Restoring Sacisaurus

Figure 1. The skull of Sacisaurus reconstructed and restored. Small black image from Ferigolo and Langer 2006.

Figure 1. The skull of Sacisaurus reconstructed and restored. Small black image from Ferigolo and Langer 2006. The tooth number is from the text, 10 in the maxilla, 15 in the dentary. 

Sacisaurus agudoensis (Ferigolo and Langer et al. 2006, Fig. 1) Carnian, Late Triassic period, ~225 mya, 1.5 m in length, is one of the oldest dinosaurs yet found. Only a few bones are known (Fig. 2). Here Sacisaurus nests as the basal poposaurid, descended from a sister to the basal phytodinosaur, Pampadromaeus. It has been an oversight, here repaired, that Sacisaurus has not been included in the large reptile tree until now.

The skull includes paired predentary bones at the tip (more on this below). The antorbital fenestra remains large. The teeth have an expanded crown. The large postorbital indicates a larger orbit with a raised cranium relative to the rostrum, which is largely unknown.

Sacisaurus from Ferrigolo and Langer (2006).

Figure 2. Sacisaurus from Ferigolo and Langer (2006).

Only one cervical has been found and it had a parallelogram-shaped centrum for an elevated skull. The caudals were elongate as in other dinosaurs.

The euarchosauriformes, now inlcuding Sacisaurus as a basal poposaurid.

Figure 3. The euarchosauriformes, now inlcuding Sacisaurus as a basal poposaurid. Click to see complete reptile tree.

The scapula was elongate. The pelvis was more like that of Pampadromaeus with an angled pubis. The hind limbs were robust.

Altogether, nothing much new to say here other than this taxon shares many traits with Panphagia and Pampadromaeus. The mandible is more robust in Sacisaurus. The antorbital fenestra is smaller. The addition of Sacisaurus to the large reptile tree (Fig. 3) did not disrupt the topology.

About that predentary
Ferigolo and Langer (2006) report, “In two specimens (Figure 3C–F), it is possible to recognize that the depressed mandibular rostral portion is formed by a subtriangular separate ossification, the caudal margin of which extends obliquely below and above the mental foramen. This demarcation is not visible in other mandibles in which the bone is apparently fused to the dentary.” So, this fusion can be present or not in later related taxa.

They also report, “Also unlike ornithischians, the teeth of Sacisaurus are not markedly inset from the lateral margin of the bearing bones, and the upper series does not reach the caudal end of the maxilla.” 

Denticles are present on the teeth, but at this scale, they are so small as to be serrations, like those of their ancestors among the theropod dinosaurs.

Ferigolo and Langer (2006) report, “If the pelvic bones assembled from the type-locality (Figure 2) belong to Sacisaurus, the new taxon represents one of the three putatively propubic ornithischians, the others being Pisanosaurus (Sereno 1991) and Silesaurus (Dzik 2003).”

Ferigolo and Langer (2006) comment on, “other referred material posses theropod features such as a ventrally excavated ectopterygoid with a strongly curved jugal process, long prezygapophises on the distal caudal vertebrae, and a well-developed fibular flange in the tibia.” And such holdover or plesiomorphic traits are to be expected at the base of new clades.

Interestingly, they report, “The ilia of Sacisaurus and Silesaurus are also atypical for dinosaurs, but resemble those of poposaurid rauisuchians.” And here’s where it gets interesting.

So what is the predentary?
According to Ferigolo and Langer (2006), “the predentary does not seem to represent a neomorphic structure of these dinosaurs. Instead, its homology to the mentomeckelian bone, and possibly also to parts of the rostral portion of most vertebrate mandibles, is proposed here. In this case, it does not correspond to a dermal bone as most of the lower jaw, but to an ossification preceded by cartilage.”

Massospondylus dentaries with a place for a missing predentary. Did this jaw have and lose the predentary? Phylogenetic bracketing says yes.

Figure 4. Massospondylus dentaries (medial view left, lateral view right) with a place for a missing predentary. Did this jaw have and lose the predentary? Phylogenetic bracketing says yes. From Barrett 2009.

And did prosauropods have, then lose the predentary?
Phylogenetic bracketing says yes. And Massospondylus kaalae (Fig. 4) seems to be missing something at the tip of its jaws. Worth looking into.

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
Barrett PM 2009.
 A new basal sauropodomorph dinosaur from the upper Elliot formation (Lower Jurassic) of South Africa. Journal of Vertebrate Paleontology 29(4):1032-1045.
Ferigolo and Langer 2006. A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone. Historical Biology, 2006; 1–11, iFirst article

wiki/Sacisaurus

4 thoughts on “Restoring Sacisaurus

  1. Wait a second. A month ago you had sauropodomorphs sister to Ornithischia, now you have them sister to ‘Paraornithischia’. If Sacisaurus didn’t change the topology, what did?

    For Sacisaurus’ cervical series, only an atlantal intercentrum is known. You’d know this if you read the text instead of depending on the reconstruction. Its denticles are actually large, as seen in figure 3J, just like most sauropodomorphs and ornithischians. The ectopterygoid is too large, so was excluded by Nesbitt (2011), probably belonging to the larger saurischian ilium and femora that were mixed in with the type material. Long distal caudal prezygapophyses and a fibular crest on the tibia wouldn’t make sense as symplesiomorphies in your tree, since they’re absent basally in your sauropodomorphs, ornithischians, Panphagia and Pampadromaeus.

    As the rest of Massospondylus kaalae’s holotype has plenty of broken surfaces (note the bone fragments adhering to the dentary in your picture, and the tooth tips), the anterior dentary tip is probably broken too. Note phylogenetic bracketing actually suggests it lacked a predentary even in your tree with your fictional “paired predentaries”. Remember you don’t order your characters, so “paired predentaries” and median predentaries have nothing to do with each other when running your matrix. Even if you misidentify that bit of sediment in Daemonosaurus as a predentary, parsimony would argue it is a median element as you have the taxon in Ornithischia. Thus if M. kaalae has a predentary in your tree, it would just have to reverse in other sauropodomorphs and would add a step to your tree. Or more likely it would just be an extra step to evolve the predentary in M. kaalae, since other massospondylids lack predentaries (note Crompton and Attridge, 1986 postulating a keratinous beak in M. carinatus has long been discredited).

    As for Sacisaurus being the basalmost ‘paraornithischian’, your characters are no doubt…
    – Maxillary teeth less than twice as tall as wide is only true of some of Sacisaurus’ maxillary teeth, which is the case for most every other dinosaur in your tree (even theropods). Note too you coded the toothless Shuvosaurus, Lotosaurus and Effigia as having short teeth.
    – A straight dentary is also present in M. kaalae (miscoded by you), some Thecodontosaurus (miscoded by you), Daemonosaurus (miscoded by you), every ornithischian you use (your basal three miscoded by you) and Panphagia (miscoded by you), so would not place it in ‘Paraornithischia’.
    As none of your other ‘paraornithischian’ characters are scorable in Sacisaurus (the straight ventral maxillary margin presumedly shifted to ‘Paraornithischia’+Sauropodomorpha when your topology changed, I’d say your placement of it in that clade is firmly rejected.

  2. Unrelated to the post (for the most part) but I noticed that you have Scelidosaurus basal to Scutellosaurus. In fact you have it basal to all thyreophorans, including osteoderm-free taxa like like Heterodontosaurus which traditionally nests as basal to Scutellosaurus. Now, as Mickey has pointed out previously on his page, I am no cladistic genius BUT my current research (which is much better supported than my previously published cladograms) is looking into basal thyreophora (see my SVP abstract from last year). None of my trees (using TNT) recover such an unconventional topology like yours. What is leading you to this unusual conclusion?

    • Cerapoda and a hypothetical (all ancestral 0s) outgroup. Changing to just one of those doesn’t affect the topology further up the tree.

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