Revision of the flexible crinoid genus Ammonicrinus and a new hypothesis on its life mode
From: Acta Palaeontologica Polonica(Vol. 56, Issue 3.)
Publisher: Polska Akademia Nauk, Instytut Paleobiologii
Document Type: Report
Length: 12,319 words
The lecanocrinid Ammonicrinus (Flexibilia) is newly interpreted based on new material from the Middle Devonian of the Rhenish Massif (Eifel and Bergisches Land, Germany). The species have echinoid-like tubercles on the attachment and on the column, which bear articulated spines. The intraspecific variability of the column is discussed for three facies-controlled morphotypes, herein classified as standard "exposed-" or "encased roller-type" and the rare "settler-type". New specimens have floating transitions between different plate sculpturing and between those individuals with none or one to several columnals with herein termed "lateral columnal enclosure extensions" on the proximal-most, barrel-like dististele and the following mesistele, which is solely distinguished by these extensions. Based on this interpretation, Ammonicrinus kongieli is evaluated as a subjective junior synonym of Ammonicrinus sulcatus. The latter species was first recognised from the Eifel (Germany). "Ammonicrinus wachtbergensis", from the upper Eifelian of the Eifel, is declared a subjective junior synonym of Ammonicrinus doliiformis. The first nearly complete specimen of Ammonicrinus kerdreoletensis is described from the lower Eifelian of Vireux-Molhain (southern Ardennes, France). Two new species are described: Ammonicrinus jankei sp. nov. and Ammonicrinus leunisseni sp. nov. A functional morphologic trend in perfecting the crown encasement by continuous modification of the lateral columnal enclosure extensions of the mesistele from the Eifelian to the Givetian, indicates a vagile benthic "predator"-driven evolution of ammonicrinids in the Eifel area. Several ammonicrinid species are herein defined as spined soft-bottom dwellers, feeding in low-intensity current water, possibly through a self-produced water flow. The first known postmortem encrusting epizoans on ammonicrinid endoskeletons are reported. Key words: Crinoidea, Flexibilia, Ammonicrinus, Devonian, Eifel, Rhenish Massif, Germany.