“There’s a very faint dimple here,” Sterling Nesbitt says. He holds a palm-sized fossil up to the light. The fossil is a pelvic bone. It belonged to a creature called Teleocrater rhadinus. The slender, 2 meter (6 foot) long reptile ran on all fours and lived 245 million years ago. That’s about 10 million to 15 million years before scientists think dinosaurs first appeared.
Nesbitt is a paleontologist at Virginia Tech in Blacksburg. He tilts the bone toward the overhead light. That illuminates a small depression in the fossil. The dent is about the size of a thumbprint. It marks the place where the leg bone fit into the pelvis. In a true dino, there would be a complete hole in this hip socket, not just a depression. The dimple is like a waving red flag: Nope, not a dinosaur.
The hole in the hip socket probably helped dinosaurs position their legs beneath their bodies. Without it, their legs would have splayed to the sides like a crocodile. Until recently, that hole was among a handful of telltale features — physical traits — used to identify a dino.
Another no-fail trait was a particular depression at the top of the skull. Until, that is, Teleocrater mucked things up. The creature lived before dinos. But it had the "dinosaur" skull depression.
The list of “definitely a dinosaur” features was once quite long. Over the past few decades it has been shrinking. That’s thanks to new discoveries of close dino relatives such as Teleocrater. An April 2017 report of Teleocrater’s skull depression knocked yet another tell-tale trait off the list.
Today, only one feature is still unique to Dinosauria. That’s the great and diverse group of animals we know as dinosaurs. They inhabited Earth for about 165 million years. Then, some mix of asteroid impact and volcanic eruptions wiped out all dinos except birds.
“I often get asked ‘what defines a dinosaur,’” notes Randall Irmis. He’s a paleontologist at the Natural History Museum of Utah in Salt Lake City. Ten to 15 years ago, scientists would list perhaps half a dozen features, he says. “The only one to still talk about is having a complete hole in the hip socket.”
Recently, scientists have dug up lots of new info about dinosauromorphs (Dy-no-SOR-oh-morfs). That’s a group that includes dinosaurs and dino-like creatures that lived right before and along with early dinosaurs. New discoveries are now calling some of those dino-diagnostic features into question. And that has been shaking up long-standing ideas about what the dino family tree should look like.
Today, only one fossil feature can be attributed solely to members of Dinosauria. That is a complete hole in the hip socket of a fossil.
Several others, including the four below, are no longer surefire dinosaur signs:
1. Until Teleocrater came along, only dinosaurs were known to have a deep depression at the top of the skull. This was an attachment site for some jaw muscles probably related to bite strength.
2. Dinosaurs and some other dinosauromorphs such as Silesaurus opolensis have an enlarged crest on the upper arm bone where muscles attached.
3. Along with dinosaurs, the dinosauromorphs S. opolensis and Asilisaurus kongwe may have had epipophyses. These are bony projections at the back of the neck vertebrae.
4. An extra (fourth) muscle attachment site, called a trochanter, at the point on the femur that meets the hip is also found in the dinosauromorph Marasuchus lilloensis.
In 1841, British paleontologist Sir Richard Owen coined the term “dinosaur.” Owen was thinking about fossil remains from three giants. There was a carnivore named Megalosaurus. There was the plant-eating Iguanodon. And then there was the heavily armored Hylaeosaurus. These animals shared several important features with one another but not other animals, he showed. (In particular, he noted, the creatures’ giant legs were upright and tucked beneath their bodies. Plus, each of the animals had five vertebrae fused together and welded to the pelvis.)
Owen decided the animals should be classed together as their own biological group, or taxon. He named it “Dinosauria.” That roughly translates to “fearfully great lizards.”
Stephen Brusatte is a paleontologist at the University of Edinburgh in Scotland. Back in Owen’s day, he says, it was a bit easier to spot similarities between fossils. Why? "There were so few dinosaurs," he notes. The more fossils you find, he points out, the harder it can be to know which are most closely related. “With every new discovery, you get a different view of what features define a dinosaur,” he notes. “It’s nowhere near as clear-cut as it used to be.”
The largest extinction of species on Earth is known as the “Great Dying.” It happened about 252 million years ago. That was at the end of the Permian Period. About 96 percent of marine species and 70 percent of land species died out.
The period that followed was the Triassic. It spanned from 252 million to 201 million years ago. During that time, new reptilian species emerged and flourished. This was the age of early dinosauromorphs and crocodylians. (Those last are the ancestors of crocodiles.) And, of course, there were the true dinosaurs themselves. No one knows exactly when dinos arose. Scientists suspect it was likely some 230 million years ago.
For tens of millions of years, dinos lived beside numerous other reptile types. But at the end of the Triassic, dramatic climate change helped trigger another mass extinction. Dinosaurs somehow survived. They went on to dominate the planet during the Jurassic Period.
Paleontologists once assumed the dinos were somehow superior. Some physical traits, the thinking went, helped them outcompete other reptiles. “But that’s not borne out by new dinosaur relatives,” Nesbitt says. Dinosaurs were quite similar to dinosauromorphs, researchers found. The new trove of dinosauromorph fossils unveils a repeating pattern of parallel evolution. (That’s when similar features evolve independently in two different lines of organisms.) Dinos and other dinosauromorphs developed features such as lengthening legs or having legs directly under the body. In short, Nesbitt says, dinos “are not doing anything different than their closest relatives.”
As a result, many paleontologists now suspect there’s another reason for dinos’ rapid expansion in the Jurassic. The mass extinction at the end of the Triassic left open holes in their ecosystem. These are known as “ecological niches.” Maybe a species that ate fish went extinct. Now that fish-eating role would be available to other species. Dinos might have been taken advantage of such opportunities. This could have let them diversify and flourish.
But that doesn’t explain why dinos survived the mass extinction at the end of the Triassic. What made them different from non-dino dinosauromorphs(and most of the crocodylians), which out? For now, no one knows.
Maybe dinosaurs had anatomical traits that helped them survive, suggests Max Langer. He’s a paleontologist at the University of São Paulo in Brazil. If true, he says, “We don’t know what those features were.”
Uprooting the family tree
To identify the animal that left behind a fossil, paleontologists pore over its bones. They note every one of their bumps, grooves and holes. They might measure the length of the leg’s tibia bone. Or they may count the digits on a forelimb. Before powerful computers were available, scientists would note which species shared bumps and grooves on their bones. They would then use these traits to group those species into evolutionary trees. (These are like a family tree, only linking species instead of individual people.) And they assessed whether those traits (also called characters) were inherited from a common ancestor, or were being passed along for the first time to their descendants.
Langer now calls that approach old-fashioned. Today, scientists use computer algorithms (or problem-solving rules) to help build elaborate evolutionary trees. A fossil’s traits still are the raw data used to create those trees. And the analyses of what order they should fall into will be only as good as those data. Different researchers may focus on different features. They may also interpret the fossils differently. Those issues hit home among dino researchers last year. That’s when a team proposed a fundamental reordering of the dinosaur family tree.
For some 130 years, the basic structure of that family tree had changed little. Dinosaurs were split into two main lines, based on the shape of their hips. Both lines had the hole in the hip socket. That’s still considered unique to all dinosaurs.
One line was known as the ornithischians (Or-nih-THISH-ee-uns). That group includes giant plant-eaters, such as the three-horned Triceratops and plate-armored Stegosaurus. A pubis (PU-bis) is one part of the pelvis in vertebrate animals. In ornithischians, the pubis pointed down toward the tail. The pubis bone in the other line of dinosaurs pointed down toward the front. It created a hip shape that was shared by long-necked sauropods such as Brachiosaurus (BRAK-ee-oh-SOR-us). It’s also found in carnivorous theropods, such as Tyrannosaurus rex. Having similar hips, sauropods and theropods were long considered close “sister” groups. Ornithischians, meanwhile, were seen as distant kin.
That changed in March 2017. That’s when three scientists proposed upending that hip-based approach to arranging dinos on the family tree.
Graduate student Matthew Baron and vertebrate paleontologist David Norman were working at the University of Cambridge in England. They were joined by paleobiologist Paul Barrett of the Natural History Museum in London, England. Together they published a paper in Nature. It observed that ornithischians had been somewhat ignored in past analyses. They were a really diverse bunch. Among them wese dinos sporting a spectacular array of frills and armors and horns and crests.
The three researchers decided to see how differently the family tree would look if it was based on the inclusion of many more ornithischians. So the team incorporated some 457 different fossil traits. These came from 74 species of all kinds of dinos and dino relatives.
The new family tree might as well have come from a totally different forest. It shuffled the three big dino groups around. Baron and his coauthors found that the ornithischians had more than 20 features in common with predatory theropods. It now put ornithischians and theropods together into a new group. And it suggested sauropods had split off from their line at an earlier date.
Their paper made a splash. However, many paleontologists were not convinced.
Indeed, Irmis says, one concern was just how much these analyses relied on judgments that might differ from person to person. Which species a study included clearly affected how the tree turned out. Plus, he notes, a slight difference in how one person interprets the structure of each fossil or skeletal trait could add up, trait after trait, leading to a “huge difference.”
Langer, Brusatte and several of their colleagues decided to tackle this problem head on. “When the paper [by Baron's group] came out, there was this flurry of excitement,” Brusatte recalls. “But a lot of us noticed right away that there wasn’t a huge amount of description about the [characteristic features of the bones].” Some scientists now became concerned that the fossils in the analysis may not have been examined all that well. Perhaps some features had not been described in enough detail. If so, those ignored details might dramatically alter the results.
Langer, Brusatte and the rest of their team traveled around the world to visit the fossils included in the original paper. They reassessed all 457 traits described — in person. The team had been expecting to cast doubt on the new tree. Maybe they’d even debunk it.
But that none of that happened.
Instead, they showed that the old hip-based dino tree from 130 years ago still offered the best fit to the dino dataset used by Baron, Norman and Barrett.
But the original tree wasn’t that much more likely to be correct than the new tree, they found. “This is the thing that really blew us away,” Brusatte says. In fact, the hip-based tree wasn’t even that much better than an older, third tree. It had grouped ornithischians closer to the other herbivores in the family, the long-necked sauropods. This other tree grouped the fierce theropods on a tree limb of their own.
Langer, Brusatte and their coauthors described their new assessment last November in Nature.
Right now, there is "great uncertainty" about the basic structure of the dino family tree, that paper concluded. “It seems that the flood of new discoveries over the past decades has revealed unexpected complexity.”
Adds Brusatte: “We shouldn’t rewrite the textbooks just yet.” He says, “We’ve taken what we thought was a certainty and turned it into a mystery — and a big mystery, at that.”
The dinosaur family tree has three main branches: herbivorous ornithischians, long-necked sauropods and fierce theropods. Their relationships may be shifting.
Source: M.C. Langer et al/Nature 2017
Based on hip shape, sauropods and theropods were thought to be more closely related to each other than to ornithischians.
A March 2017 analysis of a longer list of ornithischian species concluded that ornithischians and theropods are closely related.
A November 2017 analysis upheld the traditional view but found that other arrangements are almost equally likely —including a view that clusters herbivorous ornithischians and sauropods together.
How the different dino groups relate may seem like needlessly detailed analyses. But Nesbitt argues that it’s not. The evolutionary tree is the framework for scientific discussions of dino evolution, dino origins and what links all dinos. “It makes it difficult to ask questions about how features are evolving if we can’t have some agreed-upon taxonomy,” he adds, meaning traits that characterize particular branches in the family tree.
So without an agreed-upon tree, it’s hard to figure out which traits might have helped dinosaurs survive that mass extinction at the end of the Triassic. Each change to of the tree seems to highlight different features as being particularly important, Langer says. “If you don’t know how the tree is arranged, you can’t say which feature characterizes [dinosaurs].”
In its November 2017 Nature paper, Langer’s team asks researchers to do more of the dull, routine work of characterizing dino fossils. “We proposed that we need more … anatomical descriptions and definition of characters,” Langer says. He admits “it’s boring.” However, he adds, it’s also very important.
As Nesbitt cradles the Teleocrater pelvic bone, he turns to a tall cabinet of wide, shallow drawers. He slides one open. This drawer is filled with dozens of carefully labeled boxes. Each holds one or more Teleocrater bones. They had been collected during a 2015 expedition in East Africa to Tanzania’s Ruhuhu Basin.
The first known fossils of Teleocrater rhadinus were actually discovered in the 1930s. Those were just a few bits of vertebrae, pelvis and limb bones. And they sat ignored in London’s Natural History Museum for decades.
The Ruhuhu Basin fossils found in 2015 date to between 247 million and 242 million years ago. The site contains an abundant and diverse mix of fossils from Triassic animals. These include relatives of crocodylians and giant-headed amphibians. And there were ancient reptillian relatives of modern mammals, called cynodonts.
Nesbitt worked in the Ruhuhu Basin in 2010. On his 2015 return, he hoped to find more evidence that would help identify the mysterious Teleocrater.
And he hit pay dirt. His team found a bone bed containing at least three Teleocrater individuals. The fossils included a jawbone and part of a skull known as the braincase. The skull was particularly exciting. Based on a host of features, Teleocrater was clearly a nondinosaur. Yet it’s skull had a depression, just like a true dino.
Paleontologists tend to say that finding more fossils from early dinos and their close relatives will be key to better understanding dinosaurs. They thought it was the surest way to fill in the gaps on the family tree and generally tidy it up.
Nesbitt laughs. “Now we have way more fossils,” he says, “and it’s way messier.”
algorithm A group of rules or procedures for solving a problem in a series of steps. Algorithms are used in mathematics and in computer programs for figuring out solutions.
amphibians A group of animals that includes frogs, salamanders and caecilians. Amphibians have backbones and can breathe through their skin. Unlike reptiles, birds and mammals, unborn or unhatched amphibians do not develop in a special protective sac called an amniotic sac.
anatomy (adj. —) The study of the organs and tissues of animals. Or the characterization of the body or parts of the body on the basis of its structure and tissues. Scientists who work in this field are known as anatomists.
array A broad and organized group of objects. Sometimes they are instruments placed in a systematic fashion to collect information in a coordinated way. Other times, an array can refer to things that are laid out or displayed in a way that can make a broad range of related things, such as colors, visible at once. The term can even apply to a range of options or choices.
asteroid A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.
basin (in geology) A low-lying area, often below sea level. It collects water, which then deposits fine silt and other sediment on its bottom. Because it collects these materials, it’s sometimes referred to as a catchment or a drainage basin.
birds Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.
braincase A part of the skull that protects the brain.
carnivore (adj. carnivorous) An animal that either exclusively or primarily eats other animals.
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
coauthor One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.
colleague Someone who works with another; a co-worker or team member.
crest in zoology) A tuft of feathers or skin — or comb (as on a chicken) — atop the head of a bird or other animal.
crocodilians Large, semi-aquatic reptiles that evolved more than 80 million years ago. Many are long extinct. Their living descendants include alligators, caimans and crocodiles.
cynodont A group of long-extinct therapsid reptiles with some mammal-like features. They lived from about 260 million to 175 million years ago. Their fossils have turned up in East China, South Africa, South America, and North America. Most reptiles swallowed food whole or in large pieces. Cynodonts had a mouth structure and teeth that would have allowed them to chew their food and digest it quickly. That should have let them use the nutrients more quickly too.
descendant A blood relative of a person who lived during a previous time.
digit A structure, like a finger or toe, at the end of the limbs of many vertebrates.
dinosaur A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. For many decades, they have been distinguished by their hips. But a 2017 analysis now calls into question that characterization of relatedness based largely on hip shape.
dinosauromorphs Dinosaurs and the reptiles that were their ancestors or were dinosaur-like but lacked the full complement of true-dinosaur traits.
ecology (adj. ecological) A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
ecosystem A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.
evolution (adj. evolutionary) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. Or the term can refer to changes that occur as some natural progression within the non-living world
evolutionary tree A sort of family tree for species that organizes them onto parts of the trunk and branches based on their similarity to other species, both living and extinct.
evolve (adj. evolving) To change gradually over generations, or a long period of time. In living organisms, such an evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. These can lead to new traits, such as altered coloration, new susceptibility to disease or protection from it, or different shaped features (such as legs, antennae, toes or internal organs). Nonliving things may also be described as evolving if they change over time. For instance, the miniaturization of computers is sometimes described as these devices evolving to smaller, more complex devices.
expedition A journey (usually relatively long or over a great distance) that a group of people take for some defined purpose, such as to map a region’s plant life or to study the local microclimate.
extinct An adjective that describes a species for which there are no living members.
extinction The permanent loss of a species, family or larger group of organisms.
forelimb The arms, wings, fins or legs in what might be thought of as the top half of the body. It’s the opposite of a hindlimb.
forest An area of land covered mostly with trees and other woody plants.
fossil Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.
frill (in zoology) An upward-curving bony plate extending behind the skull of some reptiles, including dinosaurs.
fundamental Something that is basic or serves as the foundation for another thing or idea.
Great Dying A period of time roughly 252 million years ago when at least 70 percent of all land species and 96 percent of ocean species went extinct. Ot was the greatest mass extinction in Earth’s history. These species fell victim to changing climate and ecological changes.
herbivore A creature that either exclusively or primarily eats plants.
Jurassic Lasting from about 200 million to 145.5 million years ago, it’s the middle period of the Mesozoic Era. This was a time when dinosaurs were the dominant form of life on land.
kin Family or relatives (sometimes even distant ones).
limb (in physiology) An arm or leg. (in botany) A large structural part of a tree that branches out from the trunk.
lizard A type of reptile that typically walks on four legs, has a scaly body and a long tapering tail. Unlike most reptiles, lizards also typically have movable eyelids. Examples of lizards include the tuatara, chameleons, Komodo dragon, and Gila monster.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.
marine Having to do with the ocean world or environment.
mass extinctions Any of several periods in the distant geological past when many — if not most — of the larger animals on Earth disappeared forever. One that occurred as the Permian period gave way to the Triassic, sometimes called the Great Dying, led to the loss of most fish species. Our planet has experienced five known mass extinctions. In each case, an estimated 75 percent of the world’s major species died off in a short time period, typically defined as 2 million years or less.
Megalosaurus A large, carnivorous, bipedal (walked on two legs) dinosaur of the mid-Jurassic period, whose remains have been found in Europe. It is notable for being the first dinosaur ever discovered (in the late 1600s) and named.
niche A small or narrow pocket that sets something apart, or perhaps offers a region of protection.
ornithischians One of the two major groups of dinosaurs, the other being the saurischians. Ornithischian hip bones were arranged like those of birds. Despite this similarity, ornithischians are not related to birds.
paleobiology The study of organisms that lived in ancient times — especially geologically ancient periods, such as the dinosaur era. Scientists who works in this area are known as paleobiologists.
paleontologist A scientist who specializes in studying fossils, the remains of ancient organisms.
parallel evolution Similar traits that show up in two or more species, but not because they inherited them from a common ancestor. Instead, two or more species living in a similar environment independently evolved the same basic feature as an adaptation to their conditions.
pelvis (adj. pelvic) Bones that make up the hips, connecting the lower spine to leg bones. There is a gap in the middle of the pelvis that is larger in females than in males and can be used to tell the sexes apart.
Permian A time in the distant geologic past, about 250 million to 300 million years ago. Many reptiles rose to prominence on land; these were not yet dinosaurs. Many large invertebrates ruled the oceans during this period. But most would die off at the end of the Permian, as it gave way to a new geologic period known as the Triassic.
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).
predator (adj. predatory) A creature that preys on other animals for most or all of its food.
pubis One of the bones making up the pelvis in vertebrate animals.
replication (in experimentation) Getting the same result as an earlier test or experiment — often an earlier test performed by someone else. Replication depends upon repeating every step of a test, step by step. If a repeated experiment generates the same result as in earlier trials, scientists view this as verifying that the initial result is reliable. If results differ, the initial findings may fall into doubt. Generally, a scientific finding is not fully accepted as being real or true without replication.
reptile Cold-blooded vertebrate animals, whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles.
sauropod A very large, four-legged, plant-eating dinosaur with a long neck and tail, small head and massive limbs.
skeptical Not easily convinced; having doubts or reservations.
skull The skeleton of a person’s or animal’s head.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
Tanzania A nation in East Africa that sits just south of the equator. It’s well known in science communities as home to the East African Rift valley, where many hominid fossils have turned up. With a small population, many big wildlife species remain, including lions, rhinos, elephants, zebras, hippos, cheetahs and giraffes. The country is bordered by the Indian Ocean to the east; Kenya and Uganda to the north; Ruwanda, Burundi and Democratic Republic of Congo to the west; and Zambia, Malawi and Mozambique to the south. It became a country in 1964. It’s official capital is Dodoma.
taxa (sing. taxon) The groupings that distinguish organisms and their relatedness to each other. Examples in ascending order: species, genus, family, order, class, phylum, kingdom and domain.
taxonomy The study of organisms and how they relate or have branched off (over evolutionary time) from earlier organisms. Often the classification of where plants, animals or other organisms fit within the Tree of Life will be based on such features as how their structures are formed, where they live (in air or soil or water), where they get their nutrients. Scientists who work in this field are known as taxonomists.
tibia (in vertebrate animals) The inner — and normally larger — of two hind or leg bones that run between the knee and ankle. In people it’s known as the shin bone.
theropod A usually meat-eating dinosaur that belonged to a group whose members are typically bipedal (walk on two legs). They range from small and delicately built to very large.
trait A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.
Triassic Period A time in the distant geologic past, about 200 million to 250 million years ago. It’s best known as the period during which dinosaurs first emerged.
trove A collection of valuable things.
Tyrannosaurus rex A top-predator dinosaur that roamed Earth during the late Cretaceous period. Adults could be 12 meters (40 feet) long.
unique Something that is unlike anything else; the only one of its kind.
vertebrate The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes amphibians, reptiles, birds, mammals and most fish.
Journal: M.C. Langer et al. Untangling the dinosaur family tree. Nature. Vol. 551, November 2, 2017, p. E1. doi: 10.1038/nature21700.
Journal: S.J. Nesbitt et al. The earliest bird-line archosaurs and the assembly of the dinosaur body plan. Nature. Vol. 544, April 27, 2017, p. 484. doi: 10.1038/nature22037.
Journal: M. G. Baron et al. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature. Vol. 543, March 23, 2017, p. 501. doi: 10.1038/nature21700.
Journal: M.C. Langer. The origins of Dinosauria: Much ado about nothing. Palaeontology. Vol. 57, Part 3, 2014, p. 469. doi: 10.1111/pala.12108.
Journal: M. C. Langer et al. Non-dinosaurian dinosauromorpha. The Geological Society of London, Special Publications, Vol. 379, February 13, 2013, p. 157. doi: 10.1144/SP379.9.
Journal: S. L. Brusatte et al. The origin and early radiation of dinosaurs. Earth-Science Reviews. Vol. 101, July 2010, Pages 68-100. doi: 10.1016/j.earscirev.2010.04.001.