Earlier in the last seven blogs we looked at characters Nesbitt (2011) used to define Archosauriformes and nested clades up to Ornithodira with a special emphasis [his not mine] on the nesting of pterosaurs within all these clades. Nesbitt (2011) was satisfied that pterosaurs nested well here, but did not test competing candidates among the Fenestrasauria and Tritosauria, the only clades that actually provide a gradual accumulation of pterosaurian traits.
In Today’s Installment
Nesbitt (2009, 2011) nested Silesaurus, Sacisaurus, Eucoelophysis, Asilisaurus, Pseudolagosuchus and Lewisuchus within the Silesauridae, the sister clade to the Dinosauria and together these form the Dinosauriformes. The large reptile tree did not test all these taxa, but Lewisuchus nested with the basal bipedal croc, Pseudhesperosuchus). At this point the Nesbitt (2011) study includes more pertinent taxa than the large reptile tree, which was focused on broader patterns throughout the Reptilia. I apologize ahead of time for the several characters I cannot comment on around the braincase and pelvis.
Here the Nesbitt study and the large reptile tree are coming to closer accord because pterosaurs are finally out of the inclusion list. However, the large reptile tree found several of the above taxa to nest within the Dinosauria (along with the poposaurs and Lotosaurus), not just outside it. According to Nesbitt (2011)m “This clade (Silesauridae + Dinosauria) is supported by the following 13 unambiguous synapomorphies:
1) Anterior tympanic recess on the lateral side of the braincase present (101-1); Can’t comment on this not readily visible character.
2) Auricular recess extends onto internal surface of epiotic/ supraoccipital (133-1); Can’t comment on this not readily visible character.
3) Atlantal articulation facet in axial intercentrum, shape concave with upturned lateral borders (178-1); Can’t comment on this not readily visible character.
4) Crest dorsal to the supraacetabular crest/rim confluent with anterior extent of the anterior (= preacetabular) process of the ilium (265-2); Can’t comment on this not readily visible character.
5) pubis more than 70% or more of femoral length (278-1); Trait shared with putative outgroup, Poposauridae.
6) extensive medial contact between the ischia, but the dorsal margins are separated (291-1);
Trait shared with putative outgroup, Poposauridae.
7) sharp ridge (= dorsolateral trochanter of some) on the dorsolateral margin of the proximal portion of the femur (307-1); Can’t comment on this not readily visible character, but Nesbitt (2007) notes a sharp proximal ridge (not sure if this represents the same ridge), on Effigia and Shuvosaurus, two putatitve outgroup taxa.
8) Straight transverse groove on the proximal surface of the femur (314-1); Can’t comment on this not readily visible character.
Figure 1. Ankle of Effigia. Arrow points to tibial distal flange oriented toward fibula.
9) Posterolateral flange of the distal portion of the tibia nearly contacts or contacts fibula (334-1); Perhaps shared with Effigia, but apparently present on Lotosaurus among putatitive outgroups.
10) Anterior edge of the proximal portion of the fibula tapers to a point and arched anteromedially (342-1); Apparently present in Effigia among putative outgroups.
11) Midshaft diameters of metatarsals I and V less than II–IV (384-1); Present in Poposaurus and Effigia among putative outgroups.
12) Distal articulation surface of metatarsal IV deeper than broad (391-1); Can’t really comment on this not very visible character.
13) and metatarsal IV length subequal to or shorter than metatarsal II (395-1). Present in Lotosaurus, Shuvosaurus, Poposaurus and Effigia among putative outgroups.
These 13 character states represent a significant increase in the knowledge of character-state transformations immediately outside Dinosauria.”
Many of these traits are found in poposaurids, considered closer to rauisuchians than dinosaurs in the Nesbitt (2011) study. Poposaurs are considered to be dinosaurs in which the calcaneal tuber redeveloped in the large reptile study. This was shown
to be convergent with the redevelopment in crocs.
Within the Silesauridae, the base of the clade (a composite taxon consisting of the hips and hind limbs of Pseudolagosuchus
together with the front of Lewisuchus
) is “well resolved and is supported by four unambiguous character states including: foramina of the hypoglossal nerve (XII) nearly aligned in a near anteroposteriorly plane (113-1); rugose ridge on the anterolateral edges of the supraoccipital (127-1); cervical centra 3–5 longer than middorsal (181-1); notch ventral to the proximal head of the femur (304-1).
” I can’t comment on such minutia. However, I earlier
made the case that Lewisuchus
is closer to the basal pre-croc, Pseudhesperosuchus
nested with Silesaurus
Within the Silesauridae, taxa closer to Silesaurus share the following “seven” (I only counted six) unambiguous synapomorphies: 1) anterior extent of the dentary tapers to a sharp point (155-1); 2) dentary teeth absent in the anterior portion (166-1); 3) maxillary and dentary crowns apicobasally short and subtriangular (173-1); 4) sacral ribs shared between two sacral vertebrae (208-1); 5) straight medial articular facet of the proximal portion of the femur (309-1); 6) distal condyles of the femur divided posteriorly between a quarter and a third the length of the shaft (324-1).
That sharp and toothless anterior dentary may be the predentary or something like it. I wonder if the sacral rib and distal femur characters are shared with Lotosaurus?
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.