Gorzelak P and Zamora S (2016), Understanding form and function of the stem in ...

ECB-ART-50007

PeerJ 2016 Jan 01;4:e1820. doi: 10.7717/peerj.1820.

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Understanding form and function of the stem in early flattened echinoderms (pleurocystitids) using a microstructural approach.

Gorzelak P , Zamora S .

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Pleurocystitid rhombiferans are among the most unusual echinoderms whose mode of life has been long debated. These echinoderms are usually interpreted as vagile epibenthic echinoderms, moving over the sea bottom by means of a flexible stem. Although their life habits and posture are reasonably well understood, the mechanisms that control the movement of stem are highly controversial. Specifically, it is unknown whether the stem flexibility was under the control of muscles or ligamentary mutable collagenous tissues (MCTs). Here, we reconstruct palaeoanatomy of the two Ordovician pleurocystitid rhombiferans (Pleurocystites and Amecystis) based on stereom microstructure. We show that the articular facets of columnals in pleurocystitid rhombiferans are composed of fine labyrinthic stereom. Comparison with modern echinoderms suggests that this type of stereom was associated with muscles implying that their stem was a muscular locomotory organ supporting an active mode of life.

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Figure 1. Morphology and microstructure of pleurocystitids.(A) pleurocystitid reconstruction (modified after Parsley, 1970); (B–D) stereom organization in outer proximal (B), inner proximal (C) and distal columnal (D); (E) functional model for pleurocystitid stem flexibility, muscles in relaxed (blue) and contracted (red) position, dotted lines delineate the position of the axial canal (modified after Donovan, 1989); (F) stereom fields for Recent echinoderms (dark shaded area shows the stereom field indicative of muscles) (taken from Clausen & Smith, 2005); 1, fine labyrinthic stereom of articular surfaces of Pleurocystites; 2, coarse labyrinthic stereom of the interior of columnal of Pleurocystites; 3, fine labyrinthic stereom of articular surfaces of Amecystis; 4, coarse labyrinthic stereom of the interior of columnal of Amecystis.

Figure 2. Stereom microstructure of the Late Ordovician pleurocystitids and a recent crinoid Metacrinus rotundus.Stereom microstructure of the Late Ordovician pleurocystitids from Canada (Pleurocystites (A–H) and Amecystis (I–M, O)) and a recent crinoid Metacrinus rotundus (N) from the Suruga Bay under SEM (A, B, E, F, I, J, M, N), optical (C, G, K, L) and CL (D, H, O) microscope; (A) microperforate layer of the distal columnal latera; (B) fascicular stereom near the lumen of the distal columnal; (C) longitudinally cross-sectioned distal columnals articulated with fulcrum (arrows) showing irregularly arranged trabeculae and pores displaying no alignment; (D) transversally cross-sectioned distal columnal showing irregularly arranged pores near the articular surface (E) fine labyrinthic stereom with needle-like projections on articular surface of distal columnal; (F) fine labyrinthic stereom on articular surface of distal columnal (G) longitudinally cross-sectioned proximal columnals showing irregularly arranged trabeculae and pores displaying no alignment; (H) longitudinally cross-sectioned proximal columnals (highlighted with dotted lines) showing irregularly arranged pores; (I) microperforate layer (ML) and coarse labyrinthic stereom of proximal columnal; (J) the contact (dotted line) between coarse and fine labyrinthic stereom near the articular surface of proximal columnal; (K) longitudinally cross-sectioned proximal columnals showing irregularly arranged trabeculae and pores displaying no alignment; (L) longitudinally cross-sectioned distal columnals articulated with fulcrum (arrow) showing irregularly arranged trabeculae and pores displaying no alignment; (M) fine labyrinthic stereom with needle-like projections on articular surface of proximal columnal; (N) fine labyrinthic stereom with needle-like projections of the muscle fields in brachial of Recent crinoid; (O) longitudinally cross-sectioned proximal columnals (the contact between cement is highlighted with dotted line) showing irregularly arranged pores.

References [+] :

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