Monstrocrinus
Monstrocrinus is an extinct genus of stalked crinoid from the Lower to Middle Devonian, found in Brazil, Europe and, ambiguously, Algeria. It is most notable for the flattened spine structures that attach to its bowl-like calyx plates, at the base of its theca.
Traditionally, the genus was interpreted as a stalk-less free-living crinoid, drifting throughout substratum while its axe-like spines provided anchorage. However, this view was later challenged as re-examination of the original specimen revealed a previously unknown point of attachment for the column. From this a new mode of life was inferred, were the stalk would elevate the crown of the crinoid to the water, and the spines would serve as stabilizing fins, allowing the animal to maintain an efficient feeding posture in moving water. Water would move from where the theca meets the stalk towards the mouth, also passing through the arms.
History of study
Schmidt described a genus of crinoid known from remains found in a former brickwork mine in Osterbachtal, in the modern-day German town of Arnsberg, Sauerland. He had previously cited the fossil in an earlier work, describing it as "with unusually monstrous, axe-shaped spines on the basals and radials", whence the name of the genus, Monstrocrinus. The original materials were disarticulated spines and calyx plates found in different localities. Even so, he used this material to describe the new genus. Schmidt originally interpreted this genus as a stalkless crinoid, passively drifting through the sea floor, while the "monstrous floating spines" aided in giving it stability. He explained the apparent absence of a stalk through the lack of a "stem scar" on the arboreal side of the calyx, where the opening would have been closed by the growth of the surrounding plate. He also assumed from the disarticulated arms of M. granosus that all specimens must have had short arms. However, both this and the interpretation of Monstrocrinus possessing no stalk would later be proven inaccurate. Schmidt named two species in his 1941 publication; the type species M. securifer, found on the sites around Olpe, North Rhine-Westphalia, and M. granosus, from the Mandeln Formation.Le Mann, described a single plate from the Pena Negra Formation of Loscos, Spain. This specimen has an elk-antler-like spine, which resulted in Le Mann describing this as the new species M. aliformis. He also hypothesised on the evolution of Monstrocrinus, writing that the spines of Acanthocrinus first evolved the forked spines of M. granosus, which later became the antler-like spines in M. aliformis. However, this new species would later be synonymised with M. securifer, meaning that this model has since become dated. Le Mann also wrote of two isolated calyx plates from the Chefar [El Ahmar Formation] of Algeria, identifying them as Monstrocrinus sp.. However, these specimens cannot be conclusively attributed to the genus.
Scheffler described a specimen of M. securifer from the Maecuru Formation of Northern Brazil, however this specimen would later be proposed to be M. incognatus or a new species. In 2009, Scheffler et al. described additional material representing the calyx and crown, which showed that the arms divided twice. In 2011, Scheffler et al. also described a new species, M. incognitus, from specimens found in the Pimenteira Formation of the Parnaíba Basin, Northeastern Brazil. According to the 2011 publication, the specific name derives from the Latin word "incognitus", meaning disguised or unknown, referring to the unusual genus and the difficult history of interpreting its paleoecology.
More recently, Bohatý et al. described a new, more complete specimen that had its stalk, spines and crown preserved, as well as re-examining the original specimens and the previously cited works. They discovered that the original holotype and paratypes actually do have a canal connecting to a stalk, which wasn't present in Schmidt's original illustrations, as well as parts of the disarticulated crown and parts of the stem, which were originally interpreted as belonging to another genus.