Most of the species that once existed on Earth are extinct
The concept of extinct biodiversity includes species that have disappeared from the planet.
Most of the species that at some point existed on Earth (99.9%) are extinct. Many of them have disappeared as a result of mass extinctions due to sudden global environmental changes. At the same time, the extinction of species for various evolutionary reasons is a natural phenomenon, and some species of the Earth’s biota are lost every year. It is widely estimated that there are currently around 10 million species on Earth, of which only 1.2 million have been described by scientists [1].
However, in the long history of our planet, it is not only the extinction of species that sets the tone – we also know of times when new species were abundant. For example, the world’s biodiversity increased enormously just over half a billion years ago, in the early Cambrian era. At that time, a large number of new multicellular organisms emerged, who became the ancestors to many modern animals. The so-called Cambrian explosion is thought to be caused by the addition of large amounts of oxygen to the environment [2].
Biota in different geological eras
From the Paleozoic era (540–250 million years ago), it is possible to observe the formation and disappearance of species on Earth, because by that time, the organisms had developed a hard skeleton (for example, shells in molluscs, skeletons in fish) that have survived as fossils.
During the oldest Paleozoic era, in the Cambrian period, all life existed in the sea; on land, there was nothing but a lifeless stone desert. Algae, sponges, Cnidaria, bacteria, worms, molluscs, as well as more complex animals with external skeletons, such as trilobites, were found in the seas [3].
In the post-Cambrian Ordovician period, there was a new surge in biodiversity in the seas. The seas were full of diverse life, dominated in number by the brachiopods, while the fear of the seas was the nautiloids, absolute top predators at a length of up to 16 meters. The number of trilobites decreased slightly, but remained quite high. The first so-called jawless fish also appeared in the Ordovician. The plants were represented by algae and primitive land plants [4].
For example, the first leeches appeared in the seas of the subsequent Silurian period, and alongside sea scorpions, vertebrate fish, or so-called true fish, began to rule. Trilobites were still widespread. In addition, there were numerous brachiopods, molluscs, sea lilies, corals and others. By the end of the period, the first arachnids and millipedes appeared [3]. Vegetation was still dominated by algae, but the first primitive vascular plants have also been found to have existed in the Silurian period – primitive ferns [5].
During the Devonian period, plants conquered the land and formed a lush primeval forest. Terrestrial arthropods spread rapidly and predecessors of four-legged vertebrates appeared. Due to the significant development and rapid spread of fish, Devonian period is also called the Age of Fishes.
In the Carboniferous period, in terms of fauna, terrestrial vertebrates, such as primitive amphibians (Stegocephalia) and reptiles, developed rapidly. The seas were then dominated by cartilaginous fish [6].
In the second half of the Permian period, conifers, cycads, and Ginkgoales appeared. On land, among vertebrates, reptiles and the ancestors of mammals began to dominate, and the predominance of amphibians began to decline sharply. Bony fish developed and diversified rapidly in the sea [7].
The Mesozoic era (250–65 million years ago) could be called the Age of Reptiles. During the Triassic period, reptiles became the most numerous animals, but it also saw the emerging of primitive mammals and true bony fish [8]. During the Jurassic period, the ancestors of birds, sea crocodiles, and sea turtles appeared. Until the end of the Cretaceous period, the land was dominated by dinosaurs who had already developed during the Triassic era. In vegetation, flowering plants began to develop rapidly [9].
The last geological era that continues even today is the Cenozoic era. In the Cenozoic era, insects became the most abundant invertebrates, while mammals and birds became the most abundant vertebrates. Flowering plants became predominant in the vegetation [10].
Findings of extinct biodiversity in Estonia
Fossils of the Cambrian, Ordovician, Silurian, and Devonian period can be found in Estonia. They are most abundant in the limestone layers of the Ordovician and Silurian periods.
A large number of fossils have been found in Estonia, from trilobites [11], Ordovician sea urchins, and Silurian sea scorpions to primitive fish from the Silurian and Devonian periods [12]. However, there are also findings from much later eras, such as mammoth bones, which has occurred on a few occasions in Estonia [12].
Five plus one mass extinctions
Five mass extinctions can be distinguished when it comes to the extinction of species, all of which have resulted in the extinction of more than 50% of animal species at once [13]. We are currently in the middle of the sixth extinction, the man-made one [14]. Five mass extinctions [13]:
- one of the reasons for the extinction at the end of the Ordovician period (approximately 445 million years ago) was a great ice age, which significantly lowered sea levels. It is likely that nearly 85% of all species became extinct then;
- the extinction event at the end of the Devonian period took place 360–375 million years ago. About 75% of the species became extinct as a result of several events (for example, climate cooling, active volcanic activity). Species dependent on coral reefs in the warm seas particularly suffered during this era. For example, the Placodermi became extinct then; their fossils have been found in large numbers in Estonia as well;
- the largest known extinction on Earth to date took place in the Permian period about 250 million years ago, when the vast majority of all marine and land species became extinct. This was caused by natural disasters (for example, increased volcanic activity, acidification of the seas);
- the Triassic-Jurassic extinction event took place 205 million years ago. The reasons for this event are understood the least. A large number of archosaurs, most therapsids, and large amphibians were destroyed. The dinosaur era began;
- the Cretaceous-Paleogene extinction event, thought to have been caused by an asteroid falling somewhere in the Gulf of Mexico, occurred 65.5 million years ago. Three quarters of all species disappeared, including dinosaurs. Mammals and birds then received an evolutionary opportunity.
In the Anthropocene, human activity has the greatest impact on the extinction of species. The main risk factor is the loss of habitats, which the International Union for Conservation of Nature (IUCN) estimates is the highest risk factor for as many as 85% of species in the Red List of Threatened Species. Climate change also plays a very important role in the extinction of species. For example, due to the warming of the sea water, coral reefs are in great danger, but we do not have to go so far to look for those who suffer: our own bog bird, the willow ptarmigan, has become very rare, most likely due to snowless winters, and ringed seals do not have a birthing ground in ice-free winters. Excessive hunting also plays a role in the extinction of species. In addition to climate change, it is considered to be one of the main causes of the extinction of mammoths, aurochs, and woolly rhinoceroses [13].
Last modified: 01.12.2021
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[1] Kuresoo, R. Keskpaik, R. 2018. Kaduvate liikide kannul. Liigirikkuse kahanemine inimese ajastul. Tartu Ülikooli loodusmuuseum ja botaanikaaed. Tartu - https://www.natmuseum.ut.ee/sites/loodusmuuseum/files/pildid/Kaduvate_liikide_kannul_EST%20.pdf
[2] Ennet, P. 2019. Hapniku hulk kujundas kambriumis elurikkust. Novaator - https://novaator.err.ee/937034/hapniku-hulk-kujundas-kambriumis-elurikkust
[3] H. Pärnaste. 2013. Trilobiidid. Eesti Loodusmuuseum, 2013
[4] Olesk, A. Elu sügavaimad mõõnad. Tarkade Klubi. August 2009
[5] Laansoo, U., Puusepp, T., Kaur, K. Iidsete aegade taimed botaanikaaias. Maaleht, 25.01.2013. https://maaleht.delfi.ee/artikkel/65579834/iidsete-aegade-taimed-botaanikaaias
[6] https://et.wikipedia.org/wiki/Paleosoikum#Karbon
[7] https://et.wikipedia.org/wiki/Paleosoikum
[8] http://spikerdaja.blogspot.com/2009/08/geograafia-triias.html
[9] http://vana.loodusajakiri.ee/horisont/artikkel224_217.html
[10] https://www.hariduskeskus.ee/opiobjektid/loodus/?MAA_TEKE_JA_ARENG___GEOLOOGILINE_AJASKAALA
[11] Pärnaste, H. 2012. Suurte silmadega väike mereelukas. Eesti Loodus 6-7/2012 - http://eestiloodus.horisont.ee/artikkel4651_4623.html
[12] https://www.looduskalender.ee/n/node/3875
[13] Kuresoo, R. Keskpaik, R. 2018. Kaduvate liikide kannul. Liigirikkuse kahanemine inimese ajastul. Tartu Ülikooli loodusmuuseum ja botaanikaaed. Tartu. Lk 8-11 - https://www.natmuseum.ut.ee/sites/loodusmuuseum/files/pildid/Kaduvate_liikide_kannul_EST%20.pdf
[14] Liikide kuues väljasuremine – kas ka Eestis? Novaator - https://novaator.err.ee/637228/liikide-kuues-valjasuremine-kas-ka-eestis