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.
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.
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.
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.
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).
* 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.
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.