The most primitive jawed vertebrates are the heavily armoured placoderms. For a long time, placoderms were considered to be a monophyletic group or a clade. Although there were many different families, all placoderms had the same kind of dermal armour. Some scientists still argue that placoderms are monophyletic. However, recent phylogenetic studies have suggested that they are actually a paraphyletic assemblage of armored fishes branching off the gnathostome stem. For our purposes, we'll be following this recent evidence and using a simplified phylogeny with placoderms as a paraphyletic grade. Most scientists today consider placoderm the most basal group of jawed fishes. These heavily armored fishes were hugely successful during the Devonian. They lived in both marine and freshwater environments, in every continent except for South America. Even their eyeballs were mostly covered in bone. They were only around until the end of the Devonian, but in that time they dominated pretty much every aquatic environment. Let's look at the few examples of these jawed pioneers. <i>Dunkleosteus</i>, best known from the Cleveland Shale, is perhaps the most famous example. It was one of the first vertebrate superpredators and one of the largest predatory fish to have ever existed, up to nine meters long. <i>Dunkleosteus</i> was a member of a group of placoderms called the Arthrodira, which means jointed neck. Their head and trunk armour was composed of many different large plates. Between the head and trunk shield was a mobile joint, as well as a gap called the nuchal gap. When <i>Dunkleosteus</i> opened its mouth, it could do so by moving its head upwards and back, closing the nuchal gap, while its lower jaw swung down and outward. This resulted in a huge, fast, and powerful bite. Most placoderms, including arthrodires, also had specialized jaw bones that acted like self-sharpening scissors. These would have functioned like teeth, allowing them to not only grasp prey, but also slice through it. Not all placoderms were gigantic predators, though. This grade of vertebrates occupy just about every ecological niche that is possible for an aquatic vertebrate. They were incredibly diverse and specialized. Some were ray-like bottom feeders. Some resembled living rat fish. And some were specialized to feed on shellfish, while others found food in the mud or ambushed passing prey. Miguasha was home to one of the most well known placoderms <i>Bothriolepis canadensis</i>. <i>Bothriolepis</i> is a member of a group of placoderms called the Antiarchi, bottom-feeding fishes with long trunk shields and weird jointed armour on their pectoral fins. This armor gives the impression of arthropod legs since the fins were entirely encased in bone. What do you think these weird fins could have been used for? A, gripping the estuary bottom. B, burying themselves in sediment. C, controlling their movement through the water. Or D, all of the above are plausible. D is the correct answer. Antiarch fins have puzzled paleontologists for decades, with plenty of hypotheses being put forward. Maybe they use them to grip the substrate in fast-flowing water or to fling up sand and bury themselves. Most recently it's been suggested that they use their fins like rudders to move up and down in the water column. The short answer is, we don't know. But any of these are plausible hypotheses. The antiarchs, and the ratfish-like ptyctodont, placoderms were the most basal group of vertebrates to fertilize their eggs internally with claspers. Male anatomical structures used for mating. Ptychtodon claspers are similar, but not homologous to the claspers in modern sharks. And shark claspers are extensions of the pelvic fins, whereas placoderm claspers were unique structures located behind the pelvic fins. And they performed the same function but didn't evolve from a homologous structure in a shared ancestor. What kind of evolution are shark and placoderm claspers an example of? A, convergent evolution. B, divergent evolution. C, parallel evolution. Or D, congruent evolution. Placoderm and shark claspers are similar structures but they're not inherited from a common ancestor. This means that they are an example of convergent evolution, so A is the right answer. In addition to having pelvic claspers, ptychtodonts gave birth to their young instead of laying eggs. In 2009, a ptychtodont placoderm from the Gogo Formation of Australia was described with an unborn embryo in mineralized umbilical cord. This extraordinary fossil was named <i>Materpiscis attenboroughi</i>. <i>Materpiscis</i> means, mother fish. And the specific epitaph honors famous naturalist, Sir David Attenborough, who drew attention to the significance of the Gogo fish sites in his 1979 series Life on Earth. Embryos have also been found in other placoderms, like arthrodires, so it's possible that all placoderms fertilize their eggs internally and gave birth to their young. Placoderms' internal skeletons were mostly made of cartilage, not bone. So we a lot more about their heads than the rest of their bodies. At first glance they look a lot like osteostracans, especially the bottom dwelling forms. We do have a few very good fossils like <i>Ptyctodon</i> that confirm that they did have paired pelvic fins like all jawed vertebrates and unlike osteostracins. They also had two dorsal fins and heterocercal or epicercal tails, which they inherited from their jawless ancestors. Their trunks and tails were quite slender compared to the anterior part of their body. Placoderms have traditionally been thought of as slow or inefficient swimmers, weighed down by heavy armour, ambushing prey only in short bursts. However, some researchers suggest their armour may have actually served to make them better swimmers, because it gave them something solid to flex their muscles against. Placoderm vertebrae consisted of cartilaginous centra, and small y-shaped bone elements called neural and haemal arches, above and below the notochord. However, the main structural element was still the notochord. The notochord works really well as a support for small fish, but it's still flexible. It can only resist muscles to a certain point. Having the notochord as the main resistance to the longitudinal muscles of the trunk means those muscles can't be very strong compared to how strong those muscles can be in vertebrates with bony, internal skeletons. Bone is much more resistant to the pulling and pushing of muscles then the notochord is. The longer a notochord is, the less efficient it becomes at storing and directing the energy that the swimming muscles exert. Think of a flexible tree branch. A short piece of a flexible branch can resist bending and bounces back pretty well. But the longer it gets, the easier the branch is to bend, and the less springy it is. In small fishes, the notochord suffices quite well. But once vertebrates got big, the notochord was no longer enough. And placoderms got very big. Big doesn't necessarily mean slow though, and at least some placoderms, like the arthrodires, were probably active swimmers. It's possible that their armour actually allowed their tails to be flexible, while keeping the front part of the body pretty stiff. Their armour may have also acted like an exoskeleton, providing extra resistance to the trunk muscles and protecting their internal organs. With such success during the Devonian, what do you think happened to these armoured fishes? Placoderms diversify spectacularly during the Devonian, but didn't survive the second of the big five mass extinctions, which took place at the end of the Devonian. There are a number of hypotheses as to why placoderms didn't survive. A commonly stated hypothesis is that they were out competed by sharks. But sharks didn't really diversify until after placoderms died out. Placoderms may have been simply at a disadvantage, because they were so specialized. Placoderms had become so diverse that each group occupied a very specific niche and when conditions changed at the end of the Devonian, it's possible that they just couldn't adapt quickly enough. Their relatives the more derived gnathostomes had lived in the shadows of the placoderms through the Late Silurian and the Devonian, and would radiate into the niches left empty after their extinction. The more derived gnathostomes are eugnathostomes, and they're made up of the cartilaginous fishes which include sharks, rays, and their relatives. It also includes the bony fishes, like your pet gold fish, lung fish, and all tetrapods, including you. This is the next major played in our evolutionary tree. We've explored Chordata, Vertebrata, and now Eugnathostomata. So, what makes a eugnathostome a eugnathostome? There are few things that we share with sharks and not placoderms. For one thing, our jaw muscles are lateral to the skeleton of our jaws. In placoderms, the palatoquadrate cartilage attached to the skull roof and the jaw muscles pass medially to it, internal to the jaw skeleton. Eugnathostome palatoquadrate cartilages are curved inwards, not outwards, and the jaw muscles pass laterally on the outside. The eugnathostomes also have true teeth. Recently, some researchers have suggested that some derived placoderms had teeth. But, they didn't grow the same way that teeth in Chondrichthyes and Osteichthyes grow and they were not replaced like our teeth are. Whether these teeth in placoderms are homologous with crown gnathostome teeth is still something that's being debated, and it's a bit beyond the focus of this course. Extant eugnathastomes, at least, replace their teeth in a way that placoderms didn't. Let's take a quick look at some of the branches of the eugnathostome tree before we focus on our own lineage in the next lesson. Some of the earliest eugnathostomes to appear in the fossil record are the acanthodians. And acanthodians were small torpedo shaped fishes with big eyes and spines on the leading edges of all their fins, except the caudal fin. Their scales and spines have been found from the Early Silurian, maybe even the Ordovician. They were also a very widespread group of fishes, and they're found all over the world, up until the Permian. Acanthodians were around for 150 million years. To put that in perspective, that's more than twice the amount of time that separates us from the end Cretaceus extinction event. Our own genus, <i>Homo</i>, has only been around for about two and a half million years. If we can persist for another 148 million, we're in good shape. There have been quite a few recent phylogenetic studies featuring acanthodians, mostly because we don't know quite where they fit in. They share features with both living gnathostome groups: the chondrichthyans, and osteichthyans. At first, they were thought of as spiny sharks. And then, for a long time, they were considered to be the sister group to the bony fishes. Nowadays, there are a couple of competing hypotheses. Acanthodians could all be stem chondrichthyans, or they could be split up on the chondrichthyan, osteichthyan, and even the gnathostome stems. Although, we're not really sure where they fit in the gnathostome family tree, acanthodians from MOTH have helped us to better understand some interesting evolutionary questions. One of the reasons MOTH is extraordinary is because of its preservation of acanthodian and early chondrichthyan fossils. These little fishes had cartilaginous endoskeletons like modern sharks and cartilage doesn't preserve as well as bone, because of this, typical acanthodian fossils are disarticulated, scales, fin spines, or bony jaw elements. The hard parts, but not usually the whole fish. At MOTH, whole fishes have been found. They're rare, but not as rare as elsewhere in the world. And some of these fishes have interesting structures we don't see anywhere else, like tooth-like scales. Acanthodians were one of the oldest vertebrate groups to have teeth. Because of their age and position on the phylogenetic tree, it's possible that they represent some of the primitive conditions of toothed gnathostomes. One of those conditions seems to be strange tooth-like scales around the mouths of some acanthodians from MOTH. These structures were the first fossil intermediates between scales and teeth and they help support the hypothesis that teeth are homologous with scales of the living crown group gnathostomes. We're members of the Osteichthyes, but the Chondrichthyes are our closest living, non-bony fish cousins. Chondrichthyes includes all sharks, rays, and chimaeras. It's a hugely diverse and successful group, and we could spend a whole course talking just about the sharks. For this lesson though, we'll give them a brief, honorable mention on our way to bony fishes. Chondrichthyes is united by a single unambiguous synapomorphy: their cartilage calcifies in prisms. Instead of being replaced by bone, the cartilage in chondrichthyans calcifies with chains of tiny apatite crystals covering the surface of cartilage linked together by collagen. Because chondrichthyan endoskeletons were cartilaginous, they also have a pretty poor fossil record. But we have isolated scales that are probably chondrichthyan from the Silurian. It's not until the Devonian that we find numerous articulated shark fossils and the best known of these is <i>Cladoselache</i>. <i>Cladoselache</i> lived in the water column above what is now the Cleveland Shale, along with <i>Dunkleosteus</i>. It was probably food for the big arthrodire, being much smaller. It was about two meters. The <i>Dunkleosteus</i> is eight meters. The fossils of <i>Cladoselache</i> from the Cleveland shale are spectacularly preserved, including traces of skin, muscle fibers and even internal organs in some cases. This early shark had very few scales, big wing like pectoral fins and a streamlined body, and a deep crescent shaped tail fin with powerful lateral keels. <i>Cladoselache</i> is one of the best known early chondrichthyans, but it wasn't primitive. This was a fast swimming predator, adapted for speed. Fossils of <i>Cladoselache</i> had been found with fish, that it had eaten tail first, meaning, it caught the prey on the run, so to speak. Chondrichthyans would undergo a major radiation after the extinction of the placoderms and become incredibly diverse. There are over 1,000 living species of chondrichthyans, and they still occupy pretty much every marine environment in the world and some are found in fresh water too. They're a fascinating group of animals. But for our next lesson we'll return to our own roots, in the bony fishes. The next step in vertebrate evolution would be a literal one. The evolution of the vertebrate limb, and the fish tetrapod transition.
