Systematic Classification of Life ep10 - Osteichthyes

by: AronRa

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[Music] the last episode of this series started with early vertebrates and explored the subsequent evolution of their jaws and also of paired pectoral fins this development covered the entire Ordovician period from the end of the Cambrian to the early Silurian a span of nearly 45 million years ending almost 444 million years ago we looked at some of our closest cousins among those early vertebrates of the ordovician and silurian but we didn't show much to their environment at that time which we should because it wasn't just that the oceans were full of weird fish where none of them look like anything were used to there were jellyfish starfish corals and crustaceans but not a lot else that we'd find familiar they were mollusks than - including the biggest animal yet known from that time the 20-foot long 300-pound or thesaurus our then jawless ancestors were not at the top of that food chain this was a time of ammonites trilobite since grapdelites and some of the top predators were Europe turrets monstrous scorpion like arthropods some as big as alligators it was a different world as you can say apart from North America Western Europe and northern Europe seen here's three separate land masses everything else was merged together into a super continent called Gondwana land and the sea was higher than higher than it's ever been because there was no ice anywhere so most of the land was underwater which is fine because nothing yet lived on land either plants nor animals and there wouldn't have been enough oxygen in the air yet anyway instead there was well more than ten times the carbon dioxide level we have now and the climate was consequently an average of 7 degrees Celsius hotter than today imagine summer temperatures of a hundred and thirty degrees Fahrenheit then something significant happened a substantial percentage of co2 was pulled out of the atmosphere a number of hypotheses have been suggested for the catalyst including a blast of cosmic rays from a supernova and one study of strontium isotopes in the geologic record indicates that co2 was taken up through rock weathering which was high then at the same time that volcanic activity and solar output were both apparently low with mountain ranges rising out of the sea absorption into the newly exposed rock could have taken co2 levels 3000 parts per million and this drop in greenhouse gas evidently initiated an ice age lasting a half a million years with so much water now trapped in mile thick glaciers on land sea levels fell by two hundred and sixty feet or roughly 80 meters exposing nearly all of the world's coral reefs and vast regions of each continental shelf where most everything lives the dramatic drop in both sea level and global temperatures brought on the first major extinction event extinctions happen all the time any number of species occasionally die out and due course for whatever reason there are noticeable extinctions at the end of every geologic period that's how we tell them apart but this was a mass extinction there have been five such major extinction level events in geologic history where more than three-quarters of all animal life was killed off relatively quickly we are currently living in and bringing about a sixth such event the Anthropocene extinction where deforestation pollution overconsumption and anthropogenic climate change are killing everything at an alarming rate the same sort of thing happened albeit for natural reasons much slower and in Reverse at the end of the Ordovician such that 85% of all animal species were extinct by the beginning of the Silurian period throughout the ordovician and silurian and even into the Devonian they were quite a lot of armor-plated fish and many of the new lineage with jaws were still armor-plated these were known as placa derms but there was an arms race going on Ordovician fish were sensibly armored in order to fend off the different species of europe druids and ammonites around at that time but now that fish had biting jaws they turned the tables on their attackers of course most fish feed on other fish Dunkleosteus was a 30 foot long giant plaque Edurne from the Devonian with a set of choppers that could bite through anything it didn't have teeth but rather bony shearing plates like a huge industrial can opener and it had the most powerful bite of any animal that has ever lived before or since so no amount of armor was going to protect you from this thing but even as brutally powerful as this living tank was it was slow moving like a tank - and not very maneuverable so the best defense was to drop the encumbrance of all that heavy armor in favor of speed and agility to be harder to catch having pectoral fins and reducing the armor to a thin layer of bony scales gave the prototype for all modern fish a new and better design which brings us to you Nathan sumata the true jaws the pelvic fins can occur as hawks duplications of the pectoral fin which appears to be what happened in these two groups and contou diems went all out and produced a few more notice also what happened to the gills the previous fish that we showed in the last video had a series of gill openings like sharks have usually four of them but n cathode Ian's were more like modern fish in that they have therefore gill filaments concealed under a large Gill cover so it looks like they have one Gill instead of for a kentuckians are also known as spiny sharks because they're generally shark shaped and have cartilaginous skeletons except for bony spines on the leading edge of their fins likewise the earliest sharks still had Rea fins led by bony spine so the relationship is evidently a close one later on of course sharks eventually lost those spines and their refunds were covered over with skin the Sharks belong to the clade kontrick teeth having cartilaginous skeletons except for their jaws which have calcified into bone all the fish we've seen so far had skeletons made of cartilage but at this point we see the emergence of Ostia these bony fish also known as you teleost owe me with higher percentages of calcium being integrated into their skeletons further back than just their jaws and some fish like salmon for example have very little calcium and their so-called bones the same goes for a few of the more carry a typically primitive fish like this Pullip Taurus which also demonstrates another trait that distinguishes bony fish from chondrichthyes Ostia keys or you teleosts have an internal membrane called a buoyancy bladder it's just a sac along the inside of the ribs that they can inflate by taking gulps of air and this helps them adjust ballast to stay with it in a custom range of depth jealous fish were pretty much doomed to be bottom feeders and didn't need anything like this but it was a useful adaptation for biting fish on the hunt and it served another purpose too even the simplest versions of this membrane absorb oxygen which meant that some of these fish could breathe if they had like if they found themselves in warm stagnant oxygen depleted water and some of the most primitive type fish also grew a distension of that bladder that was more heavily ciliated on the inside absorbing even more oxygen and more heavily Vaska lated on the outside to circulate that oxygen throughout the body so even the earliest bony fish had lungs you know those will come in handy in the next videos so even if you'd rather not admit to having evolved you understand what it means taxonomically to have a cranium and vertebrae and a jaw all made out of bone right so use your lungs to take a deep breath and sigh and accept your classification according to all those criteria then move on to the next video because there's big changes coming and steps need to be taken to prepare



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Recapping the Ordovician, and moving on to the Silurian and Devonian. Here is a link to the entire series playlist. https://www.youtube.com/playlist?list=PLXJ4dsU0oGMLnubJLPuw0dzD0AvAHAotW

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