Paleontologists have detailed the anatomy of the enigmatic Devonian tentaculites.

Modern biodiversity is the result of a lengthy evolution of the living world, which has undergone numerous crises and fundamental transformations. On the other hand, we currently observe the remnants of past diversity in life. After all, not all major systematic groups have survived to this day or left descendants.

Consequently, there are significant questions regarding the interpretation of certain fossils. Such "problematic" organisms frequently move across major taxa. They can easily turn out to be entirely different from what they seem.

In recent decades, tentaculites (Tentaculita) have sparked considerable debate. There are still disagreements about the time range of their existence; however, it is certain that a substantial number of these invertebrates existed during the Ordovician period, and they reached their peak during the Devonian. Tentaculites primarily inhabited the seafloor, although some may have been capable of swimming in the water column.

Tentaculites varied in size from one millimeter to five or even seven centimeters. They were nestled in elongated conical shells, which were intricately sculptured: with constrictions, longitudinal lines, bulges, and so on. The specific "pattern" depended on the systematic group.

The shells of tentaculites, preserved in sedimentary rock, somewhat resemble bolts and screws with threads. This has led to speculation about the supposed creation of "building materials" by ancient civilizations.

However, no remnants of soft bodies have been found within the shells. It is likely that they were entirely contained within long, narrow tubes and could extend outward to filter food particles from the water. Presumably, the tentacles of the tentaculites protruded outward, with a mouth situated between them, while the entire structure was covered by a lid on the outside.

Yet, all these details remained at the level of conjecture. Hence, there are significant discrepancies in the interpretation of Devonian "bolts."

A new article in Historical Biology (An International Journal of Paleobiology) has clarified tentaculites considerably. Its authors described 540 "casts" of the internal cavities of shells found in southern Armenia near the Noravank monastery. This marked the first successful discovery of tentaculites in the country. Moreover, 41 fossils retained traces of muscle attachment of retractor muscles on their shells—these were used by the animal to retract into its shell.

There are seven known groups of tentaculites, distinguished by the structure of their retractors, and the new article describes six of them. In all cases, the muscle attachment points are located exclusively at the apex of the shell and share a similar shape. Thus, the muscles of the tentaculites were directly attached to the exoskeleton, which brings them closer to mollusks, brachiopods, and bryozoans. At the same time, tubicolous annelids (which have long been associated with ancient "tubular" organisms) lack this feature.

New data have shown that tentaculites were bilaterally symmetrical animals. This rules out any relation to cnidarians like jellyfish and corals. Therefore, it has been concluded that the mysterious Devonian fossils belong to the group Lophotrochozoa, which also includes modern annelids, mollusks, bryozoans, brachiopods, and flatworms. It is likely that Tentaculita were a stem group of the bryozoan type (Bryozoa), to which they bear a particular resemblance.