Lituitid cephalopods from the upper Darriwilian and basal Sandbian (Middle-Upper Ordovician) of Estonia
Abstrakt
Cephalopods of the order Lituitida Starobogatov, 1983 from the late Darriwilian and early Sandbian strata of Estonia are revised herein. During that time, the lituitids reached their peak abundance and were among the key components of cephalopod faunas on Baltica, as well as in other mid- and low-latitude palaeogeographic locations.
The Estonian lituitids mostly come from localities representing more off-shore environments with deeper-water sedimentation in the Middle Ordovician. This supports previous assumptions that lituitids preferred pelagic environments and were capable of long-distance migration.
In the studied lituitid collection, two genera were identified -LituitesBertrand, 1763 andAncistrocerasBoll, 1857. Three species -Ancistroceras ristnensissp. nov.,Ancistroceras vahikuelaensissp. nov. andLituites nehatuensissp. nov. - were newly established.
However, species determinations were often hindered by high intraspecific variability and fragmentary preservation. Investigation of cameral deposits using median sections showed that: connecting rings in chambers with deposits are commonly broken or missing; oriented fragments of connecting rings are present, sometimes overgrown by primary deposits; several generations of primary deposits can be developed; simultaneous presence of both broken and intact connecting rings in the same specimen is usual.
These observations elucidate the formation of cameral deposits in lituitids. Currently, this process is explained either by passive deposition from cameral fluids, or active secretion by cameral mantle.
The evidence collected herein from Estonian lituitids reveals, however, that both modes of formation might have occurred successively during the lituitid ontogeny and that a gradual, life-time destruction of connecting rings was possibly involved.