If a time machine allowed you to swim in the Jurassic oceans 180 million years ago, you might not be surprised by some familiar animals like turtles, jellyfish, squid and fish. Eventually, you would get to see something very different, like an ichthyosaur and ammonite molluscs. But that's not even close to being the most absurd and fascinating thing about these oceans lost to time.
There was a chance you'd come across a real giant sea monster, an enchanting and disturbing floating colony that would eventually collapse under its own weight and sink to become the most beautiful fossil in the world. That's your story. My name is Abner and welcome to ABC Land!
Walking through a museum and admiring the exhibition, sometimes you see an object that completely takes your breath away, something that will never leave your memory. All fossils are beautiful and impressive in some way, some are visually spectacular and tell amazing stories. But in my opinion, the most beautiful fossil in the world is like a mind-boggling work of art: the largest colony of crinoids in the world, a Jurassic giant of over 100 square meters.
Known as sea lilies and comatulids, crinoids are often confused with aquatic plants. However, they are a type of Echinoderm, the animal group that includes sea urchins, beach crackers and starfish. These marine filter feeders are very old: they appear in the Ordovician, more than 450 million years ago, and about 600 species live in all the world's oceans, in shallow and deep waters even today.
But crinoids are now just a shadow of what they once were. Situated in the small German town of Holzmaden is the Hauff Museum of the Prehistoric World. This museum is world famous for containing some of the finest fossils from the Lower Jurassic, collected by four generations of the Hauff family.
Continuing the family legacy, Rolf Hauff is the current director of the museum, collecting, preparing and studying fossils. There is the most beautiful fossil in the world, a huge colony of crinoids associated with a trunk, found in 1908. It is so huge that it had to be cut into parts to later be assembled and displayed.
The process of taking a raw rock and removing all the excess to display the fossil is called cleaning or preparation. It is an extremely delicate service, which can be done with drills, air jets and even acid depending on the rock surrounding the fossil. The preparation of the most beautiful fossil in the world took 18 years, and it was painstaking and patient work.
Like an impressive work of natural art, this giant colony was home to hundreds of individuals, small and large, some of them exceeding an incredible 20 meters in length, with cups one meter in diameter. They all belonged to a single species, called Seirocrinus subangularis, which had a long stalk with a huge calyx and numerous feather-like arms used to filter food from the water. Instead of anchoring themselves to rocks at the bottom of the ocean, like current crinoids, these crinoids established themselves in giant tree trunks, Hauff's is 12 meters long, and should have been bigger initially.
But how did this colony form and how did it become the most beautiful fossil in the world? It all started in the huge Jurassic coniferous forests that were close to beaches or rivers. Eventually, extreme weather events such as storms, tsunamis and floods would drive many of these tree trunks into the ocean.
There, they could float for a long time until they found another land mass. At the beginning of the Jurassic, Pangea was breaking up, which means that there were two great oceans: the Tethys Sea, surrounded by Pangea, and Panthalassa, which would become the Pacific Ocean. Panthalassa means "all ocean", it was a vast global ocean that occupied more than 70% of the planet's surface.
Objects floating there could go decades without finding solid ground. Once afloat, these logs made a great place for filter feeders to anchor themselves. With the crinoids attached to the underside of this giant floating log and numerous encrusting oysters, the driftwood became a veritable ecosystem, substrate for various creatures.
Dangling underwater, they took advantage of the association, having a free pass to navigate the warm tropical oceans, developing a huge filter-feeding community capable of reproducing. Once attached to the substrate, Seirocrinus became immobile, so swimming crinoid larvae had to first establish themselves on a trunk before maturing and colonizing. Many of the crinoids associated with logs were adults, so by comparing their growth rates with living relatives of Seirocrinus, we realize that these drifting megacolonies may have existed for over ten years, and perhaps even over 20 years for the larger logs.
This greatly exceeds the life expectancy of modern equivalent drift systems. The trunk rotted over time as it was battered by the sun, consumed by worms, and infiltrated by water. This, combined with the total weight of the colony, with oyster crinoids and other associated animals, finally caused the trunk to sink.
The entire system encountered the anoxic ocean floor, an environment with absurd pressure, in the absence of light and without disturbance. There they were buried, where they were perfectly preserved. Jurassic oceans were much warmer than modern ones, so the water held less dilute oxygen gas.
The water column was stratified, with an oxygenated surface and an anoxic bottom. The absence of oxygen allowed organisms to mineralize before completely decomposing, preserving the skeletons of whatever sank with astonishing perfection. These fossil colonies are among the largest invertebrate communities ever found in the fossil record.
Given the unlikely circumstances surrounding the preservation of this floating colony, it may be quite surprising to learn that this is not a unique discovery, quite the contrary! Exceptional colonies like this are known for around 100 fossils of varying sizes, though none as large as Hauff's specimen. Practically all are from Holzmaden and nearby, other species of crinoids were also found clinging to the trunks.
But similar fossils have been found elsewhere in the world, showing that this adaptation allowed these ancient crinoids to travel far across the Jurassic paleoceans. An analogy can be drawn between them and modern driftwood systems. When trees are carried out to sea, they become important habitats for plant and animal communities such as bivalves, anemones and crustaceans such as barnacles.
These floating logs attract fish, turtles and seabirds that feed on the associated organisms. Like their modern counterparts, these giant ancient floating colonies not only acted as homes for crinoids, but their large size provided protection for a diverse animal ecosystem. Although they have not been found in direct association, a variety of marine animals are preserved in the same rocks as these crinoids, including ammonites and belemnites, which are molluscs, but also fish and marine reptiles such as ichthyosaurs.
It is reasonable to assume that they interacted with Seirocrinus colonies in a similar way to modern day. This fossil colony is a portrait of a strange 180-million-year-old ecological community.
