What traits separate phytodinosaurs from theropods?

Yesterday we looked at the origin of dinosaurs in the context of and contra the recent Baron et al. 2017 paper. Today we’ll look at the basal split between basal phytodinosaurs, like Eodromaeus (Figs. 1, 2), with the closely related basal theropods, like Tawa (Fig. 1).

Figure 1. The theropod Tawa compared to the closely related phytodinosaur, Eodromaeus.

Figure 1. The theropod Tawa compared to the closely related smaller phytodinosaur, Eodromaeus.

Placed side-by-side to scale
Tawa and Eodromaeus are similar overall, though the plant-eaters were initially smaller. The details (below) demonstrate the initial steps toward herbivory that characterize the Phytodinosauria, distinct from the Theropoda and basal Dinosauria from which they evolved (contra Baron et al. 2017).

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

How do basal phytodinosaurs differ from the basal theropods?
Here’s the LRT list:

  1. lateral rostral shape: convex and smoothly curved (also in ancestral Herrerasaurus and Gracilisuchus);
  2. premaxilla/maxilla angle 25–45º;
  3. naris shape in lateral view almost round (not longer than tall or taller than long);
  4. postfrontal has no contact with the upper temporal fenestra;
  5. opisthotic oriented laterally without posttemporal fenestrae;
  6. palatal teeth (only on basalmost taxa);
  7. maxillary tooth depth ≤ 2x width in lateral view;
  8. last maxillary tooth at mid orbit (also in Herrerasaurus);
  9. olecranon process present (convergent in Buriolestes);
  10. metacarpals 2 and 3 align with m1.1 (except Eodromaeus);
  11. acetabulum laterally oriented (no ventral deflection, as in basal theropods);
  12. femoral head with neck and offset (appears later in theropods);
  13. penultimate manual phalanges not the longest in each series;
  14. loss of pubic boot (likely plesiomorphic because outgroups to Herrerasaurus do not have a pubic boot).

Summary and significance
Compared to the closely related theropod Tawa, the overall similar phytodinosaur Eodromaeus had a taller rounder rostrum, shorter teeth, a higher coronoid process, a longer dorsal region with more robust dorsal vertebrae, reduced gastralia, a more robust pectoral girdle and forelimb with shorter, less raptorial fingers, a deeper pubis and ischium with more robust hind limbs. The shorter teeth and larger belly together with the more robust limbs and back are traits seen in a wide variety of herbivores, even if only transitional at this early stage.

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.

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2 thoughts on “What traits separate phytodinosaurs from theropods?

  1. Reduced gastralia? Do we know whether Eodromaeus had gastralia? Absence from the reconstruction doesn’t mean anything, they’re routinely omitted from reconstructions.

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