Last updated: April 28, 2024
Article
Why Dinosaur Skulls Are Rare
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Preservation Bias
In paleontology, some things are just more likely to get preserved than others. This principle is called, "preservation bias." Whether or not something gets preserved is often just due to the nature of what the thing is. For example, solid body structures, like bones, don't typically decay as quickly as soft body parts, like organs. This is why bones are the most common fossilized body parts. Likewise, a femur is more likely to get preserved than a skull. This is because femurs are solid and structurally stronger, so they aren't as easily crushed as a skull is. The bones of big animals are not only more resistant to decay than the bones of littler creatures, they're also larger. Because there's more bone there to be preserved, it's the bones of bigger animals that tend to survive and be found by paleontologists. Of course, it helps that bigger bones are easier to notice in a landscape than little ones.
In paleontology, some things are just more likely to get preserved than others. This principle is called, "preservation bias." Whether or not something gets preserved is often just due to the nature of what the thing is. For example, solid body structures, like bones, don't typically decay as quickly as soft body parts, like organs. This is why bones are the most common fossilized body parts. Likewise, a femur is more likely to get preserved than a skull. This is because femurs are solid and structurally stronger, so they aren't as easily crushed as a skull is. The bones of big animals are not only more resistant to decay than the bones of littler creatures, they're also larger. Because there's more bone there to be preserved, it's the bones of bigger animals that tend to survive and be found by paleontologists. Of course, it helps that bigger bones are easier to notice in a landscape than little ones.
Preservation at the Carnegie Quarry
One of the most astounding discoveries of the Carnegie Quarry was the number of skulls that were preserved there. Not only did paleontologists find several skulls from dinosaurs, they also found bones and skulls from smaller animals that lived among the dinosaurs, too. The reason so many skulls were preserved here is because of what the environmental conditions were like back when the dinosaurs were alive. In the Late Jurassic, northeastern Utah was home to a large, flat floodplain. The dinosaurs lived alongside an ancient river. Thanks to the presence of freshwater molluscs fossilized in the Morrison sandstone, paleontologists believe that the dinosaurs of the Carnegie Quarry died during droughts. The Morrison stone the dinosaurs are fossilized in was once the bottom of a river. The positioning and state of the bones suggests that the dinosaurs were likely in various states of decay along the riverbank when the rains returned. As the river swelled with water, the current swept the dinosaurs downsteam until they were caught at a bend or a sandbar and buried in the sediment. The dinosaurs would've experienced a quick burial at the bottom of the river, which covered their bones and slowed their decay. Fortunately, the sand-sized grains at the bottom of the river were just the right size for preservation. They quickly filled the empty spaces of even the most delicate structures, from large dinosaur skulls down to the bones of very small creatures like Glirodon grandis and Hoplosuchus kayi.
One of the most astounding discoveries of the Carnegie Quarry was the number of skulls that were preserved there. Not only did paleontologists find several skulls from dinosaurs, they also found bones and skulls from smaller animals that lived among the dinosaurs, too. The reason so many skulls were preserved here is because of what the environmental conditions were like back when the dinosaurs were alive. In the Late Jurassic, northeastern Utah was home to a large, flat floodplain. The dinosaurs lived alongside an ancient river. Thanks to the presence of freshwater molluscs fossilized in the Morrison sandstone, paleontologists believe that the dinosaurs of the Carnegie Quarry died during droughts. The Morrison stone the dinosaurs are fossilized in was once the bottom of a river. The positioning and state of the bones suggests that the dinosaurs were likely in various states of decay along the riverbank when the rains returned. As the river swelled with water, the current swept the dinosaurs downsteam until they were caught at a bend or a sandbar and buried in the sediment. The dinosaurs would've experienced a quick burial at the bottom of the river, which covered their bones and slowed their decay. Fortunately, the sand-sized grains at the bottom of the river were just the right size for preservation. They quickly filled the empty spaces of even the most delicate structures, from large dinosaur skulls down to the bones of very small creatures like Glirodon grandis and Hoplosuchus kayi.
How Many Skulls Were Found?
In total, paleontologists uncovered a whopping 14 dinosaur skulls during the initial Carnegie Quarry excavations between 1909 and 1924. More skulls were found when the remains of the Carnegie Quarry, now the "Wall of Bones," were excavated between the 1950s and 1990s. Two skulls, both from Camarasaurus, remain on the wall today. Also housed in the Quarry Exhibit Hall is a nearly-complete skull of the predatory dinosaur, Allosaurus fragilis. This skull was removed from the wall, and is currently housed in a glass case within the building. (A photograph of this skull is visible at the top of the article.)
In total, paleontologists uncovered a whopping 14 dinosaur skulls during the initial Carnegie Quarry excavations between 1909 and 1924. More skulls were found when the remains of the Carnegie Quarry, now the "Wall of Bones," were excavated between the 1950s and 1990s. Two skulls, both from Camarasaurus, remain on the wall today. Also housed in the Quarry Exhibit Hall is a nearly-complete skull of the predatory dinosaur, Allosaurus fragilis. This skull was removed from the wall, and is currently housed in a glass case within the building. (A photograph of this skull is visible at the top of the article.)
Common Questions About Skulls
The fancy science word for skull holes is called, "fenestra," or "fenestrae" if you're referring to more than one skull hole. However, these words are only used for skull holes other than the orbits (eye holes) and nares (nostril holes). Scientists figured out pretty early on that different kinds of animals had different numbers of holes in the skull. Today, scientists group these animals into lineages, called "clades," based on the number of fenestrae they have. For example, most reptiles belong to the Sauropsida clade. This group can be further divided into more clades, with crocodiles, dinosaurs, birds, and lepidosaurs (which includes all modern reptiles) sitting in the Diapsida clade. Diapsids usually have two sets of fenestrae, one behind the eye (temporal fenestrae) and another above the eye (supratemporal fenestrae). In living reptiles, these holes serve as important attachment sites for muscles, like the ones that help close the jaw. Dinosaurs also had another set of fenestrae in front of the eye (antorbital fenestrae). Scientists believe that these fenestrae allowed extra room for air sacs, which were an important part of the dinosaurs' respiratory (breathing) system. Some diapsids, like modern birds, have since lost a few of their fenestrae, but they're still grouped in the Diapsida clade because their ancestors came from that group.
At some point, most of us learn to tell what kind of food an animal eats by looking at its teeth. Carnivores (meat eaters) tend to have sharp, pointy teeth. Herbivores (plant eaters) tend to have flat teeth. Omnivores (animals that eat both meat and plants) often have a mix of sharp and flat teeth. All these things are usually true... for mammals. Mammal teeth are often highly specialized. A mammal can have teeth of many different shapes and sizes in its mouth, where each kind of tooth does a different job. But reptiles are different. While the teeth in a reptile's mouth can be different sizes, all the teeth are usually the same shape. Unlike most mammals, reptiles usually don't need to chew their food or crack bones open. Instead, they just take a bite and swallow their food whole, so their teeth don't need to be fancy. This also seems to have been true for dinosaurs.
Many sauropods had small peg-like or pencil-shaped teeth only in the front of their mouths. Based on the shape and scratch patterns on the teeth of dinosaurs like Apatosaurus, Diplodocus, and Barosaurus, scientists think their teeth worked like a rake. They probably fed by biting onto a plant and pulling their head backwards. The teeth would pull fresh new plant material into the dinosaur's mouth, which it likely swallowed whole. Some sauropods, like Camarasaurus, had broad spoon-shaped teeth. These more robust teeth probably allowed it to feed on tougher, woodier plants.
Many sauropods had small peg-like or pencil-shaped teeth only in the front of their mouths. Based on the shape and scratch patterns on the teeth of dinosaurs like Apatosaurus, Diplodocus, and Barosaurus, scientists think their teeth worked like a rake. They probably fed by biting onto a plant and pulling their head backwards. The teeth would pull fresh new plant material into the dinosaur's mouth, which it likely swallowed whole. Some sauropods, like Camarasaurus, had broad spoon-shaped teeth. These more robust teeth probably allowed it to feed on tougher, woodier plants.
Most dinosaurs that ate meat belonged to the theropod group. Meat-eating dinosaur teeth were shaped a bit differently depending on what they ate. Dinosaurs that ate fish tended to have teeth like a crocodile, long and cone-shaped. Today, animals with teeth like this are very good at catching and holding slippery prey, like fish or squids. Dinosaurs that ate flesh, like Ceratosaurus and Allosaurus tended to have teeth that curved backward into their mouth. Today, animals with teeth like this have an easier time keeping their prey from escaping. Generally, the teeth of meat-eating dinosaurs were wider where they rooted into the mouth, and tapered down to a sharp point. They were often serrated, too. This means they had sharp, jagged edges like a steak knife, which helped them cut through meat.
Yes! Being a dinosaur tooth was a tough job. Even though dinosaurs probably didn't chew, plant eaters likely spent most of their day just ripping plants from the environment and swallowing them whole. Meat-eating dinosaurs probably didn't eat as often as plant eaters did. However, they would've had to use their sharp, serrated teeth to slash at, capture, and hold struggling prey. Fortunately for them, dinosaurs never ran out of teeth. Unlike most mammals, anytime a dinosaur lost a tooth, a new one would just grow back in the empty space. This could happen as many times as the dinosaur needed throughout its life.