The Late Ordovician mass extinction is generally considered to occur in two distinct pulses. The first pulse began at the boundary between the Katian and Hirnantian stages of the Late Ordovician Period. This extinction pulse is typically attributed to the Late Ordovician glaciation, which abruptly expanded over Gondwana at the beginning of the Hirnantian and shifted the earth from a greenhouse to icehouse climate. Cooling and a falling sea level brought on by the glaciation led to habitat loss for many organisms along the continental shelves, especially endemic taxa with restricted temperature tolerance. During this extinction pulse there were also several marked changes in biologically responsive carbon and oxygen isotopes. Marine life partially rediversified during the cold period and a new cold-water ecosystem, the ""Hirnantia" biota", was established.