Exceptional preservation of the endolithic trace fossil Dendrina belemniticola Mägdefrau, 1937 in the Upper Maastrichtian greensand of Nasiłów (central Poland)

The rosette-shaped endolithic trace fossil Dendrina belemniticola Mägdefrau, 1937 is common in biogenic substrates in the Upper Cretaceous chalk facies of Europe but badly preserved. Chalk specimens are affected by an intensive calcite cementation. Basically all parts of borings are secondarily sealed if their internal diameter is less than the size of the cement crystals and application of casting-embedding techniques fails to document the morphology of such borings. Therefore, up to now only the macro-morphology of Dendrina-borings could be detected in specimens of the chalk facies. In contrast, biogenic calcitic substrates found in the Upper Maastrichtian greensand of Nasiłów in central Poland remained almost unaltered diagenetically. In these specimens calcite cementation did not occur and endolithic traces can be studied at an exceptional high-resolution. Accordingly, new morphological features of the ichnospecies D. belemniticola can be described. Casts of the Dendrina-boring systems show a cover of unbranched hair-like extensions. Those extensions spread into the substrate from all surfaces of the boring and give the Dendrina-casts a hairy or furry appearance. Most likely they had an exploratory function in limited substrates, i.e. locating the boundaries of the substrate and investigating cavities within it. The proof of hair-like extensions that could have enabled sensory functions can explain the stenomorphic reactions of the Dendrina-producer for the first time. Some casts of the hair-like xtensions exhibit bulbous swellings. Most probably these structures represent the second phase of the boring process, followed by further expansion and fusion that constitute the typically verrucose Dendrina-rosette. The endolithic trace D. elemniticola can functionally be compared with a foraminiferan test, and a test-less foraminiferan species is considered to be the trace-making organism.