Paleogene Period: 65.5-23 Million Years Ago
The second largest mass extinction marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. The Paleogene Period marks the beginning of the Cenozoic Era and extends from 65.5 to 23 million years ago. The Paleogene time Period is subdivided into the Paleocene, Eocene and Oligocene Epochs. Paleogene is derived from Greek and means "ancient birth". The Paleogene period is most notable for the adaptive radiations of mammals and birds following the extinction of non-avian dinosaurs. Both mammals and birds would evolve into essentially modern forms during the Paleogene.
Oligocene Epoch: 33.9-23 Million Years Ago
Palmoxylon of the Catahoula Formation
Flowering plants or angiosperms (Magnoliophyta) make their first unmistakable appearance during the Early Cretaceous 140 Ma (Kenrick & Davis 2004, p. 195). Traditionally, angiosperms have been divided into monocots and dicots. Woody deciduous trees such as oak, elm, and maple are familiar examples of dicots. Grasses and palms are well known examples of monocots. Among angiosperms, dicots have a more extensive fossil record than monocots. One might expect this to be the case since today dicots outnumber monocots six to one. In addition to this fact, most monocots are herbaceous plants, which may not as readily fossilize as the woody dicots (Stewart & Rothwell, 1993, pp 487 & 488). The coastal states of Texas, Louisiana, Mississippi and Alabama are therefore special in that they possess late Eocene and Oligocene deposits in which the silicified remains of palm wood are common (Berry, 1916, p. 233). In fact, petrified palm wood or Palmoxylon is the state stone for Texas and the state fossil for Louisiana. The state stone for Mississippi is petrified wood and much of the fossil wood found in the state is Palmoxylon.
The genus name Palmoxylon is derived from the Latin word for palm tree, palm, and the Greek word for wood, xylo (Borror, 1988, p. 69 & 111). Palm trees actually do not produce wood; although, they do produce fibrous, wood-like stems. The woody cylinder stems of angiosperm dicots and gymnosperms, such as sequoias, spruce and pines, are produced from secondary growth that adds girth to the stem and consists primarily of secondary xylem made of cellulose and lignin. In fact, wood is often defined as secondary xylem (Raven, Evert & Curtis, 1981, p. 664). Palm tree trunks result from only primary growth and reach their adult diameter near ground level.
Palm tree trunks consist of individual vascular bundles embedded in a groundmass of living parenchyma cells. In cross-section the vascular bundles can give a spotted appearance to the palm fiber.
Each vascular bundle or fibrovascular bundle consists of a small vascular portion of one to four (usually two) large vessels surrounded by numerous fibers that thicken into a bundle cap on one end. Fibers provide structural support. Vessels conduct water. Phloem or food conducting tissue is found between the vessels and bundle cap.
In the center of the stem vascular bundles are spaced far apart. Towards the periphery of the stem the vascular bundles become more numerous and crowded. Longitudinal cuts reveal that the vascular bundles form rod-like structures.
In general, fossil palm fiber is easy to identify; however, monocot fiber is fairly uniform in appearance and yields little specific taxonomic information. Identification of palm tree species is difficult because their appearance is so generic. The vast majority of collectors are happy to identify their fossil specimen as belonging to the genus Palmoxylon.
Satisfactory palm specimens can be rare. Full rounds are unusual with most specimens representing fragments of trunks. Palm trunks are made from primary growth and much of the stem is composed of living parenchyma cells. The parenchyma tissue is not as resistant to decay as the wood of gymnosperms and dicot angiosperms. Thus, good preservation of intact palm trunks is less likely. The lack of good preservation combined with the generic appearance of the palm fiber explains why there is less scientific systematic work on palms versus gymnosperms and angiosperm dicots.
Still, to the keen observer differences in vascular bundle structure and ground tissue can be observed between specimens. Some species have fibrous bundles that appear as small roundish bundles composed of sclerenchyma cells or fibers (Tidwell, 1998, p. 248). Fibrous bundles are made of the same cells that make up bundle caps.
Some of the best if not the finest permineralized palm fiber, commonly known as petrified palm wood, comes from the Catahoula Formation in Louisiana, Mississippi and Texas. The Catahoula formation consists of sandstones, sand, clays, and conglomerates. Rivers and streams flowing across broad coastal plains 24 to 30 million years ago deposited sediments making up the Catahoula Formation (Matson, 1916, p. 226; Paine & Meyerhoff, 1968, p. 92; John, 2001, p. 6). These rivers were powered by an uplift of the Rockies. Volcanic activity during the Oligocene in West Texas and Central Mexico explains the volcanic origin of many types of sediments found in the Catahoula. Palms along with other tropical plants grew along a near shore environment that bordered the Oligocene Gulf of Mexico (Berry, 1916, pp 227 & 228). In Louisiana the Catahoula Formation forms a belt across the central part of the state revealing that the ancient palm groves, beaches and deltas that made up this environment lay a further 200 kilometers inland than today's coastline (Daniels & Dayvault, 2006, p. 398). This Oligocene environment had two elements necessary for forming good permineralized specimens, a chance for quick burial and a volcanic silica source.
Palmoxylon is the most abundant plant fossil from the Catahoula Formation and often exhibits excellent preservation. In the past many weathered specimens could be found on the surface or reworked in more recent deposits. Berry, in his 1916 paper, describes 7 Palmoxylon species from the Catahoula Formation, providing a key to their identification (p. 234). Differences in vascular bundles and ground tissue are used to key out the species. The species described include: P. ovatum, P. mississippense, P. texense, P. lacunosum, P. cellulosum, P. remotum and P. microxylon. Plates are included that provide illustrations showing cross-sections.
Louisiana Palmoxylon may be unmatched worldwide for its fine preservation and color. The permineralization with silica is so fine that cell structure is faithfully preserved. When tapped, specimens produce a sound not unlike fine china. The red, yellow, white and lavender colors invite our imaginations to dream of ancient sunrises and sunsets.
To borrow a phrase from Mary White, the finest of Louisiana Palmoxylon specimens are truly semi-precious gemstones that serve as keys to the geologic past.
Eocene Epoch: 55.8-33.9 Million Years Ago
The Eocene epoch extends from 55.8 to 33.9 million years ago. Eocene is derived from Greek and means "dawn of the modern". This name is well chosen as representatives of most modern orders of birds and mammals appear during this epoch.
Primary Producers & Reefs
Coccolithophorids experienced a mass extinction at the K/T boundary. Diatoms and dinoflagellates were less affected. By the Eocene all three groups had recovered; however, from then on dinoflagellate and coccolithophore diversity declined. Diatoms, on the other hand, have increased in diversity. Today, diatoms are the most diverse group in the plankton.
Interestingly, the success of diatoms may be tied to the evolution of grasses. Grasses sequester silica into their tissues in the form of phytoliths. Phytoliths add structural support to the plant. Phytoliths may also produce a gritty texture making the plant distateful to herbivores. The Silica released from phytoliths during plant decay is easily dissolved in water and washed in rivers and seas. The influx of phytolith silica into the oceans would benefit diatoms as they construct their frustules from silica. Increase in diatom diversity and abundance is correlated with the appearance of phytoliths during the Late Cretaceous and with the expansion of grasses in the Eocene, Oligocene and Middle Miocene (Kooistra, Gersonde, Medlin & Mann, 2007, pp. 228 & 229). Diatoms, dinoflagellates, and coccolithophores make up the dominant primary producers in todays oceans.
Grube Messel or the Messel Pit is a Fossil Laggerstaten near the town of Messel, Germany that was declared a UNESCO World Heritage Site in 1995. From 1875 to the 1960's the site was developed as an open pit oil shale mine. Over the years many fossils were found during the mining operations. The sediments that formed Grube Messel were layed down in a restricted lake basin within the Rhine Rift Valley during the Early Eocene 49 million years ago. Faulting and volcanic activiey were common within this rift valley. The Grube Messel stratigraphy and biota suggest the basin was part of a lush subtropical environment with extensive rivers and lakes. Some interpret the deposit as representing a crater lake. The flora and fauna preserved in the Messel shale represent a number of environments that surrounded the lake including: open water, swamp, bank-side, damp forest and drier elevated regions.
Among plants angiosperms or the flowering plants are the most common at Messel; although, gymnosperms and ferns are also present. Ferns, swamp cypresses, grasses (such as rushes and sedges), lilies, palms, laurels, tea, grapevines, citrus, and walnut represent environments that were inundated with water or moist. Pines, beeches, chestnuts, and oak grew in drier conditions farther from the lake.
Among arthropods beetles are the most common and beautiful in the Messel shale. Click beetles, weevils, jewel, dung, stag, water, longhorn, rove and leaf beetles are represented. The larvae of a particular genus of water beetle Eubrianax are interesting because extant forms live in waterfalls were oxygen levels are high. The elytra or wing cases of some beetle specimens exhibit a colorful iridescence.
Paleocene Epoch: 65.5-55.8 Million Years Ago
The Paleocene Epoch extends from 65.5 to 55.8 million years ago. Paleocene is derived from Greek and means "ancient-recent". This name is well chosen for the organisms that survied the K/T extinction event would evolve into primitive representatives of modern lineages by the end of this epoch.
Primary Producers & Reefs
During the Cretaceous diatoms, dinoflagellates, and coccolithophores assumed their dominant role as the base of marine ecosystems. The K/T extinction event at the end of the Cretaceous would impact but, not end the dominate role achieved by these three groups. Coccolithophorids did experience a major mass extinction at this time while diatoms and dinoflagellates were less affected. However, during the Paleocene all three groups underwent adaptive radiations to regain their pre-K/T diversity.
Coccolithophorid and dinoflagellate diversity waxes and wanes with global sea level changes. The cyclic process of continental rifting and reassembly, which opens and closes ocean basins is known as the Wilson Cycle. During continental rifting new ocean basins are created and ocean levels rise. Coccolithophorid and dinoflagellate diversity increases during this time. As continents reassemble and ocean basins close the diveristy of these two groups decreases. These trends have been observed in the fossil record from the Triassic onward. Diatoms do not follow this trend. From the time that the Paleocene ends to the present day coccolithophorid and dinoflagellate diversity steadly declines, while diatom diversity increases. Today diatoms are the most diverse plankton (Kooistra, Gersonde, Medlin & Mann, 2007, pp. 228 & 229).
The K/T extinction event caused reef systems to collapse. Evidence of reef systems during the Paleogene remains rare to non-existent except for a few coral associations found at high latitudes in cold waters that survived the extinction event. The recovery of reef systems was very slow, taking some 8 million years. Rudist bivalves that had dominated many reef systems during the Cretaceous would not survive into the Paleogene. Scleractinian corals that survied the K/T impact event would start to diversify during the later part of the Paleocene and revive the coral reef system (Stanley, 2001, p. 31).
The K/T extinction event had a significant impact on plant life in what is now North America. One third of higher level plant taxa went extinct and for a short time ferns became dominant over the angiosperms and conifers in North America (Stanley, 1987, p. 157). Ferns often colonize areas damaged by forest fires and this early Paleocene fern spike probably resulted from the damage done by the meteor impact. However, plant life would soon recover and the adaptive radiation of flowering plants that had started in the Cretaceous would continue.
A trend in global warming during the Paleocene is supported by both terrestrial and oceanic sources. By the late Paleocene vegetation was adapted to a warm, moist global climate. Angiosperm trees, shrubs, conifers and ferns helped to make up forested areas stretching to both poles. Willis and McElwain in their book The Evolution of Plants explore the different biomes present during the late Paleocene and early Eocene. Lets take a look at some of the highlights of their work.
The first forests recognizable as "modern rainforests" appear duirng the Palecoene. Characteristics of modern tropical rainforest vegetation evolved during the late Paleocene such as multistratal canopies, tall trees, vines, epiphites, shade tolerant trees and large leaves with drip tips. Representatives of families found in modern tropical rainforests were major components of these Paleocene forests such as Tiliaceae, Elaeocarpaceae, Simaroubaceae, Sapindaceae, Araliaceae, Proteaceae, Dipterocarpaceae and Olacaceae. Palms were common and diverse. Representatives of the conifer families Araucariaceae, Podocarpaceae and Ginkgoaceae were rare.
Subtropical vegetation covered much of the northern continents and extended up to 50 degrees in both the northern and southern hemispheres. The vegetation was a mix of what is now tropical and temperate elements. Representatives of angiosperm families Anacardiaceae, Anonaceae, Burseraceae, Cornaceae, Lauraceae, Sapindaceae, Sabiaceae, Vitaceae, Menispermaceae and Icacinaceae were present. The subtropical forests were lined by mangrove swamps along the coasts of Europe and Tasmania. Nypa palms dominated in these coastal swamps.
In higher latitudes (present day Canada, Southern Greenland, Asia, Argentina and Antarctica) the subtropical vegetation gave way to warm temperate evergreen forests dominated by oak (Quercus), beech (Betula), laurel (Larus) and magnolias (Magnolia).
In very high latitudes, up to 70 degrees, cool temperate deciduous polar forests fluorished in what is now Canada, Greenland, Siberia and Antarctica. These polar forests have no modern analogue. The polar forests were a mix of angiosperm dicots and conifers. The polar forests of the northern and southern hemispheres differed in tree composition. In the Northern Angiosperms included oak (Quercus), walnut (Juglans), beech (Betula), poplar (Populus), maple (Acer) and alder (Alnus). Conifers included larch (Larix), Dawn Redwoods (Metasequoia), golden larch (Pseudolarix) and bald cypress (Taxodium). In the southern hemisphere deciduous angiosperms and needle-leaved gymnosperms were rare. Evergreen conifers such as Araucaria, Podocarpus, Dacrydium and Nothofagus (southern beeches) were common (Willis & McElwain, 2002, pp 202-207).
Both the tropical and subtropical forests of the Paleocene and Eocene left behind deposits of coal and bauxite. Bauxite is the main ore of aluminun and coal is an important fossil fuel. The Paleocene aged Fort Union Formation outcrops in Montana, Wyoming, and the Dakotas. The Fort Union Formation contains economically important deposits of coal, uranium and coalbed methane. The fossil flora associated with the formation includes algae, fungi, bryophtes, ferns, conifers, cycads and ginkgos. Angiosperms include maple (Acer), breadfruit (Arctocarpus), birch (Betula), Hickory (Carya), chestnut (Castanea), Cercidiphyllum, mountain mohogany (Cerocarpus), Cinnamomum, Fig (Ficus), walnut (Juglans), Magnolia, Parvileguminophyllum, Sycamore (Plantanus), wing nut (Pterocarya), oak (Quercus), locust (Robinia), willow (Salix), soapberry (Sapindus), Sassafras, keati tree (Zelkova), elm (Ulmus), Phoenicites and Sabalites (Tidwell, 1998, pp 48-53). The sandstones, shales and coal beds of the Fort Union Formation represent river, lake, and swamp environments. The Flora indicates a subtropical to warm temperate climate.
The increase in global temperatures that started in the early Paleocene would continue into the Eocene. Maximum global temperatures were reached sometime during the middle Eocene. In fact, this time interval (65 - 45 Ma) was one of the warmest periods in Earth's history (Willis & McElwain, 2002, p. 194). Forested areas expanded to the poles, which were free of ice.
A sudden global warming event is associated with the end of the Paleocene. This event is known as the Paleocene-Eocene Thermal Maximum (PETM). This event is marked by a increase in average global temperature of 6 degrees Celsius over a 20,000 year time span. The warming is correlated with an increase in atmospheric carbon dioxide levels. Oceanic and atompsheric currents were affected by this event. There was a rise in ocean levels. Benthic foraminifera suffered a mass extinction event. Interestingly, planktonic foraminifera and dinoflagelates flourished during this same time. The adaptive radiation of mammals and flowering plants continued and by the Eocene representatives of most modern angiosperm plant famalies and mammalian orders had appeared.
Benton, M.J. (2005) Vertebrate Palaeontology [3rd Edition]. Blackwell Publishing: Main, USA.
Berry, E.W. (1916). The Flora of the Catahoula Sandstone. U.S. Geological Survey Professional Paper 98 M: 227-251.
- Borror, D.J. (1988). Dictionary of Word Roots and Combining Forms. California: Mayfield Publishing Company.
Daniels, F.J. and Dayvault, R.D. (2006). Ancient Forests: A Closer Look at Fossil Wood. Western Colorado Publishing Company: Canada.
Grimaldi, D. & Engel, M.S., (2005). Evolution of the Insects. New York: Cambridge University Press.
Jehle M. (2006). Turtles: Business as usual. Paleocene mammals of the world. http://www.paleocene-mammals.de/index.htm
John, C.J. (2001). Louisiana Geofacts: Land: Public Information Series No. 6. Louisiana Geological Survey: Louisiana State University.
Kemp, T.S. (2005). The Origin and Evolution of Mammals. New York: Oxford University Press.
Kenrick P. and Davis, P. (2004). Fossil Plants. Smithsonian Books: Washington.
Kooistra, W.H.C.F., Gersonde, R., Medlin, L. K. & Mann, D.G. (2007). The Origin and Evolution of the Diatoms: Their Adaptation to a Planktonic Existence. In Falkowski, P.G. Knoll, A.H. [Eds] Evolution of Primary Producers in the Sea. (pp. 133-163). China: Elsevier Academic Press.
Matson G.C. (1916). The Catahoula Sandstone. U.S. Geological Survey Professional Paper 98M: 209-226.
Mayr, G. (2009). Paleogene Fossil Birds. Berlin: Springer
Paine W.R. and Meyerhoff A.A. (1968). Catahoula Formation of Western Louisiana and Thin-Section Criteria for Fluviatile Depositional Environment. Journal of Sedimentary Research, vol 38, pp 92-113.
Prothero, D.R. (2004). Bringing Fossils to Life: An Introduction to Paleobiology [2nd edition]. New York: McGraw-Hill.
Raven, P.H., Evert, R.F., & Curtis, H. (1981). Biology of Plants [3rd Ed]. New York: Worth Publishers, Inc.
Rose, K.D. (2006). The Beginning of the Age of Mammals. Baltimore: The Johns Hopkins University Press.
Stanley, G.D. Jr. (2001). Introduction to Reef Ecosystems and Their Evolution. In Stanley, G.D. Jr. [Ed] The History and Sedimentology of Ancient Reef Systems (1-39). New York: Kluwer Academic/Plenum Publishers.
Stewart W.N. and Rothwell G.W. (1993). Paleobotany and the Evolution of Plants [2nd edition]. Cambridge University Press: Cambridge.
Tidwell, W.D. (1998). Common Fossil Plants of Western North America. [2nd Ed]. Washington: Smithsonian Institution Press.
White, M.E. (1991). Time in Our Hands: Semi-Precious Gemstones: Keys to the Geologic Past. Reed Books Pty Ltd: Sydney, Australia.
Willis, K.J. & McElwain, J.C. (2002). The Evolution of Plants. New York: Oxford Univeristy Press.