Kuzmina T.V.*, Malakhov V.V. 2025. Brachiopod development in light of Nielsen’s “folding hypothesis” // Invert. Zool. Vol.22. No.1: 68–80 [in English].
Invertebrate Zoology Department, Lomonosov Moscow State University, Leninskie Gory 1-12, 119234 Moscow Russia.
Tatyana Kuzmina: kuzmina-t@yandex.ru ORCID https://orcid.org/0000-0001-7978-7608
Vladimir Malakhov: vmalakhov@inbox.ru ORCID https://orcid.org/0000-0003-1637-2354
* Corresponding author
doi: 10.15298/invertzool.22.1.06
ABSTRACT: Brachiopoda is a phylum of marine invertebrates with a fossil record dating back to the Early Cambrian. Despite their extensive paleontological history, the evolution of their body plan remains debated. Claus Nielsen (1991) suggested the “folding hypothesis” based on data on development of the craniiform brachiopod Novocrania anomala. During metamorphosis, the larva N. anomala folds ventrally, with both juvenile shell valves derived from the dorsal side. It has been hypothesized that this process may reflect the evolutionary origin of the brachiopod body plan: brachiopods evolved from a creeping ancestor that curled ventrally. This study revisits the “brachiopod fold hypothesis” by analyzing data on development of three brachiopod subphyla: Craniiformea, Rhynchonelliformea, and Linguliformea. Analysis of rhynchonelliform development reveals that “folding” occurs during embryonic and larval stages, influencing the arrangement of larval structures. In rhynchonelliform larva (cephalula), dorsal and lateral setal pairs are positioned at a right angle relative to the anterior-posterior axis of the larva, while in craniid larva (chaetotrocha), setae are arranged sequentially along the dorsal side. In early cephalula, the mantle rudiment forms obliquely to the anterior-posterior axis, with the dorsal side closer to the larval apical lobe. The dorsal pair of setal bundles is also located closer to the apical lobe than the lateral pair. As the cephalula grows, the dorsal sector of the mantle expands, causing the setal bundles to align. We propose that this dorsal expansion reflects the folding process, with the early cephalula corresponding to the chaetotrocha stage during folding at metamorphosis, and the competent cephalula corresponding to the folded juvenile stage observed in craniids. In discinid and lingulid brachiopods, development suggests a trend toward earlier, more embryonized “folding processes”. Data on fate map construction of discinids reveals dorsal expansion in the embryo, contributing to both dorsal and ventral valve formation in juveniles. In lingulids, both shell valves develop from the dorsal side of the embryo. Our findings suggest a progressive embryonization of the folding process in brachiopods: in craniids, “folding” occurs during metamorphosis; in rhynchonelliforms, during embryonic and larval development; and in linguliforms, at early embryonic stages.
KEY WORDS: Brachiopoda, development, ontogeny, evolution, folding hypothesis.