There is nothing like flying in the current through the crystal clear waters of Komodo National Park!
What makes this area so attractive? It POPS with millions of colorful crinoids. Crinoids are so feathery and delicate, yet their parts are hard enough to have left a robust fossil record. Today we will explore the fascinating evolutionary history of the crinoids through my underwater photos and fossils.
First, lets briefly consider the structure of crinoids
The very first crinoids, as well as some deep water species found today, were attached to the ocean bottom by a stalk made of up columnals. The stalk connected to a cup (or calyx), which then gave rise to five arms.
Fossil crinoids first appeared in the mid Paleozoic Era
approximately 490 million years ago (recall that the three eras are Paleozoic, Mesozoic and Cenozoic/recent). They quickly diversified; over 5000 different species proliferated and coated the the sea floor. They nearly became extinct during the catastrophic mass extinctions at the end of the Paleozoic Era, but rebounded and diversified again in the early Mesozoic Era.
Check out these early crinoids
Note that they have very few stiff and unbranched arms, and that they have a stalk.
Increased predation by sea urchins during the Mesozoic Era led to new adaptations
The sea urchins developed sharp vicious teeth which allowed them to penetrate the defensive armor of the early crinoids. Only the crinoids that could adapt survived. Multiple structural and behavioral changes occurred:
1. Motility: The early crinoids were anchored to the ocean bottom by a stalk and could not escape predation. Later crinoids lost the stalk and acquired the ability to walk or swim. (watch a video here).
2. “Pseudoplanktonic adaptations”: Some crinoids such as Scyphocrinites had air filled bladders which allowed them to float in the water column, away from the sea urchins on the ocean bottom. Still others (Pentacrinites) attached to the bottoms of floating logs and drifted with the logs.
3. Flexible and increasingly branched arms: Early crinoids, such as Onychocrinus and Jimbacrinus, had only 5 stiff arms. Later crinoids, such as Pentacrinites had ten or more highly branched arms. The flexibility and branching allowed the crinoids to avoid some predation. It also allowed them to filter more water and trap more food.
4. Autotomy: (self amputation). Much like their relatives the starfish, crinoids developed the ability to discard branches or arms when a predator attacked. This would either satisfy or distract the predator while the crinoid escaped. Lost arms could be regenerated.
5. Nocturnal behavior: Some species became nocturnal, hiding in cracks on the reef during the daytime to avoid predation.
What are crinoids like today?
Today there are 500 species of crinoids, divided into two major groupings. The comatulid crinoids are stemless and motile, and they are prevalent in many different ocean environments. These are the crinoids we see while diving. Only a few species of stalked crinoids remain, living only in deep water (>300 feet), where the darkness provides cover from predatory fishes.
Find out more