Early Scientific Illustration of Hyolithids
Ca. 1860's / 1880's
 
Hyolithids are a group of extinct marine animals that lived during the Cambrian period, approximately 540 to 485 million years ago. They were widespread throughout the oceans during this time, and their fossils have been found on every continent. Hyolithids had a distinctive cone-shaped shell that was open at both ends, with a flat base and a pointed apex. They ranged in size from a few millimeters to several centimeters in length.
 
The classification of hyolithids has been a subject of debate among paleontologists for many years. Some researchers have placed them in the phylum Mollusca, while others have suggested that they may belong to the phylum Annelida or a group of their own. Recent studies have suggested that hyolithids may be related to brachiopods or to a group of extinct animals called helcionelloids.
 
Hyolithids likely lived in shallow marine environments and were filter feeders, using their tentacles to capture small organisms and organic particles from the water. Some species of hyolithids may have been capable of swimming, while others were likely sessile or attached to the ocean floor. They are believed to have gone extinct during the end-Permian mass extinction, which occurred approximately 252 million years ago. Despite their widespread distribution and abundance during the Cambrian, hyolithids remain a poorly understood group of animals, and their exact evolutionary relationships are still the subject of ongoing research and debate among paleontologists.