The Mesozoic Era is the correct answer! The Mesozoic Era lasted about 186 million years. The Triassic Period (252-201 mya) saw the first dinosaurs and the breakup of the supercontinent Pangaea. The Jurassic Period (201-145 mya) saw the diversification of dinosaurs (including giant sauropods, stegosaurs, and theropods). The Cretaceous Period (145-66 mya) saw the rise of flowering plants, hadrosaurs, ceratopsians, and the extinction of non-avian dinosaurs. The term "dinosaur" was coined by Sir Richard Owen in 1842, meaning "terrible lizard." Dinosaurs are named using Latin and Greek words. Dinosaurs are not extinct! Birds are theropod dinosaurs (avian dinosaurs). The discovery that birds evolved from dinosaurs is one of the most important findings in paleontology. Feathers have been found on many theropod dinosaurs (including Velociraptor). Dinosaurs are distinguished from other reptiles by their upright stance (legs directly under their body, not sprawled like lizards). Dinosaurs dominated terrestrial ecosystems for over 160 million years – far longer than humans have existed (humans have been around for only about 300,000 years).

Tyrannosaurus rex means "Tyrant Lizard King"! T. rex had the strongest bite force of any terrestrial animal (estimated 8,000-12,000 pounds per square inch). It could bite through bone. T. rex had tiny arms (only about 3 feet long) with two fingers. The function of the arms is debated (possibly for holding prey or helping it stand up). T. rex had excellent vision (binocular vision like a hawk) and a keen sense of smell. It could run at speeds of 10-25 mph (estimates vary). T. rex had a lifespan of about 30 years. The largest T. rex specimen is named "Sue" (Field Museum, Chicago). T. rex was not the largest theropod; Spinosaurus was larger. T. rex lived at the end of the Cretaceous, just before the asteroid impact that caused the extinction of non-avian dinosaurs. T. rex is one of the most iconic dinosaurs in popular culture. The first T. rex skeleton was discovered by Barnum Brown in 1902. T. rex is a theropod (bipedal carnivorous dinosaur). Some paleontologists argue that T. rex was primarily a scavenger (like a vulture), but most evidence suggests it was an active predator that also scavenged.

Triceratops means "three-horned face"! Triceratops was about 30 feet (9 meters) long and weighed 6-12 tons. It had a large frill that may have been used for display, species recognition, or protection of the neck. The horns may have been used for defense against predators (like T. rex) or for fighting rivals. Triceratops is a ceratopsian dinosaur. Other ceratopsians include Protoceratops, Styracosaurus, and Centrosaurus. The frills and horns varied among species. Triceratops was one of the last non-avian dinosaurs before the extinction event. Many Triceratops fossils have been found, including "Trike" and "Hatcher." The first Triceratops fossil was discovered in 1887. Triceratops had a beak (like a parrot) and a battery of teeth (up to 800 teeth) for grinding plants. They were replaced continuously throughout life. Triceratops had a short, pointed tail. It walked on four sturdy legs. Some paleontologists have argued that Torosaurus is actually an adult Triceratops, but most evidence suggests they are separate genera. Triceratops lived alongside T. rex, Edmontosaurus, Ankylosaurus, and Pachycephalosaurus. Triceratops is a favorite among dinosaur enthusiasts. It has appeared in many movies and documentaries.

Stegosaurus lived in western North America! The first Stegosaurus fossil was discovered in Colorado in 1876. Stegosaurus was about 30 feet (9 meters) long, 14 feet (4.3 meters) tall, and weighed 5-7 tons. The plates on its back were likely used for display or thermoregulation (absorbing or releasing heat). The tail spikes (thagomizer) were used for defense against predators like Allosaurus. The term "thagomizer" was coined by cartoonist Gary Larson (The Far Side). Stegosaurus is a thyreophoran (armored dinosaur). Other thyreophorans include Ankylosaurus (club-tailed) and Scelidosaurus. Stegosaurus had a small head with a toothless beak and small cheek teeth. It swallowed stones (gastroliths) to help grind plant matter in its stomach. Stegosaurus walked on all fours. Its front legs were shorter than its hind legs. Stegosaurus had a low-slung body. There are several species of Stegosaurus: S. stenops (most common), S. ungulatus, and S. sulcatus. Stegosaurus lived in the Jurassic Period, long before T. rex and Triceratops. It coexisted with sauropods (Apatosaurus, Brachiosaurus) and theropods (Allosaurus, Ceratosaurus). Stegosaurus is one of the most recognizable dinosaurs. It has appeared in many movies (including Jurassic Park).

Real Velociraptor was turkey-sized! The Jurassic Park filmmakers used Deinonychus (another dromaeosaurid) as a model but kept the name Velociraptor because it sounded more dramatic. Velociraptor had feathers (quill knobs on its arm bones). It was a pack hunter (based on fossil evidence of multiple individuals found together). The sickle claw was about 2.5 inches (6.5 cm) long. Velociraptor may have used it to pin down prey (like the Protoceratops fossil with a Velociraptor locked in combat – the "Fighting Dinosaurs" fossil). Velociraptor was a theropod (bipedal carnivore). It had a long, stiff tail for balance. Velociraptor had large eyes and excellent vision. It was likely an active predator. Velociraptor lived in a desert environment (the Gobi Desert, now in Mongolia and China). The first Velociraptor fossil was discovered in 1923. Velociraptor is a dromaeosaurid (from Greek "dromaeus" meaning "running lizard"). Other dromaeosaurids include Deinonychus, Utahraptor, and Microraptor. The discovery of feathered dinosaurs (like Microraptor) supports the link between dinosaurs and birds. Velociraptor is a popular dinosaur due to Jurassic Park, even though it was depicted inaccurately.

Brachiosaurus ate plants! It was one of the tallest dinosaurs (only Sauroposeidon was taller). Its height allowed it to reach food high in the trees. Brachiosaurus had a long neck (about 30 feet / 9 meters). It may have swallowed stones (gastroliths) to help grind plant matter in its stomach (like birds). Brachiosaurus is a sauropod (the group that includes the largest land animals ever). Other sauropods include Diplodocus, Apatosaurus (formerly Brontosaurus), Argentinosaurus (largest), and Dreadnoughtus. Brachiosaurus lived in North America (Colorado, Utah, Oklahoma, and Wyoming). The most complete Brachiosaurus skeleton is in the Humboldt Museum in Berlin (it was discovered in Tanzania, Africa, and named Giraffatitan). Brachiosaurus had nostrils on top of its head (it was once thought they were used as a snorkel, but that is now disproven). They were probably for display. Brachiosaurus had a small head relative to its body. It had about 50 teeth. Brachiosaurus laid eggs (about the size of soccer balls). It lived in herds (based on fossil trackways). Brachiosaurus was featured in Jurassic Park (the first dinosaur shown). The Brachiosaurus in Jurassic Park was actually a Giraffatitan (the African form). Brachiosaurus is one of the most iconic sauropods.

Asteroid impact is the leading theory! The asteroid was about 6-9 miles (10-15 km) wide. It released energy equivalent to billions of atomic bombs. The impact threw debris into the atmosphere, blocking sunlight for years (impact winter). Plants died, then herbivores, then carnivores. The extinction event is called the Cretaceous-Paleogene (K-Pg) extinction (formerly K-T extinction). Not all dinosaurs died: birds survived and evolved into the thousands of bird species today. Other survivors: mammals (which diversified after the extinction), turtles, crocodiles, snakes, lizards, and amphibians. The extinction of non-avian dinosaurs allowed mammals to become the dominant land animals. Without the asteroid, humans might never have evolved. Other theories have been proposed (volcanism, sea level change), but the asteroid impact is the most supported. The idea was first proposed by Luis Alvarez (Nobel laureate) and his son Walter Alvarez in 1980. They discovered an iridium layer (iridium is rare on Earth but common in asteroids) at the K-Pg boundary. The Chicxulub crater was found in the 1990s. The asteroid impact also caused a massive tsunami (hundreds of meters high). The extinction was rapid (geologically speaking). Some dinosaurs may have survived for a few thousand years after the impact, but none survived. The extinction event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era (the "Age of Mammals").

Spinosaurus may have been the largest carnivorous dinosaur! It had a long, narrow skull (about 6 feet / 1.8 meters) filled with conical teeth (like a crocodile). It likely ate fish (piscivore), but may also have eaten other dinosaurs. Spinosaurus fossils were first discovered in Egypt in 1912. Most Spinosaurus fossils were destroyed during WWII (in a bombing raid on Munich). New fossils have been found recently, revealing that Spinosaurus had short legs and a paddle-like tail, suggesting it was semi-aquatic (spent much of its time in water). Spinosaurus is a spinosaurid theropod. Other spinosaurids include Baryonyx (Europe) and Suchomimus (Africa). The sail on its back may have been used for display, thermoregulation, or species recognition. Spinosaurus lived in what is now the Sahara Desert (but was a lush, riverine environment in the Cretaceous). Spinosaurus was featured in Jurassic Park III (killing the T. rex). The depiction in the movie was speculative (the fossil record is incomplete). The study of Spinosaurus is ongoing; new discoveries may change our understanding. Spinosaurus is one of the most unusual theropods. Its adaptations for swimming are unique among large dinosaurs. Spinosaurus is a favorite among dinosaur fans due to its unique appearance.

Ankylosaurus lived in North America! Ankylosaurus was about 20-30 feet (6-9 meters) long, 6 feet (1.8 meters) tall, and weighed 4-6 tons. Its body was covered with bony armor, including eyelids (eyes were protected). The tail club was made of fused bones (osteoderms) and could be swung with great force. Ankylosaurus had a beak (for cropping plants) and small leaf-shaped teeth. It likely ate ferns, cycads, and other low plants. Ankylosaurus is a thyreophoran (armored dinosaur) in the family Ankylosauridae. Other ankylosaurs include Euoplocephalus and Saichania. Ankylosaurus had a low-slung body (close to the ground). It moved slowly. It may have used its tail club to break the legs of predators or to fight rivals. The tail club was also used for display. Ankylosaurus fossils are rare (only a few partial skeletons). The first Ankylosaurus fossil was discovered in 1906 by Barnum Brown (who also discovered T. rex). Ankylosaurus lived alongside T. rex, Triceratops, Edmontosaurus, and Pachycephalosaurus. Ankylosaurus is sometimes called the "living tank" of the Cretaceous. It is a popular dinosaur due to its unique armor. Ankylosaurus has appeared in Jurassic Park (in Jurassic World: Fallen Kingdom).

Pteranodon is NOT a dinosaur! Pterosaurs evolved earlier than birds and were the first flying vertebrates. They had wings made of skin stretched from an elongated finger to their body. Pteranodon (Late Cretaceous) had a wingspan of up to 20-30 feet (6-9 meters). The largest pterosaur was Quetzalcoatlus (wingspan up to 35-40 feet). Marine reptiles: plesiosaurs (long necks, four flippers), ichthyosaurs (dolphin-like), and mosasaurs (giant marine lizards) lived in the oceans. These were also not dinosaurs. The Loch Ness monster is often depicted as a plesiosaur (but no evidence exists). Dinosaurs are archosaurs (like crocodiles and birds). Pterosaurs are also archosaurs, but they are a separate group. The distinction is important to paleontologists. "Dinosaur" refers specifically to members of the clade Dinosauria (which includes birds). All non-avian dinosaurs went extinct 66 million years ago, but birds (avian dinosaurs) survived. Pterosaurs and marine reptiles also went extinct at the K-Pg boundary. The term "dinosaur" is often misused to describe any prehistoric reptile. Many "sea monsters" and "flying reptiles" are not dinosaurs. You have learned about the diversity of dinosaur groups, their characteristics, and the distinction between dinosaurs and other prehistoric reptiles. You are now a dinosaur expert!