miércoles, 21 de octubre de 2009

Periodo Cretácico

El nombre Cretácico procede de la palabra latina creta, que significa tiza. Las capas de tiza y esquisto de esta época se amontonaron sobre el lecho de los mares. El Cretácico duro desde hace 145 hasta 66 millones de año, dividida en la parte inferior (desde 145 hasta 98 millones de años) y la superior (desde 98 hasta 66 millones de años).

En el cretáceo inferior los animales herbívoros fueron los más destacados. Además en esta hubo más especies de dinosaurios que en cualquier otra era. Entre los carnívoros que habitaban se encontraban los ornitomímidos avanzados, los primeros dromeosáuridos, y algún carnosaurio. De los herbívoros el más destacado era el sauropodo. Otra especie de dinosaurios totalmente distinta, los ceratopsios, dinosaurios con cuernos, emergieron por primera vez.

El cretácico superior fue una época de muchos cambios drásticos. En este tiempo fue cuando en el continente Pangea realmente se vieron los cambios de su separación y su esparcimiento. Gradualmente iba formando los continentes que hoy tenemos. Esta separación del continente creo nuevos ambientes y climas para los habitantes, causando el proceso de evolución en organismos. Durante esta época el mayor cambio fue la aparición de las flores y otras plantas. A finales del período Cretácico aparecieron los esbeltos pinos perennes. Surgieron los árboles que mudan sus hojas, higueras, palmeras y el árbol del pan, que ahora crecen sólo en las zonas cálidas del planeta. Surgieron serpientes, en el cretáceo tardío, que eran la evolución de un animal del Cretácico temprano llamado Pachyrhachis, que tenía cuerpo de serpiente y cabeza de lagartija. Durante este tiempo fue que se dieron fenómenos naturales e interestelares que causaron una extinción masiva. Los animales sobrevivientes, evolucionaron y progresaron a la era Cenozoica.

The Berriasian epoch of the Cretaceous period retained that was seen in the last epoch of the Jurassic period. Snowfall was common in higher latitudes and the tropics became wetter [than during the Triassic and Jurassic]. At the end of the Berriasian, temperatures increased again, and stayed mostly constant until the end of the period. This was due to the production of mass quantities of carbon dioxide by volcanic activity.Warm-adapted plant fossils have been found in areas as north as Alaska and Greenland, and dinosaur fossils were found within 15 degrees of the Cretaceous South Pole. There were weaker global winds, which contributed to less upwelling and more stangnant oceans than today.This is evidenced by widespread black shale deposition and frequent anoxic events. Sediment cores show that tropical sea surface temperatures may have been as warm as 42°C (107°F). [17°C (31°F) warmer then at present, and have averaged around 37°C(99°F)].

The mass extinction that occurred during the Cretaceous period was known as the Cretaceous-Tertiary extinction event (now known as the Cretaceous-Paleogene extinction event). There have been a few theories of the cause of extinction, these being an impact event (an asteroid crashing on Earth), increased volcanism, and marine regression. In this scenario changes, terrestrial and marine communities suffered a great costs that led to the beginning of a new era.

The consequence of an impact would include a dust cloud that would block sunlight for a year or less, and sulfur acid aerosols in the contaminated the stratosphere. This led to a 10-20% in sunlight reaching the Earth’s surface. In the least, it would take 10 years for the aerosol to dissipate, which would cause the death of plants, phytoplankton, and organisms that depend on them for food (which would also count the predators of these organisms). Animals whose food chain depended on detritus (non-living particulate materials) had more of a chance of surviving. The reentry of ejecta (material expelled in a stellar explosion[revise]) caused a short but intense pulse of infrared radiation, which killed exposed organisms. Global firestorms could have been a result from the heat pulse and falling incendiary fragments, and high O2 levels would’ve supported combustion. If widespread fires continued, once the dust clouds settled, the increase in CO2 would cause a greenhouse effect. Depending on what type of rock it was, it could’ve produced acid rain, but research suggests the effects were minor[should look for]. It is important to note that impact theories can only explain rapid extinctions, since the dust clouds and sulfuric aerosols would wash out possibly less than 10 years. There is evidence that possible multiple impact events occurred that caused extinction.


Some scientists think that the Deccan Traps flood basalts caused the extinction. Previously it was linked to the idea that it was a gradual extinction, but recently it has been found that the traps erupted over 800,000 years during the Cretaceous-Paleogene boundary, which would be responsible for the extinction and delayed biotic recovery. The Deccan volcanisms could have caused the extinction in several ways. The release of dust and sulfuric aerosols would’ve blocked sunlight and reduced photosynthetic plants. Once they dust cleared, the increased CO2 levels would’ve increased the greenhouse effect.



Another extinction theory is marine regression. There is no clear evidence for the regression, but the one accepted as the most likely is that mid-ocean ridges became less active, and sank in their own weight. The continental shaft, the most species-rich part of the sea, would be greatly reduced, and cause a marine mass extinction. Yet, this change is insufficient to cause the observed level of ammonite extinction. The regression would also cause climate changes by disrupting ocean and winds current by reducing Earth’s albedo, increasing global temperature. There would also be a loss of epeiric seas. These losses greatly altered habitats, removing coastal plains, that 10 million years ago would be a host for diverse communities. As well, expansions of freshwater environments would occur, so animals who preferred marine environments, would have suffered.



There is also a suggestion that all of the 3 major postulated events; impact event, volcanism, and marine regression, caused the extinction. The main difference between the multi-cause hypothesis and the single-cause hypothesis is that the multi-cause hypothesis suggests that the single causes were or not enough to cause the extinctions or not likely to produce the taxonomic pattern of extinction.

En Norte América la extinción terrestre se ve la mayor discrepancia cuando comparas la abundancia en el record de fósiles de la época maastrichtian y el crecimiento masivo de helecho. El helecho es parte de la evidencia porque cuando un organismo se descompone en la tierra, el helecho obtiene los nutrientes del organismo, por ende crece. La época de “campanian” mostraba una gran variedad de organismos en las capas de rocas; en la maastrichtian los organismos más destacados y de gran tamaño (Tyrannosaurus, Ankylosaurus, Pachycephalosaurus, Triceratops y Torosaurus) comenzaban aparecer en las capas de rocas. Además hay fósiles de plantas que muestran la decadencia en la flora. En las capas de sedimento se encuentran una gran cantidad de polen de las angiospermas.

En los cuerpos de agua se vio una decadencia abrupta del plancton a finales del periodo. Los animales marinos gradualmente fueron extinguiéndose entre ellos se encuentran organismos de gran tamaño. Los amonoides son fósiles encontrados que nos demuestran de esta extinción que fue gradual.

Varios Fósiles:

· Tyrannosaurus rex- hell creek, montana

· Triceratops horridus sauropsid (dinosaurio)- Norte América

· Hemíptera( insecto) - Brazil

· Audoliceras Heteromorph (Amonoides)- Russia

· Paleoctopus newboldi (pulpo)- Lebanese Lagerstatt, Haqel, Lebanon

· Pinacosaurus granger (dinosaurio) – desierto del gobi en china

· Lepidoptera (Moth)- Liaoning Province of China

· Cathayornis yandica (ave)- Liaoning Province of China

· Male spider- Liaoning Province of China

· Habrohagla curtivenata (insecto)-Lioning China

· Uintacrinus socialis (colonia crinoids)- Kansas Chalk

· Protopteryx fengningensis (ave)- Hebei Province, China

· Dictyoptera (cucaracha)- Liaoning Province of China

· Antedon pinnulata (crinoids)- Hagel, Lebanon

· Geocoma libanotica (estrella marina)- Hagel, Lebanon

· Paratriakis curtirostris (tiburón) and Guadryella gaudryi (pez)- Haqel, Lebanon

· Annelida; incertae sedis (peces y crustaceos)-Lebanese Lagerstatt, Haqel, Lebanon

· Aipichthys velifer (pez)- Lebanese Lagerstatt, Haqel, Lebanon

· Class Insecta (insect)- Liaoning Province of China

· Eoentantiornis buhleri (ave)- Liaoning Province of China

· Rhinobatos hakelensis (pez)- Haqel, Lebanon

· Prochelonicera (amonoides- desierto de sahara en marueco

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