Palaeodiversity, palaeobiology and palaeoecology of middle devonian crinoids from the Eifel type region

Abstract

This doctoral thesis deals with crinoids from the Middle Devonian (U. Eifelian to L. Givetian) of the Eifel Synclines (western Rhenish Massif, Germany) and secondary with U. Eifelian to U. Givetian crinoids of the eastern Rhenish Massif. The study focuses on new recovered material and on material deposit in historical collections. Since the classic monographs of the early 19th century, crinoids are nearly unstudied in modern view. They are only periphery mentioned within the standard works Treatise on Invertebrate Paleontology and Fossil Crinoids. The Eifel has to be characterised as the global hotspot of Middle Devonian crinoids. Because of the high diversity, selected groups of each of the four occurring Palaeozoic subclasses are studied in the course of this work: 1. The subfamily Cupressocrininae (subclass Cladida); 2. the family Hexacrinitidae (subclass Camerata); 3. the genus Stylocrinus (subclass Disparida); 4. the genus Ammonicrinus (subclass Flexibilia). Altogether, four families, eight genera and 66 species are described taxonomically. 10 new species are erected newly. Based on the excellent three-dimensional preservation of the partly autochthon conserved skeletons and their ecological-/facial response, the Eifel crinoids gave important information about the palaeodiversity, palaeobiology and palaeoecology: Regeneration processes in cupressocrinitids and hexacrinitids correspond with that features defined for recent echinoderms. Because of their important functions, the regeneration of injured arms is more perfect than those of affected cups. Hexacrinites contra balanced the general smallness of the regenerative arms by an increased pinnulated surface. Pre- and postmortem skeletal modifications are distinguishable based on the presence or absence of a stereomatic response. In cupressocrinitids, obviously genetically modified anomalies must be separated from external caused skeletal modifications. The function morphology of Ammonicrinus indicates that the nutriment flow of several species was obviously enabled by an active ligament pumping mechanism of the stem via slowly stiffening and relaxing of their mutable connective tissues under ionic balance. The arms of Stylocrinus shows internally inclined edges adjoining laterally with adjacent brachials in an interlocking network to stabilise the closed arm crown and may allow settling in hydrodynamic turbulent environments. The cups of Hexacrinites show sloping morphologies in turbulent environments. Hexacrinites and Ammonicrinus show phylogenetic trends that obviously evince a predator driven evolution (e.g. platyceratid gastropods). Biogenous caused skeletal modifications in hexacrinitid-cups can be attributed to platyceratid gastropods. Epizoan encrusting of bryozoans, microconchids, corals and poriferas mostly occurred postmortem, while the bryozoan genus Cyclopelta premortem encrusted the stems of cupressocrinitids. Stylocrinus-morphotypes are controlled by the ecological and facial framework. The stratigraphic distribution of several taxa, e.g. of Robustocrinites, was controlled by regional-geological events. This have bearing on the fluctuation of the palaeodiversity: Within the Rheno-Ardennic Massif it can be shown that the Middle Devonian crinoids of the Eifel Synclines are linked to carbonatic shelf environments and displaced the crinoid associations of the U. Pragian to L. Eifelian, e.g. of the Hunsruckschiefer, which are adapted to deeper water habitats and show more filigree skeletal morphologies. With the establishment of biostromal developments, this association dominates up to the L. Givetian with successive increasing of the diversity and individual numbers. Within the L. Givetian, this palaeodiversity collapse presumably because of successive increasing of the sea level (Lower Givetian Crinoid Decline), although, outside the Eifel, this association can be traced up to the U. Givetian of the Bergisches Land and the Lahn-Dill region. With beginning of the Frasnian, a crinoid association, which is dominated by camerates, sets in and can be recognised within the Rheno-Ardennic Massif up to the Frasnian/Famennian boundary. This Melocrinites-Megaradialocrinus association was abruptly replaced by an extremely low diverse amabilicrinitid-dominated fauna, which already has a Carboniferous character, and is the response of the Frasnian-Famennian Event (Frasnian-Famennian Crinoid Decline).