The oldest spiral Nogrobs (Polychaeta, Serpulidae) from the Pliensbachian/Toarcian boundary (Oujda Mountains, eastern Morocco)

Deep within the ocean, on the soft, muddy floors where few creatures can find a foothold, a specialized group of marine worms has evolved a unique way to survive. These are serpulids, tube-dwelling worms that build hard, calcified homes around their soft bodies. While most of these worms glue their tubes firmly to rocks or shells, a few rare lineages have developed a different strategy: they live unattached, drifting or resting on the soft sediment. To do this, they often adopt a spiral shape. This spiral acts like a snowshoe, distributing the worm's weight so it does not sink into the mud. Scientists call this specific life cycle a "morphogenetic countdown program," a fancy term for a predictable sequence of growth: the worm starts by attaching to a tiny object, grows into a tight spiral, breaks free from its anchor, and then lies flat on the seafloor, sometimes growing a short, straight chimney-like tube at the end to reach the water above.

For decades, the fossil record of these spiral worms was a puzzle. The oldest known examples were thought to be much younger, and the evolutionary path connecting the attached ancestors to the free-lying descendants was unclear. Furthermore, the internal structure of their shells—the microscopic arrangement of the crystals that make up their tubes—had rarely been studied in these ancient, spiral forms. Without this microscopic evidence, it was difficult to tell if similar-looking shells belonged to the same family or were just coincidental look-alikes. Understanding when and how these worms first learned to live on the soft seafloor helps scientists reconstruct the history of marine life and how creatures adapt to changing environments over millions of years.

A team of researchers has now pushed the timeline of this evolutionary story back significantly. In the rugged Oujda Mountains of eastern Morocco, they discovered the oldest known fossils of a spiral serpulid worm, dating back to the boundary between the Pliensbachian and Toarcian stages of the Early Jurassic. The fossils were found in layers of marl and marly limestone, rocks that were once soft mud on a shallow seafloor at the edge of the ancient Tethys Ocean. The team, led by Manfred Jäger and including collaborators from institutions across Europe and Morocco, identified a new species, which they named Nogrobs (Tetralysis) zekkarensis. This discovery is significant not just for its age, but because it confirms that the ability to live unattached on soft sediment evolved much earlier than previously thought, appearing almost simultaneously with the first non-spiral members of the same genus.

The researchers collected nearly one hundred tubes of this new worm from two specific rock layers. By examining them under microscopes and taking high-resolution photographs, they observed a distinctive shape: a tight, coiled spiral that usually has three and a half turns. The tubes are marked by four rounded ridges running along their length, giving them a somewhat square or trapezoidal cross-section. Interestingly, the worms showed a strong preference for coiling in a left-handed direction, with about three-quarters of the specimens spiraling that way. Most of the fossils followed the expected pattern of growth: a tiny, straight initial tube that attached to a solid object (like a shell fragment), followed by the tight spiral that broke free to lie on the soft bottom. However, about 18 percent of the specimens were different; they remained attached to a solid object, such as a small bivalve shell, for their entire lives. This suggests that while the species had mastered the art of living on soft mud, some individuals still clung to the older, attached lifestyle, perhaps representing a transitional stage in their evolution.

To understand the biology of these ancient worms, the team looked inside the tubes. They polished the fossils and etched them with a weak acid to reveal their microscopic structure. They found that the tube walls were made of two distinct layers. The outer layer consisted of tall, spherical crystals, while the inner layer was made of irregularly oriented prismatic crystals. This specific combination of microscopic structures had never been seen before in the genus Nogrobs, which was previously thought to have simpler, single-layered walls. The presence of this complex, two-layered structure in such an ancient species suggests that the evolutionary split between different types of these worms happened very early in their history.

The discovery also prompted a broader re-evaluation of how these worms are classified. The researchers reviewed the entire family of fossil serpulids and proposed a new system that divides the genus Nogrobs into six subgroups based on their shape, the presence of spiral growth, and their microscopic structure. They revived an old name, Tetralysis, for the group of worms that are obligatorily spiral, meaning they always form a tight coil. They also introduced a new genus, Annuloserpula, for a different group of Early Jurassic worms characterized by numerous ring-like bumps along their tubes. Additionally, they corrected a naming error for a known species from England, renaming it Nogrobs (Tetraditrupa) folkestonensis to avoid confusion with a different species that shared the same name.

Perhaps the most surprising finding was the coexistence of two very different forms of these worms at the same time and place. In the same Moroccan rock layers, the team found the new spiral species alongside a non-spiral species from a different subgroup. This indicates that the evolutionary paths leading to the attached, non-spiral worms and the free-lying, spiral worms had already diverged by the Early Jurassic. The fact that the spiral worms were already so specialized, with complex internal structures and a specific growth program, suggests that their ancestors had been evolving for even longer, likely in the Late Triassic or Early Jurassic, a time for which few fossils of these creatures have been found.

The study also shed light on the community of life surrounding these worms. The Moroccan fossils were found alongside ammonites, belemnites, and various shellfish, painting a picture of a rich, shallow marine ecosystem. Some of the worm tubes were encrusted with tiny, brown, tube-like structures. The researchers could not identify the owners of these tiny tubes with certainty, but they noted that they might be juvenile versions of the same worm, or perhaps a different, unknown species of tube-dwelling worm that lived on the shells of the larger ones. This interaction highlights the complex web of life that existed on the ancient seafloor, where creatures competed for space and resources even on the soft, shifting mud.

By combining detailed descriptions of the worm's shape with a first-ever look at its microscopic shell structure, this research provides a clearer picture of how marine life adapted to the soft seafloor millions of years ago. It confirms that the "snowshoe" strategy of living unattached on soft sediment is an ancient innovation, one that appeared in the fossil record much earlier than previously documented. The discovery of these worms in Morocco does not just add a new species to the list; it rewrites the timeline of their evolution, showing that the transition from a fixed, attached life to a free-lying existence was a complex process that unfolded over a vast stretch of geological time.