Quick–tell me what a dinosaur is!

Well, you can’t exactly tell me, but take a second to picture a dinosaur in your head. Chances are you’re imagining something with any of the following features:

• A long tail
• Scaly skin and maybe some feathers
• Very large
• Sharp teeth and scary claws
• Long, towering necks
• Horns and neck frills and tail clubs, oh my!

You definitely see these features in some dinosaurs, but take a look at this animal. Would you say it’s also a dinosaur?

I have some bad news for those of you who said yes–it’s not a dinosaur. I also have another piece of bad news. The features you imagined in your head, while definitely present in some dinosaurs, are not what define all dinosaurs scientifically.

Why is this the case? And what makes a dinosaur…well, a dinosaur?

A (Very Brief) Family History

To really define what a dinosaur is (notice I’m using present-tense verbs; we’ll get to that later), we need to grasp two critical things. First is the established fact that dinosaurs are scientifically defined by a unique set of characteristics. The second is a very bare-bones (pun intended) understanding of the family history of dinosaurs and their lineage.

250MillionYearsAgoAndMe

So, that funky animal in that earlier picture I showed you? Like I said, it’s not a dinosaur. Rather, it is an animal by the name of Postosuchus, and it is a dinosaur cousin. It’s actually more closely related to our modern crocodilians than dinosaurs, but here’s the key: all of these animals descend from a common ancestor–the archosaurs.

This group of “ruling reptiles” commanded control over most of the Triassic, radiating and diversifying in a world trying to re-establish itself after the worst extinction the planet had ever seen. They evolved and expanded to fit a wide array of niches, eventually dominating terrestrial, aquatic, and aerial habitats. As the Triassic went on, this clade of reptiles broke into two major subgroups: the bird-line (avemetatarsalians) and crocodile-line (pseudosuchians) archosaurs.

Yes, you heard that right–bird-line archosaurs. Because birds are dinosaurs. Not cousins or close relatives–actual avian theropod dinosaurs. If you’re looking out a window right now and you see a goose or a magpie or an emu or a hawk, you’re looking at a dinosaur. Sorry–tangent over. (#BirdsAreDinosaurs)

The Family Line Splits Again, and Again, and Again…

A stem group of avemetatarsalians–the ornithodirans–arrived on the scene and would begin setting the stage for the proto-dinosaurs known as dinosauromorphs. The ornithodirans would also give rise to another iconic group of Mesozoic reptiles–pterosaurs.

While dinosaurs and pterosaurs would grow to be vastly different and specialized from each other, they would retain some common characteristics including an upright gait and an S-curved neck. After the pterosaur/dinosauromorph split, a few more evolutionary steps occur before we see true dinosaurs appear around the early-Late Triassic, some 230 million years ago (mya).

Bird-line archosaurs were small-to-medium size, while their croc-line cousins were grew much larger. Postosuchus, for example, was somewhere near 13 feet long. These first dinosaurs, which included species like Herrerasaurus and Eoraptor, continue that small-size trend. Additionally, they were bipedal–a condition that would actually change in some groups of dinosaurs as they diversified throughout the Mesozoic. But, at last, we had official (dare I say legit?) dinosaurs. In a few million years from their first appearance, they would begin to conquer the world.

Products of a Complex World

Just describing the steps from one group of animals to another doesn’t truly tell the whole story of dinosaur evolution. Understanding their defining features, which we’ll go over shortly, requires recognizing the context in which those features evolved.

At the time in Earth’s history when dinosaurs rose up, all of the world’s landmasses had been squished into one supercontinent–the infamous Pangaea. And, while the name conjures up romantic images of a lush, rich paradise, it was anything but. Pangaea’s coastline was limited, and because it had a vast continental interior, little ocean humidity reached inland. As a result, the supercontinent was a dry, arid landscape with restricted climatic zones in which animals could live.

It’s no surprise, either, that in the zones dinosaurs were able to develop, they were having to deal with their archosaur cousins who were busy dominating the majority of terrestrial environments. Dinosaur size was both an advantage and a disadvantage, and they weren’t widely distributed or abundant relative to other groups of animals living in the Triassic.

All these signs point to these animals being destined to struggle trying to gain a foothold. That was the case for somewhere between 20-30 million years (myr) after the first true dinosaurs had arrived on the scene. It would take two extinction events to severely impact the archosaurs, allowing dinosaurs to enter new ecological niches and flourish for the next 140 myr.

Defining Dinosaurs

While they developed all sorts of crazy, weird, and fascinating adaptations, all dinosaurs had common traits that persist throughout the geologic timescale. Because they were so diverse, when you go looking for a scientific definition, you get something as confusing as this: “Dinosauria is a group that contains the most recent common ancestor of birds–like a pigeon walking by on the sidewalk–and the non-avian dinosaur Triceratops…” Thanks, Smithsonian Magazine, Brian Zwitek, and Hans-Dieter Sues. That really clears things up…

All kidding aside, this definition can feel a little vague. However, it’s a starting point. Think about it like this: birds (I will say this over and over until it sticks) are dinosaurs. Specifically, they’re avian theropod dinosaurs. Modern birds like pigeons are the most recent ancestor of the saurischians, one of the two major dinosaur groups. Likewise, Triceratops represents the last living members of the second group, the ornithischians. So, if you take both of these animals and trace them both back to their most recent common ancestor, the animals that fall into the group these lineages encircle can be considered dinosaurs.

However, how can we be certain that an animal falls within this group? It’s all got to do with a handful of synapomorphies–characteristics shared by a group of organisms passed down from their last common ancestor. From Eoraptor and Herrerasaurus to Triceratops and Columbia livia (common pigeon), the same traits exist across all dinosaur species and persist today. But what do those look like?

It’s All In The Hips (And Legs)

One of the most definitive clues in determining whether a fossil is a dinosaur or not is its hips. In fact, we categorize the two major groups of dinosaurs by how their hips are shaped: the saurischians (reptile-hipped) and the ornithischians (bird-hipped). As of recently, these groups have been called into question, but that’s a blog post for another day.

In the ancestral condition of archosaurs, their legs and hips are splayed outwards, forcing them into a sort of sprawling locomotion. If you watch how modern crocodiles walk, that motion is actually close to the ancestral condition.

As dinosaurs evolved, their sprawling archosaur legs came directly underneath their body. Because of this, a number of muscular and skeletal changes occurred around the hips and legs that we now use in helping identify their fossils.

One of the most iconic features this adaptation led to was an opening in the hip joint (the acetabulum) where the head of the femur sits. In most vertebrates–and especially in archosaurs–this joint is backed by a wall of bone. Dinosaurs, however, lacked this. Instead, the three bones of their hips (the ilium, ischium, and pubis) created the border of the socket, and the center–where the ball of the femur connects to and sits in–was open.

When dinosaurs brought their legs under their bodies, a number of rearrangements in their hip and leg muscles occurred. All archosaurs had a caudofemoralis musculature–a large set of muscles that extend from the tail to the femur. These muscles helped retract and pull back their legs during locomotion. Unlike other archosaurs and most other animals, dinosaurs had a bone feature, known as the fourth trochanter, where these muscles attached to the femur. This feature is also asymmetrical, which may have been related to more powerful and efficient limb motions.

Another muscle–the puboischiofemoralis–also saw some changes. Extending from the pubis to the femur, this muscle brings the hindlimbs forward and towards the midline of the body. Paleontologists have inferred the presence of these muscles with the presence of bone features like the fourth trochanter. These musculature changes are also associated with the elongation of the pubic bone–another feature unique to dinosaurs.

Dinosaurs also had a particular set of sacral vertebrae that articulate (join) with the pelvis between the left and right ilium bones–the top-most bones of their pelvic girdle. All dinosaurs–as well as some stem dinosaurs–always had three or more of these vertebrae, sometimes having as many as five or six. This feature was key in connecting the vertebral column and the hindlimbs, and likely helped to brace the hindlimbs as they became more muscular and swift.

My Limbs!

Two distinct features in dinosaur limbs set them apart from their archosaur ancestors: changes in the ankle and foot, as well as the arm.

The ankle and foot features, unsurprisingly, are related to the realignment of dinosaurs’ hips and legs. Their tarsal bones (those between the metatarsals and the leg) differ from their ancestors in significant ways.

In both dinosaurs and archosaurs, two tarsal bones–the astragalus and the calcaneum–comprise the ankle. In archosaurs, these bones are more even in size and form a sort of peg-and-socket joint. This joint caused a rotational movement between these bones, likely contributing to archosaurs’ sprawling stance and locomotion.

Dinosaurs, on the other hand, had more of a hinge-joint ankle, and the tarsals are rearranged to make this possible. The astragalus bone is larger than the calcaneum, and it was firmly pressed against the smaller tarsal, as well as the tibia and fibula. These bones were united by an ascending process (a tongue-like flange), and all these changes restricted the movement of the joint. The fibula was also reduced, causing it to make far less contact with the astragalus. As a result, these changes in the tarsals helped keep dinosaurs’ posture to a more erect orientation, as well as forcing the limbs to move parallel to the vertebral column.

The metatarsals–the bones below the tarsals and the digits–were also significantly different in dinosaurs. They were bunched together and elongated in all bird-line archosaurs (pterosaurs included), and the outer and inner metatarsals of each foot were reduced in size.

Because of this, the middle three bones formed a simplified, paddle-like structure. This allowed the metatarsals to act as a single, unified structure. Basically, they’re almost a third major bone of the hindlimb. These bones also wouldn’t come into contact with the ground during the movement–only the dinosaurs’ toes. This sort of feature can actually be seen in horses, gazelles, and other rapid-running animals today.

Now, let’s get serious about the humerus for just a second. Dinosaurs had this bony feature on their humerus known as the deltopectoral crest. Many groups of animals have this feature, but it’s mostly discrete. In dinosaurs, however, it was crazy prominent. This crest is identified by a big flange that runs down about a third of the humerus.

This bone feature anchored the deltoid muscles of the shoulder and the pectoralis muscles of the chest to the humerus. Contractions of the pectoralis, empowered by this feature, would bring the arms closer to the body. Very likely, this meant dinosaurs had a great amount of power in pulling their forelimbs to their bodies.

Hamlet Would Have Some Words About This Skull

There are a ton of features of dinosaurs skulls that made them unique from other animals–particularly, their archosaur cousins.

First, they had these two bones towards the back of the skull called the jugal and quadratojugal. Both of these formed the cheeks of a dinosaur. These bones meet each other in both archosaur and dinosaur skulls, but how this occurs differs between both groups of animals. In archosaurs, the jugal (the bone further towards the front of the cheek) tapers and meets with the quadratojugal (the further back cheekbone) at a simple overlap point. In dinosaurs, this meeting is very different.

Instead of that simple overlap, the dinosaur jugal splits into two prongs which then clasp onto the anterior process–a projection of bone that moves towards the front of the animal–of the quadratojugal. To describe it way more simply (and perhaps a bit reductionist), the front cheekbone attaches to the back cheekbone with two joining points instead of one. Why this dinosaur-specific feature is a thing is not entirely clear, but it was likely this made the articulation of these two bones stronger.

This is a very bad analogy, but imagine you have two pieces of paper you want to join together with tape. You’ve got two options: you can either overlap them and put the tape between them, or use two different pieces of tape to join them together. If you go to test the strength of the bond, chances are you’ll find the one with overlap is weaker than the one joined with two pieces of tape.

Again, bad analogy, but it’s sort of a way to understand what this feature of dinosaurs might have been capable of. And, in this context, we can understand how it would be a functional adaptation. The muscles of a dinosaur’s skull–particularly its mandibular muscles–are unusually large and extensive. We know this because of the presence of a deep depression (fossa) on the top of the skull where these muscles would have attached. In dinosaurs, this feature is deeper and extends forward more than it does in archosaurs.

We know from the fossil record that lots of dinosaurs had very strong bite forces. Tyrannosaurus rex, in particular, had the strongest bite force of any land animal known–over 12,000 pounds. The presence of both the cheekbone arrangement and the deep muscle attachments on the top of the skull tell us that early on, dinosaurs adapted their skulls to be strong biters–much more powerful than their close relatives.

The features of dinosaur jaws were complemented by adaptations in dinosaurs’ necks. They had these unique projections of bone (epipophyses) on their neck vertebrae. These protruded from the sides of what are known as postzygapophyses, back-facing projections on the neural arch (curved bone that creates the canal for the spinal cord to travel through) that join with the two front-facing projections of the vertebrae behind it.

The epipophyses’ primary function is to act as attachments for the neck muscles. These muscles are used in extending, rotating, and reinforcing the neck and back. It’s also even debated if dinosaur epipophyses helped neck muscles extend to the back and thorax. Similar features are seen in archosaurs, but the ones in dinosaurs are more robust, increasing the available attachment area for the muscles. As a result, we can infer they had much stronger necks than their archosaur counterparts.

Putting the Pieces Together

So, if you’ve gotten this far and still have no idea what I’m talking about (which is often the case for me), I’ll try to sum up the defining characteristics of dinosaurs as simply as possible. If you’re looking at a fossil find any of these synapomorphies (universal traits passed down by the most recent common ancestor), then you know you’ve got a dinosaur on your hands:

• Hips with an open joint socket
• An elongated pubis
• Three or more sacral vertebrae
• Tightly grouped ankle bones where one is bigger than the other
• Asymmetrical muscle attachment points on the femur
• Bunched-together bones between the ankles and toes
• A humerus with a big, long, dope-ass crest on it
• Cheekbones with two attachment points (rather than overlapping)
• A deep fossa in the top of the skull
• Extended protrusions of bone (epipophyses) on the neck vertebrae

If you see these on a fossil–which you probably wouldn’t see all of because it’s very rare to find complete fossils–then the chances of it being a dinosaur are in your favor. But I’m sure you might be looking at this list and wondering to yourself: why is it that these somewhat abstract features define what a dinosaur is? And what do these features mean?

Dinosaurs: Diverse AF

The reign of the dinosaurs went uninterrupted for 140 myr. It was only the K-PG extinction event that usurped their throne and gave it to us dumb ol’ mammals. That’s a significant amount of time when you consider the history of life on earth.

The first complex organisms begin to show up on earth somewhere between 500-600 mya, and it took about 150-200 myr after that for the first terrestrial animals to appear. If you consider just the span of time complex life has been terrestrial–around 300-350 myr–dinosaurs were the dominant group of animals for almost half of it. And if you bring in the fact that birds are dinosaurs, then this long-living clade has been present for two-thirds of terrestrial life and one-third of complex life on earth.

This long stretch of time dinosaurs have been present on earth–particularly when they were the dominant terrestrial creatures–inevitably means one thing: diversity. Dinosaurs lived in a changing world for a very long period of time, and with those changes came new survival challenges.

Whether climate, ecology, competition, or threat-related, the major diversification of shape, size, diet, stance, etc. forces us into a difficult position. If we want to define all of these different animals as dinosaurs, we need to have a common framework. Luckily, these particular synapomorphies present themselves in some manner or another across all dinosaurs, helping us sort, identify, and classify the over 1,000 species we’ve found thus far.

Understanding Dinosaur Evolution and Success

These common traits mean one thing: they were all present in the most recent common ancestor of the dinosaurs. We can point all the way back to the Triassic and identify:

• When and where the first dinosaurs evolved
• What set them apart from their ancestors
• The complex paleoecological and paleobiological interactions that occurred as they rose to prominence

These inferences all go towards answering a complicated question: why are dinosaurs the way they are? There were benefits to the earliest dinosaurs bringing their legs underneath their bodies (because other archosaur cousins experienced similar evolutions), as well as strengthening their arms, jaws, and necks. They were also small- to medium-sized, laid eggs, may have had feathers, and started off as predators.

So what were they adapting to? Out-running their big, bad archosaur siblings? Catching prey that was fast and difficult to hold onto? Were they exploiting a more energy-efficient method of movement to get at prey sources other archosaurs couldn’t chase or get access to? Or maybe it was accidental–the fastest dinosaurs and stem-dinosaurs were the ones who survived, and they repurposed that speed while developing other adaptations? There are some ideas out there, but we’re still working towards the answers. It could be all, it could be none.

Dinosaurs clearly started off as fast movers (and therefore likely more warm-blooded than their archosaur cousins) who found ways to survive in a world dominated by the “ruling reptiles.” Their relative lack of abundance and dispersal appear to suggest they weren’t necessarily destined to become the dominant animals we know of today; evolution may have simply favored them as two extinction events forced the archosaurs back and opened up new niches for dinosaurs to explore. Whatever the answers are, they’re fascinating and need to continue being explored.

Header image credit: ABelov2014 (CC BY-SA 3.0)

References

Brusatte, Stephen L. Dinosaur Paleobiology. Oxford: John Wiley & Sons, 2012.

Naish, Darren, and Paul M. Barrett. Dinosaurs: How They Lived and Evolved. Washington, DC: Smithsonian Books, 2016.

Nesbitt, Sterling J. “The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades.” Bulletin of the American Museum of Natural History 352 (2011): 1-292. doi:10.1206/352.1.

Sereno, Paul C. “Basal Archosaurs: Phylogenetic Relationships and Functional Implications.” Memoir. Society of Vertebrate Paleontology 2 (1991): 1. doi:10.2307/3889336.

Note: I am not a paleontologist, so I won’t be surprised if I got some of this information wrong. If you spot any errors, please leave a comment or reach out via my Contact page so I can fix them.  Thank you!