Baltica

Baltica, ancient land mass or terrane (crustal block geologically distinct from its surroundings) that consisted of about two-thirds of the present continent of Europe. It was bounded to the south-east by the important Tornquist-Tessyre Line, which is a geological structural line extending from the North Sea at a point about 300 km (186 mi) east of Aberdeen, through the south of the Danish peninsula, cutting the north German coast near the island of Rügen, and extending south-eastwards to the south-east (Carpathian margin) of Poland and on into the Black Sea. To the east, Baltica was bounded by the Ural Mountains, which extend northwards into Novaya Zemlya. Some publications have postulated that the Taimyr Peninsula of northern Siberia may have also formed part of Baltica, but this seems unlikely. North-westwards, the ancient continent was bounded by the Scandian mountains of Norway and western Sweden. These mountains were formed as part of the Caledonian Orogeny of the Silurian and Early Devonian when Baltica collided with another substantial palaeocontinent, Laurentia (which today forms most of North America, Greenland, and Svalbard/Spitsbergen), and substantial nappes were thrust over the old shield of Baltica. The continental crust underlying Baltica is very thick and formed of Proterozoic and other ancient, largely metamorphic rocks. The origins of the terrane are uncertain, but may have occurred at about 750 million years ago on the splitting up of the postulated old supercontinent of Rodinia. From palaeomagnetic research, Baltica can be shown to have moved from an equatorial position at about 750 million years ago down to high southern latitudes (between 40° and 70° South) in Vendian times (650 million years ago), when it experienced severe glaciation. It stayed at this position until early Ordovician times (480 million years ago) before returning slowly northwards again. Near the end of Ordovician time (443 million years ago) it collided with Avalonia, and this combined terrane collided relatively soon after this in Silurian times with the large palaeocontinent of Laurentia (most of modern North America) to form the even larger supercontinent of Laurussia.

During most of the Lower Palaeozoic, and particularly in the Early Ordovician, the oceans surrounding Baltica were wide enough to prevent interchange of shallow-water sea-bed-dwelling animals, and thus the seas around Baltica were colonized by many endemic taxa. A striking example is the brachiopod Lycophoria, the only genus known from its family (and the family itself is without close relatives), which occurs in rock-forming abundance in the Early Ordovician of Estonia, Sweden, and elsewhere on Baltica, but which is not recorded from outside that continent. However, as the Ordovician progressed and the oceans surrounding Baltica narrowed, more sea-bed-dwelling animals originating in neighbouring terranes, such as Avalonia and Laurentia, migrated to Baltica. In late Palaeozoic times the Baltic margin of Laurussia collided with Siberia and its accreted terranes in the Uralian Orogeny.