Pisanosaurus mertii

Pisanosaurus is an unusually-preserved and controversial dinosaur(?) from the Late Triassic of Argentina. The first challenge is preservation - Pisanosaurus is known from fragmentary remains, some of which eroded away before the skeleton was discovered. The eroded bones left behind a natural mold in the rocks that preserved their shape (on one side), which is better than nothing, but not ideal (and some of the rock molds have since become damaged or lost). Perhaps because of this limited amount of information, it’s debated whether Pisanosaurus is the earliest known and most primitive ornithischian dinosaur, or whether it’s actually a silesaurid. Making this an even stickier-wicket, while most phylogenetic analyses have found silesaurs to be just outside of true dinosaurs, some studies find them to be basal ornithischians, in which case Pisanosaurus would be both a silesaur and an early ornithischian dinosaur.

Clearly we will need either more specimens of Pisanosaurus, or more clarity on silesaur relationships (or better yet, both) to settle this debate.