Ancient Craniiform Brachiopod: A Newly Discovered Species with a Unique Shell Shape and Lifestyle
Scientists from HSE University, MSU, and Tallinn University of Technology have studied a fossil species of ancient brachiopods that lived in a warm sea in what is now northern Estonia more than 445 million years ago. These ancient brachiopods developed a cup-shaped shell with a protective 'cap' that shielded them from overgrowth by other marine organisms. The study has been published in Palaeogeography, Palaeoclimatology, Palaeoecology.
About 445 million years ago, the territory of present-day Estonia was covered by a warm, shallow sea. On its seafloor lived a tiny creature only a few millimetres long—a brachiopod called Pocillocrania rubeli. Despite its small size, this species had one of the most unusual shells among its relatives; it developed a cup-shaped shell with a protective 'cap.' A new study has shown that this distinctive shell evolved as an adaptation to living on the surface of bryozoan colonies.
Bryozoans are colonial aquatic animals that grow continuously by budding, gradually building a calcareous skeleton. If the ventral valve of the brachiopod, which was attached to the substrate, had remained flat—as it does in most species of this brachiopod group—the growing bryozoan colony would eventually have blocked its access to water and suspended food particles.
Both bryozoans and brachiopods generate feeding currents using their tentacles and capture suspended food particles from the water, thereby competing for the same resource. By rising above the surface of the colony, the brachiopod not only avoided overgrowth but also gained a more advantageous position for filter feeding.
'Interestingly, a modern relative of this fossil species—brachiopods of the genus Neoancistrocrania—also has a cup-shaped ventral valve. This species lives on corals, and this valve shape allows it to rise slightly above the surface of the growing colonial organism,' explains Elena Temereva, Professor at the Joint Department with RAS Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, HSE Faculty of Biology and Biotechnology.
The shape of the dorsal (upper) valve of the fossil brachiopod was equally unusual: while the ventral valve formed a deep bowl, the dorsal valve resembled a wide-brimmed sombrero slightly recessed into it. At the same time, the valves were movable and could open and close rapidly. When the dorsal valve was slightly raised, a gap formed between the two valves, allowing water to enter the mantle cavity. Inside was the lophophore, a filtering organ that circulates water and extracts food particles from it.
One of the most important results of the study was the reconstruction of the animal’s internal anatomy. Muscle imprints—among the few soft-tissue features that can be preserved in fossils—allowed the scientists to reconstruct almost the entire muscular system of this extinct brachiopod. It was found that this small species had five pairs of muscles controlling the movement of both the valves and the lophophore. The researchers suggest that valve opening was primarily driven by hydraulic pressure from internal body fluids. When the muscles contracted, the valves closed abruptly, likely serving as a defensive response to danger. In addition, auxiliary oblique muscles allowed the valves to rotate slightly relative to each other.
The researchers believe that these movements may have had a practical function. The hard shell surface likely attracted organisms seeking a substrate for attachment. Small rotations of the valves could have helped the brachiopod dislodge unwanted settlers before they had time to establish themselves, effectively shaking them off its skeleton. In addition, the scientists were able to reconstruct the position and structure of the lophophore.
By reconstructing the morphological features of brachiopods, scientists are gradually restoring the structure of entire ancient marine ecosystems.
'We do reconstructions not for their own sake but to understand how these organisms lived. Ideally, we would like to understand how ancient seas were structured, what processes occurred within them, and what the environment was like,' concludes Prof. Temereva.
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