Slow Diffusion onto Land

John Merck

Link to Temnospondyli cladogram and phylogram cheat-sheets
Link to Reptiliomorpha cladogram and phylogram cheat-sheets


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 crown-group Tetrapoda. Their common ancestry is concealed within Romer's Gap. Their descendants (along with some "stem tetrapod" lineages emerge from it well-differentiated.)

Tetrapoda: Opinions vary as to where this name should be stuck to the tree:

For our purposes, the crown-group definition applies.


Phylogeny: Tetrapoda consists of two major lineages with living members:

Greererpeton burkemorani
Noteworthy plesiomorphies - Evolving sensory modalities: Mentioned previously, all of the special senses needed to be adjusted for life on land. For most, these changes left no fossil record (vision, olfaction). In two, however, there are clear osteological correlates. :

The Amphibian Stem:

A synopsis of temnospondyl phylogeny after Schoch, 2013 and Schoch, 2018
Temnospondyli: (Carboniferous - Cretaceous/Quaternary (?)) The name originally coined for basal tetrapods with rhachitomous vertebrae, but has become one of the better-supported monophyletic groups. Fabulously diverse and speciose. Small to large and aquatic to mostly terrestrial. (Morphometric analysis of vertebrae by Carter et al., 2021, suggest that ancestrally, adult temnospondyls were terrestrial but experienced repeated reversions to semi- or fully aquatic ecologies.) Temnospondyls were a primary component of the late Carboniferous and Permian land biota, and experienced a Triassic radiation of aquatic forms. Morphologically they varied from unspecialized to rather stout, short-tailed forms. Very few evolved the eel-like shape so common in embolomeres.


Temnospondyl Diversity: The problems and competing hypotheses of temnospondyl phylogeny are beyond our scope. What follows is a review of the major groups recovered by Schoch, 2013 and Schoch, 2018.

Capetus - an edopoid - from Palaeos
Edopoidea: (Carboniferous - Permian) A basal and plesiomorphic group of large predators, ranging from the more terrestrial Capetus to the aquatic Nigerpeton.

Dendrerpeton acadianum (Carboniferous) a basal amphibian from Wikipedia
"Dendrerpetontidae": (Carboniferous) A basal and plesiomorphic group of small predators of uncertain monophyly. Interesting because it consists of:

Capetus - an edopoid (left) and Doleserpeton, a member of Rhachitomi (right)
Rhachitomi: (Carboniferous - ?) Remaining temnospondyls belong to Rhachitomi. Synapomorphies include:

Dvinosaurus primus by D. Bogdanov from Wikipedia
Dvinosauria: (Late Carboniferous - Early Triassic) First representatives of a recurring theme in temnospondyl evolution - paedomorphic adults that retain larval characteristics including gills as an adult. Speciose and common during the Permian.

Acanthostomatops from Carroll, 2008
Zatracheidae: (Late Carboniferous - Permian) Aquatic temnospondyls with flattened spiny armored skulls. An internarial fenestra stimulates idle speculation about its purpose. Link to reconstruction of Zatrachys.

Cacops - a dissorophid - from Wikipedia
Dissorophoidea: (Late Carboniferous - Quaternary) A diverse group of small temnospondyls including:

Eryopiformes: (Permian - Cretaceous) The large Mesozoic radiation of aquatic temnospondyls and their basal Permian relatives.



The archegosaur Prionosuchus from Dinopedia
Breathing in Amphibia: Recall that even in stem-tetrapods who relied on lungs to obtain oxygen, Witzmann, 2015 notes that the eliminate CO2 through the gills because this is more efficient in an aquatic environment. But gills impose limitations because they must be kept moist. Tetrapods have found two ways around this problem: But note: most ancient amphibians retained fishy scales, preventing them from exchanging gasses through the skin. It follows that these (especially big ones) should also retain their gills as a means to eliminate CO2. In fact, the Witzmann, 2015 survey of the branchial skeletons of early tetrapods indicates that among temnospondyls, only: Appear completely to have lost their gills as adults. Gills were lost very early on the reptiliomorph side, whose members relied upon lungs for all gas exchange.

The Reptiliomorph Stem

Synapomorphies and trends:

Reptiliomorph diversity:

Chroniosaurus dongusensis by D. Bogdanov from Wikipedia
Chroniosuchia: (Permian - Triassic) Late-stage survivors of the Triassic. Distinctive features include:

Hypotheses of phylogeny place them all over the tree, from a position outside crown group Tetrapoda to one close to Amniota. All authors agree that they are not on the Amphibia branch.

Seymouria baylorensis (Permian)
Seymouriamorpha: (Latest Carboniferous - Permian) Small animals with aquatic larvae but adults ranging from the terrestrial Seymouria to paedomorphic and more aquatic forms.

And yet, they present striking plesiomorphies:

Lepospondyli: (Carboniferous - Permian (?)) An amazingly diverse group of small tetrapods characterized by trends toward:

Major lepospondyl groups:

Lepospondyl problems: But that is all so "20th Century." In the last decade, the lepospondyl roster had eroded significantly. Consider:

This leaves only Nectridia and a handful of of "microsaur" grade animals remaining in Lepospondyli. Do they actually belong there? The lepospondyl death-watch continues.

Before proceeding toward Amniota, we pause to tie up a loose end:

Lissamphibia (?) - the living amphibians

Traditionally regarded as monophyletic, Lissamphibia contains all three groups of living amphibians and one fossil group:

Megophrys montana


(Early Triassic - Quaternary) Frogs are very highly derived for specialized forms of locomotion, hearing, and prey-capture. Although most hang out near water, only a few actually feed in the water, and their adaptations are not as useful there. Among their idiosyncrasies:

Triadobatrachus massinoti from Carroll 2009.
Most fossil frogs share these features (although some have more dorsal vertebrae). Our earliest glimpse is from Triadobatrachus from the Early Triassic. Although plesiomorphic in many ways, and probably unable to jump, it shows the initial stages of many froggie adaptations of the cranium and postcranium.

Batrachuperus sinensis - dermal skull roof shaded. Modified from Carroll 2009


(Late Triassic - Quaternary) Salamanders. Generally less specialized than other lissamphibians and frequently adapted for life in the water. Although their water-breathing larvae are less specialized, we often see paedomorphic, permanently aquatic salamanders, and many of their anatomical specializations seem like adaptations to aquatic life. As in frogs, scales are completely lost, facilitating cutaneous breathing.

Cranial synapomorphies:

Postcranial synapomorphies:

Fossil salamanders largely resemble living ones. Examples include:

Dermophis mexicanus from Wikipedia


(Late Triassic - Quaternary) Caecilians. As weirdly derived as frogs but in the opposite direction - as limbless burrowers (although some are secondarily aquatic.) Caecilians retain small scales and do not breathe cutaneously.


Ichthyophis glutinosus - dermal skull roof shaded. Modified from Carroll 2009
Features of the skull:

Eocaecilia micropodia from Carroll 2009
Eocaecilia: (Early Jurassic) Only one fossil caecilian suggests their ancestral form. Noteworthy for the retention of limbs, however many derived caecilian adaptations are clearly visible. Its skull is plesiomorphic in the retention of postparietals, jugals, and (maybe) tabulars.

Celtidens ibericus, a Cretaceous albanerpetontid from Yacimiento de las Hoyas


(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.

Celtidens ibericus, Dermal skull-roof shaded. Modified from Carroll 2009

Up close, the skull of an albanerpetontid looks like a salamander attempting to become a caecilian. The neurocranium is strengthened by the fusion of prootics, opisthotics, and exoccipitals into an otic bone, however the caudate cheek-gap and hinged squamosal are still evident. Ruta and Coates, 2007, find albanerpetontods to be stem-gymnophionans. One clear synapomorphy of albanerpetontids:

Yaksha perettii from Science News
Daza et al., 2020 illuminated albanerpetontid ecology with their description of Yaksha perettii, a mid-Cretaceous member preserved in amber. Yaksha sports an elongate entoglossal process of the hyoid arch analogous to those of chameleons, suggesting that like them, it was a "ballistic" ambush predator that captured its prey by projecting its tongue. Was it just Yaksha, or is this a general feature of Albanerpetontids?

Lissamphibian relationships:

Batrachian ear, operculum shaded. Modified from Carroll 2009

What we definitely know:

Batrachia: (Early Triassic - Quaternary) The last common ancestor of anurans and caudates. Synapomorphies include:

Lissamphibian phylogenetic hypotheses:

Beyond this point, four major hyoptheses exist, reflecting two big issues:

Potential synapomorphies of Lissamphibia, if monophyletic:

Hypothesis I: Monophyletic Lissamphibia as members of Lepospondyli: Laurin and Reisz, 1997 find Lissamphibia to be nested within Lepospondyli as the sister taxon of Lysorophia, with Gymnophiona as the sister taxon to Batrachia.



Hypothesis II: Monophyletic Lissamphibia as members of Dissorophoidea: Beginning with the first application of cladistic methods (EG Milner, 1988), Lissamphibia has been found monophyletic and nested within Temnospondyli. Ruta and Coates, 2007 have recently confirmed this result, with Lissamphibia nesting inside amphibamid dissorophoids.



Hypothesis III: Polyphyletic Lissamphibia with Batrachia as member of Dissorophoidea and Gymnophiona nested within "microsaurs.": Other recent, credible phylogenetic results (E.G. Anderson 2007) recover a pattern in which: