Slow Diffusion onto Land

John Merck

Tetrapoda

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.

Synapomorphies:

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. :


Eoherpeton watsoni an Early Carboniferous anthracosaur from Carroll 2009.
Anthracosauria: (Carboniferous - Triassic) Aquatic to mostly terrestrial vertebrates of the late Paleozoic. Although this groups membership varies with each new publication, it seems monophyletic.

Identifying characters:

Relationships: Until recently seen as the basal members of Reptiliomorpha. Some characteristics, including the tabular parietal contact and the large pleurocentra are shared with other reptiliomorphs, including amniotes. However recent analyses have placed them outside of Tetrapoda, in part due to the persistence of plesiomorphies like caudal fin rays (Clack, 2011).

Anthracosaur superlative: Embolomeri: (Carboniferous - Triassic) A speciose group including aquatic predators characterized by fully embolomerous vertebrae - intercentra and pleurocentra disk-shaped and roughly equal. These included eel-shaped aquatic predators of the Carboniferous and Permian such as Archeria Pholiderpeton.

The Amphibian Stem:


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. 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.

Synapomorphies:


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.


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.

Synapomorphies:

Diversity:



The archegosaur Prionosuchus from Dinopedia
Breathing in Amphibia: Witzmann, 2015 notes two key facts: It follows that ancient amphibians who retained their scales (especially big ones) should also retain their gills as a means to eliminate CO2. In fact, his survey of the branchial skeletons of early tetrapods indicates that among temnospondyls, only: Appear completely to have lost their gills as adults.


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 froms.

And yet, they present striking plesiomorphies:


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


Rhynchonkos stovalli, a microsaur (left) and Paleothyris acadiana, and amniote (right) from Carroll 2009
Potential convergence or synapomorphy with Amniota:

Major lepospondyl groups:

Lepospondyl problems: But there is growing suspicion that Lepospondyli, as traditionally understood, may not be monophyletic. Consider:

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

Anura:

(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

Caudata:

(Late Jurassic - 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

Gymnophiona:

(Early Jurassic - 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.

Morphology:



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

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.


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:


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 Leopspondyli: 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.

Pros:

Cons:

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.

Pros:

Cons:

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:

As we speak, the weight of opinion seems to support door # 2 - Monophyletic Lissamphibia as temnospondyls, but only slightly. Changing the assumptions and search algorithms on the same matrix can break Lissamphibia up or force it to jump between amphibian and reptiliomorph stems. When Pardo's team publish their results, everything might change. Stay tuned.

Additional reading: