New Research Adds to the Understanding of Ceratopsid Evolution

New research by Dr. Caleb Brown published today in the journal PeerJ analyses two isolated ceratopsid horncores and contributes to our scientific understanding of the evolution of ceratopsid cranial display structures.

Some of the diversity of ceratopsid cranial display structures. Illustration by Julius Csotonyi.

Some of the diversity of ceratopsid cranial display structures. Illustration by Julius Csotonyi.

Ceratopsidae were a diverse and abundant group of large-bodied horned dinosaurs from the Late Cretaceous that are well-documented in the fossil record (e.g., Triceratops and Centrosaurus). They represent one of the last major evolutionary diversification of non-avian dinosaurs before the Cretaceous-Palaeogene mass extinction 66 million years ago; however, there is a lack of scientific understanding of early ceratopsid evolution due to the rarity of known specimens.

There are two known groups within Ceratopsidae: centrosaurines and chasmosaurines. The most significant differences between these two groups are their cranial ornamentation, specifically frill length and horn size. The last decade has seen dramatic increase of newly described species within both groups, specifically specimens occurring in geologically earlier time periods. Despite these new discoveries, our understanding of the early evolution of chamosaurines is more limited than centrosaurines.

A centrosaurine skull. Note the large nasal horn, the short frill, and the spikes at the top of the frill. Illustration by Julius Csotonyi.

A centrosaurine skull. Note the large nasal horn, the short frill, and the spikes at the top of the frill. Illustration by Julius Csotonyi.

A chasmosaurine skull. Note the long frill, large postorbital horns above the eye sockets and small nasal horn. Illustration by Julius Csotonyi.

A chasmosaurine skull. Note the long frill, large postorbital horns above the eye sockets and small nasal horn. Illustration by Julius Csotonyi.

In Dr. Brown’s new research, he analyzed two isolated ceratopsid horncores to better understand the evolution of ceratopsian cranial display structures. The first specimen was collected in 1989 by Museum staff (but was not prepared until recently) and is a large, nearly complete bottom half of a left horncore (a horn situated above the eye socket) and includes sinus cavities at the base of the horncore. The second specimen was collected this past fall and is a smaller left postorbital horncore that was probably from a juvenile ceratopsid.

These two postorbital horncores are from the Foremost Formation in Southern Alberta and are about 78.5 million years old. They are the only horncores described from that formation and represent some of the geologically oldest ceratopsid material in the world. In contrast to the younger formations in the Late Cretaceous Belly River Group, such as the Oldman and Dinosaur Park formations, dinosaurs from the older Foremost Formation are much rarer, less diverse, and more poorly understood. These two horncores add to our growing understanding of the early evolution of postorbital horns in this group.

Top: The first specimen, the lower half of a left postorbital horn core that includes sinus cavities at the base of the horn core. Bottom: The second specimen, a smaller left postorbital horn core that was probably from a juvenile ceratopsid.

Top: The first specimen, the lower half of a left postorbital horn core that includes sinus cavities at the base of the horn core. Bottom: The second specimen, a smaller left postorbital horn core that was probably from a juvenile ceratopsid.

These two postorbitals represent another appearance of long-horned forms in early occurring centrosaurines. This supports evidence that this is a plesiomorphic (ancestral) condition for this family. Previous research shows that there is an emerging pattern where the postorbital horns of many centrosaurines exceed in size the horns of their contemporary chasmosaurine counterparts. This pattern is reversed later in the Cretaceous Period, with an evolutionary decrease in the size of postorbital horncores in centrosaurines, and an enlargement of postorbital horncores in chasmosaurines.

The large postorbital horncore (first specimen) projected onto the skull of a hypothetical centrosaurine.

The large postorbital horncore (first specimen) projected onto the skull of a hypothetical centrosaurine.

Although incomplete, these newly described postorbital horncores add to the current literature of the evolution of ceratopsian display structures and early ceratopsid groups. “This research shows that even small, isolated specimens can add to our overall knowledge of the evolution of prehistoric animals,” stated Dr. Brown.

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