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 atmosphere experienced considerable 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 heavy volcanic eruptions were at the root of these rapid and stunning changes.
The temperate/sub-tropical atmosphere of the Eocene epoch was likely precipitated by seismic performance that violently tore Greenland and northwest Europe apart and it's ensuing heavy volcanic eruptions that released 2000 gigatons of methane and carbon dioxide (both potent greenhouse gases) into the oceans and atmosphere practically 55 million years ago (based on literal, 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 atmosphere 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 antique lakebeds - Lake Goslute, Lake Uinta, and Fossil Lake - that consist of parts of Colorado, Utah and Wyoming - that consist of sedimentary deposits of shale, limestone, mudstone, sandstone, and volcanic ash that hold the remains of possibly billions of fossils (fish, large vertebrates, aquatic vertebrates, invertebrates, arthropods, plants along with some well-preserved flowers with stamens still intact) fluctuating from tiny fragments to exquisite, well-preserved whole specimens, provides a microcosmic sight of the Eocene past.
The atmosphere 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 heavy 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 recession of the subtropical/temperate atmosphere that had existed over much of the Earth. Short-term cooling also occurred when the heavy volcanic performance blocked a considerable division of sunlight for decades.
Green River Formation fossil evidence (volcanic ash in its sedimentary layers) indicates that the first of these colossal eruptions took place in the middle of 50.7 and 49.7 million years ago. The shock was severe, continuing 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 straight through natural option and necessity) occurred likely because of the coming 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 connection does not exist in the middle of plant and pollinator - such relationships are diffuse tantalizing many separate types of pollinators (e.g. Bees, butterflies, hummingbirds) for an personel blossom). Thus, had such heavy volcanic performance not occurred, co-evolution, though probable for ecological reasons, likely would have been less widespread and more gradual.
Prior to the introduction of volcanic-induced cold, based on paleobotanical evidence and the fossil description 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, contribution no incentive to pollinating organisms since their sole objective was reproduction. Consequently, they did not expend power on producing nectar repositories.
3. Eocene flowers were generally small, fluctuating 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 in the middle of 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 particular 8 mm. Flower), and crane flies (20 found in a fossil specimen that consisted of a particular 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 in the middle of evolution of the flower and other parts of the plant based on fossil evidence consisting of the corporal 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 fluctuating 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 practically 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 considerable creature diet changes. Thus it is likely that butterflies evolved over time to subsist on nectar with the only reminder of their past confidence on leaves occurring during their caterpillar stage.
Upon introduction of atmosphere change, initiated by the heavy 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 extensive by practically 10% every million years in a series of rapid bursts (ranging in size from 2.4-189 mm 46 million years ago to practically 250-940 mm 1.3 million years ago).
2. The appearance of greater numbers of larger flowers practically 40 million years ago, likely considerable 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. Diagnosis 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 often 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 confidence on wind and water pollination.
4. The appearance of dragon flies, herbivorous snout beetles (based on the presence of Coleoptera curculiondine fossils) and an fullness 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 tantalizing colors (at least a hundred million years after some arthropod and avian species had independently advanced tantalizing colors for mating reasons) and aromatic scents (to attract insect pollinators), development of bilateral symmetry with fused parts (to sustain and guide public 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 heavy volcanic eruptions.
6. A allowance in diversity of bees (especially those of higher groups with advanced eusocial behavior - preparing of hierarchal roles such as queen, sterile worker, etc.) practically 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 fullness of angiosperms - someone else contradiction of co-evolution.
Based on the profound changes in atmosphere and diversity of flora and fauna that occurred during the Eocene epoch and supporting fossil and geological evidence, one can halt that Eocene evolution and extinctions were unleashed by heavy volcanic eruptions that altered the atmosphere, topography, and atmosphere of the Earth. While a direct connection in the middle of 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 quality to adapt to a changing landscape carefully their status - extinct or extant while the fate of flora because of their flexibility (utilization of manifold means of pollination) and resilience (ability of angiosperms to certainly affect 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 modern and Rapid in the World's Largest Flowers. Current Biology. Vol. 18 No. 23. 9 December 2008.
Michael S. Engel. Monophyly and widespread extinction of advanced eusocial bees: Insights from an unexpected Eocene diversity. Pnas. 13 February 2001.
Eocene Evolution, Extinction Unleashed by heavy Volcanic Eruptions