Eocene Evolution, Extinction Unleashed by Massive Volcanic Eruptions

in Million

The Eocene epoch (55.8 ± 0.2 to 33.9 ± 0.1 million years ago) is known as the "New Dawn" (eos = dawn in Greek; kainos = new in Greek) because of the vast transformations that occurred during its span. The climate experienced significant changes while archaic flora and fauna became extinct and modern flora and fauna emerged. Based on fossil evidence, ice core samples and geological studies, it appears that massive volcanic eruptions were at the root of these rapid and stunning changes.

The temperate/sub-tropical climate of the Eocene epoch was likely precipitated by seismic activity that violently tore Greenland and northwest Europe apart and it's ensuing massive volcanic eruptions that released 2000 gigatons of methane and carbon dioxide (both potent greenhouse gases) into the oceans and atmosphere approximately 55 million years ago (based on precise dating of volcanic ash layers around Greenland and the Faeroes Islands).

With the lava from these volcanic eruptions forming layers up to 7 kilometers thick at the mid-Atlantic ridge and fissures around North America and Europe, the climate rapidly warmed over a period of 100,000 years (the Paleocene-Eocene Thermal Maximum) triggering major extinctions and rapid mutations. At its peak, ocean temperatures reached 32° C (90° F) with tropical climates extending as far north as the 45th parallel based on the presence of nypa (tropical palm) fossils in Great Britain.

The Green River Formation, which consists of three ancient lakebeds - Lake Goslute, Lake Uinta, and Fossil Lake - that comprise parts of Colorado, Utah and Wyoming - that consist of sedimentary deposits of shale, limestone, mudstone, sandstone, and volcanic ash that hold the remains of perhaps billions of fossils (fish, large vertebrates, aquatic vertebrates, invertebrates, arthropods, plants including some well-preserved flowers with stamens still intact) ranging from tiny fragments to exquisite, well-preserved whole specimens, provides a microcosmic glimpse of the Eocene past.

The climate of the Green River Formation during the Eocene epoch was moist temperate/sub-tropical. Average winter temperatures ranged from 15-20° C (59-68° F) based on the presence of crocodile fossils (crocodiles can only survive in areas of constant warmth).

Per Fossil Butte National Monument's historical guide, "a lush green forest of palms, figs, cypress, and other sub-tropical trees and shrubs [grew] nestled among the mountains during this period. Willows, beeches, oaks, maples, and ferns grew on lower slopes, [while] a spruce and fir forest [occupied] cool mountainsides." At the same time diverse and abundant animal life existed in and around the lakebeds that were then filled with warm water. "The lake shore was alive with crocodiles and turtles; insects, dog-sized horses, and early primates inhabited the land; birds and bats mastered the air."

This diversity of life extended all the way to Alaska and Greenland where lush forests of ashes, beeches, chestnuts, elms, magnolias, maples, poplars, and willows grew. Even a few palm and banana trees took root in the far north while alligators swam in the arctic. "There were no such things as ice caps. [T]he Arctic was a vast swamp; Antarctica's mountains were speckled with just a few tiny glaciers."[1]

Then sometime around 50 million years ago, the first of hundreds of massive volcanic eruptions began. They spanned all of Mexico and parts Arizona, Colorado, Idaho, and New Mexico. Over a period of 35 million years, the eruptions launched 400,000 cubic km into the atmosphere. As ash consisting of millions of tons of iron rained down on the oceans, it initiated a feeding frenzy of photosynthetic algae that sucked billions of tons of carbon dioxide from the atmosphere. This provided long-term cooling that resulted in the formation of the polar ice caps and retreat of the subtropical/temperate climate that had existed over much of the Earth. Short-term cooling also occurred when the massive volcanic activity blocked a significant percentage of sunlight for decades.

Green River Formation fossil evidence (volcanic ash in its sedimentary layers) indicates that the first of these gargantuan eruptions took place between 50.7 and 49.7 million years ago. The shock was severe, lasting about 500,000 years. A widespread fish kill resulted and vast transformation of the diversity and type of flora and fauna (archaic forms became extinct; modern forms evolved through natural selection and necessity) occurred likely because of the arrival of a long period of cold accompanied by snowfall and ice.

This shock likely triggered co-evolution and the rapid progression towards specialization despite the appearance of an intertwined evolutionary process (especially since a monogamous relationship does not exist between plant and pollinator - such relationships are diffuse involving many different types of pollinators (e.g. bees, butterflies, hummingbirds) for an individual blossom). Thus, had such massive volcanic activity not occurred, co-evolution, though probable for ecological reasons, likely would have been less extensive and more gradual.

Prior to the introduction of volcanic-induced cold, based on paleobotanical evidence and the fossil record of arthropods and other pollinating organisms, the following can be conclusively determined:

1. Angiosperms (flowering plants) were already the dominant type of plant-form.

2. Eocene flowers exhibited morphological features consistent with wind, water and unspecialized insect pollination. At the time, most were inconspicuous and unisexual, offering no incentive to pollinating organisms since their sole objective was reproduction. Consequently, they did not expend energy on producing nectar repositories.

3. Eocene flowers were generally small, ranging in size from 3-13.5 mm. Consequently, Eocene insects were generally also small - ants 3-9 mm, flies 8-9 mm, Bittacus mecoptera 7-8 mm, and crane flies (Diptera myiomorpha) 4-16 mm long.

4. The vast majority of Eocene flowers had numerous, well-developed stamen numbering between 12-15 that protruded from their floral axis. Such stamen were exposed to exploit wind and unspecialized insect pollination. Key pollinators were bees (which emerged about 30 million years after the appearance of the first angiosperms), ants (10 found in a fossil specimen that consisted of a single 8 mm. flower), and crane flies (20 found in a fossil specimen that consisted of a single 3 mm. blossom), and beetles (Coleoptera).

5. Most Eocene flowers had four well-developed petals. Species with 3, 5 and 6 petals also existed.

6. Most Eocene flowers exhibited a high-degree of radial symmetry with well-developed perianth parts (though a few had underdeveloped perianth parts) spaced equally apart of generally the same dimensions.

7. Eocene angiosperms exhibited a high-degree of conformity between evolution of the flower and other parts of the plant based on fossil evidence consisting of the physical attachment of foliage and reproductive features.[2]

8. Many herbaceous weedy angiosperms were aquatic - from lily pads; though herbaceous weedy terrestrial angiosperms also existed (supported by the presence of fossil specimens consisting of small bees ranging in size from 1.5-3 mm) along with terrestrial woody species such as maples, based on fossil evidence.

9. Specimens of specialized pollinators (e.g. butterflies) were rare until approximately 40 million years ago. At this time, such organisms likely subsisted on plant leaves (and possibly gymnosperm spores) and served as accidental pollinators, corroborated by paleobotanical and paleontological evidence (leaf galling and mining) especially since the earliest butterfly fossils date back 130 million years when angiosperms (many without petals) were just emerging in lakes and the fact that diets can change - Robins, which normally subsist on worms during warm seasons revert to berries in the winter, especially when the ground is frozen. This is consistent with findings that interglacial warming that occurred about 1.3 million years ago resulted in significant vertebrate diet changes. Thus it is likely that butterflies evolved over time to subsist on nectar with the only reminder of their past reliance on leaves occurring during their caterpillar stage.

Upon introduction of climate change, initiated by the massive volcanic eruptions, a transition towards specialization began. The following are noticeable trends based on fossil evidence:

1. Emergence of larger flowers with the appearance of Rafflesiaceae about 46 million years ago. When the evolution to larger size began, Rafflesiaceae blossoms expanded by approximately 10% every million years in a series of rapid bursts (ranging in size from 2.4-189 mm 46 million years ago to approximately 250-940 mm 1.3 million years ago).

2. The appearance of greater numbers of larger flowers approximately 40 million years ago, likely necessary due to shorter growing seasons after the introduction of cold winters. Such a trend is supported by Green River Formation fossil specimens - one showing a specialized flower with an approximate 2-inch size and the other consisting of a well-preserved bat (Icaronycteris index) skeleton with food and waste still inside its body. Analysis of the ingested food and unspent waste exhibited ingestion of algae, pollen, and anthropods (insects). In a contradiction to co-evolution, insect size generally remained stable; it did not expand. Limitations exclusive to amber (encasing generally small insects and botanic specimens) are not applicable to fossils since such specimens have frequently consisted of intact dinosaurs and other large vertebrates.

3. Emergence of greater numbers of flowers with nectaries to attract pollinating organisms due to a transition to bi-sexuality and diminished reliance on wind and water pollination.

4. The appearance of dragon flies, herbivorous snout beetles (based on the presence of Coleoptera curculiondine fossils) and an abundance of moth and butterfly fossils; snout beetles have long snouts while butterflies and moths have specialized mouthparts and tongues for nectar collection corroborating the emergence of flowers with nectaries.

5. Emergence of flowers with bright colors (at least a hundred million years after some arthropod and avian species had independently developed bright colors for mating reasons) and aromatic scents (to attract insect pollinators), development of bilateral symmetry with fused parts (to assist and guide social insects and birds) and emergence of fruits and large seeds (to attract mammals and birds) to optimize their visits since wind and water pollination was too imprecise for the shorter growing seasons that resulted from the massive volcanic eruptions.

6. A reduction in diversity of bees (especially those of higher groups with advanced eusocial behavior - establishment of hierarchal roles such as queen, sterile worker, etc.) approximately 30-40 million years ago, despite their generalist nature (pollinating large and small flowers alike), the dominant role they play in pollination and the vast expansion in diversity and abundance of angiosperms - another contradiction of co-evolution.

Based on the profound changes in climate and diversity of flora and fauna that occurred during the Eocene epoch and supporting fossil and geological evidence, one can conclude that Eocene evolution and extinctions were unleashed by massive volcanic eruptions that altered the atmosphere, topography, and climate of the Earth. While a direct relationship between flora and fauna exists with regard to many aspects of co-evolution, indirect relationships also exist indicative that co-evolution may have been coincidental and accidental. It is likely, especially with regard to fauna, that their inability or ability to adapt to a changing landscape determined their status - extinct or extant while the fate of flora because of their flexibility (utilization of multiple means of pollination) and resilience (ability of angiosperms to positively influence soil composition) was more dependent on purely climactic conditions than fauna evolution or extinction despite the fact that some synchronized changes such as floral development to match the mouthparts of pollinating insects and birds had taken place.

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[1] Michael Reilly. Ancient Volcanic Blasts Kicked Off Modern Ice Ages.Discovery News. 19 June 2009. 24 August 2009. http://dsc.discovery.com/news/2009/06/19/eruptions-ice-age.html

[2] Conrad C. Labandeira. Paleobiology of middle Eocene plant-insect associations from the Pacific Northwest.Rocky Mountain Geology. June 2002

Additional Sources:

Charles C. Davis. Floral Evolution: Dramatic Size Change Was Recent and Rapid in the World's Largest Flowers.Current Biology. Vol. 18 No. 23. 9 December 2008.

Michael S. Engel. Monophyly and extensive extinction of advanced eusocial bees: Insights from an unexpected Eocene diversity.PNAS. 13 February 2001.

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William Sutherland has 1 articles online

William Sutherland is a published poet and writer. He is the author of three books, "Poetry, Prayers & Haiku" (1999), "Russian Spring" (2003) and "Aaliyah Remembered: Her Life & The Person behind the Mystique" (2005) and has been published in poetry anthologies around the world. He has been featured in "Who's Who in New Poets" (1996), "The International Who's Who in Poetry" (2004), and is a member of the "International Poetry Hall of Fame." He is also a contributor to Wikipedia, the number one online encyclopedia and has had an article featured in "Genetic Disorders" Greenhaven Press (2009).

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Eocene Evolution, Extinction Unleashed by Massive Volcanic Eruptions

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This article was published on 2010/03/26