Dinosaur skulls lie. Not deliberately, but by absence. The bones that form a dinosaur’s head are thin, loosely hinged, and among the first to scatter when a carcass begins to decompose. What paleontologists usually find, if they find cranial material at all, are fragments: a partial snout here, a cheekbone there, enough to suggest a shape but not enough to confirm one. For stegosaurs specifically, a group that roamed Europe, Africa, and the Americas for tens of millions of years, the skull has remained one of paleontology’s more stubborn blind spots.
That changed in Riodeva, a small municipality in the Spanish province of Teruel, where excavators working a site called “Están de Colón” pulled a partial skull from Late Jurassic sediment that was preserved well enough to show what previous finds could only approximate. The snout outline was intact. The braincase structure was readable. The facial bone geometry, the kind of anatomical detail that tells researchers how a species actually looked and how its head functioned, was there to be examined rather than guessed at.
The specimen belongs to Dacentrurus armatus, the species that has anchored the European stegosaur record since its first scientific description in 1875. According to researchers at the Fundación Conjunto Paleontológico de Teruel-Dinópolis, who led the excavations and authored the study, it is the most complete stegosaurian skull ever recovered on the continent. Their findings were published in the peer-reviewed journal Vertebrate Zoology, exactly 150 years after the species entered the scientific literature.
One Skull, 150 Years of Questions Finally Answered
Completeness alone would have made this find significant. What the researchers extracted from it goes further.
Examining the Riodeva skull gave the team enough anatomical resolution to identify a new autapomorphy, a physical feature exclusive to Dacentrurus armatus that distinguishes it from every other stegosaurian species. The source material does not name the specific feature, but its identification was considered consequential enough to formally revise the species’ scientific diagnosis, the definition that paleontologists use when deciding whether a newly found fossil belongs to this species or another.
That kind of revision has downstream effects. Species diagnoses are the data points on which evolutionary family trees are constructed. An incomplete or incorrect diagnosis introduces errors that propagate through every subsequent analysis that relies on it. For Dacentrurus armatus, a species studied for a century and a half, the Riodeva skull offered the first opportunity to test and correct that foundation against genuinely well-preserved cranial material.
“The detailed study of this exceptional fossil has allowed us to reveal previously unknown aspects of the anatomy of Dacentrurus armatus,” lead researcher Sergio Sánchez-Fenollosa said in a statement accompanying the publication. The skull came from the Villar del Arzobispo Formation, sediments laid down roughly 150 million years ago during the Late Jurassic, recovered during excavations led by the Fundación Dinópolis and its affiliated research teams in Teruel.
The Family Tree Just Got Redrawn
The skull study did not stop at anatomy. Armed with new cranial data, the research team built a revised stegosaurian data matrix and ran a fresh phylogenetic analysis using Maximum Parsimony, a standard method for mapping evolutionary relationships from physical characteristics. What came back challenged a classification that had been gaining acceptance.
Some researchers had proposed merging Stegosaurus, the North American giant most people picture when they hear the word stegosaur, with Wuerhosaurus, a Chinese genus, into a single taxonomic group based on perceived shared features. The Teruel team’s analysis does not support that merger. Their results treat the two as distinct genera, a conclusion that, if accepted by the wider field, reverses a classification that had been treated by some as settled.
The analysis also formally proposed a new grouping called Neostegosauria, defined to include medium to large stegosaurian species that spread across Africa and Europe during the Middle and Late Jurassic, North America in the Late Jurassic, and into Asia through the early Cretaceous. The team placed Isaberrysaura mollensis within Huayangosauridae as a confirmed stegosaur and ran Mongolostegus exspectabilis through a stegosaurian phylogenetic framework for the first time.
That last result suggests a lineage of early stegosaurids may have survived in Asia considerably longer than previously recognized, pushing the group’s known range into the late Early Cretaceous.
Beneath the hillside, the story is still being written
The skull is the headline, but Riodeva has not finished producing. The “Están de Colón” site has also yielded additional postcranial bones from the same adult individual, still being studied, along with something rarer still: juvenile specimens of Dacentrurus armatus from the same locality.
Finding adults and juveniles of the same species at a single site is unusual in the stegosaur record, and the scientific value is specific. Juvenile material from the same population as a known adult allows researchers to track how the animal’s skeleton changed as it grew, separating age-related variation from true species-level differences.
That distinction has created persistent problems in dinosaur taxonomy, where juveniles of one species have sometimes been misidentified as adults of another. The Riodeva collection, if it delivers on what the researchers describe, would give scientists a developmental window into Dacentrurus armatus that no European site has offered before.
Further publications from the site are expected as the remaining material is analyzed and described.