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Home , Haikouichthys, Living fossil, Myllokunmingia, Ordovician, Silurian

A short story

Leathem Mehaffey, III, Fall 201993 The First ( Chordata)

• Chordates (our phylum) first appeared in the , 525MYA.

94 , Chordates and

• Invertebrates are all not chordates • Generally invertebrates, if they have hearts, have dorsal hearts; if they have a nervous it is usually ventral. • All vertebrates are chordates, but not all chordates are vertebrates. • Chordates: • Dorsal • Dorsal nerve chord • Ventral heart • Post-anal tail • Vertebrates: Amphioxus: archetypal • Dorsal spinal column (articulated) and skeleton

95 Origin of the Chordates

96 Note the rounded extension to Possibly the oldest the head bearing sensory : 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/

• Somewhere around 488 million ago something happened to cause a change in the of the , 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 as the index ) was thought to be due to excessive warmth and low oxygen. • But some current findings point to an oxygen spike due perhaps to into the tropics, driving rapid 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 . • 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 • reefs appeared (though the were tabulate corals, not the modern scleractinian corals) • Mollusks became apex predators of the oceans, including the (), clams and snails. The • The first land appeared. Ordovician • became the first animals to invade the new habitat: land. Period • The first vertebrates (early ) appeared.

Orthocerus

102 Invertebrates of the Ordovician 103

Eurypterid The Ordovician/ 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 .

• Cooling due to CO2 removal through weathering, particularly in the recently uplifted , leading to a short but intense . • 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 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 . 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 . Plants of the Silurian 106

Rhyniophytes date from the middle of the Silurian until the end of the Early .

The Silurian saw the emergence of vascular land plants. They are considered to have arisen in fresh-water, where algae developed 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: , and ().  While land plants provided food, the main problems to be overcome were desiccation and gravity. 109 Adaptations to Land

 Desiccation: chitinous  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 . As the Ordovician progressed, heavily armored fish called placoderms appeared. Some placoderms reached 30 feet long. By the Silurian fish had developed . of 111 Jaws

 By the Silurian, some 440 MYA, the early gill arches of fish had evolved into jaws, a major advance in .  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 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 () 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 footprints of a terrestrial date from 400 MYA. Plants of the 113 Devonian

 Plants continued to make evolutionary progress during the Devonian. , horsetails and grew to large sizes and formed Earth’s first forests. By the end of the Devonian, were the first successful , growing up to 98 feet tall with a trunk diameter of more than 3 feet. They did not have true leaves but -like structures connected directly to the branches. There is evidence that they were deciduous, as the most common 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 () and ray-finned (). The Sarcopterygii gave rise to the Ripidistia, the ancestors of all and (Dipnoi).

Trout, an actinopterygian

(Crossopterygii = Sarcopterygii) , 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 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 and Acanthostega.  They were still rather fish- like. Although they had lungs, they also had gills for use in the water.