Knowing that we share an ancient history with dogs, birds, snakes, frogs, sharks, and all the vertebrates alive today, let's piece together an amazing evolutionary story. How did vertebrates go from worm-like marine animals like <i>Metaspriggina</i>, to the modern diversity of forms? In this lesson, we'll continue to examine some key fossil localities that tell us about some important evolutionary steps in that story, that took place long before dinosaurs roamed the Earth. Our first such step took place when all vertebrates were fishes. What is a fish? Choose the definition that you think describes all fish. Is it A, any animal that swims? B, any animal with fins? C, any animal with gills? Or D, none of the above? D. D is the correct answer. These definitions are the kinds of things people usually say when asked the question, what is a fish? The fact is, no single definition can set apart what we typically call fish from all other animals. All fishes are definitively vertebrates. But all the answers could also apply to things that aren't vertebrates, like mollusks. Some other vertebrates, like whales and dolphins, also swim and have flippers. But they are not fish. Fish is a term we'll use a lot in this course, and we use it a lot in our day-to-day lives. But it isn't a valid term from the perspective of phylogenetic taxonomy, at least not they way we use it. We generally use 'fish' to describe aquatic vertebrates that have gill openings, lack limbs with digits, and are normally ectothermic. And because this group excludes tetrapods, it is not a clade, or a single lineage. Such informal groups are called grades or paraphyletic assemblages, indicating that the group does not include all the descendants of a single lineage. Fish is still a useful descriptive term for any vertebrate that is not a tetrapod on the phylogenetic tree. So, we'll still use it that way in this course. Let's review some basic extant fish anatomy, in case you're not familiar with these aquatic relatives of ours. Let's review some basic
extant fish anatomy, in case you're not familiar with these aquatic relatives of ours. Like all vertebrates, fishes are bilaterally symmetrical. And they have a well-developed head with sensory organs. Fishes in general respire using gills, are most often covered with bony scales, and propel themselves using fins. There are two main types of fins, median fins and paired fins. The median fins include the caudal fin or tail fin, the dorsal fin, and anal fin. Now there may be more than one dorsal, and one anal fin in some fishes. The paired fins include the pectoral fins and the pelvic fins. And these paired fins are connected to, and supported by, pectoral and pelvic girdles, at the shoulder and hip; in the same way our arms and legs are connected to, and supported by, pectoral and pelvic girdles. This arrangement is something we inherited from the ancestors we share with fishes. They are homologous structures. We could consider <i>Haikouichthys</i>, the Cambrian vertebrate, to be one of the very first fishes. But, it doesn't look much like the fishes we're familiar with. For one thing, it had no bony scales. You can see that it had a rudimentary, median fin-like structure, but nothing really recognizably like a caudal fin, or any paired fins. <i>Haikouichthys</i> would have swum by flexing its myomeres against its notochord. But, it was probably not a very strong or fast swimmer. The first major step in vertebrate evolution would change that. The evolution of vertebrate bone. Bone would've made effective armour, but it would eventually also allow for the evolution of new swimming adaptations, like more powerful fins and tails. To explore these adaptations, we're going to travel along the vertebrate phylogenetic tree. In this lesson, we'll be exploring some of the earliest vertebrates, those without jaws. To help keep everything in perspective, we've produced an interactive phylogenetic tree, that you can use as a reference, as we encounter different groups. Why don't you take some time before we get going, open up the tree, and play? One thing you may have noticed about our phylogenetic tree, is that there are a lot of branches representing extinct groups, or taxa, that you may have never heard of. Many groups of early vertebrates left no descendants. And there's nothing quite like them alive today. If we take a look at a simplified tree of extant taxa, that is, groups with living representatives, you can see that animals that we might call fishes are generally divided into two main groups. Those with jaws and those without. We're most familiar with jawed fishes: the cartilaginous sharks and their relatives, and the bony fishes and their relatives, which include ourselves. But what about the jawless vertebrates? In this lesson, we're going to be examining extinct, jawless vertebrates. We'll discuss them in more detail soon, but for now, just take a look at these four living vertebrates. Does it look to you like any of them lack jaws? If so, which ones? A, Hagfish, B, Eel, C, Lamprey, and, or D, Rope fish? More than one answer might be correct, so check all that apply. If you think all these vertebrates have jaws, leave all the boxes blank. Although all these vertebrates are long, slender fishes, the eel and rope fish have jaws, while the lamprey and hagfish don't. So answers A and C are correct, both being living, jawless vertebrates. Hagfish and lampreys are the last remaining members of a large grade of fishes, called the agnathans. Meaning, without jaws. Although lampreys seem to share more morphological characters with jawed vertebrates than with hagfish, recent molecular research has provided some evidence that they are most closely related to each other. We call two such groups sister taxa. If the sister group relationship between lampreys and hagfish is correct, they would form a clade, called the Cyclostomata, which means round mouths. Now, they lack many characters found in later agnathans and jawed vertebrates, such as bone and paired fins. However, the lack of these characters should not be confused with being primitive. These are highly specialized animals, adapted to very particular niches. Hagfish are mainly deep sea carrion feeders, and many lampreys are parasites that feed on other living fishes. Cyclostomes have a long evolutionary history and have undergone many changes that separate them from the last common ancestor of jawed and jawless fishes. Unfortunately, because hagfish and lampreys lack bony tissues, very little is known of their fossil lineage. To learn more about the vertebrate body plan, and how it evolved from that of its soft-bodied ancestors, we have to turn to the extinct, armored agnathans, the ostracoderms. We'll be examining the features of three major groups of ostracoderms in this lesson. They're called the Pteraspidomorphi, the Thelodonti, and the Osteostraci. Each of these groups possess some important vertebrate features that are still present in extant taxa. We can even trace the ancestry of a few of our own features back to these agnathan groups. Ostracoderms is another informal term, meaning shell skin. It includes all the members of a large and diverse grade of jawless, armoured fossil fishes. Ostracoderms were the dominant vertebrates for almost a 100 million years. What made them so successful? The major innovation for this group was the acquisition of armour made of hard tissues, and particularly, a special vertebrate tissue, bone. In lesson one, we talked about two different kinds of bone found in vertebrates. What kinds of bone were they? A, endochondral bone and chondral bone. B, flat bone and round bone. C, intramembranous bone and endochondral bone. Or D, dermal bone and intramembranous bone. Vertebrates have two main types of bone. Bone that forms around cartilage is called endochondral bone. And bone that forms without a cartilage scaffold, is called intramembranous bone. So, C Is the correct answer. Dermal bone is a type of intramembranous bone. The bone found in the earliest fishes was almost entirely dermal bone. Although all vertebrate bone tissues are made up of the same materials, we can recognize different tissue types based on their textures. When we look at a cross section of some early vertebrate bone, it looks a bit like a layer cake. Those layers each have different properties, and, when added together, make for a more flexible, adaptable and much stronger structure than one made up of only one type of tissue. The whole is greater than the sum of its parts. In ostracoderms, the dermal bone itself was covered on its external surface by a sculptured or ornamented layer of a material called dentine. Dentine, or dentine, is the material that makes up the bulk of your teeth. It's denser and harder than bone, but softer than enamel. And enamel is the hard, densely mineralized tissue that caps your teeth. Dentine is produced by special cells called odontoblasts, and these cells form tiny tubes in the mineralized tissue. Different types of dentine have different patterns, or arrangements of these tubes. Below the dentine layer was a layer of spongy bone. And spongy bone, as the name implies, is filled with cavities, and looks in cross-section like a sponge. The cavities were filled with blood vessels, nerves, or bone-producing cells. And this is the same type of bone that forms in the inside of the long bones of your arms and legs. And it's softer and more flexible than dentine. The spongy bone was underlain in turn by a harder, rigid basal layer composed of thin sheets of bone tissue, without any cell spaces, called lamellar bone. In very early vertebrates, none of the bone enclosed any of the bone producing cells as it does in later vertebrates, including us. In this acellular bone, the cells depositing the bone material receded from the ossification area before they became trapped. Bone is one of the things that makes a vertebrate a vertebrate, but why is it so important? What is the function of bone? A, skeletal support, B, mineral storage, C, protective armour, or D, all of the above? Bone provides a very solid support for the attachment of muscles, improving movement efficiency and power. It acts as a storage place for phosphates and other minerals that animals use and excrete as part of their normal metabolism. And, perhaps most obviously, in our very early ancestors, bone provides very effective protection for vulnerable body parts like brains and hearts. So D is the correct answer.