Plant life is limited to marine algae. The end of the Cambrian Period is marked by a huge decline in trilobite numbers, possibly due to a glacial incursion. Ordovician Period 490-443 million years ago North America, Europe, and Africa merge. Shallow seas cover much of North America at the beginning of the period, then recede, leaving a thick layer of limestone. Later in the period, the seas recover North America, depositing quartz, sandstones, and more limestone.
Marine invertebrates are still the dominant form of life on Earth.
Silurian Period 443-417 million years ago The North American, European, and Asian land masses are situated on or near the equator. Gondwana sits in the south polar region. Fish evolve jaws. Late in the Silurian Period, the first sharks appear. The earliest land plants are represented by leafless, vascular plants called psilophytes.