Enigmacursor

MEANING: Mysterious runner

PERIOD: Late Jurassic

CONTINENT: North America

Enigmacursor is a small basal ornithischian dinosaur that grew to about 1.5 m in length. Typical of early members of this group, it had a relatively long neck and a small head with a beak. The dense hips and long slender legs suggest Enigmacursor was well adapted for running, an essential trait that the small herbivore would have used to evade predators.

Abstract from paper: Although their remains have been known since the 1870s, the small, bipedal ornithischian dinosaurs from the Upper Jurassic Morrison Formation of the western USA remain poorly known. The historic type specimens are incomplete and poorly preserved and have recently been designated as nomina dubia. Here, we describe a recently collected, partial but three-dimensionally preserved skeleton of a new small-bodied ornithischian from the Morrison Formation of Colorado, USA, that we name Enigmacursor mollyborthwickae gen. et sp. nov. The skeleton includes substantial portions of the axial and appendicular skeleton and, when scored into a phylogenetic analysis, is shown to be a non-cerapodan neornithischian, whose closest relative is Yandusaurus hongheensis from the Late Jurassic of China. The discovery of Enigmacursor enhances the diversity of ornithischian dinosaurs from the Morrison Formation and provides new information on their anatomy. In addition, it demonstrates that there is additional cryptic diversity of small-bodied Morrison Formation ornithischians, suggesting they were a more diverse component of these Late Jurassic ecosystems than was previously realized.

Enigmacursor is from the Late Jurassic. The Late Jurassic was a dynamic period, spanning from about 162 to 143 million years ago. The continents were continuing to drift apart, and the supercontinent Pangaea had fully split into Laurasia in the north and Gondwana in the south. This continental rearrangement led to the formation of large inland seas and shallow coastlines that fostered diverse ecosystems. The climate during the Late Jurassic was warm and humid, with lush forests of conifers and ferns that stretched across much of the continents, creating a rich ecosystem where dinosaurs flourished the dominant land animals.

Dinosaurs continued to diversify through the Late Jurassic, with some of the most famous species evolving in this time. Many well-known sauropods, such as Brachiosaurus and Apatosaurus roamed the land, exhibiting niche partitioning with their selectively distinct neck positions. Alongside them, stegosaurs became widespread, their plates and spikes making them one of the era's most recognizable groups. Theropods like Allosaurus and Ceratosaurus were the apex predators, evolving large, powerful bodies and sharp teeth that allowed them to hunt the gigantic herbivores. The early evolution of birds was taking place, setting the stage for the numerous species that would fill the skies in the eras to come.

Unlike the dramatic mass extinctions that marked the beginning and end of the Mesozoic, the Jurassic Period ended without a sharp boundary. As the continents continued to drift, ecosystems gradually transformed into unique habitats that supported the more specialized dinosaur species of the Cretaceous.

Enigmacursor is a basal ornithischian. Ornithischia is one of the two major clades of dinosaurs, sister to the saurischia, which includes theropods and sauropods. Basal ornithischians were small, lightly built herbivores or omnivores and were typically bipedal, with long hind limbs and relatively short forelimbs, well-suited for swift movement. Their body plans were often simple, lacking the extreme specializations seen in later ornithischians like ceratopsians or hadrosaurs. Many had simple, triangular skulls with leaf-shaped teeth, suitable for cropping low vegetation, and some evidence suggests they may have lived in groups. Though they form the base of the ornithischian family tree, they do not belong to any of the more derived subgroups and instead exhibit a mix of ancestral traits.

Basal ornithischians are mostly known from the Jurassic. The absence of definitively known ornithischians from the Triassic has puzzled paleontologists, though some propose that certain Triassic animals, like the silesaurs, may fill this gap. However, these relationships remain debated. As the Jurassic progressed, basal forms gave rise to a wide array of more specialized ornithischians, but these early dinosaurs remain a crucial and still somewhat mysterious chapter in the evolutionary history of the group.