Main dinosaur groups

Sauropods (meaning “reptile foot”) are the long-necked dinosaurs. They come in two main flavors: the gigantic vertical-necked titanosaurs (including Brachiosaurus, Patagotitan, and Alamosaurus) and the horizontal-necked, whip-tailed diplodocoids (including Diplodocus, Brontosaurus, and Amargasaurus). Sauropods and their basal relatives, sauropodomorphs, have been around since the late Triassic (about 231 million years ago).

Theropods (meaning “beast foot”) are the bipedal, carnivorous dinosaurs. These include such fan favorites as Tyrannosaurus, Allosaurus, Spinosaurus, Carnotaurus, Velociraptor, and all modern birds. Theropods have also been around since the late Triassic.

Ceratopsians (meaning “horned face”) are the horned, frilled dinosaurs. (Exactly what it says on the tin for once!) These include Triceratops, Protoceratops, Psittacosaurus, and others. Ceratopsians didn’t arise until the late Jurassic, about 161 million years ago, and only hit their stride in the late Cretaceous, about 100 million years ago.

Ornithopods (meaning “bird foot”) are the “duck-billed” dinosaurs. These include Iguanodon, Edmontosaurus, Parasaurolophus, Maiasaura, and others. Ornithopods, like ceratopsians, didn’t arise until the late Jurassic and didn’t hit their stride until the late Cretaceous.

Thyreophorans (meaning “shield bearer”) are the armored dinosaurs. They come in two main flavors: the plate-backed stegosaurs (including Stegosaurus, Kentrosaurus, and Gigantspinosaurus), which were numerous in the Late Jurassic, and the heavily-armored ankylosaurs (including Ankylosaurus, Nodosaurus, and Euoplocephalus), which took over as the stegosaurs declined and lasted until the end of the Cretaceous.

Other important clades

Tetrapods are the group of vertebrates with four limbs, including amphibians, mammals, and reptiles (which includes birds). Even though some tetrapods later lost their limbs, like snakes and whales, they are still considered tetrapods.

Amniotes are the clade of tetrapods that lay amniotic eggs, including mammals and reptiles. The eggshells of amniotic eggs aren’t permeable to water, which means the eggs can be laid on land without having to worry about them drying out (and if they are laid in the water, they’ll drown). This was a huge deal during the Carboniferous Rainforest Collapse when the climate all over the planet got a lot drier, and allowed mammals’ and reptiles’ ancestors to get a competitive edge over the amphibians.

Synapsids (meaning “together face”) are the clade of animals including mammals and their ancestors, all the way back to when they diverged from the ancestors of reptiles. They can be distinguished by the single temporal fenestra (hole) on either side of the skull behind the eye socket. They are a subgroup of amniotes.

Sauropsids (meaning “reptile face”) are the clade of animals including reptiles and their ancestors, all the way back to when they diverged from the ancestors of mammals. They have either two temporal fenestrae behind each eye (diapsids), or none (anapsids). They are a subgroup of amniotes.

Archosaurs (meaning “leader reptile”) are the clade of reptiles including crocodylians, birds, dinosaurs, pterosaurs, and phytosaurs. It is the sister group to Lepidosaurs, and a subgroup of Sauropsids. Yes, crocodiles and birds are more closely related to each other than either one is to lizards.

Lepidosaurs (meaning “scale reptile”) are the clade of reptiles including lizards, snakes, and tuatara. It is the sister group to archosaurs, and a subgroup of sauropsids.

Pterosaurs (meaning “wing reptile”) are a group of flying archosaurs closely related to dinosaurs, but are not dinosaurs. They come in two main flavors: “rhamphorhynchoids” and pterodactyloids. “Rhamphorhynchoids” (meaning “beak snout”) are a paraphyletic group of more basal pterosaurs, which usually had long tails and toothy jaws. Pterodactyloids (meaning “winged finger”) are a monophyletic group of more advanced pterosaurs, which usually had reduced tails, keratinous beaks, and extreme headgear. A notable subgroup of pterodactyloids are the azhdarchids, the late Cretaceous giant pterosaurs that were the largest creatures ever to fly.

Pelycosaurs (meaning “basin reptile”) are a paraphyletic group of very basal synapsids that lived from the late Carboniferous to the middle Permian. They included the apex predators of their time, such as the famous Dimetrodon, as well as large herbivores such as Edaphosaurus. Many had huge sails on their backs for some still-unclear reason.

Temnospondyls are large extinct stem-amphibians that first arose in the Carboniferous, which were among the first vertebrates adapted for life on land. Many early temnospondyls had way more fingers and toes than we’re used to–Acanthostega had eight on each foot! And it’s just chance that those with five per foot happened to survive the end-Devonian extinction and give rise to the rest of the vertebrates. The last known temnospondyl, Koolasuchus, survived until the early Cretaceous. Special shout-out to Prionosuchus, a giant temnospondyl pretending to be a crocodile.

Phytosaurs are large extinct crurotarsan archosaurs that lived during the Triassic that looked a lot like crocodiles but were not crocodylians.

Anamniotes are the paraphyletic clade of anything that’s not an amniote (reptile or mammal). This includes non-tetrapod fish and amphibians.

Prototherians (meaning “first beast”) are the group of egg-laying mammals that today only includes the monotremes (echidnas and platypuses). They’re not very closely related to other living mammals, their lineage having split off in the Late Triassic.

Metatherians (meaning “changed beast”) are the group of mammals including marsupials and their ancestral cousins. They tend to hail from Australia or South America and tend to be less intelligent than comparable placental mammals.

Eutherians (meaning “true beast”) are the group of mammals including placental mammals and their ancestral cousins. All living mammals that aren’t marsupials or monotremes are placentals.

Placoderms (meaning “plate skin”) were a group of armored fish that lived from the Silurian period to the Devonian period. They had both an internal skeleton AND an exoskeleton, so we have a very good idea of what they looked like from fossils. The largest one known, Dunkleosteus, was twenty feet long and weighed about a tonne, and was the apex predator of its time. Placoderms didn’t have teeth, instead using the edges of their exoskeletal plates to bite.

Chondrichthyes (meaning “cartilage fish”) are the group of fish containing sharks, rays, and chimeras. Their skeletons are made of cartilage rather than bone. They are not very closely related to other fish, their lineage having split off way back in the Silurian period.

Osteichthyes (meaning “bone fish”) are the group of fish containing ray-finned fish and lobe-finned fish (which includes tetrapods, which includes humans). Their skeletons are made of bone, and they don’t have an exoskeleton.

Actinopterygii, also known as ray-finned fish, is the subgroup of osteichthyes (bony fish) that includes most modern fish.

Sarcopterygii, also known as lobe-finned fish, is the subgroup of osteichthyes (bony fish) that includes coelacanths, lungfish, and all tetrapods (including humans).

Eurypterids (meaning “broad wing”), also known as sea scorpions, are a group of very small to very large carnivorous aquatic arthropods that were very successful throughout most of the Paleozoic Era (from the Ordovician to the Permian). Some were predators and some were filter-feeders. The largest known eurypterid, Jaekelopterus, is the largest known arthropod ever. Since eurypterids were so common and were made entirely of hard parts, they fossilized very well and thus their biology is well-understood.

Pseudosuchians (meaning “false crocodile”) are the group containing crocodiles and their ancestral cousins. They used to be incredibly diverse, especially in the Triassic–there were sail-backed, herbivorous crocs (Lotosaurus); hoofed crocs (Boverisuchus); “cat” crocs (Pakasuchus) and “cheetah” crocs (Araripesuchus); fully-aquatic crocs with fins and flippers (Dakosaurus); tank-like armored, spiky crocs (Desmatosuchus); and many more.

Ornithodira is the group containing pterosaurs and dinosaurs (which contains birds).

Cynodonts (meaning “dog tooth”) are a subgroup of synapsids (stem-mammals) that includes mammals and their close relatives, but excludes basal synapsids like Dimetrodon, Gorgonops, and the dicynodonts.

Dicynodonts (meaning “two dog tooth”) are an offshoot of synapsids (stem-mammals) that were warm-blooded, roly-poly, beaked, and toothless except for two large tusks. They aren’t our ancestors, but ancestral cousins.

Phylogeny terms

A monophyletic group, or a clade, is a group of organisms defined as an ancestor and all its descendants.

A paraphyletic group is a group that contains all the descendants of a common ancestor minus one or two. For example, “reptiles” is commonly used to mean lizards, snakes, and crocodiles but not birds, which is a paraphyletic group. This is sometimes a useful concept in taxonomy (see “stem-group” below) but isn’t a valid phylogenetic clade.

A polyphyletic group is when you’re confused and doing it wrong and group organisms by something other than ancestry. For example, a theoretical “Haemothermia” group, including all warm-blooded animals (most mammals and all birds), would be a polyphyletic group.

A wastebasket taxon is a para- or polyphyletic group whose name continues to be used because the true structure of the phylogenetic tree isn’t known. Organisms of ambiguous affinity keep getting shunted into this group until the tree finally gets untangled later. Examples include “Insectivora”, “Condylarthra”, and possibly Symmetrodonta (though this one is still waiting to be untangled).

A crown-group is a monophyletic group of living organisms connected by a common ancestor, and this ancestor, and all its other descendants.

A stem-group is a paraphyletic group of all the extinct organisms that are more closely related to a certain crown-group than to other crown-groups. For example, all dinosaurs can be called stem-birds; all synapsids can be called stem-mammals; metatherians can be called stem-marsupials, etc.

A genus (plural: genera) is the taxonomic ranking below “family” and above species, and is the most common ranking paleontologists work with, even though it’s not super well-defined. The scientific name of an organism’s genus is called its generic name, and is the first word in the binomial nomenclature system. For example, in Tyrannosaurus rex, “Tyrannosaurus” is the generic name.

A species is the taxonomic ranking below genus, and is defined as the largest group in which any two individuals of the appropriate sexes can produce fertile offspring. The scientific name of an organism’s species is called its specific name, and is the second word in the binomial nomenclature system. In Tyrannosaurus rex, “rex” is the specific name.

An ancestral trait is a trait that evolved early on in a lineage. For example, mammals are ancestrally furry–each furry group of mammals descended from an ancestor that was furry, rather than evolving fur independently many times.

A derived trait is a trait that evolved later on in a lineage. For example, bipedalism is a derived trait in hominids.

A secondarily acquired trait is when a group loses an ancestral trait, and then gains it back again as a derived trait later on. For example, whales are secondarily aquatic–they evolved from aquatic fishlike tetrapodomorph ancestors way back in the Devonian, those tetrapodomorphs came up on land and became terrestrial, and then proto-whales went back into the ocean in the Paleocene.

A sister clade is the other side of the fork, whenever a phylogenetic tree forks. For example, Amniota is the group including synapsids (mammals and stem-mammals) and sauropsids (reptiles and stem-reptiles). So synapsida is the sister clade to sauropsida.

An autapomorphy is a derived trait that’s unique to one group. For example, all archosaurs have a fourth trochanter, a ridge on the inside of the thighbone that serves as a muscle attachment point, and no other animals have this.

A synapomorphy is a trait that’s ancestrally unique to one group, but later members of the group may have secondarily lost it. For example, archosaurs may be ancestrally feathered, but many derived archosaurs such as crocodylians and many dinosaurs were secondarily scaly.

Homoplasy is when a trait is gained or lost independently in unrelated groups, often due to convergent evolution. For example, electric eels and elephantfish both separately came up with a way to produce electricity. A trait that is prone to homoplasy is called a homoplastic trait.

A homology is a trait that is derived from an ancestral trait. For example, a bat has the same arm bones in the same order in its wing as a whale has in its flipper, though each bone is a vastly different shape. Those are considered homologous structures.

An analogy is a convergent trait derived from different starting points. For example, a whale developed its flipper independently from a plesiosaur’s flipper, but both flippers look nearly the same. Those are considered analogous structures.

Paleontological Methods

Bone histology is the practice of cutting a thin cross-section of bone and looking at it under a microscope. Tons of information can be deduced about an animal from its bone histology, including its gender, growth rate, resting metabolic rate, and more.

Melanosome analysis is the technique of deducing what color a fossil animal was by looking at its pigment-containing organelles under a microscope. The pigments have long since degraded, but fortunately the melanosomes have distinctive shapes and organizations that carry information.

Osteological correlates are textures left on the surface of a bone by the tissue that used to be attached there. Muscles, fat, skin, and horns all leave distinctive marks on bone.

Leg Anatomy

Plantigrady is when an animal’s toe and foot bones (phalanges and metatarsals) are in contact with the ground while walking or running. Examples of plantigrade animals include humans, bears, sauropod dinosaurs (on their back legs), and rodents.

Digitigrady is when an animal’s toe bones (phalanges) are in contact with the ground while walking or running, but the foot bones (metatarsals) are raised off the ground. Examples of digitigrade animals include dogs, cats, birds, and humans on tiptoe.

Unguligrady is when an animal walks and runs on the tips of its toes or on its nails, and the entire foot is raised off the ground. Examples of unguligrade animals include horses, deer, and ballerinas on pointe.

Wing Anatomy

A patagium (plural: patagia) is a membrane that an animal uses to fly or glide. Prefixes can be added to this word to indicate where on the body the membrane is located.

A brachiopatagium (“brachio” means “arm”) is a membrane between the arm and the body. In pterosaurs, it’s the main large wing membrane.

A propatagium (“pro” means “before”) is a membrane between the wrist and the shoulder or neck, on the top part of the wing. It occurs in both bats and pterosaurs.

An uropatagium (“uro” means “tail”) is a membrane between the hind leg and the body that includes the tail. Bats have this.

A cruropatagium (“cruro” means “shin”) is a membrane between the hind leg and the body that does not include the tail. Pterosaurs have this.

A dactylopatagium (“dactylo” means “finger”) is a membrane between two fingers, as in bats.

A plagiopatagium (“plagio” means “flat”) is a membrane between a finger and the body, as in bats.

Aquatic tail types

Hypocercal tails have a larger bottom lobe and smaller top lobe. The only living creatures with this are flying fish, but in the past metriorhynchids and mosasaurs both sported this type of tail.

Heterocercal tails have a larger top lobe and smaller bottom lobe. Modern sharks and certain ray-finned fish like sturgeon have this type of tail, as well as placoderms like Dunkleosteus in the past.

Homocercal tails have top and bottom lobes that are about the same size. Most ray-finned fish have this type of tail, as well as ichthyosaurs in the past.

Swimming styles

Anguilliform swimming is what eels do, in which the entire body moves back and forth and large fins aren’t necessary. It’s not very fast, but maneuverable and allows the fish to fit in tight spaces.

Carangiform swimming is what most fish do, where the tail is mostly responsible for propulsion and the rest of the body is held more stiffly. It’s an intermediate style between anguilliform and thunniform.

Thunniform swimming is what the fastest fish do, like tuna, sailfish, and sharks. The body is very stiff and the tail is very powerful.

Specialized behaviors

Fossorial creatures are optimized for digging and burrowing, such as moles, clams, bees, and Oryctodromeus.

Arboreal creatures are optimized for life in trees, such as monkeys, geckos, tree snakes, squirrels, many birds, and Suminia.

Scansorial creatures are optimized for climbing. This frequently overlaps with arboreal, but some non-arboreal creatures are scansorial, like mountain goats, and some non-scansorial creatures are arboreal, like perching birds.

Cursorial creatures are optimized for running long distances, such as horses, ostriches, wolves, certain spiders, and Moros.

Saltatorial creatures are optimized for hopping, such as kangaroos, jerboas, and Scleromochlus. This is particularly common among desert-dwelling creatures.

Graviportal creatures are optimized for being massive and have specialized skeletal structures for carrying their weight around. The groups that have done this the most successfully are elephants and sauropods.

Oophagy is the practice of an embryo eating unfertilized eggs while still in the womb.

Adelphophagy is the practice of an embryo eating other embryos while still in the womb.

Durophagy is the practice of eating hard foods, such as shellfish or bones. They often need specialized stout teeth to crush these materials.

Myrmecophagy is the practice of eating colonial insects. Examples include anteaters, armadillos, pangolins, numbats, and aardwolves.

Heterodont dentition is when an animal has many types of differently-shaped teeth for different purposes. It’s more common in mammals, but is also found in some reptiles. For example, we humans have molars, premolars, incisors, and canines, while sharks just have a whole bunch of the same type of flat, serrated tooth.

Homodont dentition is when an animal has just one type of tooth, repeated. It’s the ancestral condition, and is common in reptiles and less common but also found in mammals.

Gastroliths, also known as gizzard stones, are rocks ingested on purpose by an animal that help grind up food in their stomach, or help decrease buoyancy in aquatic or semiaquatic animals. Groups known to use gastroliths are herbivorous birds, crocodylians, pinnipeds (seals and sea lions), some amphibians, sauropods, certain theropods, and plesiosaurs.

A xerocole is an animal specialized for life in a desert environment.

Diurnal animals are active during the day.

Nocturnal animals are active during the night.

Crepuscular animals are active during dawn and dusk.

Cathemeral animals are active at sporadic times throughout the day and night.

Estivation is the habit of entering a state of torpor when it’s too hot. Hibernation is entering a state of torpor when it’s too cold.


Endothermy (meaning “internal heat”) is warm-bloodedness, or keeping your body temperature higher than the environment at almost all times.

Ectothermy (meaning “external heat”) is cold-bloodedness, or allowing your body temperature to change to match the environment.

Homeothermy (meaning “same heat”) refers to endotherms that can’t hibernate, estivate, or achieve a state of torpor.

Heterothermy (meaning “different heat”) refers to endotherms that can hibernate, estivate, go into torpor, or otherwise temporarily change their body temperature.

Mesothermy (meaning “middle heat”) refers to an intermediate state between endo- and ectothermy. Some examples of animals that are sometimes considered mesotherms are monotremes (platypuses and echidnas), lamniform sharks (like makos and great whites), billfish (swordfish and marlins), leatherback sea turtles, and sauropod dinosaurs.

Gigantothermy (meaning “gigantic heat”) refers to animals that maintain a high body temperature just by being enormous, despite a relatively slow metabolism. Larger gigantotherms run hotter, which is not true of endotherms. Examples include leatherback sea turtles and sauropod dinosaurs.


Integument is the “covering” of an organism. In animals, it refers to the skin, scales, fur, feathers, or other features on the outside of an animal. In plants, it refers to the rind, husk, shell, etc.

Pycnofibres are fuzzy hair-like structures found on most pterosaurs. We now believe that archosaurs may have been ancestrally fluffy, so pycnofibres may be derived from the same feature that feathers are derived from. Functionally, pycnofibres look a lot like fur.

Osteoderms (meaning “bone skin”) are bones that grow in the skin, unattached to the rest of the skeleton, usually used for armor. They evolved independently in many groups, such as lizards, amphibians, crocodylians, dinosaurs, phytosaurs, and armadillos.

Keratin is a tough protein found in horns, hair, hooves, feathers, claws, and beaks.

Chitin is a tough protein found in arthropod shells, claws, and other hard parts.

Pinnae are external ear-flaps that are ancestral to mammals. They also may have independently appeared in some advanced notosuchian crocodyliformes in the Mesozoic. (Isn’t that cool to imagine–crocs with expressive, dog-like ears?)

Osteological correlates are textures left on bone by overlying tissues. For example, when skin lies almost directly on the bone, as on a crocodile’s face, the bone underneath is more rugose, or rough and pitted. Concentrated lines or circles of rugosity imply large soft-tissue structures rooted there, like horns or crests. Smooth bone implies soft overlying tissue. Muscle and tendon attachment sites also leave distinctive marks on bone.

Melanosomes are pigmented organelles in certain animal cells that are responsible for what color the animal appears. Thankfully for paleontologists, the shape and arrangement of melanosomes also correlate with color, so we can use those properties to figure out what color things were even when the original colors have been lost.

Countershading is a form of camouflage in which an animal’s belly is a lighter color than its back. It’s extremely common in marine animals because it’s very effective: when viewed from above, the dark back blends in with the darker deep water, and when viewed from below, the light belly blends in with the bright sky. However, it’s also common in terrestrial animals, because when the sun hits the darker back and the lighter belly is in the shade, it creates an optical illusion that makes the animal’s 3D shape harder to figure out.

Dactyly (toe arrangements)

Zygodactyly is when two toes point forward and two backward, like a parrot.

Anisodactyly is when three toes point forward and one backward, like a majority of birds.

Tridactyly is when three toes point forward and zero backward, like a rhinoceros or Tyrannosaurus.

Didactyly is when two toes point forward and zero backward, like a deer or ostrich.

Monodactyly is when one toe points forward and zero backward, like a horse or Vespersaurus.

Fauna Sizes

Microfauna are tiny creatures usually smaller than 100 microns across (less than the width of a hair) that can only be seen with a microscope. Examples include tardigrades (water bears), dust mites, copepods (small crustaceans), small nematodes, and rotifers.

Meiofauna are tiny creatures between 45 and 1000 microns across (0.045-1mm). This term usually refers to benthic creatures, which live in the seafloor. Examples include seed shrimp, certain tiny wasps and beetles, pinworms, hairybacks, and many foraminifera.

Mesofauna are tiny creatures between 100 and 2000 microns across (0.1-2mm). This term usually refers to creatures that live in terrestrial soils. They aren’t large enough to move soil particles around, so they live in the spaces between them rather than actually burrowing. Examples include springtails, coneheads, tiny spiders, tiny scorpions, potworms, and many other types of tiny arthropods.

Macrofauna are tiny creatures larger than 500 microns across (>0.5mm), which are definitely visible to the naked eye. This term refers to creatures that live in both marine and terrestrial soil environments. Terrestrial examples include earthworms, pillbugs, snails, and ants; marine examples include crustaceans, sea stars, bristle worms, bivalves, and sponges.

Megafauna is a not-very-well-defined term that usually refers to any animals human-sized or larger.


Angiosperms (meaning “vessel seed”) are flowering and fruiting plants. They are the dominant type of plant on the planet today, and arose in the Cretaceous Period. This includes obvious things like flowers and fruits, but also grasses, grains (which are a type of grass), cacti, and palm trees.

Gymnosperms (meaning “naked seed”) are non-flowering seed plants, including conifers, ginkgos, and cycads. They are the second most common terrestrial plant today, only dominating in certain ecosystems like taiga, but they include the tallest organisms on earth, and the second-longest-lived (giant basal spicule sponges get older).

Prehistoric Radiation and Extinction Events

The Great Oxidation Event was an extinction event that ended the Archean Eon and kicked off the Proterozoic. During the Archean (4 to 2.5 billion years ago), life consisted of many types of chemosynthetic prokaryotes (simple single-celled organisms), including some photosynthesizers that produced oxygen. During the Great Oxidation Event, the photosynthesizers exponentially multiplied and took over, creating so much oxygen that many of the anaerobic organisms went extinct, and, due to chemical reactions involving oxygen, creating the first “Snowball Earth” (Huronian glaciation).

The Cambrian Explosion was a radiation event that spanned the Cambrian period, 541 to about 516 million years ago, in which all the main types of modern animals quickly appeared. There are many things that could be considered causes for this, such as the advent of mouths, eyes, predation, burrowing behavior, the end-Ediacaran extinction, etc.

The Great Ordovician Biodiversification Event (GOBE) was a radiation event that spanned the Ordovician period, 485 to 444 million years ago, in which the “foundations” of modern phyla laid by the Cambrian explosion were filled in by diverse animal groups. Different types of marine ecosystems emerged, with different organisms living in, say, reefs than in deep water, and differences in populations depending on location. This contrasts with the Cambrian, which had basically one type of biome and one biota globally.

The Carboniferous Rainforest Collapse (CRC) was a climatic shift and minor extinction event that occurred in the late Carboniferous period (305 million years ago) in which the previously cosmopolitan, wet, dense rainforests that characterized the Carboniferous period gave way to the deserts of the Permian. This was caused by two major factors: the formation of the supercontinent Pangaea, which, by creating more inland area, made the climate much drier; and the global cooling and glaciation caused by the super low levels of carbon dioxide, due to the rainforests that had been working overtime since the beginning of the Carboniferous but failing to decompose (trees store carbon dioxide when they’re alive, but decomposition releases carbon dioxide back into the atmosphere; in the Carboniferous, trees were so new that nothing had figured out how to digest the tough wood yet). Together, these two factors swiftly (over just a couple thousand years) ended the reign of amphibians and arthropods, the dominant life-forms of the time, and jump-started the evolutionary trajectory of amniotes (reptiles and mammals), which do better in dry, lower-oxygen conditions.

The Mesozoic Marine Revolution (MMR) was a radiation event that spanned the Mesozoic era, 251 to 66 million years ago, in which the ocean environment, niches, and interactions between oceanic animals became a lot more complex. Predators found ways to crush shells and dig for burrowing animals, which forced the prey to develop spiky, tough shells, toxins, and other defenses to survive. This was basically Round 2 of the GOBE in terms of new types of ecosystems created and increased niche partitioning and food web complexity.

The Carnian Pluvial Event (CPE) was a climatic shift and minor extinction event in the early Late Triassic period (230 million years ago) in which the previously dry, mild climate became warm and wet, probably caused by a combination of volcanic activity leading to global warming, and the creation of a new mountain range that cut off winds between ocean and continent, generating monsoons. This caused numerous extinctions and subsequent radiations, and jump-started the rise of the dinosaurs.

The Cretaceous Terrestrial Revolution (KTR) was a radiation event in the Late Cretaceous period (125-80 million years ago) in which flowering plants (angiosperms) appeared and quickly took over the world, drastically changing many environments and the animals within them. Prior to this, there were no grasslands, no true rainforests since the Carboniferous Rainforest Collapse, and few frugivores. The worldwide domination of angiosperms created these new biomes with new niches for animals to exploit.

The Great American Biotic Interchange (GABI) was an event that began around 2.7 million years ago in the Pliocene when South America, which had been an island since the Cretaceous, collided with North America, forming the Isthmus of Panama. This allowed animals to travel from one continent to the other. Though animals did move both ways, the North American fauna tended to outcompete the South American. One famous South American animal that successfully emigrated to North America was Titanis, the last of the terror birds, which ended up as far north as Tennessee.


Elastin is an elastic protein found in vertebrates’ connective tissues.

Resilin is an elastic protein found in invertebrates’ connective tissues. It is superior to elastin in most ways.

Keratin is a hard protein found in vertebrates’ nails, hair, and horns. “Kera” comes from the Greek for “horn” (it’s the same root as ceratopsian and ceratosaur).

Chitin is a hard protein found in invertebrates’ shells. It is the invert equivalent of keratin. Detecting keratin versus chitin is how scientists found out that the Tully Monster is probably a vertebrate.

Reproductive strategies

Precocial animals are relatively capable when born, and have a short period of dependence on parents, if any. Examples include chickens, horses, crocodiles, and pterosaurs. Animals that are extremely precocial and must fight for their lives immediately after hatching, like sea turtles and brush turkeys, are known as superprecocial.

Altricial animals are helpless when born and have an extended childhood during which they depend on their parents for food, protection, and education. This strategy allows for adults with wider skillsets than precociality, but requires parental investment. Examples include songbirds, dogs, and humans.

r-strategists are animals that have many offspring and invest little or no parental care. Offspring of r-strategists are often precocial, but not always. Examples include clams, many fish, rats, and others.

K-strategists are animals that have few offspring and invest lots of care. Offspring of K-strategists are often altricial, but not always. Examples include elephants, sharks, humans, and others.

Sexual dimorphism is when the males and females of one species are physically different from one another in ways apart from genitalia. Depending on how close to monogamous an animal is, this ranges from zero sexual dimorphism in purely monogamous species like gibbons and crows, to mild sexual dimorphism in close-to-monogamous species like humans (males are larger and have facial hair), to extreme sexual dimorphism in harem-forming species like peacocks and elephant seals, in which the male and female are often mistaken for two different species.

Growth and development

Ontogeny is the way an organism grows and changes as it gets older. Most creatures don’t retain the same proportions when old as they had when young–for example, human babies have gigantic heads in proportion to their bodies compared to adults. An ontogenetic sequence is a picture showing various life stages of an organism.

Neoteny is the retention of juvenile traits into adulthood, and is one method by which evolution can take a giant leap forward. For example, our closest invertebrate relatives, the sea squirts, are sessile as adults, but have a larval form that is free-swimming and has a primitive backbone. A long time ago, one of these larvae refused to grow up, and became a free-swimming, backboned adult. This was such a successful strategy that that one critter was able to become the ancestor of all vertebrates.

Isometry means an animal maintains the same proportions as it grows: the young are like mini adults.

Allometry means an animal changes its proportions as it grows, since different body parts grow at different rates. For example, a human baby’s head and eyes are proportionally much bigger than an adult’s, and a baby’s arms and legs are proportionally much shorter. The proportions change as the baby grows up.


“Splitters” are paleontologists who habitually like to create more taxonomic groups. “Lumpers” are paleontologists who like to group other groups together and have less groups.

The Bone Wars were a period in the late 1800s (the “Gilded Age” of America) when rival paleontologists Edward Drinker Cope and Othniel Charles Marsh competed to see who could describe the most dinosaurs and reap the most dinosaur-related fame. They used ruthless tactics like hiring hit men to destroy the other’s fossil sites and slandering each other in the newspaper, and often rushed publication, generating confusion when later scientists tried to make sense of their findings. However, they described over a hundred new species of dinosaur, brought dinosaurs into the public awareness, and inspired a lot more excavation in the western United States and elsewhere the following decades. There was no winner of the Bone Wars–both men were financially and socially ruined by the end.

A niche is a general role that an ecosystem makes available for organisms to live within. If two organisms occupy the same niche, they will compete, resulting either in the local extinction of one, or the partitioning of the niche into two niches through increased specialization. For example, in the Mesozoic, ornithopods filled the niche of medium-sized grazer; in the Cenozoic, this niche is filled by cows and other ruminants.

Niche assimilation is when one species fills multiple niches at different points in its development, so effectively that it actually decreases overall species diversity in the environment.

A junior synonym happens when a new genus is described based on a new fossil (e.g. Stygimoloch) but later found to belong to an existing genus (e.g. Pachycephalosaurus). This can happen when the new fossil is a juvenile that looks different from the adult form, an elderly individual that looks different from the norm, or when the new fossil is re-analyzed later and is found to be just not different enough to merit its own genus. The “junior” part of the term comes from the practice that the name that’s published first (the senior synonym) takes precedence. In 2010, a study tried to lump Torosaurus as fully mature Triceratops; in that case, even though Torosaurus are older individuals than Triceratops, the name “Triceratops” was published first, so it would be the senior synonym while Torosaurus would be the junior synonym. However, other studies have since refuted this claim.

Radiation is when a lineage of organisms diversifies and many new species arise in order to fill open niches. Radiation events often occur after extinction events, since many niches are left open by the organisms that didn’t make it, and other organisms rush to fill the space.

Ontogeny Recapitulates Phylogeny is the outdated theory (also known as Recapitulation Theory) that ontogeny, specifically embryonic development, mirrors the evolutionary process the organism’s ancestors went through. For example, in the very early stages of development, human embryos look very similar to those of a more “primitive” animal, like a fish, and as the human embryo grows it goes through developmental stages that correspond to looking like an amphibian, a reptile, etc. This is much more of an effect than a cause, however, and does not hold up under all (or even most) conditions. Now it’s just a well-known “biological myth”.

A Lagerstätte (meaning “storage place”) is a fossil deposit with exquisite preservation. Famous examples include the Burgess Shale, Ghost Ranch, and the La Brea Tar Pits.

The fraternal birth order effect is the biological effect responsible for gay men having disproportionately more older brothers than straight men.

Taphonomy is everything that happens to an organism after death–scavenging, burial, fossilization, etc.

A ghost lineage is a line of descent we know must exist because we have fossil or living evidence for its descendents, but we haven’t found any fossil evidence for the ancestors. Take lizards for example: until recently, the oldest fossil lizards were known from Middle Jurassic England, but molecular analysis indicates that they split off from their closest relative, the tuatara, at least as far back as the Middle Triassic, making this a ghost lineage. However, new analysis of a fossil known as Megachirella, from the Middle Triassic, revealed that it was in fact an early lizard, closing that gap and fulfilling scientists’ predictions nicely. Read a more detailed explanation of ghost lineages here.

Endemism is when an organism is only found in a single time and place; for example, emus are endemic to Australia.

Cosmopolitan distribution is when an organism is found everywhere, the opposite of endemism.

Exaptation is when a trait used for one thing is co-opted for another purpose. For example, opposable thumbs are useful for swinging between branches, but also for making tools and manipulating small objects.

The semilunate carpal is a bone found in the wrists of birds and their close relatives, which allows them to fold their wings the way they do. It’s a carpal shaped like a half moon, hence the name. For more info on how bird wings work, check out my wing anatomy post!

Insular dwarfism and insular gigantism are phenomena that occur when a population of animals isolated from the main population, often on an island. The small often become large due to lack of predation, and the large become small due to scarcity of resources, leading to situations where ducks are similar height to elephants.

Preservation bias is the concept that some organisms, and some parts of organisms, fossilize better than others. This leads to systematic over- or underrepresentation of certain organisms in the fossil record, which we have to account for when trying to figure out the composition of an ancient ecosystem.

Coevolution is when two or more species influence each other’s evolutionary paths, resulting in highly specialized and complex interactions between them. Examples include leafcutter ants and their cultivated fungi, humans and domestic plants, mistletoebirds and mistletoe, and flowering plants and pollinators.

The RNA World Hypothesis is the leading explanation for how life arose from nonliving substances on early Earth. RNA and phospholipid membranes are compounds that spontaneously arise under certain conditions, and RNA is able to act as a catalyst for certain other chemical reactions. If one of those reactions was making more of itself, natural selection comes into play and life is born.