Eesti Loodusmuuseum/Näitus/Müstiline ürgmeri/ENG

The Secrets of Ancient Seas
exhibition in the Estonian Museum of Natural History

Estonia is a maritime nation. For a big part of the last 600 million years, the territory of Estonia has been influenced by various seas, whose history is told by sedimentary rocks from that period. The sediment layers deposited on top of one another in seawater are like geological book pages inscribed with lithified information on animals and plants that lived long ago. Rocks also contain hints of environmental conditions and climates in different periods.

We invite you to dive for a time travel on the seabed in the territory of Estonia from over half a billion years ago to the present. We will acquaint you with digitally created marine creatures and their genuine fossils from the collections of the museum.

THE EDIACARAN

ESTONIA AT THE SOUTH POLE

The world of 600 million years ago differed significantly from today’s world. The single supercontinent Rodinia, from which no present-day land fragment had split yet, was situated around the South Pole. During the Ediacaran period, the palaeocontinent Baltica – the territories of present-day Eastern Europe and Scandinavia – broke apart from Rodinia, with the area of Estonia at its centre, and set out for its over 200-million-year-long independent journey from the South Pole towards the equator.

LIFE STARTS TO EVOLVE

In the Ediacaran period, the Earth gradually recovered and warmed after the Cryogenian ice age, one of the harshest in Earth’s geological history. Evolution of life was only just taking its first steps. During this period, a freshwater sea invaded the area of present-day Estonia from what is the east direction by today’s geography. This glacier-fed sea was cold and poor in life. It was inhabited by bacteria, green algae and acritarchs – unicellular algae still mysterious to science. Various wormlike creatures were crawling in the bottom mud and the ancestors of jellyfish may have been swimming in the water column.

All early life forms known from the Ediacaran were soft-bodied, that is, lacked both an inner skeleton and a protective outer skeleton.

Jellyfish. Ancient jellyfish-like cnidarians may have lived in the waters here because fossils of such creatures have been found in Northwest Russia.
Algae. Plantlike algae grew either attached to the seabed or floating freely in the water column.
†Planolites. The burrow-like fossils found from the rocks here may have been left by minute worms that once bustled in the bottom mud.
†Charnia. These plantlike animals were probably some of the earliest more complex multicellular creatures in Earth’s history.

THE CAMBRIAN

BALTICA SEPARATES

About 500 million years ago, the majority of land was united in the supercontinent Gondwana, which extended from the Northern Hemisphere across the equator to the South Pole. Just the small palaeocontinent Baltica, which comprised the territory of present-day Estonia, and a few other smaller continents were drifting independently from the South Pole towards the equator. As Baltica still remained rather close to the South Pole in the Cambrian period, the shallow sea in its inland remained cool and still rather poor in life.

THE CAMBRIAN EXPLOSION

The Cambrian period saw a worldwide rapid appearance and spread of marine life, which has come to be known as the Cambrian explosion. Favourable environmental conditions enabled previously soft-bodied creatures to start growing protective mineral shells, or exoskeletons. This afforded those primitive creatures protection from predators and allowed them to colonise nutrient rich coastal areas exposed to strong waves. New taxonomic groups of animals appeared, such as trilobites, the ancestors of modern arthropods. The shallow sea covering the territory of Estonia also hosted various ringed worms, beard worms, molluscs and moss animals.

In the Cambrian, the territory of Estonia was shaken by a powerful meteorite explosion, which created the Neugrund crater, an about ten-kilometre-diameter impact crater in present-day Northwest Estonian coastal sea.

†Micromitra undosa. Brachiopods were solitary invertebrates that attached to the seabed by means of a stalk or burrowed into mud.
†Hazelia palmata. Sponges are the most primitively built immobile multicellular creatures.
†Halkieria. A bottom-dwelling creature reminiscent of present-day slugs, with tiny shells resembling the valves of a bivalve shell at both ends of its body.
†Stromatocystites balticus. Some echinoderms were starfish-like invertebrates that lived attached to the seabed.
†Hydrocephalus. Trilobites were the ancient arthropods. The back of their body was covered with a hard outer skeleton; many species had well-developed compound eyes.
†Scenella discinoides. Ancient mollusc with calcareous shell who looked like seashells today

THE ORDOVICIAN

THE PERIOD OF SEAS

The Ordovician was characterised by worldwide intense volcanic activity, a meteorite shower, and rapid evolution of life. The Ordovician ended with an ice age, which, either directly or indirectly, led to the second-largest biotic extinction in Earth’s history. By about 460 million years ago, the independent palaeocontinent Baltica, including the territory of present-day Estonia, had shifted from the South Pole to subequatorial tropical waters. The ocean level was high and nearly the entire Baltica continent, including the present Baltic countries and a big part of Scandinavia, was flooded by a shallow tropical sea.

ICE AGE ON ITS WAY

The ice age was preceded by global climatic cooling, while the Baltica continent enjoyed a mostly tropical climate as it was drifting closer and closer to the equator. The warm shallow sea here supported a thriving marine life dominated by animals with strong protective calcareous shells. The already widespread trilobites, sponges and worms were supplemented by new animal groups — corals, graptolites, stromatoporoid sponges, and predatory cephalopods known as nautiloids.

In the Ordovician, the present-day area of Hiiumaa was shaken by a meteorite impact that created the approximately four-kilometre-diameter Kärdla crater.

†Ambonychia orvikui
†Phragmolites. The Ordovician sea was home to various molluscs, including numerous bivalves and snails.
†Conularia. A now extinct ancient cnidarian creature whose origin is still unclear.
†Hyolithes. Hyoliths were ancient conical-shelled mud-burrowing creatures whose origin is still unclear.
†Pharostoma. Trilobites were distant ancestors of modern arthropods.
Nautiloids. Predatory cephalopods that could grow several metres long.
†Dictyonema. Graptolites were ancient, now extinct colonial bushlike chordates who lived either floating in the water column or attached to the seabed.

THE SILURIAN

THE TERRITORY OF ESTONIA ON THE EQUATOR

During the Silurian, three separate continents – Baltica, Avalonia and Laurentia – rapidly shifted towards one another. At the end of the period, a triple collision of these continents took place on the equator, resulting in the formation of a new continent – Laurussia. The territory of Estonia, which had so far remained part of the Baltica palaeocontinent, now got positioned on the plain in front of the Caledonide Mountains, which had started to rise along the collision zone between Laurentia and Baltica. The shallow sea with changing sea level that covered Europe 400 million years ago sometimes covered Estonian territory or left it as dry land.

THE FIRST BONY FISH

During the Silurian, the first bony fish appeared in the seas, while the first mosslike plants and arthropods appeared on land. The tropical sea covering the area of Estonia was inhabited by corals, sea lilies, sponges, brachiopods, moss animals, graptolites, various bivalves and other molluscs, as well as predatory nautiloids. The now extinct stromatoporoid sponges reached a particularly wide distribution. Some completely new evolutionary branches also arose. Of these, these waters hosted predatory sea scorpions and representatives of several fish groups.

By the end of the Silurian, the global average temperature was up to ten degrees higher than today.

†Eurypterus. Eurypterids, or sea scorpions, were up to 2.5-metre-long predatory arthropods that have gone extinct by now.
Nautiloids. Predatory cephalopods had either straight, curved or spiral shells depending on species.
†Birkenia robusta. An extinct up to 10-centimetre-long jawless fish, probably the ancestor of modern lampreys.
Moss animals. Sedentary colonial marine animals that are still widespread today.
†Rugose corals. A now extinct order of solitary or colonial corals that once lived in coastal seas.
Sea lilies. Sea lilies, or crinoids, belong to the phylum of echinoderms and formed extensive thickets in Silurian seas.

THE DEVONIAN

THE PERIOD OF RED SANDS

The map of the Devonian period is dominated by two larger landmasses and a number of smaller continents. The ancient supercontinent Gondwana sprawled over the South Pole, while the nascent Laurussia lay on the equator. The present territory of Estonia was situated in the central part of Laurussia, on the plain in front of the Caledonide Mountains. A hundreds of meters thick layer of reddish sands from the Caledonides was deposited in South Estonia and the adjacent areas during the Devonian.

FISH-RICH TERRITORY OF ESTONIA

A period of greenhouse climate that had started already several dozen million years earlier, in the Silurian, continued through the Devonian and for further 100 million years on end. An exceptionally rich biota thrived also in the warm tropical seas of the Devonian. Rapid diversification and mass spread of fish took place in the ocean. A peculiar group known as Placoderms became dominant among fish but went extinct by the end of the period. They lived also in the sea that covered part of present-day Estonia and could grow up to two metres in length. Also the first amphibians are known from the Devonian, as are treelike terrestrial plants that grew up to several dozen metres tall.

†Archaeopteris
†Pseudosporochnus. Early treelike plants resembled giant ferns and grew 2–10 metres tall.
†Psammolepis alata (depicted) and †Drepanaspis. Up to 1.5-metre-long jawless fish that once lived in these waters, resembling modern rays.
†Microbrachius dicki. The first animals known to have used internal fertilisation to produce offspring.
†Pteraspis. An extinct jawless fish whose up to 20-centimetre-long front part of the body was covered with a protective armour of bony plates.
†Panderichthys. This extinct species from the class of lobe-finned fish represents an evolutionary stage between bony fish and early four-legged animals.

HAS THE AREA OF ESTONIA BEEN HOME TO DINOSAURS?

The Mesozoic was the era of dinosaurs and reptiles. Rocks from this period are absent in Estonia because the area was subject to terrestrial conditions for several hundred million years and no rocks were formed during that time. Besides, a surface rock layer as thick as dozens or hundreds of metres has been eroded away by several continental ice sheets of later periods. The next entry in the fossil record of this area dates from just a few dozen thousand years ago. There is still reason to speculate, however, that various representatives of dinosaurs might have roamed also these neighbourhoods.

Dinosaur bone finds closest to Estonia originate from Russia, Ukraine and several other locations in Europe

ICE AGE

THE TERRITORY OF ESTONIA GETS COVERED WITH ICE

The Quaternary is the most recent period in Earth’s history, spanning from about 2.6 million years ago to the present. During the Quaternary, harsh ice ages have alternated with interglacial periods of milder climate. We currently live in an interglacial period following the most recent major glaciation. The world of the Quaternary is familiar to us. The boundaries of continents have been somewhat altered by sea level fluctuations over the last couple of million years but, in general, the positions of continents on the world map are already familiar to us.

During the latest ice age, the entire territory of Estonia got covered with an about kilometre-thick sheet of ice, which bulldozed huge quantities of loose ground and older rocks out of its way.

THE PRESENT

FORMATION OF THE BALTIC SEA

Gradual warming of the climate and the latest retreat of ice from the areas of Europe and North America started about 12,000 years ago. This was the end of the most recent ice age and beginning of the postglacial period, or the Holocene. The Holocene has also been termed Anthropocene, the epoch of humans, as it encompasses the entire recorded history, the birth of great civilisations, and technological development. The world of the Holocene is today’s world. Over the last ten thousand years, the ocean level has risen by up to 35 metres, while ground level in the Northern Hemisphere, especially in areas that have emerged from under ice sheets, has risen by up to 180 metres as a result of rebounding from the weight of ice sheets.

MODERN MARINE ANIMALS

Climate in the Holocene has been relatively stable. No significant developments have taken place in the fauna or flora during the Holocene but just the distribution ranges of animals and plants have shifted either southwards or northwards according to changes in climatic conditions. The Baltic Sea is poor in species because its water salinity is too high for freshwater species but too low for oceanic ones. Various algae and small fish are found here, and larger mammals such as seals and porpoises can also be encountered occasionally.

The Baltic Sea was formed as a result of a ground level rise following the latest ice age.

Baltic herring and flounder (depicted). Baltic herring and flounder are up to 20-cm-long bony fish widespread in the Baltic Sea.
Harbour porpoise. Porpoises are marine mammals represented by a small population also in the Baltic Sea.
Moon jellyfish. Moon jellyfish is a common inhabitant of the Baltic Sea.
Golden star tunicate. This filter-feeding invertebrate lives attached to stones and rocks in the well-lit zones of the Baltic Sea.
Common starfish. Common starfish, a predatory inhabitant of the Baltic Sea, is the only starfish species that tolerates lowsalinity water.
Brown algae. Green, brown and charophyte algae are found in the Baltic Sea, inhabiting different depth zones of the seabed.