Late Devonian to Permian

 

Records of the expansion of life onto land

Information to know

The basis for the information

Comments

The Late Devonian experienced a mass extinction in marine animals.

Paleontologists compiled information from scientific references globally to figure out how many different kinds of animals there have been at each interval in Earth history.

The Late Devonian extinction was one of five Phanerozoic mass extinctions. It seems to have been the most gradual of the five, taking place over several million years. The cause remains poorly known.

Land vertebrates – amphibians arise at the end of the Devonian. These amphibians still have many fish-like characteristics.

Fossils in Greenland and Pennsylvania, among others.

The primary anatomical differences include modification of limb bone structure from that of lobe-finned fish, modification of skull, and modification of vertebrae so animal could support itself and move on land.

“Bony fish,” which today include 99% of all fish species, diversify in the seas and become the dominant kind of fish.

Marine fossil record.

 

The first forests occur in the Late Devonian, dominated worldwide by a plant known as Archaeopteris[1].

Archaeopteris had fern-like leaves and reproduced through spores rather than seeds.

Forests were extremely important, creating new habitats and also influencing rates of weathering and erosion.

Giant forests thrived in swampy areas worldwide during the Carboniferous. These swamps were dominated by lycopod trees (related to “club mosses” of today), but included other plants related to horsetails, “tree-ferns,” and other plants now extinct.

There are excellent records of these forests because of the wet environment. So much organic matter accumulated that O2 was quickly used up by bacteria, causing enormous sedimentary accumulations. These formed coals.

Ithaca’s electricity comes from coal from Pennsylvania – we are burning wood from the Pennsylvanian Period.

 

Most continents come together in “supercontinent” known as “Pangea” during the Permian, creating a more extreme and arid environment in the interior of Pangea.

Coal swamps mostly disappear. Rocks from this interval are characteristic of dry climates.

Climates are moderated by water bodies such as lakes and oceans, which maintain a relatively stable temperature. The bigger the landmass and further from a major water body, the more extreme and arid the interior of the landmass will be.

Reptiles arise in the early Carboniferous, and are similar in many anatomical respects from their amphibian predecessors. They presumably have amniotic (hard-shelled) eggs.

Bones have been found in early Carboniferous rocks as close as Pennsylvania. Reptiles are defined in part on the basis of their eggs, but no eggs have yet been found from this interval.

Organisms have a wide variety of characteristics, some of which are preserved in skeletons, but some of which involve soft parts, eggs, or other features that are only rarely found in the fossil record.

Reptiles are relatively insignificant until later in the Carboniferous, and become the dominant land animal in the Permian.

Based on the fossil record of bones worldwide.

The amniotic egg and reptilian characteristics such as scaly skin enable reptiles to live in very dry habitats. This enabled reptiles to move into new drier habitats, and to thrive when climates turned drier.

The dominant reptiles of the late Carboniferous and Permian are synapsid or so-called “mammal-like” reptiles that would eventually lead to mammals.

There are especially rich records from South Africa, but they are known worldwide.

Other reptiles such as the predecessors to dinosaurs and modern groups of reptiles were present, but not as abundant.

One mammal-like reptile well-known to the public is Dimetrodon, popularly known as the “sail-back reptile.” Dimetrodon is not a dinosaur, nor did it live at the same time as dinosaurs.

 

Many decades ago popular authors put Dimetrodon in children’s dinosaur books, and the error has been propogated ever since.

The Permo-Triassic ended with the biggest mass extinction ever known, wiping out most species of animals on both land and sea.

Fossil records worldwide have been compiled to give us information about changes in diversity. This extinction, however, was recognized by the mid-1800s, however, since it was so obvious.

The extinction is the basis for the boundary between the Permian and the Triassic Periods.

 

Concepts:

 

Science as a process

 

Science is the process of figuring out how the world works by testing out ideas through watching nature to see if it actually looks or behaves like we thought it would. We employ such common sense thinking in our daily lives, but science takes the method to its most rigorous, logical extreme. Laboratory sciences involve testing ideas by setting up an experiment and watching what happens. Historical sciences (sciences that deal with natural events from the past) like paleontology cannot literally replicate history, so we have to look for telltale evidence left behind as our observations. The best way of doing this with some confidence is to look for consistency among a wide variety of independent of evidence – confluence of evidence. This gathering of evidence to make a case for a particular viewpoint is why doing paleontology is so often compared to gathering detective evidence for a criminal case.

 

Based on this, it is easy to see that questions that fall within the realm of science are “testable” – that means that you must potentially be able to make some kind of observations to show an idea is wrong. For example, the color of most ancient organisms seems to be permanently lost, so cannot be studied scientifically, even though the issue is interesting. Ideas that are not testable are not necessarily false, but they simply cannot be investigated by science. Some ideas that cannot currently be studied in a scientific way may be candidates for science at some later time, if we find new forms of information that relate to the truth or falseness of an idea.

 

It is generally assumed that a scientific hypothesis is more likely to be true if it explains all the available observations. A hypothesis that explains the extinction of the dinosaurs through a disease fails to explain why so many other types of organisms, on land and in the sea, also disappeared at the same time. Simple intuition suggests that an ecological crisis that could affect many kinds of organisms all at once is more likely to be true.  Famous examples of hypotheses that elegantly explained dozens of kinds of information were biological evolution and plate tectonics.

 

Adaptations to land

 

Emergence onto land was a long process. The fossil record of this emergence is rather poor because fossils tend to form where sediments accumulate, which is usually in aquatic settings. Many organisms, such as amphibians and spore-bearing plants, were still tied to aquatic environments during at least one stage of their lives. The key innovations that allowed plants and animals to move onto land without requiring wet habitats involved preservation of water throughout life, including reproduction and growth of the empbryo.

 

Animals of the Devonian-Carbinferous are a bit like animals of the Cambrian, in that early on significant disparity developed, even before diversity itself was not extremely high. In this case, moving into a new habitat, disparity increases before diversity.

 

Mass extinctions

 

Most of the animals that have ever lived are now extinct. Most of these lived a few million years and then went extinct, as part of what is known as “background” extinct in order to distinguish it from “mass extinction.” Mass extinction is an unusually high rate of extinction over a relatively short time and that may drive extinct a wide variety of organisms. There are five that really stand out: near the end of the Ordovician, near the end of the Devonian, at the Permian-Triassic boundary, near the end of the Triassic, and at the Cretaceous-Tertiary boundary.

 

The Cretaceous-Tertiary boundary is most famous because it involved the extinction of the dinosaurs and of other large aquatic and flying reptiles. It also involved, however, many other organisms on land and in the sea, including plants, invertebrate animals, and protests.

 

The Permian-Triassic extinction, however, is the largest. Over ½  the families of marine animals went extinct, and nearly ¾ the families of land vertebrates. It has been estimated that if over ½ the marine families went extinct, then over 90% of the species must have gone extinct. It included all species of skeletonized corals (today’s corals seem to have arisen from a different group of coral-like animals), all trilobites (they were most extinct by then anyway, however), most species of brachiopods, and many other marine organisms; it included many of the mammal-like reptiles as well.

 

Mass extinctions are extremely important because it would seem that many organisms went extinct in unusual ecological catastrophes. Often the organisms that are ecological significant after a mass extinction are not those that were important beforehand. Many people are aware that it wasn’t until dinosaurs disappeared that mammals diversified into many habitats and became the ecologically important animals they are today. Similarly, brachiopods (for example) were a key part of marine environments of the Paleozoic, but have been fairly insignificant since the Permian-Triassic boundary.

 

The Permian-Triassic extinction

 

Hot topic

The cause of the Permian-Triassic (P-Tr) extinction has been intensively studied in the past decade, in part because it is such an important problem to solve to understand the history of life, but fueled by new data, especially from places such as China that were previously behind the “Iron Curtain.” 

 

Traditional ideas about the P-Tr involve changing climate and sea level. It was unknown over how long a period the extinction may have taken place, because the geological record is rather poor over the boundary because of low sea level (remember that low sea level means less continental seas and continental shelf for sediments to accumulate and fossils to form). Climate change, however, did not seem tightly connected to extinction.

 

It has been suggested that limited area alone could have caused extinction of marine faunas. Bigger areas can hold more species. As Pangea closed, some coastline was lost where the continents sutured together. And as sea level dropped, areas for living in continental seas decreased. Further, continental slopes are steeper than continental shelves, so a sea level drop leaving shelves dry meant organisms had to live on the steeper slope, further decreasing the area they could live. These factors along may account for part of the decline of marine families.

 

Further, although land animals would have the same amount of area when Pangea formed, the different animals on the formerly separated land masses could have competed, losing further diversity. This loss of geographic distinction is called a loss of  “provinciality.”

 

Yet, looking broadly at diversity through geological time, and across areas as big as continents, it is not clear that sea level and habitat area are very tightly connected to global diversity.  Might there be other reasons?

 

Ocean circulation is deeply affected by continental configuration. Water bodies get stagnant if the surface water doesn’t occasionally sink to the bottom with fresh oxygen. If the global ocean grew stagnant, organic matter would rot to the point that all the oxygen would be used up and carbon dioxide would accumulate to lethal concentrations; thus the next time the ocean somehow did overturn, a lethal injection of carbon dioxide would be burped into the atmosphere and mixed with the shallow marine waters. This hypothesis was invented to explain geochemical signals at this boundary.

 

In about 1980 it was proposed that an extraterrestrial impact had caused the Cretaceous-Tertiary extinction. Though sounding like a tabloid story, many forms of evidence proved to be consistent with this idea, including the element Iridium that first gave rise to the idea (high concentrations of iridium is known only from meteors and the Earth’s interior), tektites (small spherules of meteoritic material), shocked quartz, and other evidence. Just in the past few years evidence seems to be mounting for an impact at the P-Tr boundary. This is an active field of research and doubtless new articles will be published in the coming months.

 

 

 

 



[1] Yes, the name Archaeopteris is very similar to the name of the primitive bird Archaeopteryx. “Archae” means ancient in Latin, so is a common prefix of words in paleontology. American English tends to drop to “e” -- note also the field of “archeology.”