Early Danian species of the echinoid genus Tylocidaris (Cidaridae, Psychocidarinae) from eastern Denmark

Primary spines of the echinoid genus Tylocidaris Pomel, 1883 were formerly widely used as key index fossils for the Danian (Early Palaeocene). In recent years their value as such has been discredited and tylocidarid species distribution has been shown to be diachronous in relation to dinoflagellate biostratigraphy as well as be facies-dependent to the extent that they are missing from important facies types of the Danian. Difficulties in establishing a correlation between the Danian strata of Denmark and southern Sweden have also affected their practical use. A biostratigraphical study based on specimens collected bed by bed at Stevns Klint and additional studies at Karlstrup quarry (both localities situated in the Danian Stage type area, eastern Denmark), have provided evidence that the Early Danian tylocidarid zonation of Denmark and southern Sweden is essentially the same with T. oedumi Brünnich Nielsen, 1938 and T. abildgaardi Ravn, 1928 being restricted to two distinct zones, and thus not co-occurring as suggested by Brotzen (1959) for the Limhamn locality in Sweden. One major discrepancy between the Danish and Swedish biozonation schemes has thus been eliminated. Material collected at Karlstrup quarry shows that an early Middle Danian rosenkrantzi Zone is present at that locality, this zone not having been recorded from Denmark so far. Especially interesting amongst newly collected material are tests of T. oedumi and T. abildgaardi. Finally, bryozoan mound growth is discussed. It is suggested that the growth of Early Danian mounds was affected by palaeocurrents which swept sediment away from intervening troughs, thus allowing hardground formation to take place there. The troughs must have existed for a considerable time span. At Stevns Klint, a large trough structure shows an impressive series of consecutive (incipient) hardgrounds. In the bryozoan mounds, net growth rate is shown to have been rather slow, allowing typical T. oedumi to evolve via ’overlap forms’ to T. abildgaardi during the formation of the first series of mounds. A new model for tylocidarid evolution in the type Danian is proposed, with the lineage oedumi-’overlap forms’- abildgaardi-ravni-bruennichi becoming extinct in the late Middle Danian, then being replaced by vexilifera, originating from a lineage oedumi-rosenkrantzi-vexilifera.