In a wind-swept stretch of Morocco’s Middle Atlas Mountains, paleontologists digging through Jurassic-era stone have unearthed a 168-million-year-old thigh bone. The bone belonged to a cerapodan, a small, plant-eating dinosaur that ran on two legs and would eventually give rise to more familiar names like Iguanodon, Hadrosaurus, and Triceratops.

“It had a groove on the back of the head of the femur (the bit that fits in the hip socket) for a ligament, and we only see that in cerapodans,” said Dr. Susannah Maidment, a paleontologist at the Natural History Museum in London and lead author of the study, published in Royal Society Open Science.

For decades, scientists have pieced together the story of cerapodans primarily through footprints and later fossils from the Cretaceous period, when they were widespread across the planet. But earlier chapters of their saga, during the Middle Jurassic, remained largely blank—thanks to the sheer rarity of rock from that period holding fossil clues. Now, these gaps are slightly narrower.

A Tiny Clue from a Vast Era

The fossil, a proximal left femur, was unearthed from the El Mers III Formation near Boulemane, Morocco—a Middle Jurassic site dating back around 168 million years. That’s around 2 million years older than the previous record-holder fossil for cerapodans, a dinosaur named Callovosaurus from the UK.

At first glance, the fossil might appear unremarkable. But close inspection told a different story. The bone’s architecture—the shape of the femoral head, its separation from the shaft by a distinct neck, and the presence of subtle ridges and grooves—placed it squarely within Cerapoda, a group of ornithischian (bird-hipped) dinosaurs known for their bipedal stance and, eventually, sophisticated chewing systems.

The difference may seem slight, but in evolutionary terms, it signals that these dinosaurs had already split off into their own branch much earlier than we thought. According to the research team, which also included scientists from the University of Birmingham and Morocco’s Sidi Mohamed Ben Abdellah University, this tiny femur adds weight to the idea that early ornithischians were already evolving into distinct groups by the Middle Jurassic.

A Hidden Hotspot for Dinosaur Evolution

The Bathonian rocks of the El Mers III Formation have now delivered not one, but several record-breaking fossils.

The formation has already provided the earliest known ankylosaur (Spicomellus afer) and one of the oldest stegosaurs (Adratiklit boulahfa). Now, it can lay claim to the oldest cerapodan as well. That’s three major lineages of ornithischians represented in a single geologic window.

“The Middle Jurassic appears to have been a critical time in the origin and establishment of dinosaur-dominated terrestrial ecosystems,” the authors note. And yet, because so few fossils from this period have been discovered, our understanding remains limited.

The researchers suggest the ecological upheaval caused by the Toarcian Oceanic Anoxic Event—a massive climate disturbance that warmed the planet and altered ecosystems—may have opened new niches for dinosaurs like cerapodans to exploit. “Intervals of climatic upheaval are frequently considered responsible for major biotic turnovers,” the team wrote.

Why This Femur Matters

Cerapodans are better known from the Cretaceous Period, when they evolved into a spectacular array of herbivorous giants. But the early history of the group has remained obscure.

Before this find, most Middle Jurassic evidence came from footprints—vague imprints of large, plant-eating dinosaurs that hinted at their existence but told us little about their anatomy or evolution. The only prior body fossil from this period was the aforementioned Callovosaurus.

In contrast, the new fossils designated “USMBA 001” confirm that cerapodans had already evolved—and diversified—by the Bathonian stage of the Middle Jurassic. The discovery also places their origins firmly in Gondwana, the southern half of the ancient supercontinent Pangaea, challenging older models that focused heavily on Laurasian (northern) fossils.

While the bone offers tantalizing clues, it doesn’t provide enough information to name and designate a new species. Still, it may be only the beginning.