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One-hundred and fifty million years ago, long before T-Rex reigned as the tyrant king of the dinosaurs, another fearsome theropod ruled the land his name, Allosaurus (AL-oh-SORE-us) Fragilis. Spanning a dynasty from 150 million to 144 million years ago during the late Jurassic period, Allosaurus is famous from the Cleveland-Lloyd Quarry in Utah, part of the Morrison Formation and one of the best-known ancient dinosaur graveyards in the world where the remains of 44 Allosaurus specimens lay entombed. Victims of a predator trap, the Allosaurus specimens from this Quarry are some of the actors in an epic prehistoric tragedy that unfolded 150 million years ago; this is the tale of that fateful day.

Cleveland-Lloyd Quarry:

Before we can understand this epic tale and the part that Allosuaurs plays, we must look at the stage where this drama unfolds. The Cleveland-Lloyd Dinosaur Quarry well known for containing the densest concentration of Jurassic dinosaur fossils ever found, is a paleontological site located near Cleveland, Utah in the San Rafael Swell, a part of the geological layers known as the Morrison Formation.

Morrison Formation:

The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States, which has been the most fertile source of dinosaur fossils in North America and covers an area of 600,000 square miles. Composed of mudstone, sandstone, siltstone, and limestone with a light gray, greenish, and red sediment, the Morrison Formation was a Jurassic waterway shortly after Lauraisa split into the continents of North America and Eurasia.

As North America moved north and passed through the subtropical regions, the deposits from drainage basins carried by streams and rivers became the Morrison Formation. Above is a map of the Earth as the contents traversed the globe due to plate tectonics.

Above is a map of all the dinosaur bones found preserved within the Cleveland-Lloyd Quarry. Looking at this grapic two things are abundantly clear.

First, the amount of carnivores present at the Cleveland-Lloyd Quarry is staggering. Indicated in green, the theropods present are predominately Allosaurus. Accompanying Allosaurus are the remains of Ceratosaurus (SERR-at-oh-SORE-us) – the most dragon like-dinosaur known and relative of Abelisaurus, Stokesaurus – the first tyrannosauroids and primitive ancestor of Tyrannosaurus Rex, Mashosaurus (MARSH-o-SORE-us), Torvosaurus (TORE-voh-SORE-us) the biggest carnivore of its time – as big as Allosaurs and heavier too, and Tanycolagreus – a primitive coelurosaurian theropod.

Second, the herbivores present are fewer but noteworthy. In yellow are the remains of several massive sauropods, Camarasaurus (CAM-a-ra-SORE-us) – 25.4 tons and Brontosaurus Excelsus (BRONT-oh-SORE-us) – 18 tons. Denoted in blue are the remains of an Ornithopod Camptosaurus (CAMP-toe-SORE-us), and indicated in red are the remains of several Thyreophora or armored dinosaurs known as Stegosaurus.

Upon looking at the remains encompassing a wide variety of different dinosaurs, it is clear that something unusual attracted the assemblage of paleofauna to this location. Carnivores outnumbering herbivores points to scenes in which predators flocked to the site. What drew the diverse number of theropods to this site 150 million years ago? This Jurassic crime scene preserved in stone – the Cleveland-LLoyd Quarry – is a predator trap.

What is a Predator Trap? How Does it work?

A predator trap results from a natural hazard where prey animals become trapped or incapacitated, and the predators attracted to the struggling prey suffer the same fate. As more and more animals become ensnared in the trap, additional predators and scavenges become attracted to this mounting accumulation of carrion until a wide variety of animals are tragically caught and killed by the natural hazard. The predator trap can appear many times, accumulating countless victims over millions of years.

Unlike the dry landscape today, 150 million years ago, the Cleveland-Lloyd Quarry was a freshwater lake, a source of water that drew many dinosaurs to its shores. Much like Africa today, this area had wet and dry seasons. The wet seasons, with torrential rains and floods, would fill the landscape with water. As the dry season approached, the water would dry up; dense mud would then take the place of the water. It is this weather cycle that sets the stage for what happens next.

How did the Cleveland-Lloyd Quarry Drama Unfold?

While it’s impossible to say exactly how the events unfolded at the Cleveland-Lloyd Quarry 150 million year ago, fossil evidence suggests a probable interpretation of that fateful day. Brad Cypy. (2014, July 13) Jurassic Fight Club: Bloodies Battle {Video File}. YouTube

Cleveland-Lloyd Quarry. 9:00 am – Act 1; Scene 1. Adult Stegosaurus and Offspring Enters, Stage Right.

A Stegosaurus and her young searching for water approach the shrinking lake bed. The hardened upper crust buckles under their massive weight, causing their feet to sink into the soft muck below and become affixed. The cries of distress from the Stegosaurus pair attract a lone carnivore, Ceratosaurus (SERR-at-oh-SORE-us).

Cleveland-Lloyd Quarry. 9:30 am. – Act 1; Scene 2. Ceratosaurus Enters Stage Left.

The fossil record shows that a lone Ceratosaurus was the first to arrive at the scene at Cleveland-Lloyd Quarry. One of the rarest killers, Ceratosaurus, is 13 feet tall, 20 feet from snout to tail, and weighs 3,000 pounds, a mid-size predator in the Late Jurassic. Ceratosaurus may not have been massive, but his oversized teeth that worked like a meat slicer made him a fearsome beast. (Pim, 2019)

Ceratosaurus has the most wicked teeth of any predator that walked the Earth because he has two different kinds of teeth, a trait rare in theropods. The upper teeth were long and blade-like, and the lower teeth were conical and more powerful. Ceratosaurs would use the lower jaws to grab ahold of flesh and bring down the top teeth like a meat slicer cutting a chunk out of his prey. Ceratosaurus’s bite allowed him to compensate for two weaknesses: diminished forearm claws and smaller leg muscles, reducing speed and agility. Not equipped for battle, Ceratosaurus is made to slash and dash. Dinosaurs: The Complete, Up-To-Date Encyclopedia. (Holtz & Rey. 2007).

Observing the Stegosaur pair trapped in the mire, Ceratosaurus attacks the smaller Stegosaurus. Taking massive chunks out of the struggling Stegosaurus pair.

Cleveland-Lloyd Quarry. 10:00 am. – Act 1; Scene 3. A small group of Allosaurus Approach the Lakebed. Center Stage.

The most common dinosaur found at the Cleveland-Lloyd Quarry site is Allsoaurus. Allosaurus predates Tyrannosaurus Rex by 80 million years and is the first giant theropod to roam North America.

The name Allosaurus means “Other Reptile”. At its time, there was nothing as fearsome as Allosaurus. The first of the giant meat-eating dinosaurs, this carnivore was Tyrannosaurus Rex before there was Tyrannosaurus Rex. Possessing a rare trait in theropods, Allosaurus had “Freddie Krueger” like hands with three fingers armed with a long, sharp sickle claw used to slash and tear flesh. 13 feet tall, 40 feet from snout to tail, and weighing up to 8,000 pounds (4 tons), Allosaurus is the butcher of the Jurassic. (Pim, 2019).

Allosaurus, equipped to take on big game hunted sauropods, some of the largest animals the planet has ever seen during the Late Jurassic. A long, rigid tail provided excellent balance. Coupled with massive leg muscles, Allosaurus is a powerful, fast runner with incredible agility. Dinosaurs: The Most Complete Up-To-Date Encyclopedia (Holtz & Rey, 2007).

Endocasts of the Allosaurus brain reveal that in addition to being the first giant theropod, Allosaurus was also the smartest carnivore in the Late Jurassic. Allosaurus was capable of advanced social behaviors such as cooperative hunting and planning. Like modern-day birds, Allosaurus was very social and lived in small groups.

A roaming group of Allosaurus arrive at the dried lake bed, attracted by the sound of disruption and the smell of fresh blood. The Allosaurus see the Stegosaurus pair stuck in the thick mire and a single Ceratosaurus. The fossil record shows that a single Ceratosaurus was present at the Cleveland-Lloyd Quarry, its bones crushed by another dinosaur.

Cleveland-Lloyd Quarry. 10:30 am – Act 1; Scene 4. The Allosaurus Kill the Ceratosaurus and feed on the Stegosaurus Pair.

Attempting to defend its meal against the Allosaurus group, the Ceratosaurus has an altercation with the Allosaurus. Small and not robust like the much more derived Allosaurus, the Ceratosaurus is out-competed, crushed, and killed by better-equipped theropods.

Splade toes allow the theropods to walk on top of the hardened crust of the dried lake bed without sinking into the muck below. Circling the herbivores, the Allosaurus group continues to feed on the larger struggling Stegosaurus. Armed with plates along its back and spikes at the end of its tail, the Stegosaurus uses all its defenses to the best of its ability. The fossil records show that one of the Allosaurus has pathologies indicative of a Stegosaur strike by the spikes on its tail. The other Allosaurus, underestimating the Stegosaurus’s ability, takes a step back when another group of dinosaurs enters the scene.

Cleveland-Lloyd Quarry. 11:00 am. – Act 2; Scene 1. Camarasaurus Herd Enter, Stage Left.

Traversing the Jurassic landscape on yearly migrations, a traveling herd of Camarasaurus severely parched arrived at the water source. Due to the ongoing drought resulting from the dry season, the much-needed water had transitioned to a thick, soupy mud. Motivated by desperate thirst, the Camarasaurus herd approached the moist, thick muck.

Witnessing the unfolding bloody scene, driven by intense thirst, the leading Camasasaurus cautiously approaches the dried lake bed. Victim of his weight, the Camasaurus breaks through the hardened top layer and becomes entrapped by its weight, and the 25-ton Camarasaurus’s feet sink deep into the silt. Unable to free its feet, the Camarasaurus cried in distress. The Allosaurus, determining that attacking the Stegosaurus is too risky, sees the Camarasaurus as an easier target and approaches the struggling titan.

Unable to free its feet, the Camarasaurus cried in distress. Another group of nearby Allosaurus heard the cries of anguish and approached the scene. Upon observing the entrapped Sauropod, the carnivores assumed a free meal was at hand, hot-footed toward the struggling prey.

Cleveland-Lloyd Quarry. 11:30 am. – Act 2; Scene 2. The Noise, Smell of Blood, and Carnage Attract More Predators.

Adding to the chorus of distressing calls of anguish, the entrapped herbivores, and battling carnivores attract more predators to the scene.

The fossil record of this event shows that under the feet of the Camarasaurus lies the crushed body of an Allosaurus. At some point during the struggle, the Camarasaurus freed its front legs and reared up on its hind legs, crushing an Allosaurus running towards it.

More carnivores arrive, attracted by the carnage as the battle rages. Allosaurus, the most common animal at this site, fought and killed each other as they competed for the carcasses. As more carnivores became trapped in the thick mire, the predator trap claimed more victims. The noise at Cleveland-Lloyd Quarry on this day 150 million years ago would have been deafening. It is evident from the events recorded in stone that the land flowed with blood.

~ End Scene ~

This Jurassic crime scene is a resource for dinosaur bones and allows us to understand how these animals relate to each other. The largest dinosaur graveyard, Cleveland-Lloyd Quarry, is still revealing its secrets.

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Resources:

Holtz, Thomas. R. Jr. & Rey, Luis V. Dinosaurs. The Most Complete, Up-To-Date Encyclopedia. New York, Random House, 2007.

Pim, Keiron & Pastori, Fabio. Dinosaurs The Grand Tour. New York, The Experiment, 2019.

Ransom-Johnson, Evan & Csotonyi, Julius. Dinosaur World. Kennebunkport, Maine, Applesauce Press, 2023