Earliest styliolinids from the Wenlock of Saaremaa Island (Estonia): paleoecological and evolutionary implications

The new tentaculitoid Styliolina giga n. sp. is described from the Wenlock of Saaremaa Island, Estonia. It is the earliest definite styliolinid as other known species are all reported from the Devonian. The peculiarities of the accompanying fauna indicate that Styliolina giga n. sp. could have inhabited a marginal marine environment, which had restricted connections to the ocean. The occurrence of Styliolina
in the Homerian could indicate that styliolinids were first to evolve from benthic tentaculitoids and were ancestors of nowakiids, but the annulated shells of nowakaiids resemble more closely those of ancestral Choniconarida. Presumably still nowakiids gave rise to styliolinids and Silurian records of nowakiids are missing due to a sampling bias or because they were too rare to be preserved. The earliest
large styliolinids probably inhabited only in restricted marginal marine environments and later (Devonian) colonization of open marine environments led to increased predation pressure that favoured evolution of small forms.