Let me introduce you to graptolites. These odd little prehistoric creatures were colonies of tiny marine animals which formed protective cup-like structures around their bodies and filter-fed on microscopic plankton. They’re classified as hemichordates, and their closest living relatives are a similar member of that group called pterobranchs.
They appeared in the fossil record between about 490 and 320 million years ago, and came in a wide range of shapes and sizes. There were single rows, double rows, branches, spirals, forks, and even net-like forms. The earliest types lived attached to the sea floor, but later ones floated around freely near the surface of the ocean and could reach lengths of up to 1.5m (~5ft). Some may have attached themselves to seaweed and floating debris, others are thought to have dangled from their own little bubble-like flotation rafts.
Their remains are so numerous and widespread that they’re very useful as “index fossils”, allowing paleontologists to precisely date the age of the rocks they’re found in.
(More Information: 1, 2, 3)