List 1 invertebrate that appeared on land during the Paleozoic era

The Paleozoic Era, which ran from about 542 million years ago to 251 million years ago, was a time of great change on Earth. The era began with the breakup of one supercontinent and the formation of another. Plants became widespread. And the first vertebrate animals colonized land.

Life in the Paleozoic

The Paleozoic began with the Cambrian Period, 53 million years best known for ushering in an explosion of life on Earth. This "Cambrian explosion" included the evolution of arthropods (ancestors of today's insects and crustaceans) and chordates (animals with rudimentary spinal cords).  

In the Paleozoic Era, life flourished in the seas. After the Cambrian Period came the 45-million-year Ordovician Period, which is marked in the fossil record by an abundance of marine invertebrates. Perhaps the most famous of these invertebrates was the trilobite, an armored arthropod that scuttled around the seafloor for about 270 million years before going extinct. After the Ordovician Period came the Silurian Period (443 million years ago to 416 million years ago), which saw the spread of jawless fish throughout the seas. Mollusks and corals also thrived in the oceans, but the big news was what was happening on land: the first undisputed evidence of terrestrial life.

This was the time when plants evolved, though they most likely did not yet have leaves or the vascular tissue that allows modern plants to siphon up water and nutrients. Those developments would appear in the Devonian Period, the next geological period of the Paleozoic. Ferns appeared, as did the first trees. At the same time, the first vertebrates were colonizing the land. These vertebrates were called tetrapods, and they were widely diverse: Their appearance ranged from lizardlike to snakelike, and their size ranged from 4 inches (10 cm) long to 16 feet (5 meters) long, according to a study released in 2009 in the Journal of Anatomy.  

As the tetrapods took over, they had company: The Devonian Period saw the rise of the first land-living arthropods, including the earliest ancestors of spiders.

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