Sometimes separated into the Mississippian and Pennsylvanian Periods to denote coal-bearing stratigraphy, the aptly named Carboniferous (actually meaning "coal-bearing") Period was host to well-established terrestrial ecosystems including large swaths of forest and dense swamplands. Along with the expanse of plant life and photosynthesis came a rise in atmospheric oxygen, which supported a major evolutionary radiation of several groups of terrestrial arthropods, including the arachnids, myriapods, and hexapods, the same three arthropod clades that first made their way onto land in the Silurian Period. Not only did these organisms diversify, however, they also got big—really, really big! For example, the giant millipede Arthropleura, shown here crawling on a downed tree, could reach up to about 2.5 meters (over 8 feet) long and 50 centimeters (or over a foot and a half) wide! The skies had giant bugs too; Meganeura, a predatory ancestor to modern dragonflies and damselflies, reached a wingspan of about 75 centimeters (2 and a half feet). In the water, bivalved molluscs and brachiopods were abundant, as were stalked crinoids, like those seen here attached to the submerged portion of the same fell tree. Crinoids, still alive today in deeper ocean settings, were so abundant in the Carboniferous, in fact, that their disarticulated skeletal remains formed densely packed limestones called encrinites. Not only is the state fossil of Missouri a Carboniferous stalked crinoid named Eperisocrinus missouriensis, but if you'd like to see crinoid fragments up-close, you just need to visit The Columns, made from Burlington Limestone, on the Francis Quadrangle here at Mizzou. Crinoids, as echinoderms, are closely related to starfish, sea urchins, sand dollars, and sea cucumbers—but perhaps more importantly, they fall on the same side of the evolutionary tree of animals as do the vertebrates (known as deuterostomes based on how their embryonic development). Other interesting fauna of the Carboniferous Period shown here include the strange flying-fish-like Iniopteryx and and the infamous Mazon Creek enigma, Tullimonstrum, or "the Tully monster", known from Carboniferous deposits in neighboring Illinois (and in fact now the official state fossil of Illinois). Geologically, the Variscan-Alleghanian-Ouachita orogeny took place during the Carboniferous Period, responsible for building the Central Pangean Mountains.