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Dinosaurs (excluding birds) evolved into several groups during their nearly 170 million-year history, from around 235 million to 66 million years ago. The very first true dinosaurs were most likely tiny, bipedal creatures with a carnivorous or omnivorous diet, comparable to Argentina's Eoraptor.
The oldest authentic dinosaur fossils come from the early part of the Late Triassic. However, it is quite improbable that we have discovered the first dinosaurs, and the group most likely emerged in the Middle Triassic or Early Triassic.
We have a basic idea of dinosaurs' closest cousins. The nearest major group to dinosaurs seems to be flying pterosaurs. Dinosaurs are classified as Archosauria in a broader sense. Archosaurs, the "ruling reptiles," are represented today by crocodiles and birds.
Therefore, paleontologists can include living dinosaurs like birds and crocodilians when speculating about characteristics of dinosaurs that did not fossilize, such as behavior and soft tissues. The only issue is that many dinosaurs had habits and anatomy that were nothing like current crocodiles or birds, but these parallels can help narrow down the options.
A British paleontologist named Harry Seeley suggested categorizing dinosaurs into two groups based on hip shape in 1888. These are the Saurischia (lizard-hipped) and Ornithischia groups (bird-hipped).
The carnosaurs were the prominent theropods of the Late Jurassic and Early Cretaceous periods. The Allosaurus is the most well-known member of this group. After generating titans like the Carcharodontosaurus and the Giganotosaurus, carnosaurs grew considerably scarce in the Late Cretaceous.
The remarkable Spinosaurus, also abundant in the Cretaceous period, is part of a group that branched apart after the reign of the ceratosaurs but before the arrival of the carnosaurs.
Saurischia — The Lizard-Hipped Dinosaurs
Saurischians have two hip bones that point opposite from one another, like lizards. Birds, by lineage, are saurischians, but instead of ornithischians—the emergence of real "bird hips" happened after the saurischian-ornithischian divergence.
Theropods and sauropodomorphs are the two main subgroups of saurischians. Theropods, which include birds, are the archetypal bipedal carnivorous dinosaurs, ranging from Coelophysis to Tyrannosaurus.
Sauropodomorphs include the massive quadrupedal, herbivorous long-necked, long-tailed sauropods — like the Brachiosaurus — and their "prosauropod" relatives.
Theropod translates to 'beast foot,' and this category encompasses all carnivorous (meat-eating) dinosaurs.
Theropods evolved from little predators with long, thin bodies, similar to Coelophysis. Several of them became huge creatures early on. One of these early lineages gave rise to the Abelisaurs, the main theropods of the Southern Hemisphere landmass throughout the Cretaceous period. These theropods are related to the horned Ceratosaurus.
While carnosaurs were the dominant giant theropods, another grouping, coelurosaurs, began evolving as much smaller theropods. Due to their tiny size and thin-walled hollowed bones, Jurassic coelurosaurs are poorly documented.
Still, we do know that they diverged into many branches throughout the Jurassic, which then branched and gained notoriety during the Cretaceous:
- The Alvarezsauroidea are small, light-bodied theropods having single big claws for hands and arms. To give you some perspective, "small" would be less than 10 feet long, "medium" is almost 20 feet long, and "large" is more than 20 feet long.
- The Avialae is comprised of birds and their closest relatives.
- Deinonychosauria is theropods that vary in size from small to medium and have "killer claws" on their second toe. Earlier members of the deinonychosaurs and earlier avialans are often confused.
- Ornithomimosauria, also called 'Ostrich mimics,' are small to medium-sized creatures that are largely toothless and likely herbivorous to omnivorous.
- The Oviraptorosauria are small to medium-sized theropod dinosaurs with a diversity of head morphologies (mostly toothless and typically crested) and hands that likely filled various niches.
- Therizinosauria, on the other hand, resembles dinosaurian sloths with elongated tails and lengthy necks with smaller heads; they are assumed to have been medium- to large-sized herbivores.
- Tyrannosauroidea included tyrannosaurids and their near relatives, several of which thrived as smaller predators alongside Allosaurus in the American Western. Obviously, they are best known for Tyrannosaurus rex.
Sauropodomorphs emerged as tiny bipedal creatures with omnivorous or carnivorous diets but soon turned herbivorous and gigantic body sizes. Plateosaurus, for one, could grow to be far more than 30 feet in length and four tons heavy by the end of the Triassic period.
Although these Triassic "prosauropods" featured the classic sauropodomorph long neck, smaller head, lengthy tail, and massive body, only a handful of species could move on all fours.
This trait was shared by the very first true sauropods, which must have aided subsequent sauropods in attaining such massive sizes. After all, it's way easier to be over 75 feet long and weigh 20 tons if all of your feet are on the floor.
During the Jurassic, sauropods grew to be the biggest land animals ever identified. Their fossils have been uncovered on every continent. They were particularly common at the end of the Jurassic period, with large boneyards of sauropod fossils discovered in China, Tanzania, and the United States, like the Dinosaur National Monument.
The early sauropods bore almond-shaped teeth and angular skulls, with some having incredibly elongated necks. During the Jurassic period, sauropods were divided into two major groups.
The Macronarians, for example, retained their larger teeth for a time. Brachiosaurus and Camarasaurus are two well-known Jurassic macronarians. The diplodocoids, which include the Apatosaurus, Brontosaurus, and Diplodocus, acquired pencil-like teeth and long, low bodies.
The sauropod population appears to have dwindled as time passed through the Early Cretaceous. Most lineages met extinction by the midpoint of the Cretaceous, to be succeeded by the Titanosaurs, a group of macronarians.
Titanosaurs grew plump wide bodies with pencil-like teeth, and most types possessed one or more bony masses that might have been used as mineral deposits. Just before the end of the Cretaceous period, Titanosaurs had spread to all continents.
However, something appears to have transpired in North America since sauropod fossils were absent throughout the Late Cretaceous era until the advent of the Titanosaur Alamosaurus in the southwest United States at the end of the age.
The origins of the ornithischians, the third and final major category of dinosaurs, remain unknown. The ornithischians are more challenging to categorize since they encompass multiple distinct groupings. Nonetheless, all ornithischians have a unique "beak bone" (the predentary) near the lower jaw's end.
Some experts believe that the Triassic Silesauridae (four-legged beaked creatures) are probably early ornithischians rather than dinosaur relatives. However, this is disputed. During the Jurassic period, the ornithischians were divided into numerous different groups. The majority of these emerged during the Cretaceous period.
The majority of ornithischians fall into one of the five categories.
- The low quadruped Ankylosaurs were covered in bone armor called osteoderms on their upper and lower bodies. The osteoderms on some of these animals grew into spikes or plates as they evolved. The ankylosaurids, a branch of the ankylosaur family, have bone tail clubs.
- It's common knowledge that Ceratopsians were the horned dinosaurs, although this isn't always the case, like the Psittacosaurus and Protoceratops. But the rostral beak bone on the front of the upper jaw of all ceratopsians gave them a parrot-like appearance.
Late Cretaceous species like Triceratops, whose wide bony plumes, large forehead horns, and formidable shearing jaws evolved from Ceratopsians, flourished throughout that period.
- Ornithopods started out as comparatively tiny, bipedal, herbivorous dinosaurs, but some of them, like Camptosaurus, quickly got much bigger. Larger ornithopods, like Iguanodon, reached lengths of about 30 feet by the course of the Cretaceous and could walk on two or four legs on will.
During the Late Cretaceous, a lineage of hadrosaurs, the "duck-billed dinosaurs," developed. Hadrosaurs, unlike ducks, were terrestrial creatures with formidable arrays of hundreds of stacked teeth, unlike ducks, which had wide bills. There were several with ornate crests.
- The Stegosaurs were among the earliest to spread far and wide throughout the Jurassic period. Huge bony plates differentiated Stegosaurus. However, most stegosaurs possessed smaller, spike-like plates instead of huge ones on their backs. The tails of Stegosaurs were similarly adorned with pairs of spikes. Cretaceous-era Stegosaurs appear to have been extinct by the end of the era.
- Among the five categories, Pachycephalosaurs are among the least known. (In fact, no one has ever found the hand of a pachycephalosaur.) In the Late Cretaceous period, they are well documented in Asia and North America as plant-eating bipeds. Their skulls that have been exaggerated into dome shapes, often with bone clusters or spikes, are their most well-known feature.