Diandongosuchus and its the poposaurid “synapomorphies”
Li et al. (2012) introduced us to Diandongosuchus, which they concluded was a basal poposaurid close to the aquatic quadruped, Qianosuchus (Fig. 1). Added to the large reptile tree, however, it nested between Proterochampsa and the Phytosauria, not far from Chanaresuchus and derived from the BPI 2871 specimen of Youngina (Fig. 2).
Figure 1. More strange bedfellows created by the Nesbitt (2011) study. None of these taxa belong together! Yet they nest as close relatives in the Li et al. (2012) study. Here Diandongosuchus appears twice, once in scale with the others and once larger to see the details.
The Li et al. (2012) study based on Nesbitt (2011) recovered 14,040 trees, and in a second analysis, 105,300 trees. It did not include several taxa of importance. The large reptile tree, based on several times more taxa, was completely resolved.
By adding Diandongosuchus the Li et al. (2012) study changed the tree topology. The large reptile tree added Diandongosuchus without change.
Figure 2. The real sister taxa and close relatives of Diandongosuchus beginning with the Youngina with the longest, lowest rostrum, BPI 2871, and moving forward toward the parasuchians. Diandongosuchus further separates the new putative Proterochampsa from the original. Diandongosuchus may have had an anterior process on the ilium, hidden beneath the femur.
The Li et al. (2012) study recoved 5 unequivocal synapomorphies supporting the poposaurid status of Diandongosuchus.
1) length of the anterodorsal (nasal) process of the premaxilla greater than the anteroposterior length of the premaxilla. This trait is shared by Prolacerta through chanaresuchids, but not phytosaurs, which have greatly displaced nares. Among poposaurids only highly derived Effigia shares this trait, and then just barely. The trait is present in Qianosuchus and Vjushkovia, an ancestral taxon to Qianosuchus, not included in the Li et al. and Nesbitt studies.
2) The posterior (maxillary or subnarial) process of the premaxilla restricted to the ventral border of the external naris. Not sure what this means, it is so common.
3) the centrum of cervical 3 is longer than the axis centrum. Again, this is so common.
4) the presence of a thickened process on the proximal portion of the pubic apron. as above, quite common.
5) the bone wall thickness to the shaft diameter of the femur at the midshaft >0.2 but <0.3. I can’t comment on bone wall thickness to shaft diameter of the femur on a specimen crushed to two dimensions.
Nesbitt (2011) reported Qianosuchus was the basalmost poposaurid, but also wrote, “Much of the morphology of the skeleton of Qianosuchus resembles that of Ticinosuchus because it has short pelvic elements and at least four osteoderms per presacral vertebra.” The large reptile tree nested these two together.
Nesbitt (2011) reported, “Qianosuchus shares the following important character states with other poposauroids:
1) anterodorsal process (= nasal process) of the nasal greater than the anteroposterior length of the premaxilla; While true of Qianosuchus, this has not been established in any other poposauroid but Effigia, which is otherwise distinctly different. It is more similar to that found in certain choristoderes and chanaresuchids.
2) posterodorsal process (= maxillary process, = subnarial process) of the premaxilla restricted to the ventral border of the external naris; This trait is common and plesiomorphic for reptiles.
3) anterodorsal margin of the maxilla borders the external naris; this trait is NOT found in Diandongosuchus and its pararchosauriform kin. So why was it included?
4) concave anterodorsal margin at the base of the dorsal process of the maxilla; Due to the rostral, narial and antorbital fenestra proportions in Diandongosuchus, the dorsal process near the antorbital fenestra is quite far removed from the anterior portion of the maxilla, so this trait is essentially not the same as in the Qianosuchus and Effigia in which these elements are closer together.
5) foramina for entrance of cerebral branches of internal carotid artery into the braincase positioned on the ventral surface; Sorry I don’t do foramina. Too tiny and obscure.
6) cervical ribs slender and elongated; They are short and leaf-shaped in Diandongosuchus, so quite different.
7) distal expansion of neural spines of the dorsal vertebrae absent; this is a common and plesiomorphic character.
8) sacral rib of primordial sacral one articulates with the anteriorly directed process of the ilium; with the femur in place, there is no way to determine whether or not Diandongosuchus had an anterior process of the ilium. Likely it does considering its phylogenetic position with our without it.
(9) Insertion of a sacral vertebra between the first primordial sacral vertebra; Li et al. (2012) count only two sacrals in Diandongosuchus, but the third anterior one is narrow and unidentified in Li et al. (2012, fig. 6) just above the broken left ilium, close to the label “fsr1+2″.
(10) Concave ventral margin of the acetabulum of the ilium; not present in Diandongosuchus.
(11) Thickened process on the proximal portion of the pubic apron. Plesiomorophic in all post Thadeosaurus taxa.
(12) Distal end of the fibula rounded or flat (symmetrical). Again fairly common trait.
You’ll remember my two main problems with the Nesbitt (2011) study were a lack of dichotomy in many characters (i.e. IS vs IS NOT) and a reliance on characters that were difficult to view and judge, especially in crushed specimens.
In a few days I’ll post the synapomorphies of Diandongosuchus and parasuchians according to the large reptile tree.
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.
Reference
Li C, Wu X-C, Zhao L-J, Sato T and Wang LT 2012. A new archosaur (Diapsida, Archosauriformes) from the marine Triassic of China, Journal of Vertebrate Paleontology, 32:5, 1064-1081.
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.
2012-09-05 17:45:10
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