SVP abstracts 16: A 3D aïstopod points to yet another transition to land

Marjanović and Jansen 2020 suggest
a transition to terrestrial life independent from any crown-group tetrapods in the snake-like microsaur aîstopod clade. In the LRT that clade includes extant aquatic snake-like caecilians. In the LRT terrestrial and fossorial snakes likewise had aquatic ancestors by convergence.

From the Marjanović and Jansen 2020 abstract:
“A complete, articulated, three-dimensional and stunningly well-prepared skeleton from the Saar-Nahe basin (western Germany) phenetically resembles Oestocephalus, but achieves a lower head-to-body length ratio by possessing more elongate and more numerous vertebrae.”

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Continuing from the Marjanović and Jansen 2020 abstract:
“Despite the rather young ontogenetic age indicated by size and skull proportions, the shape range of the dorsal scales is that of Colosteus, including rhombic scales around the dorsal midline.”

Figure 5. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

Figure 2. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

Continuing from the Marjanović and Jansen 2020 abstract:
“As in the “nectridean” Keraterpeton, the dorsal scales bear microscopic honeycombed sculpture; we also report this in Oestocephalus.”

Figure 3. Keraterpeton, basal to the Diplocaulus clade in the LRT.

Figure 3. Keraterpeton, basal to the Diplocaulus clade in the LRT.

Continuing from the Marjanović and Jansen 2020 abstract:
“Such sculpture is also seen on the ventral scales of the new specimen, which are nonetheless as narrow as in other aïstopods.”

Figure 4. Phlegethontia overall with neck and sacral bones colored red. The 'gill bones' are removed. They are gastralia.

Figure 4. Phlegethontia overall with neck and sacral bones colored red. The ‘gill bones’ are removed. They are gastralia.

Continuing from the Marjanović and Jansen 2020 abstract:
“The presence of the braincase and the first complete, undistorted aïstopod palate is confirmed by μCT; hyobranchial bones, endochondral girdles or a tail-fin skeleton are absent. The tail tapers to a point, is not laterally flattened, and the scales do not leave room for a soft-tissue tail fin; no gill slit is apparent in the scale cover behind the head.”

These indicators of terrestrial life contrast with the mandibular lateral-line canal previously identified in Coloraderpeton and suggest that the new specimen, together with the phlegethontiids from the contemporaneous fossil forest floor of Chemnitz (eastern Germany), represents a transition to terrestrial life independent from any crown-group tetrapods.”

The basalmost taxon in this legless clade is nearly legless Acherontiscus, (Fig. 5) considered an aquatic animal due to a few lateral lines on the skull. Living legless microsaurs, the caecilians, are also secondarily aquatic. The authors consider their new taxon and Phlegethontia (Fig. 4) secondarily terrestrial.

In a similar fashion 
extant snake ancestors in the LRT were aquatic, making most living snakes secondarily terrestrial, by convergence. Derived sea snakes and others, like the water moccasin, went back to an aquatic existence making the snake-like morphology rather flexible with regard to niche.

Figure 6. Acherontiscus is a basal taxon in the aïstopod clade.

Figure 5. Acherontiscus is a basal taxon in the legless aïstopod clade.

Continuing from the Marjanović and Jansen 2020 abstract:
“Yet, despite the stem-tetrapodomorph plesiomorphies in the braincase, lower jaw and scales of Aïstopoda, a preliminary phylogenetic analysis of an improved and greatly enlarged dataset finds no support for a whatcheeriid-grade position, and less support for a more crownward colosteid-grade position (as recently proposed) than for an amphibian one.”

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Figure 6. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Continuing from the Marjanović and Jansen 2020 abstract:
“Only Andersonerpeton, an isolated lower jaw described as an aïstopod, joins Densignathus in the whatcheeriid grade. Redescriptions of additional “nectrideans” and other supposed “lepospondyls” will be needed to resolve this conundrum.”

Figure 6. Living caecilian photo.

Figure 7. Living caecilian photo.

According to Wikipedia,
Aïstopoda include: Lethiscus, Ophiderpeton, Oestocephalus, Coloraderpeton and Phlegethontia among taxa tested by the large reptile tree (LRT, subset Fig. 6) nesting in the clade Microsauria. Aïstopods have been variously grouped with other lepospondyls, or placed at or prior to the batrachomorph-reptiliomorph divide. However, a cladistic analysis by Pardo et al. (2017) recovered Aistopoda at the base of Tetrapoda.

The aîstopod, Lethiscus, is from Viséan strata 340 mya,
coeval with Silvanerpeton, the last common ancestor of all reptiles in the LRT. There are no legless taxa proximal to reptiles in the LRT (subset Fig. 6).


References
Marjanović D and Jansen M 2020. A complete, three-dimensional early Permian aïstopod (Tetrapodomorpha) illuminates the phylogeny, ontogeny and terrestrialization of early limbed and limbless vertebrates. SVP abstracts 2020.

wiki/Aistopoda

Mellivora enters the LRT in a clade of giant honey badgers

Finally we know more about an extinct placental clade
that no one else recognized as an extinct placental clade. Clade members in the LRT included Patriofelis, Sarkastodon and Kerberos (Fig. 1). Now a living member, the honey badger, Mellivora capensis (also Fig. 1), enters the LRT within this clade.

Marsupials or placentals?
The problem is: these three extinct hyper-carnivores have been traditionally considered creodonts and within that clade: hyaenodonts and oxyaenids.
In the large reptile tree (LRT, 1730+ taxa) creodonts are marsupials. Distinct from them, but convergent in many ways, Mellivora, Patriofelis, Sarkastodon and Kerberos nest as clade members within the placental clade, Carnivora. This newly recognized honey badger clade nests between hyper-carnirorous wolverines + short face bears and the stylinodontid + earless seal clade

The placental honey badger clade
dentally converges with the marsupial creodont clade. Don’t put your trust in teeth, as we learned earlier.

According to BioWeb.uwlax.edu
honey badgers are members of the weasel clade, Mustelidae, apart from other mustelids. In the LRT, all derived members of Carnivora, including cats, dogs, bears, seals and sea lions are all derived from the mink/weasel (genus Mustela).

Figure 1. The honey badger clade, Kerboros, Patriolfelis and Sarkastodon. The only living representative is Mellivora to scale.

Figure 1. The honey badger clade, Kerberos, Patriolfelis and Sarkastodon. The only living representative is Mellivora to scale.

Mellivora capensis (originally Viverra capensis Scherber 1777; Fig. 2) is the extant honey badger or ratel, traditionally considered close to weasels. This carnivore has few natural predators because of its thick skin and ferocious defensive abilities.

Figure 1. The honey badger (Mellivora capensis) skull.

Figure 2. The honey badger (Mellivora capensis) skull.

Imagine the unreasonable viciousness of a honey badger
expanded to the size of Sarkastodon (Fig. 1).

Figure 2. The honey badger (Mellivora capensis) skeleton.

Figure 3. The honey badger (Mellivora capensis) skeleton.

This 3:20 minute honey badger video on YouTube
went viral (95.5 million views) awhile back. Quite the character, now finally understood phylogenetically.

The LRT solves problems
others don’t even think about. Adding taxa is the solution to many phylogenetic problems.


References
Schreber, JCDv 1777. “Das Stinkbinksen”. Die Säugethiere in Abbildungen nach der Natur mit Beschreibungen. Erlangen: Wolfgang Walther. pp. 450–451.

wiki/Honey_badger
wiki/Oxyaenidae

Origin of tetrapod herbivory, effects on local plant diversity

Brocklehurst, Kammerer and Benson 2020
discuss the origin of tetrapod herbivory and its effects on local plant diversity. Plant diversity will not be covered here, but we can discuss basal tetrapod herbivory.

According to the authors,
“Time-series regression analysis supports a negative relationship of plant richness with herbivore richness but a positive relationship of plant richness with minimum herbivore body size. This is consistent with studies of present-day ecosystems… Thus far, there has been little discussion on how the origin and early evolution of herbivory, either in arthropods or tetrapods, affected pat- terns of community richness in plants.”

Brocklehurst, Kammerer and Benson 2020 report,
“The earliest tetrapod herbivores appear in the fossil record in the Pennsylvanian, although the precise time of origin for this behaviour is uncertain due to the difficulty of assessing diet in extinct organisms. Potentially herbivorous taxa appear in the Bashkirian (earliest Pennsylvanian)-aged Joggins Formation: pantylid microsaurs with robust palatal dentition, a possible adaptation for grinding plant material or crushing the thick exoskeletons of arthropods (or both). More probable herbivores are known from the Kasimovian stage later in the Pennsylvanian: specimens of the diadectid Desmatodon from the Conemaugh Group of Pennsylvania.”

The authors make no distinction
between reptiles and pre-reptiles (= amniotes and anamniotes) in their paper. And that’s fine. Neither do they put their herbivores into a phylogenetic context. And that’s fine. They don’t take a stand whether diadectids were reptiles or not. They are just concerned about herbivory… and that’s fine.

Reptile herbivory
is a subject touched on in 2012, following a placodont herbivory paper (Diedrich 2011). That old cladogram needs to be revisited again based on the large number of added taxa since then. Even so, many of the hypotheses advanced eight years ago still hold up today.

From 2012:

“The separation between plant-eaters and insect-eaters formed the basal split in the large reptile tree. The emergence of Limnoscelis and its kin and Lanthanosuchus and its kin from this clade bear further scrutiny. I’m sure there’s a story brewing there. Turtles also emerged from this clade of herbivores.

“Among charted lepidosauromorphs we don’t see any other herbivores until Iguana and even it supplements with insects when young. Let me know if I’m missing any others.

“On the archosauromorph side, Edaphosaurus is the first herbivore with several therapsid clades not far behind. Thereafter we don’t see any until the Placodontia (if they were indeed herbivores and not shell crushers), the Aetosaurs and the Phytodinosauria.”

Getting back to the present day:
In the large reptile tree (LRT, 1698+ taxa) herbivory seems to appear on one branch only at the basal dichotomy of reptiles immediately following Silvanerpeton. Many, and potentially all of the basal taxa in the new Lepidosauromorpha are herbivores. Most are universally accepted herbivores: captorhinids, diadectids + bolosaurids + procolophonids, pareiasaurids and caseasaurids. The taxa surrounding and interweaving with these obvious herbivore clades could also be herbivores based on phylogenetic bracketing. These include: Milleretta, Concordia, Cephalerpeton (Fig. 1) and even Limnoscelis.

Figure 1. Opisthodontosaurus (above) with missing bones in color. Black lines represent the referred specimen, OMNH 77470 scaled to fit the holotype, OMNH 77469, here in ghosted lines. Colors represent missing bones.

Figure 1. Opisthodontosaurus (above) with missing bones in color. Black lines represent the referred specimen, OMNH 77470 scaled to fit the holotype, OMNH
77469, here in ghosted lines. Colors represent missing bones.

No taxa in the basal Archosauromorpha
are obvious herbivores until the sailback synapsid, Edaphosaurus.and again in the therapsid clade Anomodontia. Insectivory seems to have evolved into carnivory most of the time.

Among microsaurs
Pantylus and Stegotretus seem to stand alone as likely herbivores among anamniotes (basal tetrapods) confirming Brocklehurst, Kammerer and Benson 2020.

According to Brocklehurst, Kammerer and Benon
(their figure 1) throughout most of the Carboniferous plants maintained their greatest diversity. That changed during the latest Carboniferous when plants experienced a large diversity reduction coeval with the arrival of tetrapod herbivores. The authors conclude, “We find that the early Permian diversification of herbivorous tetrapods constrained the α-diversity of plants for the rest of the Palaeozoic.”

A chronological tree of tetrapods
can be found here. As you’ll see, only a few, rare taxa are known before the Late Carboniferous. Thereafter, and especially in the Early Permian, fossils from a variety of clades are known, including herbivores. The question is: Did tetrapods suddenly flourish? Or did the world change and tetrapods suddenly find themselves in areas that tended to fossilize better?

In other words,
would figure 1 of Brocklehurst, Kammerer and Benson look about the same if just insectivores and carnivores were included? Perhaps their conclusions would have been stronger if insectivores and carnivores were also included, just to be sure it was indeed the herbivores responsible for a reduction in plant diversity and not the planet at the time.


References
Brocklehurst N, Kammerer CF and Benson RJ 2020 The origin of tetrapod herbivory: effects on local plant diversity. Proceedings of the Royal Society B 287: 20200124. http://dx.doi.org/10.1098/rspb.2020.0124
Diedrich CG 2011. Fossil middle triassic “sea cows” – placodont reptiles as macroalgae feeders along the north-western tethys coastline with pangaea and in the germanic basin. Natural Science 3 (1) 9-27 (2011)

https://pterosaurheresies.wordpress.com/2019/10/16/you-heard-it-here-in-2011-diadectids-are-amniotes/
http://reptileevolution.com/reptile-tree2.htm

Lethiscus: oldest of the tetrapod crown group?

Figure 1. Lethiscus stock skull, drawing from Pardo et al. 2017 and colorized here.

Figure 1. Lethiscus stocki skull, drawing from Pardo et al. 2017 and colorized here. Note the loss of the postfrontal and the large orbit. Pardo et al. nest this taxon between Acanthostega and Pederpes in figure 3. There is very little that is plesiomorphic about this long-bodied legless or virtually legless taxon. Thus it should nest as a derived taxon, not a basal plesiomorphic one.

Pardo et al. 2017
bring us new CT scan data on Lethiscus stocki (Wellstead 1982; Viséan, Early Carboniferous, 340 mya) a snake-like basal tetrapod related to Ophiderpeton (Fig. 2) in the large reptile tree (LRT, 1018 taxa), but with larger orbits.

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Figure 2. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Lethiscus is indeed very old (Middle Viséan)
but several reptiles are almost as old and Tulerpeton, a basal amniote, comes from the even older Late Devonian. So the radiation of small burrowing and walking tetrapods from shallow water waders must have occurred even earlier and Tulerpeton is actually the oldest crown tetrapod.

Figure 2. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles). None of this is supported by the LRT.

Figure 3. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles), resurrecting an old idea not supported in the LRT. Actually not much of this topology is supported by the LRT.

Pardo et al. nested Lethicus
between Acanthostega (Fig. 4) and Pederpes (Fig. 3) using a matrix that was heavily weighted toward brain case traits. Ophiderpeton and Oestocephalus (Fig. 2) were not included in their taxon list, though the clade is mentioned in the text: “Overall, the skull morphology demonstrates underlying similarities with the morphologies of both phlegethontiid and oestocephalid aïstopods of the Carboniferous and Permian periods.” So I’m concerned here about taxon exclusion. No other basal tetrapods share a lateral temporal fenestra or share more cranial traits than do Lethiscus, OphiderpetonOestocephalus and RileymillerusAll bones are identified here as they are in Pardo et al. so bone ID is not at issue. I can’t comment on the Pardo team’s braincase traits because so few are examined in the LRT. Dr. Pardo said they chose taxa in which the brain case traits were well known and excluded others.

Figure 4. Acanthostega does not have much of a neck.

Figure 4. Acanthostega is basal to Lethiscus in the Partdo et al. tree.

Pardo et al. considered
the barely perceptible notch between the tabular and squamosal in Lethiscus (Fig. 1) to be a “spiracular notch” despite its tiny size. I think they were reaching beyond reason in that regard. They also note: “The supratemporal bone is an elongate structure that forms most of the dorsal margin of the temporal fenestra, and is prevented from contacting the posterior process of the postorbital bone by a lateral flange of the parietal bone.” The only other taxon in the LRT that shares this morphology is Oestocephalus, Together they nest within the Lepospondyli (Fig. 3) in the LRT. I think it is inexcusable that Pardo et al. excluded  Ophiderpeton and Oestocephalus. 

Figure 4. Subset of the LRT with the addition of Lethiscus as a sister to Oestocephalus, far from the transition between fins and feet. Here the microsaurs are not derived from basal reptiles

Figure 4. Subset of the LRT with the addition of Lethiscus as a sister to Oestocephalus, far from the transition between fins and feet. Here the microsaurs are not derived from basal reptiles

Summarizing,
Pardo et al. report, “The braincase and its dermal investing bones [of Lethiscus] are strongly indicative of a very basal position among stem tetrapods.”  and “The aïstopod braincase was organized in a manner distinct from those of other lepospondyls but consistent with that seen in Devonian stem tetrapods.” It should also be noted that the skull, body and limbs were likewise distinct from those of other lepospondyls, yet they still nest with them in the LRT because no other included taxa (1018) share more traits. ‘Distinct’ doesn’t really cut it, in scientific terms. As I mentioned in an email to Dr. Pardo, it would have been valuable to show whatever bone in Lethiscus compared to its counterpart in Acanthostega and Oestocephalus if they really wanted to drive home a point. As it is, we casual to semi-professional readers are left guessing.

Pardo et al. references the clade Recumbirostra.
Wikipedia lists a number of microsaurs in this clade with Microbrachis at its base, all within the order Microsauria within the subclass Leposondyli. Pardo et al. report, “Recumbirostrans and lysorophians are found to be amniotes, sister taxa to captorhinids and diapsids.” The LRT does not support this nesting. Pardo et al. also report, “This result is consistent with early understandings of microsaur relationships and also reflects historical difficulties in differentiating between recumbirostrans and early eureptiles.” Yes, but the later studies do not support that relationship. Those early understandings were shown to be misunderstandings that have been invalidated in the LRT and elsewhere, but now resurrected by Pardo et al.

Ophiderpeton granulosum (Wright and Huxley 1871; Early Carboniferous–Early Permian, 345-295mya; 70cm+ length; Fig. 2, dorsal view)

Oestocephalus amphiuminus (Cope 1868; Fig. 2,  lateral views) is known from tiny immature and larger mature specimens.

Figure 7. A series of Phlegethontia skulls showing progressive lengthening of the premaxilla and other changes.

Figure 5. A series of Phlegethontia skulls showing progressive lengthening of the premaxilla and other changes.

A side note:
The recent addition of several basal tetrapod taxa has shifted the two Phlegethontia taxa (Fig.5) away from Colosteus to nest with Lethiscus and Oestocephalus, their traditional aistopod relatives. That also removes an odd-bedfellow, tiny, slender taxon from a list of large robust stem tetrapods.

References
Pardo JD,Szostakiwskyj M, Ahlberg PE and Anderson JS 2017. Hidden morphological diversity among early tetrapods. Nature (advance online publication) doi:10.1038/nature22966
Wellstead CF 1982. A Lower Carboniferous aïstopod amphibian from Scotland. Palaeontology. 25: 193–208.
Wright EPand Huxley TH 1871. On a Collection of Fossil Vertebrata, from the Jarrow Colliery, County of Kilkenny, Ireland. Transactions of the Royal Irish Academy 24:351-370

wiki/Acherontiscus
wiki/Adelospondylus
wiki/Adelogyrinus
wiki/Dolichopareias
wiki/Ophiderpeton
wiki/Oestocephalus
wiki/Rileymillerus
wiki/Acherontiscus