The basic construction of Isocrinida (including species surviving on present deep-sea bottoms) is a rigid body capsule (calyx) with a fixed number of sutured plates and a flexible tegmen, plus three kinds of flexible appendages:
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The arms support rows of pinnules and tube feet for passive filter feeding in a current coming from the aboral side. In order to increase the filter area, the original five arms can multiply by subsequent bifurcations.
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(2)
On the aboral side, an articulated stem elevates the crown from the substrate into zones of higher current velocity. It grew longer by generations of nodals formed at the base of the calyx and by intercalation of internodals between them.
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The radiating cirri articulating with the nodals can be used for various purposes. They become longer by addition of new cirrals in the root tuft. These could alter shapes during subsequent accretionary growth.
Originally, all three kinds of appendages may have grown by introducing new ossicles at their tips; but with the specialization of the distal ossicles for particular functions (holdfasts in the stem; hooks in the cirri), growth zones had to be displaced. This basic outfit was modified in adaptation to a pseudoplanktonic life style.
In the following chapters, observed changes are interpreted as heterochronic shifts in a basic morphogenetic program.
Arms and ramuli
On driftwood, crinoids faced two problems: (1) the crowns had to hang down rather than standing up and the stem was exposed to tensional rather than compressional stress; (2) as a log moves with the water, there is no velocity gradient near the substrate other than by wave action. Therefore, all modern, but smaller, driftwood-dwellers (bryozoans, bivalves, barnacles, tunicates) create their own filter current.
Another tendency of pseudoplanktonic crinoids is to enlarge the filter fan relative to the calyx. This is facilitated by the absence of coarse sediment suspended during storms. So the arms could remain splayed all the time, held together by ciliary (?) adhesion between opposed pinnules. To this end, driftwood dwellers increased the number of arm branchings beyond the two or three usual in benthic forms—only that the following branchings are heterotomous. This means that in each axillary one branch continues to bifurcate at regular distances. The other branch (ramulus) is not only thinner; in pentacrinids it also grows-on without further branchings.
The determination of axillaries appears to differ as well. Isotomous branching probably started at the growing tip of the initial arms. As there was, at this stage, no left/right difference between the resulting branches, the number of interaxillaries could be even or uneven. In heterotomous arms, however, the tip sections are in all stages longer than the interaxillar ones. So these axillaries were probably established at some distance behind the tips. As the numbers of interaxillaries are always even (Simms 1989), new heterotomous axillaries were determined once the pinnules of the previous ramulus had grown sufficently. This is probably why axillaries bear no pinnules (Simms 1989, p. 18). After this, interaxillar sections could grow only by accretionary enlargement of ossicles, while the number of ossicles remained unchanged.
In this peramorphic process, some pecularities require special explanations: (1). The regular distancing of branching points. After the first two branchings (Fig. 1), one always counts 8, 10, 12, 14, or 16 brachial ossicles between branching points. The distance they cover corresponds to the length of the pinnules along the adjacent ramuli. That the intervening brachials always come in even numbers has its reason in an asymmetry that in Isocrinida cannot be recognized in aboral view: adorally, each brachial bears a single pinnule that is alternatingly positioned on the right or left side of the arm. In a one-sided system based on heterotomous bifurcations, a new ramulus can form only in phase with the proper step. (2) The switch from equal to endotomous dichotomy happening after the first two isotomous branchings (i.e. at the 20-arm-stage) means that the branch on the one side continued to grow as an arm with the potential for further branching, whereas the other was downgraded and attenuated into a long ramulus that in pentacrinids could not branch any further.
One effect of this transformation was that the ramuli run parallel to each other, rather than radiating, i.e., a banana-road system is established in a radially symmetric regime. As a second effect, the branch that grew-on as an arm became straight and thereby fit for a coupling function, which can be inferred from a strange taphonomic behavior. During landing, dragging, and compaction, the arms became passively splashed. Nevertheless, adjacent arms tend to keep in touch. This suggests that the tips of the pinnules adhered to each other.
Pinnules
These simple rows of minute ossicles serve various purposes:
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They equally spread the ultimate food catchers (tube “feet”). For this purpose, they line the arms and ramuli on either side. But as in all echinoderms (except in the compound plates of cidaroid echinids) one ossicle can bear only a single tube foot or triplet, pinnules alternate. This is why in side view of uniserial arms and ramuli, pinnules appear only in half the number of brachials and ramulars.
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They border the sides of the ambulacral grooves, in which mucus-bound food particles are transported to the mouth. Therefore, basal pinnulars tend to expand into polygonal plates suturing with their neighbors. That the corresponding tube feet helped in the food transport can be inferred.
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In addition, pinnules along the basal parts of the arms may protect the vulnerable tegmen of the body capsule. For this purpose, almost all pinnulars may give up their flexible connections and become sutured. Still this armor did probably not fuse with the tegmen (there were still tube feet on the oral side!). Such additional protection became important when the tegmen disproportionally expanded, as happened in adult Seirocrinus. Because the transformation relates to scaling and could be easily accomplished (but only in this direction!), “incorporation” of pinnules (Simms 1989, p. 23) is unsuited as a feature separating the two genera.
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Although it made pseudoplanktonic crinoid fossils so visually attractive, the adhesive function of pinnules received less attention. It is expressed by the fact that overall deformation during landing did never lead to entanglement of the pinnules and that torn-apart arms rarely lost the connection between their pinnules. Yet, this function (probably working like velcro strips in modern textiles, shoes, and camera boxes) was essential for pelagic crinoids, because it held the filter fan open without energy expenditure.
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The alternative view (proposed by reviewers) that this effect is due to mutable collagen in the arms is unlikely, because it does not explain the stronger adherence towards the arm tips.
Constructionally, the adhesive function explains why pinnules are longest along the margins of endotomous arms facing the opposite ones—even though they probably formed aboral folds in the splayed filter fan.
This leads to the question about the twin rows of pinnules on arms and ramules. In aboral views of fossil crowns they cannot be seen. Were they reduced to adhesive tubercles?
As some degree of adhesiveness may also exist in modern benthic crinoids, the mechanism involved will hopefully be elucidated in the future.
At the same time, adhesive connections must have been spotty enough to let water flow through. In this way the velcro couplings may have served not only for stabilizing the splayed crown, but they also formed a sieve keeping oversized objects away from the food-catching tube feet on the downcurrent side.
As there was no limit to arm growth and the introduction of new ramuli, the food-catching apparatus became enormously enlarged relative to the size of the body capsule. This is particularly true for Seirocrinus, where the crown may reach 80 cm in diameter, but with a calyx of only about 3 cm. This transformation, however, was possible only in the pelagic realm, where crowns had no more to be closed against sediment stirred up during a storm or against predators (regenerated arms are rare in pelagic crinoids).
Stem
As mentioned above, the introduction of primary columnals (nodals) had switched early on from the tip of the stem to its connection with the body capsule. Because this connection grew larger with age, an unmodified stem would have tapered away from the crown. Benthic crinoids overcome distal tapering by accretionary growth of the columnals, in diameter as well as length (Fig. 2). In order to maintain the necessary flexibility of the stem, the columnals of townet filtrators remained very thin and enlarged mainly in diameter. At the same time, their outlines became rounded (Fig. 1). Nevertheless, radial growth of the columnals was insufficient to completely offset distal tapering of the stem in pseudoplanktonic species.
The optimal flexibility distribution along the stem depends on life styles. The stems of benthic crinoids resemble the stems of trees: they should be most flexible near the top and become stiffer towards the base. Accordingly, nodals as well as internodals keep growing thicker and wider until an almost square axial section is reached. Proximal flexibility is maintained by the continuous addition of relatively thin nodals and a limited number of internodals. As observed in living benthic forms, crowns are passively bent downcurrent, so that they become flushed from the aboral side.
In claiming a pseudoplanktonic life style for Seirocrinus (Seilacher et al. 1968), stem flexibility was a major argument. It differs from benthic crinoids in a fabricational sense by the unlimited intercalation of internodals (more than 1,000 between two nodals). Only in the most distal part of the stem is this intercalation halted in order to develop a cirral root tuft. However, as Baumiller and Ausich (1996) found out, the aspect ratio of columnals is no reliable predictor of stem flexibility. Therefore, it is important to judge it by also measuring the minimal turning radii at accidental bends in many specimens relative to their distance from the crown. In Seirocrinus this method shows that stem flexibility increased away from the crown, opposite to what one observes in most benthic crinoids. One reason for this is the remnant tapering of the Seirocrinus stem away from the crown.
Crinoids also evolved the intercalation of internodals between preexisting columnals as an additional method of secondarily lengthening the stem. In proximal stem sections this process is expressed by a nested hierarchy of columnals in numbers of 1, 3, 7, 15,…..between the primary nodals. In Isocrinida, the difference is accentuated by the fact that only the nodals bear cirri and their corresponding sockets (Fig. 2).
By heterochronic shifts in the three growth modes (nodal, internodal, and accretional), the stem could be locally modified for specific functional requirements, such as a higher flexibility in the root section to prevent breakage during storms, as well as additional (but flexible) anchorage by crowded cirri (root tuft). This is in contrast to mud-dwelling crinoids, where rootlets are rigid (Seilacher and MacClintock 2005).
Cirri
The nodal cirri of Isocrinida are a derived feature. The transformation of their distalmost ossicle into a hook (which required new cirrals to be introduced at the base) indicates an original holdfast function of these organs. It is preserved in modern representatives, where a particular kind of connection (cryptosymplexy) on the distal side of each nodal allows older parts of the stem to be shed off. Thereby cirri at the broken end can readily switch into function by grasping onto firm substrates or by actively sinking the stem into soft mud (Isselicrinus Fujiwara et al. 2005; Seilacher and MacClintock 2005).
Secondary transformation for additional functions is possible as the cirrals grow larger farther away from their point of origin. It can result in non-cylindrical cross sections, as shown by the laterally compressed cirri that free-living comatulids use for grasping to the substrate (Donovan 1993) or by the alate cirrals in Pentacrinites (Fig. 1). Intercalation, analogous to the internodals of the stem, is unknown in cirri.
Obviously, the most critical question in the present case is the function of the cirri in Pentacrinites, because
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By their distal hook they provide flexible attachment to a substrate bouncing in the waves (anchoring function).
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Nevertheless, cirral development is not retarded. Rather it is completed next to the crown, thereby obstructing the passive aboral filter current of ordinary crinoids.
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Rhombic (“alate”) cross sections do not improve the anchoring function; but theoretically they would allow undulation in order to swim Hauff (1984
) or, more likely, to replace the ordinary passive filter current by an active one in the same (aboral to oral) direction. This would make sense for early stages attached to a substrate that drifts with the ambiental current.
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Densing of cirri by reducing the intercalation of none-cirrate internodals added to the blocking effect, but enhanced the power of the hypothesized active ventilation.
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This model, however, is in conflict with the fact that the cirri of modern isocrinoids are not equipped with muscles, which would be necessary to make such a machinery work.