True detectives, paleontologists seek to reconstruct the scenes, analyze the facts and bring to life the actors of a past more than 10 thousand years old written on the rocks.
When we think of paleontologists or paleontology, we may imagine scientists working with bones of extinct animals, fossils of well or poorly preserved invertebrates, or impressions of plant remains. But paleontology is more than just digging up, restoring, and assembling bones; Using evidence that researchers find in the rocks, they try to discover who is responsible for what they are seeing. In this search both knowledge and intuition play a very important role.
Two types of fossils are usually found: the first corresponds to the internal or external molds left by the animal’s body after death. But sometimes these molds are not found, but only the footprints and traces of the organisms: trace fossils, also known as ichnofossils (from the Greek ikhnos, which means footprints). The latter are indirect evidence of ancient life and reflect the activity of organisms in the past. The study of it is specifically called paleoichnology.
Paleoichnology studies the footprints or ichnites of fossil animals that have been preserved imprinted in rocks. These fossil traces of vertebrates and invertebrates are usually the impression that they left during some of their activities – moving, swimming, searching for food or resting – in the sediment or on other organisms, and they give us information about their nature and shape. of life. But even if it is unknown which organism left its mark, it allows us to know, for example, the environment where it lived, whether it was a lagoon, a deep sea or a beach area.
Ichnology: a young science
The bases of ichnology, the search and tracking of footprints, must have been practiced since prehistory, when hunters tracked down signs of their prey: their feces and other indirect evidence. But ichnology is a relatively young science.
The term invertebrate ichnology, that is, invertebrate trace, began to be used at the beginning of the 19th century, when paleontologists confused some ichnofossils with algal fossils. The first time the term ichnology was used in the title of a publication was in 1858. Edward Hitchcock, professor at the Amherst College in Massachusetts, United States, published an article on the tracks of Mesozoic vertebrates in the Connecticut Valley, in the state of the same name.
The first significant findings in vertebrate ichnology took place in the 1920s, with the discovery of dinosaur eggs and nests. Protoceratops in Mongolia. Eggs and nests are indirect evidence of reproductive behavior, and although some dinosaur eggs preserve remains of the embryo, they are properly considered ichnofossils because they are structures built with the aim of facilitating the development of young animals.
For a long time, ichnology was represented by the studies of dinosaur footprints by Roland Bird, a paleontologist at the United States Museum of Natural History. The rise of the discipline took place in the second half of the last century with the work of Adolfo Seilacher, professor of geology and geophysics at Yale University. These works marked innovations and the beginning of new concepts and classification methods, according to the behavior that could be observed and the way in which ichnofossils were preserved. In the last years of the 20th century, vertebrate ichnology was reborn, focused mainly on dinosaur footprints and eggs.
In search of clues
Like many other researchers, paleontologists carry out part of their work in the field, with the particularity that the materials they search for are remains or traces of organisms that died millions of years ago.
When paleontologists have not yet found the specific site where they are going to work, the trip to the field is called prospecting. Armed with hammers, magnifying glasses, brushes and a range of different needles (similar to those used by dentists), they set out in search of clues. They also use geological maps to locate the substrate where fossils are most likely to be found: sedimentary rocks that belong to very old layers. As these very ancient layers are normally found deep underground, paleontologists look for places where they have been exposed, for example the bottom of ravines.
Once they find a site (which curiously, almost always occurs on the last day of the study trip, when supplies and travel expenses have been exhausted, and it is necessary to return), they look for different types of ichnofossils, including burrows and perforations. Burrows, like those of polychaetes (marine worms), snakes and some modern rodents, are excavations made in a material that is not completely compacted. Drillings are excavations made in a completely compacted material such as rock or wood; An example is what wood-boring mollusks (teredos) make on docks.
Today, paleontologists can distinguish between the marks left by amphibians, reptiles, birds, mammals, and invertebrates; You can even confirm which organism left a particular trace at the level of order, family, and sometimes even genus.
Classification
Currently, ichnofossils are classified based on the behavior of the organism that produced them. The classifications, which were created by adding to the Greek word ikhnos (imprint) Latin roots, includes the following:
NAME
DESCRIPTION
CHARACTERISTICS
Repichnia
(of creeperscreep)
Tracks of crawling and movement of legless crawling animals. They are usually linear.
Cubichnia
(of lairbed)
Rest tracks. They are the least common and usually reproduce the shape of the belly of the organisms that produce them.
Pascichnia
(of pascofeed, graze)
Nutrition clues. Of organisms that eat mud, moving over it. They generally have a sinuous layout.
Fodinichnia
(of fodiodig or dig up)
Feeding galleries. They are those left by organisms that were looking for food in the sediment. They have very varied shapes.
Domichnia
(of domushome)
Dwelling galleries.
More or less superficial tubes and perforations, generally simple, where certain invertebrates (crustaceans, worms, pelecypods) were permanently housed. Their layout varies between linear and U-shaped.
Fugichnia
(of drainescape)
Escape tracks left by organisms fleeing adverse conditions or a predator.
Agrichnia
(of agrofield)
Cultivation traces.
Structures for the care and cultivation of animals or plants, with a branched shape, such as “plantation” systems of bacteria and fungi for biochemosynthesis.
Clues of movement such as impressions of an animal’s forelimbs or hindlimbs are also looked for; for example, footprints of dinosaurs or mammals; or like those of the camelids and felines that can be seen in Pie de Vaca, Puebla. Likewise, traces of legless animals that drag their bodies (snails, worms, worms) are sought, such as those that can be seen in the Chicontepec Formation, on the banks of the Cazones River in the state of Puebla.
On some occasions the information obtained gives some clue to the eating habits of ancient organisms. This is the case of gastroliths (of gastrostomach and lithos, stones), boulders that animals ingested to facilitate digestion. This behavior is still observed in birds: lacking teeth, they ingest rocks to mechanically crush the food. Inside the body of the animal, the gastroliths occupy the space that housed the internal organs, and have a slightly more polished surface. The large sauropods swallowed stones to enter a muscular gizzard that served as the stomach. In this way they were able to crush and grind the hard leaves of the conifers they fed on, making them more digestible. A large number of gastroliths have been found in Kansas, United States, belonging to groups of reptiles such as crocodiles, plesiosaurs and dinosaurs.
On the other hand, there are coprolites, which are fossilized feces, very useful for reconstructing the diet of extinct organisms and which provide additional information to understand how they crushed it. For example, a coprolite of T. rex in Saskatchewan, Canada, 44 cm long, 15 cm high and 13 cm wide, where remains of an unidentified small mammal can be seen.
Studying a site is rarely completed in a single trip, and to protect it from rain, cold, air, snow, heat and intense sun, as the case may be, the site is covered with a kind of splint: first, paper is placed hygienic or newspaper, then jute sacks, and finally, a mixture of plaster and lime. The next trip, with the fossils already located, is called recovery.
From the field to the desktop
Once the fossils have been located, extracted (when possible) and transported to the laboratory, the scientific adventure is not yet over. You could say it’s just beginning…
Most of the time an investigation takes is spent in a laboratory where fossils are extracted – with infinite patience – from the sediment, cleaned and glued together with an alcohol-based resin. The latter ends up evaporating and the resin takes the place of the missing pieces. Of course, it is very important to take photographs during this process, in case some parts of the fossils are lost or modified.
Some paleontologists use the footprints and marks of organisms to learn about their behavior and the place where they lived; that is, to know the interaction between organisms and their environment, and thus also better understand the diversity of ancient environments.
For this, ichnofossils have several advantages over fossils. It is more likely that an animal that moves, rests or tries to escape from a predator prints the trace of its body somewhere, than that the latter is preserved. Consequently, this evidence is usually more abundant than the remains of the organisms that created it. On the other hand, fossil ichnites have a greater chance of being preserved compared to body fossils, and can be found in…