Ceratosaurus nasicornis

Ceratosaurus nasicornis was a distinctive large theropod of Late Jurassic western North America. It was smaller and less heavily built than the largest contemporary Allosaurus, but carried a proportionally deep skull with long blade-like teeth, a prominent bony horn core above the nose and elevated ornament over both eyes. The nearly complete Garden Park holotype makes its skeleton unusually informative. Ceratosaurus also preserves postcranial osteoderms—small bones formed in the skin—an exceptional feature among known theropods.

Its fossils occur between approximately 154 and 148 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

The skull was deep and narrow in front view, with a fused nasal structure rising into the famous midline horn core. Paired lacrimal prominences formed lower ornaments above the eyes. The jaws carried large, laterally compressed and serrated teeth, including unusually long upper teeth. The arms were short but retained four digits; modern re-examination identified reduced metacarpals and phalanges that early preparations had misunderstood. Strong hind limbs supported a bipedal body, while tall tail vertebral spines and chevrons produced a comparatively deep tail.

• Large, blade-like nasal horn core formed by the fused nasal bones
• Raised bony ornaments above both eyes
• Deep skull with long, recurved and serrated cutting teeth
• Short forelimbs retaining four digits, two of them strongly reduced
• Fused sacral and pelvic elements in the adult holotype
• Deepened tail with tall neural spines and chevrons
• Small postcranial osteoderms associated with the neck and tail midline

Marshall P. Felch collected the holotype USNM 4735 at Felch-Marsh Quarry in Garden Park, Colorado, in 1883. Othniel Charles Marsh named Ceratosaurus nasicornis in 1884 and emphasized its horn, fused pelvic bones and unusual metatarsus. The specimen later passed from Yale to the Smithsonian. Charles W. Gilmore prepared, mounted and comprehensively described it in 1920. A revised osteology in 2000 incorporated additional Ceratosaurus skeletons, while a 2016 study corrected important details of the holotype forearm and hand.

Discovery credit: Marshall P. Felch.

Naming authors: Othniel Charles Marsh.

Ceratosaurus lived on the seasonally dry floodplains represented by the Morrison Formation. Braided rivers, ponds and wooded corridors interrupted broad fern- and cycad-rich open areas. The fauna included enormous sauropods, stegosaurs and ornithopods together with other large predators such as Allosaurus and Torvosaurus. The type specimen came from the same Garden Park fossil region that produced important Allosaurus material.

Ceratosaurus was an active carnivore capable of cutting flesh with its deep jaws and long teeth, but no fossil fixes a narrow prey preference or hunting technique. Its horn core and brow ornaments were probably covered by keratin and may have served visual display, species recognition or physical interaction; their exact role cannot be tested directly. Bone histology published in 2025 indicates exceptionally rapid growth compared with later ceratosaur lineages. Claims that its deep tail made it a specialized swimmer remain speculative.

• Ceratosaurus is currently the only theropod known with securely identified postcranial osteoderms.
• The holotype preserves a nearly complete skull and much of the skeleton from a single animal.
• Its hand retained four digits even though only two carried substantial functional claws.
• The Smithsonian remounted Ceratosaurus for the Deep Time hall after digitally revisiting the historic skeleton.

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1. 01
   The Paleobiology Database

   Open source
2. 02
   Osteology of the carnivorous Dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus

   Charles W. Gilmore · Bulletin of the United States National Museum · 1920

   Open source
3. 03
   The Principal Characters of American Jurassic Dinosaurs Belonging to the Order Theropoda

   Othniel Charles Marsh · Geological Magazine · 1884

   Open source
4. 04
   Ceratosaurus (Dinosauria, Theropoda): A Revised Osteology

   James H. Madsen Jr., Samuel P. Welles · Utah Geological Survey Miscellaneous Publication 00-2 · 2000

   Open source
5. 05
   New information on the forearm and manus of Ceratosaurus nasicornis, with implications for theropod forelimb evolution

   Matthew T. Carrano, Jonah Choiniere · Journal of Vertebrate Paleontology · 2016

   Open source
6. 06
   Osteohistology of the unusually fast-growing theropod dinosaur Ceratosaurus

   Riley Sombathy, Patrick M. O'Connor, Michael D. D'Emic · Journal of Anatomy · 2025

   Open source
7. 07
   Ceratosaurus nasicornis Marsh, 1884: USNM 4735

   Smithsonian National Museum of Natural History

   Open source
8. 08
   Unearthed: Building the Smithsonian’s Dinosaurs

   Smithsonian Libraries and Archives · 2019

   Open source