Maybe this isn’t Gracilisuchus after all…

Lecuona and Desojo (2011) recently described a pelvis and hind limb they ascribed to Gracilisuchus. Unfortunately, the specimen was larger than the holotype (Fig. 1) and had distinct traits that nested it several nodes away from Gracilisuchus (Fig. 2) in the large reptile tree. There were more bones associated with this specimen and a note requesting more data was sent to Dr. Desojo. In the meantime, in an effort to compare apples to apples only the pelves and hind limbs of both specimens are shown here.

Figure 1. The Gracilisuchus holotype and the new specimen, PVL 4597 compared to scale. The PVL specimen is re-reconstructed below in the addendum image, so do not . There are six specimens of Gracilisuchus. PULR 08 appears to have no pedal 5 phalanges, distinct from the PVL specimen which has distinct metatarsal lengths.

Figure 1. The Gracilisuchus holotype and the new specimen, PVL 4597 compared to scale.  There are six specimens of Gracilisuchus. I don’t know anything about any of their hindquartes other than Romer’s figure. PULR 08 appears to have no pedal 5 phalanges, distinct from the PVL specimen which also has distinct metatarsal lengths. Given Romer’s data, which has been called into question, these two are not conspecific nor congeneric, but are otherwise probably related. They both share a narrower mt4. The specimen on the left has a primitive pelvis like that of Decuriasuchus and Scleromochlus. The new specimen has a more derived pelvis, like that of a basal dinosaur and Pseudhesperosuchus. They say this ilium belongs, but it appears too large compared to the pubis and ischium. They say the ilium attributed by Romer is questionable, but it doesn’t match that of Tropidosuchus.

Addendum
Below a new reconstruction of PVL4597 with a flipped ischium (I mistook left for right earlier). A lateral view of the femur and a better tarsus. 
There may be problems with Romer’s illustration. If so, Romer apologizes. There are many obvious similarities between that and this reconstruction. There are also a few differences.

Addendum figure. This is a better rendition of the new hindquarters of the PVL specimen. Apologies for any earlier inaccuracies. Thanks to M. Mortimer for pointing out I had mistaken a right ischium for a left one and tarsal inaccuracies. If there are further problems, please let me know, or contact Romer.

Addendum figure. This is a better rendition of the new hindquarters of the PVL specimen. Apologies for any earlier inaccuracies. Thanks to M. Mortimer for pointing out I had mistaken a right ischium for a left one and tarsal inaccuracies. If there are further problems, please let me know, or contact Romer. Ladies and gentlemen, I present you the basalmost dinosaur – and a sister to Gracilisuchus. Apparently PVL 1045 has a perforated acetabulum. 

With the score changes the PVL specimen shifted its original position and moved to the base of the Dinosauria, derived from a sister to Gracilisuchus. More on this in a future blog. Just cleaning this one up now.

Phylogenetic Analysis
Entering the traits of the PVL specimen into the large tree recovered a nesting only one node away from Gracilisuchus at the base of the Dinosauria. Forget Lagerpeton, already, this and Trialestes are the taxa you dino-heads need to be looking at !!! So, based on the data presented the two specimens were not far from each other, but not congeneric.

So What is the PVL Specimen?
When the PVL specimen is entered into the large reptile tree matrix (subset shown in Fig. 2) it nested several nodes away from Gracilisuchus, so the two are not congeneric. The new PVL specimen nests at the very base of the Archosauria, or just beyond it, depending whether you include the Decuriasuchus clade or not. The PVL specimen shares a ventrally-directed pubis with the preceding clade of Qianosuchus to aetosaurs. Gracilisuchus has a metatarsal 4 shorter than 3. In the PVL specimen these two metatarsals were subequal. The PVL specimen probably needs its own generic name — but I’d like to see the rest of the specimen before commenting further.

They Used the Brusatte et al. (2010) Matrix/Tree
Unfortunately,  Lecuona and Desojo (2011) based their analysis on Brusatte et al. (2010) which found such oddball sister taxa as Scleromochlus/Pterosauria/Phytosauria, Euparkeria/Proterochampsidae, Phytosauria/Aetosauria and Revueltosaurus/Ornithosuchidae. None of these paired taxa resemble one another more than alternate sister taxa listed here.

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
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.
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 

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