Dinosaur outgroup taxon: Turfanosuchus

It was the Middle Triassic when
a sister to Turfanosuchus evolved into the basalmost dinosaur, Herrerasaurus (Figs. 1, 2)

Figure 1. Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.

Figure 1. Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur. Lower image to scale.

Figure 2. Skull of Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.

Figure 2. Skull of Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.

Quick one today.
I’ll let the pictures tell the story…

References
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto
Reig OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.
Young CC 1973. [On a new pseudosuchian from Turfan, Sinking (Xinjiang).] Memoirs of the Institute of Vertebrate Paleontology and Paleoanthropology of the Academia Sinica, Series B 10:15-37.

wiki/Herrerasaurus
wiki/Turfanosuchus

Effigia palate and occiput

Updated March 13, 2015 with a new palate figure based on the photo, not the original drawing in Effigia and Shuvosaurus was modified in turn.

We looked at the poposaur Effigia earlier here and here. Having never attempted a reconstruction of the palate I do so here.

effigia588

Figure 1. Effigia. is an odd derived poposaur with tiny hands and no teeth. In competition with dinosaurs, poplars did not fare as well. The dentary and predentary have been modified here from prior attempts to more closely match the mandible of Shuvosaurus (Fig. 3).

Effigia okeeffeae (Nesbitt and Norell 2006, Nesbitt 2007) Carnian, Late Triassic, ~210 mya, ~ 2 m in length, was originally considered an early theropod dinosaur by Colbert, who collected the specimen in the late 1940s but never removed it from its jacket. A recent reassessment by Nesbitt and Norell (2006) and Nesbitt (2007) nested Effigia among the poposauridis. It is an odd bipedal poposaur and perhaps the most derived member of a clade composed almost entirely of odd derived members. The reconstruction of the skull has been controversial. Perhaps only a direct tracing and shifting of the elements can solve this puzzle. All the pieces in the disarticulated fossil will come together precisely if they are correctly reassembled.

The palate 
It is possible that the palatine (Fig. 2). was misidentified originally as the right ectopterygoid. If so, then the palate resembles that of known sister taxa, like Shuvosaurus (Fig. 3)..

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). Click to enlarge.

Due to the long premaxilla
and the short maxilla the Effigia palate shifts most of the palatal elements into a smaller space. Even so all maintain their original and typical connections to the other skull elements.

Figure 3. Shuvosaurus, a sister to Effigia, has a similar palate in this reconstruction, but it was not reconstructed like this originally.

Figure 3. Shuvosaurus, a sister to Effigia, has a similar palate in this reconstruction, but it was not reconstructed like this originally.

You really can’t talk about
the palate of Effigia without comparing it to its sister, Shuvosaurus (Fig. 3). Here the main triangular part of the pterygoid must be imagined, but the quadrate processes are present and quite robust. The palatines frame the internal nares posterior to the palatal processes of the maxilla and premaxilla.

effigia-occiput588

Figure 4, the occiput of Effigia colorized here to segregate elements. That’s the central supraoccipital in pink flanked by two opisthotics in lavender, all displace dorsally. Originally they were framed by the squamosals in gold. Quadrates in red and basisphenoid in purple.

Effigia occiput
The above image (Fig. 4, Nesbitt 2007) is a CT scan of the Effigia occiput colorized to aid identification of the elements. The occiput is so inclined it is almost continuous with the palate. Originally the supraoccipital + opisthotics were identified as the two parietals with no median element recognized. Neither the supraocipital or the opisthotic were identified otherwise.

Effigia References
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

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

wiki/Shuvosaurus

 

The manus of Effigia revisited

The manus of Effigia is tiny.

Figure 1. Effigia. Note the tiny hands.

Figure 1. Effigia. Note the tiny hands.

Really tiny.
Even so, it is one of the few manus examples from a wide range of post-Proterosuchus and pre-Herrerasaurus taxa. The Effigia manus (Figs. 1, 2) includes only 3 metacarpals and 2 phalanges, plus two carpals. Another metacarpal, one that looked like mc1 of Coelophysis and measuring 12×5.5 mm was described by Nesbitt (2007).

Not much to go on, but PILs and sister taxa (even distantly related) can help (Fig. 1) make the restoration — in this case two restorations: one if you switch the two longest metacarpals, one if you don’t. I think you should, to match another poposaurid manus, that of Poposaurus (Fig.1), which we looked at earlier here so metacarpals 1-3 align. In either case, tiny digit 5 appears to be missing, so the known digits are 2-4, not 3-5.

Figure 1. The manus of Effigia reconstructed. Metacarpal 1 was described with measurements. Here two possible hands can be restored, but only when one switches the longest metacarpals. That's the one I think will prevail.

Figure 1. The manus of Effigia reconstructed. Metacarpal 1 was described with measurements. Here two possible hands can be restored, but only when one switches the longest metacarpals. That’s the one I think will prevail.

From Nesbitt (2007):
“Portions of the right manus were recovered somewhat articulated with both the radius and the ulna. Metacarpals III, IV, and V were found articulated together and a potential metacarpal I and the carpals were found together nearby. Unfortunately, the identity of the two carpals cannot be determined with certainty. The proposed metacarpal I proximal portion is broken, shifted ventrally, and appressed against the shaft. Metacarpal I is estimated to have been 12 mm long and 5.5 mm wide. It has a dorsoventrally compressed shaft and the distal end is asymmetrical; only the medial side bears a rounded articular surface; the lateral side tapers to a sharp edge. The morphology is strongly reminiscent of metacarpal I of Coelophysis and other theropods. Metacarpal II was not preserved.

“A much more robust phalanx is also present and may belong with either metacarpal III or metacarpal I. This phalanx indicates that at least one of the digits was not as reduced as much as digit I.”

If you shift the two longest metacarpals and add appropriate phalanges that fit the few established PILs, then you get a very poposaur-ish manus. If you don’t, you get an odd sort of manus, very autapomorphic. The carpals were matched to those of aetosaurs, Lotosaurus and Herrerasaurus, the closest known sisters with known carpals. If tiny distal carpals were present, they were not recovered.

On a side note,
the manus of Lotosaurus is looking ‘funky’ when compared to other popoosaurs. If anyone has good data on that piece of anatomy, please send it.

References
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
wiki/Effigia

Is Nyasasaurus the oldest dinosaur?

Nesbitt et al. (2014) report, “Here, we describe Nyasasaurus parringtoni gen. et sp. nov., which is identified as either the earliest known member of, or the sister–taxon to, Dinosauria. Nyasasaurus possesses a unique combination of dinosaur character states and an elevated growth rate similar to that of definitive early dinosaurs.”

Nesbitt et al. (2012) did not provide a reconstruction. The specimen is only known from a few Middle Triassic (Anisian) vertebrae and a proximal humerus. Here (Fig. 1) is a reconstruction that seems to fit pretty well based on the holotype and referred specimens of Nyasasaurus placed onto the bauplan of a large and slightly larger (for the cervicals) specimen of Turfanosuchus, a basal archosaur at the base of the Poposauridae (now nesting basal to archosaurs in the large reptile tree). This was, what you might call, an early ‘false start’ mimicking the actual rise of the Dinosauria.

Figure 1. Click to enlarge. Nyasasaurus bones placed on an enlargement of Turfanosuchus, a middle Triassic basal archosaur, not a dinosaur. Dinos and crocs all started out as tiny bipeds.

Figure 1. Click to enlarge. Nyasasaurus bones placed on an enlargement of Turfanosuchus, a big. middle Triassic basal archosaur, not a dinosaur. Dinos and crocs all started out as tiny bipeds, also derived from Turfanosuchus, but smaller.

Nesbitt et al. ran their analysis and found Nyasasaurus to nest between 1) Lewisuchus and dinosaurs; 2) basal to Ornithischia; or 3) as the sister to Dilophosaurus, a Jurassic dinosaur. I didn’t run an analysis as my characters would not resolve relationships based on so few parts.

The large reptile tree finds two small specimens, Trialestes and an unnamed specimen incorrectly referred by Lecuona and Desojo (2011) to Gracilisuchus (PVL 1259) at the base of the Dinosauria. The latter is as old as Nyasasaurus, fulfilling chronological predictions.

Nesbitt et al. note a ventrally elongate deltopectoral crest on the humerus, but that assumes a short humerus. The crest is not so elongate if the humerus is a little longer.

Nesbitt et al. note three sacral vertebrae, but basal dinos don’t have three sacrals, only two. Turfanosuchus also has only two, but look at the size difference! Poposaurus, a sister taxon, has five sacrals. So Nyasasaurus is something else. Nesbitt et al. note hyposphene–hypantrum intervertebral articulations in the pre sacral vertebrae. Sorry, not much about that in Turfanosuchus data. In Turfanosuchus, as in Nyasaurus, the cervical vertebrae are laterally concave.

References
Lecuona A and Desojo, JB 2011. Hind limb osteology of Gracilisuchus stipanicicorum (Archosauria: Pseudosuchia). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 102 (2): 105–128.
Nesbitt SJ, Barrett PM, Werning S, Sidor CA and Charig AJ (posthumously) 2012. The oldest dinosaur? A Middle Triassic dinosauriform from Tanzania. Biology Letters 9: 20120949.

 

 

Poposaurs – Topology Shift

Yes, I was wrong. And it’s time to man-up.
Earlier, based on available data, the large reptile tree nested poposaurids with phytodinosaurs and attributed the appearance and growth of the calcaneal tuber in certain poposaurs to convergence with the Crocodylomorpha. So the earlier data recovered poposaurs as dinosaurs with an odd ankle, not crocs with a “massive convergence” with dinosaurs, which is still the widespread hypothesis (see Nesbitt 2011 and others).

That seemed to make sense — except some poposaurs, like Lotosaurus and Asilisaurus (Fig. 1), appeared a little too early in the Triassic. They seemed to be anachronistic, and that can be a red flag.

So going back to the phylogenetic analysis,
I reexamined certain specimens, discovered a few items not originally presented (I trusted original tracings instead of making my own from in situ photos) and I found several bad scores. The newly recovered tree finds poposaurids derived from Turfanosuchus, a taxon that earlier stood alone at the base of the Archosauria (basal to crocs and dinos). Now things seem to make more sense, phylogenetically, chronologically (Fig.1) and morphologically. See if you agree…

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. Among these, only Silesaurus and Asilisaurus lost the calcaneal tuber.

Figure 2. The Euarchosauriformes featuring a new nesting for the Poposauridae.

Figure 2. The Euarchosauriformes featuring a new nesting for the Poposauridae.

Now the odd thing is: 
Poposaurs appear to provide a sort of preview to what would eventually evolve in the Dinosauria itself, likely filling similar niches in earlier strata.

Turfanosuchus and Poposaurus (Fig. 1) were convergent with theropods. The larger Asilisaurus was convergent with sauropodomorphs. The remainder were convergent with various ornithischians, even down to the toothless predentary they shared by convergence. Lotosaurus was a stegosaur mimic. Shuvosaurus was a Dryosaurus mimic. Silesaurus was a Camptosaurus mimic, down to losing the calcaneal tuber. Sacisaurus was a little Agilisaurus mimic. Effigia was still the oddball with those vestigial hands and back-sloped braincase.

So poposaurids are not dinosaurs. They are also not basal to rauisuchidae (contra Nesbitt 2011), but were derived from basal rauisuchia like Decuriasuchus and Vjushkovia. They are the most basal archosaurs. Basal poposaurs were the last common ancestors of crocs and birds. From their basalmost taxon, a sister to little Turfanosuchus, both tiny basal bipedal crocs and tiny bipedal basal dinos evolved.

Size
Poposaurs, in the form of Nyasasaurus, Asilisaurus and Lotosaurus (Fig. 1), were the first archosaurs to evolve substantial size in the Middle Triassic. Crocs and dinos remained small until the late Triassic (mid-Triassic for the basal Herrerasaurus) when they had their great radiation and poposaurs began to fade. This is an unrecognized faunal turnover.

Discovering and correcting errors is what scientists do. 
And I was happy that these new insights appeared.

What took so long?
Inattention to red flags. We should all look more closely at problems. They lead to new insights.

M.M. I hope this helps the cause. And yes, I have made and will make changes to earlier posts on this subject.

References
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.
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Nesbitt SJ, Irmis RB, Parker WG, Smith ND, Turner AH and Rowe T 2009. Hindlimb osteology and distribution of basal dinosauromorphs from the Late Triassic of North America. Journal of Vertebrate Paleontology 29 (2): 498–516. doi:10.1671/039.029.0218

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