Sillosuchus – an odd, big shuvosaurid poposaurid

Updated April 22, 2014 to reflect the new basal archosaur position of poposaurids.

Figure 1. Sillosuchus (the large one) compared to Shuvosaurus (in silhouette).

Figure 1. Sillosuchus (the large one) compared to Shuvosaurus (in silhouette) to scale. Note the difference in ilium shape and orientation.

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.

Figure 2. Because it is rather odd, it is difficult to wrap your mind around the way the pelvis looks and operates. The femur has no offset head and appears to articulate with the overhanging portion of the ilium.

Figure 2. Because it is rather odd, it is difficult to wrap your mind around the way the pelvis looks and operates. The femur has no offset head and appears to articulate with the overhanging portion of the ilium. The vertebrae have large articulating surfaces and more gracile centrum shafts, saving weight while building size.

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).

Figure 3. Two views of the same Sillosuchus restoration from a museum in Japan.

Figure 3. Two views of the same Sillosuchus restoration from a museum in Japan. Much of this appears to too robust, but the skull is toothless, which is probably correct.

Earlier
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.

References
Alcober O and Parrish JM 1997. A new poposaurid from the upper Triassic of Argentina. Journal of Vertebrate Paleontology 17:548–556.

4 thoughts on “Sillosuchus – an odd, big shuvosaurid poposaurid

  1. I had no idea of the existence of poposaurs until a couple of years ago. Up until then, I had thought that all early non-dinosaurian archosaurs were crocodile-like quadrupeds like Postosuchus. Then I found out about creatures like Effigia and Shuvosaurus. I’m amazed that nature had begun experimenting with dinosaur-esque forms among animals which were actually more closely-related to crocodiles, in some cases evolving into even bird-like forms like Effigia.

    When I was growing up, I was repeatedly taught that one of the reasons why dinosaurs did so much better than their archosaurian contemporaries (commonly refered to as thecodonts) was because they could literally and figuratively run circles around them. With these new discoveries, the answer does not seem as clear-cut as it used to be. One wonders if the dinosaurs actually faced much stiffer competition from these dino-imposters than we were originally led to believe. If so, then it makes the question of why dinosaurs succeeded and these other animals failed all the harder to answer. No doubt scientists will be scratching their heads for years to come trying to solve THAT problem.

    One last thing. Based upon the skeletons (both illustrations and mounts) that I have seen, it appears that the tail is much too short to balance out that long body. I’ve frequently seen illustrations and skeletal mounts of poposaurs and rauisuchians showing these animals with their front legs off of the ground, and standing or running bipedally. However, these animals seem to be far too front-heavy to make bipedalism, even occasional/facultative bipedalism, practical. I believe that these creatures were actually quadrupedal.

    • Thanks for your thoughts. Try to think of the quadrupedalism and bipedalism as spectrum, with some reptiles able to do both to a greater or lesser degree. Look for tiny hands on facultative bipeds. And remember, 19 living lizards are capable of bipedal locomotion, but only a few can stand still while bipedal.

      • Yes, I know that there are several species of lizard which are capable of running on two legs (the Australian Frilled Lizard, the Collared Lizard, and the Basilisk just to name three). Your tip regarding small hand size was intriguing, and it set me off on an internet search of thecodont hand images. Effigia and Sillosuchus do indeed have rather tiny hands, and surprisingly so too does Postosuchus. One wonders how useful these tiny hands with their short stubby fingers would have been at walking or grabbing things.

        Even so, with creatures such as Postosuchus or Prestosuchus, with their enormously huge heads and massive bodies, I highly doubt that they would have been good bipedal walkers, as I have seen in some illustrations. I believe, though, that they might have been good at standing on two legs for short durations, very much like the lizards which you spoke of, but were quadrupedal 99% of the time. Perhaps they could run for very short distances (when I say “short” I mean only a few yards) or rear up on their hind legs when battling rivals, like modern-day Komodo Dragons, who will actually stand up and wrestle with each other. That would be an amazing thing to see – two male 20-foot Postosuchus grappling with each other Komodo Dragon-style! That’s a great image for some paleo-art! I think I’ll get to drawing that and adding it to my paleo/history/art blog.

    • One thing that has been suggested to separate things like silesaurids from their dinosaurian kin is their metabolism. At SVP this year several talks looks at the metabolism of silesaurids and found they had a lower metabolic rate, as determined by bone growth, compared to early dinosaurs. They suggested this difference may have played a role in outcompeting their non-dinosaurian contemporaries, especially after the terminal Triassic extinctino.

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