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The Osteodontia are a large and diverse group of primarily herbivorous reptiles. Having evolved from within the Rynchocephalians, Osteodonts are sometimes classified in the clade Rynchocephalomorpha. Initially appearing as small, lizard-like creatures, Osteodonts eventually became the dominant herbivorous land vertebrates, with numbers similar to the Ornithischia (bird-hipped dinosaurs).

Anatomy Edit

Skeletal Structure Edit

The main defining characteristic of the Osteodonts is the fusion of the teeth to the maxilla and mandible. Though there is a distinct amount of variety with the dentition of different families, all Osteodonts have two pairs of serrated 'teeth' on the upper mandible and one pair of 'teeth' on the lower mandible, which create a shearing motion when the jaws are closed, a design that is adept at cutting through tough vegetation. The premaxilla and dentary bones form a beak that is covered with a keratinous material, aiding the Osteodonts in the cropping of vegetation.

Osteodonts retain their ancestor's gastralia, which are now fused to the ribs and expanded to accommodate for more developed internal organs, especially the digestive organs. The uncinate processes of the ribs are still present, and have become more developed as to provide stronger support of the scapula muscles. The Osteodonts are generally mesopubic, with the pubis pointing downward, although some species are more opisthopubic.

Organs Edit

Various organs have become much more developed, especially in the case of the lungs and the heart. Osteodonts have a four-chambered heart, much like mammals, which allows it to more effectively pump blood through the circulatory system. The lungs have gone through the most extensive modification; alongside the two main lungs, which have developed bronchi, there are two large thoracic sacs, two smaller cervical air sacs, and a interclavicular air sac. The air sacs aid in the transmission and storage of oxygen in the body, giving Osteodonts a respiratory system in which only birds are more sophisticated.

The majority of Osteodonts are hindgut fermenters, with an expended large intestine and caecum, both of which contain fermenting bacteria. The caecum is also projected to the side of the gut, rather than connected to the main passageway, and can push the fermented food back to the small intestine with the aid of muscles surrounding the digestive tract, allowing the food to be further digested before it continues through the large intestine. An exception to this design are the Suisaurids, which have a non-fermenting digestive system that allows an omnivorous diet.

Sensory Organs Edit

Osteodonts have developed more advanced ear structure than their ancestors, with an earhole and eardrum being developed, and the adipose tissue in the inner ear being replaced with fluid, as in most other amniotes. Another major change is that the middle ear now has two bones, with the quadrate forming the "incus" of Osteodonts. The outer bone sends vibrations into waves through the fluid and to the inner bone, which sends the vibrations of the sound to the inner ear, in which the highly developed hair cells in the cochlea send the information of the sound as a signal to the brain. These hearing organs and system are unique to Osteodonts, and allow them to have a much more acute and efficient sense of hearing as well as a wide range of detectable frequencies, giving Osteodonts comparable hearing to mammals and Maurosodontids.

Evolution Edit

Osteodonts can trace their evolution back to the Rhynchocephalians, an order that was once prevalent and diverse in the Mesozoic, but eventually declined to the tuataras of New Zealand by the Holocene. Despite being threatened by invasive species such as rats, the tuatara populations began to gradually increase through conservation efforts placed by humans. Following the ice age near the end of the Cenozoic, five million years in the future, the Tuatara had been introduced to most of mainland New Zealand, as well as some parts of Australia, where they would develop a more omnivorous diet than their island ancestors.

The tuatara's descendants, the Rynchocephalomorphs, would retain a form similar to lizards through the Postocene, with some species evolving into forms similar to prehistoric rhynchosaurs, having also developed an entirely herbivorous diet. However, following the Postocene-Gobian extinction that would wipe out most large mammal species, the Rynchocephalomorphs would diverse, eventually evolving into the first true Osteodonts by the Megistian.

Although the Osteodonts would originally be undermined by other herbivorous reptiles, such as the Scutosquamids, they would eventually outcompete them and diversify across Borealia by the end of and after the Basilozoic, which is largely in part to the unique dentition they have allowing them to develop a more advanced method of consuming food. Osteodonts have further developed this dentition into a beak, similar to dicynodonts, with the most advanced forms bearing similarities to Ornithischian dinosaurs.

Trianychia Edit

Trianychids ("three claws") are Osteodonts that are characterized by three toes, and the equal distribution of weight on either one of all of these toes. The claws have been developed into a more nail-like structure, which further developed into a structure similar to the hooves of ungulate mammals. Some Trianychids (Elaphosaurids and Suisaurids) have either only a singular large hoof or a hoof with two dewclaws; other Trianychids (Bysaurids) have three distinct hooved or heavily nailed toes.

Elaphosauridae Edit

With their name meaning "Deer Lizards", Elaphosaurids fit their name perfectly; nimble, agile grazers that live in herds, these are almost convergently identical to the deer, antelope and horses of the Cenozoic era. Being found throughout the majority of what is today Africa, Eurasia and N. America, they are the most widespread of the Osteodonts, with a possible number of over 200 species identified. They range in size from the small, shrub-foragers of the North American and Central African forests, standing at about 14-19 inches tall at the shoulder, to the gigantic grazers of the Eurasian Steppes, which stand at almost 89 inches tall (2.26 meters), making them larger than the tallest breed of horse.

Bysauridae Edit

Named for their Bovine-like adaptations, Bysaurids are the largest Trianychids. Largely found about nothern Borealia, particularly modern day N. America and Asia, Bysaurids are reminiscent of present-day Bovines and Rhinos, with characteristically large heads, short necks and broad chests. Most species are solitary animals, with a few exceptions, whom travel in migratory herds across the continent.

Suisauridae Edit

Suisaurids, as their name implies, are similar to modern day pigs and peccaries, both in behavior and anatomy. Like the Bysaurids, Suisaurids have large heads and short necks, but they have characteristically daintier and lighter builds, allowing them to be more agile. Their most unique feature, however, if their omnivorous diets; unlike other families, which are purely herbivores, Suisaurids supplement their diet with invertebrates, small lizards, birds and mammals, and even carrion. Some species are even more carnivorous than herbivorous, actively hunting out smaller animals while supplementing their diet with foliage.

Pentanychia Edit

Pentanychids ("five claws") are Osteodonts that have retained their ancestors' five toes, and lack the 'hoof' of Trianychids. While they're less advanced than Trianychids, Pentanychids are also less restricted in their movement; they're aren't obligate quadrupeds, and in fact contain a number of purely bipedal species.

Mysauridae Edit

The Mysaurids, or "Mice Lizards", consists of the smallest and most primitive of the Osteodonts. They are the most like their distant sphenodontian ancestors, retaining their small stature and sprawling posture, even though they act more like rodents than lizards. Resembling smaller Anomodonts, such as Diictodon, Mysaurids are sociable, burrowing animals that often feed on low-lying grasses and plant roots, although some have taken up arboreal lifestyles. As such, most are found in parts of Borealia closer to the equator, with more vegetation and stable climates and humidity, although some can be found as far north as northern Eurasia and N. America, much closer to the North Pole.

Elegansauridae Edit

The Elegansauridae consists of species convergent with herbivorous marsupials, such as kangaroos and wallabies, with a bipedal, hopping gate instead of a quadrupedal stance like their relatives. Being generalist herbivores, Elegansaurids are common throughout Borealia, and even on some offshore islands. Elegansaurids have smaller forelimbs, being bipedal, but they are still used to grasp objects, such as plants.

Dynamisauridae Edit

Dynamisaurids are perhaps the most bizarre family of Osteodonts. These reptiles are characterized by a burly, robust build (hence their name, "Robust Lizards"), smaller heads and long arms adorned with strong, sharp claws. These specialized creatures are convergent with a number of other animals, such as the extinct giant ground sloths or chalicotheres, and even therizinosaurid dinosaurs like Nothronychus. Like all of these creatures, Dynamisaurids are herbivores adapted to grasp tree branches with their powerful limbs to feed on the succulent leaves. Thus, these reptiles are only relatively common in more forested regions, although a few species have made their home in more arid regions.

Hydrosauridae Edit

The Hydrosauridae consist of reptiles completely adapted for life in the water, with a sleek, smooth build, fins and flukes for better locomotion, and a viviparous mode of reproduction. Convergent with modern day marine mammals, such as cetaceans and sirenians, Hydrosaurids are the first purely aquatic herbivorous reptiles in history, having become fully aquatic around 99 MyF. These reptiles, having little competition for the various species of sea plants that fill the waters of the future, are as successful as their cousins on land. They range from the smallest species of 177.8 cm (70 in), often found in freshwater or coastal mangrove environments, to 12 meter (40 foot) long giants that often roam in more open oceans. Hydrosaurids can be found throughout the oceans, although they are generally more common in southern waters, closer to the equator where more flora can be found.

Tetranychia Edit

Tetranychids ("four claws") are Osteodonts characterized by having four toes on each foot. These Osteodonts are significantly larger than their Pentanychid and Trianychid relatives, with some Tetranychids being the largest land animals of their time, growing to the size of even some sauropod dinosaurs.

Cyrtodontidae Edit

Cyrtodontids are characterized by having the front teeth on both the upper and lower jaws curve forward, creating two pairs of slicing 'incisors' adept at cutting through vegetation. These Osteodonts are fairly small for their suborder (still larger than most other families), with the largest being a little larger than a hippo. They also live like hippos as well; spending large amounts of time in water, feeding on aquatic plants or cooling down in the heat. Cyrtodontids are primarily found around the equator, often in areas with abundant amounts of vegetation.

Dolichirhisidae Edit

The "long-nosed lizards", as their name implies, are characterized by having an elongated proboscis, as well as a pair of teeth that have formed into tusks. Dolichirhisids have developed the same adaptations as advanced proboscideans for feeding, using their trunks and tusks to grasp plants, making them adept foragers and browsers of all but the tallest of vegetation. These Osteodonts are often found in northern Borealia, particularly Eurasia and North America, although some species can be found much farther to the south, and even on some coastal islands.

Decaspondylusidae Edit

Decaspondylusids are named for their ten neck vertebrae, which have developed to support an elongated neck. The smallest of these Osteodonts can grow to the size of a moose, while the largest reach and even surpass some sauropod dinosaurs in size, making Decaspondylusids the largest land animals of their time, and among the largest animals in natural history. Unlike their relatives, they have a cosmopolitan distribution, often feeding on the highest trees that even Dolichirhisids, the next largest herbivores, cannot reach.

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