Tetrapoda - the definition!, Life strategies, love, and homology among the amphibians

The Devonian extinction seems to have wiped out the water-breathing digit-bearing vertebrates like Acanthostega and Ichthyostega, however many basal stegocephalians held on in the Early Carboniferous. Among them was the (unknown) last common ancestor of all living land vertebrates - the ancestor of Tetrapoda.

Tetrapoda as traditionally conceived contains the vertebrates with fingers and toes. Like pornography, we feel we know tetrapods when we see them, but when we try to define them explicitly, they get slippery.

Actually applying the term to the cladogram is very problematic for two reasons:


Synapomorphies:


Caerorhachis bairdi (Carboniferous) a basal reptiliomorph from Wikipedia
Phylogeny: Tetrapoda consists of two major lineages:
Crown-groups: Node-based monophyletic groups specifically defined to encompassed all living diversity. E.G.: Lissamphibia.

Dendrerpeton acadianum (Carboniferous) a basal amphibian from Wikipedia
Amphibia: For now, we consider Amphibia. A basal member of this group like Dendrerpeton (right) does not look terribly different form a basal reptiliomorph like Caerorhachis (above) however the "amphibian" lineage acquired certain derived features early on, including:

Most fossil "amphibians" belong to a large group called Temnospondyli that is probably paraphyletic with respect to Lissamphibia. This is a diverse group of tetrapods ranging from the Early Carboniferous to the Cretaceous, although their heyday was the late Paleozoic and earliest Mesozoic. They lived in or near a variety of fresh water and shallow marine aquatic habitats and exhibited a wide range of body forms, some being stout with broad heads, some flattened, and some elongate. Head shapes, likewise varied from long snouted to broad headed forms. All were aquatic predators of some stripe or another. Dendrerpeton is a basal member of this diverse group. It is less than a meter in length, but many temnospondyls achieved the size of crocodiles, and displayed varying degrees of aquatic or terrestrial specializations.

A few interesting highlights:

(Want to know more? For all your temnospondyl needs, check out the temnospondyl entries on Darren Naish's wonderful Tetrapod Zoology blog.)



Capetus by D. Bsogdanov from Wikipedia
The well-known temnospondyls display:


Each group has specific synapomorphies with which we needn't concern ourselves here. Many, however, involve the loss or non-differentiation of dermal bones of the skull. Compare the crania pictured above with that of Doleserpeton, a temnospondyl, to get the idea:


Lissamphibia: The living amphibians


Rana catesbeiana, the bullfrog (left) in palatal view (stapes in blue); Cryptobranchus allegheniensis, the hellbender (right)

Doleserpeton, a derived temnospondyl.

Ruta et al. (2003) reconstruct Lissamphibia as the sister taxon of derived temnospondyls like Doleserpeton (Permian) above. Some potential synapomorphies of Lissamphibia and derived temnospondyls are clear: Potential synapomorphies of Lissamphibia:

Diversity:

Dermophis mexicanus from Wikipedia
Gymnophiona: Early Jurassic - Recent. Commonly called caecilians.



Celtidens ibericus, a Cretaceous albanerpetontid from Paleocritti.com
Albanerpetontidae: (Jurassic - Neogene) A minor group of extinct lissamphibians. Distinguished by features of cranial osteology, including non-pedicellate three-cusped teeth. Resembling scaly salamanders. For us, their important role is to remind us that the loss of scales in Amphibia only occurred inside Lissamphibia. Thus, we should not assume, as many artists do, that ancient amphibians had naked skin like that of frogs and salamanders.


Ambystoma tigrinum the tiger salamander
Caudata - Salamanders: Late Jurassic - Recent



White-lipped tree frog - Litoria infrafrenata
Anura - frogs:


Of these, Caudata and Anura are closest relatives. Caudata and Anura are united by several synapomorphies, including:

Take-home message: We may sometimes refer to lissamphibians as "primitive" with respect to amniotes, but they are highly evolved. There is no living tetrapod that even begins to resemble the last common ancestor of Tetrapoda.

Next week we will discuss several biological issues of Amniota. For now, we introduce the group to facilitate our discussion of Tetrapoda generally.

An evolutionary conundrum: This tetrpod overview has highlighted reproduction to set up a question: In Sarcopterygii ancestrally, was fertilization internal or external? Examination of cladogram shows the optimization of the character to be ambiguous.



Stevenson Memorial by Abbott Thayer from Wikipedia

Test of Homology

Of course, an alert observer would object to the last point. Spermatophores clearly don't look homologous with phallodia and penises and the latter two may not be homologous either. Having said that, though, how do we really know it? Cladists rely on three tests to falsify hypotheses of homology:


BTW, I have listed these tests in order of increasing strength. We would, for instance, reject the homology of the angel arm and wing, no matter how similar they looked.