Taphonomy

The crinoids that have been recovered from the Otekaike Limestone occur in two states of preservation: most, including Eagle’s (Reference Eagle2007, Reference Eagle2008) material, consist of dissociated elements that include brachials, pinnules, cirri, centrodorsals, and only in a few instances, centrodorsals with a radial circlet attached; the specimen of R. aotearoa n. comb. collected near Waipati described herein is remarkably complete and articulated, consisting of a centrodorsal, many cirri of several cirrals (none complete), with a radial circlet, radiating arms of as many as 21 brachials (none complete), and numerous pinnulars (Fig. 3, Supplemental Data 1, 2, 3). This dichotomous nature of preservation may indicate that different taphonomic processes were responsible: in the first instance, a period of postmortem decay, disarticulation, and burial, and in the second, rapid burial of an intact, perhaps live individual.

For fossil feather stars, the high degree of disarticulation and incompleteness is by far the most common mode of preservation. This is perhaps not surprising given that their multiplated skeletons are not particularly resistant to postmortem processes and disarticulate easily (Meyer and Meyer, Reference Meyer and Meyer1986). Most crinoid skeletal elements are only loosely held together by slender fibers of soft tissue (ligament and muscle) that decay quickly, whereas those articulations that are more tightly bound (e.g., syzygies) tend to be specialized for autotomy and fail even more quickly either due to the active response of the organism to the death-related stress or because they are not resistant to decay. The more frequent recovery of fossil centrodorsals or centrodorsals with a basal and radial ring is consistent with the fact that the centrodorsal is the largest element, and the articulation between it and the radials is nonmoveable, tightly sutured, and not specialized for autotomy. Experiments on extant feather stars confirm that postmortem disarticulation may be rapid, with high potential for differential sorting of elements soon after death even with minimal transport (Meyer, Reference Meyer1971; Liddell, Reference Liddell1975; Meyer and Meyer, Reference Meyer and Meyer1986; Baumiller, Reference Baumiller2003). The Otekaike feather stars described by Eagle (Reference Eagle2007, Reference Eagle2008) had to experience some period of decay prior to burial given that they are highly disarticulated: centrodorsals were recovered either as single elements or, in a few instances, with articulated radials, but with no other elements attached. Yet the material appears autochthonous, given the absence of abrasion and the fact that other dissociated elements (brachials, pinnules, cirri) were also recovered. Eagle (Reference Eagle2007) noted the presence of epibionts on some echinoid fragments at these localities, which is also consistent with rather slow sedimentation rates and the potential for exposure on the sediment–water interface of some duration prior to burial.

The preservation of the highly articulated Waipati specimen of R. aotearoa n. comb. indicates that this specimen was buried while still intact, perhaps while still alive, and that the decay of soft tissues occurred after burial. It has been generally assumed that this type of preservation implies little transport, but experiments suggest that live crinoids or their fresh (nondecayed) carcasses can remain fully intact even when moved by turbulent, sediment-laden flows over substantial distances (Baumiller, Reference Baumiller2003; Baumiller et al., Reference Baumiller, Gahn, Hess and Messing2008a). The Waipati specimen of R. aotearoa n. comb. is preserved with the long axes of the arms aligned parallel to the oral–aboral axis, in what has been referred to as a ‘shaving brush’ posture (Fig. 3, Supplemental Data 1, 2, 3). This may well represent feather stars’ response to trauma, as this posture has also been recognized in experiments with live specimens tumbled for over one hour in sediment-laden water (Baumiller et al., Reference Baumiller, Gahn, Hess and Messing2008a, fig. 1.1). Counterintuitively, the intact R. aotearoa n. comb. specimen may have experienced transport prior to burial, whereas the highly disarticulated feather stars described by Eagle (Reference Eagle2007, Reference Eagle2008) may have been buried in situ, though following a longer period of decay on the sediment–water interface. The existence of such a range of hydrodynamic conditions is consistent with the interpretation of the Otekaike sediments as representing an upper to mid-shelf environment occasionally subjected to currents/storms capable of producing concentrations of aligned penatulaceans (Eagle, Reference Eagle2007).

An unusual aspect of the pristinely preserved Waipati specimen of R. aotearoa n. comb. is its close association with fragments of calcitic bivalves and echinoid tests. Whereas the presence of both larger and smaller fragments entombed within the matrix between the arms indicates a lack of sorting, the long axes of these fragments are aligned parallel to the long axes of the crinoid’s arms, suggesting some process of preferential alignment. In addition, the shell fragments have sharp, angular edges, show no evidence of encrustation, and although most are ~1 cm in their largest dimension, several are larger than 3 cm (Supplemental Data 3). The production of such angular fragments might generally be interpreted as the result of in situ compaction (Zuschin et al., Reference Zuschin, Stachowitsch and Stanton2003; Zatoń and Salamon, Reference Zatoń and Salamon2008), but that seems unlikely in this instance because: (1) most fossils show no structural deformation, and (2) judging from the shape, size, and surface detail of the angular fragments, it is evident that they represent multiple individuals with only parts of their skeletons present. It is plausible that biological agents may have been involved in producing these fragments (Oji et al., Reference Oji, Ogaya and Sato2003; Cintra-Buenrostro, Reference Cintra-Buenrostro2007; Stafford et al., Reference Stafford, Chojnacki, Tyler, Schneider and Leighton2012; Salamon et al., Reference Salamon, Gorzelak, Niedźwiedzki, Trzęsiok and Baumiller2014).

Ecological and evolutionary implications

Several features of the Otekaike crinoids are worthy of note. First, the described fauna from a single locality, Ardlogie, consists of nine feather star species (Eagle Reference Eagle2007, Reference Eagle2008; this study). This is a slightly lower diversity than noted by Messing et al. (Reference Messing, Meyer, Siebeck, Jermiin, Vaney and Rouse2006), who found 12 species of feather stars living on sandy substrates between 10 and 20 m depth at Lizard Island, Great Barrier Reef, Australia. However, the latter are part of an extremely rich fauna of Lizard Island consisting of over 50 species (Hoggett and Vail cited in Messing et al., Reference Messing, Meyer, Siebeck, Jermiin, Vaney and Rouse2006) dominated by reef-dwelling feather stars, and five of the 12 Lizard Island species observed on soft substrates are also known to occur on reefs. Given that feather stars from Ardlogie likely lived in deeper water and not in vicinity of reefs, the diversity of this Oligocene locality must be considered strikingly similar to that at a comparable modern setting described by Messing et al. (Reference Messing, Meyer, Siebeck, Jermiin, Vaney and Rouse2006). This suggests that alpha diversity in this type of an environment has not changed dramatically since the Oligocene. Admittedly, this is but a single comparison from one habitat, but it does shed some light on the history of feather stars, hinting that their past diversity may be greatly underrepresented by their known fossil record.

A second noteworthy aspect of the Otekaike feather stars relates to the presence of a regenerating arm in the Waipati specimen of R. aotearoa n. comb. (Fig. 4.2). The extraordinary ability to regenerate is a characteristic of all echinoderms, and in crinoids regenerating body parts are commonly encountered in extant specimens and have been reported in numerous fossil stalked crinoids (see Oji, Reference Oji2001; Gahn and Baumiller, Reference Gahn and Baumiller2010 for reviews). However, regeneration has not previously been recorded in a fossil feather star. This may seem surprising given that observations of living populations reveal extremely high regeneration frequencies, sometimes with all individuals regenerating one or more arms (Mladenov, Reference Mladenov1983; Meyer, Reference Meyer1985; Schneider, Reference Schneider1988; Nichols, Reference Nichols1994; Oji, Reference Oji1996; Baumiller, Reference Baumiller2013a). However, as was previously discussed, fossils of intact feather stars are so rare that opportunities for recognizing regeneration are very few. Yet data on regeneration are crucial for evaluating hypotheses related to functional morphology and evolutionary history of crinoids. For example, today’s success of feather stars relative to stalked crinoids in terms of their bathymetric distribution, abundance, and diversity has often been linked to the various ways in which they have been able to cope with interactions with other organisms (Meyer and Macurda, Reference Meyer and Macurda1977; Messing, Reference Messing1997; Gorzelak et al., Reference Gorzelak, Salamon and Baumiller2012). Several in situ observations have confirmed that such interactions do occur in the Recent (Magnus, Reference Magnus1963; Fishelson, Reference Fishelson1972, Reference Fishelson1974; Conan et al., Reference Conan, Roux and Sibuet1981; Meyer and Ausich, Reference Meyer and Ausich1983; Meyer et al., Reference Meyer, LaHaye, Holland, Arenson and Strickler1984; Vail, Reference Vail1987; Messing et al., Reference Messing, RoseSmyth, Mailer and Miller1988; Baumiller et al., Reference Baumiller, LaBarbera and Woodley1991; Nichols, Reference Nichols1994; Baumiller et al., Reference Baumiller, Mooi and Messing2008b; Bowden et al., Reference Bowden, Schiaparelli, Clark and Rickard2011; Stevenson et al., Reference Stevenson, Gahn, Baumiller and Sevastopulo2017), often involving sublethal damage to the crinoid, including injuries to arms and/or pinnules. Consequently, regenerating arms have been assumed to represent repair of injuries and used as a proxy for antagonistic interactions (Meyer, Reference Meyer1985; Schneider, Reference Schneider1988; Oji, Reference Oji1996; Aronson et al., Reference Aronson, Blake and Oji1997; Baumiller and Gahn, Reference Baumiller and Gahn2003, Reference Baumiller and Gahn2004, Reference Baumiller and Gahn2013; Gahn and Baumiller, Reference Gahn and Baumiller2005; Baumiller, Reference Baumiller2013b). Using this logic, the regenerating arm in the Waipati specimen of R. aotearoa n. comb. not only provides the first fossil example of this phenomenon in a feather star, but supports the claim that they suffered sublethal predation in the geologic past.

Another interesting feature of the intact specimen of R. aotearoa n. comb. from Waipati relates to the position of the regenerating arm, more specifically to the articulation between the distal brachial of the remaining portion of the original arm (IIIBr1) and the proximal brachial of the regenerating portion (IIIBr2). This articulation, IIIBr1–2, is a syzygy, an articulation type that is generally thought to facilitate the shedding (autotomy) and regrowth of arms in some crinoids, including feather stars (Wilkie, Reference Wilkie2001). In extant crinoids, autotomy at syzygies can occur in response to mechanical stimulation; in addition, syzygies are almost always the site of arm regeneration (Holland and Grimmer, Reference Holland and Grimmer1981; Mladenov, Reference Mladenov1983; Oji and Okamoto, Reference Oji and Okamoto1994). Drawing on these observations, Oji and Okamoto (Reference Oji and Okamoto1994) presented a compelling argument that the function of syzygies is to reduce damage and mortality from predators, and presence and specific placement of syzygies have been linked to the success of feather stars in handling predation pressure (Oji and Okamoto, Reference Oji and Okamoto1994; Baumiller, Reference Baumiller1997, Reference Baumiller2008). So whereas syzygies have been documented in fossil feather stars because of their distinct morphology, data on their placement, which is taxon specific and highly predictable (Hess and Messing, Reference Hess and Messing2011), requires intact specimens, and confirming the autotomy function necessitates a regenerating arm. In this regard, the intact specimen of R. aotearoa n. comb. is of special importance as it: (1) provides data on the position of syzygies in Conometridae, a family with no extant representatives, and (2) confirms syzygial autotomy function in a Paleogene feather star.