 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
Paleocene
Introduction
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).
Marine Invertebrates
Ammonites, belemnites and rudist bivalves were
conspicuously missing from the Paleocene marine environments. Nautiloids
persisted and filled the empty niches of some ammonites. Gastropods and
bivalves diversified with many modern genera appearing for the first
time. Echinoids in the form of Sea Urchins first appear in the Ordovician.
These regular, round echinoids with pentaradial symmetry had long spines
and were adapted for grazing along the sea bottom. Irregular echinoids
with
a
bilateral
symmetry superimposed on a radial symmetry appear in the Jurassic. These
echinoids had smaller spines and were adapted for moving forward, burrowing
and feeding on detritus. Both regular and irregular echinoids survived
the K/T event. Sand
dollars
make their
first
appearance
during the Paleocene and undergo a great adaptive radiation during the
Cenozoic. Among the irregular echinoids sand dollars are the most flattened
and are adapted as shallow, rapidly burrowing detritus feeders (Prothero,
2004, p. 336). Lobsters and crabs continued to be important predators
and increased in diversity
and abundance.
The
biota
of our
modern seas devoloped from these Mesozoic survivors.
Fish
Numerous groups of sharks and bony fish were hit hard at the end of the Cretaceous. The hybodont sharks and many primitive forms of modern fish were lost. Open marine predators, those living along the continental shelf and in shallow seas were affected much more than deep sea forms. Evidence suggests that 18% of shark families became extinct (Benton, 2005, p. 185).
Modern sharks and rays (Neolselachii) underwent an adaptive radiation during the Cretaceous. These groups recovered after the K/T event and continued to diversify. Otodus obliquus, a fossil mackerel shark, makes it first appearance during the Paleocene. Transistions in fossil teeth of Otodus indicate that this genus gave rise to the Carcharocles lineage.
Modern
fish (Teleostei) underwent an adaptive radiation duirng the
Cretaceous and early Paleogene. In the mid-creteaceous ancestors
of Salmoniformes (smelts, salmon, and trout) diversified.
During the Late Cretaceous and early Paleogene the more advanced
spiny teleosts (Acanthomorpha) underwent an adaptive
radiation. Spiny teleosts, which include such familiar fish
as perch, bass, cod, tuna, seahorses, flatfish and puffers,
have spiny rays, which can be erected when the fish is threatened.
They
also
possess ctenoid or comb-like
scales. Spiny teleosts produce thrust from rapid tail
fin movements instead of bending the whole body allowing
them to reach great speeds like the Tuna at 70 km/h. Spiny-rayed
fish represent the crown group of Teleostei.
Amphibians
Amphibians were
little affected by the K/T extinction event. This is a curious pattern
when one considers that amphibians are sensitive to many environmental
changes. Lissamphibians
(Infraclass Lissamphibia) or the modern amphibians include the extinct
albanerpetontids and the living anurans (frogs and toads), the urodeles
(newts and salamanders), and the gymnophionans (caecilians). Representatives
of all these groups became established during the Jurassic. Within
these groups organisms representing new families appear during the
Paleocene including: the giant salamanders and hellbenders (family
Cryptobranchidae), the aquatic salamanders known as mudpuppies or
water dogs (family Proteidae), the very diverse family of New World
frogs Leptodactylidae, and true toads (family Bufonidae).
Reptiles
While non-avian dinosaurs, flying and large marine reptiles would go extinct at the end of the Cretaceous ancestors of modern reptiles not only survived through the K/T boundary but, would undergo great adaptive radiations.
During the Paleocene both extinct and modern forms of turtles (order Testudines) were the most diverse and abundant reptiles. Turtles were particularly successful in freshwater streams and lakes. While the very large marine turtles of the Cretaceous were gone, many sea going turtle families continued to flourish. Terrestrial turtles were relatively rare during this time. Some have suggested that the Paleocene could be called the "age of turtles" as their diversity and abundance peeked at this time (Jehle, 2006, Turtle page).
Representatives of crocodilians and their extinct relatives (superorder Crocodylomorpha) were much more diverse and abundant during the Paleocene than they are now. In fact, crocodilians were the largest organism to survive the K/T extinction event. Representatives of today's true crocodiles (suborder Eusuchia), which include crocodiles, alligators, caimans, and gharials evolved into terrestrial and semiaquatic lineages.
Lizards
and snakes (order Squamata) were also abundant and diversified
during the early Cenozoic. Lizards (suborder Lacertilia) first
appear during the Jurassic but continued to diversify in the
Paleocene. The worm lizards (family Rhineuridae) make their first
appearance during
the Paleocene. Snakes (suborder Serpentes) evolved from lizards
and made their first appearance during the Cretaceous. Snakes
diversified during the Paleocene, particularly those that fed
on small rodents.
Mammals
Mammals first appear at about the same time as Dinosaurs in the Triassic period. During the entire reign of the dinosaurs, mammals remained small never getting any bigger than a cat. Living mammals are divided into egg-laying mammals, pouched mammals, and placental mammals. Representatives of these three groupings make their first appearance duirng the Cretaceous. The mammals that survived the K/T extinction event had the opportunity to explore many new, empty niches. Mammals underwent a great adaptive radiation during the Paleocene and have been so successful during the Cenozoic era that it is often referred to as the “age of mammals”.
Monotremes are egg-laying mammals and are represented by the duck-billed platypus from Australia and the echidna of Australia and New Guinea. Monotremes are the most primitive living mammal. The Monotreme fossil record is scant and dates back to the Cretaceous. Monotrematum, a relative of the platypus, is from the Paleocene of Patagonia and is the first monotreme to be found outside Australia.
Pouched mammals are known as marsupials. Today, most marsupial species live in Australia, which include the familiar koalas and kangaroos. North America has one marsupial, the opossum. South America is home to many species of opossums.The marsupial fossil record dates back to the Mid-Cretaceous of North America. Cretaceous marsupials of North America were numerous and diverse, but remained small. During the Paleocene South American marsupials evolved into small insectivores, omnivores, and medium to large carnivorous forms with lifestyles like dogs, cats, and bears. The adaptive radiation of South American marsupials during the Paleocene resulted in three lineages.
Didelphimorphs (Order Didelphimorphia) radiated into small to medium sized opossums specialized as insectivores and omnivores. Didelphids went extinct in both Europe and North America during the Miocene. South American forms migrated back to Central and North America in the Pliocene/Pleistocene during the Great American Biotic Interchange.
Sparassodontia (Borhyaenoids) is an extinct order of specialized South American carnivores. Mayulestes, a hedgehog-sized animal is the oldest known borhyaenoid (Rose, 2006, p. 85). Many of these carnivores would evolve body shapes and lifestyles like those of placental dogs, bears, and cats (Benton, 2005, p. 315). These organisms serve as an excellent example of convergent evolution.
Paucituberculata is the order that includes living caenolestids or shrew-opossums. Many extinct forms of this order had teeth adapted to a rodent-like diet.
More than 95% of all living mammals are placentals. Placental differ from marsupials in having a reproductive system that allows the fetus to develop within the female for a longer period of time. Embryos of placental mammals are connected to the mother’s uterus wall by the placenta organ. The placenta supplies the developing embryo with maternal nutrients and allows embryo waste to be disposed by the maternal kidneys.
The fossil record of placental mammals dates back to the Early Cretaceous. At the beginning of the Paleocene placental mammals remained small, with very few reaching the size of a modern day sheep, but this would soon change by the Late Paleocene (Benton, 2005, p. 329). Placental mammals in North America, Europe and Asia underwent a great adaptive radiation during the Paleocene. Multiple groups of placental mammals evolved into medium to large rooters and browsers. Tillodonts (order Tillodontia) evolved into bear-sized rooters and browsers. Pantodonts (order Pantodonta) evolved into sizes and forms reminiscent of pigs, hippos, and sloths. In addition to these orders and many more, an important group of archaic ungulates, the condylarths, evolved into both herbivores and carnivores.
Archaic ungulates know as the “Condylarths” specialized into larger herbivores and interestingly, carnivores. Modern placental ungulate orders are believed to lie within this basal group (Kemp, 2005, p. 234). Dinocerates (order Dinocerata), also known as uintatheres, were the largest mammals of the late Paleocene and early Eocene of North America and Asia. These rhinoceros-like organisms possessed bony protuberances on their skulls and males were equipped with a pair of long, sabre-like canines. Uintatheres went extinct in the Oligocene.
The largest Paleocene carnivores were the mesonychids (family Mesonychidae). Mesonychids were wolf-sized, meat-eating ungulates! Their fossil range is Paleocene to Early Oligocene. Dissacus was a coyote-sized mesonychid that lived in Asia and North America during the Paleocene and Eocene. Ankalagon from the Paleocene of New Mexico was a bear-sized mesonychid.
Most of mammalian orders that flourished during this time would become extinct. However, a few mammals that evolved during the Paleocene represent modern orders, like flying lemurs, bats, primates, odd-toed ungulates, and rodents. Representatives of all other modern orders of placental mammals appear by the early Eocene (Benton, 2005, p. 333).
Birds
Modern birds (Superdivision Neornithes) underwent major adaptive radiations during the Cenozoic. Fossil evidence for the first modern bird orders dates to approximately 65 million years ago. Molecular evidence points to an origin 100 million years ago or more. Modern birds can be placed into two groups based upon the morphology of their palate.
The
palaeognaths (Division Palaeognathae) are represented today by
the small tinamous birds of South and Cental America and the
ratites. Ratites are flightless birds including the
ostriches of Africa, rheas of South America, emus of Australia,
Cassowaries of New Guinea, and the kiwis of New Zealand. All
of these birds evolved from a flying ancestor. The tinamous birds
are the only living palaeognath representatives that retain
flight.
The majority of living birds are neognaths (Division Neognathae).
Neognaths, with over 9,000 species, are the most diverse tetrapods.
A few representatives of both palaeognaths and neognaths are known from the Cretaceous. However, both groups diversified and became abundant during the Cenozoic. The first great adaptive radiation occurred during the Eocene but, diversification had already started in the Paleocene. Among the palaeognaths, birds related to rheas and tinamous-like birds make their first appearance during the Paleocene. Among the neognaths, birds related to magpie geese, penguins, tropic birds, and owls make their first appearance. Although related to crown groups at the family level, these fossil birds represent stem groups. A crown group is a monophyletic group or clade that includes the last common ancestor to extant (living) members as well as that ancestors extinct descendents. A stem group is paraphyletic and is closer to the crown group than any other living group. The stem group is not descended from the common ancestor to the crown group.
Terror birds, bony-toothed birds, presbyornithids and representatives of Gastrornithiformes also make their first appearance during the Paleocene. Let's take a closer look at these now extinct neognath families.
Presbyornithids (Presbyornithidae) were a family of waterbirds that can best be described as "wading ducks". Presbyornithids had a duck-like skull and long legs. They were specialized for filter feeding in shallow saline lakes (Mayr, 2009, p. 52).
Bony-toothed birds (Pelagornithidae) represent an extinct family of large to very large marine birds. Their fossils range from Paleocene to Pliocene. The spiny projections along the beak that give these birds their name are not true teeth. Pelagornithids were very successful birds and achieved a global distribution in the early Paleogene. During the Paleogene the smallest representatives of this family were the size of an albatross while the largest species had wingspans of 4 meters. Neogene species evolved into very large birds with wingspans of up to 6 meters (Mayr, 2009, pp 55-59).
Representatives of the family Phorusrhacidae (terror birds) make their first appearance during the Paleocene. These carnivorous birds were equipped with raptor-like beaks. Phorusrhacids were flightless birds and ranged from 1 to nearly 3 meters in hight. Phorusrhacids became the dominant predators in South America during the Cenozoic (Mayr, 2009, p. 141).
Gastornithiformes is an order of large flightless birds that makes its first appearance during the Paleocene. In form, they might remind one of an ostrich but more heavily built and equipped with a massive beak. Gastornis (formerly known as Diatryma) may be the best known representative. Gastornithids lived in forested environments. Early interpretations of the fossil specimens indicated a carnivorous diet. More recent analyses have suggested herbivore which, would include hard items such as seeds and twigs (Mayr, 2009, p. 47). Eventually, gastornithiformes diversified and spread across Asia, Europe and North America.
Insects
Overall, the Paleocene insect fossil record is poor. However, at the family level insects seem to have been little affected by the K/T extinction event. Coprinisphaera is the fossil burrow of a scarabaerine dung beetle, which makes its first apperance during the Paleocene. Coprinisphaera lived from the Paleocene to the Pleistocene and had a wide geographic range being found in South America, Antarctica, Africa and Asia. These nests coincide with the evolution of the first ecosystems to have abundant mammalian herbivores (Grimaldi & Engel, 2005, p. 50).
Plants
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.
Global Warming
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.
Bibliography |
Benton,
M.J. (2005) Vertebrate Palaeontology [3rd Edition].
Blackwell Publishing: Main, USA. Jehle M. (2006). Turtles: Business as usual. Paleocene mammals of the world. http://www.paleocene-mammals.de/index.htm Kemp, T.S. (2005). The Origin and Evolution of Mammals. New York: Oxford University Press. 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. Mayr, G. (2009). Paleogene Fossil Birds. Berlin: Springer Prothero, D.R. (2004). Bringing Fossils to Life: An Introduction to Paleobiology [2nd edition]. New York: McGraw-Hill. Rose,
K.D. (2006). The Beginning of the Age of Mammals.
Baltimore: The Johns Hopkins University Press. Tidwell, W.D. (1998). Common Fossil Plants of Western North America. [2nd Ed]. Washington: Smithsonian Institution Press. Willis, K.J. & McElwain, J.C. (2002). The Evolution of Plants. New York: Oxford Univeristy Press. |