Gorzelak P et al. (2014), Reassessing the improbability of a muscular cri...

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Sci Rep 2014 Aug 13;4:6049. doi: 10.1038/srep06049.

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Reassessing the improbability of a muscular crinoid stem.

Gorzelak P , Głuchowski E , Salamon MA .

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Muscular articulations in modern stalked crinoids are only present in the arms. Although it has been suggested that certain coiled-stemmed fossil taxa may have been functionally adapted to utilize muscles, evidence supporting this interpretation is lacking. Here, we use cathodoluminescence and SEM to reveal the skeletal microstructure of the enigmatic coiled-stemmed taxon Ammonicrinus (Flexibilia). Based on the well-established link between skeletal microstructure and the nature of infilling soft tissues in modern echinoderms, we reconstructed the palaeoanatomy of the Middle Devonian ammonicrinids. We show that their median columnals with elongated lateral columnal enclosure extensions (LCEE) have stereom microstructure unexpectedly resembling that in the crinoid muscular arm plates. In particular, large ligamentary facets, that are present on each side of a transverse ridge, are mainly comprised of fine galleried stereom that is indicative of the mutable collagenous tissues. In contrast, fine labyrinthic stereom, commonly associated with muscles, is situated in the periphery on each side of the surface of elongated LCEE. Our findings thus strongly suggest that the muscles may have also been present in the stem of ammonicrinids. These results reassess the previous hypotheses about evolution of muscles in crinoids and provide new insights into the mode of life of Ammonicrinus.

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	Figure 1. Geological map of western part of the Holy Cross Mountains and location of study site.Modified from ref. 35.
	Figure 2. Morphology and microstructure of Ammonicrinus.(a) Reconstruction of life time position (Modified from ref. 16). (b) Schematic draw of median columnal joint surface with elongated lateral columnal enclosure extension showing the orientation of investigated thin sections. (c) Distribution of stereom microstructure in transversal section. (d) Distribution of stereom microstructure in tangential section. (e) Distribution of stereom microstructure in longitudinal section. (f) Distribution of stereom microstructure in oblique-longitudinal section of the lateral columnal enclosure extension. (g) Schematic draw of median columnal joint surface with reduced lateral columnal enclosure extension showing the orientation of the investigated thin sections. (h) Distribution of stereom microstructure in transversal section. (i) Distribution of stereom microstructure in tangential section. (j) Distribution of stereom microstructure in longitudinal section. (k) Distribution of stereom microstructure in oblique-longitudinal section of the lateral columnal enclosure extension. (l) Schematic draw of distal columnal showing the orientation of the investigated thin sections. (m) Distribution of stereom microstructure in longitudinal section. (n) Distribution of stereom microstructure in transversal section.
	Figure 3. Photomicrographs of median columnal of Ammonicrinus (tangential section).(a) Optical (TL), (b) Polarizing (PL) and (c) Cathodoluminescence (CL) views. (d) CL-activated UV-VIS spectrum of the intense orange-red luminescent ammonicrinid columnal showing Mn2+ emission maximum at ~615ânm. Note that the stereom microstructure is greatly enhanced under cathodoluminescence.
	Figure 4. Results of geochemical analyses of Ammonicrinus.(a) SEM back-scattered electron (BSE) image of the polished ammonicrinid columnal showing the contrast between materials of lower (darker areas = stereom) versus higher atomic numer (lighter areas = cement). (b) Results of EDS analyses of the stereom (s) and cement (c).
	Figure 5. Cathodoluminescence (CL) photomicrographs of median (a-i) and distal (j-l) columnals of Ammonicrinus.(a) Transversal section showing medium to fine labyrinthic stereom near the periphery of the plate margin (region of the lateral columnal enclosure extension). (b) Transversal section showing the medium to coarse labyrinthic stereom in the centre of the section. (c) Transversal section showing coarse rectilinear and fine labyrinthic stereom (arrow) in the periphery of the plate margin (region close to the centre of the articular surface). (d) Longitudinal section close to the lateral columnal enclosure extension showing coarse rectilinear and/or galleried stereom and fine labyrinthic stereom (arrows) near the echinoid-like tubercles in the periphery of the plate margin. (e) Longitudinal section of the articular surface showing fine galleried stereom underlain by coarse labyrinthic stereom. (f) Oblique-longitudinal section of the lateral columnal enclosure extension showing medium to fine labyrinthic stereom. (g) Tangential section showing coarse rectilinear stereom in the periphery of the plate margin. (h) Tangential section showing coarse labyrinthic stereom underlain by fine galleried stereom near the axial canal. (i) Tangential section showing coarse labyrinthic to rectilinear stereom and fine labyrinthic stereom (arrows). (j) Longitudinal section showing fine galleried stereom (lower left) and coarse galleried to rectilinear stereom (upper right). (k) Longitudinal section showing fine galleried stereom near the lumen and perpendicular coarse galleried to rectilinear stereom near the periphery of the plate margin. (l) Longitudinal section showing coarse galleried to rectilinear stereom and microperforate layers in the periphery of the plate margin.
	Figure 6. Microstructural organization of the facet surface of median ammonicrinid columnals with elongated (a-d) and reduced (e-h) lateral columnal enclosure extensions in SEM.(a) Boundary (dotted line) between fine galleried (left) and fine labyrithic (right) stereom. (b) Higher magnification of (a). (c, d) Higher magnification of fine labyrithic stereom. (e) Medium galleried stereom. (f, g) Higher magnification of (e). (h) Fine galleried stereom of the articular surface.
	Figure 7. Stereom fields in recent echinoderms.Taken from ref. 32. Dark shaded areas correspond to the stereom fields indicative of muscles and mutable collagenous tissues (MCT). The range and mean of the six stereom types of Ammonicrinus are indicated. 1-4 stereom types identified in median and proximal columnals. 5-6 stereom types identified in distal columnals.
	Figure 8. Reconstruction of life time position of Ammonicrinus.(a) Enclosure of the ammonicrinid crown in response to the external stimuli. (b) Schematic sketches of LCEE (lateral columnal enclosure extensions) of the mesistele in coiled (above) and uncoiled (below) positions. Modified from ref. 28. Inner ligaments (blue), outer ligaments (green), muscles (red).

References [+] :

Clausen, Palaeoanatomy and biological affinities of a Cambrian deuterostome (Stylophora). 2005, Pubmed