The possible actiniarian sea anemone burrow Bergaueria hemispherica from the Pliensbachian (Lower Jurassic) of the Lusitanian Basin (Central Portugal)

Abstract

Abstract The shallow marine carbonate deposits of the S. Miguel Formation in the Lusitanian Basin reveal erosional events exposing stiff substrates with abundant Bergaueria hemispherica . The ichnoassociation represented by B. hemispherica , Thalassinoides suevicus , Spongeliomorpha isp., Asterosoma ludwigae and Rhizocorallium commune is dated from lowermost Pliensbachian by the association with ammonites from the jamesoni Biozone, brevispina Subzone. The unlined forms of B. hemispherica include peripheral and bottom burrow concentrations of shell debris, annular-like structures in the burrow margin and paired burrows-to-linear clusters that are comparable with the penetrative burrowing mechanism, anchoring and feeding behaviours of sea anemones. From the ethological point of view, and comparing with modern behaviours of burrowing anemones, the B. hemispherica from the Lusitanian Basin may represent temporary burrowing and relocation of actiniarian anemones. Their comparison with either thenarian or athenarian actiniarians may evidence a preferred surficial feeding strategy on the organic-rich muddy sediments they burrowed.