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.
13 Synapomorphies
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 and
Pseudolagosuchus 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?
Tomorrow: Dinosauria
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.
References
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.
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The Pterosaur Heresies 
1. Poposaurids lack an anterior tympanic recess (Effigia- Nesbitt, 2007 fig. 20; Shuvosaurus- Nesbitt, 2007 pg. 24; Lotosaurus- coding in Nesbitt, 2011).
2. Nesbitt miscoded Effigia (Nesbitt, 2007 pg. 29) and Shuvosaurus (ditto, and Lehane, 2005 pg. 37) as lacking a supraoccipital component to the auricular recess).
3. Coded as absent in Lotosaurus and Shuvosaurus, though I can’t determine it in either.
4. Poposaurus does not have a ridge on the dorsal preacetabular edge (Long and Murry, 1995 figure 150A; uncoded by Nesbitt), nor does Shuvosaurus (ibid. fig. 166D), and Nesbitt coded Lotosaurus similarly.
5. You’re right here, and Nesbitt codes them as such.
6. Here, Nesbitt codes poposaurs except Poposaurus as having extensive ischial contact including the dorsal margin, but I would disagree and code Poposaurus that way too (Weinbaum et al., 2007; fig. 6A). Effigia (Nesbitt, 2007 fig. 43B), Shuvosaurus (Long and Murry, 1995 fig. 165C), Sillosuchus (Alcober and Parrish, 1997 fig. 5I) and Lotosaurus (coding by Nesbitt) all differ from silesaurids and dinosaurs in suturing the dorsal margin of their ischia too.
7. You can see the dorsolateral trochanter in figure 12 of Langer and Benton (2006). Nesbitt codes it as absent in all poposaurs, though I can’t confirm it in any. Indeed, if anything Poposaurus (Colbert, 1961 fig. 34A) seems to have it.
8. Poposaurs have this, as coded by Nesbitt. You can see it in e.g. Poposaurus (Weinbaum et al., 2007- fig. 9) and Effigia (Nesbitt, 2007 fig. 44A).
9. You can see in figure 49F of Nesbitt (2007) that the distal tibia of Effigia did not contact the fibula based on their facets in the astragalus, so your figure 1 is unecessary. Nesbitt elso codes this as absent in Poposaurus, Shuvosaurus and Lotosaurus. What’s your reference for the latter having it?
10. Effigia does not have a sharp anterior tip to its proximal fibula, as seen in Nesbitt (2007, fg. 46C). Poposaurus, Shuvosaurus and Lotosaurus are also coded as lacking it.
11. Here I agree with you and not Nesbitt, Effigia (Nesbitt, 2007 fig. 47B), Shuvosaurus (Long and Murry, 1995 fig. 170D) both have a slender metatarsal I. Nesbitt codes Poposaurus and Lotosaurus the opposite. What’s your source for Poposaurus?
12. Nesbitt codes Effigia and Shuvosaurus as having this character, so it agrees with your topology.
13. You’re right here and Nesbitt codes them that way.
So of the 13 characters, Nesbitt codes 4.5 of them as being present in poposaurs, while I code 6 of them as being present (2,5,8,11,12,13) and can confirm 5 of them are absent (1,4,6,9,10).
Of the four silesaurid characters, the first three are from Lewisuchus and the last from Pseudolagosuchus (the supposed femur of Lewisuchus is a tibia). I hope you’te not denigrating them by calling them minutia.
According to Nesbitt’s codings, Lotosaurus lacks all three postcranial characters you listed, though shuvosaurs have the first one.
By minutia I only mean that these are hard to see in most taxa. Lotosaurus is the odd one among the poposaurids, so yes, it could be the only one among them not to have certain characters or conversely -to- have certain traits.
with regard to the Lotosaurus ankle, I have a photo of a mount focusing on the feet. I only wish this taxon could be better described given the apparent completeness of the museum mounts I used for reference. If I’m wrong, please send better data.