Blastoids were an extinct group of marine animals that once flourished in ancient oceans. Often referred to as “sea buds” because of their distinctive, bud-like body shape, these creatures are distantly related to modern animals like starfish and sea urchins. Their fossilized remains offer a glimpse into the diverse life forms that inhabited Earth’s past seas. As an extinct group, blastoids represent a unique branch on the tree of life.
Physical Characteristics of Blastoids
The main body of a blastoid, called the theca, had a bud-like or tulip-shaped form. This structure was composed of numerous interlocking plates made of calcium carbonate, providing rigidity and protection. They exhibited five-fold symmetry, with body parts arranged in patterns of five around a central axis.
Delicate, feathery appendages known as brachioles extended from the theca, used for feeding. These structures were slender and branched, capturing food particles from the surrounding water. Many blastoid species were attached to the seafloor by a segmented stalk, or column, which elevated their bodies into the water column, resembling a flower on a stem. Internally, blastoids had specialized respiratory structures called hydrospires, which were folded sacs located beneath the plates of the theca, for gas exchange.
Blastoid Lifestyle and Habitat
As adults, blastoids maintained a sessile lifestyle, fixed in one location on the ocean floor. This attachment allowed them to gather nutrients from passing water currents. Their feeding method involved suspension feeding, where the brachioles formed a net-like structure to filter microscopic food particles from the seawater. This passive strategy relied on ocean currents to bring them food.
They thrived in clear, shallow, and often turbulent marine environments. These habitats were suitable because constant water movement ensured a continuous supply of food particles. They inhabited carbonate platforms and reef environments, where the seafloor provided stable attachment points for their stalks. Their presence in these specific settings indicates an adaptation to well-oxygenated waters with abundant suspended organic matter.
Geological History and Extinction
Blastoids first appeared during the Ordovician period of the Paleozoic Era. They diversified and reached peak abundance during the Mississippian sub-period, which is sometimes referred to as the “Age of Blastoids.” During this time, they were a common component of marine ecosystems across various continents.
The entire group of blastoids ultimately disappeared during the Permian-Triassic extinction event, often termed “The Great Dying.” This event, occurring approximately 252 million years ago, was the most severe extinction in Earth’s history, wiping out an estimated 90% of all marine species. Massive volcanic eruptions, particularly the Siberian Traps, are thought to have triggered extreme environmental changes, including widespread ocean acidification, global warming, and oxygen depletion, which contributed to the demise of blastoids and other marine life forms.
Relationship to Modern Echinoderms
Blastoids are classified within the phylum Echinodermata, which encompasses living marine animals. This classification places them as ancient relatives of modern animals such as sea stars, sea urchins, sand dollars, and sea cucumbers. Their shared five-fold symmetry and internal skeleton of calcium carbonate plates reflect this evolutionary connection.
They are often compared to crinoids, commonly known as sea lilies, due to their superficial resemblance and shared stalked morphology. A primary distinction lies in their body plans: blastoids had a more compact, bud-shaped theca with their feeding brachioles directly attached to it. Crinoids, in contrast, feature distinct, branching arms that support their feeding structures, giving them a more expansive, feathery appearance. This anatomical difference highlights their separate evolutionary paths within the broader echinoderm group.