Chapter 95: Despair in the heart


    At this point Xin didn''t stop and continued to tell Jia about the different periods. Next on queue was the Silurian Period. Silurian Period, in geologic time, the third period of the Paleozoic Era. It began 443.8 million years ago and ended 419.2 million years ago, extending from the close of the Ordovician Period to the beginning of the Devonian Period.


    The Siluriansted about 28 million years. There was a rapid recovery of biodiversity after the great extinction event at the end of the Ordovician.


    A warm climate and high sea level gave rise trge reefs in shallow equatorial seas. Tabte corals and stromatoporid sponges were the main builders of these first coral based reefs, but rugose corals and recepticulite algae also contributed. Invertebrates remained dominant because vertebrates were rtively rare.


    Sea scorpions (euryptids) reached their maximum diversity. These predators weremonly 59 inches long.


    During the Silurian, continental elevations were generally much lower than in the present day, and global sea level was much higher. Sea level rose dramatically as the extensive ciers from the Late Ordovician ice age melted.


    This rising prompted changes in climatic conditions that allowed many faunal groups to recover from the extinctions of Late Ordovician times. Large expanses of several continents became flooded with shallow seas, and mound-type coral reefs were verymon. Fishes were widespread.


    Vascr nts began to colonize coastal londs during the Silurian Period, whereas continental interiors remained essentially barren of life.


    During most of the Silurian Period, the vast Panthssic Ocean covered the northern pr regions, the supercontinent of Gondwana stretched over the southern pr region, and a ring of at least six continents spanned the Equator and middletitudes.


    The approximate orientations and locations of Silurian continents can be reconstructed using abination of paleomaic, paleoclimatic, and biogeographic data.


    The Earth''s maic field leaves its signature on volcanic rocks and certain sedimentary rocks rich in such iron-bearing minerals as maite. As rocks capable of being maized are cooled or otherwise lithified, theirponent crystals (grains) are lined up with the Earth''s maic field.


    Unless the rocks are remelted by the heat from the''s interior or reworked by erosion, they retain this signature regardless of whether they change position or not. The Earth''s zonal climate also has an effect on global patterns of sedimentation.


    The most unusual features of the Silurian that distinguish it from the present-day physical environment rte to conditions of low continental elevationsbined with a much higher global stand in sea level. Extensive continental regions were flooded by shallow seas ranging in water depth from a few to little more than 100 metres (330 feet).


    Where these seas upied a tropical to subtropical climatic zone, coral mound reefs with associated carbonate sediments were verymon. Strata forming in arid regions differ from those formed in inundated areas or other regions with high annual rainfall. The deposition of evaporites (salts) was periodically set in motion as a result of reduced ocean cirction urring in geographically restricted ces such as shallow embayments.


    The strong faunal endemism (the restriction or limiting of species to specific continents or isted regions) present during the Ordovician Period was reced during the Silurian Period by a situation where some species were distributed globally.


    Seafloor topography was muted overrge areas of flooded continental tforms, and faunas of shelly invertebrates in different regions were remarkably consistent with one another. This has allowed geologists and stratigraphers to corrteyers of Silurian rock found on different present-day continents.


    The geographic summary that follows is based on a global reconstruction specific to the Wenlock Epoch which spans the middle of the Silurian Period.


    After telling her a brief introduction Xin dived even deeper into the topic by telling her about the climate of the period.


    Broad-scale Silurian climatic conditions can be inferred by determining the positions and orientations of the paleocontinents and assuming that atmospheric cirction functioned ording to the same basic principles during Silurian times as it does today.


    The global paleoclimate was effectively driven by major contrast in the proportions ofnd and water between the Northern and Southern hemispheres. A zonally uniform climate would be expected in the Northern Hemisphere during the Silurian, because it was dominated by a North Pr ocean.


    Wind patterns must have included strong pr easterlies at high northerntitudes, prevailing westerlies at mititudes, and northeast trade winds in the tropics. In contrast, with Gondwana centred over the South Pole, climate in the Southern Hemisphere must have been dominated by the interaction of cellr air masses overnd and water. Therge continent would have experienced wide temperature variations due to summer heating and winter cooling.


    Atmospheric cirction patterns interpreted for an early Silurian summer in the Northern Hemisphere indicate high pressure over the pr ocean with a zone of low pressure around 60 Ntitude.


    Distinct high-pressure cells formed above subtropical oceans, much like the persistent Bermuda high-pressure centre over the present subtropical North Antic.


    Another zone of low pressure formed above the thermal equator, or the region of most intense sr warming. This somewhat migratory zone was the Silurian intertropical convergence zone (ITCZ), where the convergence of Northern and Southern Hemispheric trade winds caused the warm tropical air to rise, which in turn produced regr cloud cover and precipitation.


    Mostly, the ITCZ remained near the Equator, but it may have migrated slightly to the north in response to strong summer heating on Laurentia, Baltica, and possibly Kazakhstania.


    This tendency would have been strongest along the eastern margins of tropical continents, where anticyclonic cirction around subtropical highs pulled warm, moistureden air northwestward from equatorial oceans.


    At this point Jia was feeling a dizzy and nauseous she was excited about all these in the beginning but as time passed by she kept on getting pressurized by these knowledges.


    Such thing was bound to happen too. Normal human brain can digest only 5000-6000 words of new information per day. But this limit had been already exceed when Xin started exining about the different Eons.


    Observing this Xin didn''t drag it any longer and summarized everything in a few sentence.


    "So all in all what you have to remember is that during the Silurian period Earth''s continents joined together, closing the Iapetus Ocean and forming two supercontinents: Laurasia in the north, and Gondwannd to the south.


    The South American and southern African Gondwana tes moved slowly toward and then over the South Pole. At the same time the northern continents moved together and began forming Laurasia.


    ciers retreated and nearly disappeared as continental warming began. Much of the equatorialnd mass was covered by warm shallow seas.


    There were dramatic worldwide sea-level changes and oceanic turnovers (exchanges of bottom waters and surface waters) resulting in a moderate level of extinctions during the Period. The Silurian ended with a series of rtively minor extinction events linked to climate change."


    Jia heard this small exnation and was taking a breath of relief when Xin said, "Moving onto the next one which is the Devonian Period."


    Hearing this Jia froze at her ce. She endure it anymore and started to cry inside her heart. Not even in school she had learnt that much as much she was learning that day.


    ording to Xin, Devonian Period was spanning between about 419.2 million and 358.9 million years ago.


    The Devonian period was the peak of marine faunal diversity during the Paleozoic Era. New predators such as sharks, bony fishes and ammonoids ruled the oceans. Trilobites continued their decline, while brachiopods became the most abundant marine organism. A wonderful assemge in the collection has fragments of trilobite (Phacops rana milleri), brachiopod (Sulcoretepora deissi) and bryozoan fossils, all reced with pyrite. Oceanic conditions and biological richness resulted in the greatest production of carbonate during the Paleozoic Era.


    The Devonian saw major evolutionary advancements by fishes with diversification and dominance in both marine and fresh water environmentsthe Devonian is also known as the "Age of Fishes." Jawless fish and coderms (such as the giant 33 ft Duncleosteus) reach peak diversity and sharks, lobe-finned, and ray-finned fishes first appear in the fossil records.


    Finally, the changingnd and freshwater environments fostered the evolution of some fish into the first tetrapodsthe family that evolved into allnd vertebrates. These tetrapods first evolved intond animals before the end of this Period. Invertebratend animals such as scorpions, spiders, and wingless insects also began to thrive in the new environments created by the vascr nt explosion.


    While the diversification of fishes is exciting, the Devonian vascr nt "explosion" is even more spectacr. Primitive Silurian-type nts (represented by Sawdonia in this disy case) gave rise to the major vascr nt groups: lycophytes (clubmosses, spikemosses and quillworts) and euphylophytes (horsetails and ferns, and seed nts).


    These nts transformed Earth''s environments, creating extensive marsnds. The new forests, dominated by the first trees, created a new biosphere and altered global carbon cycling. Complex soils were formed,nd and water linkages were expanded, habitats became moreplex and stable, and organic matter increased both onnd and in the oceans though runoff.


    The supercontinent of Gondwana dominated the southern hemisphere, while the smaller supercontinent of Euramerica was formed near the equator and the continent of Siberiay to the north.