Updated April 22, 2014 to reflect the new basal archosaur position of poposaurids.
Sillosuchus longicervix Alcober and Parrish (1997) is known from a few gracile cervicals, a few gracile posterior dorsals through anterior caudals, the majority of a gracile pelvis and a sinusoidal femur. Sillosuchus comes from the Late Triassic of Argentina. Sillosuchus is widely considered a poposaur close to Shuvosaurus, Effigia and Poposaurus, among others. That seems to be a reasonable nesting. Unfortunately there are too few traits to add it to the large reptile tree.
An odd pelvis
The pubis is longer than the femur, but there’s no pubic ‘boot’ in Sillosuchus. The ischia are co-ossified to create a single bone. The ilium (Fig. 2) includes a portion that extends laterally, overhanging the femur, with reinforcing flanges for strength without added weight. The anterior sacral ribs are short. The posterior ones are long. The results in angled ilia, which may not be a problem as the articular surface appears to be between the top of the femur and the overhanging ilium, rather than the medial femur and the lateral face of the pelvis.
There are five sacrals and the ribs of the middle three arise from the joint between two vertebrae, rather from the central portion of each one. That seems odd, but Effigia, with four sacrals, has a similar situation in which sacrals 1 and 2 share a rib and sacrals 2 and 3 share a rib.
A restoration in Japan
A restoration, adding a skull and other skeletal elements is present at a Japanese museum display (sorry, no more data on this yet), and online here at Wikipedia. This appears to be a much too robust restoration, based on what is known of the specimen (Figs. 1, 2) and the pelvis appears to share little with the original imagery of Alcober and Parrish (1997).
we discussed the controversy of poposaurid origins. Most other paleontologists consider them to be dinosaur-like rauisuchians. The large reptile tree recovers them as basal archosaurs.
My guess is
the rest of Sillosuchus, if it is ever discovered, will likely look like Shuvosaurus, very gracile, but with some sort of unique oddity in a toothless skull.
Alcober O and Parrish JM 1997. A new poposaurid from the upper Triassic of Argentina. Journal of Vertebrate Paleontology 17:548–556.