The manus of Poposaurus revised — again

 

Revised April 23, 2014 based on further study. 

 Figure 1. Poposaurus manus as originally restored and with digit 1 switched to 5.


Figure 1. Poposaurus manus as originally restored and with digit 1 switched to 5. Note the resemblance to dinosaur and basal croc hands in this basal archosaur.

The manus of basal archosaurs is very rare.
What few clues we have indicate that metatarsals 1-3 aligned distally and digit 5 is a vestige. Revising the manus of Poposaurus to that pattern is demonstrated here (Fig. 1).

References
Gauthier JA, Nesbitt SJ, Schachner ER, Bever GS and Joyce WG 2011. The bipedal stem crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52:107-126.
Mehl MG 1915. Poposaurus gracilis, a new reptile from the Triassic of Wyoming. Journal of Geology 23:516–522.

wiki/Poposaurus

Let’s add PILs to the Poposaurus foot

and see what happens…

The question posed by Farlow et al (2014) is were the toes of Poposaurus (Figs. 1-3) splayed or nearly parallel? Farlow (Fig. 1) showed both possibilities in a digitigrade fashion. Here (Fig. 1) I added PILs (parallel interphalangeal lines, (Peters 2000, 2011) to see which possibility produced the simplest set of PILs.

Figure 1. From Farlow et al. 2014) showing the Poposaurus foot in plantigrade and digitigrade poses. In the ghosted addition I added a digitigrade configuration, but so high as in the Farlow examples. In any case, digit 1 impresses, but shares no PILs, so it acts as a vestige, no longer part of the phalangeal sets.

Figure 1. From Farlow et al. 2014) showing the Poposaurus foot in plantigrade and digitigrade poses. In the ghosted addition I added a digitigrade configuration, but so high as in the Farlow examples. In any case, digit 1 impresses, but shares no PILs, so it acts as a vestige, no longer part of the phalangeal sets. The metatarsals in ventral view are also ghosted to better show the bones that would have contributed to making a footprint. Note: the medial and lateral PILs are complete, but the transverse set is not, but becomes more so with the spreading toes.

Farlow et al. created their splayed foot by spreading the digits as far as they could go on the distal metatarsals. Another way to do this would be to rotate the medial and lateral metatarsals, creating a metatarsal arc, but this was not attempted by Farlow et al. Even a slight axial rotation of these metatarsals would have splayed the digits just a little bit more.

And that’s really all you need.

Here (Fig. 2) we look at an even more splayed foot and now we have complete PILs even in the transverse set, which is the one Poposaurus would have used for locomotion, as in birds and theropods.

Figure 2. When you splay the digits of Poposaurus just a little bit more, the transverse PILs become complete and uninterrupted. This, then, is the most likely configuration of the pes.

Figure 2. When you splay the digits of Poposaurus just a little bit more, the transverse PILs become complete and uninterrupted. This, then, is the most likely configuration of the pes. PILs work!

Now all the PIL sets (except, again, digit 1, which just had to get out of the way) are able to operate at maximum efficiency. They are complete and uninterrupted, as in all other tetrapods.

BTW, Poposaurus is basal to Silesaurus in the large reptile tree, and Silesaurus does not preserve digit 1.

Figure 1. Poposauridae revised for 2014. Here they are derived from Turfanosuchus at the base of the Archosauria, just before crocs split from dinos.

Figure 1. Poposauridae revised for 2014. Here they are derived from Turfanosuchus at the base of the Archosauria, just before crocs split from dinos.

Three days ago we took our first look at the Farlow et al. 2014 paper.

References
Farlow JO, Schachner ER, Sarrazin JC, Klein H and Currie PJ 2014. Pedal Proportions of Poposaurus gracilis: Convergence and Divergence in the Feet of Archosaurs. The Anatomical Record. DOI 10.1002/ar.22863
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41.
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605

New Farlow et al. (2014) Poposaurus foot paper

Farlow et al. (2014) has a new paper on the foot of the poposaurid, Poposaurus.

Figure 1. Revised skull reconstruction for the PEFO specimen. Here the anterior is considered a premaxilla. Those teeth are shaped like triangles, but they are very deeply rooted and exposed very little, which casts doubts on its hypercarnivory.

Figure 1. Poposaurus in lateral view. This dinosaur like reptile really is a dinosaur with a calcaneal heel.

From their abstract:
“The crocodile-line basal suchian Poposaurus gracilis had body proportions suggesting that it was an erect, bipedal form like many dinosaurs, prompting questions of whether its pedal proportions, and the shape of its footprint, would likewise “mimic” those of bipedal dinosaurs.

Bivariate and multivariate analyses of phalangeal and digital dimensions showed numerous instances of convergence in pedal morphology among disparate archosaurian clades.

Overall, the foot of Poposaurus is indeed more like that of bipedal dinosaurs than other archosaur groups, but is not exactly like the foot of any particular bipedal dinosaur clade.” 

Included is a comparison with other archosaur taxa, (Fig. 1). Note Terrestrisuchus has no calcaneal heel. It develops in the derived Protosuchus and also poposaurid dinosaurs, according to the large reptile tree.

Figure 1. Archosaur feet divided into traditional croc-line and bird-line clades

Figure 1. Archosaur feet divided into traditional croc-line and bird-line clades

These feet can be reordered according to the large reptile tree (Fig. 2). Though many taxa are missing that would fill in morphological gaps, the general trends are more clear here.

Figure 2. Same feet, reordered according to the large reptile tree. Only Terrestrisuchus and Protosuchus are croc-like archosaurs here. Poposaurs are basal dinosaurs.

Figure 2. Same feet, reordered according to the large reptile tree. Only Terrestrisuchus and Protosuchus are croc-like archosaurs here. Poposaurs are basal dinosaurs. Silesaurus converged with theropod dinos, as did Brachylophosaurus. Note the lack of a calcaneal heel on Terrestrisuchus, a basal croc and the development of one on Protosuchus. In similar fashion poposaurid dinosaurs developed a calcaneal heel. 

Farlow et al. noted several instances of convergence (homoplasy). Indeed homoplasy is present here, even in this small sample.

On a separate note, 
Farlow et al. was kind enough to publish a radiograph of an Alligator. I added PILs and they are quite precise in this living reptile.

Figure 3. Radiograph of Alligator foot with PILs (parallel interphalangeal lines) added. Hone and Bennett tried to argue against the presence of PILs but did not have the nerve to show a foot with more than three toes.

Figure 3. Radiograph of Alligator foot with PILs (parallel interphalangeal lines) added. Seems rather clear that such lines representing phalanges working in sets is indeed present here.

Poposaur footprints have not bee found yet. Farlow et al. (2014) reported, “With a digit III length of about 16 cm, Poposaurus gracilis may have been comparable to a small to midrange theropod in overall body size (somewhere between the makers of Anchisauripus sillimani and A. minusculus, in the terminology of Lull [1953]). The dinosaur-like pedal proportions of Poposaurus, and the similarity of its reconstructed footprint to those of some dinosauromorphs, suggest that some grallatorid forms could well have been made by Poposaurus and its close relatives. However, mistaking Poposaurus tracks for dinosaur (particularly theropod) tracks would be less likely to occur if digit I of Poposaurus routinely touched the ground. Furthermore, trackways made by Poposaurus would probably have a shorter stride/footprint length ratio than grallatorid trackways.”

Personal thought
Seems to me that on Poposaurus pedal digit one is going to impress creating a four-toed ichnite.

References
Farlow JO, Schachner ER, Sarrazin JC, Klein H and Currie PJ 2014. Pedal Proportions of Poposaurus gracilis: Convergence and Divergence in the Feet of Archosaurs. The Anatomical Record. DOI 10.1002/ar.22863

Sillosuchus – an odd, big shuvosaurid poposaurid

Updated April 22, 2014 to reflect the new basal archosaur position of poposaurids.

Figure 1. Sillosuchus (the large one) compared to Shuvosaurus (in silhouette).

Figure 1. Sillosuchus (the large one) compared to Shuvosaurus (in silhouette) to scale. Note the difference in ilium shape and orientation.

Sillosuchus longicervix Alcober and Parrish (1997) is known from a few gracile cervicals, a few gracile posterior dorsals through anterior caudals, the majority of a gracile pelvis and a sinusoidal femur. Sillosuchus comes from the Late Triassic of Argentina. Sillosuchus is widely considered a poposaur close to Shuvosaurus, Effigia and Poposaurus, among others. That seems to be a reasonable nesting. Unfortunately there are too few traits to add it to the large reptile tree.

An odd pelvis
The pubis is longer than the femur, but there’s no pubic ‘boot’ in Sillosuchus. The ischia are co-ossified to create a single bone. The ilium (Fig. 2) includes a portion that extends laterally, overhanging the femur, with reinforcing flanges for strength without added weight. The anterior sacral ribs are short. The posterior ones are long. The results in angled ilia, which may not be a problem as the articular surface appears to be between the top of the femur and the overhanging ilium, rather than the medial femur and the lateral face of the pelvis.

There are five sacrals and the ribs of the middle three arise from the joint between two vertebrae, rather from the central portion of each one. That seems odd, but Effigia, with four sacrals, has a similar situation in which sacrals 1 and 2 share a rib and sacrals 2 and 3 share a rib.

Figure 2. Because it is rather odd, it is difficult to wrap your mind around the way the pelvis looks and operates. The femur has no offset head and appears to articulate with the overhanging portion of the ilium.

Figure 2. Because it is rather odd, it is difficult to wrap your mind around the way the pelvis looks and operates. The femur has no offset head and appears to articulate with the overhanging portion of the ilium. The vertebrae have large articulating surfaces and more gracile centrum shafts, saving weight while building size.

A restoration in Japan
A restoration, adding a skull and other skeletal elements is present at a Japanese museum display (sorry, no more data on this yet), and online here at Wikipedia. This appears to be a much too robust restoration, based on what is known of the specimen (Figs. 1, 2) and the pelvis appears to share little with the original imagery of Alcober and Parrish (1997).

Figure 3. Two views of the same Sillosuchus restoration from a museum in Japan.

Figure 3. Two views of the same Sillosuchus restoration from a museum in Japan. Much of this appears to too robust, but the skull is toothless, which is probably correct.

Earlier
we discussed the controversy of poposaurid origins. Most other paleontologists consider them to be dinosaur-like rauisuchians. The large reptile tree recovers them as basal archosaurs.

My guess is
the rest of Sillosuchus, if it is ever discovered, will likely look like Shuvosaurus, very gracile, but with some sort of unique oddity in a toothless skull.

References
Alcober O and Parrish JM 1997. A new poposaurid from the upper Triassic of Argentina. Journal of Vertebrate Paleontology 17:548–556.

A slightly new take on the new Poposaurus skull

More thoughts on the new Poposaurus skull (Fig. 1) that we looked at earlier (since modified). Slightly raising the broken maxilla ascending process puts a new spin on the possibilities for the skull shape and brings it into line with multi-documented sister taxa like Lotosaurus and Shuvosaurus.

Figure 4. The restored skull of the PEFO specimen referred to Poposaurus based on the Nesbitt identification of the anterior as a maxilla. The blue articular is not part of the PEFO specimen, but is described as a Poposaurus articular by Parker and Nesbitt (2013) scaled to fit. Their scale bars indicate it was 4x larger, which may be a typo. As is, the elements are part of a longer, more robust skull than any other poposaurid. See the revised skull reconstruction, figure 1B.

Figure 4. The restored skull of the PEFO specimen referred to Poposaurus based on the Nesbitt identification of the anterior as a maxilla. The blue articular is not part of the PEFO specimen, but is described as a Poposaurus articular by Parker and Nesbitt (2013) scaled to fit. Their scale bars indicate it was 4x larger, which may be a typo. As is, the elements are part of a longer, more robust skull than any other poposaurid. See the revised skull reconstruction, figure 1B.

The teeth
The teeth are indeed sharp. Parker and Nesbitt 2013 described them as belonging to a hyper-carnivore. But in the large reptile tree Poposaurus nests with beaked herbivores. In the fossil the teeth really don’t descend very much beyond the jawline. They are deeply rooted and triangular, not long and recurved.

Other indicators
Earlier we noted the less than trenchant claws on the forelimb, more appropriate for an herbivore than a carnivore. The cervicals are quite robust, able to handle a taller skull. The new reconstruction, with a possible raised maxilla ascending process, opens the possibility of a taller skull with a larger orbit, also more like an herbivore and more like its sisters.

Figure 1. Revised skull reconstruction for the PEFO specimen. Here the anterior is considered a premaxilla. Those teeth are shaped like triangles, but they are very deeply rooted and exposed very little, which casts doubts on its hypercarnivory.

Figure 1. Revised skull reconstruction for the PEFO specimen. Here the anterior is considered a premaxilla. Those teeth are shaped like triangles, but they are very deeply rooted and exposed very little, which casts doubts on its hypercarnivory.

At this point it’s just best to explore possibilities, some indicated by phylogenetic bracketing. That skull remains quite incomplete.

References
Gauthier JA, Nesbitt SJ, Schachner ER, Bever GS and Joyce WG 2011. The bipedal stem crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52:107-126.
Mehl MG 1915. Poposaurus gracilis, a new reptile from the Triassic of Wyoming. Journal of Geology 23:516–522.
Parker WG and Nesbitt 2013. Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolution. Geological Society, London, Special Publications 379: 22 pp.

wiki/Poposaurus

Poposaurs to scale and the chronological Lotosaurus problem

Updated April 22, 2014 to reflect the new basal archosaur position of poposaurids.

Adding Sacisaurus (Fig. 1) as a basal member of the poposaur list adds a certain perspective.  It calls into question the early Triassic appearance of Lotosaurus (Fig.1) since all other poposaurs are Late Triassic. Either the geological setting for Lotosaurus was poorly calibrated, or these poposaurs all had a much earlier origin, in the Permian, which appears unlikely. The other possibility is that Lotosaurus is not a poposaur after all, but an offshoot of another Permian root. This might be interesting…

Poposaurs now nest as basal archosaurs. So the chronology problem goes away.

Figure 1. Poposaurs to scale and in phylogenetic order (top to bottom). Sacisaurus is at the base. Silesaurus and Lotosaurus are derived. Poposaurus is one of the largest, along with Lotosaurus.

Figure 1. Poposaurs to scale and in phylogenetic order (top to bottom). Sacisaurus is at the base. Silesaurus and Lotosaurus are derived. Poposaurus is one of the largest, along with Lotosaurus. Pisanosaurus, basal ornithischian, does not belong in this clade.

Sacisaurus and Silesaurus could be Dinosauria (Langer and Ferigolo 2013)

Langer and Ferigolo (2013)
write in their abstract: “All previous numerical phylogenies supported a non-dinosaur dinosauromorph affinity for Silesauridae, but the reanalysis of one of those studies suggests that a position within Dinosauria is not unlikely, with silesaurids forming the basal branch of the ornithischian lineage.”

Nice!
We looked at Sacisaurus and Silesaurus earlier. A nesting closer to the Dinosauria starts to jive with the large reptile tree which nests Sacisaurus and Silesaurus within the poposauridae within the Dinosauria. Unfortunately the Langer and Ferigolo (2013) data, sparse as it is, is weakly supported, according to the authors.

They conclude:“Regardless of its poor support, the outcome of this phylogenetic study is so novel that minor comments are mandatory. Most studies understand the rise of dinosaurs as a relatively rapid burst of diversity and anatomical change during the Late Triassic, coupled with the initial cladogenic events of the saurischian and ornithischian lineages (Brusatte et al. 2010; Langer et al. 2010; Irmis 2011). In contrast, the possible nesting of Middle Triassic silesaurids into the Ornithischia branch suggests a long fuse model for dinosaur radiation, with its basal (Ornithischia–Saurischia) split occurring during the late, perhaps even early, Middle Triassic, but their rise in diversity/disparity postponed until the later in that Period. Clearly, additional data are required to fully test this hypothesis.”

Unfortunately,
Daemonosaurus was not included in this study. Neither were Pampadromaeus, Panphagia or Lotosaurus. The addition of these taxa may shed more light on the situation, as they did in the large reptile tree.

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
Langer MC and Ferigolo J 2013. The Late Triassic dinosauromorph Sacisaurus agudoensis(Caturrita Formation; Rio Grande do Sul, Brazil): anatomy and affinities. From: Nesbitt, S. J., Desojo, J. B. & Irmis, R. B. (eds) 2013. Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin.  Geological Society, London, Special Publications, 379, first published on April 23, 2013, doi:10.1144/SP379.16

Restoring Sacisaurus

Figure 1. The skull of Sacisaurus reconstructed and restored. Small black image from Ferigolo and Langer 2006.

Figure 1. The skull of Sacisaurus reconstructed and restored. Small black image from Ferigolo and Langer 2006. The tooth number is from the text, 10 in the maxilla, 15 in the dentary. 

Sacisaurus agudoensis (Ferigolo and Langer et al. 2006, Fig. 1) Carnian, Late Triassic period, ~225 mya, 1.5 m in length, is one of the oldest dinosaurs yet found. Only a few bones are known (Fig. 2). Here Sacisaurus nests as the basal poposaurid, descended from a sister to the basal phytodinosaur, Pampadromaeus. It has been an oversight, here repaired, that Sacisaurus has not been included in the large reptile tree until now.

The skull includes paired predentary bones at the tip (more on this below). The antorbital fenestra remains large. The teeth have an expanded crown. The large postorbital indicates a larger orbit with a raised cranium relative to the rostrum, which is largely unknown.

Sacisaurus from Ferrigolo and Langer (2006).

Figure 2. Sacisaurus from Ferigolo and Langer (2006).

Only one cervical has been found and it had a parallelogram-shaped centrum for an elevated skull. The caudals were elongate as in other dinosaurs.

The euarchosauriformes, now inlcuding Sacisaurus as a basal poposaurid.

Figure 3. The euarchosauriformes, now inlcuding Sacisaurus as a basal poposaurid. Click to see complete reptile tree.

The scapula was elongate. The pelvis was more like that of Pampadromaeus with an angled pubis. The hind limbs were robust.

Altogether, nothing much new to say here other than this taxon shares many traits with Panphagia and Pampadromaeus. The mandible is more robust in Sacisaurus. The antorbital fenestra is smaller. The addition of Sacisaurus to the large reptile tree (Fig. 3) did not disrupt the topology.

About that predentary
Ferigolo and Langer (2006) report, “In two specimens (Figure 3C–F), it is possible to recognize that the depressed mandibular rostral portion is formed by a subtriangular separate ossification, the caudal margin of which extends obliquely below and above the mental foramen. This demarcation is not visible in other mandibles in which the bone is apparently fused to the dentary.” So, this fusion can be present or not in later related taxa.

They also report, “Also unlike ornithischians, the teeth of Sacisaurus are not markedly inset from the lateral margin of the bearing bones, and the upper series does not reach the caudal end of the maxilla.” 

Denticles are present on the teeth, but at this scale, they are so small as to be serrations, like those of their ancestors among the theropod dinosaurs.

Ferigolo and Langer (2006) report, “If the pelvic bones assembled from the type-locality (Figure 2) belong to Sacisaurus, the new taxon represents one of the three putatively propubic ornithischians, the others being Pisanosaurus (Sereno 1991) and Silesaurus (Dzik 2003).”

Ferigolo and Langer (2006) comment on, “other referred material posses theropod features such as a ventrally excavated ectopterygoid with a strongly curved jugal process, long prezygapophises on the distal caudal vertebrae, and a well-developed fibular flange in the tibia.” And such holdover or plesiomorphic traits are to be expected at the base of new clades.

Interestingly, they report, “The ilia of Sacisaurus and Silesaurus are also atypical for dinosaurs, but resemble those of poposaurid rauisuchians.” And here’s where it gets interesting.

So what is the predentary?
According to Ferigolo and Langer (2006), “the predentary does not seem to represent a neomorphic structure of these dinosaurs. Instead, its homology to the mentomeckelian bone, and possibly also to parts of the rostral portion of most vertebrate mandibles, is proposed here. In this case, it does not correspond to a dermal bone as most of the lower jaw, but to an ossification preceded by cartilage.”

Massospondylus dentaries with a place for a missing predentary. Did this jaw have and lose the predentary? Phylogenetic bracketing says yes.

Figure 4. Massospondylus dentaries (medial view left, lateral view right) with a place for a missing predentary. Did this jaw have and lose the predentary? Phylogenetic bracketing says yes. From Barrett 2009.

And did prosauropods have, then lose the predentary?
Phylogenetic bracketing says yes. And Massospondylus kaalae (Fig. 4) seems to be missing something at the tip of its jaws. Worth looking into.

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
Barrett PM 2009.
 A new basal sauropodomorph dinosaur from the upper Elliot formation (Lower Jurassic) of South Africa. Journal of Vertebrate Paleontology 29(4):1032-1045.
Ferigolo and Langer 2006. A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone. Historical Biology, 2006; 1–11, iFirst article

wiki/Sacisaurus

Poposaur mandibles

There’s still the question of Effigia’s mandible hanging out there.
The question is: “Is that a predentary or a dentary at the tip?” Fig. 1). Nesbitt (2007) says dentary. I say predentaries. Let’s look at the evidence.

To answer that,
I took a comparative survey of poposaur mandibles (Fig. 1), looking for evolutionary patterns and thereby strive to provide an update to the predentary/dentary question. Surprisingly, in the case of Effigia, when you add in the splenials, which neither Nesbitt nor I did before, the mandibular fenestra becomes substantially reduced. That may be similar to what one sees in Lotosaurus, in which the elements are not jumbled. And that provides more substance to the “predentary” argument. Other than Lotosaurus, the closest sister is Shuvosaurus, which is known from an incomplete mandible (Fig.1) showing similar patterns over the remaining portions. Shuvosaurus has something similar to what I saw in Daemonosaurus, that others consider something else. In any case, at some point, something interesting developed in front of the dentaries in certain phytodinosaurs.

The other question is,
when something similar to a predentary appears in front of the dentary, as in Sacisaurus (Figure 1), should it be considered a “beak” rather than a premaxilla? This bone may be paired, as it is in Sacisaurus, rather than a single median bone, as in the predentary of Heterodontosaurus (Fig. 1).

Figure 1. Poposaur (and kin) mandibles. Here are Daemonosaurus, Poposaurus, Pisanosaurus, Heterodontosaurus, Sacisaurus, Lotosaurus, Effigia and Shuvosaurus. The mandibles of Lotosaurus and Effigia appear to share a common heritage of design.  In Effigia the splenial reduces the mandibular fenestra helping to clarify the identify of the dentary and premaxilla (or beak).

Figure 1. Poposaur (and kin) mandibles. Here are Daemonosaurus, Poposaurus, Pisanosaurus, Heterodontosaurus, Sacisaurus, Lotosaurus, Effigia and Shuvosaurus. The mandibles of Lotosaurus, Shuvosaurus and Effigia appear to share a common heritage of design. In Effigia the splenial reduces the mandibular fenestra helping to clarify the identify of the dentary and premaxilla (or beak). The extension of the angular to the predentary is unique to this clade.

If all these other mandibles had a premaxilla or beak (or the possibility of one), is there any reason to suspect that Effigia did not?

The original reconstructions of the Effigia mandible
introduced us to the largest mandibular fenestra I have ever seen relative to the size of the jaw. The new reconstruction reduces the fenestra length and, no doubt, produces a stronger jaw with the splenial (lavendar to iris blue bone) laminated to the medial side and edges.

Typically the mandibular fenestra splits the surangular from the angular,
as it does in Heterodontosaurus. However, in Lotosaurus the mandibular fenestra develops largely below the dentary with very little surangular and angular exposure. In Shuvosaurus the same pattern could play out, but unfortunately the key parts are missing (perhaps due to a very large mandibular fenestra?). This is a different pattern than in ornithischians, saurischians and theropods. And this pattern is also different from rauisuchians. Among euarchosauriforms, only in aetosaurs does the very large mandibular fenestra develop largely below the dentary. In others, the fenestra develops midway or beneath the surangular and it doesn’t get to the size seen in Effigia and Lotosaurus.

One final point
The suture between the two premaxillae in Effigia is convoluted like a puzzle piece. In this way they are locking themselves together, convergent with the central or fused premaxilla of ornithischians, but homologous with the premaxilla in Lotosaurus and Shuvosaurus.

If I’m wrong, show me some data. At this  point, at least it’s worth talking about.

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
Ferigolo J and Langer MC 2006. “A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone”Historical Biology 19 (1): 1–11. online pdf.
Nesbitt SJ and Norell MA 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society B 273:1045–1048. online
Nesbitt S 2007. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History, 302: 84 pp. online pdf

AMNH Effigia webpage

wiki/Effigia

Finally, a Poposaurus skull! Or is it?

With a new skull reconstruction, this post was modified Nov 2, 2013.
Previously Poposaurus (Fig. 1) was only known from post-cranial remains. A complete Poposaurus, lacking a skull and a few cervicals, was described by Gauthier 2012 (Fig. 1).

Recently Parker and Nesbitt 2013, reported on a partial maxilla, dentary and prearticular, along with associated pubis and ischium (PEFO 34865). They matched their new find to prior Poposaurus specimens (Fig. 2), such as YPM 057100. In consideration of the PEFO tooth they wrote, “We fully describe the cranial elements and demonstrate that P. gracilis was a toothed hypercarnivore.” 

Poposaurus (in gray) together with the new skull, the new pelvis and the skulls of sister taxa.

Figure 1A. Poposaurus (in gray) together with the new skull, the new pelvis and the skulls of sister taxa according to the large reptile tree, which, in this case, agrees with Parker and Nesbitt (2013). Shuvosaurus and Effiga, the toothless ones, are above. Pampadromaeusa and Silesaurus are below. Close to the neck are the new elements with the rest of the skull restored. The new pelvis floats above the nearly complete Gauthier et al. (2011) specimen, YPM VP 057100. For details on the new skull see figure 4.

There’s more than one way to rebuild that skull.

Figure 1. Revised skull reconstruction for the PEFO specimen. Here the anterior is considered a premaxilla. Those teeth are shaped like triangles, but they are very deeply rooted and  exposed very little, which casts doubts on its hypercarnivory.

Figure 1B. Added Nov. 2, 2013. Revised skull reconstruction for the PEFO specimen. Here the anterior is considered a premaxilla. Those teeth are indeed shaped like triangles, but they are very deeply rooted and exposed very little, which casts doubts on hypercarnivory.

The pelvis – is this a match?
The new pelvis, PEFO 34865, (Fig. 3) is indeed most similar to that of the YPM Poposaurus pelvis (among the pelves I’ve been able to see), but, they’re not the same. Several traits unite the two pelves including the kink in the anterior ilium, the widely diverging  ventral elements and the semi-open acetabulum (a dinosaur trait). The curved pubis and pubic medial flange with obturator foramen separates them. Rauisuchid taxa with a curved booted pubis include Arizonasaurus, which Parker and Nesbitt (2013) consider to be poposauroid. However, the large reptile tree separates Arizonasaurus from poposaurids.

Figure 1. Poposaurus pelves. In black YPM VP 057100. Above in halftone, the ilium and pubis of PEFO 34865. They may be closely related, but these are not the same species or genus.

Figure 2. Poposaurus pelvis below, Arizonasaurus pelvis above and the PEFO specimen in the middle.  They may be closely related, but these are not the same species or genus. The kink in the anterior ilium, the widespread pubis and ischium and the open acetabulum unites the PEFO and YPM pelves. The curve of the pubis and the higher posterior ilium unite the PEFO and Arizonasaurus pelves. The PEFO specimen appears to be unique and poposaurian, but not Poposaurus. To YPM pelves are shown. I don’t know which is correct the in-situ specimen with a ventrally open acetabulum and what appears to be an overlapping pubis, or the repaired one.

Hypercarnivory?
With regard to the Parker and Nesbitt statement about hypercarnivory, let’s take another look at the post-cranial skeleton of Poposaurus (Fig. 1, YPM specimen). Are those tiny harmless hands those of a hypercarnivore? By comparison, the hands of Postosuchus (Fig, 3) look equally harmless, except for that can opener claw on digit 1. The elongated metatarsals of Poposaurus are also traits shared with dinosaurs, especially phytodinosaurs that depend on fleeing to avoid predation. Postosuchus (Fig. 3), by contrast, has small feet and short metatarsals relative to the tibia. Take poposaurs out of the Rauisuchia (because they don’t belong) and there are no rauisuchians with long metatarsals.

Postosuchus.

Figure 3. Postosuchus. Hands and feet are rare in rauisuchids, but here manual digit 1 has a large trenchant claw and the other unguals are tiny. The hands of Poposaurus (Fig. 1) do not have any large claws.

Parker and Nesbitt (2012) report, “The remains of poposauroids have long been confused with those of early dinosaurs because of the striking convergences (Nesbitt & Norell 2006; Nesbitt 2007; Nesbitt et al. 2007; Gauthier et al. 2011). The extraordinary disparity of poposauroid body plans, locomotor styles and dentition is unique within Pseudosuchia; however, the evolutionary sequence of acquisition of these features as well as the loss of others within the clade is incompletely understood.”

 

Figure 4. The restored skull of the PEFO specimen referred to Poposaurus based on the Nesbitt identification of the anterior as a maxilla. The blue articular is not part of the PEFO specimen, but is described as a Poposaurus articular by Parker and Nesbitt (2013) scaled to fit. Their scale bars indicate it was 4x larger, which may be a typo. As is, the elements are part of a longer, more robust skull than any other poposaurid. See the revised skull reconstruction, figure 1B.

Figure 4. The restored skull of the PEFO specimen referred to Poposaurus based on the Nesbitt identification of the anterior as a maxilla. The blue articular is not part of the PEFO specimen, but is described as a Poposaurus articular by Parker and Nesbitt (2013) scaled to fit. Their scale bars indicate it was 4x larger, which may be a typo. As is, the elements are part of a longer, more robust skull than any other poposaurid. See the revised skull reconstruction, figure 1B.

About that ventrally concave (bent) maxilla…
Fellow poposaur, Silesaurus has a bent maxilla and a straight premaxilla ventral rim. Lotosaurus has a straight maxilla and a bent premaxilla. All rauisuchians have a convex ventral maxilla with longer teeth. 

About that tooth…
Sure that tooth has that carnivore look, but more than half is buried in the maxilla. That likely preserved the tooth with the bone. Ancestral taxa, such as Pampadromaeus, have similar sharp, even recurved teeth. More derived poposaurids do not. The teeth of the PEFO specimen trend toward less recurved and more symmetrical.  Daemonosaurus, nested with basal ornithischians, but has hyper-carnivorous teeth, so there is precedent and analogy.

About that antorbital fenestra…
Other poposaurids have a tall open antorbital fenestra. the PEFO specimen does not. It more closely resembles that of rauisuchians. However, a small, but not elongated, antorbital fenestra is found in Daemonosaurus and other ornithischians.

About that robust dentary…
Other poposaurids don’t have a robust anterior dentary, but Pisanosaurus does. Rauisuchids have a robust anterior dentary.

About that robust skull…
The robust skull of the PEFO specimen stands out from the more gracile skulls found in other poposaurids (Fig. 1), but this is not unheard of in other phytodinosaurs. The armored ornithischians, like Scelidosaurus, also reduce the antorbital fenestra. Perhaps the post-crainia of the PEFO specimen is likewise armored and perhaps secondarily quadrupedal.

The poposaur list – different here
Among their poposaurids, Parker and Nesbitt (2013) include the long-necked Qianosuchus. In the large reptile tree that taxon nests with Ticinosuchus and aetosaurs. Parker and Nesbitt include the sailbacks Arizonasaurus and Xilosuchus among the herbivorous poposaurids. The large reptile tree nests those with the similarly carnivorous rauisuchians.

Poposaurus should have a phytodinosaur (herbivore) skull
All known poposaurs recovered in the large reptile tree are either toothless or, like Pisanosaurus and Silesaurus, have a plant-eater morphology and teeth. However, the sisters of the ancestors of poposaurs (like Pampadromaeus) had teeth typical of carnivores. Among the ornithischia, Heterodontosaurus retained fangs. We should also expect the skull of Poposaurus to be relatively short, like that in Daemonosaurus, Heterodontosaurus and Lotosaurus, with a large orbit and either a large antorbital fenestra.

At the base of the Phytodinosauria the teeth do not yet reflect an herbivorous diet, whether in the sauropodomorpha, ornithischia or poposauridae. Dinosaurs were diverging rapidly. Less perfect designs became extinct more quickly than the better morphologies. The PEFO specimen is among these. It’s type did not last long.

So is PEFO 34865 a Poposaurus?
The pelvis of the PEFO specimen is very close to Poposaurus, but it is also distinct. What remains of the skull does not closely resemble any other taxon in the archosaur clade of the large reptile tree. The skull is robust, which is expected in consideration of the robust cervicals. The anteriormost dentary is missing. The question is, were paired predentaries present? Or just a continued uplift of the mandible tip?

Incomplete remains are always a challenge. Parker and Nesbitt (2013) write, “Poposauroids display a complicated pattern of unusual character suites unlike any other group of pseudosuchians or any other archosaur group in the Triassic.” Let’s keep this discussion going.

Final passing thoughts
Parker and Nesbitt (2013) report, “Interestingly, the morphology of the neural spines that make up the sail in Lotosaurus adentus differs significantly from those of Arizonasaurus and Ctenosauriscus, supporting the idea that it was independently derived (Butler et al. 2011; Nesbitt 2011).”  That was promoted long ago by the results of the large reptile tree. I’m glad to see others are finally starting to catch up to this. Now let’s get poposaurids back into the Dinosauria, where they belong1

References
Gauthier JA, Nesbitt SJ, Schachner ER, Bever GS and Joyce WG 2011. The bipedal stem crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52:107-126.
Mehl MG 1915. Poposaurus gracilis, a new reptile from the Triassic of Wyoming. Journal of Geology 23:516–522.
Parker WG and Nesbitt 2013. Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolution. Geological Society, London, Special Publications 379: 22 pp.

wiki/Poposaurus