This is a very successful group of animals, not the most diverse, not the most species, not the most abundant, but the largest animals that are dominant predators, dominant herbivores in virtually every environment. So this is the last group of animals uh we’re, going to learn about chordate features and the groups of chordate animals. These guys are deuterostomes they’re related to the echinoderms they share. 18S ribosomal rna sequences. There are fossil evidence that show that the chordates are all related. A nice sequence of when the organisms first appeared in the fossil record and intermediates between existing groups, and we can link them to the echinoderms and to each other by looking at their anatomy and studying of their embryos. And in fact, all chordates show four features at some point in their life cycle. Some of them show these features as really common in and prominent in the adult organism, but for many of them it’s in the embryo, and these features may disappear in the adult animal. So what are these features? There’S, four or five, depending on how you go? Chordates all have pharyngeal, slits or gills used in fishes for feeding or respiration. They have a structural rod. The notochord that’s inside the body that’s made up of pulpy tissue enclosed in a fibrous capsule it’s, replaced in vertebrates later by bone, bony tissue of cartilage or or a bone that vertebrae or cartilage. They have a nervous system. That’S dorsal most uh protostomes have a ventral nervous system, and not only is it dorsal it’s hollow so it’s, this tubular nerve, cord and the anterior end may be developed into a brain, and then they have a post anal tail.

That includes some skeletal materials, the notochord or other materials in the uh in the region, past the anus and they they may also show an an endostyle it’s, a a structure that that develops into the thyroid gland in many vertebrates. But it is involved in feeding and producing some hormones in early chordates, so these chordates have these features at some point in their life cycle. Okay, they may also be distinguished. There are other characters that distinguish the chordates. They have a really well developed muscle blocks that are segmented called somites, and so this appears in them and most of them use these muscles against their notochord or their vertebrae to produce movement. This turns out to be a really nice plant and it’s. Seen in lots of different animals, the earliest fossils, the earliest chordates, look very much like larval fish, and they also resemble kind of the the uh animal here on the upper right. That’S, a cephalochordate amphioxus where they have a nice stiff, notochord, a big basket, pharyngeal, slits or gills – that they use to feed an endostyle that secretes mucus for trapping particles, and then they have the dorsal hollow nerve, tube and the post anal tail. Okay. So we see these other groups and they show up very early in the cambrian period and we can have nice fossils of them. There are still living examples of cephalochordates and urochordates here’s, a a common urochordate. These are sometimes called sea, squirts or tunicates.

They really don’t. Look much like a chordate as an adult it’s, just a basket with an incurrent and an x current siphon, and so it’s got a big gill basket in between, but it’s got a full digestive tract and a fairly complex anatomy. But no notochord as an adult kind of hard to even find the nervous system here, no post anal tail. But if you look at the larvae, the tunicate larvae looks just like what it’s supposed to look like there’s a notochord in the tail. There are gill, slits there’s, a nerve tube, and so it looks exactly like it’s supposed to um here’s a diagrammatic picture of that okay. So so this looks very much like it’s supposed to, and these show up early in the fossil record, probably the best example. We see of a simple chordate are the cephalochordates amphioxus here’s, another view of that um they’re again: filter feed, earrings, they’re they’re, taking in a lot of water in their mouth, straining out the particles and letting the water pass out through these slits and then through this Atrial pore that’s over in this region, here uh very, very nice, developed larvae, our organism and capable of swimming and burying itself in the sand and doing what it needs to do. Okay. Well, these are small groups with several hundred, maybe several thousand species, but the big group of chordates are the vertebrates. This is a subphylum with probably a hundred thousand species or more it has a notochord in the larvae that is usually replaced by vertebrae in the adult.

They develop an internal skeleton of cartilage or bone. They have for uh, develop a well developed head with a brain in the anterior region and have well developed organs. The first of these guys to appear in the fossil record appear very early. They are the agnetha or the jawless fishes they had bone and bony plates, but they they lacked jaws, like closing jaws for like biting that you find in all other vertebrates. They had a round sucking mouth and there are still some of these left. The lampreys and hagfishes and you can see in the center bottom there instead of having jaws, they had these rasping plates in their mouth and they can use that to feed. And so these are parasites of fishes or kind of scavengers that are are able to effectively live in the deep ocean. So we still have several thousand species of these and again appeared very early um and some of them had bone. Although most of these are cartilaginous. Now, even as adults, they maintain a notochord, they have lots of gill, slits um, but they’re very simple. They lacked paired appendages, apparently for some of them um and they they have um. You know they don’t have a full cranium. They have just a kind of a plate: okay by the devonian period somewhere around 400 million years ago. We see more advanced fishes like the chondrichthyes. These are the cartilaginous fishes sharks, skates rays, rat fishes. They have active jaws and a cartilaginous skeleton, with paired appendages.

Very good predators, very advanced fish highly you know, have highly adaptive behaviors some really well developed organs, internal fertilization with with live birth and some of these sharks, and so so can be quite advanced thousands of species, including some very dominant predators that have been around for Millions and millions of years some of the the largest fishes are cartilaginous, fishes the whale sharks and basking sharks. So quite big, not primitive, well developed. Having really good features all right, then there are bony fishes. They have true bone uh bone. Is nice it’s a really durable material but it’s heavy and the cartilage or the bony fishes uh one group, the actinopterygia? Not only do they have a bony skeleton, they have bony scales, so they they have these these heavy materials here and so they’ve also developed a swim bladder, an internal kind of cavity which they secrete air into that can regulate their buoyancy. The actinopterygie are the most uh diverse group of fishes. These are the ray finned fishes. They have these bony rays in their fins that are strong but very flexible and allow them to fold their fins back when they want to swim fast or fan them out when they want to do to control themselves. They appeared at almost the same time as the cartilaginous fishes. They include a really diverse set of fishes. Some lose a swim bladder. Some have it well developed some swim incredibly fast and are nearly warm blooded.

They maintain muscle temperatures that are almost as high as some of the land animals, even though they’re swimming in cold waters so really well developed sets of groups very, very diverse species, wise they’re dominant in the oceans. They are the dominant group and they’re large, not as large as the cartilaginous fishes, but still larger than any of the invertebrates. At this point, one group of bony fishes the sarcopterygii have referred to as a lobe finned fishes because they have fins with central bony elements and are quite muscular. These guys also have swim, bladders and a perculum that’s, a gill cover a bony gun cover and they have an extensive fossil record they’re large, but not as diverse as the other bony fishes, but we’re we’re fairly sure that these fishes some of these fishes gave rise To all land animals land vertebrates, the tetrapods there there are some lung fishes – still existence the dip noise that have their fins placed on the bottom of the body and can be used for walking. They have a lung that’s used to breathe uh oxygen in in from the air in because they occur in shallow low oxygenated waters and they even have mary’s. They show lots of features that link them to the amphibians and they were in the right place at the right time so that we we know amphibians develop from the sarcopterygii and amphibians means you know partly in land, partly in water. These guys have good legs, but they show a lot of features that are are clearly related to fish.

They breathe through their skin and mouth as well as lungs. They have a heart. That’S shows that their circulatory system is kind of an adaptation of what the fish had they their reproduction is tied to water. They have external fertilization, they produce larval form that are very fish like that show the basic chordate features and then go through a metamorphosis where they will lose some of those features, develop legs and then exist on the land. We know that the amphibians have linkages to fish and then have linkages to the reptiles. Some groups of these a cladogram shows that all the other land vertebrates are in the same clade and that clade is based upon the development of an egg that allows the rest of the tetrapods to take their reproduction away from water so that they can lay their Eggs on land and the larvae can develop within the egg or the egg can be retained, and so these are the amniotes and they’re a monophyletic group of vertebrates. They include the mammals, several groups of reptiles, including turtles, lizards, uh, the sphenodons um, the crocodilians and then the birds. When we talk about chordates, it gets a little harder because the chordate groups that we know the general term reptile is is probably not a very good. Phylogenetic group, all living reptiles, have amniotic eggs, do some sort of thoracic breathing, which is different from amphibians, which means they they generate negative pressure within their thorax to kind of suck air in uh amphibians.

Just swallow air with their mouth. The reptiles have impermeable skins with bony plates and keratin keratinized scales, okay, but they represent distinct groups and shown here the lizards which includes lizards and snakes, they’re crocodilians, the phenodons, a very, very kind of restricted group. And then the turtles reptiles were more abundant and were more diverse in prior to the cretaceous extinction. There’S, a big group of them, the archosaurs, which includes modern birds and crocodiles and alligators, have more advanced features and other reptiles. Their hearts are divided so that their circulatory system is efficiently can use the lung. They have hollow bones and and air sacs that allow them to circulate air one way through their lung. They have parental care and are basically endothermic and which maintains. That means they maintain an internal temperature that’s high and warm all the time. Okay. The last group i want to talk about are the mammals that’s us. We have hair and again maintain a warm internal body temperature. We also have internal development, we’ve modified the placenta and the extra embryonic membranes to to support the embryo developing to a small version of the adult, and then we nourish it after birth with mammary glands, and we have teeth adaptations that set us apart. So in summary, chordate body plan, pharyngeal, gills or slits, a notochord, a dorsal tubular nerve cord and a post anal tail.

https://www.youtube.com/watch?v=Kl8HbcfOoj4