Belaústegui et al.—Echinoderm ichnology and other related processes
mussel shells was removed by the action of acids discharged by the asteroid Cosmasterias lurida during predation. It seems, however, very unlikely that these traces can be recognized in the fossil record. It was also reported by Gordillo and Archuby (2012) that, although some of the mussel shells may be crushed during these attacks, the resulting fracture patterns are indis- tinguishable from those caused by physical factors.
Crinoid bioturbation structures.—Most crinoids exhibit a ses- sile lifestyle, but it is known that some stalked crinoids (coma- tulids and isocrinids) are able show locomotion. Although some aquarium observations have been carried out (e.g., Messing et al., 1988), Baumiller and Messing (2007) recorded for the first time the active locomotion of the isocrinid Neocrinus decorus by in situ observation with a submersible at a depth of 420m near Grand Bahama Island. The resulting traces produced by the ‘elbow-crawl’ locomotion (Baumiller and Messing, 2007) con- sist of a central and rectilinear groove (i.e., the mark left by the drag of the stalk) probably flanked by the imprints of the power strokes of the arms (Fig. 1.11). Since the submarine video filmed by Baumiller and Messing (2007) only permits clear observa- tion of this central groove, these authors also figured the traces produced by the crawling comatulid Davidaster rubiginosa on a muddy substrate under aquarium condition; proposing that these traces could be comparable with those left by the arms of the isocrinid N. decorus. Recently and for the first time in the trace fossil record,
Neto de Carvalho et al. (2016) observed a definite crinoid crawling trace and erected the new ichnotaxon Krinodromos bentou. This trace fossil consists of an irregular trail, bordered by shallow and large grooves, associated at its very end with an isocrinid crinoid body fossil from the Middle Jurassic of the Cabeço da Ladeira Lagerstätte (Portugal). Despite the good knowledge of the different anchoring
strategies of crinoids in soft sediments (Seilacher and MacClintock, 2005; Donovan, 2006), some of which are potential producers of bioturbation structures, no modern or fossil traces have been ascribed to them.
Traces associated with mitrate body fossils.—Associated with body fossils of the Devonian stylophoran Rhenocystis latipedunculata, Rahman et al. (2009) erected the ichnotaxon Vadichnites transversus as horizontal straight to curved traces with low relief protuberances or closely spaced fine ridges trans- versely oriented across a shallow groove. This new ichnotaxon is argued to demonstrate that the appendage of R. latipedunculata was used in locomotion and that this movement took place appendage-first. Rahman et al. (2009) interpreted these trace fossils as having been produced just before death probably in response to catastrophic burial by turbidity current deposits. AlthoughVallon et al. (2016) do not recommended the use of this ethological category, arguing that it is based on holistic inter- pretations, rather than on trace fossilmorphology, the ichnogenus Vadichnites could be considered as an example of mortichnia. Finally, in order to obtain a rapid understanding of all these
ichnotaxa, a recompilation of ichnogenera diagnoses is shown in Table 2. According to the last ichnotaxonomic studies, only the diagnoses of those broadly accepted ichnogenera have been included.
Traces not directly produced by echinoderms but closely related to them
Echinoderms possess multi-element skeletons composed of thousands of diverse ossicles (e.g., Donovan, 1991; Kroh and Nebelsick, 2010). After death and under ‘normal’ conditions (i.e., excluding rapid burials, dysoxic conditions, etc.), the dis- articulation of most parts of echinoderm skeletons, except for many echinoids that are commonly preserved as complete denuded tests (see Belaústegui et al., 2012 and references therein), is very fast (days to weeks) and their separate single ossicles may become very important components within the sediment (Ausich, 1997; Kroh and Nebelsick, 2010; Dynowski, 2012). Occasionally, the abundance of these ossicles on the seafloor may be condensed by the activity of burrowing organ- isms (e.g., the ichnogenus Crininicaminus, see below). In addition, the unique compositional and morphological features of the echinoderm skeletons promote their rapid growth and regeneration (Kroh and Nebelsick, 2010). These qualities may facilitate the preservation of an ichnological record (e.g., predation, parasitism) on or in their skeletons.
Traces produced with or in echinoderm ossicles and/or spines.—Ettensohn (1981) erected the ichnogenus and ichnospecies Crininicaminus haneyensis, to describe cylindrical burrows with a lining mainly composed of crinoids ossicles from the Carboniferous of east-central Kentucky, USA. Subsequently, from the late Permian Kamiyasse Formation (northeastern Japan), Seike et al. (2014) erected the new ichnospecies C. giberti differing from C. haneyensis by the arrangement of the crinoid stem plates (horizontal to the long axis of the tunnel in C. haneyensis, and vertical in C. giberti). In both cases, the authors attributed its probable construction to the burrowing activity of tube-dwelling worms. Neumann et al. (2008) erected the ichnospecies
Trypanites mobilis for borings produced in bulbous spines of psychocidarid echinoids from the Late Cretaceous (Cenoma- nian) to early Paleocene (Danian) strata of the North Sea Basin (Denmark). These authors proposed that these borings were produced post-mortem by sipunculid worms, which would have used these spines as mobile domiciles on soft-bottom habitats.
Bioclaustrations on echinoderm stereom.—Embedment struc- tures or bioclaustrations consists of cavities produced by endo- symbiontic organisms that live within, by partially inhibiting, the growing skeleton of a host organism (e.g., Tapanila, 2005; Cónsole-Gonella and Marquillas, 2014). Tapanila (2005) included all these traces within the new ethological category Impedichnia; although subsequent authors do not recommend its use (Vallon et al., 2016, and references therein). In any case, bioclaustrations produced in the stereom of different echino- derms, mainly crinoids, have been recorded and several ichno- taxa have been erected to name them. The ichnogenus Tremichnus was erected by Brett (1985) to
include simple circular-parabolic pits, with or without asso- ciated stereom swellings, primarily produced by a combination of embedment (i.e., inhibition of stereom growth) and true
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