Fossil specimen of Seirocrinus subangularis from Houston Museum, Texas. Seirocrinus is a stalked crinoid or a sea lily, cousins of sea stars that are still alive today in our oceans, although they are often limited to greater depths.
However, Seirocrinus along with a few other genera, are an interesting case: it grows on floating logs, and it may reach up to 20 meters in total length. These pseudoplanktonic crinoids are most abundant in Jurassic seas, and seem to fill a niche no longer seen in most marine animals, save perhaps the tiny goose barnacles (Lepas, Dosima), which float on the ocean's surface through a similar strategy.
One can only imagine what an interesting sight it must have been. Gigantic flower-like creatures, growing on logs, with stems tall as most trees. Floating just beneath the water's surface, with ammonoids, fish and ichthyosaurs swarming around it, just as sargassum mats today attract vagrant fauna.
Further Reading
Image Source