Early Cretaceous

The Early Cretaceous (geochronological name) or the Lower Cretaceous (chronostratigraphic name) is the first part of the Cretaceous (Cretaceous) period. This time in Earth's history lasted from about 143.1 million years ago to 100.5 million years ago (Ma).

During the Early Cretaceous, the world looked very different from today. The continents were moving and had not yet formed the shapes we see now. Many new kinds of plants and animals appeared, including some of the earliest flowering plants and many types of dinosaurs. This period helps scientists understand how life on Earth has changed over millions of years.

Geology

Scientists study the age of old events, like the Barremian–Aptian boundary. They found that a big change in the ocean, called the early Aptian Oceanic Anoxic Event 1a, happened about 123.1 million years ago. This helps us learn when this boundary occurred.

During this time, huge areas of volcanic rock formed in many places. One big area formed in the South Pacific Ocean about 120 million years ago. Another formed in the Indian Ocean around the same time. These volcanic rocks changed the shape of the Earth’s surface.

The Earth’s plates also moved. The Atlantic Ocean grew wider, moving South America away from Africa. Greenland became its own piece, and new seas formed between continents. The Pacific Ocean also grew larger, and new land areas formed near the Arctic.

Evolution

During the Early Cretaceous, many new kinds of dinosaurs appeared, including ceratopsians, spinosaurids, carcharodontosaurids, and coelurosaurs. Some dinosaurs that lived in the Late Jurassic were still around.

This time also saw the first flowering plants, called angiosperms. One of the oldest families of these plants, called Archaefructaceae, was found in China.

The Early Cretaceous was when the first modern birds evolved. A fossil of an early mammal named Sinodelphys, found in China, shows that mammals began to diversify from Asia then. This mammal was more related to marsupials than to placental mammals and had feet good for climbing trees. Another fossil, Steropodon, is the oldest known monotreme, a mammal that lays eggs, and it lived in what is now Australia.

Oil shale

Oil in the Prudhoe Bay Oil Field comes from rocks formed in the Triassic Shublik Formation, a zone from the early part of the Cretaceous period, and the Lower Jurassic Kingak Shale.