Fossil Records
Dinosaurs are divided into 2 groups: Saurischian and Ornithischian.
Saurischian dinosaurs are the ancestors of birds and have similar hip structure to lizards. Their pubic bone points down and forward, have skulls with large, distinct openings between the eye sockets and nostrils, an opening below nostrils, elongation of bones on neck, and wedge-shaped ankle bone. Saurischian dinosaurs include Sauropodomorphs and Theropods.
Sauropodomorphs have a small head with grinding or nipping teeth, big bone structures, long neck useful for grazing, and a long tail for counterbalancing the neck. These characteristics show that they're herbivores, such as Brachiosaurus and Diplodocus.
Theropods have sharp teeth/beak with well-developed jaws resulting flexible jaw muscles, short clawed hands, long, sturdy, clawed feet, and large eye sockets on skull (indicating good vision). These features prove that they are carnivores, such as T-Rex and Velociraptor.
Ornithischian dinosaurs are only herbivores. They have similar hip structures to birds, but are not ancestors of birds. Their pubic bone points down and towards the tail, have wider pelvis than the Saurischians, resulting more stabilization while moving, and skulls have small openings between eye socket and nostrils. There are 3 groups, Ornithopods, Thyreophora, Marginocephalia.
-Ornithopods have no body armour, walk with 2 or 4 legs, and are from early Jurassic to Cretaceous periods. Ex/Lambeosaurus (large, duck-billed with a head crest) and Dryosaurus (small well-developed leg structures for fast-running and self-sharpening teeth)
-Thyreophorans: 4-legged walk, armed plates on body, and from early Jurassic to Cretaceous periods. Ex/ Ankylosaurids (armed) and Stegosaurids (plated and spiked)
-Marginocephalia: have horns or thick skulls, lived during the Cretaceous period. Ex/Ceratopsians (horned and beaked) and Pachycephalosaurus (dome-headed with huge bony skulls).
According to fossil records, there is a controversy between birds and dinosaurs. Some believe that birds haven't evolved from dinosaurs because birds evolved from some reptiles long before the birth of dinosaurs. The dinosaurian bird-ancestor are tree-climbers or evolved flight by chasing prey from ground up. Fossils show well-developed curved claws and wings with elongation of bones over time. Those who do believe birds evolved from dinosaurs are supported by the form of astragalus (bone in ankle), furculum (fused collarbones), and positions of the toes (3 forward 1 back footprints) are similar in dinosaurs and birds. They believe changes are caused by evolution, so only the similarities will help trace back the ancestor. The Archaeopteryx, first found bird in southern Germany, is a crow-sized skeleton covered in feathers. However, the found structures didn't support the characteristics of a bird but a dinosaur: long bony tail, claws on wings, and teeth instead of a beak. A variety of dinosaurs take the appearance similar to birds and vice versa. Some evidences suggest that bird-like dinosaurs existed earlier than the Archaeopteryx, but fossil records were incomplete and fragmentary to provide enough evidence.
Dinosaur fossils were found throughout the Mesozoic era (251.0-65.5 mya) but not in the sedimentary rocks of the Cenozoic era (65.5 mya-), which means they probably existed 64-66 million years ago. The first dinosaurs appeared in the Triassic Period (248-208 mya), and expanded rapidly through the Jurassic (199.6 to 145.5 mya) and Cretaceous (145.5 to 65.5 mya) Periods. But what caused them to disappear?
Reasons for extinction are only theories and hypothesis, so they can not be validated for sure.
1)Signor-Lipps Effect proposed by Philip W. Signor and Jere H. Lipps: organisms may have gone extinct in the fossil record before they actually do since the record of organisms is never actually complete. This means, although fossils show that it's a gradual extinction, it could be a sudden one instead. For example, some species are uncommon, so finding their fossils would be even rarer. Since the 1st and last appearance of a species determines its range of existence, that rare species seems to disappear from the fossil before its extinction period.
2)Alvarez Hypothesis (ex/ K-T extinction.): an asteroid caused dust in the atmosphere, resulting climate change; few fossils are found at current cold areas. Asteroids have more iridium than Earth’s crust, so the thin iridium layer of the strata must be made of dust from the dust cloud of the asteroid soot layers. This layer of heavy dust could've caused massive wildfires due to the dryness in the air, which wiped out many habitable zones since land was only/mostly composed of forests, grasslands, and prairies. The dust also blocked sunlight, preventing photosynthesis for plants. As a result, insufficient amount of food for herbivores, which then impacts carnivores. Over time, the harmful particles in the air would fall onto the ground and pollute the waters. Other evidences for the impact include melted rocks, shocked quartz in the rocks and small glass spheres of the K-T boundary, and the Chicxulub crater near Mexico that created sediments and markings on the ocean floor and areas in the States. The K-T extinction occurred around 65 mya, and marked the end of Cretaceous period and Mesozoic era.
3)Other reasons include natural disasters (besides extraterrestrial impacts), supported by fossils found near volcanic rocks and similarities of fossil records found over different continents due to plate tectonics (seperation and recombination of land masses), gradual decline of dinosaurs due to mammal competition (increase of mammal fossils found over the periods), and climate change (major drop in temperature and including acid rain).
Dinosaurs are divided into 2 groups: Saurischian and Ornithischian.
Saurischian dinosaurs are the ancestors of birds and have similar hip structure to lizards. Their pubic bone points down and forward, have skulls with large, distinct openings between the eye sockets and nostrils, an opening below nostrils, elongation of bones on neck, and wedge-shaped ankle bone. Saurischian dinosaurs include Sauropodomorphs and Theropods.
Sauropodomorphs have a small head with grinding or nipping teeth, big bone structures, long neck useful for grazing, and a long tail for counterbalancing the neck. These characteristics show that they're herbivores, such as Brachiosaurus and Diplodocus.
Theropods have sharp teeth/beak with well-developed jaws resulting flexible jaw muscles, short clawed hands, long, sturdy, clawed feet, and large eye sockets on skull (indicating good vision). These features prove that they are carnivores, such as T-Rex and Velociraptor.
Ornithischian dinosaurs are only herbivores. They have similar hip structures to birds, but are not ancestors of birds. Their pubic bone points down and towards the tail, have wider pelvis than the Saurischians, resulting more stabilization while moving, and skulls have small openings between eye socket and nostrils. There are 3 groups, Ornithopods, Thyreophora, Marginocephalia.
-Ornithopods have no body armour, walk with 2 or 4 legs, and are from early Jurassic to Cretaceous periods. Ex/Lambeosaurus (large, duck-billed with a head crest) and Dryosaurus (small well-developed leg structures for fast-running and self-sharpening teeth)
-Thyreophorans: 4-legged walk, armed plates on body, and from early Jurassic to Cretaceous periods. Ex/ Ankylosaurids (armed) and Stegosaurids (plated and spiked)
-Marginocephalia: have horns or thick skulls, lived during the Cretaceous period. Ex/Ceratopsians (horned and beaked) and Pachycephalosaurus (dome-headed with huge bony skulls).
According to fossil records, there is a controversy between birds and dinosaurs. Some believe that birds haven't evolved from dinosaurs because birds evolved from some reptiles long before the birth of dinosaurs. The dinosaurian bird-ancestor are tree-climbers or evolved flight by chasing prey from ground up. Fossils show well-developed curved claws and wings with elongation of bones over time. Those who do believe birds evolved from dinosaurs are supported by the form of astragalus (bone in ankle), furculum (fused collarbones), and positions of the toes (3 forward 1 back footprints) are similar in dinosaurs and birds. They believe changes are caused by evolution, so only the similarities will help trace back the ancestor. The Archaeopteryx, first found bird in southern Germany, is a crow-sized skeleton covered in feathers. However, the found structures didn't support the characteristics of a bird but a dinosaur: long bony tail, claws on wings, and teeth instead of a beak. A variety of dinosaurs take the appearance similar to birds and vice versa. Some evidences suggest that bird-like dinosaurs existed earlier than the Archaeopteryx, but fossil records were incomplete and fragmentary to provide enough evidence.
Dinosaur fossils were found throughout the Mesozoic era (251.0-65.5 mya) but not in the sedimentary rocks of the Cenozoic era (65.5 mya-), which means they probably existed 64-66 million years ago. The first dinosaurs appeared in the Triassic Period (248-208 mya), and expanded rapidly through the Jurassic (199.6 to 145.5 mya) and Cretaceous (145.5 to 65.5 mya) Periods. But what caused them to disappear?
Reasons for extinction are only theories and hypothesis, so they can not be validated for sure.
1)Signor-Lipps Effect proposed by Philip W. Signor and Jere H. Lipps: organisms may have gone extinct in the fossil record before they actually do since the record of organisms is never actually complete. This means, although fossils show that it's a gradual extinction, it could be a sudden one instead. For example, some species are uncommon, so finding their fossils would be even rarer. Since the 1st and last appearance of a species determines its range of existence, that rare species seems to disappear from the fossil before its extinction period.
2)Alvarez Hypothesis (ex/ K-T extinction.): an asteroid caused dust in the atmosphere, resulting climate change; few fossils are found at current cold areas. Asteroids have more iridium than Earth’s crust, so the thin iridium layer of the strata must be made of dust from the dust cloud of the asteroid soot layers. This layer of heavy dust could've caused massive wildfires due to the dryness in the air, which wiped out many habitable zones since land was only/mostly composed of forests, grasslands, and prairies. The dust also blocked sunlight, preventing photosynthesis for plants. As a result, insufficient amount of food for herbivores, which then impacts carnivores. Over time, the harmful particles in the air would fall onto the ground and pollute the waters. Other evidences for the impact include melted rocks, shocked quartz in the rocks and small glass spheres of the K-T boundary, and the Chicxulub crater near Mexico that created sediments and markings on the ocean floor and areas in the States. The K-T extinction occurred around 65 mya, and marked the end of Cretaceous period and Mesozoic era.
3)Other reasons include natural disasters (besides extraterrestrial impacts), supported by fossils found near volcanic rocks and similarities of fossil records found over different continents due to plate tectonics (seperation and recombination of land masses), gradual decline of dinosaurs due to mammal competition (increase of mammal fossils found over the periods), and climate change (major drop in temperature and including acid rain).