Fossilised remains of land animals are extremely rare in Iceland. A scattering of fossilised mammal bones have been found in sedimentary layers in Vopnafjörður in East Iceland that date from the Pliocene. These bones are believed to be those of a deer. Finds from the Miocene consist of fossilised water fleas in sedimentary layers in Mókollsdalur and Langavatnsdalur and an imprint of a freshwater shell in Fnjóskadalur. Sedimentary layers from the same epoch have yielded several well-preserved insect fossils and traces of insect activity on fossilised plant remains. These insects include one species of beetle, aphids, and bibionids.
Marine mammal fossils and fossilised shell remains are more common. One of the most important sites for marine animal fossils is found at Tjörnes in North Iceland. The sedimentary strata at this site have been extensively researched. Scientists have classified them into three main sequences, separated by lava strata. Tectonism due to continental drift has caused subsidence and uplift in the region. As a result, some sedimentary strata at Tjörnes formed in shallow marine waters, while others formed above sea level. Sedimentary sequences at Tjörnes contain marine deposits, lacustrine (lake) deposits, fluvial (river) deposits, and tillite. Many strata contain an abundance of fossils, providing invaluable information about the climate and the marine environment at the start of the Ice Age.
The oldest sequence is just over 500 m thick and is known as the Tjörnes sequence (Tjörneslögin). Most species of fossilised marine animals found in Icelandic sedimentary strata are represented here in the Tjörnes sequence. Molluscs, crustaceans, and foraminifers are the most common fossils in the Tjörnes sequence, but the fossilised remains of fish and marine mammals have also been found, including fossilized seal, walrus, and whale bones.
The Tjörnes sequence is divided into three biozones, each named for the dominant or characteristic species within that biozone.
The Venerupis zone is the oldest. Above this is the Mactra zone. Within these zones, marine deposits formed in shallow ocean waters alternate with thin lignite or brown coal strata formed in wetlands. Today, many mollusc species in the Venerupis and Mactra zones are found only in warmer waters than those found around Iceland. The mollusc fauna in the Mactra zone indicates that the ocean is now cooler than it formerly was.
The Serripes zone is the youngest and uppermost. It sits above the Mactra zone. The boundary between the Serripes and Mactra zones corresponds to the Gilbert-Gauss geomagnetic reversal around 3.6 million years ago. In the lowermost part of this zone, most of the cold-blooded mollusc shells associated with the Mactra zone disappear. In their place come new mollusc species, which live at a similar sea temperature as found in Icelandic waters today. Around 80 shell species have been identified. The origins of just over 20 of these species can be traced to the Pacific Ocean.
The Bering Strait began to open around 5 million years ago. Around 3.6 million years ago, the closure of the Panama Strait triggered changes in ocean circulation in the North Pacific Ocean. At the same time, many marine animals migrated through the Bering Strait into the Atlantic Ocean. Around 125 new species of ocean molluscs came this way, including Buccinum undatum, Neptunea despecta, Serripes groenlandicus, and Macoma calcarea. Changes to the fossil species in marine sedimentary strata from this time can be observed at many locations in Europe. Scientists consider the appearance of Serripes groenlandicus and Neptunea despecta in sedimentary strata in the Atlantic Ocean as marking the beginning of the Ice Age.
At the end of the last Ice Age and during the early Holocene, glaciers retreated rapidly. The sea level rose, submerging inland areas. Marine sedimentary deposits formed, containing subfossils. These sedimentary deposits, together with the ancient marine organisms that have been preserved in them, have improved geoscientists’ understanding of climate and sea-level change in Iceland at the end of the last Ice Age.