algae baltica cambrian carbon dioxide carboniferous cenozoic coal continent cretaceous cryogenian devonian europe extinction event fish forest fossil geology gondwana ice age invertebrate jurassic laurentia mesozoic mississippian neogene neoproterozoic ordovician oxygen paleogene pangaea pannotia permian phanerozoic plant proterozoic quaternary silurian snowball earth south pole supercontinent trace fossil trichophycus pedum trilobite vertebrate wikimedia commons