Two New Species from New Mexico Help Fill Gap in Evolution of Horned Dinosaurs
Two new transitional species of plant-eating horned dinosaurs have been unearthed in New Mexico, the United States.
Navajoceratops sullivani and Terminocavus sealeyi. Image credit: Ville Sinkkonen Denver Fowler.
The newly-discovered dinosaurs roamed the Earth approximately 75 million years ago (Cretaceous period).
Named Navajoceratops sullivani and Terminocavus sealeyi, both species belong to Ceratopsidae, the same family as famous horned dinosaurs Triceratops, Centrosaurus, and Styracosaurus.
Their fragmentary skulls were uncovered from the Hunter Wash Member of the Kirtland Formation in New Mexico.
The specimens are intermediate in age between two previously known ceratopsid dinosaurs Pentaceratops and Anchiceratops.
Pentaceratops lived 75.3 million years ago in New Mexico and had a distinctive deep notch on the back border of the frill, and a pair of spikes at the center of the frill that turn outwards like the wings of a butterfly.
Anchiceratops had no notch in its frill and lived 3.8 million years later in what is now Canada.
In the 1990s, Texas Tech University paleontologist Thomas Lehman proposed that Pentaceratops might have been the ancestor of Anchiceratops.
Navajoceratops sullivani and Terminocavus sealeyi are intermediate in shape between these two dinosaurs and show how the notch in the frill became even deeper through time and eventually closed in on itself, explaining the lack of a notch in Anchiceratops.
“The two intermediate skulls form important links in a 5 million year lineage stretching from Utahceratops through Pentaceratops, to Anchiceratops,” said study authors Dr. Denver Fowler and Dr. Elizabeth Freedman Fowler from Badlands Dinosaur Museum and Museum of the Rockies.
The parietal frills of Navajoceratops sullivani (top) and Terminocavus sealeyi (bottom). Image credit: Fowler Freedman Fowler, doi: 10.7717/peerj.9251.
The new specimens revealed a splitting event deep in the evolutionary history of long-frilled ceratopsids (chasmosaurines), after which a Pentaceratops lineage evolved a progressively deepening notch in the frill, contrasting against its sister group, the Chasmosaurus lineage, which evolved a progressively shallower notch.
“The origin of this evolutionary split occurred during the Late Cretaceous period, when a vast interior seaway flooded the lowlands of North America dividing it into eastern and western subcontinents,” the paleontologists said.
“A short period of especially high sea level 85-83 million years ago brought the edge of the sea very close to the young Rocky Mountains.”
“For hundreds of miles across what is now central Utah to southern Alberta, the coastal plain would have been as little as 5-10 km wide, providing very little habitat for dinosaurs.”
“This would have effectively cut off northern and southern populations, which then probably evolved in isolation into two distinct lineages. However, after 83 million years ago, the sea receded from the mountain front, allowing northern and southern populations to mix again.”
The team’s paper appears in the journal PeerJ.
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D.W. Fowler E.A. Freedman Fowler. 2020. Transitional evolutionary forms in chasmosaurine ceratopsid dinosaurs: evidence from the Campanian of New Mexico. PeerJ 8: e9251; doi: 10.7717/peerj.9251
This article is based on text provided by Dickinson Museum Center.
