Early Cambrian small carbonaceous fossils (SCFs) from an impact crater in western Finland

Abstract

We describe an assemblage of small carbonaceous fossils (SCFs) and acritarchs from cored siltstones of the Lappajärvi impact structure, west-central Finland. Previous studies had detected a depauperate acritarch biota ascribed to a deep Proterozoic origin—this age, however, was based on recovery of long-ranging poorly age-diagnostic sphaeromorphs. To resolve the age and provenance of these crater sediments, we applied low-manipulation processing techniques optimized for retrieval of larger organic-walled microfossils. Our study revealed a previously undetected assemblage containing numerous metazoan SCFs consisting of flattened ‘protoconodonts’ (grasping spines assignable to total group Chaetognatha) and a distinctive fossilised chaeta, possibly representing the oldest known annelid remains. Phylogenetically problematic fossils include various acritarchs (large Leiosphaeridia sp., Tasmanites tenellus, smaller sphaeromorphs, Synsphaeridium, Archaeodiscina and Granomarginata) and filamentous forms (Palaeolyngbya- and Rugosoopsis-like filaments, Siphonophycus), likely representing prokaryotic or protistan grades of organisation. As well as adding new diversity to an emerging SCFs record, these data substantially refine the age of these sediments by more than half a billion years, to an early Cambrian Terreneuvian age. More specifically, the assemblage is equivalent to that of the Lontova Formation from the Baltic States and northwest Russia, but is previously unreported from Finland. Identification of Lontova-type SCFs/organic-walled microfossils at Lappajärvi further constrains the poorly resolved extent of maximum flooding during the early Cambrian in Baltica. Renewed attention should be directed to strata that have thus far produced only biostratigraphically long-ranging or ambiguous palynological assemblages—‘SCF-style’ processing can reveal hitherto undetected, age-informative microfossils that are otherwise selectively removed in conventional palynological studies.