Chordates - CrashCourse Biology #24

Key Takeaways

next time someone ask you who you think you are just given the facts you're a mamalian amniotic tetral Copan ostean naal vertebra cranial cordate yeah it's a mouthful and in order to understand what it means you're going to have to understand the most complex group of animals on Earth and what it takes to get from this to [Music] this the phm cord dat accounts for all 52,000 species of vertebrates on Earth and several thousand species of invertebrates together they range from Tiny brainless filter feeders all the way up to Scarlet Johansson now you know by now that when we talk about classifying animals we're really talking about their shared ancestry each new branch on this tree marking an important new evolutionary Milestone and just like with tissue layers and segmentation and simpler animals there are traits we can look for to track the evolution of cord dates by the time all of those traits appear in one organism will have arrived at the most complex class within the most complex film the mammals but first let's start with the fundamentals we've talked before about synapomorphic traits traits that set a group of animals apart from its ancestors and from other groups that came from the same ancestors Cates share four synapomorphies that make us who we are each of them is present at some point in every cordat life cycle how about a volunteer to demonstrate these traits ah I see that the lancelets are raising their mouth Parts the lancelets also known as seil Cordata literally head cords are one of the three sub Fila of cordat and unlike almost all other cordat these tiny brainless invertebrate filter feeders retain all four of these characteristics for their entire lives you probably already know where most of these traits are going to appear since the philm is named after it the spinal cord or at least something that resembles a spinal cord first there's the Nota cord a structure made of cartilage that runs between an animal's digestive tube and its nerve cord in most vertebrates a skeleton develops around the not cord and allows the muscles to attach in humans the not cord is reduced to the discs of cartilage that we have between our vertebrae second we have the nerve cord itself called the dorsal Hollow nerve cord a tube made of nerve fibers that develops into the central nervous system this is what makes cordat different from other animal Fila which have solid ventral nerve cords meaning they run along the front or stomach side third all chordates have Fair andal slits in invertebrates like the lancelet here they function as filters for feeding in fish no other Aquatic animals they're gill slits and in land dwelling vertebrates like us they disappear before we're born but that tissue develops into areas around our Jaws ears and other structures in the head and neck and finally we can't forget our fourth synapomorphy the post anal tail which is exactly what it sounds like it helps Propel aquatic animals through the water makes our dog look happy when she Wags it and in humans it shrinks during embryonic development into what is known as the coxic or tailbone it's right here and trust me when it comes to tail placement post anal is the way to go these four traits all began to appear during the Cambrian explosion more than 500 million years ago and today they're shared by members of all three cordate sub Fila even if the animals in those sub Fila look pretty much nothing like each other for instance our new friends here in seilo Cordata are the oldest living sub film but you can't forget the other invertebrate group of cordat the UR cordada literally tail cords there are over 2,000 species here including SE Squirts and if you're confused about why this ended up in a filum with us it's because they have tadpole like larvae with all four cordate characteristics the adults which actually have a highly developed internal structure with a heart and other organs retain the fenal slits but all the other cordate features disappear or reform into other structures the third and last and most complex sub filum is the vertebrata and has the most species in it because its members have a hard backbone which is allowed for an explosion in diversity from Tiny minnows to the great blue whale you can see how fantastic this diversity really is when you break down vertebrata into its many many classes from slimy sea snaky things to us warm and fuzzy mammals and as these classes become more complex you can identify the traits they each developed that gave them an evolutionary Edge over the ones that came before for example how's this for an awesome trait a brain vertebrate with a head that contains sensory organs and a brain are called craniates they also always have a heart with at least two Chambers so since this is science you got to have to know that there's going to be an exception for every rule that you're going to have to remember and the exception in this case is the mixy or hagfish the only vertebrate class that has no vertebra but is classified with us because it has a skull this snake likee creature swims by using segmented muscles to exert Force against its notic cord whatever hagfish closely related to it is the class petrona otherwise known as lamp Rays the oldest living lineage of vertebrates now these have a backbone made of cartilage and maybe even more important a more complex nervous system with the Advent of a backbone reer it's getting larger developing more complex skeletons and becoming more effective at catching food and avoiding Predators but did you notice anything missing lampre and other early vertebrates are agnathans literally no jaws and if you want to be able to chew food it really helps to have a jaw in teeth most scientists think that the jaw evolved from structures that supported the first two fenal slits near the mouth and teeth well the current theory is that they evolved from sharp scales on the face Naes or jaw mouths arrived on the scene 470 million years ago and one of the oldest and most successful groups of Naes that have survived to the present day are the class kries the cartilage fish you know them as the Sharks and skates and rays and as their name says their skeleton is made up mostly of cartilage but they show the beginnings of a calcified skeleton conans haven't changed much over the past 300 million years or so and their success stems from the paired fins that allow for efficient and swimming and those jaws for biting off delicious hunks of Flesh if we're going to eventually get to the mammals we need bones and we find those with the evolution of fish meet ekes which technically means the bony fish unlike cartilagenous fish members of this group have a mineralized endoskeleton now ekes is sometimes considered a super class because it includes a whole slew of diverse classes that descended from it there's actually some controversy among taxonomists about what to call it the main thing to know is that the majority of all vertebrates fall under ostes and that includes you it's broken up into two main groups which themselves include a bunch of classes the first is the actinopteri or rined fishes and with 27,000 species pretty much every fish you've ever heard of is in this group Rin fishes evolved in fresh water spread out into the oceans and some eventually then came back to fresh water in the second group things start to get really strange and interesting these are the lob finned fishes or the sarop G and name derived from Bones surrounded by muscle found in their pectoral and pelvic fins and that sounds like something that could be used for walking lob fins include the celant which consist of one living species lung fish which gulp air into their lungs and tetr pods which have adapted to land with four limbs this is weird right even though land animals clearly are not fish since tetr pods evolved from bony fish they are filed under this group these tonomous man I want to party with them sometimes but first imagine that you're a fisherman off the coast of South Africa and the Western Indian Ocean about 75 years ago just put that in your brain and hold on to it and you've just pulled up a fish that no one has ever seen not only that you've caught a fish that was thought to have become extinct 75 million years ago this is exactly what happened in 1938 when Captain hendrik gusen hauled up a cican and it has mystified scientists ever since a second population has since been identified near Indonesia in 1999 but the deep sea creatures remain extremely rare the cicin Fascinate scientists because of its paired lobe fins they extend from the body like legs and move in an alternating pattern in other words they move more like a horse than like a fish and in fact those paired fins are supported by the very same bones that we have in our arms and legs the cican also has a hinged joint in the skull so it can widen it mouth to eat large prey as well as thick scales that don't exist on any living fish it's not good eatting but why would you want to eat what's essentially a living fossil all right and we're talking about tetr pods which of course means four feet and getting those four feet onto land was really awesome for those early creatures because that meant that they could escape the increasingly brutal and predatory world of the ocean tetr pods gradually replace their fins with Limbs and developed entirely new body parts that were never seen before like necks with the help of of additional vertebrae that separated the body from the head the first tetr pods are today found in the class amphibia which were the first creatures to develop a three- chambered heart there are more than 6,000 known species of amphibians like frogs and salamanders most of which begin life as tadpoles in water and then later develop legs and lungs in a digestive system and often migrate to land for adulthood but amphibians lay eggs that don't have shells so they dehydrate quickly so they have to be laid in water so this leads us to our next evolutionary milestone for the cord dates the amniotic egg amniote are tetrapods that have eggs adapted for life on land a group that includes reptiles birds and mammals the amniotic egg was crucial for the success of land dwellers allowing embryos to develop in their own private pond of the amniotic sac often surrounded by a hard shell in the case of reptiles and birds the class reptilia represents the earliest amniotes like amphibians they have a three-chambered heart but they're totally terrestrial and here's where we find our dinosaurs and our snakes and turtles and lizards you haven't hear reptiles described as cold blooded this does not mean that their blood is cold they're actually ectothermic which means that they absorb external heat as their main source of body heat hence the lizard that likes to lay in the sun all day the oldest group of reptiles the archosaurs mostly disappeared when most of the dinosaurs died out 65 million years ago but two lineages of argosaurus survived one includes the modern reptiles crocodiles and alligators and the other is the type of dinosaur that we now call Birds the class AES there are big obvious differences between these two surviving argosaurus one is designed for for like eating and fighting big animals while the other is designed for flying around and being graceful and stuff the not so obvious but equally important differences that birds are endotherms which means that they can crank up their metabolism to regulate their body temperature making all that heat requires big furnace which is provided thanks to the evolution of a four chambered heart there's only one other group of animals that developed this trait independently of birds by the way and it allowed them to spread through the planet and I'm talking of course about the class mamalia otherwise known as amniotes that have hair three special ear bones and mamory glands and most mammals have evolved to dispense with a hard egg shell altogether the embryo avoiding predation and other environmental dangers by developing inside the mother's body in this class of cord dates you'll find me Dame Judy Dench your dog your cat Shamu the Orca African elephants the South American Pudu and 5,300 other known species of mammals it all began with a simple ancestor more than 500 million years ago in this crazy cordate film we finally made it and now you know exactly who you are thanks for watching this episode of Crash Course biology we hope that it was helpful and that you feel like a smarter person there's review stuff next to me that you can click on go to those parts of the videos that you maybe want to watch a little bit more of reinforce it in your brain thanks to everybody who helped put this together and if you have any questions for us you can get in touch with us on Facebook or Twitter or of course down in the comments below goodbye e