Dino of the Week: Pisanosaurus

our feathered friend Pisanosaurus

Type Species: Pisanosaurus mertii

Classification: Dinosauria - Ornithischia - Heterodontosauridae*

Time Period: Late Triassic

Location: Argentina

Diet: Herbivore

several Pisanosaurus flee a roving rauisuchian

The small, herbivorous dinosaur Pisanosaurus lived in what is now Argentina during the Carnian stage of the Late Triassic, around 228 to 216 mya. This dinosaur was lightly-built and grew to about three feet in length and probably weighed around five to twenty pounds. Pisanosaurus was found in a locale that, during its day, was a volcanically active floodplain covered by dense forests with a warm and humid climate subject to seasonally strong rainfalls. As a ground-dwelling herbivore it likely fed on ferns and horsetails, and it may have sought to hide from the fast-moving theropods Herrerasaurus and Eoraptor by seeking sanctuary in the giant conifers of the highland forests that grew up along the riverbanks. It may have competed with other Carnian herbivores such as the crocodile-like rhynchosaurs, the armored aetosaurs, and two-tusked dicynodonts. It would’ve kept a close lookout for the top predators of its day: the heavyweight rauisuchians and the dreaded pseudosuchian Sarchosuchus, the Top Dog in the volcanic basin (though given Pisanosaurus’ size and agility, it may have feared being trampled more than being hunted!).

a single Pisanosaurus seeks shelter among the giant conifers
of Argentina's Late Triassic volcanic basin

Up until 2017 Pisanosaurus was viewed by most paleontologists as the oldest known ornithischian dinosaur. Ornithischians were the ‘bird-hipped’ dinosaurs, as opposed to the ‘lizard-hipped’ saurischians. Ornithischians of the later Mesozoic included ornithopods and lambeosaurs, ceratopsians and pachycephalosaurs, and stegosaurs and ankylosaurs. The dominant dinosaurs of the Late Triassic were the saurischian prosauropods and theropods, so Pisanosaurus’ presence in the Late Triassic is important (for a long time it was thought that ornithischians didn’t appear until the Jurassic). In recent years, however, some scientists have argued that Pisanosaurus wasn’t a dinosaur at all but a silesaurid. The silesaurids were ‘wanna-be’ dinosaurs, cousins to the lineage that would eventually dominate the Mesozoic. Debate still rages: “Is Pisanosaurus the earliest and most primitive ornithischian, or is it an impostor whose family line was doomed to extinction?”

* If Pisanosaurus is not a dinosaur after all (see paragraph above!), then it's classification would be more like Dinosauriformes - Dracohors - Silesauridae