Is this the footprint of Arizonasaurus?

Figure 1. Synaptichnium MNA V3425. Arrow points to direction of movement and aligns with sagittal plane. PILs and pads added.

Figure 1. Synaptichnium MNA V3425. Arrow points to direction of movement and aligns with sagittal plane. PILs and pads added. The pink manus track is another specimen.

The middle Triassic Moenkopi formation
in Arizona has provided a rich trove of fossils. An excellent footprint (MNA V3425, Fig. 1) was recently published online here and attributed to Arizonasaurus, a likely bipedal carnivorous archosauriform (Fig. 2). Arizonasaurus was derived from basal Rauisuchia, like Vjushkovia, and is most closely related to Yarasuchus and Qianosuchus according to the large reptile tree.

Figure 2. Arizonasaurus. Not sure which of the two mandibles is correct here, so both are presented. Note, neither manus nor pes is preserved in the specimen.

Figure 2. Arizonasaurus. Not sure which of the two mandibles is correct here, so both are presented. Note, neither manus nor pes is preserved in the specimen.

According to the online article,
“Paleontologist Christa Sadler has written a book, “Dawn of the Dinosaurs,” about the archosaurs of the Middle and Late Triassic in the region. Unusually detailed footprints of the large reptile, or something like it, are preserved in a slab of Moenkopi sandstone in the collections repository at the Museum of Northern Arizona, where Sadler has studied. MNA  [Museum of Northern Arizona] Colbert Collections Curator of Vertebrate Paleontology David Gillette, Ph.D., says the footprints were discovered in Wupatki National Monument in 1973.”

Figure 3. Manus impression of man v3245. Note the heavy scales here.

Figure 3. Manus impression of man v3245. Note the heavy scales here.

The LRT currently doesn’t include ichnites (footprints)
but let’s see what happens this time, since the track is so precisely imprinted. Unfortunately, Arizonasaurus does not preserve the manus or the pes (Fig. 1). Nevertheless, out of 801 candidate taxa, MNA 3425 nests with a sister to Arizonasaurus, Decuriasuchus, and is similar to the pes of other Arizonasaurus sisters, Qianosuchus and Nandasuchus, all Middle Triassic taxa. So, phylogenetic bracketing works, at least to this extent. And it just shows you don’t need a long list of character traits to successfully nest some taxa.

Figure 3. Scaly palms of two crocodilians. Digit 1 is on the left in both specimens.

Figure 4. Scaly palms of two crocodilians. Digit 1 is on the left in both specimens.

Notes on the scaly palm of MNA V3425
Dinosaur footprints do not have large scale impressions. By contrast, croc hands and feet do have large scales (Fig 3). The sisters to Arizonasaurus, Qianosuchus and Yarasuchus, both have short limbs, a long rostrum and a general crocodile-like build. Likewise Decuriasuchus was long-bodied, quadrupedal with a large foot and a presumably small hand (not preserved). In similar fashion, Arizonasaurus likely also had a large foot and small hand based on its pectoral and pelvic girdles and femur (Fig. 2), but was a likely biped.

Figure 5. Decuriasuchus does not preserve the manus, but it was probably small based on the forelimb.

Figure 5. Decuriasuchus does not preserve the manus, but it was probably small based on the forelimb.

Belated apologies
to those who tried [or continue to try] to access www.reptileevolution.com yesterday and today. Eviidently the server is down, wherever it is. I can’t access it either to make updates and repairs. Hopefully the RepEvo website will be restored soon. :  )

 

Surviving the Permian-Triassic boundary

For those of you
who typically ignore the letters to the editor, this is one exchange that you might find interesting.

Earlier Bill Erickson asked me 
“So, why, in your opinion, did diapsid reptiles suddenly — and I do mean suddenly — become so dominant beginning in or about Carnian time, and remain dominant thereafter throughout the Mesozoic, after millions of years of synapsid dominance beforehand in the mid-to-late Paleozoic and early Triassic?”

I answered
-Why- questions are very tough in Science, Bill. I don’t know the answer to your question. I don’t have an opinion either.

B. Erickson replied
“David – I’d agree for the most part, but I do think Peter Ward made a good case [in his book Gorgon.] that synapsids had a less efficient respiratory system than many archosaurs, and that lower atmospheric oxygen was a major driver in the end-Permian extinction. Of course, some synapsids, especially cynodonts, were diverse in early Triassic, and that’s another story.”

To which I replied
Bill, I have heard of Ward’s hypothesis and it makes a certain sense. Let me toss this off-the-cuff idea at you.

Synapsids, to my knowledge, survived the Permian extinction event by burrowing, or perhaps there was a part of the world they found refuge in. If the former, whether in dirt or leaf litter, both niches seem to support small to tiny tetrapods. See Pachygenelus, Megazostrodon and Hadrocodium for examples. [Well, those are all bad examples as they are all Early Jurassic, but consider the small earliest Triassic cyndont, Thrinaxodon (Fig. 1).]

Figure 1. Thrinaxodon, a burrowing synapsid from the Early Triassic was similar in size and proportion to the Late Permian ancestor of all archosauriformes, Youngoides (Fig. 2). These similar basal taxa were the genesis for all later mammals, dinosaurs and birds. 

Figure 1. Thrinaxodon, a burrowing synapsid from the Early Triassic was similar in size and proportion to the Late Permian ancestor of all archosauriformes, Youngoides (Fig. 2). These similar basal taxa were the genesis for all later mammals, dinosaurs and birds.

On the diapsid/archosauriform side, the likely aquatic proterosuchids cross the Permo-Triassic boundary, then give rise to all the familiar archosauriformes. In the water niche larger tetrapods, like crocs, are supported. As Malcolm Gladwell documented so well [in his book Outliers], an initial minor advantage can accelerate or become emphasized over time.

So, again guessing here, the largely nocturnal denizens of the burrows and leaf litter apparently played to their environment and stayed small yielding the otherwise unoccupied largely diurnal aquatic-grading-to-terrestrial taxa the larger size as they played to their niche. Maybe the diapsids just got to the outdoors/daylight niche first.

Figure 2. Updated image of various proterosuchids and their kin. When you see them all together it is easier to appreciated the similarities and slight differences that are gradual accumulations of derived taxa. Youngoides and the earliest proterosuchids were Late Permian. Others were Early Triassic and later.

Figure 2. Updated image of various proterosuchids and their kin. When you see them all together it is easier to appreciated the similarities and slight differences that are gradual accumulations of derived taxa. Youngoides and the earliest proterosuchids were Late Permian. Others were Early Triassic and later.

Along the same lines, the lepidosaur diapsids stayed relatively small and unobtrusive except for the Late Triassic sea-going tanystropheids and Late Cretaceous sea-going mosasaurs, perhaps following the same niche rules and regs as above. Pterosaur lepidosaurs also experienced much greater size in the Late Cretaceous.

Just a thought/opinion supported by what I can recall at the moment. Let me know your thoughts if you’d like to continue this thought journey. [END]

And then beyond that exchange…
I note that EarlyTriassic synapsid taxon list also includes the large dicynodont, Kanneymeira and a number of small therocephalians. Burrowing taxa are pre adapted to a nocturnal existence. The big dicynodont must have survived in some sort to refuge niche.

The standard story
includes the notion that dinosaurs and other archosauriform predators were snapping up every little synapsid they saw, so the survivors became invisible by becoming nocturnal and or really tiny… and that probably continued throughout the Mesozoic, with both clades improving generation after generation.

erythrosuchid

Figure 3. Basal archosauriforms from the Early Triassic,  including Euparkeria, Proterosuchus and Garjainia.

The twist brought to you by
the large reptile tree is the outgroup for the Archosauriforms, Youngoides, is a small, Thrinaxodon-sized terrestrial younginiform diapsid (Fig. 1). Perhaps an early affinity for rivers and lakes was the key to survival among proterosuchid archosauriforms when the P-Tr problems escalated. But also note that the small ancestors to dinosaurs, the euparkeriids, (Fig. 3) ALSO survived the P-Tr boundary as small terrestrial forms alongside the much larger terrestrial erythrosuchids, otherwise known as giant younginids.

Maybe we’ll never know…
but it’s interesting to put at least some of the puzzle pieces together.

 

 

Cladogram quirk and a new basalmost Euarchosauriform: SAM PK K6047A

I’ve claimed on several occasions 
that my cladogram was fully resolved and all of its subsets were also fully resolved. While that is typically true, everyone prefers a cladogram with more strength, where the taxa are lumped and separated by at least three points in Bootstrap Analysis. When that happens the Bootstrap tree will also be completely resolved (= all scores 50-100).

I found a quirk
And as I write this I am going to figure out why this happened and wonder if it can be repaired. Typically a bad score or several bad scores are responsible for any lack of resolution. Not sure if that’s the case this time.

Follow me
as I describe the setup and the problem. I’m looking to apply Bootstrap scores to members of the basal Archosauriformes with the addition of Teyujagua, a proterosuchid we looked at earlier.

  1. A heuristic search algorithm using PAUP of the entire cladogram: fully resolved. 
  2. The same deleting all anamniotes and lepidosaurormorpha — the new Archosauromorpha (Eldeceeon and all derived taxa) remains: fully resolved.
  3. The same deleting all basal archosauromorphs and synapsids: the protodiapsida (Myceteosaurus and all derived taxa) remains: fully resolved.
  4. The same deleting all basal protodiapsids and basal diapsids: the new younginiforms  (Spinoaequalis and all derived taxa) remains: fully resolved.
  5. The same deleting all aquatic younginiforms: the terrestrial younginiforms: Spinoaequalis + the SAM K7710 specimen(s) of Youngina and all derived taxa) remains: fully resolved.
  6. Now, working backwards: The same deleting all dinosaurs: 6 trees retained. Loss of resolution in the Protodinosauria. Several incomplete taxa based on drawings there. We’ll reexamine that clade in a later post.
  7. The same adding Herrerasaurus: fully resolved. A more complete data specimen solves the problem in #6.
  8. The same deleting all archosaurs: fully resolved.
  9. The same deleting the protoarchosaurs (the Gracilisuchus clade): fully resolved.
  10. The same deleting all the poposaurs: fully resolved.
  11. The same deleting all the Arizonasaurus/Ticinosuchus/Aetosaurus clade: fully resolved. 
  12. The same deleting all remaining Rauisuchia (Vjushkovia through Postosuchus): fully resolved.
  13. The same deleting all erythrosuchidae and ornithosuchidae: 2 trees retained. Loss of resolution at the Euparkeria node.
  14. The same adding Garjainia: fully resolved. 
  15. The same deleting all Choristodera: fully resolved.
  16. The same deleting all Chanaresuchidae and Parasuchia: fully resolved.
  17. At this point with a single tree scoring 447, I attempted a Bootstrap analysis. Basically all that is left here are the basal terrestrial younginiforms including several Youngina and Youngoides specimens, protorosaurs and basal archosauriforms up to and including all tested proterosuchids + Euparkeria, Osmolskina and Garjainia. Here  the small Proterosuchus skull (BPI/1/4016) and Elaphrosaurus rubidgei (RC59) are apparently mucking up the works, even though they do not nest together otherwise.
  18. The same adding Diandongosuchus, a basal parasuchian: fully resolved.
  19. At this point with a single tree scoring 488, warrants another Bootstrap analysis. Again Elaphrosuchus scores insufficiently distinct from several other included taxa to lack a score of 50+ across four other clades. And a very low score of 51 separates Elaphrosuchus from the BPI/1/4016 specimen of Proterosuchus. Both are skull only taxa and both have a certain amount of damage.
  20. The same deleting the new taxon, Teyujagua: Bootstrap scores all above 50. So that addition caused problems.

Are there scoring errors here? 
Or do these taxa converge? Or do two sisters lack any data points in common? Let’s find out by taking a closer look at the offending parties.

[About a day or two elapses at this point in the narrative]

Scoring errors
There were many errors around these nodes, hopefully all are now repaired. I reexamined several drawings, photos and tracings. Unfortunately fossil bones don’t come with overlying colors, so they have to be interpreted.

I also added a taxon
(Figs, 1, 2). It turned out to be a key transitional taxon. Score corrections and the new taxon boosted 5/6 of the Bootstrap scores.

Figure 1. The SAM PK K6047A specimen that Ewer 1967 attributed to Euparkeria, but is actually a closely related unnamed genus.

Figure 1. The SAM PK K6047A specimen that Ewer 1967 attributed to Euparkeria, but is actually a closely related now unnamed genus, more primitive than Euparkeria, transitional  to basal Proterosuchus specimens.

Adding the ‘other Euparkeria‘: SAM PK K6047A
While reexamining the images of the Euparkeria holotype in Ewer 1967, I compared the rostrum that has a naris (SAM PK K6047A) with the classic holotype (SAM PK K 5867) that lacks a naris (Fig. 1). The fossils did not match. The dimensions were off (orbit vs antorbital fenestra, etc.) and the teeth were different in length. Ewer provides two images of the 6047 specimen, lacking data for the middle rostrum between the pix. The tracing (Fig. 1) recovers a basal euarchosauriform with a longer rostrum and narrower orbit, more like that of its phylogenetic predecessor, the BPI/1/4016 specimen of Proterosuchus (Fig. 3). This taxon ties Euparkeria more closely (more gradual transition in traits) to Proterosuchus (Figs. 2, 3).

There is also a SAM PK K6047B specimen
and it has been named Browniella africana by Broom (1913) I have not seen it. I do not know if it resembles or was found with the 6047A specimen. Most workers consider this taxon a junior synonym of Euparkeria.

Sookias and Butler 2013
reviewed the Euparkeriidae, but did not mention the 6047A or B specimens, except, perhaps as two of the eleven specimens that comprise their hypodigm. They defined the clade in this fashion: “Euparkeriidae Huene 1920. Stem-based definition –the most inclusive clade containing Euparkeria capensis Broom 1913a but not Crocodylus niloticus Laurenti 1768 or Passer domesticus Linnaeus 1758. (new).” I have not tested all the taxa listed by Sookias and Butler, but their definition seems to be overly broad.

Figure 2. Cladogram of basal archosauriforms. Note the putative basalmost archosauriform, Teyujagua (Pinheiro et al 2016) nests deep within the proterosuchids. The 6047 specimen that Ewer referred to Euparkeria nests as the basalmost euarchosauriform now.

Figure 2. Cladogram of basal archosauriforms. Note the putative basalmost archosauriform, Teyujagua (Pinheiro et al 2016) nests deep within the proterosuchids. The 6047 specimen (green)  that Ewer referred to Euparkeria nests as the basalmost euarchosauriform now.

What do we learn here?

  1. Incomplete taxa can cause loss of resolution, as everyone knows. The addition of a more complete cousin can provide the remedy.
  2. Scoring errors also lead to loss of resolution.
  3. There is only one tree, the tree of Nature, that we are trying to model here. So there IS a correct solution to this problem.
  4. Adding taxa almost always provides traits that make phylogenetic transitions more gradual. The only exceptions are terminal taxa, those that lack descendants.
  5. Known proterosuchids still do not represent ontogenetic (maturation) stages. They are phylogenetically distinct taxa that lead to more derived clades (Figs. 2,3).
  6. There is still no evidence for the sisterhood of Euparkeria with the verified sisters Turfanosuchus and Gracilisuchus  (Sookias and Butler 2013, Butler et al. 2014).
  7. Phylogenetic miniaturization preceded and was part of the basal archosauriform radiation.
  8. It is important for professionals not to assume that different specimens represent a single species. Minor differences might turn out to be key traits as demonstrated here.
Figure 3. Click to enlarge. Updated image of various proterosuchids and their kin. When you see them all together it is easier to appreciated the similarities and slight differences that are gradual accumulations of derived taxa.

Figure 3. Click to enlarge. Updated image of various proterosuchids and their kin. When you see them all together it is easier to appreciated the similarities and slight differences that are gradual accumulations of derived taxa. In the white zone are specimens considered proterosuchids.

Finally: the value of a large gamut cladogram
becomes more valuable with every added taxon. More gradual transitions become apparent and bias is further minimized.

References
Broom R 1913. On the South-African Pseudosuchian Euparkeria and Allied Genera. Proceedings of the Zoological Society of London 83: 619–633.
Butler RJ, Sullivan C, Ezcurra MD, Liu J, Lecuona A and Sookias RB (2014). New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and the biogeography of the archosaur radiation. BMC Evolutionary Biology 14:1-16.
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
Sookias RB and Butler RJ 2013. Euparkeriidae. Geological Society, London, Special Publications published online January 24, 2013 as doi: 10.1144/SP379.6

 

 

 

Back to Vancleavea

Several years ago we looked at Vancleavea campi (Figs. 1,2 ), a Triassic aquatic reptile described by Nesbitt, et al. 2008 as an archosauriform nesting with Erythrosuchus, Euparkeria, Turfanosuchus and Doswellia (according to Wikipedia, based on published work listed above). Unfortunately, Vancleavea shares few traits with these archosauriforms. It has no antorbital fenestra, no upper temporal fenestra and no mandibular fenestra.

Figure 1. Vancleavea surrounded by purported sister taxa as figured by Nesbitt and Wikipedia. None of these taxa share more traits with Vancleavea than does Helveticosaurus, a taxon ignored since it was proposed here.

Figure 1. Vancleavea surrounded by purported sister taxa as figured by Nesbitt and Wikipedia. None of these taxa share more traits with Vancleavea than does Helveticosaurus, a taxon ignored since it was proposed here.

Not yet tested in academic publications,
the thalattosaur, Helveticosaurus, shares more traits with Vancleavea than 569 other tested taxa in the large reptile tree.

Figure 2. Vancleavea with its sister, Helveticosaurus.

Figure 2. Vancleavea with its sister, Helveticosaurus.

This counter argument
was made more than 4 years ago. To date no one else has supported or refuted the argument. Nevertheless, in the last four years Vancleavea has appeared in several cladograms without Helveticosaurus. Unfortunately this demonstrates that paleontologists are really not interested in its correct nesting node, but would rather just add new taxa to existing flawed analyses and cladograms. Testing prior work is not their strong suite. Discovery is.

Vancleavea campi  (Nesbitt et al. 2009) Late Triassic,~210 mya, ~1.2 meters in length, was originally considered a very weird archosauriform close to DoswelliaTurfanosuchusChanaresuchus and Erythrosuchus, but that’s because the authors did not compare it to Helveticosaurus with which Vanclevea shares more traits. It turns out that Vanclevea was a not-so-weird thalattosaur and a prime example of what happens when the gamut of the inclusion set is decided prior to the analysis. Vancleavea was the last in its lineage. Unlike other thalattosauriforms, Vancleavea was armored with a variety of ossified scales covering the body.

There must be dozens
of Vancleavea-like thalattosaurs yet to be discovered, judging by the variation present between it and Helveticosaurus. Even so, after adding hundreds of taxa to the large reptile tree, these two still nest together.

Not the only time a taxon’s correct nesting ignored.
These taxa are also traditionally incorrectly nested based on the results of the large reptile tree.

  1. Turtles
  2. Pterosaurs
  3. Fenestrasauria and Tritosauria
  4. Snakes
  5. Caseasauria
  6. Mesosauria
  7. Poposauria
  8. Rhynchosauria
  9. Synapsida
  10. Chilesaurus and Daemonosaurus
  11. Gephyrostegus
  12. Procolophon
  13. Cartorhynchus
  14. Youngina and Youngoides
  15. Xianglong
  16. Tetraceratops
  17. Eudibamus
  18. Doswellia
  19. Revuletosaurus
  20. Scleromochlus
  21. Pseudhesperosuchus
  22. Marasuchus
  23. Lagerpeton
  24. Teraterpeton
  25. Ticinosuchus
  26. and whatever else I’ve forgotten to list here

References
Nesbitt SJ, Stocker MR, Small BJ and Downs A 2009. The osteology and relationships of Vancleavea campi (Reptilia: Archosauriformes). Zoological Journal of the Linnean Society 157 (4): 814–864. doi:10.1111/j.1096-3642.2009.00530.x.
Parker WG and Barton B 2008. New information on the Upper Triassic archosauriform Vancleavea campi based on new material from the Chinle Formation of Arizona. Palaeontologia Electronica 11 (3): 20p.

wiki/Vancleavea

Oldest archosauromorph/archosauriform – svp abstracts 2013

From the abstract
Ezcurra et al. 2013 wrote, Archosauromorphs include all diapsids closer to crocodiles and birds than to lepidosaurs. The group has a very rich Mesozoic and Cenozoic fossil record, but the Paleozoic record is restricted to a handful of Late Permian specimens. The most informative Permian archosauromorph so far discovered is Protorosaurus speneri from the middle Late Permian of Western Europe. In addition, there are several less well known putative archosauromorphs from Russia and Africa. We review these records here and include several of them in a quantitative phylogenetic analysis for the first time. This phylogenetic analysis included a broad taxonomic sampling of basal synapsids, basal diapsids and saurians. We could not find archosauromorph apomorphies in a supposed Late Permian proterosuchid cervical vertebra from South Africa (Bernard Price Institute for Palaeontological Research specimen BP/1/4220), and consider this specimen to belong to an indeterminate amniote. BP/1/4220 possesses striking features that are not present in other amniotes of which we are aware, such as posteriorly extended, wide and almost horizontally oriented accessory processes between the postzygapophyses. A problematic reptile (University Museum of Zoology, Cambridge specimen UMZC T836) from the Late Permian of Tanzania, first described in the 1950s, was recovered in the phylogenetic analysis as a protorosaur at the base of Archosauromorpha and is probably diagnosable as a new species. The position of UMZC T836 within Archosauromorpha is supported by the presence of three well-developed laminae in the cervico-dorsal neural arches and the absence of a humeral entepicondylar foramen. The supposed protorosaur Eorasaurus olsoni from the middle Late Permian of Russia was recovered within Archosauriformes, being more closely related to crown archosaurs than to proterosuchids, implying that this species may be the oldest known archosauriform. However, the fragmentary nature of the known material of this taxon and the low character support for this position means that this identification is currently tentative. Archosaurus rossicus from the latest Permian of Russia was found to be more closely related to Proterosuchus fergusi than to other archosauromorphs and represents a valid species. The revision conducted here suggests a minimum fossil calibration date for the crocodile-lizard split of 254.7 Ma. The occurrences of Protorosaurus speneri close to the paleo-Equator and UMZC T836 in high paleolatitudes of southern Pangea imply a wider paleobiogeographic distribution for archosauromorphs during the Late Permian than previously appreciated.”

Notes
If the Archosauromorpha is indeed all diapsids closer to birds and crocs than to lizards, then the large reptile tree, as discussed earlier, divides all reptiles into the new Archosauromorpha and the new Lepidosauromorpha, both of which contain diapsid-grade reptiles as subclasses. So, there’s a flip-flop here between overarching clades and subclades. That means the lepido-archo split occurred before the advent of Westlothiana, the oldest known archosauromorph at 338 mya in the Visean, Mississippian (early Carboniferous), itself a taxon derived from a sister to the most primitive archosauromorphs, Brouffia and Casineria.

Archosauriforms, Younginids and Protorosaurs
What Ezcurra et al. are really looking at are basal Archosauriformes, as mentioned in their abstract, but in the large reptile tree this clade is derived from younginids, which in turn are derived from protorosaurs and thadeosaurs. Youngina BPI 3859 and Thadeosaurus from the Late Permian 260 mya were coeval with Protorosaurus, so the last common ancestor of these three predates them, probably back to 280-300 mya. Like Garjainia and Proterosuchus, Archosaurus is also derived from Youngina. At about this time the major split between aquatic and terrestrial archosauromorphs took place as derived mesosaurs appear at 290 mya.

Lepidosaurs also appear in the Late Permian 255 mya in the form of Lacertulus coeval with Paliguana, the most primitive Lepidosauriform… no relation to the oldest archosauriforms.

References
Ezcurra MD, Butler R and Scheyer T 2013. The Permian archosaurmorph record revisited: a new species from Tanzania and the potentially oldest archosauriform. Journal of Vertebrate Paleontology abstracts 2013.

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.

Erythrosuchids are GIANT Younginids!!

Sure Proterosuchus (Fig. 2) nests at the base of the Archosauriformes, but it had already strongly evolved into the long, low, hook-jawed morphology of other proterosuchids following the path set out by younginids like this one.

Less derived and very much more like the even more basal, Youngina is the a basal erythrosuchid, Garjainia. A comparison is warranted, has been overlooked, and is long overdue.

Figure 1. Garjainia is just a giant Youngina, as shown here. Arrow points to Youngina to scale with Garjainia, a basal erythrosuchids.

Figure 1. Garjainia is just a giant Youngina, as shown here. Arrow points to Youngina to scale with Garjainia, a basal erythrosuchid. We’re bypassing Proterosuchus by the way.

Garjainia is just a giant Youngina.
And size makes all the difference. With that large, tall skull, small pectoral girdle, separate pubis and ischium and relatively short tail, Garjainia shares several traits with Youngina (Fig. 1) that are not present in Proterosuchus (Fig. 2), a more basal archosauriform that took a different, more amphibious path.

The large reptile tree recovered Youngina basal to Prolacerta and Proterosuchus.

Figure 2. The large reptile tree recovered Youngina basal to Prolacerta and Proterosuchus. Other Youngina nested between Prolacerta and Proterosuchus. Still others nested closer to choristoderes.

Skull
In Garjainia the skull is relatively larger. The naris and rostral profile are raised, the teeth are larger but fewer, the maxilla is deeper and convex ventrally, the orbit is smaller, the antorbital fenestra included a fossa dorsal to the fenestra. The lateral temporal fenestra was invaded posteriorly by the squamosal and quadratojugal. Whereas sweet gentle Youngina was a sweet little insect-eater that occasionally chomped on small vertebrates, Garjainia was a fearless giant (for its time) killer and everything was on its menu.

Neck, torso and tail
To carry such a large skull the cervicals were larger and taller. The chest was deeper and the neural spines were taller to anchor larger back muscles. The tail was relatively shorter with shorter, taller vertebrae. It was also more heavily muscled.

Pectoral girdle and fore limbs
The pectoral girdle was little changed but the humerus and forearm were much more robust in Garjainia, again to carry the weight and move rapidly when necessary.

Pelvic girdle and hind limbs
The ilium and ischium both extended posteriorly. The pubis lost virtually all connection to the ischium and became transversely wider rather than anteroposteriorly long, creating the basic archosauriform pubis that so many of them retained. The hind limbs were more robust, of course.

Youngina is NOT ancestral to Lepidosauromorpha!
Wikipedia promotes a completely misguided (yet traditional) family tree of the Reptilia that nests Youngina and Thadeosaurus then Claudiosaurus as basal to Archosauromorpha + Lepidosauromorpha. Nothing could be further from the truth! This comes from a misguided and traditional hypothesis that basal lepidosauromorphs had a complete lower temporal bar, as in Sphenodon. This is wrong, as we learned earlier. This mistake also comes from not having a large gamut reptile tree that documents where every one of 344 taxa nest in complete resolution, rather than cherry picking a few favorite taxa to test.

Nesbitt 2011
Nesbitt 2011, for reasons unknown, put the proto-rhynchosaur Mesosuchus beyond the base of the Archosauriformes, rather than Youngina (taxon exclusion). He employed the very derived Erythrosuchus in his tree, rather than the more primitive Garjainia. For good measure he threw in the thalattosaur, Vancleavea. We talked about that earlier here, here and here.

Figure 3. Sister taxa according to Nesbitt 2011. Erythrosuchus, Vancleavea and Tropidosuchus.

Figure 3. Sister taxa according to Nesbitt 2011. Erythrosuchus, Vancleavea and Tropidosuchus.

Sorry that traditional paleontologists have messed things up so badly. Trying to fix it one step at a time.