Marasuchus skull restored

Updated March 13 with text and figure corrections and overlooked references.

Very few skull bones are known for Marasuchus, the tiny theropod-like dinosaur or proto-dinosaur. Here’s a shot at a restoration of the skull (Fig. 1).

Figure 1. Marasuchus skull restored. My what big teeth you have! Based on the maxilla and occiput, this appears to be a long, low skull. Looks like a little basal theropod, like Tawa. Line drawing from Theropod Database in which M. Mortimer moved the 'quadrate' to the postorbital, based on Bonaparte 1975.

Figure 1. Marasuchus skull restored. My what big teeth you have! Based on the maxilla and occiput, this appears to be a long, low skull. Looks like a little basal theropod, like Tawa. Line drawing from Theropod Database in which M. Mortimer moved the ‘quadrate’ to the postorbital, based on Bonaparte 1975.

Marasuchus would make a pretty good little basal theropod/basal dinosaur EXCEPT,

  1. each cervical is too short
  2. no cervicals have epipophyses
  3. the pubis is too short
  4. and it has no boot
  5. the ischium is too deep and V-shaped along its entire length.
  6. the femur lacks some grooves and bumps found in sister taxa
  7. the proximal tibia has a lateral bump does not reach the posterior rim
  8. distal tarsal 4 is not flat
  9. the astragalus has a larger facet for a larger fibula

Marasuchus is also smaller than basal dinosaur/theropod sisters (Fig. 2) and, considering this list, one wonders if some of these traits are due to neotony, the juvenilization of traits when a taxon experiences generational miniaturization.

Figure 1. To scale compared to Marasuchus, Agnosphitys cromhallensis (Fraser et al. 2002) is known from a selection of uncrushed bones, all of which resemble those from Marasuchus, but slightly larger with a relatively longer rostrum and shorter arms. These two represent a separate and distinct lineage of theropods.  Click to enlarge.

Figure 2. To scale compared to Marasuchus, Agnosphitys cromhallensis (Fraser et al. 2002) is known from a selection of uncrushed bones, all of which resemble those from Marasuchus, but slightly larger with shorter arms. These two represent a separate and distinct lineage of theropods.  Click to enlarge.

If not as a basal theropod close to the odd theropods, Procompsognathus and Segisaurus, then where else could Marasuchus more parsimoniously nest? Most of the above traits can be found individually far from bipedal dino-types, but the suite cannot be found elsewhere. I think we have to rely on maximum parsimony here.

Your thoughts?

References
Bonaparte JF 1975. Nuevos materiales de Lagosuchus talampayensis Romer (Thecodontia – Pseudosuchia) y su significado en el origen de los Saurischia, Chañarense Inferior, Triasico Medio de Argentina [New materials of Lagosuchus talampayensis Romer (Thecodontia – Pseudosuchia) and its significance on the origin of the Saurischia, Lower Chañares, Middle Triassic of Argentina]. Acta Geológica Lilloana 13:5-90.
Romer AS 1971. The Chanares (Argentina) Triassic reptile fauna X. Two new but incompletely known long-limbed pseudosuchians: Brevoria 378: 1-10.
Romer AS 1972. The Chanares (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: 53-73

Theropod Database

wiki/Marasuchus

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2 thoughts on “Marasuchus skull restored

  1. The skull you use was created by me for my 4-9-2013 blog post critiquing of your placement of Marasuchus. I in turn modified it from Bonaparte (1975), who you should also cite in your references. As stated in the caption of my figure, Sereno and Arcucci (1994) found that “though Bonaparte thought the anterior fragment was the premaxilla and the posterior margin was the antorbital fenestra, this isn’t true.” Thus the margin of the antorbital fenestra you show isn’t real, and the actual margin would be somewhere posterior/dorsal to it. Perhaps more troubling is that the concave dorsal line you seem to think is the parietal’s dorsal margin is actually the suture of the prootic and opisthotic FOR the parietal. This means the skull roof itself is unknown. Related to this, you place the braincase much too far posteriorly, as the paroccipital process (in section) is the back of that ‘V’ you put above the quadrate condyles. These are just more examples of how your incorrect interpretations lead to incorrect codings which lead to flawed trees. I know you have good intentions, but you have to gain a greater comprehension of anatomy and the literature (in addition to cladistic practices) before your enthusiasm can lead to trustworthy results.

  2. Thank you, Mickey. I recognized the hand of Bonaparte when I found the image on Google, did not trace it back to the Theropod Database, where evidently it was you who moved the putative quadrate to the postorbital. Good job. I wish you would have attempted a restoration, that way I would not have had to have been the first.

    On the maxilla issue, I agreed with Sereno and Arcucci that the anterior maxilla was not the premaxilla, but could find no alternative to the posterior margin of the antorbital fenestra issue. You said it’s ‘somewhere posterior/dorsal to it’, but I need data, which has not been published.

    With regard to the brain case, you’re absolutely correct about the absence of the parietal. I did not compare to sisters and I should have. I did not attempt to rotate the Bonaparte drawing in the restoration, but left it as is.

    Mickey, you did so well with these corrections, but then you launched into your slippery slope hypothesis, blackwashing the entire site for some oversights that had no effect on matrix scores. In the end, however, you’re correct, in Science you should not trust, you should test. Thank you again. I appreciate your help! Keep writing when you find an issue.

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