Full Story: ZME Science (3/23)
A 168-Million-Year-Old Dinosaur Bone Sheds Light on a Hidden Evolutionary Era
Researchers push back the origin story of cerapodan dinosaurs by two million years.
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.
