With two pairs of semi-circular canals, heterostracans could probably detect movement of their heads up and down and side to side. However, they would have a little harder time keeping in balance if they rolled around the long axis of their bodies. Because this is the function of the third semi-circular canal that they probably didn't have. Then again, being bottom-dwelling, they probably didn't roll very often while swimming. The pteraspidomorphs are probably the most primitive of the ostracoderms. The second ostracoderm group we'll look at in this lesson, the Thelodonti is less well understood. They represent a very large and diverse assortment of fossil fishes with many different forms, and are only really united by features of their scales. They may not represent a true clade. It could instead be an assortment of multiple, independent, lineages. Rather than the large heavy shields of the heterostracans, thelodonts had unique thelodont scales; many tiny tooth-like hollow scales covering their body. Thelodont means hollow tooth. They had many gill openings arranged in an arc starting immediately posterior to their eyes. There were two main types of thelodont. Some had bodies that were flattened dorsoventrally, or depressed. And others were flattened laterally, or compressed. Now, here's a question for you to think about. If all fossils are flattened, how do we know whether they are depressed or compressed? The way the fossils are preserved, or their taphonomy, tells us something about the shapes of the living fishes. When fishes die, their bodies sink through the water column and are buried in the muck at the bottom. If a fish is flattened more one way than the other, their bodies are more likely to end up lying on the flat sides. It's like when you toss a coin. It's flattened one way and it's most likely to land on one of its flat sides rather than standing up on its edge. Depressed thelodonts are almost always found with their dorsal or ventral sides facing up, parallel to the layers in the rock they're in. They also had an interesting new development for vertebrates, which were paired lateral fin-like structures. Like the corneal spines in heterostracans, these probably helped provide stability in the water. Until the 1990s, all known thelodonts were depressed. But then, two researchers from the University of Alberta, Michael Caldwell and Mark Wilson, discovered a whole new type of vertebrate with thelodont scales in the MOTH locality. These are the compressed thelodonts, called the Furcacaudiformes. Furcacaudiformes were similar to their depressed cousins in that they were covered with tiny, hollow, tooth-like scales, and had many gill openings arranged in an arch. However, they had deep, laterally compressed bodies. When their fossils were first discovered, they were pretty poorly preserved, and they were originally misinterpreted as depressed thelodonts. Their pectoral fins looked strangely asymmetrical, and they consistently had a weird conical groove on one side of their bodies. This puzzled researchers for decades until better preserved specimens of entire fishes were collected. With this new evidence, the researchers were able to determine that rather than looking at the dorsal or ventral sides, they were looking at the lateral sides of the fishes. The conical groove was the digestive track, and the pectoral fins were actually a dorsal and ventral fin. In addition to having a completely different body type than other early vertebrates these thelodonts had strongly forked and lobed tails. '<i>Furcacauda</i>' means forked tail. And <i>Furcacauda</i> is one of the best known genera of furcacaudiforms and the namesake of the group. It was kind of shaped like a hand. It had a relatively blunt head with big eyes and a small mouth and a wedge shaped body. Imagine my palm as the body of the fish. My thumb is where <i>Furcacauda</i>'s dorsal fin would be, and then my fingers are the lobes of it's forked tail. The notochord and lateral line probably ran through the ventral lobe of the tail. So although it was externally symmetrical it was internally hypocercal like heterostracans. Which thelodonts do you think were faster swimmers? A, compressed thelodonts or B, depressed thelodonts? Think again about modern fishes. Remember that body dwelling fishes are usually depressed. Fast fishes, like tuna, are compressed. If we look at modern fishes, the laterally compressed ones are generally better swimmers than the dorsoventrally depressed bottom dwelling fishes. So, it's reasonable to argue that the laterally compressed thelodonts were probably more efficient swimmers than the dorsoventrally depressed ones. So A is the best answer. Compressed thelodonts also had very big eyes, meaning they probably could see quite well. This is also useful if you're moving through the water quickly. In addition to a brand new body plan, furcacaudiform thelodonts also appear to have had what was previously considered to be a feature only associated with jawed vertebrates. The structure is tube-like, appears to be part of the gut, and has sharply defined front and back ends. If it's part of the gut, these sharp ends were probably sphincters, and this structure has been interpreted as a fossilized stomach. This is the oldest evidence for a stomach in the fossil record of vertebrates, and the first evidence of any kind of stomach in jawless vertebrates. So, this fossil evidence was big news. The implication is that the stomach evolved before jaws. And that's food for thought for paleontologists. In fact, fossil evidence have changed a lot of our ideas about the things unique to jawed vertebrates. The next and last group of agnathans we'll look at in this lesson had quite a few characters that used to be associated only with jawed vertebrates: the Osteostraci. The osteotracans were another group of heavily armoured agnathans. Osteostracan means bone shell. They are found in North America and Europe, from the Early Silurian to the Late Devonian. Let's look at <i>Superciliaspis</i>, an osteostracan from MOTH, for some key features of this group. Osteostracans had a dome shaped bony shield covering the dorsal surface of their heads, and these cephalic shields were essentially helmets. The term cephalos means head. The eyes were located close together on top of their heads with a pineal organ situated between them. They had several gill openings arranged in a circle on the ventral side of their bodies. Their mouths were also ventral. What kind of habitat do you think osteostracans occupied, based on their body shape? Do you think they lived A, at the top of the water column near the surface. B, at the bottom of the water column on the sea floor. C, In the middle of the water column, or D, none of the above. Osteostacans had eyes that pointed up, a mouth that pointed down and were flattened ventrally. It's likely that osteostracans spent most of their time in the mud at the bottom of the water column. So B is the best answer. Perhaps, because of this, osteostracans possessed a unique set of fields in their dorsal shield, as we can see in these specimens, <i>Cephalaspis</i>. These are specific and discrete areas of the shield where the armour was reduced to small tesserae, probably supported by exposed skin. These cephalic fields may have functioned as a kind of sensory organ, to detect changes in the pressure of the water around them, similar to a lateral line system. They were connected to the inner ears by large branching canals, hinting at such a sensory function. Osteostracan fossils provide us with insight into their internal anatomy, because some soft tissue features of the head were preserved as impressions in the cartilage of the skull. A thin layer of bone that formed around it preserved the cartilage. Osteostracans had two pairs of semicircular canals like lampreys. In some specimens we can see the impressions of the gill pouches which were large and located anteriorly. They also had two pectoral fins. Although they share some features with agnathans like dermal armour and two pairs of semicircular canals, these features are probably primitive for vertebrates as a whole. And such features are called symplesiomorphies. Symplesiomorphies are shared primitive traits. You and I both share hair with apes, like chimpanzees. But that's not a feature that united us as sister groups, because other mammals also have hair. Instead we rely on traits that unite closely related groups to the exclusion of other groups. These traits are called synapomorphies or shared derived traits. Hair is a synapomorphy for the group Mammalia, but within that group, hair is a symplesiomorphy for apes. Osteostracans are generally considered to be the sister group to jawed vertebrates. This is based on a number of synapomorphies. Compare this osteostracan with this extant jawed vertebrate, a shark. Identify which of these features are synapomorphies that unite the osteostracans with jawed vertebrates. A, vertebrate bone. B, pectoral fins. C, dorsal fin. And or D, an upward-pointing tail. More than one answer may be correct, so choose all that apply. Vertebrate bone is a feature that osteostracans inherited from the common ancestors they shared with pteraspidiforms, so that is not a synapomorphy of these two groups. The same can be said for the dorsal fins since furcacaudiform thelodonts had a dorsal fin. Osteostracans and jawed vertebrates share pectoral fins and an upward pointing tail which aren't present in more privative vertebrates. Therefore, B and D are the correct answers. They are the synapomorphies that unite these two groups. Osteostracans had several things in common with jawed vertebrates. They had cell spaces in their bone like jawed vertebrates do and unlike the acellular bone found in heterostracans. They had a special set of bones inside their eyes called a sclerotic ring, which could have helped their eyes retain their shape under the high pressure associated with deep water. Other derived traits meant that osteostracans were probably more efficient swimmers than other agnathans. The notochord in osteostracan tails went through the upper lobe of the caudal fin, rather than the lower lobe. And this kind of asymmetrical tail is called an epicercal or heterocercal caudal fin. Technically, a heterocercal caudal fin is any kind of asymmetrical tail, but in general, it's used for a tail with a notochordal upper lobe. Whereas hypocercal, or reverse heterocercal is used for a tail with a notochordal lower lobe. Now modern sharks and some bony fishes have epicercal tails today. The upper lobe of a sharks tail is larger and stiffer than the lower lobe. So, more water gets pushed by the dorsal part of the tail compared to the ventral part. Epicercal tails are able to produce a lot of forward thrust, but because the dorsal part of the tail pushes more water with each thrust, they also produce a thrust that points the head down as the shark moves forward. Sharks counter this with their wing-like pectoral fins, which produce an upward force. Osteostracans could do the same with the wing like projections of the cephalic shields, and their pectoral fins. But they may have preferred to keep their heads down, so they could find food in the mud on the sea floor. Comparing an osteostracan like <i>Superciliaspis</i> to <i>Haikouichthys</i> or <i>Metaspriggina</i> it's clear that jawless vertebrates underwent a lot of changes through their evolutionary history. Another major change was in the making. The sister group to the osteostracans had a whole new set of weaponry that would change the way vertebrates interacted with their environment and with each other. Up next, jaws.
