Blastoid

Blastoids were an extinct type of stemmed echinoderm, often called sea buds. They first appeared during the Ordovician period, along with many other groups of echinoderms, and may have originated even earlier in the Cambrian. Blastoids became most diverse during the Mississippian subperiod of the Carboniferous period.

These interesting creatures lived in the ancient seas and were related to other echinoderms like crinoids. Though they were not as diverse as their relatives, blastoids left behind many fossils that we find today, especially in rocks from the Mississippian age.

Blastoids disappeared forever at the extinction event at the end of the Permian period, around 250 million years ago. Today, they are studied by scientists to learn more about life in the ancient oceans.

Description

Blastoids were sea creatures with bodies covered in special plates made of calcium carbonate. These plates formed the main part of their body, called the theca. The theca was often attached to a stalk made of stacked disc-shaped plates. The bottom of the stalk stuck to the ocean floor with a holdfast, similar to a crinoid. Sometimes, the stalk was very short or missing, and the holdfast was attached right to the base of the theca.

At the top of the theca was the mouth. From the center of the mouth, five grooves spread out like flower petals. These grooves, called ambulacra, had thin structures called brachioles that helped catch food particles and move them to the mouth. The mouth was surrounded by five plates with openings, including the anus and entrances to special breathing organs called hydrospires. These openings helped keep different fluids separate. Waste left through a special opening near the anus.

Blastoids lived in ocean waters and fed on tiny floating creatures called planktonic organisms. They used their ambulacra to catch food, moving it from one set of grooves to another until it reached their mouth. Each of these grooves was covered by moveable plates that could open to let food in or close to keep it out.

Taxonomy

See also: List of echinodermata orders

Blastoids may have come from the Cystoids. They have two groups. Fissiculata have openings that go to their water systems through slits. Spiraculata have openings that go through canals with pores. The oldest blastoid found is called Macurdablastus. It is from the Middle Ordovician period in Tennessee, but it does not fit into either group.