet, a flattened skull with fused skull elements, stalked or pedicillate teeth, 26 or fewer presacral vertebrae, large orbits, paired occipital condyles, and the maxilla articulates with the quadratojugal. They are aquatic or tied to an aquatic environment through reproduction in that most lay eggs in water and require water for an aquatic larval stage.

A.

CLADES OF THE BATRACHOMORPHS 1. The Temnospondyl Clade 2. The Lissamphibian Clade 3. The Batrachian Clade

B.

FIGURE 1. A. Major clades of the Batrachomorpha. Lepospondyli + Amniota is the outgroup. Frost et al. (2006) confirm the monophyly of the Lissamphibia (clade 2). Both Benton (2005) and Anderson (2008) give convincing evidence that the Temnospondyls are sisters to the Lissamphibia. Extinct taxa are in red. FIGURE 1. B. The position of the Batrachomorpha (Temnospondyli + Lissamphibia) relative to the tetrapod taxa after Benton (2005).

The Temnospondyl Clade (1) The temnospondyls were amphibious with gilled larvae and adults that were aquatic, semi-terrestrial or fully terrestrial from the Pennsylvanian (Upper Carboniferous) to the Lower Cretaceous. Their heads were frog-like and many had a lateral line. Other distinctive features included a large opening in the palate and several palatine fangs. Their most distinctive feature was the cut vertebra (=temnospondyl) of the animals (see Figure 2 below), which were complex, primitive vertebrae made of separate unfused components that include the pleurocentra, intercentra, neural arches, and zygopophyses. One of the most important taxa was Eryops (Figure 3), a semi-aquatic to terrestrial predator of the lower Permian. It had a sprawling stance, was 1.8-2 m long, and weighed nearly 100 kg. Some, like Dvinosaurus (Figure 4) of the upper Permian and lower Triassic, evolved to become sexually mature as larvae. Such paedomorphic forms evolved a weaker (more aquatic) structure in which the pleurocentra were lost or highly reduced. On the other hand, Figure 5 shows Balanerpeton, an early Temnospondyl that became almost entirely terrestrial as an adult.

FIGURE 2. An illustration of the evolution of the vertebrae of tetrapods from Ichthyostega to a Temnospondyl (top right) and the amniote type (bottom right). Image from: http://www.elp.manchester.ac.uk/pub_projects/2003/MNZO0MLK/lectur27.jpg

FIGURE 3. Image of Eryops, an extinct temnospondyl. Image from: http://www.bertsgeschiedenissite.nl/geschiedenis%20aarde/amfibieen.htm.	FIGURE 4. Divinosaurus of the Permian and Triassic, was a paedomorphic temnospondyl with external gills and a lateral line. Image by Dmitry Bogdanov, Wikipedia Commons.	FIGURE 5. Balanerpeton of the Carboniferous, was almost entirely terrestrial as an adult. Image by Arthur Weasley, Wikipedia

The Lissamphibian Clade (2) All of the living amphibians are in the Lissamphibia, which includes three major groups: Gymnophona (from two Greek roots meaning “naked snake”) commonly called the caecilians, Urodela (from a Latin and a Greek root meaning “visible tail”), and the Anura (from a Greek and a Latin root meaning “no tail”). All of the extant amphibians are carnivores (or insectivores). The caecilians are blind, legless, worm-like amphibians that live a fossorial or aquatic lifestyle (see Figure 6). Their annular scales make them appear to be segmented like giant earthworms. Even though they are worm or snake-like, they have tails that are short or absent with a very elongate trunk (up to 200 vertebrae). Anderson (2008) summarizes the debate in the literature regarding the relationship between the caecilians and the rest of the Lissamphibia. Anderson (2008) supports and the enormous analysis of Frost et al. (2006) confirms the sister relationship between the caesilians and the Batrachia (Clade 3). A competing view is the the caecilians are sisters to the Lepospondyli, which would make the Batrachomorpha paraphyletic.

The Batrachian Clade (3) Urodela (also called the Caudata) are the salamanders and newts. They have four sprawling legs, a short body, and a long tail, though some taxa have lost their limbs entirely. Salamanders undergo a metamorphosis in which the larvae have external gills but otherwise resemble the adults. However, some purely aquatic taxa (e.g. Necturus) retain larval characters as adults. The largest salamanders are the Hellbenders (Cryptobranchus, Figure 7), which are nearly 2 meters long, are lungless and gilless, and respire entirely through the skin in cold well-oxygenated upland streams. Newts (see the Red-Spotted Newt, Figure 8) have a three-stage development. After they mature as an aquatic larva, they emerge to become the brightly-colored eft which can be found on the floors of damp forests. Then, they return to the water, become dull green with a swimming tail in anticipation of reproduction. The Anura (also known as Salientia) are the frogs and toads. These animals are tailless as adults (Anura is from two Greek roots meaning without tails) and a highly modified axial skeleton that provides for rigid support of the torso. The shortened forelegs and elongate, muscular hind legs are adapted to leaping. Most exhibit complete metamorphosis; however, a few groups have direct development. Frogs and toads are among the most successful vertebrates with about 4,000 described species worldwide. Xenopus, the African Clawed Frog (Figure 9), is a fully aquatic frog of sub-Saharan Africa and often used as a model for vertebrate development. Dendrobates (Figure 10) is a brightly-colored tree frog from the rainforests of Central and South America. It is one of the Poison Dart Frogs because it exudes a powerful neurotoxin in the mucus that covers its body. The obvious advertisement of its presence (red body and blue legs) is a warning to a would-be predator that this animal is dangerous.

FIGURE 6. A caecilian from Thailand looks like a brightly colored earthworm. Image from: http://people.virginia.edu/~tsr6a/2caecilian.jpg	FIGURE 7. A Hellbender, a lungless, gilless aquatic salamander of the Eastern US. Image from: http://people.wcsu.edu/pinout/herpetology/calleganiensis/bottom.jpg	FIGURE 8. Red-Spotted Newt of the Eastern US. Image from: http://www.ct.gov/dEP/cwp/view.asp?a=2723&q=325776	FIGURE 9. Xenopus, the extant African Clawed Frog. Image from the Systematic Biology Biodiversity Archive.	FIGURE 10. Dendrobates, a frog of the rainforests of Central America, is one of the poison dart frogs. Image from the Systematic Biology Biodiversity Archive.

LITERATURE CITED Anderson, J. S. 2008. Focal review: The origin(s) of modern amphibians. Evolutionary Biology. 35: 231-247. Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Frost, D. R., T. Grant, J. Faivovich, R. H. Bain, A. Haas, C. F. B. Haddad, R. O. de Sa, A. Channing, M. Wilkinson, S. c. Donnellan, C. J. Raxworthy, J. A. Campbell, B. L. Blotto, P. Moler, R. C. Drewes, R. A. Nussbaum, J. D. Lynch, D. M. Green, and W. C. Wheeler. 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History. 297: 1-297. Pough, F. H., C. M. Janis, and J. B. Heiser. 2009. Vertebrate Life. 8th ed. Benjamin Cummings. New York. pp. 688. Säve-Söderbergh, G. 1934. Some points of view concerning the evolution of the vertebrates and the classification of this group. Archiv für Zoologi. 26A (17): 1-20.

By Jack R. Holt and Carlos A. Iudica. Last revised: 08/08/2016