Evolution of Jaws Introduction A crucial innovation of vertebrate animals was that of bilateral symmetry, which led to the formation of distinct anterior-posterior, dorsal-ventral, and left-right axes to define the organization and forms of the body. This enabled animals to encounter new environments anterior (head) first, which led to a clustering of nervous tissue at the anterior end. Eventually, this would give rise to a complex nervous system and specific facial structures that reflect how they adapted to habitats over time. The jaws of an animal can tell us a great deal about how diet shaped their morphology over time in both herbivorous and carnivorous species. Specific jaw structures, like the flexible-gapes and mobile skulls of snakes contrast widely with other reptiles, for instance, the relatively inflexible lizard skulls and gapes, for which some researchers state that “mosasaurs – large, extinct marine lizards related to snakes – represent a crucial intermediate stage…possessed long snake-like palatal teeth for holding prey….retained the rigid upper jaws typical of lizards… highly flexible lower jaws” (Lee et al, 1999) which were morphologically and functionally similar to that of snakes. Since snakes can only consume what can fit through their jaws, the adaptation of a wide jaw-gape enables them to consume prey much larger than themselves. Their elastic skin also stretches to accommodate large prey. One study looked into skull size and shape alongside ecological aspects because “Skull morphology is well known to be affected by different selective pressures such as feeding performance, diet, and behavior, but habitat specializations are also expected to be of major influence” (Da Silva et al, 2018), and found that, “…the evolution of the snake skull is a clear example of balance between natural selection (ecology) and temporal regulation of morphogenesis (heterochrony)” (Da Silva et al, 2018). It is a combination of “nature and nurture” that influenced the evolution of squamate skulls. [Squamates are scaly reptiles. Heterochrony is the genetically determined Figure 1: Carnivorous Dinosaur Jaws (Klein, 2021) difference in the timing/duration/rate of a developmental process in an organism] “The highly flexible lower jaw is thus inferred to have evolved before the highly flexible upper jaw – in the microphagous common ancestor of mosasaurs and snakes – for accommodating large prey. The mobile upper jaw evolved later – in snakes- for dragging prey into the esophagus…Snakes have more rigid braincases than lizards and the partially fused joints of mosasaurs are transitional between the loose joints of lizards and rigid joints of snakes. Thus, intermediate morphologies in snake skull evolution should perhaps be sought not in small burrowing lizards, as commonly assumed, but in large marine forms” (Lee et al, 1999) Jaws first evolved in the now extinct placoderms of the Silurian period, who were the ancestors of today’s gnathostomes, ‘jawed vertebrates’. This adaptation enabled vertebrates to consume a range of prey while also providing a means of defense against predators. It is believed that, “Jawed vertebrates arose from non-jawed vertebrates that had a pharyngeal gill apparatus composed of gill bars and slits. Anterior gill bars evolved into the jaw…” (WIREs, 2018). The development of the jaw was shaped by other morphological changes such as, “…neural crest cells and key genetic pathways in development… genes involved in the formation of a jaw joint”, as well as some components of the jaw associated with coevolution of ear bones (WIREs, 2018). Whether aquatic or terrestrial, certain jaw structures/forms may emerge that are more similar among more distantly related species, if the driving force is similar i.e. dietary needs, type of prey or predator, than among their own species. Figure 2: Skulls of Reticulated PythonsNormal and Swallowing Today, imaging of dinosaurs’ forms and bodies are increasingly accurate, offering clues into how these behemoth creatures ate and lived prior to their extinction. The remaining relatives of dinosaurs include reptiles like birds, crocodilians, and snakes (Rigby, 2021) with a range of jaw structures from the toothless bird beak to the stiff crocodilian jaw and the flexible snake jaw. Though we learned about much of dinosaur-era life through fossils, there is a great deal left unknown, one mystery is that of the “effects of the Cretaceous-Paleogene (K-Pg) mass extinction on the evolution of snakes – a major clade of predators…remains poorly understood” (Klein, 2021) while a lot of knowledge about that time period before the K-Pg mass extinction was elucidated through fossil evidence. Understanding how snakes evolved to have wide gaping jaws could also shed light on how they coevolved with other innovations that emerged at the same time, altogether, painting a picture of what their ecology may have looked like i.e. the prey they hunt, predators to defend themselves from, and plants in their immediate environment. Understanding how jaw morphologies diverged and/or converged in same or different species could also illustrate how certain forms continue to serve certain animals in certain environments. Another study on dinosaur predators showed how they could “swing jaws down at a terrifying wide right angle, opening its maw nearly 80 cm to bite massive prey…The larger Tyrannosaurus rex could open its jaws at nearly the same angle, while a related plant-eating dinosaur had a much smaller gape of 43.5 degrees” (Rogers, 2015). One limitation to our knowledge of dinosaur morphologies is due to the simple fact that we do not have fossil record of muscle tissue in most dinosaurs, and studies are largely based on muscle scars like striations on the bone surface, and comparing them with known anatomies of extant vertebrates like “…crocodilians, birds, and the more distantly related lepidosaurs” (Nabavizadeh, 2019) Snakes swallow by ‘jaw walking’ i.e. the jaws attach loosely to the skull, followed by the left and right sides, which are not joined in the front, and move independently so that teeth on one side grip and pull the prey while the other side of the jaw move forward to obtain a new grip, so the opposite sides of the skull are able to ‘walk’ over the food, bringing it into its flexible body (Figure 2, American Museum of Natural History). If diet had such an impact on the size of jaw gape, then it is likely there would be signs of convergent evolution i.e. homologous structures like the wide-gaping jaw evolving in relatively unrelated lineages, shaped primarily by dietary needs. Dietary influences would also likely affect the teeth, surrounding mouth tissue/muscles and the structures of the skulls themselves. By tracing how the wide-gaping jaws manifested in both extinct and living reptile species, there Figure 3: Snake Jaw Muscles would likely also be signs of coevolution. As diet changes, due to the environment (changes in plant species affect herbivores, which are consumed by carnivores) there may be similar patterns in the way jaws developed in dinosaurs and modern snakes compared with how their teeth evolved. Teeth shapes and sizes may vary due to differences in diet, adapted to different functions like to pierce and hold prey in place or to crush bones or grind down on plant fibers. Larger and more powerful teeth may be observed in animals with less flexible jaws, as they rely on mastication to break down their prey while snakes often administer a venom that breaks the prey down into a slush before swallowing them. Reptilian teeth morphology may also vary based on their positions in the jaw, or the lack of teeth as observed in birds and snakes. In this paper, I will compare the anatomy of reptilian jaws by comparing their morphology among dinosaurs and their extant reptilian descendants. In doing so, I will illustrate how different mechanisms of bite evolved, with a focus on the wide-gape that characterizes snakes and continues to fascinate today. Though larger jaws enabled dinosaurs to consume tougher materials like plants and larger prey, the overall trend was a decrease of size that eventually aided their survival. Modern snakes are believed to have evolved from a few survivors of the asteroid impact that wiped out most of life on Earth. The asteroid impact wiped out most of the dinosaurs, save a few select species, “Other clades persisted but suffered severe reductions in diversity, including birds, mammals, and squamates” (Klein, Catherine, 2021), which coincided with a relatively quick recovery in diversification of flora and fauna (Klein, 2021). The snake jaw, specifically, must be a huge advantage evolutionarily if it first emerged in dinosaurs and continues to be found extensively today. Figure 4: AMNH- Monitor lizards feeding In general, the trend of jaw morphologies should illustrate a tendency towards more flexible jaws in carnivorous species that often must wrangle with live prey, and less flexible jaws with large blunt teeth in herbivorous species that may need to grind tough fibrous plant matter. Methods The American Museum of Natural History has extensive exhibits about dinosaurs, with fossils and artistic renderings that illustrate what we understand about life-forms and lifestyles before the asteroid impact that wiped them out. I started by visiting exhibits about modern reptiles like the crocodilians, snakes, and lizards to observe feeding behaviors and the displays of their skulls to observe their jaw structures. Next, I needed to compare the jaw structures that survived from the dinosaurs up to modern day. I visited the dinosaur exhibits, with a focus on the jaws and teeth of each fossil or skeleton. I also photographed the captions that had information pertinent to the subject of jaws. Results The earliest jawed vertebrate ancestors were the gnathostomes, including the extinct placoderms, chondrichthyans (e.g. sharks, rays, chimaeras), osteichthyans (bony fishes, tetrapods), and some other aquatic species. The placoderms are a sister clade to living gnathostomes, and their jaws were made of distinct cartilages and primitive tooth-like structures, so it is likely that jaws evolved before teeth. According to one study, “Teeth did not evolve convergently among the extant and extinct classes of early jawed vertebrates, but rather, successional teeth evolved within the gnathostome stemlineage soon after the origin of jaws” (Rucklin et. al., 2015), and specifically, “The extinct placoderms are the most primitive jawed vertebrates known, comprising either a monophyletic sister lineage to crown gnathostomes, or, more persuasively, a primitive grade of jawed vertebrates that includes a succession of sister lineages to crown gnathostomes. As such, placoderms are crucial to resolving the early evolution of teeth and jaws.” (Rucklin et. al., 2015). Jaws were an adaptation that enabled vertebrates to exploit a more diverse range of food sources, and equipped them for predation and defense against predators. The earliest jawed vertebrates, therefore, benefited from this adaptation due to its affect on diversifying food sources. Diet continued to shape jaw evolution through the dinosaurs up to today. Mosasaurs are large carnivorous sea reptiles, now extinct, with “snake-like lower jaws, but retain lizard-like upper jaws” (Lee et al, 1999), which could also be an intermediate form in the evolution of flexible snake jaws. Their jaws are beak-like in shape, and are morphologically similar to toothed diving birds from the Cretaceous period, Hesperornis- convergent evolution is presumed to be the Figure 4: AMNH- Mosasaur reason for similarities in jaw shape/structure, which is also thought to be the result of similar diets of fish. Convergent characteristics are proposed, including that of the mobile jaw joint that enabled wider gape, “Lower jaw with transverse articulation between splenial and angular bones” which are related to an “absence of bony symphysis of the rami of the two jaws.” (Gregory, 1951). There are many similarities in jaw structure between mosasaurs and Hesperornis, which were determined to have evolved independently, which could have been influenced by similarities in diet. Other features of bird jaws that mirror those of the mosasaur and modern snakes are evident in bird species H regalis and H gracilis in which “The two halves of the lower jaw were separate, or only ligamentously united at the tip. This feature contrasts sharply with all recent birds, in which the rami of the jaws are invariably fused together…It is one of the convergent anatomical resemblances to mosasaurs, which also lacked any symphysis of the jaws. Among living tetrapods, the closest analogy is found among serpents, which also lack this symphysis and are thus enabled to expand the mouth tremendously for swallowing their prey, and also to force bulky prey down their throats by alternate movements of the two halves of the jaw….somewhat similar feeding habits may be postulated for Hesperornis” (Gregory, 1951). Today’s surviving relatives from the age of dinosaurs include the squamates, which is the largest order of scaly reptile including lizards, snakes, and amphibians, and the second largest order of living vertebrates characterized, in part, by a flexible jaw. This is most evident in snakes, which are able to swallow prey much larger than themselves as a result. The jaws of monitor lizards are also relatively wide, but adapted more to allow them to crush bone when feeding on their prey instead of swallowing whole. The varanoids is a group that includes monsters, and snakes, that “all that had a hinge in the lower jaw and the postdentary bones. This hinge more flexible, enabling the animal to prey” (AMNH placard) monitor lizards, marine lizards, Gila descended from a common ancestor between the front bone (the dentary) makes the skull wallow large Figure 5: Varanoids placard Crocodilians also feed by crushing their prey with strong jaws, and mostly feed underwater- they swallow smaller animals whole, and tear apart larger prey with their powerful jaws- they can lock their jaws on prey firmly while moving its body to tear its body apart. Their teeth are more Figure 5: Crocodile Skull similar to mammals than snakes i.e. they are sest in sockets while lizard and snake teeth are attached to the side of the jawbone, and in some lizard species, are stuck along the upper edgethe sharp pointy teeth lining the crocodilian jaw are adapted to hold struggling prey in the mouth. Snakes feed differently than crocodilians and lizards, many dissolving their prey with venom rather than crunching down with powerful teeth and jaws. They swallow their prey by ‘walking’ their jaws, which aren’t “…connected like they are in mammals. At the front, each mandible is attached by a stretchy ligament. The mandibles can therefore spread apart laterally, increasing the width of the mouth. The mandibles are loosely connected at the back to the skull, allowing for much greater rotation than most animals have…move independently of each other, slowly inching the prey into the throat” (Macdonald, 2019) Backward facing teeth are also present to keep the prey in the mouth. Since snakes do not have teeth, many rely on venom to digest food, turning the prey’s organs into a slush that they can more easily suck in. Figure 6: Snake Skulls, Cobra with Prey The reticulated python has a very loosely assembled skull- the jaws are able to spread apart in the front and back with all tooth-bearing bones able to move independently, which altogether lend them extensive flexibility in capturing and swallowing diverse prey, as big as a deer. Jaw flexibility is not exclusive to snakes- it is also observed in coelacanths, a fish once believed to have been extinct since at least 65 million years ago (when dinosaurs were believed to have become extinct, the end of the Cretaceous period) and later found to be still living today. They were also once considered the closest relative of tetrapods (four-limbed vertebrates, later proven to be false because lungfish were found to be closer relatives). Like snakes, Figure : Coelecanth Fossil “Their jaws are hinged to open wide,” because they have “…an intracranial joint, a hinge in its skull that allows it to open its mouth extremely wide to consume large prey” (Bates, 2015). The exact mechanism of the wide gape is different between snakes and coelacanths, but both evolved due to dietary needs i.e. to consume larger and more motile prey that may still be alive and moving. This could be an instance of convergent evolution. Coelacanths are still alive today and while tetrapods have diversified immensely, the coelacanth remains largely unchanged i.e. changed much slower, which may be due to its inhabitance of deep oceans. Monitor lizards have a skull that is more flexible, and may represent an intermediate form between snakes and crocodilians- the front part of the skull moves on a ‘hinge’ between the front and back while independent movement of other parts of the skull enable them to capture, chew, and swallow prey. While they don’t…?? Lack of teeth evolved in avian reptiles like birds Figure 7: Monitor Lizard Skull Herbivorous dinosaurs generally have large blunt teeth that appear to be adapted to crushing and grinding tough fibrous plant matter. With the wide range of plant life, there is also a range of feeding mechanisms that evolved in their skulls. A large factor in the success of herbivorous dinosaurs like the sauropodomorphs, ornithischians, “was their ability to orally process plant materials with varied types of jaw occlusion and feeding motions (Nabavizadeh, 2019). Ornithischians are an extinct order of herbivorous dinosaurs that include the Stegosaurus, Triceratops, and duck-billed Iguanodons. One group of Ornithischians, the Euornithopods, were particularly successful and widespread, peaking in diversity with duck-billed hadrosaurs in the Cretaceous period- their jaw hinge was below the row of teeth, which Figure 8: Euornithopod skull caused the teeth to meet at the same time in a ‘clamp’ like motion. In more primitive species, the joint between the upper and lower jaws was at level with the teeth, enabling a slicing motion to break down food. The lower jaw hinge of Figure 9: Reticulated Python Jaw Euornithopods, combined with uneven enamel on teeth, altogether enabled them to crush and grind food in a particularly efficient way. Discussion Jaws emerged in the now extinct placoderms, and evolved a range of structures adapted to different selective pressures like dietary preferences. The mosasaurs and snakes share a common ancestor, and along with the Hesperornis bird, display varying extents of jaw mobility, “The movable avian quadrate permitted alternate movements of the jaws, to a lesser extent than in snakes, but probably as much as in mosasaurs” (Gregory, 1951), and the three groups share similar diet- they are all carnivorous and consume live prey. Consuming live prey is advantaged by a flexible or mobile jaw structure. Today’s monitor lizards and snakes are often equipped with venom to aid their ability to digest prey or defend themselves against predators. The presence of venom could be a reason why snakes evolved to be mostly toothless, with the exception of sharp hypodermic fangs that administer venom to their prey or predators. Venomous fangs and jaw structure continue to aid modern snakes as they hunt for food and defend themselves from larger predators Reptiles before and after the extinction of the dinosaurs had some similarities in jaw structure- herbivorous animals largely exhibited wide blunt teeth adapted to crushing fibrous plant matter, while carnivorous animals tended to exhibit sharp pointed teeth adapted to tearing flesh. Jaw synapomorphies, for example, certain variations on the jaw hinge like the wide gape of snake jaws, reflect mostly dietary influences. The mechanisms by which snakes survived the mass extinction may include their ability to burrow, an aquatic habitat that potentially buffered the impact on their physical environment, ability to hunt in low-light environments, and infrequent eating (snakes do not eat as often as many other animals), could have all played varying roles in their survival (Klein, 2021). Snakes are also categorized into two general groups based on how they feed “macrostomaton snakes, able to eat prey much larger than their own head, or microstomatan snakes limited to proportionally smaller prey” (Lyle, 2021). It was also found that five distinct types of microstomatan jaws evolved with “completely unique jaw and function”, which, “suggests that, rather than snakes ancestrally being quite similar to blindsnakes before diverging into the species we see today, it’s more likely that blindsnakes in fact, represent several distinct and highly modified evolutionary paths” (Lyle, 2021) Jaw morphology could be more representative of convergent evolution than once believed, involving multiple evolutionary paths, with similar influences i.e. diet, which could be attributable to a particular habitat. Since crocodiles feed underwater, they tend to have larger jaws that grew unimpeded by gravity on land- the large heavy skull that normally would be a burden is less so under water. This would also explain how the mosasaurs reached their enormous size Birds are well known to be the living descendants of dinosaurs, and though they evolved to have toothless beaks that seem to be shaped for an increasingly plant-based diet of Figure 10: Crocodile skeletons small seeds and berries, actually have a great deal of bite force. For example, a recent study found that finches from the Galapagos Islands have a bite force that, due to their small relative size, exceeds that of the Tyrannosaurus Rex i.e. the bite force relative to body mass of the T. Rex (8 tons) is expected to be what it is, 57,000 newtons (Weisberger, 2019) due to its large size, while that of the Finch was found to be surprisingly immense in proportion to their body mass (Finches typically weigh just 1 oz), “In fact, if a finch were scaled up to T. rex-size, the bird’s bite would then be 320 times stronger than that of its extinct cousin”, which indicates that “…the force of a T.rex bite is unsurprising when evaluated in light of the animal’s body mass and the bite strength of its ancestors” (Weisberger, 2019) The largest herbivorous dinosaur group, Ornithiscia, which includes Triceratops, Stegosaurus, and duck-billed hadrosaurs like the Iguanodon which shared “a huge variety of different jaw constructions and tooth shapes”, which Dr Ali Nabavizadeh, who studied the jaws of dinosaurs and modern relatives extensively, taking into account fossil evidence, “…estimate the relative differences in the bite forces of extinct animals…to estimate the actual biology of dinosaurs” (Pritchard, 2016) Herbivorous dinosaurs may express varying extents of aspects of skull structure, for example, “Sauropodomorph and theropod herbivores, representing several independent evolutionary origins of herbivory, emphasize rapid food acquisition, not chewing. By contrast, ornithischian herbivores emphasise traits that enhance chewing” (Benson and Barrett, 2020) which emphasize the effect of diet on jaw structure. Due to the selective pressures of diet, there was also coevolution of gut size, “increases in gut retention times and greater adult body sizes in sauropods (with correspondingly longer slower guts) and by the acquisition of gastric mills in theropod dinosaurs, the same strategy as adopted by their descendants, the birds” which accommodated for “lack of a grinding dentition” (Benson and Barrett, 2020). The herbivorous jaw coevolved with other adaptations to digesting a plant-based diet, including the gut, and primitive gizzards, which in modern birds, helps them grind down tough food. Paleontologists were perplexed by a structure in the dinosaur jaw that was once believed to be flexible like today’s snakes and monitor lizards, enabling them to entrap struggling live prey- the intramandibular joint (Rigby, 2021) Recently, they were found to be rather inflexible, for example “dinosaurs like T rex possess specialized bones that cross the joint to stiffen the lower jaw” (Fortner, quoted by Rigby 2021). If T. Rex lower jaw were stiffer rather than more motile, like the snake’s, it could have evolved out of a need to clamp down on and masticate the Figure 10: Euornithopod flesh and bones of captured prey, while snakes don’t chew on their prey, but swallow them whole. Dietary preferences, thus, shape jaw morphology. Bibliography Gregory, J. T. (1951). Convergent evolution: The jaws of hesperornis and the mosasaurs. Evolution, 5(4), 345– 353. https://doi.org/10.2307/2405679 Lee, M. S., Bell, G. L., & Caldwell, M. W. (1999). The origin of snake feeding. Nature, 400(6745), 655–659. https://doi.org/10.1038/23236 Lyle, A. (2021, April 19). Study of snake jaw structure yields new understanding of evolutionary origins [web log]. Retrieved March 28, 2022, from https://www.ualberta.ca/folio/2021/04/study-of-snake-jawstructure-yields-new-understanding-of-evolutionaryorigins.html#:~:text=Researchers%20have%20traditionally%20believed%20that,further%20along%20the %20evolutionary%20tree. Macdonald, J. (2019, June 26). JSTOR Daily. Retrieved March 28, 2022, from https://daily.jstor.org/howsnakes-swallow/. Nabavizadeh, A. (2019). Cranial musculature in herbivorous dinosaurs: A survey of reconstructed anatomical diversity and feeding mechanisms. The Anatomical Record, 303(4), 1104–1145. https://doi.org/10.1002/ar.24283 Nabavizadeh, A., & Weishampel, D. B. (2016). The predentary bone and its significance in the evolution of feeding mechanisms in ornithischian dinosaurs. The Anatomical Record, 299(10), 1358–1388. https://doi.org/10.1002/ar.23455 Nabavizadeh, A. (2015, December 21). Evolutionary Trends in the Jaw Adductor Mechanics of Ornithischian Dinosaurs. The Anatomical Record, 299(1), 271-294. https://doi.org/10.1002/ar.24283 Pritchard, A. (2016, February). News Bite: The evolution of ornithischian dinosaur jaws and bites [audio blog]. Retrieved March 28, 2022, from https://www.pasttime.org/2016/02/news-bite-the-evolution-ofornithischian-dinosaur-jaws-and-bites/. Rigby, S. (2021, April 27). T. Rex’s Alligator-like Jaw Helped It to Bite through Bone. Retrieved March 28, 2022, from https://www.sciencefocus.com/news/t-rexs-alligator-like-jaw-helped-it-to-bite-through-bone/. Rogers, N. (2015, November 6). Dinosaurs hunted with wide-gaping jaws. Science. Retrieved March 28, 2022, from https://www.science.org/content/article/dinosaurs-hunted-wide-gaping-jaws. Rücklin M, Donoghue PC, Johanson Z, Trinajstic K, Marone F, Stampanoni M. Development of teeth and jaws in the earliest jawed vertebrates. Nature. 2012 Nov 29;491(7426):748-51. doi: 10.1038/nature11555. Epub 2012 Oct 17. PMID: 23075852. Weisberger, M. (2019, January 11). Finch's Bite is 320 Times More Powerful Than T. Rex's. Retrieved March 28, 2022, from https://www.livescience.com/64472-finch-bite-beats-t-rex.html. WIREs Authors. (2018, December 13). How Fish Developed Jaws [web log]. Retrieved March 28, 2022, from https://www.advancedsciencenews.com/how-fish-developed-jaws. Yeager, M. (2017). Visualization of comparative anatomy: Jaw muscles of theropod dinosaurs and their extant relatives: Illustrating the story of Functional Morphology and Evolution (thesis). ProQuest LLC, Ann Arbor.
