Main article: Triassic Divergence
Crinozoa is the group containing all extinct and living crinoids or "sea lilies." They are represented by all major groups found in our Earth, as well as the pentacrinids which include today's agricrinids.
Natural History Edit
Crinoids emerged sometime in the Cambrian, and early groups quickly rose to prominence as the dominant suspension feeders in many marine environments. They reached their peak in the Carboniferous, when large crinoid beds dominated most of the shallow environments in the world's oceans. The Permian-Triassic Mass Extinction eliminated most of the major groups of crinoids, leaving the articulate crinoids as the only representatives. During the Triassic, these animals underwent another period of radiation but were hit hard by the Triassic-Jurassic Mass Extinction. Although they continue to have representatives in many environments to this day, they never again reached the level of success achieved in the Carboniferous.
During the Extended Triassic, the renewed diversification of the crinoids continued, resulting in the emergence of the "modern" groups of articulate crinoids. The major groups included the traumatocrinids (which became extinct at the Triassic-Jurassic boundary), the roveacrinids (becoming extinct at the end of the Cretaceous), and the groups of crinoids found to the present in our timeline (millericrinids, cyrtocrinids, bourgueticrinids, comatulids, and isocrinids). In addition, this timeline includes the pentacrinids, which survive to the present day. Finally, the sister group of the living isocrinids, called the autocrinids is prevalent in the world today.
The relationships among the living crinoids are ambiguous at best. What is known with a good degree of certainty is that the autocrinids evolved from stem group members of the isocrinids, and that the traumatocrinids, millericrinids, and cyrtocrinids tend to group together as a sister group to the other crinoids.
The traumatocrinids seemed to follow the "typical" crinoid body plan of a calyx attached to a stalk. However, traumatocrinids utilized a relatively rare mode of life, in that they tended to be attached to floating objects. This mode of life causes some confusion with the living pentacrinids, but it seems to be well-supported that they are not closely related groups.
Traumatocrinids are known exclusively from the Late and Extended Triassic, most notably from the deposits of Guanling. They are suspected to have had a more-or-less global distriubtion.
The millericrinids are a group of rare, usually stalked crinoids that utilize an unremarkable body plan similar to that of the "typical" crinoid layout.
Millericrinids are known from deep ocean environments with high currents worldwide.
Cyrtocrinids are another group of rare crinoids, but they do not seem to follow the typical body plan. They seem to have neither stalks nor arms, but little is known of them due to a lack of observation.
Cyrtocrinids are found in the typical deep ocean environments of most crinoid groups.
Bourgueticrinids are very similar in outward form to the "typical" crinoid body plan of the millericrinids.
Bourgueticrinids are widespread but rare in deep oceans.
Comatulids are unstalked, often brightly colored crinoids representing the most diverse group by species number, as well as the most common overall. They tend to move using their cirri or occasionally by swimming in the water column using their arms. Their success is likely due in large part to their ability to evade predation even in shallow water environments.
Comatulids are found in shallow water reef environments worldwide, but a few deep sea species are also represented.
Isocrinids are fairly typical crinoids in body plan and habit, except for the fact that they have numerous cirri along their stalks affording them a small degree of motility.
Isocrinids are found in deep oceans worldwide.
Autocrinids are a highly unusual and diverse group of crinoids that have taken on symbiotic algae. Their body form is similar to the isocrinids, in that they are stalked crinoids with cirri running the length of the stalk, but they have specialized adaptations for algal "farming." The arms are broadened to accommodate the symbionts, and there are specialized "wiper cirri" at the base of the calyx to groom and harvest the algae. Unlike the scleractinian corals and giant clams of our timeline, the symbiotic algae live on the surface of the crinoid rather than in its tissues. In addition, the outer edges of the arms retain the ability to capture organic matter, allowing them to function as facultative suspension feeders like their relatives. Lastly, their cirri allow them to detach from the substrate to escape predators, resulting in one of the most unusual displays in this timeline when whole "fields" of crinoids rise into the water column in response to a large disturbance.
Autocrinids are found in sandy or muddy, nutrient-rich environments across the tropical and temperate latitudes, usually in high concentrations as crinoid beds reminiscent of the Carboniferous.
Roveacrinids were suspension-feeding crinoids that lacked a stalk and spent their time floating in the water column. Thus, they had specialized arms of open water movement.
Roveacrinids became extinct at the end of the Cretaceous, but enjoyed a wide distribution in the world's oceans.
While they retain the typical crinoid body plan overall, the pentacrinids are unusual in that they have an essentially pelagic lifestyle. They drift with the current attached to driftwood or other floating objects, but some species have adapted to a sort of pseudo-colonialism with numerous animals attached to each other in free-floating clusters.
Pentacrinids are found in areas of high plankton concentration worldwide.