Gorgosaurus libratus

Gorgosaurus libratus was the common albertosaurine tyrannosaur of Alberta's Dinosaur Park Formation. Adults approached nine metres but remained more lightly built than Tyrannosaurus rex, with a shallower skull and proportionally long lower legs. More than a dozen good skeletons and skulls span young juveniles to mature adults, making Gorgosaurus one of the best dinosaurs for studying how tyrannosaur bodies and feeding changed during growth. It lived alongside the more robust tyrannosaur Daspletosaurus, although fossil occurrence alone does not reveal exactly how the two divided habitats or prey.

Its fossils occur between approximately 76.6 and 75.1 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

The adult skull was long and moderately deep, with roughened lacrimal bosses in front of the eyes, broad jaw-muscle attachment areas and thick, serrated teeth. Very short arms ended in two functional fingers, while the hind limbs and metatarsals were comparatively slender and elongate. Young animals had shallow skulls, narrow slicing teeth and especially gracile legs. Through growth, the snout deepened, jaw muscles enlarged, teeth became more robust and the body shifted from a lightly built runner toward a heavier predator capable of handling larger carcasses.

• Long, moderately deep tyrannosaurid skull
• Roughened lacrimal bosses in front of the eyes
• Large serrated teeth replaced throughout life
• Very short two-fingered forelimbs
• Long, relatively slender lower legs and metatarsals
• Exceptional juvenile-to-adult growth series

Charles M. Sternberg discovered the nearly complete skeleton CMN 2120 beside Alberta's Red Deer River in 1913 while leading a Geological Survey of Canada field party. Lawrence Lambe named Gorgosaurus libratus in 1914 and published a detailed monograph in 1917. The specimen preserved a complete two-fingered hand, then a rare feature in the tyrannosaur record. American Museum of Natural History crews soon collected several additional skulls and skeletons from the same formation. Gorgosaurus was later merged with Albertosaurus for much of the twentieth century, but modern anatomical and growth studies generally recognise the two as separate genera.

Discovery credit: Charles M. Sternberg.

Naming authors: Lawrence M. Lambe.

The Dinosaur Park Formation records river channels, forested floodplains, ponds and coastal lowlands beside the Western Interior Seaway. Gorgosaurus shared the regional fauna with horned dinosaurs such as Centrosaurus and Chasmosaurus, hadrosaurs, armoured dinosaurs, smaller theropods and the more robust tyrannosaur Daspletosaurus. The formation accumulated over roughly a million and a half years, so its entire species list should not be treated as one frozen community.

A juvenile Gorgosaurus, TMP 2009.12.14, preserves hind limbs from two young Citipes in its abdominal cavity. Their different digestive states show that they were eaten in separate meals, providing direct evidence that a five-to-seven-year-old tyrannosaur selected small prey and swallowed choice body parts. Combined with the deepening skull and thickening teeth of adults, this supports a shift toward larger prey during growth. It does not establish pack hunting, a universal menu or whether the juvenile killed rather than scavenged every prey item.

• A juvenile preserves the first stomach contents found in place inside a tyrannosaur skeleton.
• That young individual had eaten the hind limbs of two less-than-one-year-old Citipes in separate meals.
• The holotype included a complete hand with only two functional fingers.
• For decades many museum labels and papers called this species Albertosaurus libratus.

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1. 01
   Palaeobiology Collections

   Canadian Museum of Nature

   Open source
2. 02
   On a new genus and species of carnivorous dinosaur from the Belly River Formation of Alberta

   Lawrence M. Lambe · The Ottawa Naturalist 28 · 1914

   Open source
3. 03
   The Cretaceous Theropodous Dinosaur Gorgosaurus

   Lawrence M. Lambe · Geological Survey of Canada Memoir 100 · 1917

   Open source
4. 04
   Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria)

   Thomas D. Carr · Journal of Vertebrate Paleontology 19(3) · 1999

   Open source
5. 05
   Exceptionally preserved stomach contents of a young tyrannosaurid reveal an ontogenetic dietary shift in an iconic extinct predator

   François Therrien, Darla K. Zelenitsky, Kohei Tanaka and 3 coauthors · Science Advances 9(49) · 2023

   Open source
6. 06
   Foundations: articulated juvenile Gorgosaurus

   Royal Tyrrell Museum

   Open source