Reviewing old and new news from Brazil on the origin of mammals and ictidosaurs

Figure 1. Brasilodon nests with Sinoconodon as a stem mammal.

Figure 1. Here Brasilodon nests with Sinoconodon as a stem mammal (mammaliaformes).

Bonaparte et al. 2003
discovered two taxa close to the origin of mammals, Brasilodon  (Fig. 1) and Brasilitherium (Fig. 2). Originally both were considered stem mammals. In the large reptile tree (LRT, 1025 taxa, subset figure 4) Brasilodon nests with the stem mammal, Sinoconodon. However, Brasilitherium, also from the Late Triassic, nests at the base of the monotremes a clade including Akidolestes, Ornithorhynchus and Kuehneotherium. So it’s not a stem mammal. It’s a mammal. Bonaparte et al. 2003 missed that nesting due to taxon exclusion and a very interesting jaw joint that did not fit a preconceived pattern (Fig. 2 and see below).

Figure 2. Brasilitherium compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus.

Figure 2. Brasilodon compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus.

Bonaparte et al. 2003
nested Brasilodon between Pachygenelus and Morganucodon + Brasilitherium, basically matching the LRT which did not exclude monotremes and Sinoconodon.

The key skeletal trait
defining Mammalia (unless it has changed without my knowledge) has been the disconnection of the post dentary bones from the dentary coincident with the dentary articulating with the squamosal producing a new mammalian jaw joint and the genesis of tiny ear bones.

Note: that’s not happening yet
in Brasilitherium despite its phylogenetic nesting as a basal monotreme. In Brasilitherium the articular, a post dentary bone, still articulates with the quadrate (Fig. 2). So, going by the jaw joint, Brasilitherium is not a mammal. However, going by its phylogenetic nesting in the LRT, it is a mammal.

Figure 4. Therioherpeton nests at the base of the Mammaliaformes with Brasilodon, between Yanaconodon and Sinoconodon, not far from Megazostrodon.

Figure 3. Therioherpeton nests at the base of the Mammaliaformes with Brasilodon, between Yanaconodon and Sinoconodon, not far from Megazostrodon.

We’ve seen something similar occurring
at the origin of mammals, where amphibian-like reptiles (without reptile traits) have not been recognized as amniotes, based on their phylogenetic nesting in the LRT.

And, of course,
traditional workers still consider pterosaurs to be archosaurs based on their antorbital fenestra (by convergence), not their phylogenetic nesting (first documents in Peters 2000) in the LRT which solves earlier taxon exclusion problems by introducing a wider gamut of candidate sister taxa.

Th late appearance of the now convergent mammalian jaw joints
after the phylogenetic origin of mammals helps explain the two sites for ear bones in monotremes (below and medial to the posterior dentary) versus in therians (posterior to the jaw joint).

Tooth count
Basal monotremes have more teeth than any other mammals. Derived monotremes, like the living platypus and echidna, have fewer teeth, with toothless anterior jaws. This is a pattern of tooth gain/tooth loss we’ve seen before in other toothless taxa like Struthiomimus.

Recently, Bonaparte and Crompton 2017
concluded that ictidosaurs (Pachygenelus and kin) originated from more primitive procynosuchids rather than probainognathids. Pachygenelus likewise has a squamosal dentary contact, but it also retains a quadrate/articular contact as a transitional trait. They write: “We suggest a revision to the overwhelmingly accepted view that morganucodontids arose from probainognathid non- mammalian cynodonts (sensu Hopson & Kitching 2001). We suggest two phylogenetic lines, one leading from procynosuchids to ictidosaurs and the other from procynosuchids to epicynodonts and eucynodonts. One line evolves towards the mammalian condition, with a loss of circumorbital bones prefrontal, postfrontal, and postorbital), retention of an interpterygoid vacuity, a slender zygomatic arch, dentary/squamosal contact, and a long snout. The second evolves towards advanced non-mammalian cynodonts and tritylodontids with loss of the interpterygoid vacuity (present in juveniles), formation of a strong ventral crest formed by the pterygoids and parasphenoid, a very deep zygomatic arch, a tall dentary, and a short and wide snout.”

Talk about heretical!
Unfortunately, with the present taxon list, the LRT does not concur with Bonaparte and Crompton 2017, but instead recovers a more conventional lineage (Fig. 4).

Ictidosauria according to Bonaparte and Crompton:
The diagnostic features of Ictidosauria are as follows:

  1. absent postorbital arch, postorbital, and prefrontal;
  2. a slender zygomatic arch with a long jugal and short squamosal;
  3. a dorsoventrally short parietal crest and transversally wide braincase;
  4. interpterygoid vacuity;
  5. ventral contact of the frontal with the orbital process of the palatine;
  6. an unfused lower jaw symphysis;
  7. a well-developed articular process of the dentary contacting the squamosal;
  8. and a petrosal promontorium.
Figure 5. Basal Cynodont/Mammal cladogram focusing on the nesting of Brasilodon and Brasilitherium in the LRT.

Figure 4. Basal Cynodont/Mammal cladogram focusing on the nesting of Brasilodon and Brasilitherium in the LRT.

Therioherpeton (Bonaparte and Barbierena 2001; Fig. 3) also enters the discussion as a stem mammal.

Therioherpetidae according to Bonaparte and Crompton:
share several features with mammaliaforms:

  1. a slender zygomatic arch
  2. squamosal dentary contact
  3. unfuseddental symphysis
  4. petrosal promontorium
  5. transversely narrow postcanines with axially aligned cusps and an incipient cingulum
  6. and a transversely expanded brain case
  7. Therioherpetidae lack procumbent first lower incisors occluding between the first upper incisors
  8. lack an edentulous tip of the premaxilla
  9. and lack transversely widened postcanines

According to the Bonaparte team
Three distinct groups have been included in Mammaliformes.

  1. Morganucodon, Megazostrodon and Sinoconodon;
  2. Docodonta
  3. Haramiyids such as Haramiyavia

They report,
“Brasilitherium is closer to the first group than the more derived second and third groups. Brasilitherium is almost identical to Morganucodon, except that the latter has a mammalian tooth replacement pattern (single replacement of the incisors, canines, and premolars, and no replacement of the molars), double rooted molars, and the orbital flange of the palatine forms a medial wall to the orbit (Crompton et al. 2017).”

“Several features present in Procynosuchus are absent in probainognathids (sensu Hopson & Kitching 2001), but present in Ictidosauria.

  1. Interpterygoid vacuities (present only in juvenile probainognathids);
  2. a slender zygomatic arch;
  3. incisiforms present at the junction of premaxilla and maxilla;
  4. a low and elongated dentary;
  5. and an unfused lower jaw symphysis.”

Hopefully it will be seen as a credit to the LRT 
that it nested each new taxon about where the three Bonaparte teams nested them (sans the unusual Procynosuchus hypothesis), only refined a bit with the addition of several overlooked monotreme taxa, several of which have similar (to Procynosuchus) low, long skulls and rather low-slung post-crania.

Refrerences
Bonaparte JF and Barbierena MC 2001. On two advanced carnivorous cynodonts from the Late Triassic of Southern Brazil. Bulletin of the Museum of Comparative Zoology 156(1):59–80.
Bonaparte JF, Martinelli AG, Schultz CL and Rubert R 2003. The sister group of mammals: small cynodonts from the Late Triassic of Southern Brazil. Revista Brasileira de Paleontologia 5:5-27.
Bonaparte JF and Crompton AW 2017. Origin and relationships of the Ictidosauria to non-mammalian cynodonts and mammals. Historical Biology. https://doi.

 

A late (Middle Triassic) survivor of a Viséan radiation: Bystrowiella

Most of the taxa
in the large reptile tree (LRT, 1023 taxa) are bunches of leaves on bushy branches, chronicling the slow but steady march of evolution and radiation. In a few cases, like SphenodonDidelphis and Monodelphis, body parts are relatively unchanged over tens to hundreds of millions of years. That seems to be the case once again with Bystrowiella schumanni (Fig. 1), a taxon that nests with Viséan (340 mya) radiation taxa, but appears 130 million years later in the Middle Triassic (208 mya), In other words, this taxon had a long ghost-lineage.

Figure 1. Bystrowiella materials. Noteworthy are the lack of antorbtial fenestra, lack of an intertemporal, great size of the femur relative to the humerus and pectoral girdle and the possibility that disassociated armor might have belonged to this taxon, convergent with chroniosuchians.

Figure 1. Bystrowiella materials. Noteworthy are the lack of antorbtial fenestra, lack of an intertemporal, great size of the femur relative to the humerus and pectoral girdle and the possibility that disassociated armor might have belonged to this taxon, convergent with chroniosuchians (inset). Scale bars indicate a larger humerus than was drawn with the scapula graphic.

Bystrowiella schumanni (Fig. 1, Middle Triassic) was considered a bystrowianid chroniosuchid by Witzmann and Schoch 2017 despite lacking the hallmark antorbital fenestra found on traditional chroniosuchids and having the medial premaxillary teeth larger than the lateral ones, along with a long list of other distinct traits. They reported, “In sum, the postcranial skeleton of Bystrowiella is much more amniote-like than that of chroniosuchids, and one might expect this morphology in a rather terrestrial animal.”
Chroniosuchids are otherwise known from the Early Permian to Late Triassic.

By contrast
the large reptile tree (LRT, 1023 taxa) nest chroniosuchids near the base of the new Archosauromorpha branch of the Reptilia (= Amniota), not as a basal  And it nests Bystrowiella as a late surviving member of the Viséan radiation that gave us reptiles, derived from basal Seymouriamorpha close to the origin of the Leponspondyli, but distinct from the lineage. They reported, “The most conspicuous character that is shared by chroniosuchians, Gephyrostegus and higher stem amniotes is the T-shaped interclavicle, and this character distinguishes chroniosuchians from embolomeres.”

Not sure about those osteoderms
They were found separate from the skull, but match back of the skull depressions. If they do belong to Bystrowiella, then they evolved by convergence with chroniosuchids over 130 million years.

Both analyses
nest Bystrowiella near Silvanerpeton, a stem or basal amniote from the Viséan. The Witzmann and Schoch tree nests other chroniosuchids there, too, though probably due to taxon exclusion.

Outgroups in the LRT include
the basal seymouriamorph Kotlassia and the basal seymouriamorph leposponyl, Utegenia. Despite its late appearance in the fossil record, phylogenetically that puts Bystrowiella at the very base of the clade that includes all reptiles (= amniotes), which makes it a VERY interesting taxon.

References
Witzmann F and Schoch RR 2017. Skull and postcranium of the bystrowianid Bystrowiella schumanni from the Middle Triassic of Germany, and the position of chroniosuchians within Tetrapoda. Journal of Systematic Palaeontology 29 pp.

http://dx.doi.org/10.1080/14772019.2017.1336579

 

Guaibasaurus: a theropod! (Not a sauropodomorph)

Just look at it!!
With those very short, sharply-clawed forelimbs, how could anyone misidentify Guaibasaurus as ancestral to sauropods? And yet several big-name paleontologists did exactly that, most recently Baron et al. 2017.

Figure 1. Tiny forelimbs with three sharp-clawed fingers indicate that Guaibasaurus is a theropod, not a sauropodomorph. Shown to scale with related theropods Marasuchus and Procompsognathus.

Figure 1. Tiny forelimbs with three sharp-clawed fingers indicate that Guaibasaurus is a theropod, not a sauropodomorph. Shown to scale with related theropods Marasuchus and Procompsognathus. The posture of this skeleton is similar to the resting position of birds, which are also theropods.

Guaibasaurus candelariensis (Bonaparte et al., 1999, 2007; UFRGS PV0725T; Late Triassic) is known from an articulated skeleton lacking a neck and skull. Originally considered a basal theropod, later studies allied it with basal sauropodomorphs. Here this specimen nests as a basal theropod in a rarely studied clade. In the large reptile tree (LRT, 1018 taxa) Guaibasaurus nests between Segisaurus and Marasuchus + Procompsognathus (Fig. 1).

Wikipedia reports:
José Bonaparte and colleagues, in their 1999 description of the genus, found it to be possible basal theropod and placed it in its own family, Guaibasauridae. Bonaparte and colleagues (2007) found another early Brazilian dinosaur Saturnalia to be very similar to it, and placed the two in the Guaibasauridae which was found to be a primitive saurischian group. Bonaparte found that these forms may have been primitive sauropodomorphs, or an assemblage of forms close to the common ancestor of the sauropodomorphs and theropods. Overall, Bonaparte found that both Saturnalia and Guaibasaurus were more theropod-like than prosauropod-like. However, all more recent cladistic analyses found the members of Guaibasauridae to be very basal sauropodomorphs, except Guaibasaurus itself which was found to be a basal theropod or alternatively a basal sauropodomorph.”

On a similar note, Ezcura 2010 report, 
“A phylogenetic analysis found Chromogisaurus to lie at the base of Sauropodomorpha, as a member of Guaibasauridae, an early branch of basal sauropodomorphs composed of Guaibasaurus, Agnosphitys, Panphagia, Saturnalia and Chromogisaurus.” See Figure 2. We need to realize there are some phytodinosaurs, like Eoraptor, Eodromaeus, Panphagia and Pampadromaeus, that are outside of the Sauropodomorpha and outside the Ornithischia. The greater paleo community has not recognized this yet.

Figure 2. Taxa variously considered members of the Guaibasauridae. Here the top few nest with or closer to Sauropodomorpha. The bottom taxa nest with theropods in the LRT.

Figure 2. Taxa variously considered members of the Guaibasauridae. Here the top few nest with or closer to Sauropodomorpha. The bottom taxa nest with theropods in the LRT. Note the small size of Marasuchus, Agnophitys and Procompsognathus. Evidently phylogenetic miniaturization was taking place here, but in this case we know of no ancestors. Maybe someday we will..

I realize the authors
of the Guaibasaurus Wiki article can’t take a stand nor do they choose to test the hypotheses of PhDs, but I can and do here. Science is all about testing observations, comparisons and analyses. When Baron et al. nested Guaibasaurus with the sauropodomorphs, and Eoraptor + Eodromaeus with theropods, and avoided including a long list of taxa from the only other archosaur clade, Crocodylomorpha. and avoided including a long list of taxa from the outgroup to the Archosauria, the Poposaurs, then their results have to be considered suspect at least and bogus at worst. Headline grabbing is fun and lucrative for paleontologists, but not always good for paleontology. So many mistakes have been chronicled that it’s getting to the point that discoveries need to be put on simmer and only lauded when other studies validate them. On the same note, referees are not being tough enough on manuscripts.

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature  543:501–506.
Bonaparte JF;Ferigolo J and Ribeiro AM 1999. 
A new early Late Triassic saurischian dinosaur from Rio Grande do Sol state, Brazil. Proceedings of the Second Gondwanan Dinosaur Symposium, National Science Museum Monographs. 15: 89–109.
Bonaparte JF, Brea G, Schultz CL and Martinelli AG 2007. A new specimen of Guaibasaurus candelariensis (basal Saurischia) from the Late Triassic Caturrita Formation of southern Brazil. Historical Biology, 19(1): 73-82.

Several appearances and disappearances of the neck

FIgure 1. Panderichthys has no neck, but closely related Tiktaalki does have a neck.

FIgure 1. Panderichthys has no neck, but closely related Tiktaalki does have a neck.

One of the main differences between fish and tetrapods,
other than the transition from fins to feet, is the origin of the neck. famously in the amphibian-like fish, Tiktaalik (Fig. 2). The proximal outgroup taxon in the large reptile tree (LRT, 1016 taxa), Panderichthys (Fig. 1), does not have a neck. The skull and opercular bones are jammed up against the cleithrum (pectoral girdle) permitting no wiggle room. That wiggle room ultimately comes from the disappearance of those opercular bones.

Figure 1. Tiktaalik had a neck, that small space between its skull and pectoral girdle not seen in more primitive taxa.

Figure 2. Tiktaalik had a neck, that small space between its skull and pectoral girdle is not seen in more primitive taxa, which retain opercular bones, lost in Tiktaalik.

It is noteworthy
that more primitive taxa than Tiktaalik, in the Paratetrapoda, like Pholidogaster and Colosteus (Fig. 3) also lack a neck. The pectoral girdle extends beneath the posterior jaws, as in Osteolepis.

Figure 1. Colosteus relatives according to the LRT. Only Pholidogaster and Colosteus are taxa in common with traditional colosteid lists. Note the lack of a neck in Osteolepis, Pholidogaster and Colosteus.

Figure 3. Colosteus relatives according to the LRT. Only Pholidogaster and Colosteus are taxa in common with traditional colosteid lists. Note the lack of a neck in Osteolepis, Pholidogaster and Colosteus.

The first tetrapod clade,
(Fig. 9) with flat-headed Greererpeton at its base, had a neck, though not much of one. In related taxa like Gerrothorax (Fig. 4), the skull and torso were so wide that a neck would have been useless for lateral movements, but essential to help the skull rise during feeding (famously, like a toilet bowl lid!). More derived taxa in this clade, like Metoposaurus, had a little more neck represented by more space between the skull and pectoral girdle.

Figure 1. Gerrothorax, lacks a supratemporal rim and has laterally extended ribs, similar to those in Greererpeton.

Figure 4. Gerrothorax, has a wide skull and wide torso permitting little to no lateral skull movement, but vertical movement is not impeded.

The second tetrapod clade,
(Fig. 9) with Ossinodus and Acanthostega (Fig. 5) at its base, likewise did not have much of a neck. Perhaps there was less of a neck than in more basal Tiktaalik. This is a small clade with just these two members, so far.

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

Figure 5. Acanthostega does not have much of a neck. There is little wiggle room between the skull and pectoral girdle.

The third tetrapod clade,
(Fig. 9) with Pederpes and Crassigyrinus (Fig. 6) at its base likewise had very little wiggle room between the skull and cleithrum. Crassigyrinus had a short neck between its cheeks, so likely was immobile. In this clade derived members, Sclerocephalus and Eryops, document the third appearance of the neck in tetrapods. Even so, it was a very short relatively immobile neck.

Figure 5. Crassigyrinus has little to no neck.

Figure 6. Crassigyrinus has little to no neck. What neck it has is now tucked between its cheeks.

The fourth tetrapod clade
(Fig. 9) with Ichthyostega (Fig. 7) as its base, might have had some wiggle room between the skull and tall cleithrum. Not sure whether the small skull or large skull is correct. Certainly its phylogenetic successor, the reptilomorph Proterogyrinus (Fig. 8), had a substantial neck as did most of its descendants (but see below for notable exceptions).

Figure 6. Not sure which is more correct, but this Ichthyostega data shows little to no wiggle room for the larger skull, more for the smaller skull.

Figure 7. Not sure which is more correct, but this Ichthyostega data shows little to no wiggle room for the larger skull, much more for the smaller skull.

Basal reptilomorpha
and in the clade Seymouriamorpha, like Seymouria, and in the LRT leads to both Reptilia and Lepospondyli, had an increasingly mobile neck.

Figure 6. Proterogyrinus had a substantial neck.

Figure 8. Proterogyrinus had a substantial neck apart from the pectoral girdle.

The number of cervicals
remains low (under 4) in basal lepospondyls, and sometimes that number decreases to one. An exception, Eocaecilia, had 5 elongate cervicals. Basal amniotes, like Gephyrostegus, had six flexible cervicals.

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 9. 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

Notable reversals, back to lacking a neck, include:

  1. Rana, the frog.
  2. Cacops the basal lepospondyl
  3. Mixosaurus, the ichthyosaur and
  4. Eubaelana, the right whale, with short fused cervicals
Figure 5. Eubalaena australis, the Southern right whale nests with Cetotherium in the LRT.

Figure 10. Eubalaena australis, the Southern right whale nests with Cetotherium in the LRT. Here the cereals are fused and immobile.

 

 

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

Don’t give up on the origin of pterosaurs!

Evidently it is still widely held
that pterosaurs appeared suddenly without antecedent. As evidence of this failure to follow the data, I came across a blog called Tetrapod Flight in which the author, Leon Linde, writes on Monday, March 16, 2015:

  1. The first tetrapods to evolve powered flight were the pterosaurs. True
  2. These were a group of archosaurs related to the dinosaurs, but not dinosaurs themselves. False. Pterosaurs are lepidosaurs, not related to dinos. 
  3. The earliest known pterosaur was Eudimorphodon, who lived in what is now Italy around 230-220 million years ago, in the late Triassic. True enough
  4. However, while the earliest known pterosaur, Eudimorphodon had specialised multi-cusped teeth not found in any of the later pterosaurs, so it would not have been ancestral to them but rather part of a distinct pterosaur lineage that died out in the Triassic. False. Multicupsed teeth are found in several Triassic pterosaurs AND in their proximal sisters. 
  5. Furthermore, both Eudimorphodon and other late Triassic pterosaurs are “completely” developed, having all the typical pterosaur skeletal characteristics. True. That’s why they are called pterosaurs. They have all ‘the goods’.
  6. This suggests the origins of pterosaurs may lie even further back in the past, in the earlier Triassic or perhaps even in the Permian (Wellnhofer, 1991). Yes to the earlier (Middle) Triassic (Cosesaurus) and No to the Permian. 
  7. No fossils of the pterosaurs’ immediate ancestors are known. False. We have pterosaur proximal and distant ancestors going back to basal tetrapods with fins. Click here
  8. The most likely theory on their origins is that they evolved from arboreal creatures that would leap from branch to branch, flapping their forelimbs to stay airborne longer. Actually we have evidence for this scenario chronicled here.
  9. Pterosaur hips had great freedom of movement, their knees and ankles were hinge-like and their feet were plantigrade. True, True, True and False. Some beachcombers had plantigrade feet, but basal forms did not. 
  10. The knees and ankles did not permit the necessary rotation for them to move bipedally, so pterosaurs were obligate quadrupeds (though they may have had bipedal ancestors). False. Like living bipedal lizards, basal pterosaurs were bipedal and agile. We have their tracks! Later forms, especially beachcombers, were quadrupedal, and we have their tracks, too.
  11. A possible explanation for these features is that the early pterosaurs or proto-pterosaurs were arboreal creatures that evolved powerful leaping from branch to branch as an active mode of transport not dissimilar to that of arboreal leaping primates (Christopher, 1997). This reference should be Bennett 1997. Powerful leaping, fast running, yes, but without the use of the hands, which were flapping like those of birds and getting larger. Hard to develop wings when you’re using your hands on the ground. 
  12. These arboreal leapers would not have been gliders, who merely fall slowly downwards and forwards with the help of special flaps, but rather creatures utilising a quite different form of locomotion, one that led them to eventually having their forelimbs evolve into more and more sophisticated flapping airfoils. True! But not like the image below (which appeared on the blog post). This pretty but bogus image shows no flapping and no reason or benefit to having proto-wings on this dinosaur that should have been a fenestrasaur. 

Click to enlarge. A false series of pterosaur ancestors. Artist: Maija Karala.

Sadly
this state of affairs in pterosaur research shows that the general public and pterosaur artists and workers alike are still stuck in the tail-dragging age. Evidently, they have decided to shun and overlook recent data that chronicles and documents the origin of pterosaurs.  See below and here.

References
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330
Wild R 1993. A juvenile specimen of Eudimorphodon ranzii Zambelli (Reptilia, Pterosauria) from the upper Triassic (Norian) of Bergamo. Rivisita Museo Civico di Scienze Naturali “E. Caffi” Bergamo 16: 95-120.

wiki/Cosesaurus

 

 

Chinlestegophis and the origin of caecilia

Yesterday Pardo et al. 2017
described two conspecific and incomplete amphibians in the lineage of caecilians, Chinlestegophis jerkinsi (DMNH 56658, DMNH 39033, Figs. 1, 3). These long-sought specimens were discovered in the late 1990s preserved in Late Triassic burrows.

This is really big news!
Congratulations to the Pardo team!!

From the abstract:
“Here, we report on a small amphibian from the Upper Triassic of Colorado, United States, with a mélange of caecilian synapomorphies and general lissamphibian plesiomorphies. We evaluated its relationships by designing an inclusive phylogenetic analysis that broadly incorporates definitive members of the modern lissamphibian orders and a diversity of extinct temnospondyl amphibians, including stereospondyls. Stem caecilian morphology reveals a previously unrecognized stepwise acquisition of typical caecilian cranial apomorphies during the Triassic. A major implication is that many Paleozoic total group lissamphibians (i.e., higher temnospondyls, including the stereospondyl subclade) fall within crown Lissamphibia, which must have originated before 315 million years ago.”

The diagnosis:
“Small stereospondyl with a combination of brachyopoid and caecilian characteristics.”  Stereospondyls were generally large, flat-skulled aquatic taxa that had simplified and rather weak vertebrae in which the intercentrum was topped by a neural arch and the pleurocentrum was reduced to absent. According to Wikipedia, “All lepospondyls have simple, spool-shaped vertebrae that did not ossify from cartilage, but rather grew as bony cylinders around the notochord.” 

This is the opposite of
Reptilomorphs, in which the pleurocentra are large and the intercentra are smaller. Reptilomorphs generally were smaller and better adapted to terrestrial environments.

In the LRT traditional stereospondyls
(Fig. 5, pink) are mid-sized basalmost tetrapods, aquatic with a weak backbone because they are not far from fish with fins. Temnospondyls have stronger limbs and stronger backbones (Fig. 5, yellow), but typically remain large and aquatic.

Reptilomorphs 
(Fig. 5, orange) tend to be smaller with stronger limbs and vertebrae and reduce their dependence on water. Both lepospondyls (including living amphibians) and reptiles arise from this clade in the LRT.

Few microsaurs
were included in the Pardo et al study (Fig. 4) and the topology of their tree is very different from the present topology. Caecilians nest with lepospondyl microsaurs in the large reptile tree (LRT, 2014).

In addition
several skull bones are identified differently here (Fig. 1) than in the Pardo et al. study (Fig. 3). Pardo et al. identify an otic notch (that hole in the temporal region). Here that appears to be the space left open after the supratemporal has popped out during taphonomy. The supraorbital bones are all re-identified and both the lacrimal and quadratojugal are now listed in the present identification of bones. Based on conversations with Pardo and others, bone identification on several taxa may be the cause of the differing tree topologies.

Figure 1. GIF movie showing the two skulls of Chinlestegophis from Pardo et al. 2017 with DGS colors applied to both along with a revised set of bone labels

Figure 1. GIF movie showing the two skulls of Chinlestegophis from Pardo et al. 2017 with DGS colors applied to both along with a revised set of bone label based on phylogenetic bracketing among the previously excluded microsaurs close to caecilians.

Outgroup taxa should help identify the bones.
Pardo et al. recover Rileymillerus and Batrachosuchus as outgroup taxa within a large clade that includes Eryops and Sclerocephalosaurus at one base and Trimerorhachis and Greererpeton at the very base. By contrast, the LRT recovers Microbrachis and ultimately Utegenia as outgroup taxa. Microsaurs, Microbrachis and Utegenia were not mentioned in the Pardo et al. report.

First step: Learn about Rileymillerus
As usual, I knew nothing about this taxon earlier this week. Now, according to the LRT Rileymillerus nests with Oestocephalus and Ophiderpeton, two other long-bodied microsaurs with round cross-section skulls, not included in the Pardo et all study.  The apparent loss or lack of bones in the temporal region may be homologous with the lateral temporal fenestra in Ophiderpeton. That’s a rare trait among basal tetrapods.

Figure 3. Rileymillerus from Bolt and Chatterjee 2000 with colors applied.

Figure 2. Rileymillerus from Bolt and Chatterjee 2000 with colors applied. Note the lack of bone on both sides of the temples in this specimen, as in Ophiderpeton. The color (DGS) identify of the bones here is not in complete accord with Bolt and Chatterjee. As you can see, the skull has many cracks, which makes finding the sutures that much more difficult.

Unfortunately
Pardo et al. excluded most of the taxa that the LRT found were most closely related to the clade Chinlestephos + (caecilians + lysorophians) That includes Microbrachis and the rest of the microsaurs. They had good reason for doing so (see below).

Figure 3. Chinlestegophis diagram. Drawings produced by Pardo et al. At left bones colored as they labeled them. At right same bone colors rearranged to fit the new interpretation. See figure 1.

Figure 3. Chinlestegophis diagram. Drawings produced by Pardo et al. At left bones colored as they labeled them. At right same bone colors rearranged to fit the new interpretation. See figure 1. The lateral temporal fenestra is interpreted here as the spot on the skull that once held the supratemporal. No related taxa have a lateral temporal fenestra in either cladogram.

The Pardo et al. skull bone labels
differ from the present interpretation (Fig. 3). Even with such massive dissonance, Pardo et al. and the LRT both nest Chinlestegophis with caecilians and not far from Rileymillerus.

How can such a thing happen??
I can’t answer that at present. It’s frankly surprising.

Figure 4. Pardo et al. cladogram nesting caecilians as ultra-derived temnospondyls.

Figure 4. Pardo et al. cladogram nesting caecilians as ultra-derived temnospondyls. Taxa also present in the LRT are highlighted to show the general mixup of taxa that the LRT separates.

The drifting of the postorbital
In most tetrapods the postorbital is one of the circumorbital bones. In caecilians and their relatives the postfrontal takes over that spot and the postorbital drifts posteriorly, still lateral to the parietal. This observation may be one of the issues attending circumorbital and temporal bone identification arguments in this clade.

Figure 5. Basal tetrapod subset of the LRT. This cladogram includes microsaurs. When given the opportunity to nest with microsaurs, caecilians do so.

Figure 5. Basal tetrapod subset of the LRT. This cladogram includes microsaurs. When given the opportunity to nest with microsaurs, caecilians do so.

In their Supplemental Info
Pardo et al. added the traits for Chinlestegophis to the dataset of Maddin et al. 2012 (who earlier described Jurassic Eocaecilia) and found Chinlestegophis nested with Rileymillerus, close to the stem frog Micromelerpeton and strong-legged Acheloma all far from the caecilians and all derived from a sister to giant Eryops. This study did include microsaurs. Lots of them! Other mismatches include nesting the large reptile Limnoscelis between Seymouria and tiny Utaherpeton and Microbrachis, taxa that share few traits with each other in the LRT. Numerous other morphological mismatches also occur In Maddin et al. Evidently no one is using scaled reconstructions in their analyses as a final check on these mismatches. In the LRT caecilians nest with similar long-bodied, tiny-limbed taxa, which some claim is due to convergence. On a similar note, the LRT lumped and separated snakes from amphisbaenids while other trees failed to do this. So perhaps convergence is not the reason here when dealing with burrowing amphibians.

Figure 6. Maddin et al. cladogram featuring only two temnospondyls from the LRT. Here Chinlestegophis does not nest with caecilians.

Figure 6. Maddin et al. cladogram featuring only two temnospondyls from the LRT. Here Chinlestegophis does not nest with caecilians and Rileymllerus nests far from Oestocephalus.

A note from Jason Pardo
restates that the Maddin et al. study “found no close relationship between Eocaecilia and lepospondyls nor did we find a close relationship between Chinlestegophis and those taxa.”

Figure 6. Living caecilian photo.

Figure 7. Living caecilian photo. Lengths range from 6 inches to 5 feet.

All three cladograms
share few major branches in common. As everyone knows by now, the major branches are the more difficult ones to determine. And, if we can’t agree on the identify of the skull bones, of specimens, the tree topologies will have a hard time finding consensus.

Wikipedia reports,
“Currently, the three prevailing theories of lissamphibian (extant amphibians) origin are:

  1. Monophyletic within the temnospondyli
  2. Monophyletic within lepospondyli
  3. Diphyletic (two separate ancestries) with apodans (=caecilians) within the lepospondyls and salamanders and frogs within the temnospondyli.”
Figure 8. Skull of Microbrachis in several views. Here is where the postorbital leaves the orbit margin and drifts posteriorly. Compare to Chinlestegophis above.

Figure 9. Skull of Microbrachis in several views. Here is where the postorbital leaves the orbit margin and drifts posteriorly. Compare to Chinlestegophis above.

So… even the experts have not come to a consensus
on basal tetrapod topologies. The LRT agrees that the lissamphibia are monophyletic within the lepospondyli, matching option #2 above. There are many aspects of caecilians that need to be interpreted in light of their phylogeny. And we’re not coming to a consensus on that. Earlier we looked at the fusion of the cheek bones in caecilians here with the extant taxon Dermophis.

References
Bolt JR and Chatterjee S 2000. A New Temnospondyl Amphibian from the Late Triassic of Texas. Journal of Paleontology 74(4):670-683.
Maddin HC, Jenkins FA, Jr, Anderson JS 2012. The braincase of Eocaecilia micro podia (Lissamphibia, Gymnophiona) and the origin of Caecilians. PLoS One 7:e50743.
Pardo JD, Small BJ and Huttenlocker AK. 2017, Stem caecilian from the Triassic of Colorado sheds light on the origins of Lissamphibia. PNAS: 7 pp. www.pnas.org/cgi/doi/10.1073/pnas.1706752114