Main dinosaur groups

Sauropods (meaning “lizard 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.

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 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, and only hit their stride in the late Cretaceous.

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) and the heavily-armored Ankylosaurs (including Ankylosaurus, Nodosaurus, and Euoplocephalus).

Other important clades

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. 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 “lizard 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 lizard”) 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 lizard”) 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 lizard”) are a group of flying archosaurs closely related to dinosaurs, but are not dinosaurs. They come in two main flavors: “Rhamphorhynchids” and Pterodactyloids. “Rhamphorhynchids” (meaning “beak snout”) are a paraphyletic group of more basal pterosaurs, which usually had long tails and toothy jaws. Pterodactyloids (meaning winged finger-ish) 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 lizard”) 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-unknown 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 ton, 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).

Tetrapods are the group of vertebrates with four limbs, including amphibians, mammals, and reptiles (which includes birds).

Eurypterids (meaning “broad wing”) 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; hoofed crocs; “cat” crocs and “cheetah” crocs; fully-aquatic crocs with fins and flippers; tank-like armored, spiky crocs; and many more.

Pan-aves (meaning “all birds”) are the group containing pterosaurs and dinosaurs (which contains birds).

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 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.

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, 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, where the entire body moves back and forth and large fins aren’t necessary.

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, and Suminia.

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.

Integument

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 trait 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 (extinct non-crocodylian croc-lookalikes), and armadillos.

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

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.

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.

Miscellaneous

“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.

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.

A junior synonym happens when a new genus is described based on a new fossil (eg Stygimoloch) but later found to belong to an existing genus (eg 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.

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.

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 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.

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.