Did extinct these animals have cheeks? Paleontologists work from fossils of dinosaurs, so how can they know? An anatomist explains.
By Ali Nabavizadeh (@Vert_Anatomist)
An animal’s anatomy can tell us a lot about how it lived, including how it moved, how it ate, how it breathed, and just about any other physiological process involving morphology. In studying modern vertebrates, performing dissections is essential for understanding anatomical similarities and differences between various species and what these comparisons can tell us about their evolutionary relationships. Dissection can reveal how an animal’s internal structures worked together in life, such as how muscles and bones connected and worked. Comparative anatomists use dissection to study all types of vertebrates, but the vertebrate paleontologists among them are at a bit of disadvantage. Namely, the bones of their extinct study subjects are all that are left for a vertebrate paleontologist to work with when studying extinct dinosaurs. This makes questions such as “Did plant-eating dinosaurs have cheeks?” especially hard to answer.
The problem with fossils
Soft tissue rarely is fossilized because of how quickly it breaks down after death. This process usually leaves just the animal’s fossilized bones for vertebrate paleontologists to study, save a few exceptions. Because of this limitation, many vertebrate paleontologists use techniques such as the extant phylogenetic bracketing (EPB) to reconstruct soft tissues (Witmer, 1995). The EPB uses what is known of the anatomy of related living animal species to assess the likelihood of the existence and placement of certain soft tissue structures in the extinct animal. In addition to the EPB, there are also functional adaptations that need to be taken into account that may be outside of what is known of living relative species. This conceptual fusion of evolutionary relationships and functional adaptations is what vertebrate paleontologists use to reconstruct soft tissue anatomy to deduce functional morphology. Incidentally, these techniques have been instrumental as we investigate form and function in one of the most mysterious and popular groups of extinct vertebrates—the dinosaurs (e.g., Holliday, 2009).
My research in particular investigates the functional anatomy and evolution of muscles around the heads of the largely plant-eating, or herbivorous, group of dinosaurs known as ornithischians (Fig. 1). The ornithischian clade consists of the horned ceratopsians (e.g., Triceratops), the plated stegosaurs (e.g., Stegosaurus), the tank-like armored ankylosaurs (e.g., Ankylosaurus), the dome-headed pachycephalosaurs, the so-called “duck-billed” hadrosaurs, and their relatives. In my opinion, what makes these animals so fascinating is the incredible diversity of feeding adaptations they had acquired. Just as large plant-eating mammals do today, these dinosaurs used fine-tuned, specialized heads and teeth to eat diverse plant matter all around the world. This study of mine (Nabavizadeh, 2018) looks at reassessing the head muscles in ornithischian dinosaurs to investigate, not only their feeding function, but whether or not they needed “cheeks” to facilitate their feeding as mammals do.
So, did these dinosaurs have cheeks?
For over a hundred years, there has been an ongoing debate about the presence or absence of so-called “cheeks” in ornithischian dinosaurs (see Galton [1973] for a summary). These “cheeks” were thought to be present because of an inset tooth row in these animals allowing room for such a structure on the outside of the mouth. Although at its surface it might seem insignificant, it has actually been a point of contention among paleontologists as we try to figure out how ornithischian “cheeks” would have looked. The original idea was that the muscle fibers of the “cheeks” were oriented straight up and down between the upper and lower jaws along prominent outer ridges, acting like the buccinators (i.e., cheek muscles) in mammals to help keep food in the mouth.
The problem was that this muscle was totally made up and is only known in mammals (not in reptiles, as the dinosaurs were); not to mention the fact that it would have been functionally useless as a muscle. Evolving this type of novel muscle would have required an unlikely amount of muscle division and reorientation. Granted, this does not mean that these dinosaurs couldn’t have had skin flaps on the sides of their faces to keep food from tumbling out.
In my paper (Nabavizadeh, 2018), I’ve proposed a new reconstruction of head muscles that takes an already existing layer of muscle on the outer surface of the jaw and extends it forward on the lower jaw as a large fan of muscle (Fig. 2). This muscle fan would attach onto that same ridge that was previously proposed as an attachment of the alleged “cheek” muscle. A large, outwardly-directed jugal (i.e., “cheek bone”) in many ornithischians, especially the horned ceratopsians, was also a dead give-away for the likely presence of a large muscle body to run through and extend forward and down to the lower jaw.
Although at first glance this muscle arrangement seems odd, it is actually seen in a few other extinct animals, particularly those that implement a major palinal (i.e., backward-oriented) feeding stroke in their jaw mechanisms, such as the herbivorous early relatives of mammals known as dicynodonts (King, 1989) and small extinct mammals called multituberculates (Gambaryan and Kielan-Jaworowska, 1995). A fan of muscle was used for palinal feeding in these animals as a support system to lift the entire jaw as one unit to then pull the jaw back while in occlusion. Incidentally, many ornithischians (e.g., the horned ceratopsians, the “duck-billed” hadrosaurs, and the armored ankylosaurs) also incorporated a palinal feeding mechanism themselves, as has been shown by tooth wear analyses (Weishampel, 1984; Varriale, 2011; 2016; Mallon and Anderson, 2014; Osi et al., 2014; Nabavizadeh, 2014; 2016; Nabavizadeh and Weishampel, 2016). Yet, in the original reconstruction, their head muscles were constricted at the back of the jaw, which would not have allowed the jaw to be pulled backward when the animal bit down on something.
This new muscle arrangement would have greatly improved jaw leverage when the animal munched on plants. It also allows for bilateral jaw curling in some of these dinosaurs; specifically, the hadrosaurs and ankylosaurs. With this muscle orientation, the so-called “cheeks” as we know them in large herbivorous mammals were likely not present in ornithischians. Rather, although not its primary function, this large muscle would have secondarily helped keep food in the mouth while it is feeding, as mammal cheeks assist in doing. As a result, this study rewrites how we think of the overall feeding mechanics of these massive herbivores and it also helps us rethink exactly what these animals would have looked like in the flesh—especially because all we have to work with are the bare bones they left behind.
References
Galton PM. 1973. The cheeks of ornithischian dinosaurs. Lethaia 6:67–89.
Holliday CM. 2009. New insights into dinosaur jaw muscle anatomy. Anat Rec 292:1246–1265.
Gambaryan PP, Kielan-Jaworowska Z. 1995. Masticatory musculature of Asian taeniolabidoid multituberculate mammals. Acta Palaeontol Pol 40:1.
King GM, Oelofsen BW, Rubidge BS. 1989. The evolution of the dicynodont feeding system. Zool J Linn Soc 96(2):185–211.
Mallon JC, Anderson JS. 2014. The functional and palaeoecological implications of tooth morphology and wear for the megaherbivorous dinosaurs from the Dinosaur Park Formation (upper Campanian) of Alberta, Canada. PLoS One 9(6):e98605.
Nabavizadeh A. 2014. Hadrosauroid jaw mechanics and the functional significance of the predentary bone. In: Evans D, Eberth D, editors. The hadrosaurs: Proceedings of the International Hadrosaur Symposium. Bloomington: Indiana University Press. p 467–482.
Nabavizadeh A. 2016. Evolutionary trends in the jaw adductor mechanics of ornithischian dinosaurs. Anat Rec 299:271–294.
Nabavizadeh A, Weishampel DB. 2016. The predentary bone and its significance in the evolution of feeding mechanisms in ornithischian dinosaurs. Anat Rec 299(10):1358–1388.
Nabavizadeh, A. 2018. New reconstruction of cranial musculature in ornithischian dinosaurs: implications for feeding mechanisms and buccal anatomy.Anat Rec.
Ösi A, Barrett PM, Földes T, Tokai R. 2014. Wear pattern, dental function, and jaw mechanism in the Late Cretaceous ankylosaur Hungarosaurus. Anat Rec 297:1165–1180.
Varriale FJ. 2011. Dental microwear and the evolution of mastication in ceratopsian dinosaurs. Doctoral dissertation. Baltimore, Maryland: Johns Hopkins University School of Medicine.
Varriale FJ. 2016. Dental microwear reveals mammal-like chewing in the neoceratopsian dinosaur Leptoceratops gracilis. PeerJ 4:e2132.
Weishampel DB. 1984. Evolution of jaw mechanisms in ornithopod dinosaurs. Adv Anat Embr Cell Biol 87:1–109.
Witmer LM. 1995. The extant phylogenetic bracket and the importance of reconstructing soft tissues in fossils. Functional morphology in vertebrate paleontology. Cambridge: Cambridge University Press. Vol. 1. p 19–33.
Featured Image: Comparison of previous head muscle reconstruction (on left, showing hypothetical “cheek” muscle) and new head muscle reconstruction (on right) in the horned ceratopsian dinosaur Triceratops. Modified from Nabavizadeh (2018).
About the Author
Ali Nabavizadeh, PhD, is an assistant professor of anatomy in the Department of Biomedical Sciences at Cooper Medical School of Rowan University. His research investigates comparative anatomy and evolution of herbivorous dinosaurs; specifically, their cranial muscles and the functional morphology of their feeding mechanisms. He also expands these studies to other large herbivorous vertebrates, including dicynodonts and elephants.
FAQs
Did plant eating dinosaurs have cheeks? ›
Which dinosaurs didn't chew their food? Sauropods (long-necked plant-eaters) couldn't chew. They had no cheeks to keep food in their mouths and no grinding back teeth. Instead, they had peg-like teeth that raked and sliced leaves from trees.
Which dinosaurs had cheeks? ›For example, Witmer says, Triceratops and Leptoceratops, both ornithischians, have long been thought to have fleshy cheeks, which scientists believed were involved in how these plant-eaters ate. The idea that they had cheeks was based on scientists' comparison of these dinosaurs to modern-day mammals, such as sheep.
Do plant eating dinosaurs have teeth? ›Some plant-eating dinosaurs grew new teeth every couple of months, with some of the largest herbivores developing a replacement tooth every 35 days, to keep their chompers from getting too worn down on all that vegetation, new research finds.
Did hadrosaurs have cheeks? ›It is believed hadrosaurs had cheeks in order to keep food in the mouth. Researchers have long believed their unusual mouth mechanics may have played a role in their evolutionary success.
Do dinosaurs have cheeks? ›But Witmer points out that the dinosaurs' closest living relatives—birds and crocodiles—lack cheeks. Moreover, dinosaurs lack the bony ridges to which cheek muscles attach in mammals; Leptoceratops, for instance, had only a smooth shelf of bone above the teeth.
Do all ornithischians have cheeks? ›With this muscle orientation, the so-called “cheeks” as we know them in large herbivorous mammals were likely not present in ornithischians. Rather, although not its primary function, this large muscle would have secondarily helped keep food in the mouth while it is feeding, as mammal cheeks assist in doing.
Do reptiles have cheeks? ›The problem, Nabavizadeh points out, is that no birds, crocodylians, or reptiles have cheeks like mammals do. If dinosaurs had cheeks, the underlying musculature would be very different. So here's where the bones come in.
Did Iguanodon have cheeks? ›The First Dinosaur to Chew
In order to accomplish this the animal needed cheeks, living reptiles don't have these. However, the Iguanodons skull show that its teeth are on the inside of the jaw. Thus, it left room for a skin covering, acting as a cheek.
Hadrosaurs have often been called “duck-billed dinosaurs.” You don't have to look at their skulls for very long to see this analogy is wide of the mark. Not only did hadrosaurs such as Edmontosaurus have shovel-shaped, grooved beaks, but their jaws were lined with rows of cropping, crushing teeth.
How many teeth did plant eating dinosaurs have? ›The dinosaurs typically had between 20 and 30 teeth in their upper and lower jaws, and these teeth likely experienced a lot of wear as the sauropods fed on the available vegetation, D'Emic said.
What did a plant eating dinosaur have? ›
Some plant-eating dinosaurs had beaks and cheek teeth that were better suited for slicing and grinding vegetation. These features would have helped them to eat a wider variety of plants, including tough ones like cycads and horsetails.
What did plant eater dinosaurs eat? ›The likely plants that Diplodocus ate include: ferns, cycads, horsetails, club mosses, seed ferns, conifers and gingkoes. Diplodocus did not eat grass, bamboo or any other kind of flowering plants as they were not around in the Jurassic Period.
How many teeth did hadrosaurs have? ›The duck-billed hadrosaur was a toothy creature with up to 1,400 teeth, Erickson said. The teeth migrated across the chewing surface, with sharp, enamel-edged front teeth moving sideways to become grinding teeth as the teeth matured.
Did hadrosaurs have teeth? ›Rather than shedding teeth and replacing them with new ones like other reptiles, hadrosaurs' mouths contain several parallel stacks of six or more teeth apiece, forming a “highly dynamic network” of teeth that was used to grind and shear tough plant material.
What dinosaur has a duck face? ›The hadrosaurs, or duck-billed dinosaurs, received their name from their broad, flattened, elongated snouts and their toothless beaks. Their sets of grinding teeth and cheek pouches were extremely well adapted to browsing on vegetation.
How did ornithischians eat? ›Some lineages evolved jaws with dozens of close-set rows of teeth on which the enamel was more developed on either the inside or the outside. As these teeth came into use, the side with thinner enamel was worn down, so the upper and lower jaws formed an efficient grinding surface.
Is a diplodocus a reptile? ›a genus of extinct saurischian reptiles of the suborder Sauropoda. They grew to a length of 25 m and had very long tails and necks and relatively small heads. There were several species, and they were herbivorous.
How did the jaw of the ornithischians change? ›Answer and Explanation: Over time, the mandible of the ornithischians changed to develop a predentary bone. This was a hard bone connected to the anterior part of the lower jaw, often with a forward-facing point. This served as a lower beak among ornithischian dinosaurs.
How many dinosaurs are there? ›Estimates vary, but in terms of extinct non-avian dinosaurs, about 300 valid genera and roughly 700 valid species have been discovered and named.
Which dinosaur groups hips are like birds? ›Ornithischia is a strongly supported clade with an abundance of diagnostic characters (common traits). The two most notable traits are a "bird-like" hip and beak-like predentary structure, though they shared other features as well.
Do all animals have cheeks? ›
Animals that have cheeks, like humans, horses, and pigs, can create suction with their tongues, drawing water in. But cats, dogs, and other carnivores that have to open their mouths wide to catch their prey don't have cheeks and have to find other solutions, Crompton said.
Why do we have cheeks? ›The cheek aids in enzymatic digestion by the secretion of the enzymes from the parotid gland. While in mechanical digestion, the cheek aids in maintaining the food in the mouth so that it can be chewed and swallowed. The majority of the muscles in the cheek region contribute to facial expression.
Do lizards have lips? ›How do toothed reptiles such as lizards or crocodilians maintain their pearly whites? Well, lizards have lips. Their tiny, pointed teeth are almost always covered by their scaly skin.
What was the second dinosaur? ›The second validly-named dinosaur was Iguanodon, but the identification of its fossils as a distinct and extinct species was a somewhat long and arduous process.
What was the first dinosaur discovered? ›The first dinosaur recognized by science - the Megalosaurus – was described in 1824 from a partially preserved jaw.
Is Iguana a dinosaur? ›Technically, iguanas are distantly related to dinosaurs in that they share a common ancestor several hundred million years ago. While all life on Earth is distantly related, both iguanas and dinosaurs are reptiles, but they are from distant branches of reptiles, and iguanas did not descend from dinosaurs.
Why did some dinosaurs become plant eaters? ›“The plant eaters have always outnumbered carnivores in any ecosystem,” Dr. Henderson says, adding that most dinosaurs were herbivores because of how abundant plant life is. Many plants we see today were eaten by herbivorous dinosaurs.
How did dinosaurs digest their food? ›They just pulled up their food and gulped it down. The mechanical break-up may have been carried out by a 'gastric mill'. Similar to today's birds, dinosaurs may have swallowed stones with which they ground the food to a paste with their muscular stomach.
How did brontosaurus have its food? ›Brontosaurus was herbivorous and lived on land. Its long neck may have evolved to reach marshy vegetation some distance away or to reach leaves higher up in trees. Brontosaurus also consumed stones to help grind up and digest unchewed plant matter once it reached the stomach.
Did Iguanodon have cheeks? ›The First Dinosaur to Chew
In order to accomplish this the animal needed cheeks, living reptiles don't have these. However, the Iguanodons skull show that its teeth are on the inside of the jaw. Thus, it left room for a skin covering, acting as a cheek.
Do reptiles have cheeks? ›
The problem, Nabavizadeh points out, is that no birds, crocodylians, or reptiles have cheeks like mammals do. If dinosaurs had cheeks, the underlying musculature would be very different. So here's where the bones come in.
How did ornithischians eat? ›Some lineages evolved jaws with dozens of close-set rows of teeth on which the enamel was more developed on either the inside or the outside. As these teeth came into use, the side with thinner enamel was worn down, so the upper and lower jaws formed an efficient grinding surface.
What dinosaurs are chewers? ›Hadrosaurs have often been called “duck-billed dinosaurs.” You don't have to look at their skulls for very long to see this analogy is wide of the mark. Not only did hadrosaurs such as Edmontosaurus have shovel-shaped, grooved beaks, but their jaws were lined with rows of cropping, crushing teeth.
What was the second dinosaur? ›The second validly-named dinosaur was Iguanodon, but the identification of its fossils as a distinct and extinct species was a somewhat long and arduous process.
What was the first dinosaur discovered? ›The first dinosaur recognized by science - the Megalosaurus – was described in 1824 from a partially preserved jaw.
Is Iguana a dinosaur? ›Technically, iguanas are distantly related to dinosaurs in that they share a common ancestor several hundred million years ago. While all life on Earth is distantly related, both iguanas and dinosaurs are reptiles, but they are from distant branches of reptiles, and iguanas did not descend from dinosaurs.
Do all animals have cheeks? ›Animals that have cheeks, like humans, horses, and pigs, can create suction with their tongues, drawing water in. But cats, dogs, and other carnivores that have to open their mouths wide to catch their prey don't have cheeks and have to find other solutions, Crompton said.
Why do we have cheeks? ›The cheek aids in enzymatic digestion by the secretion of the enzymes from the parotid gland. While in mechanical digestion, the cheek aids in maintaining the food in the mouth so that it can be chewed and swallowed. The majority of the muscles in the cheek region contribute to facial expression.
Do lizards have lips? ›How do toothed reptiles such as lizards or crocodilians maintain their pearly whites? Well, lizards have lips. Their tiny, pointed teeth are almost always covered by their scaly skin.
How do you pronounce ornithischians? ›How To Say Ornithischians - YouTube
How long did the dinosaurs live? ›
It isn't easy to tell from dinosaurs' fossilised remains how long they lived. "Traditional" estimates based on slow, reptilian growth rates, combined with the enormous size of dinosaurs, led scientists to conclude it could be up to several hundred years.
How did the jaw of the ornithischians change? ›Answer and Explanation: Over time, the mandible of the ornithischians changed to develop a predentary bone. This was a hard bone connected to the anterior part of the lower jaw, often with a forward-facing point. This served as a lower beak among ornithischian dinosaurs.
What did a plant eating dinosaur have? ›Some plant-eating dinosaurs had beaks and cheek teeth that were better suited for slicing and grinding vegetation. These features would have helped them to eat a wider variety of plants, including tough ones like cycads and horsetails.
Why did some dinosaurs become plant eaters? ›“The plant eaters have always outnumbered carnivores in any ecosystem,” Dr. Henderson says, adding that most dinosaurs were herbivores because of how abundant plant life is. Many plants we see today were eaten by herbivorous dinosaurs.
What plants did Dinos eat? ›The likely plants that Diplodocus ate include: ferns, cycads, horsetails, club mosses, seed ferns, conifers and gingkoes. Diplodocus did not eat grass, bamboo or any other kind of flowering plants as they were not around in the Jurassic Period.