Аннотация: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.