Function and development of lateral stereom
The latera of crinoid columnals represents an interface between the animal and its external environment. A thin layer of tissue separates the solid material of the columnal from the environment and hence the lumina seen on the latera probably serve as points of connection between the external tissue and the internal stroma. For this reason alone it seems likely that most, if not all, crinoid columnals will have abundant lumina on the latera and that their absence in fossil material is an artifact of preservation. The proportion of pore space for a given area of latus may also be influenced by this requirement of connectivity between the interior and the exterior of the columnal.
In non-isocrinid crinoids, and the isocrinid Chariocrinus, the columnal latera are constructed of labyrinthic stereom. Smith (1980) found labyrinthic stereom to occur very widely and to be associated with all types of tissue. He also noted that commonly it was associated with moderate to fast stereom growth and that a reduction in growth rate could lead to the development of perforate stereom. Perforate stereom, in which the circular to elliptical lumina are arranged in a moderately to steeply en echelon configuration, is found among most of the isocrinids with the apparent exception in this sample of the Early Jurassic Chariocrinus. Smith (1980) described perforate stereom as forming during periods of slow stereom growth, with denser inner and outer layers providing resistance to bending stresses and protection against abrasion. The presence of small en echelon lumina aligned along low angle cleavage planes on the latera of etched columnals of Holocrinus dubius and of Isocrinus sp. from Kardolina suggests that the en echelon configuration seen in well-preserved columnals might perhaps reflect underlying cleavage planes. However, the significantly smaller size and spacing of the lumina on the etched columnals (<10 μm across), compared with those on well-preserved columnals (typically >15 μm), suggests that those on the etched columnals are a preservational artifact. Further work would be needed to ascertain if the configuration of stereom lumina on isocrinid columnal latera actually reflects any underlying structural control.
Phylogenetic aspects
Smith (1980) noted that some stereom arrays were restricted to particular echinoid orders, implying phylogenetic as well as ontogenetic and functional controls. Gluchowski (1982) also suggested that certain types of lateral stereom with large pores might have some phylogenetic significance. The present study indicates that the stereom of columnal latera is indeed significant for crude discrimination at high taxonomic levels, but might it also be used as a discriminant at lower levels?
Species-level discrimination
The two extant species of Endoxocrinus examined might be expected to show very closely similar lateral stereom, particularly considering suggestions that Endoxocrinus parrae and E. prionodes may even be conspecific (Meyer et al. 1978; Oji 1990). Both have the typical en echelon perforate stereom seen widely in isocrinids, but they are certainly not identical. In Endoxocrinus parrae, there is a noticably sharper demarcation between the stereom lumina and the intervening planar areas than is seen in E. prionodes, where instead the transition from the edges of the lumina to the intervening planar surface is significantly more rounded (Fig. 2a, b). Oji (1990) suggested that the macroscopic differences between these two species reflected ecophenotypic variation linked to habitat depth. The stereom differences may also perhaps be a consequence of these habitat differences but alternatively it may indicate that the two are in fact separate species after all.
Genus and family level discrimination in Early Jurassic isocrinids
Among Early Jurassic isocrinids encountered in north-west Europe several discrete genera, Isocrinus, Balanocrinus, Hispidocrinus and Chariocrinus, can be recognised (Simms 1988a, 1989a). Stratigraphic and morphologic criteria indicate ancestor–descendant relationships between three species of Isocrinus spanning the Hettangian to Pliensbachian stages; I. psilonoti, I. tuberculatus and I. robustus. Several other species were assigned to the genus, although with less certainty about their relationship to this ‘central’ lineage. A second lineage, assigned to Balanocrinus, shows similar strong evidence for ancestor–descendant relationships between four species spanning the Sinemurian–Pliensbachian stages; Balanocrinus quiaiosensis, B. subteroides, B. gracilis and B. solenotis. It had been suggested that the Balanocrinus lineage could be traced back into the Late Triassic (Klikushin 1979b, 1982), but Simms (1985, 1988a) suggested, on morphological grounds, that it arose as a neotenous offshoot from the ‘central’ Isocrinus lineage. In support of this, there is a close similarity between the lateral stereom of the Early Jurassic Isocrinus and Balanocrinus (Fig. 1b, d). This is distinct from the configuration seen in the Late Triassic (Carnian) ‘Balanocrinus’ subcrenatus, where the lumina are almost circular and widely spaced (Figs. 2f, 4).
Two other Early Jurassic species are assigned to a third genus, Hispidocrinus, on account of their distinctive morphology, with markedly stellate columnals bearing relatively small cirral scars and with relatively short noditaxes (<8 columnals) compared with Isocrinus (>12 columnals). The relationship of this genus to other isocrinids in the Early Jurassic or Late Triassic is unclear. In the type species, Hispidocrinus scalaris, the stereom on columnal latera is of classic perforate type, with near circular lumina (Fig. 1f) arranged en echelon and separated by planar intervening areas. This arrangement is different from Early Jurassic Isocrinus and Balanocrinus in which the lumina are markedly elliptical (Fig. 1a, b, d, e). However, its putative Early Jurassic descendant, Hispidocrinus schlumbergeri (Fig. 1g), has lateral stereom that more closely resembles these genera (Fig. 4), perhaps indicating a reversion back to the pattern seen in the central Isocrinus lineage.
Another isocrinid has recently been recognised as a distinct species in the latest Triassic (Rhaetian) and earliest Jurassic (Hettangian) in north-west Europe. This was figured in the 19th Century (Loriol 1884–89) as Isocrinus angulatus, although the original name was a nomen nudum (Oppel 1856–58). It was dismissed by Simms (1989a) as a synonym of Isocrinus psilonoti but abundant new material, albeit disarticulated, from the basal Hettangian Stage has revealed that these two taxa are not conspecific. Isocrinus angulatus never attains the large size of I. psilonoti, has consistently shorter noditaxes (<9 columnals) and a more markedly stellate columnal outline. In this respect, it is not dissimilar to Hispidocrinus scalaris and H. schlumbergeri and, intriguingly, the shape and configuration of stereom lumina on columnal latera (Fig. 1c) is more reminiscent of H. scalaris (Fig. 1f) than of other Early Jurassic isocrinids. Morphometric analysis of this species has yet to be undertaken but this observation suggests that its affinities may perhaps lie with Hispidocrinus (Fig. 4).
Of the Early Jurassic isocrinids examined for this study, one stands out as being anomalous. This is Chariocrinus wuerttembergicus, a small isocrinid that is common in the British Toarcian. Unlike all of the other isocrinids examined, which have perforate stereom on columnal latera, this species appears to have lateral stereom with a structure more akin to unordered labyrinthic stereom (Fig. 2c).
High-level discrimination
The distinction, based on the lateral stereom, between most of the isocrinids examined and the non-isocrinids has already been alluded to, with most of the former possessing perforate stereom and the latter labyrinthic stereom. However, other macroscopic characters can generally be used to identify phylogenetic affinities at this high level without relying on this character alone. Labyrinthic stereom has been identified on the columnal latera of representatives of four high level taxa; in Millericrinus ? alpinus (Millericrinina), Plicatocrinus inornatus (Cyrtocrinina), Democrinus brevis (Bourgueticrinidae) and the seemingly anomalous case of Chariocrius wuerttembergicus (Isocrinina). The precise phylogenetic relationship between the millericrinids and cyrtocrinids is unclear as many of their apparent similarities are actually plesiomorphic characters and hence uninformative, but they are treated together here as Millericrinida.
The occurrence of labyrinthic stereom on the columnal latera of representatives of three or four discrete taxonomic groups suggests that this is the plesiomorphic condition, and that the perforate stereom, with en echelon lumina, seen in most of the isocrinids examined, is apomorphic and arose after the isocrinid and ‘non-isocrinid’ clades had diverged (Fig. 5). Gluchowski (1982) reported labyrinthic stereom on the columnal latera of some Palaeozoic crinoids (disparids and monobathrid camerates) which, coupled with its taxonomically widespread occurrence in other crinoid taxa, also suggests that this is the plesiomorphic state for columnal latera microstructure.
Bourgueticrinids have in the past been assigned to the Millericrinida (Roux 1977; Pisera and Dzik 1979) but others have considered them as neotenous derivatives of either a comatulid or isocrinid stock (Rasmussen 1978; Simms 1988b; Simms et al. 1993). The presence of labyrinthic stereom on the latera of Democrinus columnals, although a plesiomorphic trait, is nonetheless significant for resolving this issue. Both comatulids and isocrinids pass through a ‘pentacrinoid’ larval stage in which they possess a tiny (<1 mm diameter) stem with bifascial synarthrial articula and hence, on the grounds of columnal morphology alone, the bourgueticrinids could have evolved from either group (Simms 1988b). In comatulids, the stem is lost as the centrodorsal develops while in isocrinids continued growth of the stem leads to the development of typical symplectial articula (Simms 1989b). Examination of pentacrinoid larval columnals of Balanocrinus subteroides (Fig. 3b) shows that even at this small size they have developed the en echelon pattern of perforate stereom typical of most isocrinids. This contrasts markedly with the latera of Democrinus columnals, which is entirely labyrinthic (Fig. 2d). The absence of en echelon perforate stereom on Democrinus columnals suggests that bourgueticrinids more probably are neotenous derivatives of the comatulids rather than isocrinids (Fig. 5), retaining the stem which ordinarily would be lost in adult comatulids.
The lateral stereom of the Early Jurassic Pentacrinites fossilis (Fig. 1h) appears distinct both from the regular perforate stereom of isocrinids and the more chaotic labyrinthic stereom of millericrinids, cyrtocrinids and bourgueticrinids. It might perhaps be interpreted as somewhere in between these two. The surface, although having gently convex ridges between rows of lumina, is much more planar than in typical labyrinthic stereom yet the configuration of the lumina is much more irregular than in the perforate stereom of isocrinids and, in that respect, resembles labyrinthic stereom. It is suggested here that this ‘sinuous perforate stereom’ in Pentacrinites arose independently from the apomorphic state represented by labyrinthic stereom (Fig. 5). Although some authors (Roux 1981) have suggested a close phylogenetic relationship between Pentacrinites and other isocrinids, the more widely held view is that the pentacrinitids are sister group to the comatulids s.l. (Rasmussen 1978; Simms 1988b; Simms et al. 1993). If this were the case then the distinctive arrangement of lateral stereom in Pentacrinites is an autapomorphy for that clade rather than a variation of the en echelon perforate stereom seen widely in the isocrinids.