As we continue our journey along the vertebrate tree of life, it's important to remember that vertebrates didn't evolve in a vacuum. The evolution of our earliest vertebrate relatives was shaped by global events, as was the evolution of the groups with which they co-existed. What was the world like hundreds of millions of years ago, and how did changes to that world impact the early life that filled its seas? During the Ordovician Period, much of the world's land was collected together in this super continent called Gondwana. And Gondwana included what are now Southern Europe, Africa, South America, Antarctica, and Australia. North America was equatorial, part of a continent called Laurentia. The Iapetus Ocean separated Laurentia from two other large continents, Siberia and Baltica. Shallows seas mostly covered Laurentia during the first half of the Ordovician. In fact vast areas of the planet were covered in continental shelves and shallow seas; perfect conditions for marine animals. The Early to Middle Ordovician was a time of diversification of carbonate shelled organisms. They included reef building corals, echinoderms like sea stars, arthropods like trilobites, mollusks like bivalves, and cephalopods and strange animals called brachiopods. Brachiopods look deceptively like bivalves. The earliest vertebrates like the arandaspids and astraspids swam along side these animals. Towards the end of the Ordovician, the Earth's climate changed dramatically. Major uplifts and periods of mountain building took place in areas that had been covered by shallow seas. Volcanic activity increased and ocean currents changed as the continents moved. Gondwana headed steadily south, and by the Late Ordovician, it covered the South Pole. Which of these maps would you expect to have continental glaciers? Is it A, B or C? Continental glaciers form when the continents cover the poles. So you'd expect B to have continental glaciers. B is the correct answer. Glaciation is more likely to occur when land covers the poles than when the poles are covered by oceans. The oceans absorb and retain heat better than the continents do. This means it is much easier for glaciers to form and persist on land. The vast land mass of Gondwana over the South Pole resulted in a major glaciation in the Late Ordovician. With a huge amount of water locked up in ice, global sea levels dropped dramatically and the shallow seas disappeared. This rapid cooling and sea level drop resulted in two major pulses of extinction. Taken together, these Ordovician-Silurian extinction events were the first of the 'big five' mass extinctions in Earth's history, and the second most severe. For context, the Ordovician-Silurian mass extinction was more severe than the mass extinction of 66 million years ago that ended the reign of dinosaurs. Almost all life existed in the sea in the Ordovician, and it was devastated. An estimated 60% of the existing genera went extinct over 5 million years. Fortunately for us, the early vertebrates were one of the groups that made it through. During the Silurian the polar glaciers retreated and the sea level rose, restoring the shallow seas. Laurentia, Baltica, and a microcontinent called Avalonia began to collide and close the Iapetus Ocean. The Earth entered a long, warm, greenhouse phase that was relatively stable by the mid to Late Silurian. The first moss-like land plants evolved and the first terrestrial arthropods followed. However, most life remained in the seas. The agnathans diversified and evolved along with the first members of a new vertebrate group with a new adaptation. What evolutionary novelty do you think allowed vertebrates to first become predators? A, Legs. B, Jaws. C, Claws. Or D, Teeth. At some point, probably around the time of the Ordovician-Silurian extinctions, vertebrates evolved jaws. Jaws allowed them to become predators. So B is the correct answer. Up until this point in our journey along the evolutionary tree, all vertebrates were jawless. Some groups of ostracoderms had armor plates around their mouths that were specialized for feeding. But none had anything like a biting jaw. Ostracoderms probably fed by filtering food particles from the water they swam through, or scooping up mud along the sea floor. Most were small, and vertebrates were by no means at the top of the food chain. But that would soon change. A jaw is a structure in the mouth composed of at least two opposing sections of skeletal material connected by a flexible joint. In vertebrates the sections of the jaw are made of cartilage or bone, and oppose vertically. This means the vertebrate jaw is made up of an upper jaw and a lower jaw on each side of the head. Together called the mandibular arch. The upper jaw in primitive jawed vertebrates is a cartilaginous element called the palatoquadrate cartilage, and the lower jaw is made of the Meckel's cartilage. In bony fishes, like perch, these cartilages get replaced with bone. But, these cartilages still make up the jaw in modern cartilaginous fishes like sharks. The vertebrate jaw is not homologous with pinching mouth apparatuses in non-vertebrate animals. For instance, arthropods, like insects, have mouth parts that are made of a substance called chitin. And they oppose each other laterally. The fact that so many different groups have independently evolved similar structures indicates that there is a significant advantage to biting. Jawed vertebrates are called gnathostomes, meaning jawed mouths. And they all belong to a large clade called the Gnathostomata. When jaws evolved is a bit of a puzzle because the vertebrate fossil record from the Ordovician and Early Silurian is very poor. We've found disarticulated remains that probably come from jawed fishes like scales or fragments of bone. But these aren't really definitive. What we do have are identifiable gnathostome scales from the Early Silurian and articulated gnathostome fossils from the Late Silurian. Relatively recently, fossils from the Late Silurian of China have been described that represent several different gnathostome groups. Fossils of jawed vertebrates from the Late Silurian have also been recovered from Canada. Taking all of these clues into account, gnathostomes evolved by at least the Early to middle Silurian, and had diversified and dispersed around the world by the Late Silurian. During the Silurian and Devonian, these jawed vertebrates would rapidly diversify to fill many niches previously occupied by non-vertebrate predators. Have you heard the term "niche" before? What do you thing it means? A, The role an organism fills in their environment like 'predator'. B, The place an organism lives. C, the food an organism eats. Or D, all of the above? D, is the correct answer. All of these things help define what a niche is. Rather than being only one specific thing, a niche refers to all of the factors that affect growth, reproduction, and survival of a species. A niche is animal's way of life. Think of it like the animal's job in the ecosystem. It's how a particular species makes its living. What it must do to survive. A species will survive only if all the conditions of its niche are met. This can include the food the organism eats, the physical space they occupy, the climate they do best in, or any combination of factors. When more than half of the life that existed in the Ordovician went extinct, those species no longer occupied their niches, and the resources they lived on became available to other organisms. We often see a pattern in the fossil record of a period of rapid diversification following an extinction event. This happens when the survivors take advantage of the relative lack of competition for resources. They quickly adapt to fill open niches that were formally held by the extinct organisms. It seems as if vertebrates were poised to take advantage of the niches that open up when many groups of invertebrates went extinct at the end of the Ordovician. Certainly, the evolution of jaws would have conferred an advantage. Fishes with jaws were able to grasp and prey upon other animals. And this was something that the agnathans at the time, were unable to do. They had no grasping jaws. The ostracoderms were filter feeders, or sucked up mud to find food. Enter the gnathostomes, which were active predators. Both groups diversified and thrived during the Silurian with agnathans dominating. The gnathostomes took some time to catch up, but during the Devonian they came to dominate almost every marine environment. Having opposable jaws means that gnathostomes could catch, hold, and eat other animals. This opened up a whole new arena of competition for the early vertebrates. Being able to secure more and higher quality food confers a significant selective advantage to any animal. And the early vertebrates were no exception.
