Daemonosaurus has two sister taxa: Haya and Jeholosaurus

Earlier we talked about the Late Triassic saber-toothed dinosaur, Daemonosaurus (Sues et al. 2011, Fig. 1, CM 76821) originally considered a weird basal theropod between Eoraptor and Tawa. That’s due to taxon exclusion. The real sisters of Daemonosaurus were not tested. The authors also mistakenly nested Eoraptor within the Theropoda when it is actually an outgroup, a phytodinosaur closer to Sauropodomorpha, which were only included as a suprageneric taxon, along with Ornithischia. Unfortunately that’s the same suprageneric/taxon exclusion/inclusion problem that happens so often it’s not funny anymore.

Figure 1. Daemonosaurus, Jeholosaurus and Haya nest as sister taxa. And it's easy to see why.

Figure 1. Daemonosaurus, Jeholosaurus and Haya to scale nest as sister taxa. And it’s easy to see why. Somewhere in this clade lies the origin of the predentary bone and the retroverted pubis. Long premaxillary teeth and a short rostrum are key traits. Note the infilling of the mandibular fenestra.

Solution: add taxa and avoid suprageneric taxa
In the large reptile tree Daemonosaurus did not nest with theropods, but at the base of the Ornithischia between basal phytodinosaurs like Eoraptor and the basal ornithischian, Pisanosaurus. It’s been three years since that post.

Today Jeholosaurus (Han et al 2012) and Haya (Figs. 1,2), two widely acknowledged basal ornithischians, nest with Daemonosaurus. One look at the three of them together pretty much sums up the rest of this post. Note their chronology. This is a basal clade that lasted through all three periods of the Mesozoic.

That they nest together tells me the post-crania of Daemonosaurus likely had at least a proto-ornithischian pelvis and supports my earlier observation of a proto-predentary.

Figure 1. Haya skull and post-crania.

Figure 1. Haya skull and post-crania. At present this specimen gives us the best approximation of the post-crania of Daemonosaurus, although the neck vertebrate were longer. Note the stub of a fifth toe on the pes.

Fossils of Coelophysis were present on the same block that contained the skull of Daemonosaurus, Wonder if there was a predator/prey relationship? Skull lengths were similar. Overall size was likely similar too.

References
Han F-L, Barrett PM, Butler RJ and Xu X 2012. Postcranial anatomy of Jeholosaurus shangyuanensis (Dinosauria, Ornithischia) from the Lower Cretaceous Yixian Formation of China. Journal of Vertebrate Paleontology 32:1370-1395.
Makovicky PJ, Kilbourne BM, Sadleir RW and Norell MA 2011. A new basal ornithopod (Dinosauria, Ornithischia) from the Late Cretaceous of Mongolia. Journal of Vertebrate Paleontology 31: 626–640.
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 
Xu, Wang and You, 2000. A primitive ornithopod from the Early Cretaceous Yixian Formation of Liaoning. Vertebrata PalAsiatica 38(4)318-325.

wiki/Daemonosaurus
wiki/Haya
wiki/Jeholosaurus

Spinosaurus the paradigm buster

Figure 1. Spinosaurus from Ibrahim 2014. Yes, the proportions are correct. It's a non bipedal swimming theropod.

Figure 1. Spinosaurus from Ibrahim 2014. Yes, the proportions are correct. It’s a non bipedal swimming theropod. The red bones are known. The gray ones are hypothetical.

Generally I avoid all but the most basal dinosaurs.
However, just a few posts ago we looked at Spinosaurus and Arizonasaurus, two archosauriformes with a similar dorsal sail.  Well, it seems the proportions of Spinosaurus were a little off in the hind leg department. And that makes the new data fascinating.

And POP there goes a paradigm
The Spinosaurus tale has been told by Nat Geo, Science, Nature, and other places. I found the reaction to this heretical pile of facts just as fascinating. There was shock. And there were skeptics! (something I’ve grown accustomed to from the first vampire pterosaur abstract onward).

Skeptics are good. But facts are facts.

From the Abstract
We describe adaptations for a semiaquatic lifestyle in the dinosaur Spinosaurus aegyptiacus. These adaptations include retraction of the fleshy nostrils to a position near the mid-region of the skull and an elongate neck and trunk that shift the center of body mass anterior to the knee joint. Unlike terrestrial theropods, the pelvic girdle is downsized, the hind limbs are short, and all of the limb bones are solid without an open medullary cavity, for buoyancy control in water. The short, robust femur with hypertrophied flexor attachment and the low, flat-bottomed pedal claws are consistent with aquatic foot-propelled locomotion. Surface striations and bone microstructure suggest that the dorsal “sail” may have been enveloped in skin that functioned primarily for display on land and in water.

From the Dinosaur Mailing List
There’s something fishy about the new Spinosaurus
the pelvis and hind limbs are too small.”

Note sure if these questions have been answered
1. Did Spinosaurus knuckle-walk to protect its fore claws? After all, it had to come out of the water, at least to lay its eggs. Perhaps it ventured out only to mud and sand bars as crocs do.
2. Other than display, was the sail used for thermoregulation? Keeping it dry in a hot sun would have allowed it to soak in heat. Wetting it would have cooled the sail by evaporative heat loss.
3. Was the sail used in transportation? If there was a current in the water, the orientation of the sail to the current could have been aligned for minimum impact or at right angles for maximum impact.

When we have missing parts, it is usually okay to fill them in with parts gleaned from sister taxa — until the actual parts become found. Then we have to give up our cherished paradigms and let the facts speak for themselves.

References
Ibrahim N, Sereno PC, Dal Sasso C, Maganuco S, Fabbri M, Martill DM, Zouhri S, Myhrvold N, Iurino DA 2014. Semiaquatic adaptations in a giant predatory dinosaur. Science. doi:10.1126/science.1258750.

History of Spinosaurus with old reconstruction of long hind limbs

Paul Sereno on YouTube

Arizonasaurus vs Spinosaurus

Two unrelated reptiles
evolved similar morphologies, Arizonasaurus and Spinosaurus (Fig. 1), a long rostrum filled with sharp teeth, a bipedal configuration and enormous neural spine arising from the dorsal vertebrae. One was a giant. The other about waist high. Seen here together for the first time…

Figure 1. Spinosaurus and Arizonsaurus, together for the first time. The similarities are obvious and intriguing.

Figure 1. Spinosaurus and Arizonsaurus, together for the first time. The similarities are obvious and intriguing. Spinosaurus courtesy of Scott Hartmann.

Spinosaurus is a famous giant theropod dinosaur. Arizonasaurus is none of these things. It’s a member of a clade that has no name, but arose from basal rauisuchids, like Venjukovia. It was a sister to Ticinosuchus + Aetosaurs and Yarasuchus + Qianosuchus, none of which have much of a sail back. I thought comparing these two might provide clues to their convergent looks.

Arizonasaurus comes from the Middle Triassic Moenkoepi Formation, which included fresh water and a diverse fauna. Earlier we looked at the possibility that this predator was bipedal, based on the very small pectoral girdle and very deep (for its time) pelvic girdle, almost like that of T-rex, but more gracile. Relatives include fish eaters, like long-necked Yarasuchus and plant eaters, like aetosaurs. So this is already a diverse clade that no doubt will provide many surprising morphologies in the future. Originally described as a prestosuchid rauisuchian, Brusatte et al. (2010) nested it with poposaurs. In the large reptile tree poposaurs nest a little closer to dinosaurs and basal crocs.

Spinosaurus comes from the Middle Cretaceous of northern Africa, which, at the time included tidal flats, mangrove forests and several other giant theropods. Only a few other dinosaurs had such long neural spines. The question is, where they more like sails, and aid in thermoregulation? Or did they support a buffalo-like hump of fat? Spinosaur relatives, all smaller, did not sport much of a sail back. So whatever its utility was, it was unique.

Sail backs seem to spring up occasionally and quickly around the reptile family tree. They never seem to last.

Moving on
to those long jaws, Spinosaurus was considered a quick-strike artist, feeding on everything from fish to small dinosaurs, but with that size it could have taken on any prey. No such claims have been made for Arizonasaurus, perhaps because not much of the skull is known. But the teeth were sharp

My take
I have no expertise and no stake in the hump vs. sail argument. Since these sails seem to come and go rather quickly, my opinion is they are literally a flash in the pan, thus they have no real utility and are only for show… secondary sexual traits. Popular one day, not so popular the next. The blessing probably becomes a curse over time, as the sail gets bigger, so the trait and the animal disappears. The neural spines are broad because they have “roots” that are broad, unlike Dimetrodon and like Sphenacodon.

References
Bailey JB 1997. Neural spine elongation in dinosaurs: sailbacks or buffalo-backs?. Journal of Paleontology 71 (6): 1124–1146.
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. 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.
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
wiki/Ctenosauriscus

New Farlow et al. (2014) Poposaurus foot paper

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Eoraptor Re-Reconstruction

Earlier we took note of the new nesting ofEoraptor as a basal sauropodomorph in Sereno (2013), matching its nesting in the large reptile tree.

The basal phytodinosaur, Eoraptor (Figs. 1, 2), has recently been described and illustrated (Sereno et al. 2013) in its entirety.

Figure 1. Eoraptor as illustrated by Carol Abraczinskas for Sereno et al. 2013.

Figure 1. Eoraptor as illustrated by Carol Abraczinskas for Sereno et al. 2013.

The Abraczinkas illustration (Fig. 1) portrays Eoraptor as a basic theropod, despite its nesting as a basal sauropomorph. I get the impression that the torso was done essentially freehand, especially with regard to the ribs. Fingers 4 and 5 are missing in the fossil because the matrix ends there. Descendant taxa (according to the large reptile tree), like Anchisaurus, Brachiosaurus and Iguanodon have fingers 4 and 5, so Eoraptor probably had them too. They are shown in figure 2 in pink.

Figure 2. Eoraptor based on tracing illustrations in Sereno et al. 2013, including the in situ composite image.

Figure 2. Eoraptor based on tracing illustrations in Sereno et al. 2013, including the in situ composite image. Here the ribs are shorter, fingers 4 and 5 are restored, the dorsal series is less arched, the dorsal ribs are shorter, the pelvis tilts further foreword and rides lower, the crus appears more robust and neural spines are more individualized and not generalized. Ribs are not shown from posterior cervicals. I’m struck by how robust the forelimbs are.

Several other differences in the new reconstruction more accurately reflect the in situ fossil, from which it was traced. The back was straighter without the shoulder hump found in figure 1. Other slight changes are listed in the figure two caption. Even so, this early biped appears to have had a carnivorous dentition that perhaps tasted plants occasionally.

Looks like a carnivore, except…
Sereno et al. (2013) report, “The first dentary tooth in Eoraptor, in addition, is retracted from the anterior end of the dentary, which is marked by a pair of conspicuous neurovascular foramina—features that characterize plant-eating basal sauropodomorphs. These features and the short length of the lower jaws suggest that there may have been a small keratinous beak at the anterior end of the lower jaws in Eoraptor and Panphagia. We have yet to discover a carnivorous dinosaur—or for that matter a carnivorous extant lizard—that has retained teeth for predation and that has inset these teeth from the anterior end of the lower or upper jaws (Sereno, 2012). This favors Eoraptor as a herbivore.”

Manual digits 4 and 5
In Eoraptor, Herrerasaurus and other basal dinos metacarpals 4 and 5 are tiny, almost vestigial (Fig. 2) yet in their descendant, Brachiosaurus, all five metacarpals are subequal. This is odd. Fingers and metacarpals usually disappear after they become vestiges, but not this time. Evidently metacarpals 3 and 5 re-elongated to support the weight when sauropod ancestors became quadrupedal.

Figure 3. Plateosaurus hand. Note metacarpal 4 is longer than in Eoraptor, but metacarpal 5 is not.

Figure 3. Plateosaurus hand. Note metacarpal 4 is longer than in Eoraptor, but metacarpal 5 is not.

In the prosauropod, Plateosaurus (Fig. 3), the hand is quite similar to that of Eoraptor. Metacarpal 4 is  about 3/4 the size of metacarpal 3 and three phalanges are present. Metacarpal 5 remains a vestige with a single phalanx. In the basal sauropod, Shunosaurus the lateral metacarpals are more nearly alike.  Somewhere between these two taxa, we find the origin of sauropods with longer lateral digits.

Wikipedia reports, 
“Evidence against sauropod ancestry within Prosauropoda comes from the fact that prosauropods had a smaller outer toe on their hind feet than the sauropods. Many maintain that it is easier for digits to be reduced or lost during evolution than the reverse, however there is no evidence for this. The lengthening, or gaining of extra digits is common in marine reptiles, and within the theropods digit lengthening occurred at least once. Therefore, using this as evidence against ancestral prosauropods is questionable.”

References
Sereno PC, Martînez RN and Alcober OA 2013. Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha). Society of Vertebrate Paleontology Memoir 12, 32 (Supp. to #6):83-179.

Eoraptor Confirmed as Basal Phytodinosaur

Figure 2. Eoraptor based on tracing illustrations in Sereno et al. 2013, including the in situ composite image.

Figure 1. Eoraptor based on tracing illustrations in Sereno et al. 2013, including the in situ composite image.

Abstract - We (Sereno et al. 2013) describe the basal sauropodomorph Eoraptor lunensis, based on the nearly complete holotypic skeleton and referred specimens, all of which were discovered in the Cancha de Bochas Member of the Ischigualasto Formation in northwestern Argentina. The lightly built skull has a slightly enlarged external naris and a spacious antorbital fossa with a prominent, everted dorsal margin and internal wall lacking any pneumatic extensions into surrounding bones. The tall quadrate is lapped along its anterior margin by the long, slender ventral process of the squamosal, and the lower jaw has a mid-mandibular joint between a tongue-shaped splenial process and a trough in the angular. All but the posterior-most maxillary and dentary crowns have a basal constriction, and the marginal denticles are larger and oriented more vertically than in typical theropod serrations. Rows of rudimentary palatal teeth are present on the pterygoid. Vertebral centra are hollow, although not demonstrably pneumatized,and all long bones have hollow shafts. The radius and ulna are more robust, the manus proportionately shorter, and the manual unguals less recurved than in the contemporaneous basal theropod Eodromaeus murphi. An outstanding feature of the manus of Eoraptor is the twisted shaft of the first phalanx of the pollex, which deflects medially the tip of the ungual as in basal sauropodomorphs. The long bones of the hind limb have more robust shafts than those of Eodromaeus, although in both genera the tibia remains slightly longer than the femur.

From the text - Eoraptor lunensis was placed by Sereno et al. (1993) and Sereno (1999) as the basal member of Theropoda on the basis of phylogenetic analyses that identified synapomorphies uniting Eoraptor with Herrerasaurus and other theropods.

An opposing camp emerged with the view that Eoraptor was a more basal saurischian, outside both Theropoda and Sauropodomorpha (Langer, 2004; Mart´ınez and Alcober, 2009; Brusatte et al., 2010; Langer et al., 2010).

We now regard Eoraptor as a basal sauropodomorph (Mart´ınez et al., 2011), and there are important events that led us to this new understanding. It was not until excellent remains of this dinosaur were discovered in 1996 and prepared several years later that its distinction from Eoraptor was revealed (Mart´ınez et al., 2011).

Secondly, two key discoveries came to light while working on the holotypic skeleton of Eoraptor for this monograph. We discovered that, prior to its final fossilization, slight disarticulation of digit I in the well-preserved right manus of Eoraptor (Fig. 69) had obscured a remarkable derived feature known only among large bodied basal sauropodomorph dinosaurs (Sereno, 2007b)—the medial rotation in the shaft of proximal phalanx of manual digit I that directs the tip of the ungual inward (Fig. 73D). 

We also realized that the lower jaws of Eoraptor seemed slightly short relative to the upper jaws (Figs. 16, 17) and that the anterior end of the dentaries also had vascular openings (Fig. 23) similar to those of many larger-bodied basal sauropodomorphs thought to have a small keratinous lower bill (Sereno, 2007b; Mart´ınez, 2009). By preparing between the premaxillary teeth, we were able to verify evidence from the computed tomography (CT) data that the first dentary tooth in Eoraptor, as in Panphagia (Mart´ınez and Alcober, 2009), is inset a short distance from the anterior end of the dentary.

Thirdly, the discovery of Panphagia in Ischigualasto (Martínez and Alcober, 2009) and Saturnalia in southeastern Brazil (Langer et al., 1999, 2007; Langer, 2003) highlighted postcranial features in the girdles and hind limb shared with later sauropodomorphs.

The striking similarities between Eoraptor and Panphagia and Saturnalia became apparent. 

More recently, the discovery in southeastern Brazil of wellpreserved cranial remains of Pampadromaeus (Cabreira et al., 2011) has extended the striking similarities between Eoraptor and Brazilian genera to include the skull.

We reconsider the relationships of Eoraptor and other basal dinosaurs elsewhere (Sereno and Martínez, in review). Evidence is mounting that Eoraptor and several other taxa from the Ischigualasto and Santa Maria formations (Panphagia, SaturnaliaPampadromaeus) are basal sauropodomorphs.

Based only on Sereno et al. 1993 data and whatever was online at the time
Now that several traits in Eoraptor are now published, the large reptile tree (and its limited number of characters, will be updated soon) also nested Eoraptor with Pampadromaeus and these two with Panphagia in a clade basal to the Phytodinosauria (= Sacisaurus and the poposaurs + Sauropodomorpha + Ornithischia).

This order is confirmed by Martínez et al. (2013) which found, “The analysis positions Panphagia as the basal-most sauropodomorph, followed by Eoraptor, Pampadromaeus, and a clade that includes Chromogisaurus and Saturnalia.”

So, another confirmation for a much maligned study. Nice.

References
Martínez RN, Apaldetti C and Abelin D 2013. 
Basal sauropodomorphs from the Ischigualasto Formation. Basal sauropodomorphs and the vertebrate fossil record of the Ischigualasto Formation (Late Triassic: Carnian-Norian) of Argentina. Journal of Vertebrate Paleontology Memoir 12:51-69.
Sereno PC, Forster CA, Rogers RR and Moneta AM 1993.
Primitive dinosaur skeleton form Argentina and the early evolution of the Dinosauria. Nature 361, 64-66.
Sereno PC, Martínez RN and  Alcober OA 2013. Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha). Journal of Vertebrate Paleontology Memoir 12:83-179.

Procompsognathus – What does it look like?

The small Late Triassic archosaur Procompsognathus (~60 cm length, von Huene 1921, Fig. 1) was earlier and convincingly revealed to be a chimaera by Sereno and Wild (1992). The croc skull (Figs. 2,3) did not belong to the dino post-crania. Unfortunately no reconstruction was provided. Here (Fig. 1) is a Procompsognathus reconstruction , along with Segisaurus (~1 m length, Camp 1933) an early Jurassic dinosaur, to which it was allied.

Figure 1. Procompsognathus (below) along with Segisaurus (not to scale). We don't have the actual skull of Procompsognathus, but it was likely small, but taller than wide.

Figure 1. Procompsognathus (below) along with Segisaurus (not to scale). We don’t have the actual skull of Procompsognathus, but it was likely small, but taller than wide.

Procompsognathus post-crania
The post-cranial portion of the specimen (SMNS 12591) was considered close to Segisaurus (Fig. 1) and here nests close to it, but closer to the tiny Middle Triassic theropod, Marasuchus.  Pedal digit 1 rides a little higher on the metatarsus in Procompsognathus and Marasuchus among only a few distinguishing traits.

Distinct from Segisaurus, Procompsognathus has longer, more robust hind limbs and essentially vestigial forelimbs. It is also half as large with a much longer pubis, longer cervicals with smaller cervical ribs, a higher metatarsal 1 and shorter, more robust phalanges on pedal digit 4, which also has a very long ungual.

Figure 2. SMNS 12591a, a basal croc skull close to the ancestry of dinosaurs.

Figure 2. SMNS 12591a, a basal croc skull close to the ancestry of dinosaurs. The premaxilla is unknown and has been restored here. The palatine appears in the antorbital fenestra.

SMNS 12591a – the croc skull
A basal croc, the SMNS 12951a skull, is twice as wide as tall. The quadrate leans anteriorly. Phylogenetically the skull nests in the large reptile tree at the base of the Gracilisuchus + Scleromochlus clade and next to the Terrestrisuchus + Saltoposuchus clade. So there is a good chance that the SMNS 12951a skull was attached to gracile bipedal crocodylomorph post-crania, along the morphological lines of Procompsognathus, and not too far from the base of the Archosauria.

Figure 3. The SMNS 12591a skull reconstructed. It is twice as wide as tall, a croc feature.

Figure 3. The SMNS 12591a skull reconstructed. It is twice as wide as tall, a croc feature.

Sereno and Wild (1992) described postfrontals (blue in Fig. 2), but strangely did not illustrate them (Fig. 3). Gracilisuchus and Scleromochlus also retain postfrontals but most other crocs do not. What appears to be a post dividing the antorbital fenestra in situ is actually the displaced palatine, as described by Sereno and Wild (1992).

References
Camp C 1936. A new type of small bipedal dinosaur from the Navajo sandstone of Arizona. Univ. Calif. Publ., Bull. Dept. Geol. Sci., 24: 39-56.
Huene F von 1921.
Neue Pseudosuchier under Coelurosaurier aus dem württembergischen Keuper. Acata Zoologica 2:329-403.
Sereno P and Wild R 1992. Procompsognathus: theropod, “thecodont” or both? Journal of Vertebrate Paleontology 12(4): 435-458.