Apateon and the origin of salamanders + frogs

Figure 1. Apateon overall and the skull in palatal and dorsal views. This taxon nests between Doleserpeton and Gerobatrachus in the LRT.

Figure 1. Apateon overall and the skull in palatal and dorsal views. This taxon nests between Doleserpeton and Gerobatrachus in the LRT.

Apateon pedestris (von Meyer 1844, Early Permian, 295mya; 12 cm in length) was long considered a temnospondyl in the family Branchiosauridae. Here Apateon nests between Doleserpeton and Gerobatrachus in the lepospondyl lineage of frogs, like Rana and salamanders like Andrias.

Resembling a small salamander with a long, laterally flattened tail, Apateon had a shorter rostrum and large orbits than Doleserpeton. The pineal opening was larger. The ilium was more erect. The pubis was missing. The ectopterygoid did not contact the maxilla and the palatine did so only with a narrow process. At present, no other taxa in the LRT (978 taxa) do this.

Small scales covered the body. Three pairs of external gills were present for underwater respiration. Many species are known, as well as a good ontogenetic series.

Anderson 2008 reported, 
“Branchiosaurs [including Apateon] are closely related to amphibamids, if not included in the latter group, and have been suggested to be closely related to salamanders because of shared similarities in the sequence of cranial ossification.”

“New transitional fossils like the stem batrachian Gerobatrachus have filled in the morphological gap between amphibamid temnospondyls and the earliest frogs and salamanders, and this portion of the lissamphibian origins question appears very well supported.”

The LRT recovers
Amphibamus much closer to the base of the lepospondyls, about 5 nodes distant from Apateon. Of course, neither are closely associated with temnospondyls in the LRT, despite the open palate, otic notch and other convergent traits.

Neotony
The apparent lack of gill-less adults among all of the apparent larval gilled specimens of Apateon was a cause of consternation for awhile. The new largest specimen (Frobisch and Schoch 2009) appears to indicate an adult specimen. It had partially interdigitating and tight sutures of the skull roof, a high degree of ossification and differentiation of the postcranium as compared to smaller larval specimens. Uncinate processes indicate that this specimen represents an adult. However, it lacks ossifications of the exoccipitals and quadrates, intercentra, and the coracoid as seen in metamorphosed specimens. Frobisch and Schoch conclude, “The anatomical evidence at hand clearly indicates that both life history strategies, metamorphosis and neoteny, were established in Paleozoic branchiosaurids.”

References
Anderson JS 2008. Focal Reviews: The Origin(s) of Modern Amphibians. Eovlutionary Biology 35:231-247.
Anderson JS et al. 2008.  
A stem batrachian from the Early Permian of Texas
and the origin of frogs and salamanders. Nature 453 (7194): 515–518.
Frobisch N and Schoch RR 2009. The largest specimen of Apateon and the life history pathway of neotony in the Paleozoic temnospondyl family Branchiosauridae. Fossil Record 12(1):83-90.
von Meyer H 1844. Briefliche Mittheilung an Prof. Bronn gerichtet. Neues Jahrbuch für Geognosie, Geologie und Petrefakten-Kunde 1844: 329-340.

wiki/Gerobatrachus
wiki/Apateon

Diplovertebron and amphibian finger loss patterns

Diplovertebron punctatum (Fritsch 1879, Waton 1926; Moscovian, Westphalian, Late Carboniferous, 300 mya, Fig. 1) was considered an anthracosaur or reptile-like amphibian. That is confirmed by the large reptile tree (LRT, subset Fig. 2), where it nests with  Utegenia transitional between basal seymouriamorpha, like Kotlassia, and basal amphibians, like Balanerpeton (Fig. 3), yet close to the origin of stem reptiles, like Silvanerpeton. Based on the nesting of Tulerpeton in the LRT, Diplovertebron had origins in the Late Devonian.

Figure 1. Diplovertebron nests at the base of the lineage of amphibians, close to the base of the reptiles, all derived from seymouriamorphs. Note the retention of five fingers. Wish I had better data than this.

Figure 1. Diplovertebron nests at the base of the lineage of amphibians, close to the base of the reptiles, all derived from seymouriamorphs. Note the retention of five fingers. Wish I had better data than this.

In Diplovertebron,
the vertebral structure is primitive. The notochord persisted in adults. The ribs were long and slender as in basal taxa, not shortened as in lepospondyl amphibians. Five manual digits were preserved with a 2-3-3-3-4 formula, a formula similar to amphibians, not like reptiles (2-3-4-5-5). The ilium is bifurcate with a long posterior process. The pubis did not ossify, as in several basal tetrapods including Crassigyrinus and derived Amphibia. Small scutes covered the entire torso ventrally, as in basalmost tetrapods and basal reptiles.

Figure 2. The gradual loss of basal tetrapod fingers. Unfortunately fingers are not known for every included taxon.

Figure 2. The gradual loss of basal tetrapod fingers. Unfortunately fingers are not known for every included taxon. Odd Tulerpeton with 6 fingers may result from taphonomic layering of the other manus peeking out below the top one. See figure 6.

The presence of five manual digits
in Diplovertebron and Balanerpeton (Figs. 4, 5) sheds light on their retention in Acheloma + Cacops. There is a direct phylogenetic path between them (Fig. 2). Note that all other related clades lose a finger or more. Basal and stem reptiles also retain five fingers.

Figure 2. Utegenia nests as a sister to Diplovertebron.

Figure 3. Utegenia nests as a sister to Diplovertebron.

Note the narrow frontals,
on Diplovertebron distinct from the wide frontals in Utegenia and Kotlassia, but more similar to those in Balanerpeton (Fig. 4), another basal amphibian, and Silvanerpeton, a stem reptile. Yet none have the hourglass shape found in Diplovertebron.

Figure 4. The basal amphibian, Balanerpeton apparently has five fingers (see figure 5).

Figure 4. The basal amphibian, Balanerpeton apparently has five fingers (see figure 5).

As reported
earlier, finger five was lost in amphibians,while finger one was lost in temonospondyls. Now, based on the longest metacarpal in Caerorhachis and Amphibamus (second from medial), apparently manual digit one was lost in that clade also, distinct from the separate frog and microsaur clades. In summary, loss from five digits down to four was several times convergent in basal tetrapods.

Figure 5. DGS recovers five fingers in Balanerpeton with a Diplovertebron-like phalangeal pattern.

Figure 5. DGS recovers five fingers in Balanerpeton with a Diplovertebron-like phalangeal pattern. Two 5-second frames are shown here.

Finally, we have to talk about
Tulerpeton (Fig. 6). The evidence shows that the sixth manual digit is either a new structure – OR – all post-Devonian taxa lose the sixth digit by convergence, since they all had five fingers. Finger 6 has distinct phalangeal proportions, so it is NOT an exposed finger coincident rom the other otherwise unexposed hand in the fossil matrix.

Figure 2. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here.

Figure 6. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here. Digit 6 is either a new structure, or a vestige that disappears in all post-Devonian taxa.

References
Fritsch A 1879. Fauna der Gaskohle und der Kalksteine der Permformation “B¨ ohmens. Band 1, Heft 1. Selbstverlag, Prague: 1–92.
Kuznetzov VV and Ivakhnenko MF 1981. Discosauriscids from the Upper Paleozoic in Southern Kazakhstan. Paleontological Journal 1981:101-108.
Watson DMS 1926. VI. Croonian lecture. The evolution and origin of the Amphibia. Proceedings of the Zoological Society, London 214:189–257.

wiki/Diplovertebron

Platyhystrix: closer to Acheloma than to Cacops?

Platyhystrix was a dissorophid lepospondyl anamniote that had a dorsal sail (Figs. 1, 2 ), not quite like those  of its more famous reptilian/synapsid contemporaries, Dimetrodon and Edaphosaurus.

There must have been something in the air back then,
and those dorsal sails were there to catch it!

Figure 1. Platyhistrix skull reconstructed from slightly disassociated parts.

Figure 1. Platyhistrix skull reconstructed from slightly disassociated parts. And the Lewis and Vaughn 1965 dorsal sail, distinct from the others in figure 2. The skull here appears to have a confluent naris and antorbital finestra, as in Acheloma, but there are other bones missing there, too, like most of the maxilla.

Dissorophids are traditionally nested with
temnospondyls, but here, at the large reptile tree (LRT, now 959 taxa), they arise from a sister to the basal seymouriamorph, Utegenia and continue to be generally smaller taxa (< 60cm).

Figure 2. Other Platyhystrix specimens known chiefly from dorsal spines.

Figure 2. Other Platyhystrix specimens known chiefly from dorsal spines. That old skull from Williston 1911 is missing the central area, here imagined from the more complete specimen in figure 1.

Distinct from Acheloma
the skull of Platyhysterix does not appear to have giant palatal fangs, or such large marginal teeth. The jugal nearly separates the postorbital from the supratemporal. The postorbital is larger and much knobbier.

Like Acheloma
The rostrum may include a confluent nairs/antorbital fenestra, a constricted rostrum (in dorsal view), a naris of similar laterally wavy shape, robust premaxillary ascending processes, large tabulars and other traits relatively exclusive to these two.

A fair amount of reassembly
is required of the Platyhystrix skull. The random neural spine below the lower right jaw line allies the skull with specimens that also have long neural spines.

Figure 1. Acheloma dunni skull with a confluent antorbital fenestra and naris.

Figure 3. Acheloma dunni skull with a confluent antorbital fenestra and naris.

Wouldn’t it be interesting 
to see hatchlings and juveniles of Platyhystrix? It is widely considered, along with its double-armored kin, Dissorophus, to have been fully terrestrial. So, did these two have a swimming tadpole stage? And then develop spines and armor in adulthood? Or did they converge with reptiles, laying protected eggs on land, skipping the tadpole stage? Let’s keep an eye out for little finbacks.

References
Berman DS, Reisz RR and Fracasso MA 1981. Skull of the Lower Permian dissorophid amphibian Platyhystrix rugosus. Annals of the Carnegie Museum 50 (17):391-416.
Case EC 1911. Revision of the Amphibia and Pisces of the Permian of North America. Publ. Carnegie Inst. Washington 146:1-179.
Dilkes DW and Reisz R 1987. Trematops milleri identified as a junior synonym ofAcheloma cumminsi with a revision of the genus. American Museum Novitates 2902.
Lewis GE and Vaughn PP 1965. Early Permian vertebrates from the Cutler Formation of the Placerville area, Colorado, with a section on Footprints from the Cutler Formation by Donald Baird: U.S. Geol. Survey Prof. Paper 503-C, p. 1-50.
Williston SW 1911a. A new family of reptiles from the Permian of New Mexico. American Journal of Science 31:378-398.
Williston SW 1911b. American Permian vertebrates. University of Chicago Press: 145 pp.

wiki/Acheloma
wiki/Platyhystrix

Dermophis, an extant caecilian gets the DGS treatment

Sometimes bones disappear.
Other times bones become fused to one another. The extant caecilian Dermophis (Fig. 1) might demonstrate one or the other or both. Coloring the bones helps to interpret and explain their presence despite the absence of sutures due to fusion or loss.

Figure 1. Dermophis, the extant Mexican caecilian, with bones, even if fused to one another, identified. The quadratojugal and squamosal are absent. Black and white image from Digimorph.org. Coloring the bones makes them so much easier to read and understand.

Figure 1. Dermophis, the extant Mexican caecilian, with bones, even if fused to one another, identified. The quadratojugal and squamosal are absent. Black and white image from Digimorph.org. Coloring the bones makes them so much easier to read and understand.

Dermophis mexicanus (Mexican caecilian, Peters 1880; extant) The nasal and premaxilla are fused. The maxilla, lacrimal, prefrontal and palatine are fused. The occipital elements and the paraspheniod are fused (= Os basale). The parietal and postparietal are fused. The jugal, squamosal, postfrontal and postorbital are fused. The dentary and surangular are fused. The splenial, articular and angular are fused. The pterygoid and quadrate are fused.

The cheek bones are traditionally labeled squamosals, but that may not be the whole story here. Different from nearly all other basal tetrapods (including other amphibians), caecilians shift the jaw joint forward, creating a large retroarticular process of the posterior mandible.

Dermophis lives in humid to dry soils beneath leaf-litter, logs, banana or coffee leaves and hulls or similar ground cover. It is viviparous.

Ontogeny should tell
The true identity of skull bones should be able to be determined by watching their growth from small disconnected bone buds in the embryo. Unfortunately, the references I’ve seen don’t make that growth clear in all cases. So, I’m stuck, for the present, with comparative anatomy within a phylogenetic framework that nests caecilians with Acherontiscus (Fig. 4) and kin, which have large and separate cheek bones.

FIgure 2. Eocaecilia has small limbs and a substantial tail.

FIgure 2. Eocaecilia has small limbs and a substantial tail. The tabular may be absent here unless it, too, is fused to the postorbital/squamosal. The tabular is tiny in Dermophis and probably useless.

Limbs and limb girdles
are absent in all extant caecilians and the majority of species also lack a tail. They have a terminal cloaca, like an earthworm. Limbs are vestigial in Eocaecilia (Fig. 2), and a substantial tail is present.

Figure 1. Eocaecilia skull with original and new bone identifications based on comparisons to sister taxa listed here. Like Brachydectes, the jaw joint has moved forward, beneath the jugal now fused to the quadratojugal creating a long retroarticular process, otherwise rare in amphibians. Also rare is the fusion of the squamosal with the postorbital.

Figure 3. Eocaecilia skull with original and new bone identifications based on comparisons to sister taxa listed here. Like Brachydectes, the jaw joint has moved forward, beneath the jugal now fused to the quadratojugal creating a long retroarticular process, otherwise rare in amphibians. Also rare is the fusion of the squamosal with the postorbital. Note the reduced supratomporal. here and in Dermophis.

The tentacle
Extant caecilians have a unique chemosensory organ located on the head called the tentacle. The tentacle exits the skull through the tentacular foramen (looks like an antorbital fenestra) located between the nares and orbit. Eocaecilia lacks this foramen (Fig. 3).

Figure 4. Acherotisicus has large cheek bones (squamosal, quadratojugal) that appear to fuse in Eocaecilia and Dermophis.

Figure 4. Acherotisicus has large cheek bones (squamosal, quadratojugal) that appear to fuse in Eocaecilia and Dermophis.

References
Peters WCH 1880 “1879”. Über die Eintheilung der Caecilien und insbesondere über die Gattungen Rhinatrema und Gymnopis. Monatsberichte der Königlichen Preussische Akademie des Wissenschaften zu Berlin 1879: 924–945.

Image above from Digimorph. org and used with permission.

wiki/Dermophis

Marjanovic and Laurin 2016: Basal tetrapods, continued…

rhynSorry this took so long…
As you’ll see there was a lot of work and prep involved that has been several weeks in the making. Thank you for your patience.

Earlier I introduced the Marjanovic and Laurin 2016 study
the way they did, by reporting their confirmation of the Ruta and Coats 2007 basal tetrapod topology that they were testing prior to reevaluating the data. I noted then that both studies (Fig. 5) included many so-called pre-reptiles, including  Bruktererpeton, Chroniosaurus, Solenodonsaurus, Limnoscelis, Tseajaia, DiadectesOrobates and Westlothiana,should not be in the pre-amniote inclusion set. Those taxa nest within the Reptilia in the large reptile tree (LRT, subset Fig. 4) with Silvanerpeton and Gephyrostegus at the base of the Reptilia (= Amniota). As reported earlier, those two are the amphibian-like reptiles that first developed the amniotic egg that defines the clade Amniota, a junior synonym of the Reptilia, based on the tree that recovers them at the base of both major branches, the new Archosauromorpha and the new Lepidosauromorpha early in the Viséan.

How can one readily compare two competing cladograms? 
You would not want to sit through a comparison of tens of thousands of scores for competing trees in a short blog like this. But we can compare images of taxa (Figs. 1–3. 6–8) placed in their phylogenetic order, subdivided for clarity into the three major lineages of basal tetrapods:

  1. Basalmost tetrapods and the lineage that led to Reptilia
  2. Members of the Lepospondyli
  3. Members of the Microsauria

These images will serve as a ready reference for today’s topics. As a preview, in summary:

The Marjanovic and Laurin (ML) 2016 tree nests

  1. frogs like Rana and salamanders like Andrias with microsaurs.
  2. small amphibamids, Cacops and Micromelerpeton nest with temnospondyls.
  3. basal Amniota splits into Synapsida (Caseasauria + Archaeovenator) and Sauropsida (Captorhinus, Paleothyris, Petrolacaosaurus) arising from an unknown genus basal to Diadectomorpha + Amniota
  4. The clade Amphibia arises near Solenodonsaurus + the crown-group Tetrapoda
  5. The clade Microsauria is divided into three parts separated by non-microsaurs with origins near Westlothiana.

The LRT nests

  1. frogs and salamanders nest with lepospondyls.
  2. small amphibamids, Cacops and Micromelerpeton nest with lepospondyls.
  3. basal Amniota splits into Archosauromorpha  (several basal taxa, Archaeovenator, Paleothyris and Petrolacaosaurus) and Lepiodosauromorpha (several basal taxa, Caseasauria and Captorhinus) with both major clades arising from Gephyrostegus bohemicus a late-surving Westphalian taxon, and Silvanerpeton, a Viséan taxon.
  4. The clade Amphibia arises near Balanerpeton and the amphibamids.
  5. The clade Microsauria has a single origin near Kirktonecta 

What you should be looking for
is a gradual accumulation of traits in every lineage. And look for taxa that don’t fit in the order presented. This can be done visually with these figures, combining hundreds of traits into one small package. Rest assured that all scoring by ML and the competing analysis in the LRT were done with the utmost care and diligence. So, some biased or errant scoring must have taken place in one study or the other or both for the topologies to differ so great. Bear in mind that ML had firsthand access to fossils and may have bowed to academic tradition, while I had photos and figures to work with and no allegiance to academic tradition.

First
the large reptile tree (LRT) taxa (Figs. 1–3) had two separate origins for limbed vertebrates.

Figure 1. CLICK TO ENLARGE. Basal tetrapod subset according to the LRT. These taxa lead to Reptilia, Lepospondyli and through that clade, the Microsauria. Note the convergent development of limbs and digits arising out of Osteolepis.

Figure 1. CLICK TO ENLARGE. Basal tetrapod subset according to the LRT. These taxa lead to Reptilia, Lepospondyli and through that clade, the Microsauria. Note the convergent development of limbs and digits arising out of Osteolepis.

In both studies
basal tetrapod outgroups are tail-propelled sarcopterygians having muscular fins not yet evolved into limbs with digits. Behind the skull are opercular bones that are lost in taxa with limbs. An exoskeleton of bony scales disappears in taxa with limbs. Snout to tail tip length averages 50 cm.

In the LRT
locomotion switches to the limbs in temnospondyls, which tend to be larger (1m+ and have overlapping dorsal ribs. The Greererpeton branch flattens out the ribs and skull, reducing both the tail and the limbs to likely become sit-and-wait predators. Phylogenetic size reduction and limb elongation is the trend that leads to Reptilia (Gephyrostegus). However an early exception, Crassigyrinus (Fig. 1), elongates the torso and reduces the limbs to adopt an eel-like lifestyle. Kotlassia adopts a salamander-like lifestyle from which Utegenia and the Lepospondyli arise (Fig. 2) alongside Reptilia.

Figure 2. CLICK TO ENLARGE. Subset of the LRT representing lepospondyli leading to frogs.

Figure 2. CLICK TO ENLARGE. Subset of the LRT representing lepospondyli leading to frogs.

In the LRT,
short-tailed, salamander-like Utegenia (derived from the Seymouriamorpha, Fig. 2) is a late-surving basal member of the generally small-sized clade Lepospondyli, which ultimately produces salamanders and frogs. A side branch produces the larger, temnospondyl-like Cacops, which develops a bony ridge atop the dorsal spines. Note the nesting here of Gerobatrachus as a salamander and frog relative, distinct from the ML tree (Fig. 6).

Figure 3. CLICK TO ENLARGE. Subset of the LRT focusing on Microsauria.

Figure 3. CLICK TO ENLARGE. Subset of the LRT focusing on Microsauria.

In the LRT
the Microsauria are derived here from the small basal amphibamids, Caerorhachis and more proximally, Kirktonecta. Microsaurs range from salamander-like to lizard-like to worm-like. The tail elongates to become the organ of locomotion in the Ptyonius clade. The head and torso flatten in the Eoserpeton clade.

Below
is the pertinent subset of the LRT (Fig. 4) with a representative, but not complete or exhaustive set of taxa. A summary of the tree’s differences with the ML tree is presented above. The ML tree is summarized below in three parts (6-8).

Figure 4. Subset of the LRT focusing on basal tetrapods.

Figure 4. Subset of the LRT focusing on basal tetrapods.

The Marjanovic and Laurin 2016 tree
(Fig. 5) presents a topology that is similar to the LRT in parts, but distinct in other parts, as summarized above. I realize this presentation is illegible at this column size due to the large number of taxa. Click on it to enlarge it. At the top and down the right column are basal taxa leading to temnspondyls and reptiles at bottom right. Working from the bottom up the left side are the microsaurs ending with the lissamphibians (frogs and salamanders) at the top/middle of the left column.

Figure 4. CLICK TO ENLARGE. The reevaluated Marjanovic and Laurin tree from which taxa on hand were set to match the tree topology (Figs. 5-7).

Figure 5. CLICK TO ENLARGE. The reevaluated Marjanovic and Laurin tree from which taxa on hand were set to match the tree topology (Figs. 5-7).

The ML tree
subdivides into there parts (Figs 6-8): basal taxa, some leading to temnospondyls and amphibamids; taxa leading to and including Amniota; and finally microsaurs leading to and including extant amphibians.

Figure 5. Basal tetrapods according to Marjanovic and Laurin 2016. Figures 6 and 7 lead to Amniota and Microsauria respectively.

Figure 6. Basal tetrapods according to Marjanovic and Laurin 2016. Figures 6 and 7 lead to Amniota and Microsauria respectively.

In the ML topology,
Ichthyostega, a taxon with a very large pectoral girdle, ribs, and pelvis, gives rise the the altogether smaller and more fish-like Acanthostega, which gives rise to members of the Whatcheeridae, tall-skulled Crassigyrinus and flat-skulled Osinodus. The traditional Colosteidae arise next. They have a variety of long shapes with short-legs. Oddly from this seemingly primitive clade arises small, short-torsoed, long-legged Eucritta followed by long torsoed, short-legged Proterogyrinus followed by a large clade of short-torsoed, long-legged taxa, including the >1m temnospondyls and the <30cm amphibamids.

Figure 7. CLICK TO ENLARGE. These are taxa listed on the Marjanovic and Laurin 2016 that lead to Reptilia (Amniota).

Figure 7. CLICK TO ENLARGE. These are taxa listed on the Marjanovic and Laurin 2016 that lead to Reptilia (Amniota).

In the ML tree
Gephyrostegus arises from the small temnospondyl, Balanerpeton, and and gives rise to Chroniosaurus, Solenodonsaurus, the Seymouriamorpha (including Utegenia) and the Diadectomorpha, nesting as the sister clade to the Amniota. Thus, no phylogenetic miniaturization was present at the origin of the Amniota in the ML tree. Moreover, dozens of taxa were not included here that nest at the base of the Amniota (Reptilia) in the LRT.  Basal amniotes in the ML tree are all Latest Carboniferous to Early Permian, while in the LRT basal amniotes arrived at least 40 million years earlier in the Visean (Early Carboniferous) and had radiated widely by the Late Carboniferous, as shown by the ML taxaon list. No amphibian-like reptiles made it to their Amniota.

FIgure 7. Microsauria according to Marjanovic and Laurin 2016. Here frogs and caecilians nest within the Microsauria.

FIgure 8. CLICK TO ENLARGE. Microsauria according to Marjanovic and Laurin 2016. Here frogs and caecilians nest within the Microsauria.

In the ML tree
the three microsaur clades (Fig. 5) arise from the Viséan taxon, Westlothiana (Fig. 8), which nests as a derived reptile when tested against more amniotes in the LRT. Utaherpeton is a basal microsaur in both trees, but it gives rise to the eel-like Acherontiscus and kin in the ML tree. Westlothiana further gives rise to Scincosaurus and kin, including the larger Diplocaulus. Thirdly, Westlothiana gives rise to lizard-like Tuditanus which gives rise to big-skulled Pantylus and tiny-limbed Microbrachis, shark-nosed Micraroter and Rhynchonkos. In both trees, Batropetes bucks the long-body, short-leg trend. In both trees Celtedens, representing the salamander-like albanerpetontids, gives rise to extant salamanders and frogs

So the possibilities are:

  1. Only one tree is completely correct
  2. Only one tree is mostly correct.
  3. Both trees have some correct and incorrect relationships

Problems

  1. Basal tetrapods tend to converge on several traits. For instance in the LRT, the palate is ‘open’ with narrow pterygoids in both temnospondyls and lepospondyls.
  2. Many small derived taxa lose and fuse skull bones
  3. Many taxa fuse vertebral bones as they evolve away from the notochord-based semi-encircling vertebrae of fish toward more complete vertebrae in which the neural spine, pleurocentrum and intercentrum tend to fuse, sometimes in convergent pattern, as widely recognized in basal reptiles and microsaurs.
  4. In basal tetrapods, fingers are not often preserved. So when four fingers appear their identity has to be ascertained. In the LRT mc5 and digit 5 are absent in Lepospondyls. In the LRT mc1 and digit 1 are absent in the temnospondyls. Five fingers and/or metacarpals are preserved in the few other non-amniote, basal tetrapods that preserve fingers (Proterogyrinus, Seymouria). The ML tree assumes that when four digits are present, they represent digits 1–4.

Ultimately
maximum parsimony and Occam’s Razor should rule unless strong evidence to the contrary is provided. After evidence is presented, it’s up to colleagues to accept or reject or ignore hypotheses.

References
Marjanovic D and Laurin M 2016. Reevaluation of the largest published morphological data matrix for phylogenetic analysis of Paleozoic limbed vertebrates. PeerJ. Not peer-reviewed. 356 pp.
Ruta M and Coates MI 2007
. Dates, nodes and character conflict: addressing the lissamphibian origin problem. Journal of Systematic Palaeontology 5-69-122.

Orbit size does not always equal eyeball size

Earlier we looked at the two-part orbit of Baphetes and Megalocephalus. I put forth a ‘shifting eyeball’ hypothesis, but I don’t buy into it, just to set things straight. I think the eyeball was in the dorsoposterior, more rounded portion. As we saw even earlier, basal tetrapods were evolving rostral loss of bone. So that sort of thing happened then.

Today we’ll talk about
an extreme case of tiny eyeball and enormous orbit.

Andrias davidianus (Blanchard 1871; 1.8m in length; extant) is a sister to Rana, the bullfrog and derived from a sister to Gerobatrachus. The jugal is absent. The orbit is much larger than the eyeball.

Figure 1. Skull of Andrias with skull bones identified. The jugal is absent. This extant amphibian has a tiny eyeball.

Figure 1. Skull of Andrias with skull bones identified. The jugal is absent. This extant amphibian has a tiny eyeball.

Images of the living
Andrias can be found here. You’ll be lucky if you do see the eyeball. It is very tiny. I probably overemphasized the size of the eyeball in figure 1.

References
Blanchard É 1871. Note sur une nouvelle Salamandre gigantesque (Sieboldia Davidiana Blanch.) de la Chine occidentale. Comptes Rendus Hebdomadaires des Séances de l’Académie des Sciences. Paris 73: 79.

An ‘amphibian’ with an antorbital fenestra

Surprised to find this: 
Acheloma (Cope 1882; Dilkes and Reisz 1987; Early Permian, 275 mya; aka Trematops), a trematopsid amphibamid lepospondyl basal tetrapod had a confluent antorbital fenestra and naris. Bolt 1974 considered this a “very elongate external naris” and then considered two hypotheses for its origin and use:

  1. as a nasal salt gland (rather improbable, but still possible, according to Bolt)
  2. to transfer of forces away from the antorbital bar (Bolt’s preferred hypothesis)

Bolt also noted
that earlier papers referred this morphology to a confluent antorbital vacuity, but dismissed the notion by saying, “There is no evidence that any labyrinthodont, including the ancestors of trematopsids, possessed such an [completely separate] antorbital vacuity.” IMHO, this convergent trait need not have been completely separate to qualify as an antorbital vacuity/fenestra. As Bolt noted, in nearly every case, there is a slight constriction in this vacuity marking the end of the naris and the beginning of the antorbital vacuity (Fig. 1).  A nasal flange descends inside the vacuity.

Earlier
we looked at the antorbital fenestra in other tetrapods here.

Figure 1. Acheloma dunni skull with a confluent antorbital fenestra and naris.

Figure 1. Acheloma dunni skull with a confluent antorbital fenestra and naris. Scale bar = 5 cm.

Perhaps of interest to this discussion
is the relatively large diameter palatal teeth on the vomers, palatines and ectopterygoids (Fig. 1). Bolt also found evidence for a nasal flange in related Doleserpeton and Tersomius, but not in unrelated Seymouria and Eryops.

Olson 1941 had this odd explanation:
The anterior part was for smelling, the longer posterior part was for respiration and the reason for this was the internal naris lies beneath only the posterior part. Bolt noted the shortest route was not always the only route in tetrapods. Air passages can be quite complicated.

The odd otic notch
that likely housed an eardrum in related taxa, is long and narrow in Acheloma. Dilkes and Reisz (1987) noted, “The shape of the otic notch, however, argues against an impedence-matching hearing system because the vibrational properties of the postulated tympanum would be profoundly different from one with the same surface area but circular in outline.”

Acheloma cumminsi was originally considered a temnospondyl, but here nests between Dendrerpeton and Cacops within the lepospondyls with many traits convergent with temnospondyls, like that large wide skull and large overall size. The related Acheloma dunni (Fig. 1) had giant palatal teeth.

As promised earlier:
lepospondyl traits of Acheloma and Cacops not present in temnospondyls from the character list of the LRT. Let me know if you see errors here:

  1. Ventral naris chiefly maxilla in lateral view
  2. Prefrontal separate from postfrontal
  3. Preorbital length of skull sub-equal to postorbital length of skull
  4. Naris shape in lateral view < 2x longer than tall
  5. Palatine exposure on the external skull below orbit.
  6. Squamosal posterior rim is a ‘big curve’
  7. Squamosal descends to ventral skull
  8. Mandible tip straight, does not rise
  9. Cervical centrum longer than tall
  10. Cervical neural spines not taller than centra
  11. Pleurocentra larger than intercentra
  12. Two sacral vertebrae
  13. Sacral spines not > acetabulum depth
  14. Anterior chevron shapes, not wider proximally
  15. Anterior caudal neural spines not higher than centra
  16. Clavicle shorter than scapula
  17. Humerus not ‘L’-shaped
  18. Manual metacarpals 1-3 align
  19. Longest metacarpals: 2, 3 and sometimes 4
  20. Longest manual digit: three and four
  21. Manual unguals sharp pointed
  22. Metacarpal 5 absent – except in Cacops. Acheloma has 5 carpals.
  23. Posterior ilium not longer than anterior ilium
  24. Pubic apron wide
  25. Longest metatarsals: 3 and 4
  26. Pedal 3.1 not > p2.1
  27. Overall size not > 60 cm in length

Shifting
Acheloma, Broilleus and Cacops to Eryops adds 24 steps at present. Shifting those three + Dendrepeton and Tersomius adds 17 steps at present. Shfting those five + the three members of the Amphibamus clade adds 35 steps at present.

On a side note:

Having a fifth finger on basal tetrapods (no matter how you count them, 1-4 or 2-5) is rare after Acanthostega partly because a complete manus is rare in basal tetrapods and partly because many taxa have only four fingers. Proterogyrinus, Seymouria, Cacops and basal reptiles all have five fingers preserved. Presently that’s a discontinuous list, but those five fingers could be homologous. If you know of any other related examples, let me know. I need that data.

References
Bolt JR 1974. Osteology, function, and evolution of the trematopsid (Amphibia: Labyrinthodontia) nasal region. Fieldiana: Geology 33(2): 11-30.
Cope ED 1882. Third contribution to the history of the vertebrata ofthe Permian Formation of Texas. Proc. Phil. Soc., 20: 447-461.
Dilkes DW and Reisz R 1987. Trematops milleri identified as a junior synonym of Acheloma cumminsi with a revision of the genus. American Museum Novitates 2902.
Olson EC 1941. The family Trematopidae. Journal of Geology 49:149-176.

wiki/Acheloma