Beneath the sun-baked sediments of the Morrison Formation in western Colorado rests the story of a quiet survivor. With delicate limbs, a lightweight frame, and an unexpectedly complex history, Enigmacursor mollyborthwickae was not the type to command attention through brute strength or size. Instead, this dinosaur’s story is one of survival through subtlety—its very bones a riddle locked in stone. Named for both its mysterious background and its sprightly build, Enigmacursor gives us a rare glimpse into the quiet diversity of small-bodied herbivores that shared the Jurassic landscape with giants.

Despite having existed during one of the most dynamic periods in dinosaur evolution, this nimble biped remained elusive to science for over a century. Fragmentary remains attributed to similar animals had long frustrated researchers, often relegated to the ambiguous status of nomen dubium. But the recent discovery of a well-preserved skeleton has finally lifted the veil, revealing new insights into a long-overlooked group of dinosaurs. What emerges is a more complete picture of a fast-moving, plant-eating creature that thrived in the shadows of towering sauropods.

Enigmacursor Key Facts

Enigmacursor Origins, Taxonomy and Timeline

The name Enigmacursor is every bit as telling as the dinosaur it represents. “Enigma” draws attention to the tangled, confusing lineage of small ornithischians in the Morrison Formation—species that have long been debated, reclassified, or simply left unnamed due to fragmentary fossils. “Cursor,” borrowed from Latin, means “runner,” a nod to its lean, leggy limbs that suggest it was built for speed. Together, the name captures both the historical mystery and the physical agility of this lightweight herbivore.

Classified within Ornithischia, Enigmacursor mollyborthwickae falls near the base of Neornithischia—a major dinosaur clade that includes both “duck-billed” ornithopods and “frill-necked” marginocephalians, but excludes armored thyreophorans. Unlike the more derived members of this group, Enigmacursor lacks the anatomical specializations seen in ornithopods and ceratopsians, instead aligning with other small-bodied, bipedal herbivores that occupied a more primitive evolutionary position. Its close relatives may have included similarly gracile forms like Hypsilophodon or Agilisaurus, though its exact affinities remain under study.

The type species, Enigmacursor mollyborthwickae, is the sole representative of its genus. The species name honors Molly Borthwick, whose generous contribution helped ensure the specimen’s preservation at the Natural History Museum in London.

As for its place in time, this dino roamed the earth during the Late Jurassic, roughly between 156.0 and 146.0 million years ago. This period, spanning the Kimmeridgian to the Tithonian Stages, was a time of towering sauropods, explosive plant growth, and evolutionary experimentation. The Morrison Formation, where Enigmacursor’s remains were unearthed, is one of the richest Jurassic fossil beds in North America—a window into a vibrant, competitive ecosystem.

Discovery & Fossil Evidence

The skeleton that would become Enigmacursor was uncovered between 2021 and 2022 by a private firm, Dinosaurs of America, LLC, operating in Moffat County, Colorado. The site—Skull Creek Estates—yielded a three-dimensionally preserved partial postcranial skeleton, along with several isolated teeth. What made this discovery particularly valuable was the quality and coherence of the preserved material. Although incomplete, the specimen retained enough anatomical association to provide a clear sense of the animal’s overall body plan and proportions.

Now housed at the Natural History Museum in London under the label NHMUK PV R 39000, this holotype includes multiple vertebrae, ribs, limb bones, pelvic elements, and both feet. While the skull is largely missing, the teeth suggest positions within the premaxillary region. The presence of femora, tibiae, and pedal elements offers vital clues about its locomotion and size. Notably, radiographs and 3D scans were employed to differentiate original material from reconstructed parts, aiding in detailed anatomical study.

So far, there have been no other formally named finds associated with this genus, making Enigmacursor mollyborthwickae a one-specimen taxon. But given the history of misidentified or unnamed small-bodied ornithischians in the Morrison Formation, more specimens may yet emerge under clearer taxonomic light.

Enigmacursor Size and Description

Before diving into the physical specifics, it’s worth noting that Enigmacursor’s build aligns well with other small-bodied neornithischians. Designed for agility and speed, its anatomy leans heavily toward a cursorial lifestyle, suggesting a dinosaur well-suited for darting between the legs of giants or fleeing from larger predators.

Short Description of Enigmacursor

With an elongated femur and slender tibia, this little runner had the build of a sprinter. Its vertebrae show no signs of extensive ornamentation or armor, keeping its body lightweight. The tail was likely used for balance, stiffened by overlapping tendons as seen in other similar dinosaurs. It had relatively long arms for its size, with a well-developed deltopectoral crest on the humerus, suggesting decent muscle attachment for maneuvering through dense undergrowth.

The teeth—small and denticulate—indicate a herbivorous diet, suited to cropping soft vegetation. Though most of the skull is missing, the preserved teeth resemble those of other basal ornithischians, suggesting similar feeding strategies. The feet had four functional toes, a trait retained from earlier ornithischian ancestors, and were built for agility. This supports the interpretation of Enigmacursor as a fast, bipedal herbivore, well adapted for navigating the diverse landscapes of the Late Jurassic.

Size and Weight of Type Species

Estimates put Enigmacursor mollyborthwickae at about 5.0 feet in length and just over a foot tall at the hip—around 1.3 feet. With a body mass of approximately 40–50 kg (88–110 lbs), it would have been comparable in size to a medium-sized dog. Unlike its towering contemporaries, this dinosaur likely relied on stealth and agility rather than size or strength for survival.

While there are no competing size estimates for this species at present, the measurements of the holotype offer a fairly clear picture. The femur measured 169 mm (about 6.7 inches), and the tibia stretched even longer—suggesting strong lower legs for quick movement. The combination of a lightly built body and proportionally long limbs would have made it an efficient runner, capable of quick bursts of speed.

Such proportions set Enigmacursor mollyborthwickae apart from stockier, more robust ornithischians. Its physical design speaks of a dinosaur that occupied a very specific ecological niche—small, fast, and hard to catch.

The Dinosaur in Detail

One feature that stands out is the structure of the femur. In Enigmacursor, a trochanteric fossa is present between the femoral head and greater trochanter—a condition found in some basal neornithischians. The ligament sulcus, a groove typically located on the posterior surface of the femoral head for ligament attachment, is absent—a feature that helps differentiate it from more derived ornithischians in which the sulcus is usually present.

The ilium lacks a supracetabular crest, a condition that is common among early neornithischians and contrasts with the more prominent crests seen in basal ornithischians and some ornithopods.

In the lower leg, the tibia features a medially directed, hook-like posterior condyle, a distinctive morphology not widely seen in other ornithischians. Taken together, these anatomical traits (and others) supported the recognition of Enigmacursor as a valid new genus, distinct from historically problematic taxa like Nanosaurus.

Phylogenetic analysis places Enigmacursor as a basal neornithischian outside Cerapoda (the clade exclusively comprised of Ornithipoda + Marginocephalia), with its closest known relative identified as Yandusaurus hongheensis from the Late Jurassic of China. This relationship hints at the early radiation and broad geographic spread of small-bodied neornithischians during the Jurassic.

Interesting Points about Enigmacursor

Contemporary Dinosaurs

Present in this period is Nanosaurus, another small-bodied neornithischian herbivore that, like Enigmacursor, navigated the understory of the Late Jurassic world. While both were agile bipeds, their fossil histories have diverged—Nanosaurus has long been taxonomically unstable, based largely on fragmentary material. If the two did coexist, they may have divided ecological space by favoring different plant types or feeding strategies.

Among the more imposing contemporaries was the iconic thyreophoran ornithischian Stegosaurus, a heavily armored herbivore whose low-slung body and spiked tail contrasted starkly with Enigmacursor‘s light frame. Though vastly different in size and defense, both would have browsed at similar heights, possibly targeting different vegetation or feeding at different times to reduce competition. The presence of such a formidable neighbor may have also offered incidental protection—carnivores would think twice before picking a fight near a thagomizer.

Predators were ever-present threats. The Morrison Formation hosted large theropods such as Allosaurus, Ceratosaurus, and Torvosaurus, all of which would have considered small dinosaurs like Enigmacursor easy and opportunistic prey. Its small size and agility likely helped it evade these predators, but survival depended on constant vigilance and speed in a landscape dominated by formidable hunters.

Even among the giants, diversity was more than it first appears. High-browsing sauropods like Brachiosaurus dominated the treetops, while low-slung Diplodocus raked ferns and understory plants. Their contrasting feeding styles created a vertically stratified plant world that benefited nimble foragers like Enigmacursor, which thrived in the gaps—feeding, fleeing, and surviving under the feet of giants.

Enigmacursor in its Natural Habitat

The Morrison Formation during the Late Jurassic was a rich and varied environment—semi-arid but punctuated by seasonal rainfall and lush river valleys. The terrain alternated between open woodlands, fern meadows, and floodplains. In this mixed landscape, a small, agile herbivore like Enigmacursor would have had access to low-lying vegetation, ideal for a ground-based browser.

Being an herbivore, it likely fed on soft plants—ferns, cycads, and juvenile horsetails. Its sharp, ridged teeth would have been perfect for snipping stems and leaves, but not for chewing. Swallowed vegetation would probably have been broken down in the gut, aided by internal bacteria. Moving on two legs, this dino was able to cover ground efficiently, whether searching for food or fleeing from danger.

While no direct evidence exists about its social habits, it may have lived in small groups, like some of its relatives. Herding would have provided some protection, as there is safety in numbers, especially for small prey animals. With a keen sense of hearing and wide field of vision, it would have been alert to threats at all times. In shaping its habitat, Enigmacursor may have contributed to seed dispersion and plant cycling, playing a quiet but vital role in its ecosystem.

Frequently Asked Questions

Sources

The information in this article is based on various sources, drawing on scientific research, fossil evidence, and expert analysis. The aim is to provide a comprehensive and accurate overview of Enigmacursor. However, please be aware that our understanding of dinosaurs and their world is constantly evolving as new discoveries are made.

This article was last fact checked: Joey Arboleda, 06–27–2025

Featured Image Credit: Connor Ashbridge, CC BY 4.0, via Wikimedia Commons