Knowing fossils and their age

In All You Need Is Biology we often make reference to fossils to explain the past of living beings. But what is exactly a fossil and how is it formed? Which is the utility of fossils? Have you ever wondered how science knows the age of a fossil? Read on to find out!

WHAT IS A FOSSIL?

If you think of a fossil, surely the first thing that comes to your mind is a dinosaur bone or a petrified shell that you found in the forest, but a fossil is much more. Fossils are remnants (complete or partial) of  living beings that have lived in the past (thousands, millions of years) or traces of their activity that are preserved generally in sedimentary rocks. So, there are different types of fossils:

• Petrified and permineralized fossils: are those corresponding to the classical definition of fossil in which organic or hollow parts are replaced with minerals (see next section). Its formation can leave internal or external molds in which the original material may disappear.

  

  Petrified fossil of horseshoe crab and its footsteps. CosmoCaixa. Photo: Mireia Querol Rovira

• Ichnofossils (trace fossils): traces of the activity of a living being that are recorded in the rock and give information about the behavior of the species. They may be changes in the environment (nests and other structures), traces (footprints), stools (coprolites -excrements-, eggs …) and other traces such as scratches, bites…

  

  Dinosaur eggs (nest). CosmoCaixa. Photo: Mireia Querol Rovira

  

  Coprolites, CosmoCaixa. Photo: Mireia Querol Rovira

• Amber: fossilized resin of more than 20 million years old. The intermediate state of amber is called copal (less than 20 million years) old. The resin, before becoming amber can trap insects, arachnids, pollen… in this case is considered a double fossil.

  

  Piece of amber with insects inside, CosmoCaixa. Photo: Mireia Querol Rovira

• Chemical fossils: are fossil fuels like oil and coal, which are formed by the accumulation of organic matter at high pressures and temperatures along with the action of anaerobic bacteria (bacteria that don’t use oxygen for metabolism).
• Subfossil: when the fossilization process is not completed the remains are known as subfossils. They don’t have more than 11,000 years old. This is the case of our recent ancestors (Chalcolithic).

  

  Ötzi a subfossil. It is Europe’s oldest natural mummy. He lived during the Chalcolithic (Copper Age) and died 5300 years ago. Photo: Wikimedia Commons

• Living fossils: name given to today’s living organisms very similar to species extinct. The most famous case is the coelacanth, it was believed extinct for 65 million years until it was rediscovered in 1938, but there are other examples such as nautilus.

  

  Comparison between the shell of a current nautilus (left) with an ammonite of millions of years old (right). CosmoCaixa. Photo: Mireia Rovira Querol

• Pseudofossils: are rock formations that seem remains of living beings, but in reality they are formed by geological processes. The best known case is pyrolusite dendrites that seem plants. 

Pirolusita infiltrations in limestone. CosmoCaixa. Photo: Mireia Querol

Obviously fossils became more common after the appearance of hard parts (shells, teeth, bones …), 543 million years ago (Cambrian Explosion). The fossil record prior to this period is very scarce. The oldes tknown fossils are stromatolites, rocks that still they exist today formed by the precipitation of calcium carbonate because of the activity of photosynthetic bacteria.

The science of fossils is Paleontology.

Stromatolite 2,800 million years old, Australian Museum. Photo: Mireia Querol Rovira

HOW A FOSSIL IS FORMED?

The fossilization can occur in five ways:

• Petrifaction: is the replacement of organic material by minerals from the remains of a living being buried. An exact copy of the body is obtained in stone. The first step of petrificationis  permineralization: the pores of the body are filled with mineral but organic tissue is unchanged. It is the most common method of fossilized bones).
• Gelling: the body becomes embedded in the ice and don’t suffer transformations .
• Compression : the dead body is on a soft layer of soil, such as clay, and is covered by layers of sediment .
• Inclusion : organisms trapped in amber, or petroleum .
• Impression: organisms leave impressions in the mud and the trace is preserved until the clay hardens.

  

  Fossilization processes and resulting fossils. Unknown author

  UTILITY OF FOSSILS

• Fossils give us information on how living things were in the past, resulting in evidence of the biological evolution and help to establish the lineages of living things today.
• Allow analyzing of cyclical phenomena such as climate change, atmosphere-ocean dynamics and even orbital perturbations of the planets.
• Those who are of a certain age can be use to date the rocks in where they are found (guide fossils).
  
• They give information of geological processes such as the movement of the continents, the presence of ancient oceans, formation of mountains…
• The chemical fossils are our main source of energy .
• They give climate information from the past, for example, studying the growth of rings in fossilized trunks or deposition of organic matter in the glacial varves.

  

  Fossil trunks where growth rings are observed. American Museum of Natural History. Photo: Mireia Querol Rovira

  DATING FOSSILS

  To determine the age of fossils there are indirect methods (relative dating) and direct (absolute dating). As there is no perfect method and accuracy decreases with age, the sites are often dated with more than one technique.

  RELATIVE DATING

  The fossils are dated according to the context in which they are found, if they are associated with other fossils (guide fossils) or objects of known age and it depends on the stratum they are found.

  In geology, stratums are different levels of rocks that are ordered by their depth: according to stratigraphy, the oldest ones are found at greater depths, while the modern ones are more superficial, as the sediments have not had much time to deposit on the substrate. Obviously if there are geological disturbances dating would be wrong if there were only this method.

  

  Stratigraphic timescale. Picture: Ray Troll

  ABSOLUTE DATING

  This methods are more accurate and are based on the physical characteristics of matter.

  RADIOMETRIC DATING

  They are based on the rate of decay of radioactive isotopes in rocks and fossils. Isotopes are atoms of the same element but with different number of neutrons in their nuclei. Radioactive isotopes are unstable, so they are transformed into a more stable ones at a rate known to scientists emitting radiation. Comparing the amount of unstable isotopes to stable in a sample, scientits can estimate the time that has elapsed since the fossil or rock formed.

  

  Carbon-14 cycle. Unknown author

• Radiocarbon (Carbon-14): in living organisms, the relationship between C12 and C14 is constant, but when they die, this relationship changes: the uptake of C14 stops and decay with a descomposing rate of 5730 years. Knowing the difference between C12 and C14 of the sample, we can date when the organism died. The maximum limit of this method are 60,000 years, therefore only applies to recent fossils.
• Aluminum 26-Beryllium 10: it has the same application as the C14, but has a much greater decaying period, allowing  datings up to 10 datings millions of years, and even up to 15 million years.
• Potassium-Argon (⁴⁰K/⁴⁰Ar): is used to date rocks and volcanic ash older than than 10,000 years old. This was the method used to date the Laetoli footprints, the first traces of bipedalism of our lineage left by Australopithecus afarensis.
  
• Uranium Series (Uranium-Thorium): various techniques with uranium isotopes. They are sed in mineral deposits in caves (speleothems) and in calcium carbonate materials (such as corals).
• Calcium 41: allows to date bones in a time interval from 50,000 to 1,000,000 years .

PALEOMAGNETIC DATING

The magnetic north pole has changed throughout the history of Earth and its geographical coordinates are known in different geological eras.

Some minerals have magnetic properties and are directed towards the north magnetic pole when in aqueous suspension, for example clays. But when laid on the ground, they are fixed to the position that the north magnetic pole was at the time. If we look at what coordinates are oriented such minerals at the site, we can associate it with a particular time.

Deposition of magnetic particles oriented towards the magnetic north pole. Source: Understanding Earth, Press and Seiver, W.H. Freeman and Co.

This dating is used on clay remains and as the magnetic north pole has been several times in the same geographical coordinates, you get more than one date. Depending on the context of the site, you may discard some dates to reach a final dating.

THERMOLUMINESCENCE DATING AND  OPTICALLY STIMULATED LUMINESCENCE (OSL)

Certain minerals (quartz, feldspar, calcite …) accumulate in its crystal structure changes due to radioactive decay of the environment. These changes are cumulative, continuous and time dependent to radiation exposure. When subjected to external stimuli, mineral emits light due to these changes. This luminescence is weak and distinct as apply heat (TL), visible light (OSL) or infrared (IRSL).

Fluorite’s thermoluminescence. Photo: Mauswiesel

Can be dated samples that were protected from sunlight and heat to more than 500 ° C, otherwise the “clock” is reset as the energy naturally releases.

ELECTRON PARAMAGNETIC RESONANCE (ESR)

The ESR (electro spin resonance) involves irradiating the sample and measuring the energy absorbed by the sample depending on the amount of natural radiation which it has been subjected during its history. It is a complex method which you can get more information here.

REFERENCES

• Carbonell, E. (coord) (2011). Homínidos. Las primeras ocupaciones de los continentes (pg 26-37). Ariel Historia.
• Coenraads, R. (2005). Rocas y fósiles. Libros cúpula.
• Tipos de fósiles
• New Word Encyclopaedia
• Dating rocks and fossils using geological methods
• CENIEH – Centro Nacional de Investigación sobre la Evolución Humana
• Procesos químicos de fosilización
• Museo Virtual de Paleontología
• Understanding evolution 
• Cover photo by Mireia Querol (Tarbosaurus)

The evolution of amphibians: the conquest of the land

Amphibians were the first group of vertebrates to develop limbs and to be able to leave the water to conquer the land. Even if they are seen as simple and primitive animals by most people, amphibians show a wide diversity of survival strategies which have allowed them to occupy most terrestrial and fresh-water habitats. On this entry we’ll explain some of the aspects related to their evolution, explaining how our ancestors managed to get out of the water.

ORIGIN OF THE AMPHIBIANS

Current amphibians, together with reptiles, birds and mammals are found within the superclass Tetrapoda (“four limbs”), the vertebrate group that abandoned the sea to conquer the land. These first tetrapods were amphibians and they evolved around 395 million years ago during the Devonian period from lobe-finned fish named sarcopterygians (class Sarcopterygii, “flesh fins”) within which we find the coelacanth and the current lungfish.

Specimen of coelacanth (Latimeria chalumnae) a sarcopterygian fish, photo by smerikal.

This group of fish is characterized by its fins which, instead of being formed by rays like in most bony fish, they have a bony base that allowed the subsequent evolution of the limbs of the first amphibians. Within the sarcopterygians, the nearest relatives of the tetrapods are the osteolepiformes (order Osteolepiformes) a group of tetrapodomorph fish that got extinct about 299 million years ago.

Restoration of Eusthenopteron, an extinct osteolepiform, by Nobu Tamura.

ADAPTATIONS TO LIVE ON LAND

The conquest of land was not done from one day to the other; it was possible with the combination of multiple adaptations. Some of the most important characteristics that allowed the first amphibians to leave the water were:

• Evolution of lungs, which are homologous to the gas bladder that allows fish to control its buoyancy. Lungs appeared as an additional way to get oxygen from the air. In fact, there is actually a sarcopterygian family the members of which have lungs to get oxygen from the air, for they live in waters poor on oxygen.

• Dissection of Protopterus dolloi a sarcopteryigian fish with lungs.

• Development of the choanaes, or internal nostrils. While fish present a pair of external nostrils at each side of its snout through which water passes on while swimming, the ancestors of the tetrapods only had one external nostril at each side connected to the internal nostrils, the choanae, which communicated with the mouth. This allowed them to get air through their noses using lung ventilation and this way to smell outside of water.
• Apparition of the quiridium-like limb. The quiridium is the tetrapod’s most basic characteristic. This limb is known for having the differentiated parts: the stylopodium (one bone, the humerus or the femur), the zeugopodium (two bones, the radius or tibia and ulna or fibula) and the autopodium (fingers, hands, toes and feet). While the stylopodium and zeugopodium derived from the sarcopterygian’s fins, the autopodium is a newly-evolved structure exclusive from tetrapods.

Simplified drawing of the structure of the quiridium, by Francisco Collantes.

In short, the relatives of the osteolepiformes developed the tetrapod’s typical characteristics before ever leaving water, because they probably lived in brackish, shallow waters, poor in oxygen and that dried out quickly and often.

THE FIRST AMPHIBIANS

Probably the creature known as Tiktaalik is the closest animal to the mid-point between the osteolepiformes and the amphibians. The first recorded amphibians were labyrinthodonts meaning that their teeth had layers of dentin and enamel forming a structure similar to a maze.

Cross-section of a labyrinthodont tooth, form "On the Genesis of Species", by St. George Mivart.

There were four main groups of primitive amphibians, each characterized by: a group that includes the first animals that were able to get out of water, a second group which contains the ancestors of the amniotes (reptiles, birds and mammals) and two more groups, both candidates to be the ancestors of modern amphibians.

Order Ichthyostegalia

Ichthyostegalians were the first tetrapods to be able to leave the water. They appeared at the late Devonian period and they were big animals with large wide heads, short legs and an aquatic or semi aquatic lifestyle (they probably were pretty clumsy on land). They moved around using mainly their muscular tail with rays similar to that of fish.

Fossil and restoration of Tiktaalik. Photo by Linden Tea.

Similarly to current amphibians, they presented a lateral line (sensory organ that allows fish to detect vibrations and movement underwater) and were able to breathe through their skin (they lost the cosmoid scales of their ancestors). Also, the eggs were laid in the water, from which the tadpoles emerged and later on, they suffered a metamorphosis process to become adults just like current amphibians. Subsequently ichthyostegalians gave rise to the rest of amphibian groups.

Skeletons of Ichthyostega and Acanthostega, two typical ichthyostegalians.

Clade Reptiliomorpha

Reptiliomorphs were the ancestors of amniotes and appeared about 340 million years ago. Most of them were usually large and heavy animals, which presented more advanced adaptations to live on land (laterally-placed eyes instead of dorsally-placed ones and a knobby more impervious skin). Even though, reptiliomorphs still laid their eggs in the water and had larval-stages with gills. It wouldn’t be until the late Carboniferous period when the first amniotes (animals that could lay their eggs on dry land) would emancipate completely from water.

Mounted skeleton of Diadectes a large herbivorous reptiliomorph from the American Museum of Natural History, photo by Ghedoghedo.

Order Temnospondyli

This group is one of the possible candidates to being the ancestors of modern amphibians. This is the most diverse group of primitive amphibians and it survived until the early Cretaceous period, about 120 million years ago. The temnospondyls varied greatly in shape, size and lifestyle.

Restoration of Eryops megacephalus a large temnospondylian predator, by Dmitry Bogdanov.

Most of them were meat-eaters, but some were terrestrial predators, some were semi aquatic and some had returned completely to water. Even though, all species had to return to water to breed for the fertilization was external; while the female was laying clutches of eggs in the water, the male released the sperm over them.

Mounted skeleton of Koskinonodon a 3 metres long temnospondyl, from the American Museum of Natural History, photo by Lawrence.

Within the temnospondyls we can find some of the biggest amphibians that ever lived, such as Prionosuchus, with an estimated length of 4,5 meters and about 300 kilograms of weight. Also, even though their skin was not covered with scales, it wasn’t completely smooth like in modern amphibians.

Restoration of Prionosuchus by Dmitry Bogdanov.

It is believed that this group could be the sister-taxon of modern amphibians, even though there’s one last group which could be a candidate to that post.

Order Lepospondyli

Lepospondyls were a small group of primitive animals which appeared at the early Carboniferous and disappeared at the late Permian period. Even though lepospondyls were not as numerous and smaller than the temnospondyls, they presented a wide range of body shapes and adaptations.

Restoration of Diplocaulus magnicornis, of about 1 metre long was the biggest of all lepospondyls, by Nobu Tamura.

The first lepospondyls looked superficially like small lizards, but subsequently lots of groups suffered processes of limb reduction or loss.

Restoration of Pelodosotis, an advanced lepospondyl, by Dmitry Bogdanov.

The relationship of the lepospondyls with the rest of tetrapods isn’t very clear. Different hypothesis go from some authors arguing that they are a group separated from the labyrinthodonts, some thinking that they are the ancestor of current amphibians and reptiles, and some even saying that they are the ancestors of only a portion of modern amphibians.

Restoration of Lysorophus, a Permian lepospondyl, by Smokeybjb.

As we can see, the classification of primitive amphibians can be an extremely complex thing. On this entry I tried to make a summary of the most important groups of ancient amphibians and, on the next one, we’ll center on the evolution of modern amphibians, the so-called “lissamphibians”, and we’ll look in more detail all the controversies surrounding these curious animals.

REFERENCES

The following sources have been consulted in the elaboration of this entry:

• Cover photo: Tyler Keillor and Beth Rooney.
• http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1691537/pdf/14667343.pdf
• http://www.biology.ualberta.ca/courses.hp/biol606/papers/Ahlberg+1998.pdf
• http://usf.usfca.edu/fac_staff/dever/tetrapod_review.pdf
• http://icb.oxfordjournals.org/content/early/2007/08/13/icb.icm055.full.pdf+html
• http://www.usfca.edu/fac_staff/dever/tetropodevol.pdf
• http://icb.oxfordjournals.org.sci-hub.org/content/5/2/287