A side benefit for paleontologists is that vertebrates with hard skeletons had a much better chance of being preserved. There are fragments of bone-like tissues from as early as the Late Cambrian with the oldest fossils that are truly recognizable as fishes come from the Middle Ordovician from North America, South America and Australia. At the time, South America and Australia were part of a supercontinent called Gondwana. North America was part of another supercontinent called Laurentia and the two were separated by deep oceans. These two super continents, and others that were also present, were partially covered by shallow equatorial seas and the continents themselves were barren and rocky. Land plants didn't evolve until later in the Silurian Period. In these shallow equatorial seas, a large diverse and widespread group of armored, jawless fishes evolved: the Pteraspidomorphi. The first of our three groups of ostracoderms. The Pteraspidomorphi are divided into three major groups: the Astraspida, the Arandaspida and the Heterostraci. The oldest and most primitive pteraspidomorphs were the Astraspida and the Arandaspida. You'll notice that all three of these taxon names contain 'aspid', which means shield. This is because these early fishes and many of the Pteraspidomorphi, possessed large plates of dermal bone at the anterior end of their bodies. This dermal armor was very common in early vertebrates, but it was lost in their descendants. The Arandaspida are represented by two well-known genera: <i>Sacabampaspis</i>, from South America and <i>Arandaspis</i> from Australia. <i>Arandaspis</i> has large, simple, dorsal and ventral head shields. Their bodies were fusiform, which means they were shaped sort of like a spindle, fat in the middle and tapering at both ends. Many fishes are fusiform. Why do you think that is? A fusiform shape results in the least amount of resistance from the water that fishes move through, and less resistance means less energy is wasted while swimming, which is certainly an advantage for aquatic organisms. Between their dorsal and ventral shields, arandaspids like <i>Sacabambaspis</i> had many small, rectangular or diamond shaped plates in a row, and as many as 20 tiny external openings were located at the boundaries between these plates. These openings, as in lampreys, allow water to leave the large pharynx, so they are pharyngeal openings. But they are also called gill openings, because the gills are housed in this region. The pharynx was a breathing organ, because of the presence of gills, but was also used to strain small food particles from the water that passed through it. The eyes in <i>Sacabambaspis</i> were situated at the very anterior end of their bodies, kind of like car headlights, with their jawless mouths between their eyes. They had no fins, except the caudal fin. The Astraspida from North America were also fusiform fishes with only a rounded leaf shape caudal fin and bony armour. They had dorsal and ventral shields made up of many polygonal units of bone called tesserae and had diamond shaped scales covering their tails. Their eyes were located on the sides of their heads instead of at the very front, and they had around ten gill openings, not as many as the Arandaspida. These were situated on the margin of the dorsal shield in a more dorsal position that those of the Arandaspida. The dorsal shields in astraspids had several longitudinal crests, as well as an interesting set of grooves in the plate making up their shields. These grooves likely held something that we can see in most living fishes: a sensory canal system. If you look at the flanks of many living fishes today, you'll see a groove or canal running down the sides of their bodies. This is called a lateral line system and it's a sensory system that allows fishes to detect minute changes in the pressure of the water around them. As anything moves through the water, it pushes water in front of it. Being able to sense these pushes, means the fish can sense something coming before it arrives. If you've ever tried to catch a fish bare handed, you've probably been foiled by their lateral line system. The key to the lateral line system is a special sense receptor called a neuromast. The neuromast consists of a cup like base and a jelly filled sheath called a cupola. Any movement in the surrounding water causes the fluid in the canal and the cupola to also move. Within the cup like base of the neuromast are several sensory cells with hair-like cilia that extend up into the cupola. Now when the cupola is moved by a pressure wave in the water, the hairs are bent, and this change in the signal effects the sensory cells, which effects the message that they send to the brain of the fish. This means the fish can tell the direction of the wave as well as how strong it is. A fish can sense its own movement, that of nearby predators or prey and even the water displacement of stationary objects. Have you ever wondered why fish in a tank aren't constantly running into the glass wall? It's because when they swim towards one of the walls, they push water out in front of them and this wave gets reflected back by the wall and the fish can sense this reflective wave through their lateral line system. So even though they can't see the glass, they know it's there. Such a sensory system would certainly have helped the astraspids to find their way in the marine habitat as well as helping them to avoid predators and find prey. It's not surprising that most fishes today retain a system of sensory canals like the lateral line. Astraspids are the sister group to the rest of the Pteraspidiformes, the large and diverse clade of fishes called the Heterostraci. Heterostraci means different shields or different scales. And these fishes lived from the early Silurian to the late Devonian, about 430 to 370 million years ago. In general, heterostracans look similar to the astraspids. They had dorsal and ventral head shields, fusiform bodies, large trunk scales, and no fins, except the caudal fin. But the arrangement of their scales and their bone tissues are unique. In addition, they had only one external branchial openings on each side of their bodies. There are around 300 known species of heterostracans, and they came in a variety of forms. Some of the earliest heterostracans has solid dorsal shields that were completely fused, with openings for their eyes and a sensory organ on the dorsal surface of their heads called a pineal organ. And this organ was probably photosensitive, allowing heterostracans and other animals, such as the lamprey, to sense changes in the level of light above them in the water. And this would be useful for telling them which way was up and keeping track of how deep in the water column they were. The University of Alberta has a wonderful collection of ostracoderm fossils including hundreds of heterostracan specimens. Many of them came from an extraordinary Early Devonian fossil locality called Man On The Hill, or MOTH. Now MOTH was first discovered by field scientists from the Geological Survey of Canada who contacted University of Alberta researcher, Dr. Brian Chatterton. When Dr. Chatterton went to the locality, he noticed an oddly shaped rock formation visible from his main field camp that looked like a man standing on top of the nearest hill. He called the locality Man On The Hill, after this rock formation. This locality is high up in the MacKenzie Mountains of the Northwest Territories of Canada, well above the vegetation line. The fossils weather out of a layer of rock and tumble down into a scree pile below. In order to collect these fossils, researchers must take a helicopter from the nearest town to make a base camp, then climb the steep slope daily to search through the rubble for fossils. Usually the best fossils are barely visible, with only a tiny piece of bone exposed and the rest still covered in rock. These rocks formed over 400 million years ago. But once the fossils are exposed, It doesn't take long for them to weather away to nothing. University of Alberta researchers have been collecting fossils from the MOTH locality since the 1980's and have brought back hundreds of fossil fish specimens to be housed in the university collections. In lesson one, we discussed several types of depositional environments. The rocks in MOTH are composed of finely laminated clay-rich limestone or calcite-rich shale. There are no wave ripples. What depositional environment does MOTH represent? A, lagoon environment, B, lake environment, C, continental slope environment, or D, continental shelf environment? In the Early Devonian, what is now the Mackenzie Mountains would have been under an equatorial shallow sea. The MOTH locality would have been on the continental shelf, so D is the correct answer. The most recent interpretation of the MOTH environment is that it was a deep pocket, or low, within the carbonate shelf itself. Black shale and iron rich pyrite indicate that the muck at the bottom would have been poor in oxygen. Despite this, the area was capable of supporting many diverse forms of life. Perhaps, the water column above the anoxic low was well oxygenated, and the fishes sank into the muck only after they died. This has resulted in many spectacular fossils. Among these are representatives of all the three groups of ostracoderms we're examining in this lesson. The Devonian Period, when the MOTH fish lived, is known as the Age of Fishes because all the main groups of fossil fishes had evolved by this time and thrived globally. You can find hints of this great diversification in some well-preserved fossils from the preceding Silurian Period. For example, one of the earliest and best preserved heterostracans came from a Silurian locality near MOTH, known as Avalanche Lake. And this little fish is named <i>Athenaegis chattertoni</i>. <i>Athenaegis</i> means Athena's shield, so named because its dorsal shield is covered with convoluted ridges. And these ridges reminded the researchers of snakes. You may know that in Greek mythology the Gorgon Medusa had snakes for hair, and her head wound up attached to the goddess Athena's shield. That's the origin of the name <i>Athenaegis</i>. At the time it was described, <i>Athenaegis</i> included the oldest known well preserved vertebrate tail. Heterostracans were the first group to really learn to swim using their tails to propel themselves through the water, instead of flexing their body myomeres against their notochords. The tail in <i>Athenaegis</i> was fork-shaped with many thin scales covering the two lobes. It also had rows of small bony plates between its dorsal and ventral shields, and even bony plates around its eyes. Its mouth was composed of many small plates, and it would have probably acted like a flexible scoop. Later heterostracans had similarly forked tails. Two of the best known of these hetrostracans are from MOTH are <i>Nahanniaspis</i> and <i>Dinaspidella</i>. Although their tails look symmetrical, the lower lobe was a bit bigger than the upper lobe and the notochord extended into the lower lobe. This type of a tail is called a hypocercal tail. Hypo means below and cerca means tail. Let's pause our survey of early vertebrates here for a moment and talk about fish tails. Why do they matter in terms of how vertebrates evolve? The short answer is that they effect fishes' ability to swim. The function of a tail is simple. It pushes water. And by pushing on the water behind it, a fish is able to propel itself forward. In living fishes, there's a huge variety of forms of caudal fins. Each of these tails pushes water a little differently depending upon how the fish generally swims. Thin tall, crescent shaped tails are very good for fish that swim fast over a long distance, like tuna. They're extremely efficient when it comes to moving in one direction, but they're not the best shape for a lot of rapid changes in direction. Short, broad tails are best for fast acceleration without sacrificing maneuverability. But they're not good for long distance cruising. So they're often found in ambush predators like this grouper. Most early vertebrate tails were short and broad. Which makes sense for small bottom dwelling fishes. Most early vertebrates had hypocercal tails. When the notocord extended through one lobe of the tail, that lobe is generally stiffer than the other, and also, usually bigger. These tails are asymmetrical, which results in the propulsive force they generate being in two directions. Hypocercal tails propel fishes forwards and also upwards because the ventral part of the tail pushes more water than the dorsal part. When you think about that, it makes sense that a lot of early vertebrates had hypocercal tails. These were small fishes covered in bony armoured plates, and probably would have tended to sink. If every thrust of the tail pushed them upwards as well as forwards, they would have helped to counteract the sinking effect. In addition to strong tails, many later heterostracans had dorsal spines on their head shields. Their head shields were also composed of many different plates. Many had wing-like plates, called cornual plates, that projected laterally. And these plates and spines may have helped provide stability in the water, like keels, preventing the heterostracans from rolling erratically. These heterostracans were very successful and grew quite large. Some species were up to two meters long. The endoskeletons of heterostracans were probably made of cartilage. All we know of their internal anatomy comes from impressions on the inner surfaces of their dorsal and ventral shields. On this internal view of a dorsal shield from a MOTH heterostracan you can see notches for the eyes, the pineal area, and impressions of gill pouches and a dorsal nerve cord. These two pairs of grooves here are impressions of semicircular canals. You and I have three semicircular canals; they're part of our inner ears and detect rotation of the head. Each canal is filled with fluid and contains sensory cells that are very similar to the neuromasts in the sensory lateral line canals with hair cells and a cupola. When the head rotates, the fluid moves within the canals causing the cupola and the hair cells to bend. We perceive that as a sense of tilt. These canals are responsible for the feeling you get when you go around a turn in a car, tightly, and give us our sense of balance. We have three semicircular canals, but not all vertebrates do. For instance, hagfish have one. And lamprey's have two. Based on what you know about semicircular canals and heterostracan internal anatomy, how many semicircular canals do you think heterostracans had? One? Two? Three? Or Four? This is a bit of a trick question because our fossils can't help us answer this definitively as all we have are impressions. Still, we definitely see two pairs of impressions so B is the best answer.
