Hemiprotosuchus: closer to Aetosaurus than Protosuchus

Not much written about this genus
According to Wikipedia,Hemiprotosuchus is an extinct genus of protosuchid from the Late Triassic (Norian stage) Los Colorados Formation of the Ischigualasto-Villa Unión Basin in northwestern Argentina, South America.” 

Figure 1. Hemiprotosuchus leali skull from Desojo and Ezcurra, nests with Decuriasuchus in the LRT. The variation within this clade is increased with this nesting.

Figure 1. Hemiprotosuchus leali skull from Desojo and Ezcurra, nests with Decuriasuchus in the LRT. The variation within this clade is increased with this nesting.

The specimen
(Fig. 1, image from Desojo and Ezcurra 2016) seems to be preserved as a half skull, nearly complete. Bonaparte 1969 first considered this a protosuchid like Protosuchus (Fig. 2), likely due to its low triangular rostrum and high temporal region.

Figure 2. Protosuchus skull. The high cranium and low triangular rostrum evidently made Bonaparte 1969 consider Hemiprotosuchus similar enough to Protosuchus.

Figure 2. Protosuchus skull. The high cranium and low triangular rostrum evidently made Bonaparte 1969 consider Hemiprotosuchus similar enough to Protosuchus.

After testing
in the large reptile tree (LRT, 1594 taxa) Hemiprotosuchus (PVL 3829, Bonaparte 1969; Norian, Late Triassic) nested between Ticinosuchus (Fig. 3) and aetosaurs, like Stagonolepis and Aetosaurus (Fig. 3).  That’s a long way from Protosuchus in the LRT.

In 1969 no one knew
that Ticinosuchus was basal to aetosaurs. The LRT recovered that relationship here in 2011.

Figure 3. Hemiprotosuhus image from Desojo and Ezccura 2016. Colors added. This taxon is derived from Ticinosuchus, basal to aetosaurs.

Figure 3. Hemiprotosuhus image from Desojo and Ezccura 2016. Colors added. This taxon is derived from Ticinosuchus, basal to aetosaurs.

Others (e.g. Nesbitt 2011 and works based on that cladogram)
considered Revueltosaurus (Fig. 3) a basal aetosaur. The LRT nests Revueltosaurus closer to the genesis of the Euarchosauriformes (between Euparkeria and Erythrosuchus among lesser known taxa).

Desojo and Ezcurra 2016
accepted the protorosuchian affinities of Hemiprotosuchus without further comment.


References
Bonaparte JF 1969. Dos nuevas ‘faunas’ de reptiles triasicos de Argentina. Gondwana Stratigraphy (IUGS Symposium, Buenos Aires)2:283–306.
Desojo JB and Ezcurra MD 2016. Triassic pseudosuchian archosaurs of South America. Historia Evolutiva y Paleobiogeográfica de los Vertebratos de América del Sur. XXX Jornados Argentinas de Paleontología de Vertebrados. Contribuciones del MACN No. 6: 57–66.

wiki/Hemiprotosuchus

CM 73372 reconstructed

So far as I know,
Carnegie Museum specimen CM 73372 (Fig. 1) does not yet have a name, nor has it been reconstructed. Weinbaum 2013 included this skull-less image in a Postosuchus study, which makes sense at first sight, given the size, proportions and age (Late Triassic) of both specimens. The large reptile tree (LRT, 1394 taxa) nests CM73372 close to Postosuchus, but closer to Teratosaurus and Smok. Since Teratosaurus is known from skull-only data at present, there is loss of resolution at that node.

Figure 1. CM73372 in situ and reconstructed using DGS methodology. At first glance it seems to be a biped with short fingers, like Postosuchus. In situ image from Weinbaum 2013.

Figure 1. CM73372 in situ and reconstructed using DGS methodology. At first glance it seems to be a biped with short fingers, like Postosuchus. In situ image from Weinbaum 2013.

This is an interesting taxon because
Lucuona et al. 2017 and others nest it basal to Crocodylomorpha. Weinbaum considered it a member of the Archosauria and the Paracrocodylomorpha, a clade the large reptile tree (LRT, 1394 taxa) does not recover.

According to Wikipedia
Loricata was an early name for an order that includes crocodilesalligators, and gharials, although the order is now referred to as Crocodylia. Nesbitt 2011 defined it as the most inclusive clade containing Crocodylus niloticus (the Nile crocodile), but not the extinct Poposaurus gracilisOrnithosuchus longidens, or Aetosaurus ferox. In the LRT, that clade is a junior synonym for Crocodylomorpha, since Poposaurus is a member of the proximal outgroup, the Poposauria. In traditional paleontology Loricata includes Rauisuchia and Crocodylomorpha. If so, then it also includes Poposauria and Dinosauria, but that was not the original intention of this definition.

Paracrocodylomorpha is another clade invalidated by the LRT because it includes Poposauria and Loricata. In the LRT Rauisuchia is the basal clade, followed roughly by Poposauria and Archosauria (crocs + dinos only).

You might recall,
the Nesbitt 2011 cladogram finds phytosaurs arising from a sister to the distinctly different Euparkeria. Taxon exclusion is the problem here. Nesbitt 2011 also finds Ornithosuchia (Ornithosuchus and kin) and Pterosauria forming the first dichotomy arising from a basal sister to Phytosauria. Again taxon exclusion is the problem here, yet widely accepted in the paleo community for reasons unknown (except, possibly ease of use and fear of change). We talked about other odd and topsy-turvy sister taxa recovered by Nesbitt 2011 earlier here, here and here, three blog posts in a nine-part series.

This addition of CM73372 to the LRT sets us up
for tomorrow’s discussion on basal archosaurs.

References
Lecuona A, Desojo JB and Pol D 2017. New information on the postcranial skeleton of Gracilisuchus stipanicicorum (Archosauria: Suchia) and reappraisal of its phylogenetic position. Zoological Journal of the Linnean Society, 2017, XX, 1–40.
Weinbaum J 2013. Postcranial skeleton of Postosuchus kirkpatricki (Archosauria:
Paracrocodylomorpha), from the Upper Triassic of the United States. Geological Society London Special Publications · August 2013.

wiki/Paracrocodylomorpha
wiki/Loricata

A closer look at Sikannisuchus huskyi

Earlier we looked at the skull roof of Sikannisuchus. Unfortunately, I ignored those long mandible bits and pieces. These give us more clues to restore the missing parts in lateral view (Fig. 1).

Sikannisuhus huskyi restored on must a view bits and pieces.

Sikannisuhus huskyi restored on must a view bits and pieces. Nesting with Postosuchus provides clues to the shapes of the missing parts. That’s a nice long skull. Must have been a BIG reptile.

If I made any errors, or the clues point in another direction, let me know.

References
Nicholls EL, Brinkman DB and Wu K-C 1998. A new archosaur from the Upper Triassic
Pardonet Formation of British Columbia. Canadian Journal of Earth Science 35: 1134–1142.

Revueltosaurus: closer to Aetosaurus? or to Fugusuchus?

This post was updated December 8, 2014 with a new nesting of Revueltosaurus with Fugusuchus as a basal erythrosuchid. 

Yesterday we saw a higher resolution Revueltosaurus skull in lateral view and we noted a disagreement in the phylogenetic nesting of this Late Triassic oddball.

According to Nesbitt (2011)…
Nesbitt (2011) nested the odd herbivorous Triassic archosauriform, Revueltosaurus, at the base of the aetosaurs, including Aetosaurus and Stagonolepis. Outgroups include an unresolved clade including Gracilisuchus, Turfanosuchus and Ticinosuchus. Outgroups to this clade include Riojasuchus and Ornithosuchus (= Ornithosuchidae) at the base of the “Pseudosuchia“. Wiki follows Nesbitt (2011).

According to Parker (2014?)…
William Parker, the discoverer of the twelve-specimen nest of Revueltosaurus skeletons, will soon publish a large monograph on this genus. His analysis will also nest Revueltosaurus with aetosaurs. He’s very sure of that.

Figure 1. Revueltosaurus compared to its big sister, Fugusuchus, a basal erythrosuchid.

Figure 1. Revueltosaurus compared to its big sister, Fugusuchus, a basal erythrosuchid.

According to the large reptile tree…
On the other hand, the large reptile tree does not recover the Nesbitt tree topology, but finds Revueltosaurus nests with Fugusuchus (Fig. 1) at the base of the Erythrosuchidae. To move Revueltosaurus to the Aetosauridae requires an additional 35 steps. t.

Like Aetosaurs:
Revueltosaurus is a heavily armored quadruped and a plant eater (judging by its teeth). I’m told by Parker the following traits are identical in Revueltosaurus and aetosaurs: scapula/coracoid, humerus, squamosal and armor design. You can see (Fig. 1) that despite being twice the size, Revueltosaurus has more gracile girdles and limbs and larger hands and feet than Aetosaurus. And the skull has a distinctly different shape in nearly all regards, including the squamosal. Even so…

An herbivorous rauisuchid, with several aetosaur and turtle-like traits.
On Revueltosaurus, the naris is rather unique. The lateral processes of the premaxilla have become more robust, reducing the size of the naris and moving them to an anterior position.  Revueltosaurus shares these traits with Fugusuchus.

Figure 2. Revueltosaurus pelves compared to Aetosaurus and Postosuchus. Red arrows point to limit of femoral head. Blue arrows point to acetabular shelf that roofs over the femur in rauisuchids.

Figure 2. Revueltosaurus pelves compared to Aetosaurus and Postosuchus. Red arrows point to limit of femoral head. Blue arrows point to acetabular shelf that roofs over the femur in rauisuchids. Revueltosaurus is a small slow quadrupedal plant eater, so it doesn’t have the long booted pelvis of Postosuchus. But note the similarity of the broad ischium to Aetosaurus!  Wm. Parker tells me, “It’s the plesiomorphic archosauriform condition with the very short anterior process. Lacks the deep supracetabular buttress found in rauisuchids. It actually looks like a juvenile aetosaur and it is difficult to tell them apart.” Schoch reports the aetosaur oblique posterior ridge is unlike the supraacetabular ‘boss’ of rauisuchids.

The pelvis should settle the argument
But it doesn’t. The pelvis of Revueltosaurus appears to bear a acetabular boss beneath which the femur articulates and supports. In aetosaurs the pelvis has a simpler shape without the boss. In aetosaurs the pelvis is wider at the top, which provides a broad platform for the armor, and angles inward ventrally. We don’t have a pelvis and hind limb for Fugusuchus, but Euparkeria is not far off. So is Gargainia. Both have similar elements.Here again, the different nesting sites appear to be based on employing macro-traits or micro-traits. This is a problem I have no solution for at present.

Figure 2. Revueltosaurus

Figure 2. Revueltosaurus

 

References
Hunt AP 1989. A new ornithischian dinosaur from the Bull Canyon Formation (Upper Triassic) of east-central New Mexico. In Lucas, S. G. and A. P. Hunt (Eds.), Dawn of the age of dinosaurs in the American Southwest 355–358.
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.
Parker WG., et al. 2005
. The Pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. In Proceedings of the Royal Society London B 272(1566):963–969.

wiki/Revueltosaurus

Revueltosaurus in higher resolution

An online photo of Revueltosaurus (Fig. 1) adds details and clarifies problems. There is also a Vimeo video here featuring paleontologist, William Parker, who authors the Chinleana blogsite. He also discovered 11 skeletons of Revueltosaurus from one site with two that are nearly complete or complete. From that data the complete skeleton (Fig. 1) has been recreated.

Figure 1. Revueltosaurus skull from online source (link above) color coded to bone. Above, the aetosaur, Stagonolepis, to which Nesbitt and Parker nest Revueltosaurus. In contrast, the large reptile tree nests Fugusuchus with Revueltosaurus as a derived herbivorous erythrosuchid.

Figure 1. Revueltosaurus skull from online source (link above) color coded to bone. Above, the aetosaur, Stagonolepis, to which Nesbitt and Parker nest Revueltosaurus. In contrast, the large reptile tree nests Fugusuchus with Revueltosaurus as a derived herbivorous erythrosuchid.

With this data, Nesbitt (2011) nested Revueltosaurus at the base of the Aetosauria with Aetosaurus and Stagonolepis.

In counterpoint,
The large reptile tree, employing a different character list, nested Revueltosaurus with Postosuchus, as a derived, smaller, somewhat turtle-ized rauisuchid.

We’ll look at the pros and cons of both analyses tomorrow.

References
Hunt AP 1989. A new ornithischian dinosaur from the Bull Canyon Formation (Upper Triassic) of east-central New Mexico. In Lucas, S. G. and A. P. Hunt (Eds.), Dawn of the age of dinosaurs in the American Southwest 355–358.
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.
Parker WG., et al. 2005
. The Pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. In Proceedings of the Royal Society London B 272(1566):963–969.

wiki/Revueltosaurus

Luperosuchus: an erythrosuchid, not a rauisuchian

Luperosuchus fractus (Romer 1972, PULR 04) was considered a indistinct pseudosuchian originally and later a rauisuchian by Desojo and Arcucci (2009). The large reptile tree recovers it as an erythrosuchid and a sister to Shansisuchus, which had an even larger subnarial fenestra. Earlier we looked at the two Shansisuchus specimens, noting that the referred specimen was much larger than the holotype with a distinct morphology, more like Luperosuchus.

Figure 1. Luperosuchus restored based on Romer 1971. Above: original drawing by Romer. Below tracing based on photo in Romer 1971, specimen PULR 04. At right is referred specimen PULR 057. Although related, the referred specimen strikes me as generically different with the low placement of the naris and large postorbital.

Figure 1. Luperosuchus restored based on Romer 1971. Above: original drawing by Romer. Below tracing based on photo in Romer 1971, specimen PULR 04. Extension of the qj and a deeper max gives it more of a erythrosuchid look. At right is referred specimen PULR 057. Although related, the referred specimen strikes me as generically different with the low placement of the naris and large postorbital. Analysis on PULR 057 has not been done.

The reconstruction by Desojo and Arcucci (2009, Fig. 1, above) assumes a short quadratojugal, but a longer qj (Fig. 1, below) matches sister taxa.

This one is probably a rauisuchid
Another much smaller specimen (PULR 057, Fig. 1) was referred to Luperosuchus. That seems doubtful based on the lower placement of the naris, the straighter rostral profile, the larger antorbital fenestra, the deeper pmx/mx notch and the more robust postorbital. These traits appear to lead to Ticinosuchus and the aetosaurs as other archosauriformes retain a high naris. A second possibility leads toward the euparkeriid Osmolskina. A phylogenetic analysis was not attempted due to the small number of traits shown.

References
Desojo JB and Arcucci AB 2009. New material of Luperosuchus fractus (Archosauria: Crurotarsi) from the Middle Triassic of Argentina: the earliest known South American ‘Rauisuchian’. Journal of Vertebrate Paleontology 29(4): 1311-1315. 
Romer AS 1971. The Chañares (Argentina) Triassic reptile fauna. VIII. A fragmentary skull of a large thecodont, Luperosuchus fractus. Breviora 373:1-8.

Short note: ReptileEvolution.com has just passed a million hits for this year. Between 4.2 and 5.7 thousand unique visitors access the site every month.

Reconstructing the hand of Ticinosuchus

Sometimes fossils are wonderfully preserved
and fully articulated. Sometimes they are wonderfully preserved but woefully disarticulated. At such times, most of the bones can be fit together with ease, but the bones of the fingers and toes can be vexing.

Figure 1. Ticinosuchus forelimbs. Note the scattered manual elements here reconstructed to create PILs and match sister taxa patterns.

Figure 1. Ticinosuchus forelimbs. Note the scattered manual elements here reconstructed to create PILs and match sister taxa patterns. Yellow is the radius. Pink is the ulna. Metatarsal 3 is the most robust based on sister taxa. The phalangeal pattern is 2-3-4-5-4.

Case in point: Ticinosuchus
An important taxon in the evolution of crocs and dinosaurs and other Triassic oddities is the basal rauisuchian, Ticinosuchus. It had departed from the rauisuchian ancestors so much that it is basal to the armored herbivorous aetosaurs of the Late Triassic. Most of the elements of both manus of the Ticinosuchus are present, but scattered. That doesn’t mean they’re impossible to put back together again.

Trace the parts.
Move the parts into a logical pattern (thick with thick, thin with thin, gradually tapering digits, phylogenetic bracketing patterns) then test your results to see if PILs (parallel interphalangeal lines) are produced. When all that happens, you can have high confidence in a correct solution.

Figure 2. Ticinosuchus overall, hand, foot and skull.

Figure 2. Ticinosuchus overall, hand, foot and skull. The hand is presented as originally interpreted by Krebs and by a new reconstruction based on the tracing in figure 1 and phylogenetic bracketing.

This is a long-armed quadrupedal taxon with long (longer than each metacarpal). Metacarpal 3 was the most robust. Metacarpal 5 was extremely short. Digits 3 and 4 were subequal. Digit 1 was the shortest digit, but digit 5 had smaller phalanges. Where known, sister taxa share most of these traits.

Earlier here, here and here we put the manus of an early archosauriform together.

References
Krebs B 1965. Ticinosuchus ferox nov. gen. nov. sp. Ein neuer Pseudosuchier aus der Trias des Monte San Giorgio. Schweizerische Palaontologische Abhandlungen 81:1-140.
Lautenschlager S and Desojo JB 2011. Reassessment of the Middle Triassic rauisuchian archosaurs Ticinosuchus ferox and Stagonosuchus nyassicus. Paläontologische Zeitschrift Online First DOI: 10.1007/s12542-011-0105-1

wiki/Ticinosuchus

The Sailback Arizonasaurus – a Good Bipedal Candidate

Figure 1. Arizonasaurus configured as a biped. The depth of the pubis suggests a similar length for the femur and tibia. The gracile pectoral girdle suggests a gracile forelimb. The long deep tail is based on the related Yarasuchus.

Figure 1. Arizonasaurus configured as a biped. The depth of the pubis suggests a similar length for the femur and the tibia follows. The gracile pectoral girdle suggests a gracile forelimb, perhaps smaller than shown here. The long deep tail is based on the related Yarasuchus. Looks a little like Spinosuchus, doesn’t it? And this clade is known for fish-eating.

Like the cheese, Arizonasaurus stands alone. Almost.
Phylogenetic analysis nests what is known about Arizonasaurus with Yarasuchus and Qianosuchus two sail-less ticinosuchians (not poposaurs, as envisioned by Nesbitt 2011).

Another sailback, Lotosaurus does nest with poposaurs, though, but it’s quite different and a herbivore.

Figure 2. Xilousuchus and  Yarasuchus compared.

Figure 2. Xilousuchus and Yarasuchus compared. Yarasuchus is a sister to Arizonasaurus, but has a much more robust pectoral girdle.

We don’t have any fore limbs or hind limbs for Arizonasaurus,
but we do have its pectoral and pelvic girdles.

The pelvic girdle is very deep, compared to the ancestral Vjushkovia, and sister taxa like Yarasuchus, Qianosuchus and Ticinosuchus. In these taxa the femur extends at least as far as the pubis depth and sometimes a little further. If we add such femora to the reconstruction of Arizonasaurus, it becomes essentially bipedal.

The pectoral girdle is quite small and gracile. It would be odd for massive or elongate forelimbs to be attached to such a small pectoral girdle, so here (Fig.1) an appropriate gracile short forelimb is added.

But wait, that’s not all.
That sail adds leverage and strength to the back bone, helping to hold up the elevated anterior half. Moreover, if we add on the deep tail of the related Yarasuchus, we reconstruct a substantial counterbalance (Fig. 1).

References
Butler RJ, Brusatte SL, Reich M, Nesbitt SJ, Schoch RR, et al. 2011. The Sail-Backed Reptile Ctenosauriscus from the Latest Early Triassic of Germany and the Timing and Biogeography of the Early Archosaur Radiation. PLoS ONE 6(10): e25693. doi:10.1371/journal.pone.0025693 Plos One paper
Nesbitt SJ 2003. Arizonasaurus and its implications for archosaur divergence
Sterling J. Nesbitt Proceedings of the Royal Society, London B (Suppl.) 270, S234–S237. DOI 10.1098/rsbl.2003.0066
Nesbitt SJ, Liu J and Li C 2010. A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China. Transactions of the Royal Society of Edinburgh 101 (Special Issue 3-4):271-284.
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.
Welles SP 1947 Vertebrates from the Upper Moenkopi Formation of the Northern Arizona. Univ. California Publ. Geol. Sci. 27, 241–294.
Wu X-C 1981. The discovery of a new thecodont from north east Shanxi. Vertebrata PalAsiatica 19: 122–132.

wiki/Arizonasaurus

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

Reinterpreting the Skull of Effigia Using DGS

Updated March 12, 2015 with a revised mandible for Shuvosaurus. 

The Extreme Strangeness of Effigia
Effigia okeeffeae (Nesbitt and Norell, 2006) 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.

“Extreme Convergence”
A recent reassessment by Nesbitt and Norell (2006) and Nesbitt (2007) nested Effigia among the poposaurid rauisuchians based largely on the ankle, but they noted “extreme convergence in the body plans” with ornithomimid dinosaurs. They reported that the ankle of Effigia articulated in a crocodile-normal configuration, with a morphology similar to Alligator (Figure 1). The broken and missing calcaneal “heel”would have turned proximally, like that in a sister taxon, Shuvosaurus.

The pedes of Alligator and Effigia

Figure 1. The pedes of Alligator (left) and Effigia (right) demonstrating the convergence of the structure of the ankle bones (astragalus and calcaneum). In basal archosaurs the ankle is a simple hinge, but in Alligator the hinge takes a sharp turn between the astragalus and calcaneum. The astragalus and calcaneum of Effigia articulate in a crocodile-normal configuration and their morphology is similar to Alligator. Note the peg of the astragalus inserts into and rotates on the calcaneum. There are also axes of rotation at the distal tibia/fibula and at the (evidently missing) distal tarsals. 

The Calcaneal Tuber and its Distribution
Most paleontologists assert that the calcaneal “heel” is found only in rauisuchians + crocodylians (= pseudosuchians) not dinosaurs and their kin. Without the present expanded inclusion list, prior workers were not aware of the new clade, the Paraornithischia, that nested Effigia as a sister taxon to the phytodinosauria based on more parsimoniously shared traits from head to toe. The “extreme convergence” with restricted to the ankle.

Figure 2. Comparing the DGS method of skull reconstruction to traditional methods. Above: The skull of Shuvosaurus, a sister taxon. Next: Effigia traced from a photo published in Norell and Nesbitt 2006. Note the surangular extends over the mandibular fenestra, as in other dinosaurs and a predentary is present as in other poposaurids. Next: Original tracing of Effigia skull. Bottom: Original restoration of Effigia skull from Norell and Nesbitt (2004). Note the anterior extent of the surangular.

Figure 2. Comparing the DGS method of skull reconstruction to traditional methods. Above: The skull of Shuvosaurus, a sister taxon. Next: Effigia traced from a photo published in Norell and Nesbitt 2006. Note the surangular extends over the mandibular fenestra, as in other dinosaurs and a predentary is present as in other poposaurids. Next: Original tracing of Effigia skull. Bottom: Original restoration of Effigia skull from Norell and Nesbitt (2004). Note the anterior extent of the surangular.

DGS
I have never seen the skull of Effigia, only published photos. Even so, it appears that the original reconstruction by Nesbitt (2007) contains certain errors and oversimplifications that I repaired and reidentified. The DGS (Digital Graphic Segregation) method using Adobe Photoshop enabled a test of the original reconstruction and not all the original results could be verified.

Surangular/Dentar/Premaxilla
Chief among the problems in the Nesbitt and Norell (2006) reconstruction is the identification of the long bone over the mandibular fenestra as the surangular. This arose from the identification of the mandibular tip as the maxilla. Perhaps the authors did not realize that in sister taxa the anterior bone is a predentary or a pseudopredentary (if not homologous with the predentary in ornithischians). The temporal muscles for closing the jaw were also attached to the dentary in all tetrapods, which cannot happen when the dentary is restricted to the jaw tip. Here the toothless dentary is in its standard place and a predentary precedes it at the jaw tip.

A Descending Cranium
In Effigia the posterior skull descends, but that was “fixed” in the reconstruction of Nesbitt and Norell (2006). Here (Fig. 2) what you see is what you get.

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
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