Dinosaurs consisted of a large group of reptiles that first appeared on earth during the Triassic period (between 243 and 231 million years ago). At the time, dinosaurs were the most dominant terrestrial vertebrates. In 1842, Sir Richard Owen became the first person to classify dinosaurs when he classified Iguanodon, Hylaeosaurus, and Megalosaurus into the suborder Saurian Reptiles and gave them the name Dinosauria. Harry Seeley further divided dinosaurs based on their hip structures into orders Saurischia and Ornithischia in 1887 and 1888. These two classifications are the basis of subsequent modifications in the taxonomic classifications of dinosaurs. In the 1950s, entomologist Willi Hennig made several classification changes into categories based on tracing the characteristics that dinosaurs shared with their most recent ancestors, therefore, being closely related and sharing the most features. Hennig undertook an in-depth analysis of fossil remains to determine the relationship between the different clades and their evolutionary link. Orders Saurischia and Ornithischia have two suborders each.
4. Suborder Theropoda (Order Saurischia)
Theropoda has four infraorders namely Coelophysoidea, Herrerasauria, Ceratosauria, and Tetanurae. Ceratosauria has a division, Neoceratosauria, and two subdivisions called Abelisauroidea and Ceratosauridae. The subdivision Abelisauroidea are in two family classifications named Abelisauridae and Noasauridae. Infraorder Tetanurae has three divisions; Carnosauria, Coelurosauria, and Megalosauria. Megalosauria has one subdivision, Spinosauroidea, which is further in two families Megalosauridae and Spinosauridae. Carnosauria has subdivision Allosauroidea and two-family classifications called Allosauridae and Carcharodontosauridae. Division Coelurosauria has a Family classification Coeluridae above the subdivision Maniraptoriformes which in turn has six family units namely; Alvarezsauridae, Therizinosauridae, Troodontidae, Dromaeosauridae, Tyrannosauridae, and Ornithomimidae. The only surviving Class of theropods is Aves. The name Theropoda comes from Greek words meaning "wild beast" and "foot." Theropods, including the Aves, have three toe limbs and hollow bones. Primitive proto-theropods and most theropods existed during the Triassic period in the current day Southern Africa, South America, and North America.
By the late Jurassic period, only four lineages of theropods remained, these were; allosaurs, ceratosaurs, coelurosaurs, and megalosaurs. Members of this suborder were originally carnivores although some evolved to become omnivores, herbivores, insectivores, and piscivores. Most theropods possessed blade-like teeth that had serrated edges. Theropod bodies were also diverse with some having feathers or feather-like features and others scaly skins. In some, young ones had different outer body coverings compared to their parents’. A theropod, Tyrannosaurus was among the largest dinosaurs. Different theories advance the narrative that theropods shrunk in body size for the past fifty million years. The ostrich is the largest theropod that exists to date while the troodontid Anchiornis huxleyi was the smallest non-avian theropod species weighing 3.88 ounces and was one foot in length. During their peak time, theropods averagely weighed 359 pounds. Almost all theropods had varied gait and stance but were all bipedal, including extant ones. Extinct theropods bodies were almost upright and the forelimbs greatly reduced in length. These forelimbs also varied in different species but had three fingers with claws.
3. Suborder Sauropodomorpha (Order Saurischia)
Sauropodomorpha has two clades, Thecodontosaurus and Riojasaurus, each having a family unit, Plateosauridae and Massospondylidae respectively. This suborder also has an infraorder Sauropoda that has two family units Vulcanodontidae and Omeisauridae as well as Division Neosauropoda that has Cetiosauridae and Diplodocidae family units. The taxonomic classification of this suborder also has subdivision Macronaria which has Camarasauridae family unit and Titanosauriformes infra-division that has the family unit Brachiosauridae and cohort Somphospondyli. Somphospondyli has two family units, Euhelopodidae and Titanosauridae.
The name Sauropodomorpha comes from Greek meaning "lizard-footed forms" and were large saurischian dinosaurs with long necks, long tails, and were herbivores. These quadrupeds were the largest animals to ever walk on earth. Sauropodomorphs lived in the Mesozoic Era and became extinct towards the end of the Cretaceous. Sauropodomorphs browsed tree leaves due to their long necks, small heads, and a long tail that balanced the whole animal. They also had large nostrils and a pollex with a big claw. Some grew up to 131 feet and weighed approximately 110 US short tons. Their teeth were also weak, and they had stomach stones to digest plants. These dinosaurs were initially bipedal but evolved to walk on four limbs. Other features included a short and bulky torso, a manus, 25 presacral vertebrae, ten or more cervical vertebrae, and three or more sacral vertebrae. Sauropodomorphs normally reached sexual maturity before they became fully grown adults.
2. Suborder Thyreophora (Order Ornithischia)
Suborder Thyreophora has the family unit Scelidosauridae and two infraorders Stegosauria and Ankylosauria. Ankylosauria also has two family units namely Nodosauridae and Ankylosauridae, all of which are extinct.
The name Thyreophora has a Greek origin that means “bearing a shield” and therefore members of this suborder, also called armored dinosaurs, had skeletal attributes that united them, especially the Osteoderms which was a keratin-covered bony plates or spikes that formed armored scuta. The body armor lined up in longitudinal rows along the length of the dinosaurs. Some Thyreophorans had simple, low, and collapsed scutes whereas others had complex structures that included spikes and plates. Thyreophorans also had small heads and big body sizes as well as shorter forelimbs compared to the hind limbs. These dinosaurs lived in the early Jurassic period and became extinct at the end of the Cretaceous. Ankylosaurids had a large tail club made up of distended vertebrae fused together and were armored head to tail, including on minor body features like eyelids. These dinosaurs had flat heads and very small or no neck. They also had spikes on both sides of the head. Suborder Stegosauria lived between the Middle and Late Jurassic period and had small heads with leaf-like teeth. Stegosaurs also had rows of spikes on the body.
1. Suborder Cerapoda (Order Ornithischia)
Suborder Cerapoda is of the order Ornithischia and has two infraorder units namely Ornithopoda (bird feet), and Marginocephalia (fringed heads). Most Cerapoda members had enameled teeth. Marginocephalia includes the Pachycephalosauria (thick-headed lizards) and the Ceratopsia (horned faces). Ceratopsia further has three family units; Ceratopsidae, Protoceratopsidae, and Psittacosauridae while Ornithopoda has four family units which are Hadrosauridae, Heterodontosauridae, Hypsilophodontidae, and Iguanodontidae.
Depending on the clade, Ornithopods grew to a maximum length of between 9.84252 and 29.5276 feet. Most of their front teeth were lower than the cheek teeth. The pachycephalosaurs were bipedal, had ossified tendons on their backs, and had enameled leaf-shaped teeth. Pachycephalosaurs had a distinctively marked thickening on the forehead (frontoparietal) and therefore the name dome-heads. These heads may have probably been adapted to defense and aggression. Ceratopsians had varied body sizes ranging from smaller ones like dogs to thirty feet long ones that measured up to five tons. Some members of the Ceratopsians have usually compared to modern day rhinoceros but was several times bulkier, heavier and had different behavior. Ceratops had large horns on their heads that extended from their skulls’ back reaching over the neck and shoulders. The frill (neck shield) made the animal appear as if it had the largest head ever seen on any land animal measuring up to ten feet.