Prozostrodon: how close to mammals?

Wikipedia reports, Prozostrodon was an advanced cynodont that was closely related to the ancestors of mammals.”

While true in the broadest sense
after all, even primitive Vaughnictis is “closely related to the ancestors of mammals,” in the large reptile tree (LRT, 1025 taxa) Prozostrodon nests a few nodes down from the most primitive mammals with several intervening transitional taxa.

Figure 1. Prozostrodon nests in the LRT between Thrinaxodon and the Chiniquodon clade.

Figure 1. Prozostrodon nests in the LRT between Thrinaxodon and the Chiniquodon clade, not as close to the base of mammals as Wikipedia implies.

Prozostrodon brasiliensis (originally Thrinaxodon brasiliensis, Barberena et al. 1987; Triassic; the size of a cat) was originally considered congeneric with Thrinaxodon, then closer to mammals. Here it nests between Thrinaxodon and the Chiniquodon clade. The foot was more symmetrical than in Thrinaxodon. The teeth were relatively smaller with molars no deeper than the incisors and with a larger canine.

Earlier we looked at several new addition to the Cynodontia. This is just one more.

References
Barberena MC, Bonaparte JF and Teixeira AMSA 1987. Thrinaxodon brasiliensis sp. nov., a primeira ocorrencia de cinodontes gales – sauros para o Triasico do Rio Grande do Sul. Anais do X Congresso Brasileiro de Paleontologia, Rio de Janeiro, Brazil, pp. 67-76.
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.

 

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New name and a name resurrection for two Solnhofen pterosaurs

Vidovic and Martill 2017
propose new and resurrect old generic names for two Solnhofen pterosaur specimens. Both are good and needed based on an earlier abstract (Peters 2007) and tree topology published here six years ago at ReptileEvolution.com in the large pterosaur tree (LPT, 232 taxa).

Unfortunately
Vidovic and Martill remain completely in the dark regarding pterosaur ontogeny. As we learned earlier here, here and here from several adult and juvenile specimens, pterosaurs juveniles and embryos had adult proportions and that’s why they were mechanically able to fly shortly after hatching. Vidovic and Martill report, “It is difficult to distinguish ‘G. rhamphastinus’ (Fig. 3 from the holotype of D. kochi (Fig. 2) other than by using size-related criteria.” And, “juvenile pterosaurs with small crests have been identified.”

Also unfortunately,
Vidovic and Martill still consider pterosaurs to be derived archosaurs or archosauriforms. They report, “A cladistic analysis of the Pterosauria, including all the taxa discussed here, was performed. The analysis included 104 operational taxonomic units (OTUs) comprising 99 pterosaurs and five archosauriforms as an outgroup.” We have to ask ourselves, how long will pterosaur workers remain in the dark on these basic questions that were answered years ago? Look here, here (Peters 2000, 2007) and here.

Pterodactylus wastebasket
Vidovic and Martill write: “Until relatively recently, the genus Pterodactylus Cuvier, 1809 had been a wastebasket taxon that has included many diverse pterosaurs, including some that are now recognized as basal nonpterodactyloids.” We looked at the Pterodactylus wastebasket here in 2011 (Fig. `1).

The Pterodactylus lineage and mislabeled specimens formerly attributed to this "wastebasket" genus

Figure 1. Click to enlarge. The Pterodactylus lineage and mislabeled specimens formerly attributed to this “wastebasket” genus

Wellnhofer 1970
provided catalog numbers for dozens of Solnhofen specimens. Since those numbers are simpler than their museum numbers that’s how they are named (Figs. 2, 3) at ReptileEvolution.com.

basal germanodactylids

Figure 2. Basal Germanodactylia, Three taxa preceding Germanodactylus rhamphastinus: No. 6, No. 12 and No. 23, the last renamed Diopecephalus kochi. These are all adults.

No. 23 — BSP AS XIX 3 — Diopecephalus kochi (formerly Pterodactylus kochi).
(Fig. 1, left). Seeley had it right originally. Vidovic and Martill correct a century of error when they report, “The holotype of ‘P. kochi’ was considered to belong to a distinct genus by Seeley (1871), which he  unambiguously named Diopecephalus Seeley, 1871.”

No. 64 — B St AS I 745  —
Altmuehlopterus (formerly Germanodactylus) rhamphastinus

Vidovic and Martill reported, “Many phylogenetic studies demonstrate that the two species of Germanodactylus nest together (Kellner 2003; Unwin 2003; Andres & Ji 2008; Lu et al. 2009; Wang et al. 2009; Andres et al. 2014) in a monophyletic clade, but a more focussed analysis by Maisch et al. (2004) demonstrates the genus to be paraphyletic. Maisch et al. (2004) created the nomen nudum Daitingopterus, intended for the reception of ‘G. rhamphastinus’ by placing the name in a table with no specific reference to a specimen.”

Figure 3. Germanodactylus rhamphastinus, No. 64 in the Wellnhofer 1970 catalog.

Figure 3. Germanodactylus rhamphastinus, No. 64 in the Wellnhofer 1970 catalog. Vidovic and Martill renamed this specimen Altmuehlopterus, which is fine and appropriate.

The LPT separates A. (G.) rhamphastinus from G. cristatus by two taxa.

Problems with the Vidovic and Martill 2017 tree:

  1. Lagerpeton nests with Marasuchus, both as proximal outgroups to the Pterosauria. Totally bogus. Tested, validated, real outgroups are listed here. The Fenestrasauria (Peters 2000) is overlooked in the text and references.
  2. Preondactylus and Austriadactylus nest as basalmost pterosaurs. Bergamodactylus, the basalmost pterosaur in the LPT, is excluded.
  3. Only one specimen each of Dorygnathus and Scaphognathus are employed. The LPT shows two clades of pterodactyloid-grade pterosaurs arise from various specimens of Dorygnathus while two others arise from tiny Scaphognathus specimens experiencing phylogenetic miniaturization.
  4. As a result (perhaps) toothy ornithocheirids nest with toothless pteranodontids. In the LPT ornithocheirids arise from equally tooth cycnorhyamphids while shartp-face pteranodontids arise from similar germanodactylids.
  5. The Darwinopterus clade nests as the proximal outgroup to the traditional Pterodactyloidea, when the LPT shows it to be a sterile clade with some pterodactyloid-grade traits.
  6. Altmuehlopterus (formerly Germanodactylus) rhamphastinus nests with G. cristatus
  7. Diopecephalus kochi nests with Pterodactylus antiquus.
  8. Those are the big problems. There are more, but I want to keep it pertinent.

Vidovic and Martill provide clues to their observational problems
when they note, “The genera Pterodactylus and Diopecephalus are remarkably similar.” No they aren’t! Species within the Pterodactylus clade are not even that similar!

Re: Germanodactylus and Pterodactylus,
Vidovic and Martill write: “We agree that some of the differences could be ontogenetically variable and perhaps vary between sexes, so in 1996 it seemed possible that the two species could be at least congeneric.” They disagree with the “common opinion” that the two are distinct genera. Let’s go with the evidence of a large gamut phylogenetic analysis — not opinion — or any analysis lacking so many pertinent taxa.

Vidovic and Martill 2017 rename G. rhamphastinus
Altmuehlopterus rhamphastinus. That’s good. It is generically distinct from its proximal relatives in the LPT. They report, “This name is presented as an alternative to the geographically significant name Daitingopterus (Maisch et al., 2004) which is a nomen nudum.” Not sure how all that falls. I’ll leave such issues to the PhDs.

If you like long nomenclature puzzles
you’ll like Vidovic and Martill 2017. They do a good job of running down all the names that prior workers gave to these century-old specimens. Beware that they are clueless as to the origin of pterosaurs, the ontogeny of pterosaurs and previous work on the phylogeny of pterosaurs based on a much larger taxon list of ingroup and outgroup taxa.

References
Peters D 2000. A redescription of four prolacertiform genera and implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106: 293-336
Peters D 2007  The origin and radiation of the Pterosauria. Flugsaurier. The Wellnhofer Pterosaur Meeting, Munich 27. Abstract online here.
Vidovic SU and Martill DM 2017. The taxonomy and phylogeny of Diopecephalus kochi (Wagner, 1837) and ‘Germanodactylus rhamphastinus’ (Wagner, 1851). From: Hone DWE., Witton MP and Martill DM (eds) New Perspectives on Pterosaur Palaeobiology. Geological Society, London, Special Publications, 455, https://doi.org/10.1144/SP455.12
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

wiki/Pterodactylus
wiki/Germanodactylus

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.

 

Sperm whales have faces, too!

Figure 1. This image comes from a news story on whale strandings and the contents of their stomachs. But I see two distinct faces here, like humans, chimps and other mammals with distinctive coloration patterns and variations on morphology.

Figure 1. This image comes from a news story on whale strandings and the contents of their stomachs. But I see two distinct faces here, like humans, chimps and other mammals with distinctive coloration patterns and variations on morphology.

Humans have distinct faces.
So do chimps, dogs, cows, other mammals and animals in general. We just have to see two in close proximity (as in Fig. 1) to notice the slight variation that Nature puts on pod mates and/or family members. This minor variation, of course, is the engine by which large variation can add up in isolation to produce new species, whether larger or smaller, more robust or more gracile, shorter, longer, with longer or shorter limbs, longer or shorter faces. The variations are endless, but patterns can be gleaned in phylogenetic analysis.

Look closely
and you’ll see the profile of these two beached whales are slightly different, the flippers are slightly different, to say nothing of the variations on the white patches and scars that they are partly born with and then develop during their lifespan as white scars.

Just think,
this odontocete is derived from swimming tenrecs, derived from basal placentals, derived cynondonts, etc. etc. all due to subtle variations in family members like you see here, over vast stretches of time and millions of generations.

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