GEOL 431 Vertebrate Paleobiology

Spring Semester 2023

Slow Diffusion onto Land

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:

• Clack, 2012 feels that excluding creatures with hands and feet from a group whose name means "four feet" is perverse. For her, Tetrapoda has is a classic "apomorphy-based definition."
• Coates et al., 2008 favor restricting the name to the "crown group" - i.e. the node-based group defined as the last common ancestor of living members of the group.

For our purposes, the crown-group definition applies.

Synapomorphies:

• First two cervical vertebrae specialized as the atlas and axis. These facilitate movement of the head on the neck.
• Not a synapomorphy, exactly, but somewhere before Tetrapoda, the number of digits on manus and pes had stabilized at five.

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

• Amphibia: (total-group) All organisms more closely related to frogs, salamanders, and caecilians than to Amniota.
  NOTE: This use of the term "Amphibia" excludes creatures like seymouriamorphs and lepospondyls, even though they might have ecologically resembled modern amphibians.
  
  • Lissmphibia: (Crown-group) The last common ancestor of living amphibians (frogs, salamanders, and caecilians) and all its descendants.

• Reptiliomorpha: (total-group) All organisms more closely related to Amniota than to Lissamphibia.
  NOTE: Some creatures in the reptiliomorph total-group, might have ecologically resembled amphibians despite their evolutionary propinquity to amniotes.
  
  • Amniota (Crown-group) The most recent common ancestor of mammals and birds and all of its descendants.

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. :

• Lateral line sense: Useless out of water. Thus, they are quickly lost in creatures that spend most of their time on land. Although not always preserved as bony sensory canals, the presence of such is a clear indicator of aquatic life-style.
  
  
  Schematic tetrapod ear from talkorigins.org
• Hearing: Acoustic impedance is the resistance of a system to the flow of sound through it. It is a function of the density of the medium. The density of the body is similar to that of water. Thus, sound can pass directly from water through the body into the otic capsule at full intensity. Air is much less dense than water. Thus, for airborn sound to register in the otic capsule, it must be collected and amplified. This is accomplished by an impedance matching ear:
  • Sound is collected over a broad area by a tympanum (A.K.A. ear drum) or through the jaw.
    
  • In land vertebrates, a rod-like bone called the stapes transmits the vibrations of the tympanum to a small opening in the otic capsule, the fenestra ovalis (oval window.) By being concentrated into a small area, the sound is amplified so that it can be detected by the inner ear.
    
    
    Choanate hyomandibula/stapes cladogram from Nature
  • The stapes is the hyomandibula, modified from the transmission of sound. The air filled space it occupies is called the middle ear. Together with the eustacean tube that connects the middle ear to the pharynx, it is homologous to the spiracle of aquatic vertebrates.

  • The fenestra ovalis is the ancestral vestibular fontanelle, the ventral extremity of the otico-occipital fissure.

  • The tympanum represents the developmental failure of the spiracle to penetrate the surface of the body. Instead, a thin membrane is stretched across the spiracular notch.

    The transformation of the hyomandibula into a stapes is gradual, and in some creatures, the structure probably served both load-bearing and acoustic functions. The transformation of the spiracle into an otic notch for the tympanum has no direct fossil record. When a notch is present at the posterior cheek margin, we rely on inference. We assume:

    • Acanthostega's stapes was short and massive, but rather than articulating with an articular face in the neurocranium, it's head fit into the vestibular fontanelle.
    • Creatures like Crassigyrinus with heavy stapes and extensive lateral line systems might have retained an open spiracle.
    • Creatures like Greererpeton with no notch might have no external spiracle or tympanum at all.
    • Creatures with reduced lateral line systems and slender stapes probably had tympana.

  As we will see, the impedance-matching ear evolved slowly and independently several times in different lineages.

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. (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.

Synapomorphies:

• Four-fingered manus. Note: What's lost is the oddball digit V, not derived from the digital arch. (Pes retains five toes.)
• The humerus has a distinct shaft at mid-length.
• Slender rod-shaped stapes. (Since most also have a large notch at the rear of the skull, most view as evidence for impedance-matching ear.)
• Wide interpterygoid vacuities (link to Capetus scale = 5 cm.) in the palate. In life, the skin covering these was covered by a shagreen of tiny denticles.
• Exoccipitals sutured to postparietals and form paired occipital condyles.

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.

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

"Dendrerpetontidae": (Carboniferous) A basal and plesiomorphic group of small predators of uncertain monophyly. Interesting because it consists of:

• Dendrerpeton (Late Carboniferous - right). A small form known from well-preserved specimens in Joggins Nova Scotia tree trunk casts. Its skull displays a deep otic notch and wide interpterygoid vacuities. Judging from its lack of sensory canals and preservation in a terrestrial environment, its seems to have been primarily terrestrial as an adult. Its pectoral girdle conveys a good impression of the ancestral state for Tetrapoda.
• Balanerpeton: (Early Carboniferous) The oldest known temnospondyl, also from the lacustrine environment of East Kirkton, Scotland. These, too, seem to be terrestrial animals whose bodies washed into a lake bed.

Rhachitomi: (Carboniferous - ?) Remaining temnospondyls belong to Rhachitomi. Synapomorphies include:

• Loss of the intertemporal.
• Trend toward enlarged interpterygoid vacuities. (Note: In Doleserpeton (right) the pterygoids no longer meet anteriorly.)

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.

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.

Dissorophoidea: (Late Carboniferous - Quaternary) A diverse group of small temnospondyls including:

• Branchiosauridae and Micromelerpetontidae: (Carboniferous - Permian) Small aquatic forms with healthy sensory canals.
• Dissorophidae: (Carboniferous - Permian) Stout terrestrial forms with dermal armor and deeply incised otic notches (right). This includes the sail-backed temnospondyl Platyhystrix. (Link to image)
• Amphibamidae: (Carboniferous - Permian) Small forms with large orbits and interpterygoid vacuities. In some ways convergent on amniotes:
  • Sensory canals lost.
  • Humerus with a long shaft.
  • Gastrocentrous vertebrae: with large pleurocentra, often coossified with neural arches, and small crescent-shaped intercentra.

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

Synapomorphies:

• Otic notch is restricted to the dorsal part of the cheek.
• Prefrontal and jugal in contact in margin of the orbit.

Diversity:

• Eryopidae: (Carboniferous - Permian) The basal group contains Eryops, beloved of illustrators.
• Archegosauridae: (Permian) Specialized long-snouted predators that seem to have hunted in fresh water but ventured onto land frequently in the manner of crocodilians.
• Stereospondyli: (Permian - Cretaceous) Aquatic fresh-water and marine predators that experienced a significant radiation in the Triassic.
  
  
  Rhineceps nyasaensis from Carroll, 2008
  Stereospondyl evolutionary trends:
  • Reduction of the pleurocentra. This reaches its logical conclusion in some members with proper stereospondylous vertebrae.
  • Flattening of the head.
  • Further enlargement of the interpterygoid vacuity.
  • Basipterygoid articulation (formerly a mobile joint) becomes an immobile suture.
  • Occipital condyles project far posteriorly.
  • Elongate transverse processes and neural spines.

  Stereospondyl diversity:

  • Paraphyletic grade of basal members including Rhineceps nyasaensis (right) and Koolasuchus cleelandi, from the Cretaceous of Australia, the youngest known stereospondyl. (Maybe.)
  • Trematosauria: (Triassic - Jurassic) Large forms with palate strongly integrated with neurocranium by broader basipterygoid sutures and suturing of pterygoid to exoccipitals. (Link to Thoosuchus yakovkevi.) Spectacular examples:
    • Trematosauridae: (Triassic) Long-snouted marine and fresh-water forms. Link to skull of Trematosaurus.
    • Metoposauridae: (Triassic) Exceptionally flat-headed lurkers in Triassic fresh waters. (Link to Buettneria perfecta skull and skeletal mount.
    • Plagiosauridae: (Triassic - Jurassic) Short-faced paedomorphic forms, yet still large. (Link to skull of Gerrothorax.)

  • Capitosauria: (Triassic - Earliest Jurassic) Including the largest heads of any temnospondyl, belonging to the likes of Mastodonsaurus. (Link to skull of Mastodonsaurus giganteus.)

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:

• Lissamphibians exchange gasses including CO₂ through the skin. Easy enough in aquatic or moist terrestrial environments.
• Amniotes suck it up and exchange all gasses through the lungs. Presumably the advantages of terrestrial life outweigh the cost of less efficient CO₂ elimination.

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:

• Dissorophoids
• Dendrerpetontids

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:

• Gastrocentrous vertebrae: with large pleurocentra, often coossified with neural arches, and small crescent-shaped intercentra.
• As in anthracosaurs, early members of the reptiliomorph stem display a contact between tabulars and parietals. Whether this is a synapomorphy of reptiliomorpha or a larger group is not clear.

Reptiliomorph diversity:

Chroniosuchia: (Permian - Triassic) Late-stage survivors of the Triassic. Distinctive features include:

• armor of interlocking scutes on their backs.
• Snouts perforated by antorbital fenestrae.

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.

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

• Stocky bodies and short tails, indicative of adult life on land
• Impedance-matching ears in deep embayments at the rear of the skull. The fenestra ovalis is enclosed in a long tube formed from the basisphenoid and parsphenoid - a condition unique to Seymouriamorpha.
• Swollen neural arches. (Note: this is also seen in many basal amniotes. Convergence?)
• Very large adults of the terrestrial forms like Seymouria incorporate a second sacral rib pair and lose sensory canals in skull.

And yet, they present striking plesiomorphies:

• Discosaurus: retains the anocleithrum above its cleithrum.

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

• Holospondylous vertabrae in which the pleurocentrum is fused to the neural arch. (Some basal forms have small intercentra.)
• Loss of skull bones including the supratemporal and intertemporal.
• Paired occipital condyles (convergent with Temnospondyli. Link to Pytonius.)

Major lepospondyl groups:

• "Microsauria": (Carboniferous - Permian (?)) An ecologically diverse variety of small critters, ranging from long-bodied forms with small limbs, to stout compact ones. The traditional group of all "microsaur" grade animals is paraphyletic, but recent work indicates that it contains a large monophyletic assemblage - Recumbirostra whose members are typically elongate with tiny limbs. In this and other respects they resemble modern fossorial squamates and are thought to be fossorial, themselves.
  
  
  Rhynchonkos stovalli from Carroll 2009
  A characteristic example that will figure in later discussion - Rhynchonkos stovalli (Early Permian)
  Characteristic features include:
  • A characteristic wedge-shaped atlas that inserts between the occipital condyles.
  
  Brachydectes newberryi from Carroll 2009
• Lysorophia: (Carboniferous - Permian (?)) Contains single family Molgophidae, so that the two terms are more or less interchangeable. Extremely elongate forms with:
  • Reduced limbs
  • Significant reduction of dermal skull bones, including loss of:
    • Postfrontals
    • Jugals
  • Reorientation of quadrates, displacing jaw articulation anteriorly.
  
  
  
  Urocordylus wandesfordii from Carroll 2009
• "Nectridia:" (Carboniferous - Permian) Specialized aquatic forms with:
  • Haemal arches that fuse directly to pleurocentra.
  Recent analyses (E.G. Ruta et al., 2007) suggest that this group is paraphyletic.
  
  
  Diplocaulus magnicornis from Carroll 2009
  Nectridia includes Diplocaulidae (E.G. Diplocaulus magnicornis (right and link to full skeleton). Characterized by:
  • Bizarre expansion of the posterolateral corners of the skull table into conspicuous "horns."
  • Convergent evolution of wide interpterygoid vacuities.
  Note: resting trace fossils left by these animals indicate that the "horns" were connected to the body by a fold of flesh. Sorry to generations of illustrators who have reconstructed this animal like this. In reality, it's probably more like this.
  
  

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

• Remember Aistopoda? Prior to Pardo et al., 2017, they were held to be sister taxon to Nectridia. Now evicted to a basal position on the tetrapod stem.
  
  Rhynchonkos stovalli, a microsaur (left) and Paleothyris acadiana, an amniote (right) from Carroll 2009

• Supraoccipitals: Among recumbirostrans we frequently see an endochondral supraoccipital ossification of the occipital arch dorsal to the foramen magnum. This is, otherwise, an amniote feature! Convergence or synapomorphy? Szostakiwskyj et al., 2015 describe CT scans of the microsaur Rhynchonkos and related forms, noting that their supraoccipitals (and other braincase elements) really strongly resemble those of amniotes, specifically members of Eureptilia, implying that Rhynchonkos is actually a reptile!
  
  
  The transverse flange of the pterygoid in assorted amniotes plus the recumbirostran
  Odonterpeton triangular (H) but not Rhynchonkos (I) from Mann et al., 2022

• Transverse flange of pterygoid: In palatal view, the pterygoids of amniotes are drawn into a conspicuous transverse flange anterior to the sub temporal fenestra. This feature helps to stabilize the jaw and provides a base for palatal teeth. Surprise, Mann et al., 2022 identify a rudimentary version in the recumbirostran Odonterpeton triangular.

• Indeed, Pardo et al., 2017 and subsequent analyses have found Recumbrirostra to be basal sauropsids. (We will address in a future lecture.)
  
  
  Comspicuous supraorbital foramina in a lysorophian and two recumbrirostrans from Mann et al., 2022
• What's more, Lysorophia is nested within Recumbirostra in these analyses. See potential synapomorphy at right (Mann et al., 2022).

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:

• Anura: (Early Triassic - Quaternary) Frogs
• Caudata: (Late Triassic - Quaternary) Salamanders
• Gymnophiona: (Early Jurassic - Quaternary) Caecilians
• Albanerpetontodae: (Jurassic - Neogene)

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:

Postcranial:
• No more than eight presacral vertebrae.
• Caudal vertebrae ossify as a urostyle - a single long rod.
• The ilium is very long, articulating with the sacral rib anteriorly and the femur posteriorly. Together, these two synapomorphies enable the frog to rotate its torso around its hips, extending the back while jumping.
• Tibia and fibula are fused into a single unit.
• The tibiale and fibulare (proximal tarsals) become elongate.
• Scales completely lost, facilitating cutaneous breathing.



Gastrotheca walkerii dermal skull-roof shaded. Modified from Carroll 2009
Cranial:
• Frontals and parietals fused into paired frontoparietals.
• Extremely large otic notch accommodates tympanum.
• Jaw muscles reorganized to accommodate large middle-ear cavity.
• Extremely wide interpterygoid vacuities.
• Lower jaw without teeth.
• Hyoid skeleton specialized to allow prey capture by protrusion of the tongue.

Developmental:

• The tadpole - a morphologically and ecologically distinct larval stage.

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.

Caudata:

(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:

• Otic notch and tympanum lost - impedance-matching ear reduced.
• Maxilla does not connect posteriorly with the cheek, creating a gap in the skull margin.
• In basal forms, the squamosal hinges to the neurocranium and can swing laterally, facilitating expansion of the oral cavity and improving aquatic suction-feeding.
  Q: Where else have wee seen this?

Postcranial synapomorphies:

• Distal carpals (and tarsals) 1 and 2 fused into a basale commune. This is only the beginning.
• Alone among tetrapods, the chondrification sequence of the digital arch is reversed, with digits I and II forming first, and digit V (!) last. Note that in salamanders, digit V forms as part of the digital arch! (Fröbisch and Shubin, 2011.)

Fossil salamanders largely resemble living ones. Examples include:

• Chunerpeton (Late Jurassic - China). Regarded as a stem salamander by Rong et al., 2020.
• Karaurus. (Late Jurassic - China)
• Marmorerpeton (Late Jurassic - Scotland) Closely related to Chunerpeton according to Jones et al., 2022..
• Triassurus (Late Triassic - Uzbekistan) A poorly preserved juvenile that, nevertheless, displays the characteristic cheek-gap.

Gymnophiona:

(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.

Morphology:

• The skull, although missing many bones, is compact and strong, for use as a digging impliment.
• The eye sockets are greatly reduced, and the eye is often covered by a layer of skin.
  
• A unique sense organ, the tentacle, protrudes slightly from below the eye. The tentacle can be extended slightly. (In some, the eye and tentacle are consolidated such that the eye can be made to protrude like that of a surprised Loony Toon character.

• Reproduction: Fertilization is internal. (Males have intromittant organ called the phallodium, whose homology to the amniote penis is unknown.) Caecilians can be oviparous (laying large unshelled eggs in moist ground on land) or ovoviviparous.
• Apparently, caecilians are capable of secreting oral venom that can be transmitted into their prey (Mailho-Fontana et al., 2020.)

Features of the skull:

• Otic notch and impedance-matching ear lost.
• Endochondral elements of the neurocranium fused into an os basale
• Bones of the jaw fuse into:
  • Pseudodentary anteriorly
  • Pseudoanguar posteriorly
• Behind the jaw articulation, a spectacular retroarticular process provides leverage for jaw-closing muscles that originate on the branchial skeleton.

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.

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.

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:

• Frontals are fused.

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:

What we definitely know:

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

• The operculum, a cartilaginous element that couples the stapes to the fenestra ovalis.
• The auditory sensory cells of the inner ear are divided into a papilla basilaris and a papilla amphibiorum, in contrast to other vertebrates who have only a single papilla.
• The complete loss of fishy scales and increased reliance upon cutaneous breathing.

Lissamphibian phylogenetic hypotheses:

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

• Is Lissamphibia monophyletic or polyphyletic?
• Where, on the tetrapod tree, do any lissamphibians go?

Potential synapomorphies of Lissamphibia, if monophyletic:

• Significant reduction of skull roof bones. Includes the loss of elements:
  • Supratemporals
  • Ectopterygoids
  and the absolute reduction in the amount of dermal bone.
• Palatal teeth occur in a row that parallels marginal teeth, especially on the vomer.
• Greatly reduced ribs
• Holospondylous vertebrae.
• Several soft-tissue features

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.

Pros:

• Fossils like Eocaecilia are similar to lysorophians in size and proportions.
• All groups involved have lost many of the same bones of the skull and reduced the dermal skull roof.

Cons:

• This result rests on loss and reversal characters whose actual homology is questionable and in which convergent evolution is common elsewhere on the tree.
• And we are worried about whether we understand Lepospondyli well at all.

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:

• The association is based on unique synapomorphies such as:
  • the presence of two-cusped pedicellate teeth (in which the root and crown are separated by a layer of uncalcified tissue)
  • The expansion of the interpterygoid vacuity and loss of the ectopterygoid
  in Lissamphibia and possible close relatives like the Permian Doleserpeton.
• The hypothesis' ready accommodation of new fossil taxa, such as the Early Permian Gerobatrachus (Anderson et al., 2007.)
• Its robustness when new fossil character information comes to light. (E.G. the description of the braincase of Eocaecilia by Maddin et al. 2012.)
• The fact that reinterpretations of known fossils and anatomical structures seem to reinforce it. E.G: Maddin and Anderson's 2012 identification of homologies of the caecilian otic capsule with that of frogs and salamanders.

Cons:

• A significant morphological gap separates Gymnophiona and everything else in this scheme.

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:

• Batrachia nests within Amphibamidae as the sister taxon of Gerobatrachus and Doleserpeton.
• Gymnophiona nests among "microsaurs." Possible sister taxa include Rhynchonkos.

  Pros:

  • The associations don't stretch the imagination, and account for creepy ecological similarities between caecilians and what we imagine lepospondyls to have been like.
  Cons:
  • The scheme invokes improbable convergences between gymnophionans and batrachians including:
    • Widening of interpterygoid vacuity
    • Loss of ectopterygoid
    • Loss of supraoccipital
    • Evolution of the amphibian inner ear (See Maddin and Anderson, 2012.)
    ...and conceals some sharp contrasts between gymnophionans and their supposed sister taxa (Link to contrasts)
  • And if Rynchonkos and its kin really are reptiles????
  
  Hypothesis IV: Batrachia as member of Dissorophoidea and Gymnophiona nested within Stereospondyli: Pardo et al., 2017 describe Chinlestegophis, a Late Triassic stem gymnophionan. Cool because:
  • It pushes the caecilian record farther back in time.
  • The new taxon, while sharing potential synapomorphies with caecilians, appears to be nested in Stereospondyli as a trematosaurian, close to metoposaurs!
  If true, Rhachitomi is a junior synonym of Lissamphibia. But wait! Marjanovic and Laurin, 2019 cast doubt on the Pardo et al., 2017 result.
  
  
  Funcusvermis gilmorei (Andrey Atuchin/National
  Park Service/Petrified Forest Museum Association)
  But this just in! Another Late Triassic stem gymnophionan, Funcusvermis gilmorei (Kligman et al., 2023) This one however, appears in phylogenetic analysis as a dissorophoid, pulling Gymnophiona into a sister taxon relationship with Batrachia - Door number 2 again!

  So we wait and hope. Door number 2 - Monophyletic Lissamphibia within Dissorophoidea remains the front runner for now.
  

  Additional reading:

  • Jason Anderson 2007. Incorporating Ontogeny into the matrix: A Phylogenetic Evaluation of Developmental Evidence for the Origin of Modern Amphibians. In: J. Anderson and H-D Sues Eds. Major Transitions in Vertebrate Evolution. Indiana University Press, Bloomington and Indianapolis. pp. 182-227.
  • Jason Anderson, Robert Carroll, Timothy Rowe 2003. New information on Lethiscus stocki (Tetrapoda: Lepospondyli: Aistopoda) from high-resolution computed tomography and a phylogenetic analysis of Aistopoda. Canadian Journal of Earth Sciences, 40(8): 1071-1083
  • Jason Anderson, Robert Reisz, Diane Scott,Nadia Froebischer, and Stuart Sumida, 2007. A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders. Nature 453, 515-518.
  • Robert Carroll, 2009. The Rise of Amphibians: 365 Million Years of Evolution. Johns Hopkins University Press, Baltimore. 360 pp.
  • Aja Mia Carter, S. Tonia Hsieh, Peter Dodson, Lauren Sallan, 2021. Early amphibians evolved distinct vertebrae for habitat invasions. PLOS One 16(6): e0251983
  • Jennifer Clack. 2011. Gaining Ground - The Origin and Evolution of Tetrapods. Indiana University Press. Bloomington and Indianapolis. 523 pp.
  • Jennifer Clack. 2012. A Carboniferous embolomere tail with supraneural radials. Journal of Vertebrate Paleontology, 31(5) pp. 1150-1153.
  • Michael Coates, Marcello Ruta, and Matt Friedman. 2008. Ever Since Owen: Changing Perspectives on the Early Evolution of Tetrapods. Annual Review of Ecology, Evolution, and Systematics. 2008. 39:571Ð92.
  • Juan Daza, Edward Stanley, Arnau Bolet, Aaron Bauer, Salvador Arias, Andrei Cernansky, Joseph Bevitt, Phillip Wagner, and Susan Evans. 2020. Enigmatic mid-Cretaceous amphibians were chameleon-Like ballistic feeders. Science 370:687-691.
  • Nadia B. Fröbisch and Neil H. Shubin. 2011. Salamander limb development: Integrating genes, morphology, and fossils. Developmental Dynamics, 240(5):1087-1099.
  • Marc E. H. Jones , Roger B. J. Benson, Pavel Skutschas, Lucy Hill, Elsa Panciroli, Armin D. Schmitt, Stig A. Walsh, and Susan E. Evans. 2022. Middle Jurassic fossils document an early stage in salamander evolution. Proceedings of the National Academy of Sciences, 119 (30) e2114100119.
  • Jozef Klembara, Jennifer Clack, Andrew Milner, and Marcello Ruta. 2014. Cranial anatomy, ontogeny, and relationships of the Late Carboniferous tetrapod Gephyrostegus bohemicus Jaekel, 1902. Journal of Vertebrate Paleontology, 34(4):774Ð792.
  • Ben T. Kligman, Bryan M. Gee, Adam D. Marsh, Sterling J. Nesbitt, Matthew E. Smith, William G. Parker and Michelle R. Stocker. 2023. Triassic stem caecilian supports dissorophoid origin of living amphibians. Nature 2023.
  • Michel Laurin and Robert Reisz. 1997. A new perspective on tetrapod phylogeny. In: S. Sumida and K. Martin eds. Amniote Origins. Academic Press. New York. 9-59.
  • Hillary Maddin and Jason Anderson. 2012. Evolution of the Amphibian Ear with Implications for Lissamphibian Phylogeny: Insight Gained from the Caecilian Inner Ear. Fieldiana Life and Earth Sciences. 5 :59-76.
  • Hillary Maddin, Farrish Jenkins, Jason Anderson. 2012. The Braincase of Eocaecilia micropodia PLOS One, December 2012. DOI: 10.1371/journal.pone.0050743.
  • Pedro Luiz Mailho-Fontana, Marta Maria Antoniazzi, Cesar Alexandre, Daniel Carvalho Pimenta, Juliana Mozer Sciani, Edmund D. Brodie, Jr., and Carlos Jared. 2020. Morphological Evidence for an Oral Venom System in Caecilian Amphibians. CellPress 23(7) July 4, 2020.
  • Arjan Mann, Jason D Pardo, and Hans-Dieter Sues. 2022. Osteology and phylogenetic position of the diminutive ÔmicrosaurÕ Odonterpeton triangulare from the Pennsylvanian of Linton, Ohio, and major features of recumbirostran phylogeny. Zoological Journal of the Linnean Society 2022;, zlac043,
  • David Marjanovic and Michel Laurin. 2019. Phylogeny of Paleozoic limbed vertebrates reassessed through revision and expansion of the largest published relevant data matrix. PeerJ 6:e5565
  • Angela Milner. 1988. The relationships and origins of living amphibians. In: M. Benton ed. The phylogeny and classification of tetrapods, volume I: amphibians, reptiles, birds. Clarendon Press. Oxford. 59-102.
  • Darren Naish. 2013. Tetrapod Zoology blog - Teenage Mutant Ninja Temnospondyls. retrieved from http://blogs.scientificamerican.com/tetrapod-zoology/2013/09/24/teenage-mutant-ninja-temnospondyls/, February 2015.
  • Pardo, J. D., Small, B. J., Huttenlocker, A. K. 2016. A caecilian-like temnospondyl from the Triassic Chinle Formation of Colorado and its bearing on the origins of Lissamphibia. Presentation at 76th Annual Society of Vertebrate Paleontology meetings.
  • Pardo, J. D., Small, B. J., Huttenlocker, A. K. 2017. Stem caecilian from the Triassic of Colorado sheds light on the origins of Lissamphibia. Proceedings of the National Academy of Sciences 114 (27).
  • Yu-Fen Rong, Davit Vasilyan, Li-Ping Dong, and Yuan Wang. 2020. Revision of Chunerpeton tianyiense (Lissamphibia, Caudata): is it a cryptobranchid salamander?. Paleoworld Pre-proof
  • Marcello Ruta and Michael Coates. 2007. Dates, nodes and character conflict: Addressing the Lissamphibian origin problem. Journal of Systematic Paleontology, 5:1, 69-122
  • Rainer Schoch. 2013. The evolution of major temnospondyl clades: an inclusive phylogenetic analysis. Journal of Systematic Paleontology, iFirst 2013, 1-33
  • Rainer Schoch. 2018. The putative lissamphibian stem-group: phylogeny and evolution of the dissorophoid temnospondyls. Journal of Paleontology, 93(1)
  • Matt Szostakiwskyj, Jason D. Pardo, Jason S. Anderson. 2015. Micro-CT Study of Rhynchonkos stovalli (Lepospondyli, Recumbirostra), with Description of Two New Genera. Plos|One June 10, 2015.
  • Florian Witzmann. 2015. CO₂-metabolism in early tetrapods revisited: inferences from osteological correlates of gills, skin and lung ventilation in the fossil record. Lethaia 49(4), 492Ð506.