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.
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 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.”
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.
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