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Chordates (Phylum Chordata)

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A short story Leathem Mehaffey, III, Fall 201993 The First Chordates (Phylum Chordata) • Chordates (our phylum) first appeared in the Cambrian, 525MYA. 94 Invertebrates, Chordates and Vertebrates • Invertebrates are all animals not chordates • Generally invertebrates, if they have hearts, have dorsal hearts; if they have a nervous system it is usually ventral. • All vertebrates are chordates, but not all chordates are vertebrates. • Chordates: • Dorsal notochord • Dorsal nerve chord • Ventral heart • Post-anal tail • Vertebrates: Amphioxus: archetypal chordate • Dorsal spinal column (articulated) and skeleton 95 Origin of the Chordates 96 Haikouichthys Myllokunmingia Note the rounded extension to Possibly the oldest the head bearing sensory vertebrate: showed gill organs bars and primitive vertebral elements Early and primitive agnathan vertebrates of the Early Cambrian (530MYA) Pikaia Note: these organisms were less Primitive chordate, than an inch long. similar to Amphioxus 97 The Cambrian/Ordovician Extinction • Somewhere around 488 million years ago something happened to cause a change in the fauna of the earth, heralding the beginning of the Ordovician Period. • Rather than one catastrophe, the late-Cambrian extinction seems to be a series of smaller extinction events. • Historically the change in fauna (mostly trilobites as the index species) was thought to be due to excessive warmth and low oxygen. • But some current findings point to an oxygen spike due perhaps to continental drift into the tropics, driving rapid speciation and consequent replacement of old with new organisms. 98 Welcome to the Ordovician YOU ARE HERE 99 The Ordovician Sea, 488 million years 100 ago The Ordovician Period lasted almost 45 million years, from 489 to 444 MYA. During this period, the area north of the tropics was almost entirely ocean, and most of the world's land was collected into the southern supercontinent Gondwana. • Initially very warm (ocean temperature 42oC [108oF]; slowly cooled, until climate was reasonable by the middle Ordovician. Temperature then rapidly declined, leading to intense glaciation and sea-level fall. • Oxygen levels were around 17%*, CO2 about 15X today’s value in the beginning. • Ended around 444MYA with a mass extinction that killed 85% of living species (the second most severe of all The Ordovician Climate extinction events!). *Equivalent to about 5,000 feet today 101 • The early Ordovician climate was warm and wet. Shallow seas covered most of the continents. • During this period invertebrates diversified • Coral reefs appeared (though the corals were tabulate corals, not the modern scleractinian corals) • Mollusks became apex predators of the oceans, including the cephalopods (nautiloids), clams and snails. The • The first land plants appeared. Ordovician • Arthropods became the first animals to invade the new habitat: land. Period • The first vertebrates (early fish) appeared. Orthocerus 102 Invertebrates of the Ordovician 103 Eurypterid The Ordovician/Silurian Extinction: 104 Causes No-one knows the exact cause. Here are some generally accepted hypotheses: • Glaciation: Gondwana had drifted over the south pole, thus cooling and becoming glaciated. Sea levels dropped, eliminating many shallow sea niches. Oxygen levels fell as photosynthesis declined. • Gamma-ray burst originating from a hypernova within 6,000 light-years of Earth. A ten-second burst would have immediately destroyed half of the atmospheric ozone, exposing surface-dwelling organisms, including those responsible for photosynthesis, to extreme ultraviolet radiation. • Cooling due to CO2 removal through weathering, particularly in the recently uplifted Appalachian mountains, leading to a short but intense ice age. • Heavy Metal poisoning. Toxic metals on the ocean floor may have dissolved into the water when the oceans' oxygen was depleted. * *Vandenbroucke, Thijs R. A.; Emsbo, Poul; Munnecke, Axel; et al. Metal-induced malformations in early Palaeozoic plankton are harbingers of mass extinction. NATURE COMMUNICATIONS 6 (7966) 2015 The Silurian Period 105 The Silurian period followed the Ordovician Extinction. It began around 440 million years ago and lasted until 419 MYA, making it the shortest period of the Paleozoic Era. There were as many as four glaciation events in the early Silurian, but after that it was warm and tropical. During this period vascular land plants arose as did the first Florida agnathan vertebrate fish and precursors of sharks. Plants of the Silurian 106 Rhyniophytes Cooksonia date from the middle of the Silurian until the end of the Early Devonian. The Silurian saw the emergence of vascular land plants. They are considered to have arisen in fresh-water, where algae developed spores to allow pool-to-pool dispersion. Most were only a few centimeters high and confined to wet areas. Invertebrates of the Silurian 107 Tabulate corals formed extensive reefs. Eurypterids Invasion of the 108 Land The first terrestrial animals were arthropods: centipedes, millipedes and scorpions (arachnids). While land plants provided food, the main problems to be overcome were desiccation and gravity. Arthropod 109 Adaptations to Land Desiccation: chitinous exoskeleton Respiration: book lungs Mobility: jointed legs Vertebrates of the Silurian 110 By the end of the Cambrian fish had arisen. They were jawless fish called conodonts. As the Ordovician progressed, heavily armored fish called placoderms appeared. Some placoderms reached 30 feet long. By the Silurian fish had developed jaws. Evolution of 111 Jaws By the Silurian, some 440 MYA, the early gill arches of fish had evolved into jaws, a major advance in predation. These gill arches today form your middle ear bones. The Devonian Period 112 Lasted from 416 to 358 MYA Began with a high oxygen peak; ended with a low of 12% (which then rose during the Carboniferous to greater than 30%) Most of the land mass was in the supercontinent Gondwana in the southern hemisphere, with Euramerica equatorial. Known as “The Age of Fish” due to a great diversification of fish. Bony fish (teleosts) with swim bladders and fins arose, and cartilaginous fish (sharks and rays) became common. By the end of the Devonian vertebrates appeared on land. The first fossil footprints of a terrestrial tetrapod date from 400 MYA. Plants of the 113 Devonian Plants continued to make evolutionary progress during the Devonian. Lycophytes, horsetails and ferns grew to large sizes and formed Earth’s first forests. By the end of the Devonian, progymnosperms were the first successful trees, growing up to 98 feet tall with a trunk diameter of more than 3 feet. They did not have true leaves but fern-like structures connected directly to the branches. There is evidence that they were deciduous, as the most common fossils are shed branches. Reproduction was by male and female spores that are accepted as being the precursors to seed-bearing plants. In the Devonian fish diversified into two groups: 114 lobe-finned (Sarcopterygii) and ray-finned (Actinopterygii). The Sarcopterygii gave rise to the Ripidistia, the ancestors of all tetrapods and lungfish (Dipnoi). Trout, an actinopterygian (Crossopterygii = Sarcopterygii) Latimeria, a crossopterygian living fossil Tetrapods arose from crossopterygians 115 Dipnoi (lungfish): Crossopterygian 116 relatives of terrestrial tetrapods Dipnoi appeared in the late Devonian, and some persist to today. The presence of lungs and fleshy fins with bones gave them the ability to move on land. They are freshwater fish, implying that the route from sea to land went through fresh water. Transition to 117 land: Tiktaalik Tiktaalik dates from about 375 MYA (late Devonian). It is highly significant because it shares so many fish and tetrapod characteristics. It had gills and scales and a more robust skeleton (rib cage) but could not support its weight on land. It probably had primitive lungs and gulped air when the shallows warmed and oxygen concentration dropped. Invasion of the 118 land By the late Devonian vertebrates appeared on land, probably facilitated by a rise in oxygen. The earliest fossils were Ichthyostega and Acanthostega. They were still rather fish- like. Although they had lungs, they also had gills for use in the water..
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