Howdy! In this class we'll be focused on dinosaurs. But what exactly is a dinosaur? If we find a fossil, how do we know whether that fossil is a dinosaur and not some other animal? There are many ways that the the word dinosaur is used in the popular media, and there are many misconceptions about what is called a dinosaur.
The origin of the word dinosaur is also full of confusion about what these bones we find embedded in rocks could even be, and some of that confusion continues to influence public ideas of what a dinosaur is even today. When the word dinosauria was first defined in 1841, there were only three dinosaurs that were known then to Western scientists. The first of these species was the carnivorous megalosaur. in 1824 by William Buckland.
However, Buckland didn't see megalosaurs really being any different from modern reptilian predators and maybe just a bit bigger. Something like a crocodile is basically what he thought megalosaur looked like. The second dinosaur to be coined was a guanidon, originally identified from teeth found in a quarry by Mary Ann Mantell and her husband, the physician Gideon Mantell. Gideon sort of saw himself as being more important than just a doctor, as something sort of a scientist.
And so he became a bit obsessed with the teeth, and he took them and showed them to Buckland. Buckland, however, told him they were just fish teeth. undeterred, Gideon mailed them to France to show them to the famous anatomist Georges Cuvier.
Georges mailed them right back to Gideon with a note saying that they were just rhino teeth, nothing remarkable. But Gideon was not convinced. Unrelentless, he went to London, and he spent a lot of time there in the collections of the Royal College of Surgeons, which he was a member of. He was looking through the collections of bones, etc., and he found a lot of them. various species of animals, trying to find teeth that looked like the teeth that had come from the quarry.
The curator of the collections, William Clift, glanced at them and said, well, they look like iguana teeth, just 20 times the size. And indeed, when Gideon compared them to modern iguana teeth, that is exactly how they were. They were like much larger iguana teeth.
And so that cracked the case for Gideon. He imagined that they were iguana teeth. He imagined that the teeth and some other assorted bones that he had found from the quarry altogether came from some very large iguana-like animal and about 20 times the size of a modern iguana, something that was really quite strange and very unlike anything that was alive today. He decided to name this animal, iguanasaur. A close friend of his in a letter said that that was a little redundant since iguanasaur in Greek just means iguana lizard.
And so Gideon actually changed it before publication to iguanodon, meaning iguana tooth. Gideon's claims of these giant prehistoric reptiles sparked the public interest. And Gideon and his fellow fossil collectors searched the quarry that the iguanasaur Iguana teeth had come from for more.
In 1832, Gideon described the third dinosaur species from fragments of a large armored reptile that he named Hylaeosaur. This was fragmentary because quarry workers had used explosives to extract its remains. However, no matter how fragmentary, it was now clear that there was some lost variety of gigantic reptile-like animals that once lived. gone extinct.
This led into early theories about organic transmutation, what we now call evolution, the changing of species from one to another. However, this was still years years before Charles Darwin published his ideas on natural selection in 1859. And many of these early ideas about transmutation were not well thought out, and were very much scorned by high-ranking scientists. Speaking of high-ranking scientists, Richard Owen, an aristocrat, an anatomist, who had a high-paying university professorship, and was much more higher-ranking in society than either Gideon Mantel or William Buckland, he decided that he would coin the word dinosauria to include these ...three particular animals.
Owens did not agree with those various unscientific claims of transmutation, with species evolving into new species on their own, and so he sought to interpret the giant reptiles, in his worldview, as terrible lizards, or as he... named them dinosaurs. Owen established this name, in fact, by inserting the term into the transcript of a speech that he had given the year before, in 1840, which was about to be published in written form.
This is the first time that Owen has ever published a This is rather unusual, even back then for a scientist to do, and some have wondered if Owens did this, because he had heard that Gideon Mantell intended to name the group himself. Once Gideon heard that Owens had established a name, though, he gave up. Owens'dinosaurs were terrible in the sense of the Greek dynos, meaning terrible, potent, or fearfully great, by which Owens didn't mean that they were horrible, but that they they were reptiles of great, awesome, and godlike power. You could describe the ancient Greek god Zeus as terrible in that sense.
Same sense applies to the dinosaurs. What Owens saw them as was that they had a strange combination of reptilian, bird, and mammal-like characteristics, from which he inferred that this was evidence that they were some sort of... lineage of reptiles that had been uplifted by divine force, gifted with mammal-like characteristics in some interval before the creation of true mammals.
For his role in coining the term dinosaur, which had increased the public frenzy over dinosaurs, Owens became involved in dictating the look of several dinosaur sculptures for the Crystal Palace grounds in London. Working closely with the Crystal Palace, Owens began to design the dinosaur sculptures. with the sculptor Benjamin Waterhouse Hawkins. These sculptures, which opened to the public in 1854, were based on the incomplete specimens that Gideon Mantell and Buckland had found, and they looked very different than the dinosaurs we know today, exemplifying Owen's view of dinosaurs as these enormous reptiles endowed with mammal-like characteristics via divine intervention. Following Owen, the exact traits of what defined a dinosaur remained unclear, but broadly the word came to mean reptiles of unusual size that were terrestrial, lived on land, as opposed to being aquatic.
that they walked upright like a mammal or a bird and were extinct. Furthermore, although Owens saw the dinosaurs as filled with great potency and strength, the sculptures of the Crystal Garden were instead received by the public as looking rather lazy and slow, and said in their mind that dinosaurs were slow-moving giants, not competitive with modern mammals, lazy in streams, and easily replaced by humanity and other mammals. That image largely stayed in the public mind for over a century, even though paleontologists were making constant discoveries between, leading scientists to have a somewhat more accurate view of the world. somewhat different and evolving view, which changed very rapidly at the end of the last century. For example, this T-Rex toy is from the mid-1980s and looks very different from the way the same species was depicted in a 1992 movie, Jurassic Park.
This toy rather more closely captures that depiction. That film really changed the public perception of dinosaurs and moved most folks away from the misconception of dinosaurs. conception.
Owen's initial definition of dinosaurs as terrible lizards leaves a lot to be desired. In science today, we build definitions based upon observable facts, but not all facts are equally useful. We also need to be very specific in our definitions in science because we need to communicate precisely. So now that we know more than Owens and other early dinosaur workers from 180 years ago, how might we go about developing this definition.
Sometimes folks use dinosaur just to mean something old like that class is taught by a dinosaur. So how long ago that a group of animals lived could be a way to define the group dinosaurs. Let's consider that.
If we define a dinosaur as anything we only know about through fossils, that they are So old that we have no living individuals, we have a really broad definition. Animals like this trilobite that went extinct 250 million years ago is certainly not a dinosaur. But that definition would make them dinosaurs.
We need to get more specific and say that it is only one fossil from the Mesozoic, a specific time period between about 250 million years ago and the Mesozoic. years ago and 65 million years ago. But fossil organisms like, say, an ammonite, would then be dinosaurs as well. So, I've just given two extinct invertebrate organisms, animals lacking a backbone, and you're probably thinking, well, that if we'd also included that they had an internal skeleton and that they had a backbone, the definition would work out better.
So, say, what about a mosasaur then? If we allow, if it was alive during the Mesozoic, and Mosasaurs are vertebrates, but this is still not something scientists would consider to be a dinosaur. Why not? Nor are other vertebrate organisms that we find as fossils in the Mesozoic, like small mammals that were only the size of a paperclip.
So if being a vertebrate and having bones and a backbone on land, living in the Mesozoic is still- not enough of a specific definition to include dinosaurs and exclude everything that's not a dinosaur, we still aren't specific enough. Fossil bones, footprints in the mud, and other types of fossils give us information about the physical and atomical forms of organisms that we can use to group them. We can then look at multiple detailed characteristics of the fossil and infer things about their shape or height, how they walked by putting together the fossil bones we find to visualize the complete organism. So, we might look at something we've reconstructed that had four legs, walks with its feet under its body, and has scaly skin, and say, well, now I've got a good list. This must be a dinosaur.
But we'd still fall short because ancient relatives of today's crocodiles that lived in the Mesozoic did not have the odd, sprawling, body-dragging walk that alligators and crocodiles have today. The These ancient crocodilians actually also evolved the ability to walk with their legs under their body. Walking with your legs under your body makes an animal a lot faster on land than if it waddles with its limbs out, as being able to do so is useful to multiple animals and could evolve in different groups because it gives them an advantage over those other sprawling animals. I mean, would you say that a bird is human just because we both walk on two legs? or that a snake must be a worm because they all lack limbs.
Anatomical characteristics that help animals function in the world and live their particular lifestyle can show up in lots of different unrelated groups and can cause us to put organisms in the same box, so to speak, even when they aren't all that similar. Even though anatomy is probably the best way to define a group, not all anatomical characteristics will be the same. equally useful for creating that definition.
In science we've realized that simply listing characteristics and placing organisms in boxes with similar-looking characteristics leads us to make some pretty poor definitions. But one thing we can piece together through the anatomy of organisms is how they are genetically related to each other. These definitions that rely on the evolutionary or genetic relationships among organs organisms are called phylogenetic definitions, and we use particular anatomical characteristics of the organisms to build evolutionary trees and know their phylogenetic positions. These characteristics aren't influenced by the lifestyle of organisms, but are instead features that an organism has simply because it was passed to them from their ancestors. We then define groups based on shared ancestry inferred from anatomical similarities.
These groups that we define based on shared ancestry are called clades, and a clade includes the most common ancestor of the group and all of its descendants, no matter how distant. When in geologic time the organism lived is not relevant, and whether it later evolved one or more characteristics that made it very different from the original, ...an ancestor, perhaps more adapted for some particular lifestyle, that doesn't matter either. In this class, we are primarily examining the clade Dinosauria, by which we mean the group containing the last common ancestor of... all dinosaurs, so the species that is the ancestor of everything that we could call a dinosaur, and the descendant of that particular ancestor, no matter how distant, all the descendants.
We'll talk later in the class about how we determine these relationships and test different hypotheses of who is more related to whom, but for now, know that we define these groups based on their shared ancestry. There is a common ancestor of all life on our planet. There is a common ancestor of all animals.
There is a common ancestor of all vertebrates and a common ancestor of all dinosaurs and and these various clades are nested within each other, such that all dinosaurs are also animals, all dinosaurs are also vertebrates, but not all vertebrates are dinosaurs, and not all animals are vertebrates. There are other groups of vertebrates with their own shared ancestors, such as mammals, for example. Oh, take this Dimetrodon, for example. If you want to find a toy Dimetrodon like this, Well, you will almost certainly have to go to a toy store and walk down the aisle where the toy dinosaurs are and that's where you will probably find a toy Dimetrodon. And superficially, well, a Dimetrodon looks a lot like a dinosaur.
But if we look at particular skeletal characteristics of Dimetrodon, we find some really striking differences from it and say, oh, a Tyrannosaurus Rex. One of the obvious is looking at the skulls. And with the Dimetrodon.
metrogon skull we'll see that there's actually just one hole here where the jaw muscles attach. However if we look at dinosaurs like the T-Rex we see that there are actually two holes, one above the eye and one below for those jaw muscles to attach. If we keep looking at other fossils and at more at other modern organisms we'll find that there's lots of things that share these two different ways. of attaching a jaw to a skull and we'll realize that we can actually group many organisms into how exactly the jaw attaches to the skull In fact, having just one hole in a skull is a feature shared by all mammals, including us. Whereas having two holes in the skull is a trait shared by all archosaurs.
The clay that... includes dinosaurs, crocodiles, and birds. So this Dimetrodon, it turns out, based upon the shared anatomical characteristics with other mammals and other species that are closely related to mammals, is part of this mammal group which we call synapsids. And so it's more closely related to us than it is to the Tyrannosaurus rex. And it is not a dinosaur.
So that is to say that we share a common ancestor with Demetrodon, and that common ancestor is shared with Demetrodon, that common ancestor does not lead to Tyrannosaurus rex. That's a way of thinking about what is more related to what. That Tyrannosaurus rex and crocodiles share a common ancestor, which isn't in that chain of common ancestors that leads to us and Demetrodon.
This phylogenetic definition of dinosaurs also leads us to realize that, well, birds are actually dinosaurs. That Tyrannosaurus Rex and, say, your common chicken, actually share a common ancestor that... really is quite close in time. That that common ancestor was probably alive sometime in the Triassic or the Jurassic. The idea that dinosaurs are extinct or restricted to the Mesozoic is a misconception.
due to having an incomplete definition of what a dinosaur is. Birds have anatomical characteristics that they inherited from their ancestors, characteristics that are shared with dinosaurs, but are not shared with other groups of vertebrates. One aspect, and you'll learn more about this later, is that all dinosaurs, starting right from the most common ancestor of dinosaurs, seem to have a hole.
through their hip. That same hole is also present in today's modern chicken, not to mention hawks and ducks and ostriches. So just like some vertebrates are dinosaurs, some dinosaurs are birds. And all birds are dinosaurs, such that dinosaurs are very much alive today. In this class, we will often use the phrase non-avian dinosaurs to mean all the dinosaurs that are not birds.
Dinosauria, the clade of dinosaurs, actually has birds nested within it. The non-avian dinosaurs are extinct, meaning they are no longer living on our planet. But birds are extant and still living and evolving on Earth.
We'll use these terms often. Particularly, we will say extinct to mean clades that are no longer living and extant to mean clades that are still with us. Extant can also mean that you were not extinct at that time. T.
rex and Triceratops were extant at the same time in the fossil record, for example. As we find new fossils, we try to fit them into this phylogenetic framework by integrating the anatomical characteristics of those fossils. fossils.
Sometimes this new information fits in really neatly and we immediately know where that fossil fits in the phylogenetic tree, the evolutionary relationships of all the organisms we're interested in, and what it's most closely related to. Other times the particular mix of characteristics that we see in a fossil may be something that we've never seen before and this new combination may alter our understanding of the relationship. relationships that we infer. The big groups, what is a vertebrate, what is a dinosaur, what is a mammal, those are really well supported, unlikely to change.
But we'll see that for smaller groups nested within dinosaurs, for example, there's a lot of There's sometimes uncertainty in those relationships, and uncertainty that we will need new fossil finds and more careful study to resolve. Moving forward, we'll use phylogenetic definitions of groups of organisms. These groups are determined using characteristics that the organism have due to their shared common ancestry.
Given everything we know about dinosaurs and their anatomical characters come from different from fossils we will next talk about how fossils are preserved and what processes control the fossil record beginning in the next video segment