A new stem-chordate solute from the Middle Ordovician of Estonia

The solute Maennilia estonica nov. gen. et nov. sp. (Maenniliidae, nov. fam.) from the middle Caradoc Keila Horizon of the village of Vasalemma, near the town of Keila, near Tallin, Estonia is reconstructed and described. Remarkable features include: the hydropore developed as a madreporite and penetrated by the gonopore; ringshaped major plates in the fore tail, visibly formed by the fusion of smaller plates; a symmetrical mid tail, with dorsal and ventral series of ossicles extending forwards to meet the fore-tail rings in the mid line; an anterior lobe projecting leftwards and better developed in large than small individuals; and large size. Maennilia estonica may have lived with the ventral face up. The presence of a branchial pore could not be established. Ontogenetic changes in the anterior lobe suggest that the gonad was anteriorly located in the head, as in other solutes. Maenniliidae nov. fam. comprising M. estonica and Heckericystis kuckersiana (Hecker) 1940, and with Maennilia estonica as the type species, is proposed with three autapomorphies: the gonopore penetrates the hydropore, the mid tail has a peculiar symmetrical structure and the head skeleton is relatively rigid. The Maeniliidae have rings in the fore tail visibly formed by the fusion of smaller plates. Among other solutes, only Dendrocystites sedgwicki Barrande, 1887 seems to share this feature which should therefore probably be seen as a synapomorphy of the Maenniliidae with that species. Within the chordate stem group, the Maenniliidae with Dendrocystites sedgwicki probably represent a plesion crownward of the Iowacystidae and anti-crownward of Dendrocystoides scoticus (Bather) 1913. Hecker's reconstruction of H. kuckersiana is partly mistaken. Isolated hind-tail ossicles of ”Dendrocystites” rossicus Jaekel, 1901, from the Llanvirn near Pavlovsk in the St. Petersburg area of Russia, suggest a solute of very great size i.e. with a guessed total length of perhaps 300 mm, making it by far the largest known solute.