Prehistoric Flora & Fauna

The Silurian Period

Monday, August 7, 2017

Underwater life thrived during the Silurian Period, 443 million years ago to 416 million years ago. Credit: Alena Hovorkova

The Silurian (443.7 to 416.0 million years ago)* was a time when the Earth underwent considerable changes that had important repercussions for the environment and life within it. One result of these changes was the melting of large glacial formations. This contributed to a substantial rise in the levels of the major seas. The Silurian witnessed a relative stabilization of the Earth’s general climate, ending the previous pattern of erratic climatic fluctuations.

Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Silurian Period by Encyclopedia Britannica

Coral reefs made their first appearance during this time, and the Silurian was also a remarkable time in the evolution of fishes. Not only does this time period mark the wide and rapid spread of jawless fish, but also the highly significant appearances of both the first known freshwater fish as well as the first fish with jaws. It is also at this time that our first good evidence of life on land is preserved, such as relatives of spiders and centipedes, and also the earliest fossils of vascular plants.


The Silurian is a time when many biologically significant events occurred. In the oceans, there was a widespread radiation of crinoids, a continued proliferation and expansion of the brachiopods, and the oldest known fossils of coral reefs. As mentioned earlier, this time period also marks the wide and rapid spread of jawless fish, along with the important appearances of both the first known freshwater fish and the appearance of jawed fish. Other marine fossils commonly found throughout the Silurian record include trilobites, graptolites, conodonts, corals, stromatoporoids, and mollusks.

Dalmanites Grammysia  

On the left, Dalmanites limuluris, a trilobite from the Silurian of New York. To the right, Grammysia cingulata, a bivalve from the Upper Ludlow of England.

It is also in the Silurian that we find the first clear evidence of life on land. While it is possible that plants and animals first moved onto the land in the Ordovician, fossils of terrestrial life from that period are fragmentary and difficult to interpret. Silurian strata have provided likely ascomycete fossils (a group of fungi), as well as remains of the first arachnids and centipedes.

Perhaps most striking of all biological events in the Silurian was the evolution of vascular plants, which have been the basis of terrestrial ecology since their appearance. Most Silurian plant fossils have been assigned to the genus Cooksonia, a collection of branching-stemmed plants which produced sporangia at their tips. None of these plants had leaves, and some appear to have lacked vascular tissue. Also from the Silurian of Australia comes a controversial fossil of Baragwanathia, a lycophyte. If such a complex plant with leaves and a fully-developed vascular system was present by this time, then surely plants must have been around already by the Ordovician. In any event, the Silurian was a time for important events in the history of evolution, including many “firsts,” that would prove highly consequential for the future of life on Earth.

Cooksonia Baragwanathia

Cooksonia, on the left, has usually been considered the oldest known land plant. Fossils assigned to several species are known from North America, Europe, Asia, and Africa, and from both the Late Silurian and Early Devonian. The lycophyte Baragwanathia, on the right, is structurally more complex than Cooksonia, but Silurian fossils of this plant have been found in Australia, significantly earlier than in the Northern Hemisphere.


The Silurian’s stratigraphy is subdivided into four epochs (from oldest to youngest): the Llandovery, Wenlock, Ludlow, and Pridoli. Each epoch is distinguished from the others by the appearance of new species of graptolites. Graptolites are a group of extinct colonial, aquatic animals that put in their first appearance in the Cambrian Period and persisted into the early Carboniferous. The beginning of the Silurian (and the Llandovery) is marked by the appearance of Parakidograptus acuminatus, a species of graptolite.

The Llandovery (443.7-428.2 million years ago*) preserves its fossils in shale, sandstone, and gray mudstone sediment. Its base (beginning) is marked by the appearance of the graptolites Parakidograptus acuminatus and Akidograptus ascensus. The Llandoverian epoch is subdivided into the Rhuddanian, Aeronian, and Telychian stages.

At the close of the Telychian stage, the appearance of Cyrtograptus centrifugus marks the start of the Wenlockian epoch (428.2 to 422.9 million years ago).* The fossils are found in siltstone and mudstone under limestone. Missing from the fossil record of the Wenlock was the conodont Pterospathodus amorphognathoides, present in earlier strata. This is an epoch with excellent preservations of brachiopod, coral, trilobite, clam, bryozoan, and crinoid fossils. The Wenlock is subdivided into the Sheinwoodian and Homerian stages.

The Ludlow (422.9 to 418.7 million years ago)* consists of siltstone and limestone strata, marked by the appearance of Neodiversograptus nilssoni. There is an abundance of shelly animal fossils. The Gorstian and Ludfordian stages make up the Ludlow epoch.

Platy limestone strata rich in cephalopods and bivalves characterize the Pridolian (418.7 to 416.0 million years ago),* the final epoch of the Silurian. It is marked by the appearance of the index fossil Monograptus parultimus, and also by two new species of chitinozoans (marine plankton), Urnochitina urna and Fungochitina kosovensis, which appear at the base or just above the base of the Pridoli.

Tectonics and paleoclimate

Although there were no major periods of volcanism during the Silurian, the period is marked by major orogenic events in eastern North America and in northwestern Europe (the Caledonian Orogeny), resulting in the formation of the mountain chains there. The ocean basins between the regions known as Laurentia (North America and Greenland), Baltica (central and northern Europe and Scandinavia) and Avalonia (western Europe) closed substantially, continuing a geologic trend that had begun much earlier. The modern Philippine Islands were near the Arctic Circle, while Australia and Scandinavia resided in the tropics; South America and Africa were over the South Pole. While not characterized by dramatic tectonic activity, the Silurian world experienced gradual continental changes that would be the basis for greater global consequences in the future, such as those that created terrestrial ecosystems. A deglaciation and rise in sea levels created many new marine habitats, providing the framework for significant biological events in the evolution of life. Coral reefs, for example, made their first appearance in the fossil record during this time.

Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Silurian Period by Encyclopedia Britannica

The Silurian Period’s condition of low continental elevations with a high global stand in sea level can be strongly distinguished from the present-day environment. This is a result of the flood of 65% of the shallow seas in North America during the Llandovery and Wenlock times. The shallow seas ranged from tropical to subtropical in climate. Coral mound reefs with associated carbonate sediments were common in the shallow seas. Due to reduced circulation during the Ludlow and Pridoli times, the process of deposition of evaporites (salts) was set in motion. Some of these deposits are found in northern Europe, Siberia, South China and Australia.


The Ordovician Period

Sunday, August 6, 2017

The Ordovician Period by Masato Hattori

The Ordovician Period lasted almost 45 million years, beginning 488.3 million years ago and ending 443.7 million years ago.* During this period, the area north of the tropics was almost entirely ocean, and most of the world’s land was collected into the southern supercontinent Gondwana. Throughout the Ordovician, Gondwana shifted towards the South Pole and much of it was submerged underwater.

Ordovician map

The Ordovician is best known for its diverse marine invertebrates, including graptolites, trilobites, brachiopods, and the conodonts (early vertebrates). A typical marine community consisted of these animals, plus red and green algae, primitive fish, cephalopods, corals, crinoids, and gastropods. More recently, tetrahedral spores that are similar to those of primitive land plants have been found, suggesting that plants invaded the land at this time.

From the Lower to Middle Ordovician, the Earth experienced a milder climate — the weather was warm and the atmosphere contained a lot of moisture. However, when Gondwana finally settled on the South Pole during the Upper Ordovician, massive glaciers formed, causing shallow seas to drain and sea levels to drop. This likely caused the mass extinctions that characterize the end of the Ordovician in which 60% of all marine invertebrate genera and 25% of all families went extinct.


Ordovician strata are characterized by numerous and diverse trilobites and conodonts (phosphatic fossils with a tooth-like appearance) found in sequences of shale, limestone, dolostone, and sandstone. In addition, blastoids, bryozoans, corals, crinoids, as well as many kinds of brachiopods, snails, clams, and cephalopods appeared for the first time in the geologic record in tropical Ordovician environments. Remains of ostracoderms (jawless, armored fish) from Ordovician rocks comprise some of the oldest vertebrate fossils.

Ordovician period by redcode77

Despite the appearance of coral fossils during this time, reef ecosystems continued to be dominated by algae and sponges, and in some cases by bryozoans. However, there apparently were also periods of complete reef collapse due to global disturbances.


100-Million-Year-Old Evidence of Insect Brood Care

Sunday, July 30, 2017

A team of scientists from China, Germany, Poland, and the United Kingdom, has described a new genus and species of an ensign scale insect from mid-Cretaceous Burmese amber, which preserves eggs within a wax ovisac, and several freshly hatched nymphs.

“Fossils of fragile female scale insects are extremely rare. What is unique here is the age of the discovery: 100-million-year-old evidence of brood care among insects has not been found until now,” said team leader Dr Bo Wang of the Chinese Academy of Sciences, who is also the lead author of a paper published in the journal eLife.

The fossil, named Wathondara kotejai, is from Kachin Province in northern Myanmar. It is the only Mesozoic (252 to 66 million years ago) record of an adult female scale insect.

“The generic name refers to Wathondara – goddess of earth in Buddhist mythology from Southeast Asia,” Dr Wang and co-authors wrote in the paper.

“The species is named after the late Polish entomologist Jan Koteja in recognition of his significant contribution to the study of both extant and fossil scale insects.”

The 100-million-year-old piece of amber preserves an adult female with eggs, six first-instar nymphs, and a weevil.

“It is polished in the form of a flattened ellipsoid cabochon, clear and transparent, with diameter about 11 mm, height about 5 mm, and weight about 0.8 g.”

Wathondara kotejai was trapped while carrying around 60 eggs and her first freshly hatched nymphs. The eggs and nymphs are encased in a wax-coated egg sac on the abdomen. This primitive form of brood care protects young nymphs from wet and dry conditions and from natural enemies until they have acquired their own thin covering of wax.

The behavior has been so successful for promoting the survival of offspring that it is still common in insects today. Young nymphs hatch inside the egg sac and remain there for a few days before emerging into the outside world.

The findings may even offer an explanation for the early diversification of scale insects.

“Brood care could have been an important driver for the early radiation of scale insects, which occurred during the end of the Jurassic or earliest Cretaceous period during the Mesozoic era,” Dr Wang said.

The only other direct evidence of brood care is from Cenozoic ambers, the era that extends to the present and began about 65 million years ago with the extinction of the dinosaurs.


Bo Wang et al. 2015. Brood care in a 100-million-year-old scale insect. eLife 4: e05447; doi: 10.7554/eLife.05447


Top 5 Worst Mass Extinctions

Tuesday, June 20, 2017

Information and Facts About Mass Extinctions -  National Geographic

Ordovician-Silurian extinctionglobal extinction event occurring during the Hirnantian Age (445.2 million to 443.8 million years ago) of the Ordovician Period and the subsequent Rhuddanian Age (443.8 million to 440.8 million years ago) of the Silurian Period that eliminated an estimated 85 percent of all Ordovician species. This extinction interval ranks second in severity to the one that occurred at the boundary between the Permian and Triassic periods about 251 million years ago in terms of the percentage of marine families affected. The Ordovician-Silurian extinction was almost twice as severe as the K–T extinction event that occurred at the end of the Cretaceous Period, about 66 million years ago, which is famous for bringing an end to the dinosaurs.


The Late Devonian extinction was one of five major extinction events in the history of the Earth’s biota. A major extinction, the Kellwasser event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage (the Frasnian–Famennian boundary), about 375–360 million years ago. Overall, 19% of all families and 50% of all genera became extinct. A second, distinct mass extinction, the Hangenberg event, closed the Devonian period.


The Permian–Triassic (P–Tr or P–Textinction event, colloquially known as the Great Dying, the End-Permian Extinction or the Great Permian Extinction, occurred about 252 Ma (million years) ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras. It is the Earth’s most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects. Some 57% of all families and 83% of all genera became extinct. Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after any other extinction event, possibly up to 10 million years. Studies in Bear Lake County near Paris, Idaho showed a quick and dynamic rebound in a marine ecosystem, illustrating the remarkable resiliency of life.


The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, 201.3 million years ago, and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. In the seas, a whole class (conodonts) and 34% of marine genera disappeared. On land, all archosaurs other than crocodylomorphs (Sphenosuchia and Crocodyliformes) and Avemetatarsalia (pterosaurs and dinosaurs), some remaining therapsids, and many of the large amphibians became extinct.


The Cretaceous–Paleogene (K–Pgextinction event, also known as the Cretaceous–Tertiary (K–Textinction, was a mass extinction of some three-quarters of the plant and animal species on Earth that occurred over a geologically short period of time approximately 66 million years ago. With the exception of some ectothermic species like the leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kilograms (55 lb) survived. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era that continues today.




Pleistocene Epoch

Wednesday, May 24, 2017

Pleistocene Epoch map

The Pleistocene Epoch is typically defined as the time period that began about 1.8 million years ago and lasted until about 11,700 years ago. The most recent Ice Age occurred then, as glaciers covered huge parts of the planet Earth.

There have been at least five documented major ice ages during the 4.6 billion years since the Earth was formed — and most likely many more before humans came on the scene about 2.3 million years ago.

The Pleistocene Epoch is the first in which Homo sapiens evolved, and by the end of the epoch humans could be found in nearly every part of the planet. The Pleistocene Epoch was the first epoch in the Quaternary Period and the sixth in the Cenozoic Era. It was followed by the current stage, called the Holocene Epoch.

Worldwide ice sheets

At the time of the Pleistocene, the continents had moved to their current positions. At one point during the Ice Age, sheets of ice covered all of Antarctica, large parts of Europe, North America, and South America, and small areas in Asia. In North America they stretched over Greenland and Canada and parts of the northern United States. The remains of glaciers of the Ice Age can still be seen in parts of the world, including Greenland and Antarctica.

But the glaciers did not just sit there. There was a lot of movement over time, and there were about 20 cycles when the glaciers would advance and retreat as they thawed and refroze. Scientists identified the Pleistocene Epoch’s four key stages, or ages — Gelasian, Calabrian, Ionian and Tarantian.

The name Pleistocene is the combination of two Greek words: pleistos (meaning “most”) and kainos (meaning “new” or “recent”). It was first used in 1839 by Sir Charles Lyell, a British geologist and lawyer.

As a result of Lyell’s work, the glacial theory gained acceptance between 1839 and 1846, and scientists came to recognize the existence of ice ages. During this period, British geologist Edward Forbes aligned the period with other known ice ages. In 2009, the International Union of Geological Sciences established the start of the Pleistocene Epoch at 1.806 million years before the present.

Defining an epoch

While scientists haven’t been able to determine the exact causes of an epoch, changes in ocean current, composition of the atmosphere, changes in the position of the Earth in relation to the sun are believed to be key contributors.

Overall, the climate was much colder and drier than it is today. Since most of the water on Earth’s surface was ice, there was little precipitation and rainfall was about half of what it is today. During peak periods with most of the water frozen, global average temperatures were 5 to 10 degrees C (9 to 18 degrees F) below today’s temperature norms.

There were winters and summers during that period. The variation in temperatures produced glacial advances, because the cooler summers didn’t completely melt the snow.

Life during the Ice Age

While Homo sapiens evolved, many vertebrates, especially large mammals, succumbed to the harsh climate conditions of this period.

One of the richest sources of information about life in the Pleistocene Epoch can be found in the La Brea Tar Pits in Los Angeles, where remains of everything from insects to plant life to animals were preserved, including a partial skeleton of a female human and a nearly complete woolly mammoth.

In addition to the woolly mammoth, mammals such as saber-toothed cats (Smilodon), giant ground sloths (Megatherium) and mastodons roamed the Earth during this period. Other mammals that thrived during this period include moonrats, tenrecs (hedgehog-like creatures) and macrauchenia (similar to a llamas and camels).

Although many vertebrates became extinct during this period, mammals that are familiar to us today — including apes, cattle, deer, rabbits, kangaroos, wallabies, bears, and members of the canine and feline families — could be found during this time.

Other than a few birds that were classified as dinosaurs, most notably the Titanis, there were no dinosaurs during the Pleistocene Epoch. They had become extinct at the end of the Cretaceous Period, more than 60 million years before the Pleistocene Epoch began.

Birds flourished during this period, including members of the duck, geese, hawk and eagle families. There were also some flightless birds such as ostriches, rheas and moas. The flightless birds did not fare as well, as they had to compete with mammals and other creatures for limited supplies of food and water, as a good portion of the water was frozen.

Crocodiles, lizards, turtles, pythons and other reptiles also thrived during this period.

As for vegetation, it was fairly limited in many areas. There were some scattered conifers, including pines, cypress and yews, along with some broadleaf trees such as beeches and oaks. On the ground, there were prairie grasses as well as members of the lilly, orchid and rose families.

Mass extinction

About 13,000 years ago, more than three-fourths of the large Ice Age animals, including woolly mammoths, mastodons, saber-toothed tigers and giant bears, died out.  Scientists have debated for years over the cause of the extinction, with both of the major hypotheses — human overhunting and climate change — insufficient to account for the mega die-off.

Recent research suggests that an extraterrestrial object, possibly a comet, about 3 miles wide, may have exploded over southern Canada, nearly wiping out an ancient Stone Age culture as well as megafauna like mastodons and mammoths.


10 Interesting Facts About The Geological Time Scale

Wednesday, May 17, 2017

10 Interesting Facts About The Geological Time Scale

1) The geological time scale (GTS) is a system of chronological measurement that relates stratigraphy to time, and is used by geologistspaleontologists, and other Earth scientists to describe the timing and relationships between events that have occurred throughout Earth’s history.

2) Evidence from radiometric dating indicates that Earth is about 4.54 billion years old. The geology or deep time of Earth’s past has been organized into various units according to events which took place in each period.

This clock representation shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago).

3) Different spans of time on the GTS are usually delimited by changes in the composition of strata which correspond to them, indicating major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the non-avian dinosaurs and many other groups of life. Older time spans which predate the reliable fossil record (before the Proterozoic Eon) are defined by the absolute age.

4) The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth were made in the late 18th century. The most influential of those early attempts (championed by Abraham Werner, among others) divided the rocks of Earth’s crust into four types: Primary, Secondary, Tertiary, and Quaternary.

5) The first geologic time scale that included absolute dates was published in 1913 by the British geologist Arthur Holmes.

6) Geologists have divided Earth’s history into a series of time intervals. These time intervals are not equal in length like the hours in a day. Instead the time intervals are variable in length. This is because geologic time is divided using significant events in the history of the Earth.

7) The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periodsepochs and ages. The terms “eonothem“, “erathem“, “system“, “series“, and “stage” are used to refer to the layers of rock that correspond to these periods of geologic time in Earth’s history.

8) Geologists qualify these units as “early“, “mid“, and “late” when referring to time, and “lower“, “middle“, and “upper” when referring to the corresponding rocks. For example, the lower Jurassic Series in chronostratigraphy corresponds to the early Jurassic Epoch in geochronology. The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus “early Miocene” but “Early Jurassic.”

9) The term “Anthropocene” is used informally by popular culture and a growing number of scientists to describe the current epoch in which we are living. The term was coined by Paul Crutzen and Eugene Stoermer in 2000 to describe the current time, in which humans have had an enormous impact on the environment.

10) The Carboniferous is often treated in North America as two geological periods, the earlier Mississippian and the later Pennsylvanian.


The Geological Time Scale: Timeline of Life on Earth

Wednesday, May 17, 2017

The Geological Time Scale: Timeline of Life on Earth

Evolution is a complicated subject. While everybody understands that black bears are related to grizzly bears and we can even figure they are related to extinct bears, lots of people wonder how scientists can be so sure that bears are related to salmon as well.

One evidence is rock layers specifically, what is called the geologic column. Basically, scientists have learned that rocks are stacked in layers containing fossils with the oldest fossils at the deepest layers, and the youngest, or most recent fossils, near the top. It’s as if rock layers are a vertical timeline. At the bottom of the timeline there are no fossils of modern animals. As you move towards the surface, you find fish, then amphibians, then reptiles, mammals, birds, and finally modern mammals including humans.

We’re not talking about an abstract diagram: this is the actual record of the earth’s crust, recorded in rocks around the world.

But how do we know this evolutionary sequence of layers, one on top of the other, is accurate? Why is there any order at all to rock layers?

Two laws, or principles of geology explain why rock layers are formed in this way.

Geological Timeline by Ray Troll


This law of science tells us that dirt, mud, sand and other sediments are almost always deposited in horizontal layers. As these sediments stack up vertically, they often harden, forming rock layers.

The Law of Original Horizontality was first proposed by Danish geological pioneer Nicholas Steno in the 17th century. The law states that layers of sediment were originally deposited horizontally under the action of gravity.


Rock layers are usually ordered with the oldest layers on the bottom, and the most recent layers on top. The Law of Faunal Succession explains that fossils found in rock layers are also ordered in this way.

Grand Canyon Geological Layers


There are thousands upon thousands of layers in the earth’s crust. However, scientists have grouped the layers into major groups. The most recent three layers are the Paleozoic, Mesozoic, and Cenozoic. These layers represent the last 500 million years of life on earth.

In the Paleozoic, you find fish, amphibian, and reptile fossils (in that order), but never dinosaurs, birds, modern mammals, or even flowering plants.

Think of that: despite the billions of plant fossils in the Paleozoic layer, nobody has ever found one fossil of a flower, including any kind of deciduous tree or even a single blade of grass. Why not? The obvious explaination is flowers had not evolved yet.

The next layer, the Mesozoic, is often called the age of dinosaurs. The Mesozoic has dinosaurs like crazy. Of course, dinosaurs are reptiles and that’s why you won’t find any until after the Paleozoic which contains the first reptiles. The Mesozoic also has the first flowering plants, birds, and mammals, though few if any birds or mammals that we know of today.

On top of the Paleozoic and Mesozoic is the Cenozoic. This is the current layer that is still being deposited in oceans, deserts and swamps all around the earth today. The Cenozoic is the first major layer where we find modern mammal fossils like cats, dogs, monkeys and humans. This layer, or “era” is often referred to as the age of mammals.

These three layers make up a sort of 3-layer cake. Just like a cake, the bottom layer went down first, followed by the middle and the top. Since fossils progress from fish at the bottom to humans at the top, we have clear evidence that life evolved through time.


Of course, there isn’t one place in the world to go and see every fossil animal from all time stacked one on top of the other. In fact, it’s rare to find all three major layers on top of one another. Why not?

Well the first obvious answer is that even in the world today there are places where sediments (layers) are deposited but in other places (like mountains) they are eroded. So gaps are a common occurrence in many regions.

Also, while the layers are usually deposited in a clear order, those layers are often disturbed later on by volcanoes, rivers, mountains, and shifting continents.

Look at the diagram at right. If you were to stand on the cliff to the left side of the cross section, you would see the top layer in two places. The cracks, or faults, in the rock have slid the layers out of alignment. Only when you view the entire area can you piece the original order back together.

The crust of the earth is made of several huge plates. These plates “float” on the hot, soft mantle below the crust. We can actually measure the movement of the plates using satellites in space. Every year, they shift in different directions, each on their own path. Sometimes the plates collide, causing mountains. Other times, they separate and hot magma flows up to form volcanic islands and new land. It happens slowly but surely and as it does, our nice three layer cake becomes a little messier.

It’s as if somebody slid the cake off the table, and the dog ate half of it before dad comes to the rescue. Look at any one spot and you might not find all three layers in the right order, but look at the big picture and the original order is still visible.

There are many evidences of evolution, but the geologic column remains the most obvious clue to the history of life on earth.


Related Links:

Evolution Of Dinosaurs: Faster Than Previously Believed

Sunday, March 26, 2017

Honey, who shrunk the dinosaurs? Study traces dinosaur evolution into early birds

Dinosaurs were in decline for tens of millions of years before the Earth was struck by an asteroid, ending their dominion over the planet. What was killing off dinosaurs near the end of their reign?

Scientists previously thought that dinosaurs evolved from their smaller ancestors over a period of at least 10 million years but findings of a new study suggest that the evolution occurred in less than five million years.

For the new study published in the Proceedings of the National Academy of Sciences on Dec. 7, 2015 Randall Irmis, from the Natural History Museum, and colleagues used radioactive isotope measurements for dating the zircon crystals that were found in the sediments of the Chañares Formation, which is known for its fossils of early dinosaur relatives.

The analysis revealed that the formation is between 234 million and 236 million years old from the Late Triassic period, which means that the fossils of the dinosaur’s reptile predecessors, the early dinosauromorphs, that were sandwiched in the rock layers are of the same age.

The early dinosauromorphs were like the dinosaurs sans some key features such as the former having a ball-and-socket hip that rotates easily and an additional vertebra at the end of their spine.

FOSSIL BED Early dinosaur ancestors like the pair on the right were thought to evolve around 10 million years before dinosaurs. But new dating of fossil layers in Argentina cuts that time in half, to about 5 million years. IMAGE COURTESY OF VICTOR LESHYK

Scientists have already studied dinosauromorphs but there were uncertainties about their age since biostratigraphy, the technique used to date their fossils, were not as accurate as other dating methods such as the one employed in the new study.

The findings provided evidence that the early dinosauromorphs lived between five to 10 million years earlier than previously believed revamping the long held timeline of the early dinosauromorphs evolving into dinosaurs. The study likewise offered proof that the dinosaurs evolved much faster than previously thought.

“We constrain the rate of dinosaur origins, demonstrating their relatively rapid origin in a less than 5-Ma interval, thus halving the temporal gap between assemblages containing only dinosaur precursors and those with early dinosaurs,” Irmis and colleagues wrote.

The researchers said that although the dinosaurs may have evolved rapidly, the prehistoric animals appear to have dominated paleo-Earth in a smooth and gradual manner.  It took quite a while for the  prehistoric giants to spread globally as it took them millions of year after their origin to gradually dominate the mid to high-latitude regions of the Earth.

“You don’t seem to see dinosaurs showing up and immediately taking over,” Irmis said. “It really emphasizes that there wasn’t much special about the first dinosaurs. They were pretty similar to their early dinosauromorph relatives and probably doing very similar things.”



Jaws to Ears in the Ancestors of Mammals

Sunday, March 26, 2017

Ancestors of Mammals

All the animals you see on this evogram are synapsids, the group that gave rise to the mammals. Sometimes synapsids are called “mammal-like reptiles;” however, that is misleading because synapsids are not reptiles. Synapsids and reptiles are two distinct groups of amniotes, animals that produce young that are enveloped with a membrane called an amnion that prevents desiccation. All reptiles (including birds) have eggs with amniotic membranes (which some lay and others retain inside their bodies until hatching). And of course all mammals (the clade of synapsids still alive today) reproduce using an amnion, and those that lay eggs (e.g., the platypus and echidna) produce amniotic eggs.

Mammal evogram

Like birds, crocodiles, turtles, snakes, lizards, amphibians, and most fishes, the earliest synapsids had a bone in the back of the skull on either side called the quadrate that made the connection with the lower jaw via a bone called the articular. But mammals today, including humans, use two different bones, called the squamosal and the dentary, to make this connection. How did this new jawbone configuration evolve?

For reasons we don’t fully understand, several lineages of synapsids — including the one that would eventually give rise to the mammals — began to evolve changes in the jaw joint. Originally the quadrate and articular bones formed the jaw joint, but these synapsids (e.g., Probainognathus) evolved a second pair of bones involved in the jaw articulation. The squamosal bone was positioned alongside the quadrate in the upper jaw, and the dentary was positioned alongside the articular in the lower jaw.

Skull of Probainognathus, an early synapsid.

This unusual paired condition did not last long, though. Soon, the quadrate and articular lost their function in jaw articulation and even their position in the jaw as they evolved. They became increasingly smaller and eventually migrated into the ear region, where they became the “hammer” and “anvil” of the ear. So, over time, the synapsids’ quadrate-articular jaw joint (which the rest of the tetrapods possess) was replaced by a dentary-squamosal joint (which all living mammals possess), while the quadrate and articular migrated, shrank, and became part of the complex of middle ear bones.

Evolution of the jaw joint in synapsids. Abbreviations used: a-articular, d-dentary, q-quadrate, s-squamosal.

Only in recent years has it become apparent that several lineages of synapsids, including mammals, replaced their quadrate-articular jaw joint with a dentary-squamosal joint. We don’t fully understand why these changes happened. Some evidence suggests that the change in the quadrate-articular complex improved hearing. Other evidence suggests that these changes were a byproduct of early mammals’ increasing brain size. These ideas are not mutually exclusive, of course, and more research is needed. Whatever the functional advantages may have been, the pattern of evolution in these features clearly shows another example of exaptation: the incorporation of the dentary and squamosal bones into the jaw joint, originally alongside the quadrate and articular, eventually allowed the latter two bones to acquire a completely different function and to leave the jaw articulation altogether.



Saturday, March 11, 2017

The Paleocene or Palaeocene, the “old recent”, is a geologic epoch that lasted from about 66 to 56 million years ago. It is the first epoch of the Paleogene Period in the modern Cenozoic Era. As with many geologic periods, the strata that define the epoch’s beginning and end are well identified, but the exact ages remain uncertain.

Gastornis is an extinct genus of large flightless birds that lived during the late Paleocene and Eocene epochs of the Cenozoic era. Initially thought to be a predator, new theories suggest this large bird may have been a herbivore. Paleoart by Jacek Major

The Paleocene Epoch brackets two major events in Earth’s history. It started with the mass extinction event at the end of the Cretaceous, known as the Cretaceous–Paleogene (K–Pg) boundary. This was a time marked by the demise of non-avian dinosaurs, giant marine reptiles and much other fauna and flora. The die-off of the dinosaurs left unfilled ecological niches worldwide. The Paleocene ended with the Paleocene–Eocene Thermal Maximum, a geologically brief (~0.2 million year) interval characterized by extreme changes in climate and carbon cycling.

The name “Paleocene” comes from Ancient Greek and refers to the “old(er)” (παλαιός, palaios) “new” (καινός, kainos) fauna that arose during the epoch.

South American hoofed mammals, during the Miocene and Paleocene periods

Boundaries and subdivisions

The K–Pg boundary that marks the separation between Cretaceous and Paleocene is visible in the geological record of much of the Earth by a discontinuity in the fossil fauna, with high iridium levels. There is also fossil evidence of abrupt changes in flora and fauna. There is some evidence that a substantial but very short-lived climatic change may have happened in the very early decades of the Paleocene. There are several theories about the cause of the K–Pg extinction event, with most evidence supporting the impact of a 10 km diameter asteroid forming the buried Chicxulub crater on the coast of Yucatan, Mexico.

The end of the Paleocene (~55.8 Ma) was also marked by a time of major change, one of the most significant periods of global change during the Cenozoic. The Paleocene–Eocene Thermal Maximum upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and a major turnover in mammals on land.

The Paleocene is divided into three stages, the Danian, the Selandian and the Thanetian, as shown in the table above. Additionally, the Paleocene is divided into six Mammal Paleogene zones.


The early Paleocene was cooler and dryer than the preceding Cretaceous, though temperatures rose sharply during the Paleocene–Eocene Thermal Maximum. The climate became warm and humid worldwide towards the Eocene boundary, with subtropical vegetation growing in Greenland and Patagonia, crocodilians swimming off the coast of Greenland, and early primates evolving in the tropical palm forests of northern Wyoming. The Earth’s poles were cool and temperate; North America, Europe, Australia and southern South America were warm and temperate; equatorial areas had tropical climates; and north and south of the equatorial areas, climates were hot and arid, not dissimilar to today’s global desert belts around 30 degrees northern and southern latitude.


In many ways, the Paleocene continued processes that had begun during the late Cretaceous Period. During the Paleocene, the continents continued to drift toward their present positions. Supercontinent Laurasia had not yet separated into three continents – Europe and Greenland were still connected, North America and Asia were still intermittently joined by a land bridge, while Greenland and North America were beginning to separate. The Laramide orogeny of the late Cretaceous continued to uplift the Rocky Mountains in the American west, which ended in the succeeding epoch.

South and North America remained separated by equatorial seas (they joined during the Neogene); the components of the former southern supercontinent Gondwanaland continued to split apart, with Africa, South America, Antarctica and Australia pulling away from each other. Africa was heading north towards Europe, slowly closing the Tethys Ocean, and India began its migration to Asia that would lead to a tectonic collision and the formation of the Himalayas.

The inland seas in North America (Western Interior Seaway) and Europe had receded by the beginning of the Paleocene, making way for new land-based flora and fauna.

The Western Interior Seaway


Warm seas circulated throughout the world, including the poles. The earliest Paleocene featured a low diversity and abundance of marine life, but this trend reversed later in the epoch. Tropical conditions gave rise to abundant marine life, including coral reefs. With the demise of marine reptiles at the end of the Cretaceous, sharks became the top predators. At the end of the Cretaceous, the ammonites and many species of foraminifera became extinct.

Marine fauna also came to resemble modern fauna, with only the marine mammals and the Carcharhinid sharks missing.


Terrestrial Paleocene strata immediately overlying the K–Pg boundary is in places marked by a “fern spike”: a bed especially rich in fern fossils. Ferns are often the first species to colonize areas damaged by forest fires; thus the fern spike may indicate post-Chicxulub crater devastation.

In general, the Paleocene is marked by the development of modern plant species. Cacti and palm trees appeared. Paleocene and later plant fossils are generally attributed to modern genera or to closely related taxa.

The warm temperatures worldwide gave rise to thick tropical, sub-tropical and deciduous forest cover around the globe (the first recognizably modern rain forests) with ice-free polar regions covered with coniferous and deciduous trees. With no large browsing dinosaurs to thin them, Paleocene forests were probably denser than those of the Cretaceous.

Flowering plants (angiosperms), first seen in the Cretaceous, continued to develop and proliferate, and along with them coevolved the insects that fed on these plants and pollinated them.



Mammals had first appeared in the Late Triassic, evolving from advanced cynodonts, and developed alongside the dinosaurs, exploiting ecological niches untouched by the larger and more famous Mesozoic animals: in the insect-rich forest underbrush and high up in the trees. These smaller mammals (as well as birds, reptiles, amphibians, and insects) survived the mass extinction at the end of the Cretaceous which wiped out the non-avian dinosaurs, and mammals diversified and spread throughout the world.

Pantodonts, uintatheres and xenungulates: The first large herbivorous mammals. By Kelly Taylor

While early mammals were small nocturnal animals that mostly ate soft plant material and small animals such as insects, the demise of the non-avian dinosaurs and the beginning of the Paleocene saw mammals growing bigger and occupying a wider variety of ecological niches. Ten million years after the death of the non-avian dinosaurs, the world was filled with rodent-like mammals, medium-sized mammals scavenging in forests, and large herbivorous and carnivorous mammals hunting other mammals, birds, and reptiles.

Fossil evidence from the Paleocene is scarce, and there is relatively little known about mammals of the time. Because of their small size (constant until late in the epoch) early mammal bones are not well preserved in the fossil record, and most of what we know comes from fossil teeth (a much tougher substance), and only a few skeletons.

The brain to body mass ratios of these archaic mammals were quite low.

Mammals of the Paleocene include:

  • Monotremes: The ornithorhynchid Obdurodon sudamericanum, in the family that includes the platypus, is the only monotreme known from the Paleocene.
  • Marsupials: modern kangaroos are marsupials, characterized by giving birth to embryonic young, who crawl into the mother’s pouch and suckle until they are developed. The Bolivian Pucadelphys andinus and the North American Peradectesare two Paleocene examples.
  • Multituberculates: the only major branch of mammals to become extinct since the K–Pg boundary, this rodent-like grouping includes the Paleocene Ptilodus.
  • Placentals: this grouping of mammals became the most diverse and the most successful. Members include primates, plesiadapids, proboscideans, and hoofed ungulates, including the condylarths and the carnivorous mesonychids.


Because of the climatic conditions of the Paleocene, reptiles were more widely distributed over the globe than at present. Among the sub-tropical reptiles found in North America during this epoch are champsosaurs (aquatic reptiles that resemble modern gharials), crocodilia, soft-shelled turtles, palaeophi snakes, varanid lizards, and Protochelydra zangerli (similar to modern snapping turtles).

Choristodera is an order of semi-aquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Late Triassic, to at least the early Miocene. It was named by Edward Drinker Cope in 1884.

Examples of champsosaurs of the Paleocene include Champsosaurus gigas, the largest champsosaur ever discovered. This creature was unusual among Paleocene reptiles in that C. gigas became larger than its known Mesozoic ancestors: C. gigas is more than twice the length of the largest Cretaceous specimens (3 meters versus 1.5 meters). Reptiles as a whole decreased in size after the K–Pg event. Champsosaurs declined towards the end of the Paleocene and became extinct during the Miocene.

Examples of Paleocene crocodylians are Borealosuchus (formerly Leidyosuchusformidabilis, the apex predator and the largest animal of the Wannagan Creek fauna, and the alligatorid Wannaganosuchus.

Non-avian dinosaurs may have survived to some extent into the early Danian stage of the Paleocene Epoch circa 64.5 Mya. The controversial evidence for such is a hadrosaur leg bone found from Paleocene strata in New Mexico; but such stray late forms may be derived fossils.


Birds began to re-diversify during the epoch, occupying new niches. Genetic studies suggest that nearly all modern bird clades can trace their origin to this epoch, with Neornithes having undergone an extremely fast, “star-like” radiation of species in the early Palaeocene in response to the vacancy of niches left by the KT event.

Modern Birds: The Neornithes

Large flightless birds have been found in late Paleocene deposits, including the omnivorous Gastornis in Europe and carnivorous terror birds in South America, the latter of which survived until the Pleistocene.

In the late Paleocene, early owl types appeared, such as Ogygoptynx in the United States and Berruornis in France.


Began: Cretaceous-Tertiary mass extinction
65 million years ago

Ended: 54.8 million years ago