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Experimental neoichnology of post-autotomy arm movements of sea lilies and possible evidence of thrashing behaviour in Triassic holocrinids.
Gorzelak P
,
Salamon MA
,
Brom K
,
Oji T
,
Oguri K
,
Kołbuk D
,
Dec M
,
Brachaniec T
,
Saucède T
.
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Echinoderms exhibit remarkable powers of autotomy. For instance, crinoids can shed arm and stalk portions when attacked by predators. In some species, it has been reported that the autotomized arms display vigorous movements, which are thought to divert the attention of predators. This phenomenon, however, has not been well explored. Here we present results of experiments using the shallowest water species of living stalked crinoid (Metacrinus rotundus) collected at 140 m depth. A wide range of movements of detached arms, from sluggish writhing to violent flicks, was observed. Interestingly, autotomized arms produce distinct traces on the sediment surface. They are composed of straight or arched grooves usually arranged in radiating groups and shallow furrows. Similar traces were found associated with detached arms of the oldest (Early Triassic) stem-group isocrinid (Holocrinus). This finding may suggest that the origins of autotomy-related thrashing behaviour in crinoids could be traced back to at least the Early Triassic, underscoring the magnitude of anti-predatory traits that occurred during the Mesozoic Marine Revolution. A new ethological category, autotomichnia, is proposed for the traces produced by thrashing movements of shed appendages.
Figure 1. Traces produced by autotomized arms of Metacrinus rotundus. (a–c) Straight deep grooves arranged in radiating group; (d–f) a few sets of straight parallel grooves and furrows inclined at different angles to each other; (g–i) two large arched grooves; (j–l) small short parallel furrows. (a, d, g, j) Photographs of sediment surface; (b, e, h, k) photographs of gypsum casts; (c, f, i, l) false-color depth maps of gypsum casts (for source data see the Open Science Framework, https://osf.io/b8zu2/project ”3D models of crinoid traces”, files SOM7_ichno_3D, SOM8_ichno_3D, SOM9_ichno_3D); (m) color scale of elevation. Large and small arrows indicate deep grooves left by the arm and shallow furrows made by pulling the pinnules along the substrate, respectively. Scale bars = 1 cm.
Figure 2. Current-induced sole markings (arrows) left on the sediment surface. (a) Photograph of sediment surface; (b) photograph of gypsum cast.
Figure 3. Possible trace fossils associated with articulated piece of arm of Holocrinus sp. (UBGD 30564; Thaynes Group, Lower Triassic; collection of Université de Bourgogne, Géologie Dijon, France). (a) Slab preserving articulated fragment of arm and associated traces (arrows); a photograph taken using a camera fixed to a tripod. (b) Slab preserving articulated fragment of arm and arched grooves (curved red arrows) arranged in radiating group and a few sets of indistinct straight parallel furrows (straight small red arrows with a question mark) and analogical traces (in dotted circle) produced by autotomized arms of Recent isocrinid; a photograph taken using a camera attached to an optical microscope. (c) False-color depth map of a slab; (d) false-color depth map of a slab acquired from photographs taken from binocular microscope (elevation has been increased (× 2) to enhance depth contrast); (e) color scale of elevation. For source 3D data see the Open Science Framework, https://osf.io/b8zu2/project ”3D models of crinoid traces”, files SOM10_old, files SOM11_newest. Scale bars = 1 mm; scale bar in dotted circle = 1 cm.
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