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

3 thoughts on “Another Gracilisuchus (from Brinkman 1981)

  1. As the person who sent you the Brinkman paper, I’m disappointed you didn’t get the point. The specimens Brinkman used for that reconstruction (MCZ 4116 and 4118) were both used by Romer for his original Gracilisuchus description and figures that you show in your figure 2. The seemingly flatter skull you show as “the holotype” is actually a composite “based mainly on” MCZ 4117, which is not the holotype. MCZ 4118 has “a robust neck pretty much like that of Gracilisuchus” because the neck in Romer’s description IS MCZ 4118. You’re comparing a specimen to itself. As Romer’s description and figures are based on combinations of all of these specimens, without visiting the MCZ and LPM, there’s no way to code them separately for most characters.

    Brinkman’s point in illustrating MCZ 4116 and 4118 is that 4117 is crushed dorsoventrally, so Romer’s reconstruction was wrong. My point in showing you this is that your analysis has so many superficial shape/proportion characters that taphonomic differences like crushing could cause specimens of the same species to be placed apart, and that’s exactly what happened.

    The case for PVL 4597 is basically the same. This was another specimen used by Romer for his description, though at the time it was called the Tucuman specimen. And again, instead of comparing PVL 4597 to the holotype, you’re actually comparing it to a composite of the holotype and itself. The important differences in “the holotype” you mention and code (metatarsal IV/III ratio, tarsal type, calcaneal tuber orientation, etc.) are actually illustrated and described by Romer based on PVL 4597 itself, and not the holotype. So again, you’re comparing a specimen to itself.

    • Parrish (1993) didn’t counter anything. The skull he illustrates is “very much in the shape that Romer (1972) proposed” because it was based on Romer’s drawing. Parrish’s figure caption says it was “modified after Romer, 1972a”. He just changed some of the sutures. He never disagrees with Brinkman to say the skull is uncrushed.

      Your addendum figure 1 is the holotype (LPM 64-XI-14-11), so is not the specimen Romer’s skull reconstruction is mostly based on (MCZ 4117). Romer’s skeletal reconstruction that your figure 2 is based on is a composite of both of these specimens as well as MCZ 4118, PVL 4597 and presumably MCZ 4116.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s