Trilobites lasted the entire 300 million years of the Paleozoic Period.
The Paleozoic Period lasted from 542 million to 251 million years ago and covered six separate geological ages. Before this time, single-celled organisms and invertebrates had long dominated the Earth. But during the Paleozoic, vertebrates bloomed, and the ancestors of dinosaurs and mammals appeared. The land was filled by insects, tetrapods and plants. The geological order beneath the surface of the Earth is witness to the systematic evolution of species during this time.
Cambrian
The Cambrian age lasted between 542 and 488 million years ago. This period is associated with the great flourishing of diverse life. The most complex life before the Cambrian consisted of sponges, worms, cnidaria (like jellyfish) and multi-cellular colonies, but over a span of tens of millions of years, organisms with hard parts and more complex body plans began to flourish. The brachiopod with its hinged, clam-like shell evolved in the Cambrian and still lives on today as 300 separate species. Edrioasteroids (related to modern starfish and sand dollars), nautiloids (early cephalopods), gastropods (snails and slugs), opabinia (lobed animal with a fan-shaped tail and a long proboscis appendage), arthropod trilobites and early crustaceans also appeared during this time.
Ordovician
The Ordovician age lasted between 488 and 443 million years ago. Most invertebrates like mollusks and cephalopods continued to evolve over millions of years. Coral and graptolites (an early and important planktonic life) proliferated across the Earth. Creatures like the haikouichthys, a cartilage-filled precursor to fish that developed in the Cambrian, gave way to jawless armored fish called ostracoderm in the Ordovician that probably filtered food from the seabed. The arthropod crabs evolved in freshwater and shallow lagoons. Early plants became some of the first multicellular life forms besides fungi to colonize land.
Silurian
The Silurian lasted between 443 and 416 million years ago. Fish diversified during the Silurian, evolving into jawed fish like the acanthodians. The water also became home to eurypterids (sea scorpions) and leaches. Invertebrates like brachiopods, mollusks, trilobites and hederellids (tubular, colonial animals) continued to thrive. On land, early arthropod insects began to stake out territory. Leafless plants became more prevalent, including the first signs of vascular plants like cooksonia in the middle Silurian. Nascent leafed and stemmed plants that still used spores such as baragwanathia developed late in the Silurian.
Devonian
The Devonian lasted between 416 and 359 million years ago. Fearsome fish like the 10-meter long dunkleosteus and the early shark cladoselache ruled the waters. Around the early to middle Devonian, the first four-limbed tetrapods appeared on land: eusthenopteron (a lobe-finned fish), panderichthys, tiktaalik and ichthyostega (a limbed tetrapod) are all related to modern tetrapods. Early amphibians like labyrinthodontia evolved from these first tetrapods. During the middle to very late Devonian, ferns, horsetails and seeded plants such as early trees began to appear, turning the land lush and green.
Carboniferous
The Carboniferous age lasted between 359 and 299 million years ago. The period is better known as the Mississippian (early Carboniferous) and Pennsylvanian (late Carboniferous). In the water, sharks and crustaceans diversified across the globe. Plants flourished, and the increased levels of oxygen in the atmosphere led directly to the proliferation of very large winged insects, centipedes and arachnids. While ancient amphibians were still evolving, the first reptiles had already appeared. These amniotes were known for their specially adapted eggs. Early reptiles diverged quickly into anapsids (now modern turtles and tortoises), euryapsids (ancient marine reptiles like plesiosaurs), synapsids (which led to modern mammals), and diapsids (which led to all other reptiles and birds).
Permian
The final period of the Paleozcoic was the Permian, which lasted between 299 and 251 million years ago. By this time, ammonoids, which are shelled cephalopods, early cockroaches, and cone-bearing conifers were beginning to develop. The land was being dominated by archosaurs, from which dinosaurs eventually evolved, and therapsids and cynodonts that would eventually give rise to mammals. The cynodonts had many reptile characteristics, but they also had mammal-like skulls with differentiated teeth, bulging braincases, and robust jaw musculature, galvanized by the evolution of jaw bones that had become part of the ear. Now that plants had covered the Earth, herbivores began to expand rapidly during this period. By the end of the Permian, trilobites and many ancient animals had gone extinct.
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