Lewisuchus: protoarchosaur or silesaurid?

Here’s another taxon in a paleontological argument based on phylogenetic analyses that use different character lists. We looked at similar nesting problems for other archosauriforms here and here.

Figure 1. LeFigure 1. Lewisuchus from Romer 1972. Hind limb (in blue) based on description only. Pelvis is hypothetical. Two skulls are shown, one with a descending jugal, as in sister taxa.wisuchus, a tiny and incomplete predecessor to crocs and dinos (including birds).

Figure 1. Lewisuchus from Romer 1972. Hind limb (in blue) based on description only. Pelvis is hypothetical. Two skulls are shown, one with a descending jugal, as in sister taxa. Click to enlarge.

Nesbitt (2011)
nests the small (3?-ft. long) bipedal Chañares (Middle Triassic) archosaur, Lewisuchus admixtus (Romer 1972, Fig. 1), with Silesaurus and kin. Considered a dinosauriform, or dinosaur precursor, Lewisuchus had long hind limbs and presumably short forelimbs (only the proximal humerus is known along with a small pectoral girdle) along with a vertical quadrate and sharp recurved teeth. Silesaurids were likely herbivores. Lewisuchus was not.

Figure 2. Skulls and occiputs of Silesaurus, Lewisuchus and Pseudhesperosuchus

Figure 2. Skulls and occiputs of Silesaurus, Lewisuchus and Pseudhesperosuchus for comparison. Silesaurus nests with poposaurid dinosaurs in the large reptile tree and Pseudhesperosuchus nests at the very base, or just outside the Archosauria. Nesbitt did not attempt to nest Pseudhesperosuchus. Silesaurus and Lewisuchus nest outside the Dinosauria in the Nesbitt tree.

By contrast
the large reptile tree nested Lewisuchus with the much larger protoarchosaurs, Decuriasuchus and Pseudhesperosuchus just outside the archosaurs (crocs, birds, their last common ancestor and all of its descendants. It takes 24 extra steps when Lewisuchus is moved to Silesaurus in the large reptile tree.

These traits link Lewisuchus and Pseudhesperosuchus in the large reptile tree, to the exclusion of Silesaurus

2. Skull not shorter than cervicals
3. Skull width < 1.2x height
4. Occiput with posttemp fenestra
127. Retroarticular straight
128. Ventral mandible two tier convex
129. Eight cervicals
131. Cervicals do not decrease cranially
133. Cervical neural spines taller than centra
142. Lumbar area present (short ribs)
196. Femoral head offset only
197. Femur > half glenoid-acetabulum length
198. Fourth trochanter low to absent
202. Fibula not greater than .5 tibia diameter
227. Dorsal osteoderms present

Additionally, tibia is probably > 2x ilium length, but ilium is unkonwn.

The following traits link Lewisuchus to Silesaurus to the exclusion of Pseudhesperosuchus in the large reptile tree.

28. Maxilla straight vent margin

Figure 2. Pseudhesperosuchus was recovered as a Lewisuchus sister in the large reptile tree.

Figure 2. Pseudhesperosuchus was recovered as a Lewisuchus sister in the large reptile tree.

In the Nesbitt tree,
the following traits link Lewisuchus to Silesaurus. Pseudhesperosuchus and Decuriasuchus were not included in the analysis, so Gracilisuchus and Turfanosuchus are used instead.


Figure 3. Silesaurus as a biped and occasional quadruped.

Lewisuchus shares these traits with Silesaurus but not Gracilisuchus. Pertinent notes follow in parentheses.

15. Maxillary teeth posterior edge convex (images in Romer show concave)
66. Postorbital squamosal contact restricted to dorsal margin of the elements (we don’t know the dorsal extents as the fossil stops there, Sq unknown in Silesaurus)
78. Quadrate head partially exposed laterally (also in Pseudhesperosuchus = Ps)
84. Ectopterygoid dorsal to pterygoid (not in Romer’s Gracilisuchus = Gr, also found in Ps)
87. Ectopterygoid body arcs anterodorsally (as above)
93. Basipterygoid processes directed anteriorly or ventrally at their distal tips, not posteriorly (not shown in Gr or Ps)
100. Parabisphenoid recess absent (recess present in Gr, not shown in Ps)
101. Parabisphenoid ant tympanic recess on lat side of braincase present (not shown in Gr or Ps)

The following parts are difficult to observe with existing printed data:
103. Parabasisphenoid between basal tubera and bpt processes as wide as long or wider
113. Exoccipital relative positions of the nerve XII exits aligned sub vertically, not ant/post
127. Supraoccippital regose ridge on the antlat edges present
178. Atlantal art facet in axial intercent. Concave with upturned lateral borders, nott saddle-shaped.
179. Axis dorsal margin of neural spine arcs dorsally, not expanded posterodorsally
181. Cerv vert 3-5 centrum length longer than mid-dorsal (Gr, Si and Ps are all shorter than Lew)
191. Cerv vert distal end of neural spines unexpanded (Lew is more like Gr and Ps than Si)
196. Cerv ribs slender, elongated, not short and stout (Ps ribs are slender)
197. Dors vert neural spine distal expansion absent (longer distally in Lew)
218. Scap blade height vs dista width less than 3x (true)
227. Glenoid orientation posteroventrally (since the scapula rotates, this trait changes)
401. Dorsal osteoderms absent (Romer describes “thin dorsal scales”)

Lewisuchus shares these traits with Gracilisuchus but not Silesaurus according to Nesbitt 2011.
67. Postorbital bar composed mostly of postorbital – (postorbital unknown in Silesaurus, but does not appear to be true)
154. Dentary dorsal margin of ant portion horizontal, not dorsally expanded – (Actually ant dent rises in Lewisuchus, too.)
155. Dent anterior extent rounded, not tapered to a sharp point.
166. Dent teeth full arcade
171. Tooth, crown, not mesiodistally expanded
173. Max and dent crowns shape tall and blade-like, not short and subtriangular

Traits found in Lewisuchus, but not in Gracilisuchus or Silesaurus
75. Jugal long ridge on lat surface absent
110. Opisthotic paraoccipital processes directely laterally or dorsolateraly, not ventrally
183. Third cerv vert centrum length longer than axis centrum, not subequal (could also be found in Ps)

Moving Lewisuchus to Gracilisuchus adds 41 steps in the Nesbitt tree.

Macro traits in the Peters study contrast with micro traits in the Nesbitt study. So, in the Peters study sister taxa end up looking alike because they share more macro traits. This does not happen consistently in the Nesbitt tree, as documented earlier here and here (and links there in).

Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.
Nesbitt, SJ, Sidor CA, Irmis RB, Angielczyk KD, Smith RMH and Tsuji LA 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464(7285):95-98.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna; XIV, Lewisuchus admixtus, gen. et sp. nov., a further thecodont from the Chañares beds. Breviora 390:1-13.



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