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As long as the aboral nerve entoneural center near the base of the calyx remains intact, regeneration of the entire crinoid crown may occur following near decapitation. Amemiya and Oji demonstrated this experimentally, and it has been recognized to occur in natural populations Bourseau et al. At least some crinoids have had the ability to recover from such extreme trauma since the Paleozoic.
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Springer illustrated a specimen of Taxocrinus colletti White in which the entire crown, with the exception of the infrabasal circlet and part of a single basal plate, was lost and successfully regenerated Fig. Regeneration of the crown in a flexible crinoid. B and C Regenerated crown in T.
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Arrows indicate the boundary between the basal and infrabasal plate circlets the latter is mostly within the stem facet , which roughly corresponds to the regeneration plane in B and C. The regenerated specimen USNM S lost the entire infra-basal circlet and all but a single basal plate b , which was mechanically fractured. This specimen demonstrates that flexible crinoids and presumably cladids could regenerate entire crowns as long as the aboral nerve center is undamaged.
See Springer , p. There are only a few published examples of stalk recovery in crinoids, extinct or extant. For example, Strimple and Frest figured two specimens of a Pennsylvanian flexible crinoid, Euonychocrinus simplex Strimple and Moore , which had been separated from their stalks and had successfully restored a few columnals. Ausich and Baumiller reported possible regrowth in a single, atypical Ordovician stalk composed of numerous, irregularly arranged plates.
However, although the specimen figured by Ausich and Baumiller seemingly recuperated a portion of the stalk, there is no evidence of crown recovery, and as correctly suggested by Oji , it should not be considered a legitimate example of regeneration. Moreover, because stalk replacement generally proceeds as a continuation of normal development from the base of the calyx, it may not represent true regeneration, but rather simple regrowth Nakano et al.
Occasionally, crinoids may regenerate two or more appendages in place of one. This is especially true of their arms Ubaghs ; Oji , but has also been observed among the anal tubes of Paleozoic camerate crinoids Fig.
One hypothesis to explain this phenomenon is that aberrant branching may result from the mechanical fracturing of an ossicle, followed by the initiation of multiple regeneration sites. This hypothesis could be tested experimentally in the arms of living crinoids by producing irregular mechanical fractures through the brachials. Aberrant branching as evidence for damage and regeneration in crinoids.
Arrows indicate inferred regeneration sites and points of aberrant branching. Multiple branches from a single ossicle may occur on the fractured brachials arm plates of fossil crinoids. Gahn and Baumiller figured a specimen of Holcocrinus longicirrifer Wachsmuth and Springer in which the primaxil, the plate supporting the first arm division, was mechanically damaged.
Rather than repair the plate and regenerate a new arm, the crinoid produced a second primaxil and corresponding pair of arms. Faulty regeneration has also been used to explain skeletal variation in living isocrinids. Oji noted that skeletal primibrachial variation increases with size in Metacrinus rotundus Carpenter , possibly because of the accumulation of inaccurate recuperations over time.
Depending on the extent of the damage, crinoid arms may fully regenerate within a few weeks to many months Mladenov ; personal observations ; thus, the history of regenerative events within a single individual is difficult to quantify. A more direct measure of the regenerative histories of individual crinoids may be possible through isotopic analysis.
Isotopic analysis of crinoid skeletons may provide a tool for studying the history of regeneration in single individuals, thereby revealing new insights into the frequency and magnitude of regenerative events at the scale of crinoid lifetimes. Although this approach is in its infancy and has yet to be applied to fossil crinoids, diagenetic overprinting of skeletal material may prove to be a substantial obstacle for using isotopes to study regeneration in the geologic past.
Solid bars represent isotopic differences between superadjacent brachials without regeneration. Gray arrows represent isotopic differences between superadjacent brachials with regeneration. Inset line drawings are of partial crinoid arms composed of numerous brachials that show non-regeneration white image and regeneration gray image across specialized autotomy articulations cryptosyzygies; dotted lines. Vertically ruled lines indicate muscular articulations.
One method of non-regenerative repair is the production of small filling or reparative plates platelets as have been reported to occur in the mechanically damaged tests of echinoids Ameye and Dubois ; Bonasoro et al. Such repair also characterizes the specimen of D. The gaping wound in the posterior interray of that specimen does not show evidence of regeneration; rather, reparative plates are found around the margin of the injury.
These plates are unusually small, irregularly arranged, and unfused at their margins. Although this individual may not have survived long enough for the wound to heal entirely, Wachsmuth and Springer , pl. In that specimen, the arm facets and calyx plates associated with the C-ray and adjacent portions of the B- and D-rays suffered injury but did not regenerate—new arm facets and regular calyx plating were not regained; instead the area healed over with an irregular mosaic of plates Fig.
A Lateral view of specimen, centered on E ray. B Dorsal view of specimen with CD posterior interray directed upward. C Lateral view of specimen centered on CD interray. The arm facets, c, of this species are distributed around a medially expanding disk. Note that physical trauma, presumably the bite of an unknown predator, has disrupted the symmetry of the disk A and B. Moreover, the damage and subsequent regeneration likely generated an additional or bifurcating anal tube, a A and C.
The completely closed wound was sealed with relatively small, irregularly arranged filling plates 7C. As in Fig. A specimen of Staphylocrinus bulgeri Burdick and Strimple provides a third and very unusual example of damage Fig. In that specimen, an entire basal plate appears to have been lost without injury to adjacent plates, as if it had simply popped out, and the wound was filled with reparative plates.
This may indicate that the reparative plates had begun to fuse into a single basal plate, which if completed, would have represented a case of true regeneration. A Lateral view centered on DE interray. B Close up of missing DE basal and filling plates. Note two regions of potential platelet fusion in the central and upper center portions of the missing DE basal. Finally, reparative plates have been reported from a crinoid stem facet. McIntosh and Schreiber , pl. In that specimen, the stalk was presumably separated from the crown, which was not fatal to the crinoid.
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We are not aware of any extant crinoids in which such reparative plates have been recognized in the stem facet or elsewhere. Another form of non-regenerative healing is the development of skeletal overgrowths or calluses. In the first type, crown loss was associated with the development of a holdfast-like termination, and in the more common second type, a rounded, sometimes multiplated callus or gall formed over the decapitated stalk.
In both cases, the axial canals of the stalks were infilled and overgrown by biogenic calcite. Similar overgrowths have been observed on the proximal columnals of autotomized isocrinid stalks Roux ; Emson and Wilkie Overgrowths have been recognized on crinoid stalks from fossils as old as the Ordovician, variably interpreted as being associated with the distal Warn and Strimple and proximal Donovan and Schmidt end of the animals.
The entoneural nerve center or chambered organ , located at the base of the crinoid crown, is likely responsible for regulating regeneration in both the crown and stalk Przibram ; Nakano et al. If this nerve center is damaged, neither the crown nor the stalk may regenerate.
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Thus, instances of proximal stalk overgrowths likely result from the complete removal of the crown. Experimental studies on M. However, after eight months in aquaria, the stalks lacked evidence of regeneration and, curiously, they exhibited no overgrowths. The previous section illustrates several important points about regeneration in crinoids: 1 crinoids have exhibited a wide range of regenerative abilities since their appearance in the fossil record; 2 regeneration is common among fossil and living crinoids; 3 regeneration may occur in all parts of the crinoid skeleton, but it occurs most commonly among the arms; and 4 crinoids may experience injuries from which they may recover, but not truly regenerate, as long as the aboral never center is unimpaired.
The oldest undisputed crinoids are from the Early Ordovician Guensburg and Sprinkle , During this period, crinoids show evidence of damage and recovery Springer ; Warn and Strimple ; Ausich and Baumiller ; Donovan and Schmidt ; Baumiller and Gahn ; and, as was suggested previously, their capacity to regenerate was likely inherited from stem-group echinoderms and did not evolve independently within the Crinoidea. With the aim of understanding the evolutionary history of crinoid regeneration, questions of primary interest relate to if, and how, crinoid regenerative abilities have changed through time.
The answer, simply stated, is that there is no unambiguous evidence that crinoids, as a whole, have changed in their abilities to regenerate, even with increasing predation pressure through the Phanerozoic Vermeij ; Kowalewski and Kelley ; Kelley et al. As discussed previously, there is no evidence that camerate crinoids could fully regenerate their crowns following severe trauma, even if the aboral nerve center was undamaged Figs. In contrast, regenerative abilities of the crown were present in flexibles Fig. Presumably, the ancestor of both flexibles and articulates, the cladids, also had the ability to regenerate entire crowns, but fossil evidence for this is so far lacking.
Additionally, we are not aware of any examples of reparative plates in articulates. Rather than heal damage via reparative plates, articulates may regenerate fully. Such potential for recovery would indeed represent an increase in regenerative ability, if it could be demonstrated that the poorer recovery power of camerates represented the primitive condition among crinoids. Although predation is the most frequently cited cause of injury to crinoids Ausich and Meyer ; Baumiller and Gahn , it is not the only culprit.