Prehistoric Life During the Ordovician Period

Prehistoric Life During the Ordovician Period One of the lesser-known geologic spans in the earths history, the Ordovician period (448-443 million years ago) didnt witness the same extreme burst of evolutionary activity that characterized the preceding Cambrian period; rather, this was the time when the earliest arthropods and vertebrates expanded their presence in the worlds oceans. The Ordovician is the second period of the Paleozoic Era (542-250 million years ago), preceded by the Cambrian and succeeded by the Silurian, Devonian, Carboniferous and Permian periods. Climate and geography. For most of the Ordovician period, global conditions were as stifling as during the preceding Cambrian; air temperatures averaged about 120 degrees Fahrenheit worldwide, and sea temperatures may have reached as high as 110 degrees at the equator. By the end of the Ordovician, however, the climate was much cooler, as an ice cap formed on the south pole and glaciers covered adjacent landmasses. Plate tectonics carried the earths continents to some strange places; for example, much of what would later become Australia and Antarctica protruded into the northern hemisphere! Biologically, these early continents were important only insofar as their coastlines provided sheltered habitats for shallow-water marine organisms; no life of any kind had yet conquered land. Marine Life During the Ordovician Period Invertebrates. Few non-experts have heard of it, but the Great Ordovician Biodiversity Event (also known as the Ordovician Radiation) was second only to the Cambrian Explosion in its importance to the early history of life on earth. Over the course of 25 or so million years, the number of marine genera around the world quadrupled, including new varieties of sponges, trilobites, arthropods, brachiopods, and echinoderms (early starfish). One theory is that the formation and migration of new continents encouraged biodiversity along their shallow coastlines, although climatic conditions also likely came into play. On the other side of the evolutionary coin, the end of the Ordovician period marked the first great mass extinction in the history of life on earth (or, one should say, the first for which we have ample fossil evidence; there were certainly periodic extinctions of bacteria and single-celled life during the preceding Proterozoic Era). Plunging global temperatures, accompanied by drastically lowered sea levels, wiped out a huge number of genera, although marine life as a whole recovered fairly rapidly by the start of the ensuing Silurian period. Vertebrates. Practically all you need to know about vertebrate life during the Ordovician period is contained in the aspises, especially Arandaspis and Astraspis. These were two of the first jawless, lightly armored prehistoric fish, measuring anywhere from six to 12 inches long and vaguely reminiscent of giant tadpoles. The bony plates of Arandaspis and its ilk would evolve in later periods into the accoutrements of modern fish, further reinforcing the basic vertebrate body plan. Some paleontologists also believe that the numerous, tiny, worm-like conodonts found in Ordovician sediments count as true vertebrates; if so, these may have been the first vertebrates on earth to evolve teeth. Plant Life During the Ordovician Period As with the preceding Cambrian, evidence for terrestrial plant life during the Ordovocian period is maddeningly elusive. If land plants did exist, they consisted of microscopic green algae floating on or just underneath the surface of ponds and streams, along with equally microscopic early fungi. However, it wasnt until the ensuing Silurian period that the first terrestrial plants appeared for which we have solid fossil evidence.

Estimate vs. Guess

Estimate vs. Guess Estimate vs. Guess Estimate vs. Guess By Mark Nichol What’s the difference between estimate and guess? The distinction between the two words is one of the degree of care taken in arriving at a conclusion. Estimate is from the Latin word aestimare, meaning â€Å"to value.† That term is also the origin of estimable, which means â€Å"capable of being estimated† or â€Å"worthy of esteem† (but is more often used in the latter sense), and of esteem, which means â€Å"regard† (and is usually associated with high regard). To estimate is to judge the extent, nature, or value of something, with the implication that the result is based on expertise or familiarity. An estimate is the resulting calculation or judgment. (A related term is approximation, meaning â€Å"close or near.†) Bridging the gap between a guess and an estimate is an educated guess, a more casual estimate. An idiomatic term for this type of middle-ground conclusion is â€Å"ballpark figure.† The origin of this American English idiom, which alludes to a baseball stadium, is not certain, but one conclusion is that it is related to â€Å"in the ballpark,† meaning â€Å"close† in the sense that one at such a location may not be in a precise location but is in the stadium. To guess is to believe or suppose, to form an opinion based on little or no evidence, or to be correct by chance or conjecture. A guess is a thought or idea arrived at by one of these methods. Synonyms for guess include conjecture and surmise, which like guess can be employed both as verbs and as nouns. One might also have a hunch or an intuition, or may engage in guesswork or speculation. â€Å"Dead reckoning† means the same thing as guesswork, though it originally referred to navigation based on reliable information. Near synonyms describing thoughts or ideas developed with more rigor include hypothesis and supposition, as well as theory and thesis. In summary, a guess is a casual, perhaps spontaneous conclusion, whereas an estimate is based on some thought and/or data. Want to improve your English in five minutes a day? Get a subscription and start receiving our writing tips and exercises daily! Keep learning! Browse the Misused Words category, check our popular posts, or choose a related post below:50 Handy Expressions About Hands80 Idioms with the Word TimeList of Prefixes and Suffixes and their Meanings