The influence of burrow-generated pseudobreccia on the preservation of fossil concentrations.

Burrowed discontinuity surfaces associated with condensed fossil concentrations demarcate breaks (hiatuses) in the Phanerozoic marine sedimentary record. Such intervals may be difficult to interpret in view of complex anatomy and varied fossil signatures. Transformation of a discontinuity surface into a heavily burrowed 'pseudobreccia' may further complicate the record, but this issue has remained unexploited to date. The richly fossiliferous Cretaceous-Paleogene (Maastrichtian-Danian) boundary interval exposed in Poland provides opportunities to test the influence of burrow-generated pseudobreccia on the preservation of associated hiatal fossil concentrations. Here, we document pseudobreccia anatomy and fossil-sediment relationships by three-dimensional X-ray computed tomography imaging. In the pseudobreccia zone, we identify a distinctive assemblage of late Maastrichtian fossils, which underwent subsurface preparation by burrowers in a hitherto unreported ichno-taphonomic process. Recognition of pseudobreccia is a prerequisite for the interpretation of the stratigraphy, sedimentology and fossil record of such intervals, which is of special interest for periods of major biotic turnovers, like the end-Cretaceous mass extinction.

Stable isotopes and predation marks shed new light on ammonoid habitat depth preferences.

Ammonoids are extinct cephalopods with external shells which predominated in many late Paleozoic and Mesozoic marine ecosystems. Stable isotope data from ammonoid shells constitute primary tools for understanding their palaeohabitats. However, in most sedimentary successions globally the aragonitic shells of ammonoids are dissolved during fossilisation process and therefore not available for geochemical studies. We overcome this taphonomic bias by analysing the better preservable calcitic elements of the ammonoid jaws (aptychi). We study moulds and aptychi of two successive members, temporal subspecies in our interpretation, of a scaphitid evolutionary lineage from a Late Cretaceous chalk succession in Poland. In order to reconstruct their habitat depth preferences, we apply the powerful combination of stable isotope data from aptychi and co-occurring benthic and planktic foraminifera with an analysis of predation marks preserved on scaphitid specimens. On this basis we conclude that the populations of the older subspecies led a nektic, and those of the younger subspecies, a nektobenthic lifestyle. The shift in habitat depth preferences took place probably as a response of local populations to the shallowing of the sea. Previous studies largely assumed stable depth preferences for ammonoid species, genera and even higher clades. Our study casts doubts over such generalizations by pointing out that ammonoids could have been more flexible in their depth-related behaviour than anticipated.

From pristine aragonite to blocky calcite: Exceptional preservation and diagenesis of cephalopod nacre in porous Cretaceous limestones.

Aragonite (along with calcite) is one of the most common polymorphs of the crystalline calcium carbonate that forms the skeletal structures of organisms, but it has relatively low preservation potential. Under ambient conditions and in the presence of water, aragonite transforms into calcite, the stable polymorph. Aragonite is also more soluble therefore, in water-permeable siliceous limestones (opokas) that are typical of Upper Cretaceous deposits of Poland and Ukraine, the primary aragonitic skeletons are either entirely dissolved and found as moulds and casts or transformed into secondary calcite, whereas the primary calcitic shells remain well preserved. Contrary to the common notion of the lack of aragonite in such porous carbonate deposits, we show that relics of aragonite can be preserved as a nacreous lining on cephalopod moulds or as thin, lenticular structures entrapped in neomorphic calcite. Based on the observed intermediate steps of aragonite alteration, we propose an extended model of nacre diagenesis. Among the originally aragonitic biota, only nautilids and ammonites have retained relics of pristine skeletons. Such selective preservation of only some aragonitic structures (nacre but not the prismatic aragonitic layers) points to the role of microstructural and biochemical differences between cephalopod shell layers that may set a threshold for the dissolution, dissolution/precipitation or preservation of original biomineral structures.

A new hermit crab (Anomura, Paguroidea) from the upper Albian (Cretaceous) of Annopol, Poland.

A new diogenid paguroid, Paguristes liwinskii sp. nov., is described from upper Albian phosphorite-bearing deposits near Annopol, along the east bank of the River Vistula (Wisla), east-central Poland. This new species constitutes an additional example of Early-Mid-Cretaceous macrofaunal shift, from marine reefal limestone to siliciclastic facies, triggered by the worldwide radiation of planktonic organisms. The species described here is the earliest known member of the genus Paguristes, previously recorded from the upper Santonian/lower Campanian to the Recent.