Nesbitt (2011) and nearly all prior studies considered dinosaurs monophyletic and so does the large reptile tree. The only difference are the included taxa. The large reptile tree includes Trialestes, various silesaurids and poposaurids with Pisanosaurus, all within the Dinosauria. The large tree also nested sauropods and poposaurids with ornithischians in an herbivorous clade, Phytodinosauria. On the other hand, Nesbitt (2011) followed traditional nestings with Pisanosaurus as a basal ornithischian and sauropods closer to theropods (along with pterosaurs not far off). Poposaurids were nested with rauisuchians.
Again, apologies for many of the traits here for which I have not data. Often these traits are subtle and tucked away, not commonly illustrated.
Unambiguous dinosaur characters found by Nesbitt (2011) and others include:
1) supratemporal fossa [stf] present anterior to the supratemporal fenestra;
This trait is tentatively present in Silesaurus, which preserves the frontals. Effigia rotates the stf posteriorly and drops it into a fossa, but I haven’t seen the skull in occipital view. The stf of Lotosaurus appears to be depressed in a fossa.
2) epipophyses present in postaxial anterior cervical vertebrae; Seems to be present in Effigia (could use a second opinion here).
3) apex of deltopectoral crest situated at a point corresponding to more than 30% down the length of the humerus; This trait is indeed present in theropods, sauropods and ornithischians, but not present in Pisanosaurus or other poposaurids so far as data indicates (missing in Effigia and Poposaurus).
6) fourth trochanter asymmetrical, with distal margin forming a steeper angle to the shaft; Sorry, I don’t have good data for this.
7) proximal articular facet for fibula of the astragalus and calcaneum occupies less than 0.3 of the transverse width of the element; Sorry, I don’t have good data for this.
Unambiguous synapomorphies Nesbitt (2011) first found include:
1) exocciptials do not meet along the midline on the floor of the endocranial cavity; Sorry, I don’t have good data for this.
2) proximal articular surfaces of the ischium with the ilium and the pubis separated by a large concave surface; Sorry, I don’t have good data for this.
3) cnemial crest arcs anterolaterally; Sorry, I don’t have good data for this.
4) distinct proximodistally oriented ridge present on the posterior face of the distal end of the tibia; Sorry, I don’t have good data for this.
Potential synapomorphies found by Nesbitt (2011) and others include:
1) concave ventral margin of the acetabulum of the ilium; Sorry, I don’t have good data for this.
2) Concave articular surface for the fibula of the calcaneum; Sorry, I don’t have good data for this.
3) postfrontal absent; A good character for dinos, but also found in crocs beginning with Terrestrisuchus and higher pararchosauriforms plus Doswellia. Also found in poposaurids. In crocs and pararchosauriforms it seems apparent that the postfrontal and postrobital have fused.
4) posttemporal opening absent or less than half the diameter of the foramen magnum; Sorry, I don’t have good data for this.
5) three or two phalanges on manual digit IV; A good character for dinos, four on Pisanosaurus, Scelidosaurus, Lotosaurus, Effigia, Poposaurus.
6) posterior process of the jugal is split by the anterior process of the quadratojugal; In some theropods the jugal process is so gracile that it may be overlapped rather than split. This area is often damaged. In Scelidosaurus and Heterodontosaurus the jugal is not split. This trait is also present in Lotosaurus and damaged or missing in other poposaurids.
Potential synapomorphies found originally by Nesbitt (2011) include:
1) three premaxillary teeth; Hmmm, I found four basally in dinos, reduced in Scelidosaurus and Heterodontosaurus and missing in several poposaurids. More than four teeth in the clade of Agilisaurus, Lesothosaurus and Scuttellosaurus.
3) concave emargination ventral to the proximal head of the femur; Sorry, I don’t have good data for this.
4) calcaneal tuber absent; This appears to be a good character for dinos, but does not include Pisanosaurus and poposaurids, which redeveloped the tuber, then lost it again in Silesaurus + Pseudolagosuchus. A tiny tuber is present in basal crocs. By convergence the tuber is also absent in all basal reptiles upt to Prolacerta, then absent again in the Proterochampsia.
5) interclavicle absent; A good character for dinos, including Lotosaurus and poposaurids. Absent by convergence in three other unrelated clades. Fused to sternum and clavicles in pteros.
6) proximal articular surface of the humerus separated by a gap from the deltopectoral crest. This trait may be correlated to deltopectoral crest apex located 30% down the humerus. As above, this trait is indeed present in theropods, sauropods and ornithischians, but not present in Pisanosaurus or other poposaurids so far as data indicates (missing in Effigia and Poposaurus).
I think that’s it for the Nesbitt characters. Much more work to be done. Convergence is rampant.
Tomorrow we’ll go back to a variety of subjects.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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.