Distribution of ‘key’ traits in basal tetrapods

Before the advent of phylogenetic analysis,
paleontologists attempted to define clades with a short list of synapomorphies. In this way they were getting close to the dangers of pulling a Larry Martin. Many taxa, like pterosaurs and Vancleavea were (and are) considered enigmas because they seemed to appear suddenly in the fossil record with a short suite of traits that did not appear in other reptiles. That was only true back then because paleontologists were only considering short lists of traits.

After the advent of phylogenetic analysis
considering long lists of traits, the rule of maximum parsimony allowed clades to include members that do not have a short list of key traits. For instance some reptiles, like snakes, do not have limbs, but that’s okay based on the rule of maximum parsimony as demonstrated in the large reptile tree (LRT, 977 taxa, subsets shown in Figs. 1-5).

Before the advent of phylogenetic analysis
Carroll (1988) divided basal tetrapods into labyrinthodonts and lepospondyls and presented short lists of key traits.

Labyrinthodonts

  1. evolved directly from rhipidistian fish
  2. labyrinthine infolding of the dentine
  3. palate fangs and replacement pits
  4. vertebral centra composed of more than one element
  5. otic notch
  6. large in size

Lepospondyls

  1. a heterogeneous assemblage of groups with perhaps several origins from among various labyrinthodonts
  2. simple (non-labyrinthine) teeth
  3. no palate fangs
  4. vertebral centra composed of one element
  5. no otic notch
  6. small in size

By contrast,
the large reptile tree introduces a non-traditional topology in which lepospondyls have a single origin. Below (Figs. 1-5) the distribution of several traits are presented graphically.

Figure 1. Distribution of the solid and open palate architectures in basal tetrapods in the LRT topology.

Figure 1. Distribution of the solid and open palate architectures in basal tetrapods in the LRT topology.

Open palate distribution
Basal tetrapods have a solid palate (Fig. 1) in which the pterygoid is broad and leaves no space around the medial cultriform process. Other taxa have narrow pterygoids and large open spaces surrounding the cultriform process. Still others are midway between the two extremes. Traditional topologies attempt to put all open palate taxa into a single clade. Here the open palate evolved three times by convergence.

Figure 2. Size distribution among basal tetrapods in the LRT topology

Figure 2. Size distribution among basal tetrapods in the LRT topology

The length of basal tetrapods
falls below 60 cm in Eucritta and more derived taxa. It also falls below 60 cm in Ostelepis, at the origin of Tetrapoda and Paratetrapoda.

Figure 3. Distribution of single vertebrae among basal tetrapods in the LRT.

Figure 3. Distribution of single vertebrae among basal tetrapods in the LRT. Note: Phlegethonita now nests with the microsaurs. 

Single piece centra
appear in frogs + salamanders and microsaurs. Intercentra appear in all other taxa.

Figure 6. Distribution of palatal fangs among basal tetrapods in the LRT.

Figure 6. Distribution of palatal fangs among basal tetrapods in the LRT.

Palate fangs
appear in all basal paratetrapods and tetrapods except Spathicephalus and Gerrothorax. Exceptionally, Seymouria also had palate fangs.

Figure 7. Distribution of the otic notch among basal tetrapods in the LRT.

Figure 7. Distribution of the otic notch among basal tetrapods in the LRT.

The otic notch
is widespread among basal tetrapods. Those without an otic notch include

  1. Six flat-skulled temnospondyls in which the tabular contacts the squamosal. Some of these, like Greererpeton, have figure data that lack an otic notch, but photos that have one.
  2. Salamanders and frogs that greatly reduce posterior skull bones.
  3. All microsaurs more derived than Microbrachis

Let me know
if I overlooked or misrepresented any pertinent data. This weekend I should be able to look at and respond to the many dozen comments that have accumulated over the last few weeks.

 

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The human occiput and palate

We looked at the facial portion
of the human skull earlier. Today we’ll look at the occiput and palate (Fig. 1).

Figure 1. Human occiput and palate. On most tetrapods these two are usually set at right angles to each other, but an upright stance has rotated the occiput to a ventral orientation.

Figure 1. Human occiput and palate. On most tetrapods these two are usually set at right angles to each other, but an upright stance has rotated the occiput to a ventral orientation.

There’s nothing new here. 
This is just an opportunity to educate myself on the human palate and occiput. Only the endotympanic (En) is a novel ossification. The occiput is a single bone here, the product of the fusion of several occipital bones. Can you find the suborbital fenestra? It’s pretty small here.

The asymmetry is interesting here.
Sure, this is an old adult, missing some teeth, but you’ll see other examples elsewhere.

Let me know
if you see any errors and they will be corrected. As you already know, everything I present here was learned only 48 hours earlier — or less.

Origin of the long basipterygoid process of the basisphenoid in pterosaurs

Today we talk about a throat bone, the basisphenoid.

Figure 1. Elongation of the basipterygoid process of the basisphenoid in pterosaur precursor, Cosesaurus.

Figure 1. Elongation of the basipterygoid process of the basisphenoid in pterosaur precursor, Cosesaurus. Also shown is a precursor, Macrocnemus and two derived taxa, Rhamphorhynchus and Anahanguera. 

The basisphenoid bone connect the palate to the braincase with a pair of basipterygoid (bp) processes. In almost all tetrapods the bp processes are short. They might be small and they might be big but they are never long — except in pterosaurs and their precursors among the fenestrasaurs (Fig. 1) and in sauropod dinosaurs. (If I’m forgetting others, please advise.)

Macrocnemus (Fig. 1) has typically short basipterygoid processes. But they get long and gracile in Cosesaurus (Figs. 1, 2) and more so in Rhamphorhynchus. In Anhanguera (Fig. 1) they are fused, splitting only at their base. These bones are usually the last to be reconstructed, unless they’re obvious, as in the examples above.

Basipterygoid process fusion in pterosaurs
had its origin in Haopterus in ornithocheirids. By convergence this also occurs in the big Pterodactylus longicollum and after Germanodactylus rhamphastinus among the sharp-beak pterosaurs.

That’s the needle-like cultriform process of the basisphenoid between the bp processes. Anhanguera seems to have lost or fused its cultriform process. Evidently this happens as fusion process proceeds.

Figure 2. palatal elements in Cosesaurus. Image from Elleberger 1993. Click to enlarge. 

Figure 2. palatal elements in Cosesaurus. Image from Ellenberger 1993. Click to enlarge. Some parts are easy to identify. Others are more difficult. This is not the way Ellenberger identified the elements. And it is another example of DGS. Pink elements are quadrates.

There has not been much research on pterosaur palates and basipterygoid processes.
For a long time it was thought that the maxillary portions of the palate were the palatines (e.g. Wellnhofer 1978, 1991; Bennnett 1991, 2001`). Peters (2000) solved that problem with comparisons to Macrocnemus (Fig. 1). Osi et al (2010) published their “new interpretation,” but also understood they had only confirmed Peters (2000), who also noted that sometimes the palatine and ectopterygoid fuse, as they do in Rhamphorhynchus (Fig. 1). These elements don’t fuse in Dorygnathus, as Osi et al (2010) showed.

The basisphenoids are rarely exposed in pterosaurs so it will take more digging to see how and when they joined in large derived taxa.

I don’t have a ready answer for why the bp processes elongate in Cosesaurus, other than the fact that it was holding its head more erect while walking around bipedally while flapping its forelimbs. The long bp process kept the palate low and the occipital condyle high on the fenestrasaur skull.

Which throat muscles are attached to the bp and cultriform processes? What function do they serve. I don’t know and there’s not much out there on Google, from what I can tell. Any help would be appreciated.

References
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodon and Systematics of the Pterodactyloidea. [Volumes I & II]. Ph.D. thesis, University of Kansas, University Microfilms International/ProQuest.
Bennett SC 2001. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260: 1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260: 113–153.
Osi A, Prondvai E, Frey E and Pohl B 2010. New Interpretation of the Palate of Pterosaurs. The Anatomical Record 293: 243-258.
Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Wellnhoffer P 1991.  The Illustrated Encyclopedia of Pterosaurs. London: Salamander. 192 pp.

Poposaur palates

The palates of poposaurs are poorly known Some have not been described or reconstructed (Fig.1). Others have been wrongly reconstructed or partially reconstructed (Fig. 4). Here (Fig. 1) are two poposaurs, Effigia and Shuvosaurus next to Daemonosaurus (Sues et al. 2011, also largely guessed at from broken pieces) and Thecodontosaurus, which provides more certitude. Most unfortunately, the palate of Lotosaurus has not been described or illustrated despite the presence of several specimens and museum casts. The little question is: On Daemonosaurus, which way do the ectopterygoids go? Long side against the pterygoid, as in rauisuchids? Or short side, as in Effigia and other dinosaurs?

Figure 2. Effigia palate in situ (left) and reconstructed by reassembling colored elements (at right).

Figure 2. Effigia palate in situ (left) and reconstructed by reassembling colored elements (at right).

On rauisuchians, as in ornithosuchians (Fig. 2), the ectopterygoid has a larger contact area with the lateral pterygoid and it produces a small “stem” to contact the jugal (as in Saurosuchus) or the maxilla (as in Riojasuchus). If you flip the ectopterygoid of Daemonosaurus, you get the rauisuchian type of ectopterygoid. Left as is (Fig. 1), however, you get the dinosaurian type,  and that is the preferred reconstruction here based on phylogenetic bracketing.

Click to enlarge. Euparkeriid, ornithosuchian, rauisuchian, aetosaurian, and basal archosaur palates.

Figure 2. Click to enlarge. Euparkeriid, ornithosuchian, rauisuchian, aetosaurian, and basal archosaur palates. Here are Euparkeria and Osmolskina, both euparkeriids. Ornithosuchus and Riojasuchus are ornithosuchids. Saurosuchus and Postosuchus are both rauisuchians. Stagonolepis is an aetosaur. Pseudhesperosuchus is close to the basal archosaur pattern with a much smaller ectopterygoid and smaller ectopterygoid/pterygoid contact. The original configuration is shown on the right side. A possible alternative is shown on the left. Not sure how it was preserved. I’d like to know if you have this data. If the left is correct in figure 2 (Pseudohesperosuchus), and Shuvosaurus is also correct in figure 1, these suggest that Daemonosaurus is correctly drawn in figure 1.

Silesaurus palate with missing elements restored on the right.

Figure 4. Silesaurus palate with missing elements restored on the right. Illustration (without color) from Dzik 2003 who illustrated missing elements on the left.

Silesaurus Palate The missing ectopterygoid and palatine were not illustrated for Silesaurus. Given the palates of related taxa (Fig.1), I have added the missing elements on the right here (Fig. 4) to match them. Thus these restorations are guesses that appear to make sense in context. When better data come along, we’ll make improvements.

This has been a first attempt at reconstructing the palates of several poposaurs at once based on similar morphologies in close kin. The palates should remain somewhat similar. If anyone has good data on the palates of other rauisuchians and basal dinosaurs, please forward them on.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Figure 3 is absent from this post now. Apologies. I had it in my files for several years and thought it had been published by now. It had not. 

 

References Bonaparte JF 1969. Dos nuevas “Faunas” de reptiles Triasicos de Argentina: I. Gondwana Symp., IVGS: 283-306.
Borsuk-Bialynicka M and Evans SE 2009. Cranial and mandibular osteology of the Early Triassic archosauriform Osmolskina czatkowicensis from Poland. Palaeontologia Polonica 65, 235–281.
Brusatte SL, Benton MJ, Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Chatterjee S 1985. Postosuchus, a new Thecodontian reptile from the Triassic of Texas and the origin of Tyrannosaurs. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 309 (1139): 395–460. doi:10.1098/rstb.1985.0092.
Chatterjee S 1991. An unusual toothless archosaur from the Triassic of Texas: the world’s oldest ostrich dinosaur? Abstract, Journal of Vertebrate Paleontology, 8(3): 11A.
Chatterjee S 1993. Shuvosaurus, a new theropod: an unusual theropod dinosaur from the Triassic of Texas. National Geographic Research and Exploration 9 (3): 274–285.
Dzik J 2003. A beaked herbivorous archosaur with dinosaur affinities from the early Late Triassic of Poland. Journal of Vertebrate Paleontology 23: 556-574.
Ewer RF 1965. The Anatomy of the Thecodont Reptile Euparkeria capensis Broom Philosophical Transactions of the Royal Society London B 248 379-435. doi: 10.1098/rstb.1965.0003
Rauhut OWM 1997. On the cranial anatomy of Shuvosaurus inexpectatus (Dinosauria: Theropoda). In: Sachs, S., Rauhut, O. W. M. & Weigert, A. (eds) 1. Treffen der deutschsprachigen Palaeoherpetologen, Düsseldorf, 21.-23.02.1997; Extended Abstracts. Terra Nostra 7/97, pp. 17-21.
Long R and Murry P 1995. Late Triassic (Carnian-Norian) Tetrapods from the Southwestern United States. New Mexico Museum of Natural History and Science Bulletin 4, Pp. 153-163.
Sill WD 1974. The anatomy of Saurosuchus galilei and the relationships of the rauisuchid thecodonts. Bulletin of the Museum of Comparative Zoology 146: 317-362.
Sues H-D, Nesbitt SJ, Berman DS and Henrici AC 2011. A late-surviving basal theropod dinosaur from the latest Triassic of North America. Proceedings of the Royal Society Bpublished online
Walker AD 1961. Triassic reptiles from the Elgin area: StagonolepisDasygnathus and their allies. Philosophical Transactions of the Royal Society B 244:103-204.
Walker AD 1964. Triassic reptiles from the Elgin area: Ornithosuchus and the origin of carnosaurs. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 248(744): 53-134.
Yates AM 2003. A new species of the primitive dinosaur Thecodontosaurus (Saurischia: Sauropodomorpha) and its implications for the systematics of early dinosaurs. Journal of Systematic Palaeontology 1(1):1-42. wiki/Daemonosaurus wiki/Shuvosaurus

The Pterosaur Palate: More workers coming on board.

Pinheiro and Schultz (2012) recently reported on an “unusual” pterosaur palate from Brazil. (Actually it was not so unusual, IMHO, having reconstructed hundreds of them).

They claimed that the palatal elements were misidentified by Osi et al. (2010) who echoed Peters (2000). (Actually their labeling and interpretation of Pterodactylus micronyx (BSP 1936 I 50) was identical to those earlier interpretations. Their other interpretations for the most part followed suite.

Pinheiro and Schultz (2012) reported, “Only recently was a new interpretation of the pterosaur palate made, in a study that utilized the Extant Phylogenetic Bracket to identify homologous structures in the palates of pterosaurs, birds and crocodiles.” Actually, by now everyone should know that pterosaurs belong to an extinct clade of lizards, not crocs or birds. There has never been support, except in the absence of lizards, for a pterosaur-bird-croc relationship. The long fifth toe, the long fourth finger, the ossified sternum and the extreme thinness of the egg shell are traits pterosaurs share with living lepidosaurs to the exclusion of living archosaurs. If an antorbital fenestra is key, it only takes one reminder to note that this structure appears four times by convergence within the Reptilia.

But this brings up an interesting point. Why were the palatal shelves of pterosaurs ever considered palatines (See all works by Bennett and Wellnhofer) if they are maxillary in origin in crocs and birds??

So where did Pinheiro and Schultz (2012) go right?
They correctly identified the palatal shelves as belonging to the maxilla, separated only by the vomers. Most pre-2000 studies (anything by Bennett, Wellnhofer, other early workers) mistakenly labeled these palatines. They correctly identified the pterygoids. They correctly identified the palatines in most of their pterosaurs, but they did not understand that the palatines and ectopterygoids both fuse and diverge diagonally in Pteranodon (so their ectopterygoid is the ectopalatine). This becomes obvious after a study of Germanodactylus palates.

And where did Pinheiro and Schultz (2012) go wrong? 
Other than the aforementioned misconceptions, Pinheiro and Schultz labeled the posterior portion of the shelf of the new pterosaur as the palatine, ignoring their own graphics showing the palatine and ectopterygoid merge in most pterosaurs to form a single small L-shaped bone, the ectopalatine. Tiny, fragile and easily lost, the ectopalatine is missing from the new skull fragment of their study.

Their second mistake was using only highly derived taxa, like Pteranodon, Anhanguera and Tupuxuara, to identify palatal elements. Their single basal pterosaur, the Dorygnathus of Osi et al. (2012) was not relabeled, contra their earlier pronouncement. Their identification of the premaxilla/maxilla in Dorygnathus follows without criticism the mistake made by Osi et al. (2009) reviewed earlier here.

Pinhiero and Schultz (2012) blindly follow earlier analyses that link broad-snouted, toothy Anhanguera with sharp-snouted, toothless Pteranodon. On the face of it, what were they not thinking??? (Oh, yes, they were following tradition without critical thinking).

Their drawings show only the ventral view, but the tiny ectopalatines are often dorsal to the pterygoids.

Their lateral view reconstructions were not of the specimens employed, but were of generic and non-generic relatives. BSP 1936 I 50 belongs to the cycnorhamphids, not Pterodactylus.

Earlier pterosaurheresies covered nearly every aspect of pterosaur palate evolution starting here and going on for seven (occasionally interrupted) chapters. You can also Google “pterosaur palate” under the “images” choice.

In conclusion, I’m glad to see more workers are jumping on board with the maxilla shelf identification, and correctly identifying the palatine, but not sure why they think this is such a new thing.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Ösi A, Prondvai E, Frey E, Pohl B 2010. New interpretation of the palate of pterosaurs. The Anat Rec 293: 243–258. doi: 10.1002/ar.21053.
Peters D 2000b.
 A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Pinheiro FL, Schultz CL 2012. An Unusual Pterosaur Specimen (Pterodactyloidea, ?Azhdarchoidea) from the Early Cretaceous Romualdo Formation of Brazil, and the Evolution of the Pterodactyloid Palate. PLoS ONE 7(11): e50088. doi:10.1371/journal.pone.0050088

More Therapsid Palates

Yesterday I published a bad Biarmosuchus palate based on an erroneous illustration at Palaeos, another website devoted to prehistory. Today I’ll try to rectify that error and take the process another step forward.

There are two problems I’m trying to solve.
Was Archaeothyris closer to basal therapsids, like Nikkasaurus, than the traditional therapsid ancestors, the sphenacodonts?

And did Nikkasaurus + dicynodonts and kin split from biarmosuchids and kin at the base of the Therapsida, near Nikkasaurus?

At present, both of these heretical hypotheses are recovered by the large reptile tree. However neither Archaeothyris nor Nikkasaurus, preserve the palate, to my knowledge. So we have to look at what we do have: sisters and cousins. Complicating the matter is the evolution of a more 3-dimensional palate, with a deeper vomer (in palatal view) and an advancing secondary palate coming in from the sides. Further complicating the matter is a large amount of convergence, as you’ll see.

Six synapsid palates.

Figure 1. Six synapsid palates. Bones color-coded. With Biarmosuchus the sutures are not well-preserved, but this should be an improvement on my earlier attempt. Note the wide variety here, even in closely related taxa.

Ophiacodon and Haptodus
The palates of Ophiacodon and Haptodus are closer to one another in appearance than either is to the other four. The overall shape of the palate in Haptodus is closer to Biarmosuchus, but Ophiacodon is also much longer than the absent Archaeothyris (please remember the actual sister to the most primitive therapsids was not Archaeothyris, Ophiacodon nor Haptodus, but a missing taxon nesting close to all three and closer still to Nikkasaurus, a largely ignored late-surviving, but basal therapsid).

First problem: Ophiacodonts vs. Sphenacodonts
The palates of Haptodus and therapsids had a longer vomer and a longer pterygoid posterior to the transverse process. Ophiacodon and therapsids had smaller palatal teeth. The vomer contacted the pterygoid in Haptodous and therapsids. The interpterygoid vacuity was smaller in Ophiacodon and therapsids. The maxilla produced a slight medial shelf in Haptodus. The ventral skull was pinched in at the canines in Ophiacodon and therapsids.

Yes, it’s pretty clear, neither Ophiacodon nor Haptodus makes as good of a sister to Biarmosuchus than the imaginary missing taxon somewhere in between them…something like Nikkasaurus.

Second problem: A nearly diphyletic Therapsida
Other trees find dicynodonts to be sister to gorgonopsians and both derived from dinocephalians and biarmosuchians. The large reptile tree finds dicynodonts split from all other therapsids at the base of their clade.

Therapsids are known for their canine teeth, but there are no canine teeth in Endothidon (Fig. 1), Otsheria and Nikkasaurus.  Other dicynodonts had canine teeth, often their only teeth. Since dicynodonts were plant eaters, one wonders if canine teeth in dicynodonts were secondarily evolved, as it appears when one compares Tiarajudens to its sister, Anomocephalus. The sisters to dicynodonts, dromasaurs like Suminia and Galechirus, also lacked large canines.

A suborbital fenestra is present in Regisaurus, but not in Procynosuchus, or that other taxa here. Not sure about Biarmosuchus.

The quadrates were small in Biarmosuchus and Regisaurus, but they were larger in the pelycosaurs and dicynodonts.

The choanae opened ventrally in four the synapsids, but not in  Regisaurus and Endothiodon, which had something of a secondary palate, again, likely by convergence. The choanae were displaced posteriorly in both dicynodonts and Regisaurus, likely due to convergence.

The palates of the two figured dicynodonts are not close to one another in morphology. The large variety in these six palates provides no clear guidance with regard to basal therapsid affinities. The morphological distances between these taxa is great and the palate of a key taxon, Nikkasaurus, remains unknown.

When I learn more, we’ll come back to this subject. I do know that without Nikkasaurus resolution is lost at that node in the large reptile tree. Yes, its that close and Nikkasaurus is rarely included in other phylogenetic studies.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

New Evidence for a Therapsid/Ophiacodont Relationship in the Palate ??

The palates of Haptodus, Ophiacodon and Biarmosuchus.

Figure 1. The palates of Haptodus, Ophiacodon and Biarmosuchus. More traits are shared by the latter two to the exclusion of Haptodus. Okay, that’s wrong. I had a bad bit of data here for the Biarmosuchus skull. I went with a published image, then realized the mistakes I made here. Do-over in process.

Another short one. The picture tells the story (that was written Jun 17. And sometimes that story is wrong when the data is wrong. All the copy here in red was written the day after, the 18th. I’m trying to keep up a one-a-day blog schedule and fell into “well, isn’t that convenient” trap. Hopefully today we’ll put in a few fixes. Nuf sed. See next blog. Apologies. 

Addendum. Now I see that M. Mortimer has some comments as well. Looks like we have some house cleaning here. Let’s do it and see what comes of it.