Gracilisuchus 2020: Two new specimens added to the genus

Lucuona, Desojo and Cerda 2020 bring us new information
on Gracilisuchus (Figs. 1, 3), a basal bipedal crocodylomorph in the large reptile tree (LRT, 1734+ taxa; subset Fig. 2). Gracilisuchus was originally considered an ornithosuchid by Romer (1972). Others considered it to nest between Parasuchus and Stagonolepis (Benton and Clark 1988), as the sister to Postosuchus (Juul 1994) or as a sister to Postosuchus and Erpetosuchus (Benton and Walker 2002). All suffered from taxon exclusion.

Figure 4. The PVL 4597 specimen nests at the base of the Archosauria, not with Gracilisuchus.

Figure 1. The PVL 4597 specimen (above)  nests at the base of the Archosauria, not with Gracilisuchus (below). Gracilisuchus based on holotype  PULSR8 with the skull diagram (MCZ4117)  based on Romer 1971. See figure 3 for an update on that skull.

Gracilisuchus stipanicicorum
(Romer 1972; Butler et al. 2014; Ladinian, Middle Triassic, ~230 mya, 30 cm long; holotype PULSR8) is a basal crocodilomorph. In the LRT Gracilisuchus was derived from a sister to Dibothrosuchus (Fig. 5), and preceded both Saltopus and Scleromochlus in the LRT. These three taxa are not mentioned in the Lucuona et al. text.

Taxon exclusion
is the major and continuing problem in vertebrate paleontology. The LRT is trying to repair that problem simply by adding taxa.

From the Lucuona et al. 2020 abstract:
“Gracilisuchus stipanicicorum Romer, 1972 is a basal suchian from the Late Triassic Chañares Formation (Argentina), nested in the recently erected Gracilisuchidae, along with Turfanosuchus dabanensis Young, 1973 and Yonghesuchus sangbiensis Wu et al., 2001 from China.”

Adding taxa separates these three genera in the LRT (subset Fig. 2) and invalidates any clade with only these three polyphyletic members. These three taxa and all their descendants do form an unnamed clade: Poposauria + Archosauria in the LRT, which was not their intention, nor that of Butler et al. 2014, who erected the clade with these three members with the same criticism about 5 years ago.

Figure 2. Subset of the LRT focusing on Crocodylomorpha. Matching Nesbitt et al. 2005, the LRT nests Redondavenator near the base of the Crocodylomorpha.

Figure 2. Subset of the LRT focusing on Crocodylomorpha. Matching Nesbitt et al. 2005, the LRT nests Redondavenator near the base of the Crocodylomorpha.

Continuing from the abstract:
“The six known specimens of Gracilisuchus Romer, 1972 preserve most of the skeleton, lacking only most of the shoulder girdle and forelimb. Our latest fieldwork has recovered two specimens that preserve previously unknown elements, including the humerus, radius, and ulna, as well as the femur, presacral vertebrae, and paramedian osteoderms.”

By contrast the LRT (Subset Fig. 2) separates the PVL 4597 specimen from the PULSR8 holotype. The former nests at the base of the Archosauria (Crocodylomorpha + Dinosauria) at least two nodes away at present.

By combining two or more specimens
Lecuona et al. 2020 created an unwanted and confusing chimaera. A better practice is to score each specimen individually and let those that nest together do so. THEN create a chimaera if warranted.

Figure 5. Gracilisuchus skull updated with new colors.

Figure 3. Gracilisuchus skull (MCZ 4117) updated. Note the slender fenestra between the premaxilla and maxilla, as in Dibothrosuchus.

The traditional short-faced Gracilisuchus specimen
MCZ 4116 (Fig. 4), was not mentioned in the Lecuona et al. 2020 text. Perhaps that’s a good thing since earlier the LRT nested the short-faced specimen with Trialestes.

Figure 1. The former Gracilisuchus specimens MCZ4116 and MCZ4118 with colors added.

Figure 4. The former Gracilisuchus specimens MCZ4116 and MCZ4118 with colors added.

Lecuona et al. consider Gracilisuchus to be a member
of the invalid clade, ‘Pseudosuchia’.

Bone histology studies
were performed on the two new specimens:

  1. CRILAR PV 480, “one and a half incomplete cervical centra articulated with each other and with two incomplete ribs, a series of three incomplete vertebrae articulated with one rib, dorsally in contact with the left row of the paramedian osteoderms, and posteriorly four ribs with no articulating vertebrae but in anatomical position (Fig. 2A, B), a series of six incomplete cervicodorsal vertebrae with some of their ribs preserved and half of a centrum attached posteriorly (Fig. 2C, D), and moulds of two short fragments of paramedian osteoderms”
  2. CRILAR PV 490, “two articulated cervical vertebra in contact with a short paramedian osteoderm series, one isolated dorsal vertebrae, left humerus, right ulna, right radius, left femur, and six histological sections of the femoral diaphysis and osteoderms”

Until phylogenetic analysis is performed on each specimen,
we can’t be sure that these are indeed Gracilisuchus specimens.

Figure 2. Images from Wu et al. 1993, colors and hind limbs added. Compare to skull in figure 1.

Figure 5. Images from Wu et al. 1993, colors and hind limbs added. Compare to skull in figure 1.

Until Lucuona et al. 2020 add pertinent taxa 
they will not understand the phylogenetic context of the holotype specimen, the referred specimens and all pertinent, but unrelated taxa. That means whatever they have to say about the new fossils has to be considered with some reservation. They think all this material belongs to Gracilisuchus. The LRT demonstrates at least one specimen nests apart from the holotype. Better to know with validation, than to guess.


References
Benton MJ and Clark JM 1988. Archosaur phylogeny and the relationships of the Crocodilia in MJ Benton (ed.), The Phylogeny and Classification of the Tetrapods 1: 295-338. Oxford, The Systematics Association.
Brinkman D 1981. The origin of the crocodiloid tarsi and the interrelationships of thecodontian archosaurs. Breviora 464: 1–23.
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.
Juul L 1994. The phylogeny of basal archosaurs. Palaeontographica africana 1994: 1-38.
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.
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 XX:1–40.
Lecuona A, Desojo JB and Cerda IA 2020. New information on the anatomy and histology of Gracilisuchus stipanicicorum (Archosauria: Pseudosuchia) from the Chañares Formation (early Carnian), Argentina. Comptes Rendus Palevol 19 (3): 40-62. https://doi.org/10.5852/cr-palevol2020v19a3
Parrish JM 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. Journal of Vertebrate Paleontology 13(3):287-308.
Romer AS 1971. The Chañares(Argentina) Triassic reptile fauna. Two new bu incompletely known long-limbed pseudosuchians. Breviora 378:1–10.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.

wiki/Gracilisuchus

Lagosuchus enters the LRT with Saltopus

A small partial skeleton named
Lagosuchus talampayensis 
(Romer 1971; PULR 09; Late Triassic; Fig. 1) was found on the same slab as the more completely known Gracilisuchus (Romer 1971, Fig. 1). Oddly and coincidentally, if not for the few duplicated posterior dorsals and sacrals, the two incomplete specimens could have created one more or less complete, specimen.

Figure 1. Gracilisuchus and its slab mate, Lagosuchus. Note the very few bones both specimens preserve in common are at the base of the dorsal column.

Figure 1. Gracilisuchus and its slab mate, Lagosuchus. Note the very few bones both specimens preserve in common are at the base of the dorsal column.

Back in Romer’s time in the 1970s,
Lagosuchus was considered congeneric with a second, more complete specimen before the two were separated by Sereno and Arcucci 1994. The later, more complete one was renamed Marasuchus lilloensis (Fig. 2). The two authors considered the less complete holotype of Lagosuchus to be undiagnosable.

Figure 2. Lagosuchus to scale with Marasuchus. Several proportions are different between the two.

Figure 2. Lagosuchus to scale with Marasuchus. Several proportions are different between the two.

According to Wikipedia
“Lagosuchus talampayensis is a genus of small avemetatarsalian archosaur from the Middle to Late Triassic period. It is generally thought to be closely related to dinosaurs, as a member of the Dinosauromorpha. Its fossils were found in the Chañares Formation of Argentina, the dating of which is uncertain; some sources date it to the Middle Triassic whilst others date it to the earliest Carnian.”

Just this year,
Agnolin and Ezcurra 2019 determined that Lagosuchus was indistinguishable from Marasuchus, returning to Romer’s original hypothesis of relationships.

Added to
the large reptile tree (LRT, 1623+ taxa) the nesting of Lagosuchus took place in three stages.

Stage one:
Testing all Archosauriformes (sans birds). Result: Due to the large number of skull-only and post-crania-only taxa, there was loss of resolution in the clade that recovered Lagosuchus: basal bipedal Crocodylomorpha. The rest of the tree topology was unaffected. Marasuchus, for instance, remained within the clade of least popular (most often overlooked) basal theropods.

Stage two: 
Further deletions, saving only Crocodylomorpha, Poposauria and basal Theropoda (including Marasuchus). Result: resolution was not improved in basal Crocodylomorpha due to the continued mixing of skull-only and skull-less taxa  In hindsight, this stage could have been skipped, but since every included taxon affects every other included taxon, this stage was deemed necessary.

Figure 4. Lagosuchus compared to Saltopus.

Figure 4. Lagosuchus compared to Saltopus. The lack of dorsal armor here links these two together. The larger number of laterally wider sacrals in Saltopus sets the apart.

Stage three:
Deletion of two skull-only basal crocodylomorph taxa: Yonghesuchus and the MCZ4116 specimen attributed to Gracilisuchus. Result: skull-less Lagosuchus nested with another skull-less crocodylomorph, Saltopus (Fig. 3), a slightly later taxon from Europe apparently overlooked in all prior Lagosuchus studies.

By the way,
the nesting of skull-only Yonghesuchus near the two skull-less taxa provides clues to the likely morphology of the missing skulls in Lagosuchus and Saltopus… and indicates a world-wide distribution for members of this clade.

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

Distinct from Saltopus,
Lagosuchus
 has only two laterally unexpanded sacrals and a shorter antebrachium.

Distinct from other related taxa,
the Lagosuchus and Saltopus share a lack of gastralia, a lack of dorsal scutes, a fibula greater than half the diameter of the tibia, a mesotarsal ankle joint without a calcanaeal heel and long feet (greater than half the tibia length) without compressed metatarsals. These relatively common traits are easy to overlook because they are found by convergence elsewhere, but are relatively rare at the base of the Crocodylomorpha. Everything comes back to a phylogenetic context. The LRT still works despite rampant convergence in the Tetrapoda.

After googling ‘Saltopusuchus, Lagosuchus
I would have been pleased to cite Rauhut and Hungerbühler 2000 who thought these two were closely related 20 years ago, according to the Saltopus article in Wikipedia. However, shortly after writing this line of text, I downloaded and read their paper. Rauhut and Hungerbühler report, “Characters like elongated hindlimbs, bipedality, mainly three digits used in locomotion, and cursorial habits are also present in primitive dinosauriformes like Marasuchus SERENO & ARCUCCI, 1994. Therefore, S. elginensis can only be treated as a probable dinosauriform nomen dubium.” So, they provided no analysis and they cited the wrong specimen. The holotype of Lagosuchus was not mentioned in the paper. I wonder why the Wiki writer got it wrong?

Should we give credit
to the mistaken writer of the wiki/Saltopus article? Do happy accidents count? If you know of a valid previous citation that links the holotype of Lagosuchus with Saltopus, please let me know so I can advertise that citation.

Otherwise,
the linking of Lagosuchus with Saltopus appears to be a novel hypothesis of interrelationships. This could be due to the fact that the basal bipedal crocs have been largely ignored as a clade or that both specimens have been traditionally held in low regard. Then again, the relationships of basal bipedal crocs to the clade Dinosauria has been overlooked and ignored (due to taxon exclusion) by other workers. Finally, too often, basal bipedal crocs in the LRT, like Gracilisuchus and Scleromochlus, have not been recognized as basal bipedal crocs by other workers. So, this turned out to be another case of a perfect storm meeting low hanging fruit.


References
Agnolin FL and  Ezcurra MD 2019.The validity of Lagosuchus talampayensis Romer, 1971 (Archosauria, Dinosauriformes), from the Late Triassic of Argentina. Breviora. 565 (1): 1–21.
Rauhut OMW and Hungerbühler A 2000. A review of European Triassic theropods. Gaia, 15: 75-88.
Romer AS 1971.
The Chañares (Argentina) Triassic reptile fauna. X. Two new but incompletely known long-limbed pseudosuchians. Breviora. 378: 1–10.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. XV. Further remains of the thecodonts Lagerpeton and Lagosuchus. Breviora. 394: 1–7.
Sereno PC and Arcucci AB 1994. Dinosaurian precursors from the Middle Triassic of Argentina: Marasuchus lilloensis, gen. nov. Journal of Vertebrate Paleontology. 14 (1): 53–73.

wiki/Saltopus
wiki/Lagosuchus

The skull of PVL 4597 joins its post-crania in the LRT

The PhD thesis of Agustina Lecuona 2013
on the several specimens attributed to the Middle Triassic Gracilisuchus (Fig. 2) is online (PDF). It includes the previously unpublished skull of PVL 4597 (Figs. 1, 2), which the large reptile tree (LRT, 1592 taxa) nests apart from Gracilisuchus, as the last common ancestor of all archosaurs (crocs + dinos only) with or without its skull. We reviewed Gracilisuchus yesterday, so this addition to the LRT is timely.

Figure 1. The skull of PVL 4597 in several views from the 2013 PhD thesis of A. Leucona. Colors added.

Figure 1. The skull of PVL 4597 in several views from the 2013 PhD thesis of A. Lecuona. Colors added.

The differences between PVL 4597 and Gracilisuchus are few (Fig. 2).
So, it is not a surprise that Lecuona considered them congeneric.

However,
the differences are fewer between PVL 4597 and its ancestor, Turfanosuchus (Fig. 4), and  its descendant, Herrerasaurus, the last common ancestor of dinosaurs traditionally and in the LRT. 19 additional steps are added when PVL 4597 is forced to nest with Gracilisuchus in the LRT.

FIgure 2. Comparing PVL 4597 to Gracilisuchus. Despite their many similarities, these two do not nest together in the LRT.

FIgure 2. Comparing PVL 4597 to Gracilisuchus. Despite their many similarities, these two do not nest together in the LRT. Taxon exclusion is the issue with the PhD dissertation and the use of an invalidated analysis from Nesbitt 2011.

Basal members of large clades
are sisters to basal members of sister clades (Fig. 3). We compare those taxa with one another, ignoring the more derived members.

Figure 3. Subset of the LRT focusing on basal archosaurs and their immediate ancestors.

Figure 3. Subset of the LRT focusing on basal archosaurs and their immediate ancestors.

Here (Fig. 4) are the skulls of
Turfanosuchus and Herrerasaurus, taxa closer to PVL 4597 than PVL 4597 is to Gracilisuchus is in the LRT. The long-awaited skull confirms the nesting of the post-crania.

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

Figure 4. Skull of Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.

Without PVL 4597, 
the LRT still nests Turfanosuchus and basal bipedal crocs close to the base of the Dinosauria, contra the results of other studies that generally do not include those taxa.

Unfortunately,
Lecuona’s PhD thesis employed a borrowed and flawed cladogram on which she mistakenly trusted in: Nesbitt 2011. Even though Lecuona’s revised cladogram includes the basal bipedal crocs (which nest at derived nodes in her thesis), earlier we dismantled Nesbitt 2011 in a 7-part series ending here. Rescored Nesbitt 2011 resembles the LRT.


References
Lecuona A 2013. Anatomía y relaciones filogenéticas de Gracilisuchus stipanicorum y sus implicancias en el origen de Crocodylomorpha. PhD thesis. PDF
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.

wiki/Gracilisuchus

Gracilisuchus revisited and tweaked

Revised October 30, 2019 and November 4, 2019
as I revise the holotype specimen by deleting the limbs. The nesting does not change.

The basal bipedal crocodylomorph,
Gracilisuchus (Figs. 1-3), was one of the first taxa in the large reptile tree (LRT, 1592 taxa), back when there were about 260? taxa. At the time I used Romer’s 1972 reconstruction (Fig. 5) to score data points. That turned out to be a freshman mistake. Romer filled in the missing taxa with the so-called Tucuman specimen, PVL4597, but kept the hind limbs and feet from the holotype slab. The hips and tail are not preserved in the holotype specimen PULR8 (Fig. 1).

Figure 1. Gracilisuchus revised with new, slightly longer legs  and more precise feet after tracing the holotype. Romer added a pelvis and tail that are erased here.

Figure 1.  Gracilisuchus revised with the subtraction of limbs and tail.  Romer added limbs and a tail that are erased here.

 

 

A larger cladogram problem.
Lecuona, Desojo and Pol 2017 report, “The phylogenetic relationships of G. stipanicicorum were evaluated based on an extensive phylogenetic analysis of Archosauriformes expanding a previous dataset (Nesbitt, 2011) in terms of both taxon and character sampling.”  As we learned earlier in a 7-part series, the analysis by Nesbitt 2011 and all those that followed are flawed throughout. Those analyses include unrelated taxa and exclude pertinent taxa. When scores are corrected or filled in where appropriate the resulting topology closely matches the LRT.

Here’s a good idea:
Don’t trust ANY previously published cladograms. Build your own.

This revision gave me an opportunity to update the text
on the Gracilisuchus page. That was needed due to the large number of additional taxa added near Gracilisuchus over the last 8-9 years. With the loss of limbs data, the current nestings of Gracilisuchus and a headless taxon wrongly attributed (Lecuona and Desojo 2011) to Gracilisuchus, PVL4597, did not change with these corrections. Just goes to show, it’s the taxon list, not the character list, that is key to understanding hypothetical interrelationships.

Basal Crocodylomorpha

Figure 5. Basal Crocodylomorpha, including Gracilisuchus, Saltopus, Scleromochlus and Terrestrisuchus. That’s the old Gracilisuchus pictured here, with tail and hips.

Thanks to
reader Neil P for bringing Gracilisuchus back to my attention.


References
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.
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.
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 XX:1–40.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.

wiki/Gracilisuchus

Return of the short-face Gracilisuchus MCZ 4116

Earlier we looked at the MCZ 4116 specimen attributed to Gracilisuchus (Fig. 1).

Figure 1. MCZ 4116 a short-faced Gracilisuchus compared to the holotype with a longer face.

Figure 1. MCZ 4116 a short-faced Gracilisuchus compared to the holotype with a longer face. These two nest as sister taxa at the base of the Archosauria.

 

Gracilisuchus (Romer 1972) 
nests at the base of the Archosauria in the large reptile tree. Scleromochlus and Saltopus are sister taxa. So are these short-faced specimens (Fig. 1), MCZ 4116 and 4117 (Brinkman 1981). That short rostrum looks juvenile, but note these specimens are not smaller than the holotype (Fig.1). Romer and Parrish (year?) restored the snout tip with a Gracilisuchus-like big round nasal and a very short, transverse premaxilla. As an option, I just followed existing contours and added a premaxilla similar in length to the holotype.

Could this be a juvenile of a much larger adult?

Figure 1. Gracilisuchus revised with new, slightly longer legs  and more precise feet after tracing the holotype. Romer added a pelvis and tail that are erased here.

Figure 1. Gracilisuchus revised with the subtraction of limbs and tail.  Romer added limbs and a tail that are erased here.

References
Brinkman D 1981. The origin of the crocodiloid tarsi and the interrelationships of thecodontian archosaurs. Breviora 464: 1–23.
Romer AS 1972. 
The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.

Another Gracilisuchus (from Brinkman 1981)

Asterisks* note possible changes in interpretation made during this addendum.

It’s time to add another Gracilisuchus to the data matrix

This one (MCZ4116) has been known since Brinkman (1981) and is deemed worthy of inclusion due to its phylogenetic placement (between Gracilisuchus and Scleromochlus) and distinct morphology. You might remember earlier we looked at the hind limb specimen (PVL 4597) described by Lecuona and Desojo 2011 and found it nested a few nodes away from the holotype and therefore could not be congeneric.

The second Gracilisuchus described (Brinkman 1981) with a more juvenile looking skull despite sharing a similar size with the holotype (fig. 2).

Figure 1. The second Gracilisuchus described (Brinkman 1981) with a more juvenile looking skull despite sharing a similar size with the holotype (fig. 2). It nests between Gracilisuchus and Scleromochlus. It is not congeneric, but certainly a sister to Gracilisuchus.

Basal crocodylomorphs and basal archosaurs
This taxon is important in that it demonstrates* the vertical quadrate atypical for crocs and more typical for dinos. It nests near the base of the Archosauria. It would have had a robust neck pretty much like that of Gracilisuchus (fig. 2) and tiny Scleromochlus, it’s other sister.

Gracilisuchus

Figure 2. Gracilisuchus (Romer 1972). Click for more data.

What do the larger orbit and taller skull tell us?
This specimen had even larger eyes*. The rounder rostral profile* reminds one of the basal dinosaur/phytosaur/maybe ornithischian, Daemonosaurus, which was heading toward herbivory. Not sure if the same can be said here in the case of the Brinkman specimen or not.

Addendum figure 1. Gracilisuchus. The one Romer figured.

Addendum figure 1. Gracilisuchus. The one Romer figured.

The flatter skull is a trait shared by Gracilisuchus and Scleromochlus to the exclusion of this Brinkman (1981) specimen, which indicates it is an offshoot* without any other short-rostrum relatives currently known.

Skull of Graciliisuchus according

Addendum figure 2. Skull of Graciliisuchus according to Parrish (1993), very much in the shape that Romer (1972) illustrated.

Reader M. Mortimer suggested that, according to Brinkman 1981, the skull of Gracilisuchus was not like the one illustrated by Romer, but actually closer to MCZ 4118. To counter that, Parrish 1993 reported that Gracilisuchus was represented by at least six well-preserved specimens and also illustrated one very much in the shape that Romer (1972) proposed (Addendum figures 1 and 2). 

Further addenda:

A rather complete Gracilisuchus skull, courtesy of M. Parrish.

Addendum figure: A rather complete Gracilisuchus skull, courtesy of M. Parrish. I have dodged the darkest areas in Photoshop to bring out detail.

Another Gracilisuchus,

Addendum figure 2. A taller snouted Gracilisuchus, not MC 4118 by its appearance. Photo by M. Parrish. Are the differences between these two skulls gender-based? Reconstruction based? Taphonomic? Or individual variation?

Two skulls illustrated by Brinkman 1981. Note both are partials. Below a photo of a complete skull. Above MCZ 4116 restored by reversing the taphonomic flattening/rotation to the parasagittal plane of the postorbital and preorbital areas. The lacrimal appears splint almost longwise. If so, sliding the pieces back together also reunites the front of the skull parts that were separated after death. All these changes reduce the apparent skull height.

Addendum image. Two skulls illustrated by Brinkman 1981. Note both are partials. Below a photo of a complete skull. Above MCZ 4116 restored by reversing the taphonomic flattening/rotation to the parasagittal plane of the postorbital and preorbital areas. The lacrimal appears splint almost longwise. If so, sliding the pieces back together also reunites the front of the skull parts that were separated after death. All these changes reduce the apparent skull height. Also note the position of the quadrate, rotate to the vertical, posterior side exposed, on MCZ 4117.

* So, most if not all differences in these skulls may be due to the influence of taphonomy, rotating cranial elements to the parasagittal plane and splitting other bones. There may still be differences, perhaps due to individual variation, etc. 

References
Brinkman D 1981. The origin of the crocodiloid tarsi and the interrelationships of thecodontian archosaurs. Breviora 464: 1–23.
Lecuona A and Desojo J B 2011. Hind limb osteology of Gracilisuchus stipanicicorum (Archosauria: Pseudosuchia). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 102 (2):105-128.
Parris JM 1993. Phylogeny of the Crocodylotarsi with reference to Archosaurian and Crurotarsan monophyly. Society of Vertebrate Paleontology 13(3): 287-308.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.

wiki/Gracilisuchus