ИСТИНА

The development of recent brachiopods comprises two main types. Modern Craniiformea and Rhynchonelliformea have lecithotrophic larvae; the larvae of Rhynchonelliformea have pedicle lobe. The ontogeny of Linguliformea includes planktotrophic swimming stage that is in fact a juvenile brachiopod. The larvae of fossil brachiopods may be reconstructed by the relief of first-formed region on the umbo of adult shell. Our collection contains first Ordovician representatives of the order Strophomenida (Rhynchonelliformea). Their first-formed region is asymmetrically conical on ventral valves and flattened on dorsal valves. The ventral cone narrows into tube (pedicle sheath) with 20-30 µm wide aperture on the apex. This aperture is too small for accommodating any peduncular outgrowth. This type of shell may correspond to planktotrophic stage characterized by alimentary canal with anus in the cone apex. Judging by first-formed shells of Permian productids, this type of larva preserved in the class Strophomenata up to the end of Paleozoic. The similarity in the development of strophomenids and their ancestral order Billingsellida is generally accepted. Some billingsellids also have tubular outgrowth on the umbo of ventral valve; however, it is not homologous to pedicle sheath of strophomenids. Our material clearly shows that the tube of billingsellids is formed by accreted deltidial plates of juvenile shell, which already has well-developed structures of inner skeleton. The billingsellids have well-pronounced first-formed region on the dorsal valve and lack it on the ventral valve; their larvae certainly had pedicle lobe as there is a large pedicle opening between the valves.