Eodromaeus: a very basal phytodinosaur, just barely not a theropod

Eodromaeus murphi (Martínez et al 2011; Late Triassic, 230 mya, holotype PVSJ 560 1.2m), nests in the large reptile tree at the very base of the Phytodinosauria, despite the fact that it was no doubt a meat eater with long, fang like teeth (Fig. 1) derived from its basal dinosaur precursors. Other workers, including Martinez et al 2011, have nested this taxon as a basal theropod.

Figure 1. Eodroameus murphi figures from Martinez et al. 2011. From this data the large reptile tree nested this taxon at the base of the Phytodinosauria, next to the Theropoda.

Figure 1. Eodroameus murphi figures from Martinez et al. 2011. From this data the large reptile tree nested this taxon at the base of the Phytodinosauria, next to the Theropoda. The above cladogram by Martinez et al 2016 nests Eodromaeus as a basal theropod.

Wikipedia reports:
Eodromaeus (meaning “dawn runner”) is an extinctgenus of basal theropod dinosaur known from the Late Triassic period of Argentina. It has been cited by Sereno as resembling a supposed common ancestor to all dinosaurs, the “Eve” of the dinosaurs.”

Indeed it is.

Eodromaeus now nests
at the base of the Phytodinosauria. To shift it to the base of the Theropoda adds 7 steps in the large reptile tree. To shift it down one more node to the base of the Theropoda + Phytodinosauria adds 9 steps. The following traits tend to separate Eodromaeus from basal theropods like Tawa.

  1. rostrum convex, smooth curve (also happens later in derived theropods)
  2. lack of a pmx/mx notch
  3. major axis of naris not horiz to 30º
  4. posterolateral premaxilla process present and narrower than naris
  5. retroarticular process straight
  6. three sacral vertebrae
  7. anterior caudal neural spines about the size of each centrum
  8. clavicles absent
  9. metacarpal 2 = mc3
  10. longest manual digits 2 and 3
  11. retention of phalanges on manual digit 4
  12. possible alignment of mc5 with 3 and 4
  13. lack of a pubic boot (also in segisaurs and Coelophysis).
  14. mt 1 50-75% of mt3 and mt4
  15. mt 2 and mt 3 do not align with a joint in pedal digit 1

The promaxillary fenestra
Martinez et al. 2016 report, “On the snout, an accessory pneumatic opening, the promaxillary fenestra, is present near the anterior margin of the antorbital fossa (specimen number PVSJ 560). The promaxillary fenestra is present in the basal theropod Herrerasaurus and in most later theropods, although it is secondarily closed in the early North American theropod Tawa and some coelophysoids.”

The promaxillary fenestra is also found
in the phytodinosaurs Daemnosaurus, Heterodontosaurus, Yinlong, and it is just closing in Pampadromaeus. Although not noted in Eoraptor, there appears to be a promaxillary fenestra on both sides of the holotype. BTW, this is not a character trait in the large reptile tree.

Figure 2. The Phytodinosauria and the nesting of Eodromaeus at its base along with Eoraptor and Pampadromaeus.

Figure 2. The Phytodinosauria and the nesting of Eodromaeus at its base along with Eoraptor and Pampadromaeus.

 

Palate teeth
Martinez et al. 2016 report,“A row of very small rudimentary teeth crosses the palatal ramus of the pterygoid in Eodromaeus (PVSJ 560), as in Eoraptor, the only dinosaurs known to retain palatal teeth.”

Prior success for the large reptile tree
Earlier the large reptile tree nested Eoraptor in the Phytodinosauria prior to Martinez et al. 2011) reporting the same nesting.

I don’t agree
that cervical 10 of Martinez et al (Fig. 1) is indeed a cervical because its ribs are enclosed in the torso. I don’t agree with the long torso as originally illustrated based on the in situ fossil (Fig. 3). I wish I could see the referred materials. Where are they published?

Figure 3. Eodromaeus in situ. This specimen lacks forelimbs. Note the brevity of the dorsal section compared to figure 1.

Figure 3. Eodromaeus in situ. This specimen lacks forelimbs. Note the brevity of the dorsal section compared to femur and compared to figure 1. A restored pes is shown at right.

This new data 
does not change the tree topology (now at 635 taxa). We should not expect the first phylogenetic phytodinosaur to be a plant eater any more than we should expect the first amniote to look like a lizard, or the first of any clade to have all the traits that derived members have.

Figure 5. GIF animation (2 frames) from U of Chicago YouTube video adding soft tissue to Eodromaeus. Those are protofeathers, not hairs, and happy to see scales are minimized here, like a turkey or vulture head.

Figure 5. GIF animation (2 frames) from U of Chicago YouTube video adding soft tissue to Eodromaeus. Those are protofeathers, not hairs, and not happy to see so many scales  here. Birds don’t have scales on their heads or anywhere but their feet, when the feathers are gone. Click to view.

Late addition
this YouTube video of Eodromaeus model from the U. of Chicago. The skin should be skin, like bird skin, not scales. What you see here is traditional thinking. The protofeathers looks too much like an old man’s receding hair. Based on phylogenetic bracketing, the short, straight, feather covering should be dense wherever present, especially along the back and perhaps the neck. Remember, feathers have their genesis on bird embryos on the back near the hips and Eodromaeus would have been close to the genesis of short feather quills, as seen in some ornithischia, not feathery flight feathers.

References
Martinez RN, Sereno PC, Alcober OA, Colombi CE, Renne PR, Montañez IP and Currie BS 2011. A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern PangaeaScience 331 (6014): 206–210. doi:10.1126/science.1198467PMID 21233386.

wiki/Eodromaeus

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