Marasuchus: In or out of the Dinosauria?

Figure 1. Marasuchus lilloensis (above) and Lagosuchus talampayensis (below) compared. The radius and ulna are longer in marasuchus. The hind limbs are more robust in Lagosuchus. The length of the torso in Lagosuchus is based on the insitu placement of the pectoral girdle and forelimb, which may have drifted during taphonomy.

Figure 1. Marasuchus lilloensis (above) and Lagosuchus talampayensis (below) compared from an earlier blog post. Is Marasuchus in or out of the Dinosauria? Either way it was probably close to the origin of dinosaurs. Oddly the scapulocoracoid was quite robust in Marasuchus.

Here’s the problem:
Earlier we looked at the difference between Lagosuchus and Marasuchus, too small dino-like bipeds of the Late Triassic. The Nesbitt (2011) tree nests Marasuchus (Fig. 1) outside the Dinosauria. The large reptile tree nests Marasuchus as a small theropod. Let’s look at both sides of this situation. It appears to come down to micro-traits vs. macro-traits, a theme we’ll return to over the next few blog posts.

According to Nesbitt 2011
The following traits include Marasuchus, Silesauridae and the Dinosauria.

  1. Pubis longer than ischium (also in several post-Euparkeria taxa)
  2. Proximal portion of the pubis articular surfaces with the ilium and the ischium separated by a groove or gap. (also in Postosuchus)
  3. Ischio-pubis contact present and reduced to a thin proximal contact.
  4. Ischium, proximal articular surfaces articular surfaces with the ilium and the pubis continuous but separated by a fossa.
  5. Ischium length markedly longer than the dorsal margin of iliac blade (minus the anterior process). (doesn’t seem to be true in basal dinos)
  6. Anterior trochanter forms a steep margin with the shaft but is completely connected to the shaft.
  7. Anterior trochanter shelf proximal to the fourth trochanter (insertion site for M. iliofemoralis externus) present.
  8. Proximodistally oriented groove on the lateral side of the distal portion of the tibia.
  9. Anterior ascending flange (anterior process) of the astragalus present and less than the height of the dorsoventral height of the posterior side of the astragalus.

The following traits support the Dinosauria, exclusive of Marasuchus.

  1. Exoccipitals do not meet along the midline on the floor of the endocranial cavity. (Also in Crocodylomorpha and Effigia + Shuvosaurus.)
  2. Epipophyses present in postaxial anterior cervical vertebrae.
  3. Apex of deltopectoral crest situated at a point corresponding to more than 30% down the length of the humerus .
  4. Radius shorter than 0.8 of humerus length.
  5. Proximal articular surfaces of the ischium with the ilium and the pubis separated by a large concave surface.
  6. Fourth trochanter a sharp flange.
  7. Fourth trochanter asymmetrical, with distal margin forming a steeper angle to the shaft. 
  8. Cnemial crest arcs anterolaterally.
  9. Distinct proximodistally oriented ridge present on the posterior face of the distal end of the tibia.
  10. Proximal articular facet for fibula of the astragalus occupies less than 0.3 of the transverse width of the element.
  11. Concave articular surface for the fibula of the calcaneum.

The following traits support Marasuchus outside of the Theropoda:

  1. Parabasisphenoid recess absent
  2. Parabasisphenoid ant tympanic recess absent
  3. Epipophyses absent in past axial ant cerv verts
  4. Epipophyses absent in post cerv verts
  5. Cerv verts pneumatic features absent
  6. First primorial sacral art surf circular, not C-shaped in lat view
  7. Forelimb/hindlimb ratio > 0.55 (actually this ratio IS .55. est for manus)
  8. Ischium articular surfaces continuous but separated by fossa
  9. Fourth trochanter mound-like and rounded (looks sharp and described as aliform-wing-shaped)
  10. Femur surface between lat condyle and crista tibiofibularis on distal surface smooth
  11. Tibia prox portion cenemail crest present and straight
  12. DT4 medial side, without a distinct medial process
  13. Astragalus prox art facet for fibula occupies more than .3 transverse width
  14. Astragalus post groove present
  15. Calcaneal tuber present (but Eoraptor also has a slight tuber)
  16. Calc art surf for fibula convex

The following traits support Marasuchus within the Theropoda

  1. Opisthotic ventral ramus covered by lateralmost edge of exoccipital in post view
  2. Acetab antitrochanter present
  3. [not scored by Nesbitt] Gastralia present

The following traits support Marasuchus inside the Dinosauromorpha, but outside of the Dinosauria:

  1. Exoccipital lateral surfaces with clear crest lying ant to both ext foramina
  2. Axis, dorsal margin of the neural spine arcs dorsally
  3. Glenoid posterovent
  4. Prox pubis art surface separated by a groove or gap
  5. Ischio pubis contact present and reduced to a thin prox contact
  6. ischium markedly longer than iliac blade minus ant process
  7. Tibia > femur (not all dinos)
  8. Femur postmed tuber present and small
  9. Femur ant torchanter shelf present
  10. Femur post lat portion ventrally descended
  11. Tibia prox portion cenemail crest present and straight
  12. Tibai lat side of distal portion proxdistal oriented groove
  13. DT 4 width subequal to dt3
  14. DT4 size of art facet for mt 5 less than half of lat surface
  15. Astragalus ant ascending flange present less than height of dorsventral height
  16. Astragalus ant hollow reduced to a foramen or absent
  17. Astragalus antmed corner acute
  18. Compact metatarsus
  19. Longest metatarsal > .5 tibia
  20. Mt 5 phalanges: none (but note more derived taxa have phalanges.)

According to the large reptile tree,
Marasuchus is a derived theropod nesting with Procompsognathus. Moving Marasuchus out of the Dinosauria (outside Herrerasaurus) adds 15 steps.

The following traits support the inclusion of Marasuchus with Procompsognathus in the Theropoda.

  1. Mid-cervicals shorter than mid-dorsal
  2. Chevrons wider proximally
  3. Pedal 2.1 not > p2.2
  4. Pedal digit 4 not narrower than digit 3
  5. Overall size not > 30 cm tall, 60 cm long

The following traits support the inclusion of Marasuchus with Procompsognathus and Segisaurus in the Theropoda.

  1. Dorsal transv proc shorter than centra
  2. Pubis curving ventral
  3. Pedal 1.1 aligns with mt2, mt3

The following traits support the inclusion of Marasuchus within the Theropoda (represented by the addition of Coelophysis and Tawa in the large reptile tree).

  1. Skull shorter than cervicals (also in Turfanosuchus).
  2. Ant caudal spines shorter than centra.
  3. Caudals 3x longer than tall.
  4. Tibia not shorter than femur
  5. Fibula anterior trochanter low crest
  6. Mt 1 <.5 mt 3
  7. Mt 1 <.5 mt 4
  8. Metatarsals not shorter than half the tibia (also in phytodinosaurs.)
  9. Phalanges pedal digit 5: 0 (also in most crocs)

The following traits support the inclusion of Marasuchus within the Dinosauria.

  1. Interclavicle absent
  2. Forelimb < .55 hindlimb (actually the ratio is right at .55 given the hypothetical size of the manus shown in figure 1).
  3. Ilium anter proces truncated
  4. Acetabulum semiperforate
  5. Femoral head offset and subrectangular
  6. Fourth trochanter sharp
  7. Tibia > 2x ilium length
  8. Advanced mesotarsal ankle
  9. No calcaneal tuber (actually the tuber is vestigial, but poposaurs also have a substantial tuber, a reversal convergent with derived crocs).
  10. Proximal metatarsals 1 and 5 reduced (actually, not so much in Marasuchus, 1 is not reduced in Plateosaurus, 5 is not much reduced in Tawa)
  11. Osteoderms absent

The following traits in the large reptile tree are shared by Marasuchus and other taxa outside the Dinosauria.

  1. Longest metatarsals 3-4

Summary
Even a cursory glance through this list demonstrates the Nesbitt characters are often micro-traits representing characters that must be seen in person and in close-up. These include various bumps and holes in various bones. By contrast the Peters characters are largely macro-traits, visible and able to be measured in published images. That means the sister taxa used by the Peters images are more likely to look alike overall. That also means testing the Nesbitt tree is going to take personal inspection of the specimens. What I don’t understand is why the micro-traits and macro-traits diverge so widely, assuming both were scored correctly.

I wonder if the Nesbitt tree included Segisaurus and Procompsognathus if the tree topology would change? Oddly both have advanced theropod feet with 1 and 5 reduced.

References
Fraser NC, Padian K, Walkden GM and Davis LM 2002. Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. Palaeontology. 45(1), 79-95.
Paul GS 1988. Predatory dinosaurs of the world (464). New York: Simon and Schuster.
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

wiki/Marasuchus

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