Diving with Trilobites: Life in the Silurian–Devonian Seas

During the Silurian, shallow seas covered the margins of continental landmasses, the largest of these was the Gondwana supercontinent, which extended from the South Pole to the Equator. In addition, several smaller continents, including Laurussia in the equatorial region and Siberia in the Northern Hemisphere, occupied the globe. All oceans were inhabited by a rich and highly diverse marine biota in which all modern phyla occurred. The most iconic Paleozoic animals living in these oceans—trilobites—were the first invertebrates with complex eyes, and we could try to imagine seeing life as they may have experienced it. Other contemporary members of the arthropod phylum include eurypterids (“sea scorpions”), some of which developed immense body sizes and were among the first animals leaving their footprints on land. Large areas of sea floor were covered with brachiopods—bivalved lophophorates that superficially resemble clams—who were among the most common invertebrates in the Paleozoic oceans. Among the planktonic organisms, stunning colonial hemichordates, called graptolites, thrived alongside radiolarians (single-celled protozoa), early planktonic arthropods, and diverse phytoplankton*. We also encounter the largest reefs in the Earth’s history, which were different from extant counterparts. These reef complexes were built by extinct tabulate and rugose corals, peculiar sponges (stromatoporoids), and a wide diversity of bryozoans (moss animals), together with calcified algae and bacteria. As time travellers, we will be impressed by the abundance of fishes (mainly those we call agnathans—those without jaws) living alongside the first fishes with true jaw structures. And last, but not least, all these animals thrived in the most fascinating surroundings of algal “forests” of those times. Finally, we are going to witness one of the Big Five mass extinction events, which exterminated the great reef systems along with what was once a thriving fish fauna.