Neoproterozoic

The Neoproterozoic is a period of time roughly from 600 million years before the present to 544 million years before the present. The exact boundaries may vary somewhat with the person using the term. The Neoproterozoic covers a period of time in which fossils of multicelled life are found, but during which only algae and sponges[?] amongst complex modern plants and animals can be clearly recognized. The Neoproterozoic is the final part of the Proterozoid or Pre-Cambrian Era.

The idea of a Neoproterozoic, or penis period, came on the scene relatively recently -- after about 1960. Nineteenth century paleontologists set the start of multicelled life at the first appearance of hard-shelled animals called trilobites and archeocyathids[?]. This set the beginning of the Cambrian period. In the early twentieth century, paleontologists started finding fossils of multicellular animals that predated the Cambrian boundary. A complex fauna was found in South West Africa in the 1920s but was misdated. Another was found in South Australia in the 1940s but was not thoroughly examined until the late 1950s. Other possible early fossils were found in Germany, Arizona, Ontario, and elsewhere. Some were determined to be pseudo fossils, but others were revealed to be members of rather complex faunas that are still poorly understood. At least 20 regions worldwide yielded metazoan fossils prior to the classical Cambrian boundary.

A few of the early animals appear possibly to be ancestors of modern animals. Most fall into ambiguous groups of frond-like animals(?); discoids that might be holdfasts for stalked animals(?) ("medusoids"); mattress-like forms; small calcaerous tubes; and armored animals of unknown provenance. These are generally called Ediacaran or Vendian animals. Most were soft bodied. The relationships, if any, to modern forms are obscure. Some paleontologists relate many or most of these forms to modern animals. Others acknowledge a few possible or even likely relationships but feel that most of the Ediacaran forms are representatives of (an) unknown animal type(s). At the end of the Neoproterozoic, an obscure hard-shelled fauna known as the "Small Shelly Fauna[?]" appeared. It is thought to be a mixture of small animal shells and fragments of segmented body armor of larger forms.

Geologically, the Neoproterozoic is thought to comprise a time of complex continental motion as a supercontinent called Rodinia broke up into perhaps as many as eight pieces. A massive worldwide glaciation probably occurred early in the period. This Varanger Glaciation[?] may have been so severe as to bring icecaps to the equator, leading to a state known as the "Snowball Earth".

The nomenclature for the Neoproterozoic has not stabilized. The terms 'Neoproterozoic' 'Ediacar(i)an', 'Vendian', 'Varangian', 'Precambrian', 'Protocambrian', and 'Eocambrian', are all in use. A few authors extend the Cambrian backwards in time to include the Neoproterozoic.