Stylophyllopsis rugosa, (Duncan and Wright, 1867)

Stolarski, Jarosław & Russo, Antonio, 2002, Microstructural diversity of the stylophyllid (Scleractinia) skeleton, Acta Palaeontologica Polonica 47 (4), pp. 651-666 : 656-659

Stylophyllopsis sp. cf. S. rugosa (Duncan and Wright, 1867)

Figs. 7 , 8 .

Material.— 1 specimen (IPUM−Sic.5).

Description.—Corallum trochoid, GCD = 17 mm, LCD = 15 mm, H = 20 mm (juvenile corallum portion is not preserved); corallum surface worn ( Fig. 7D ), only occasionally someremnantsoftheepithecalwallarepreserved.Septahexamerally arranged in five full cycles and some rudimentary S6 according to the formula S1–2>S3>S4>S5>S6 (see also changes in septal number in corallum ontogeny in Fig. 6 ). All septa consist of spines that lower in fossa are embedded in sclerenchyme (in available transverse sections of calice, septal spines of S1–2 are already covered with sclerenchymal deposits). S1–2 reach the calicular center, S3 extend nearly to the center, S4 to ca. 3/4 of the calicular radius, whereas S5 may reachhalfofthecalicularradiusbutoftenaredevelopedasisolatedspinesprotrudingfromthewall,similarlyasrudimentary S6. Sharp and dense granulations occur on septal spines ( Fig. 7B, E ) which, if not completely covered with sclerenchyme, may partly protrude from the septum giving its surface a spiny appearance. Axial septal edges, particularly of higher septal cycles, are dissociated into septal spines ( Fig. 7A, B ). Endotheca composed of oblique and arched dissepiments.

Though the skeleton is diagenetically altered (it has entirely calcitic mineralogy as proven by X−ray diffraction), transverse sections of spines show a distinct, star−shape pattern in organization of internal rod−shape structures. In optical microscope, rod−shape structures are dark−grey and form the core of granulations on septal spines surface. EDS mapping does not indicate enrichment of “dark rods” by Mn, Fe or Si, elements typical of diagenetic minerals. On the other hand, SEM observations of lightly etched polished sections of septa, show differences in relief between areas of “dark rods” (as seen in optical microscope) and adjacent parts of septal spines ( Fig. 8A–D ). Neomorphic crystals in regions of “darkrods”showverydistinct,fine−porouspatternofetching (pockmarked surface), in contrast to much less porous texture of etched crystals outside the “dark rods” zone.

Remarks.— Stylophyllopsis rugosa Duncan and Wright, 1867 was originally described from the Late Sinemurian ( Echinoceras raricostatum Zone ) of Honeybourne ( England). The Sicilian specimen described herein differs from the lectotype (illustrated by Duncan 1867: pl. 15: 16, and by Beauvais 1976: pl. 11: 6) in having a shorter corallum and a higher number of septa at a given calicular diameter, i.e., 80 septa at 25 mm diameter versus ca. 92 septa at 17 mm diameter (see also Fig. 6 ). However, according to the Duncan’s description (1867: 58), coralla of this species are quite variable (including forms with five full septal cycles and rudimentary septa of higher cycles) and the Sicilian specimen fits within this range of variability. Nevertheless, because of the worn surface of the Sicilian specimen, its specific attribution is given with caution.

The pockmarked surface of etched polished skeletal surfaces suggests occurrence of mineral or organic inclusion that undergo preferential etching. A similar etching pattern was observed in diagenetically altered coralla of the Cretaceous Aulosmilia (see Sorauf and Cuif 2001: fig. 5); in skeleton neomorphised to calcite, the mid−septal zone have abundant inclusions whereas septal flank areas are composed of massive calcite without inclusions ( Sorauf and Cuif 2001: 161). The mid−septal zone corresponds to the position of calcification centers in skeletons of extant corals, whereas the flank areas to the zone have longer aragonite fibers. Calcification centers, which originated during the first phase of septal formation, are composed of minute aragonite crystals and are rich in organic components, whereas zones with fibers, originated during the next, successive phases of septal growth, contain a smaller amount of organic components. As stated by Sorauf and Cuif (2001: 161): “primary septal area with greater content of organic matrix underwent diagenesis separately, and possibly earlier than the secondary skeleton, with diagenesis of primary septum preserving more inclusions”. One may thus suggest that densely pockmarked crystals arranged in star−like patterns in septal spines of Sicilian Stylophyllopsis sp. cf. S. rugosa are remnants of calcification centers developed in axes of radially arranged?trabeculae.

Occurrence.—Early Jurassic (Sinemurian), Sicily.

Figures

Fig. 7. Stylophyllopsis sp. cf. S. rugosa (Duncan and Wright, 1867), IPUM−Sic.5 with abraded surface (epitheca not preserved). A. Transverse section (enlarged part on B, E) C, D. Distal (C) and lateral (D) views of corallum (arrows mark sectioned region). B, E. Transverse section. Note septal spines (s) with radiating darker structures (arrows on E). Specimen from Sinemurian Black Limestones, Longi, Sicily.

Fig. 8. Stylophyllopsis sp. cf. S. rugosa (Duncan and Wright, 1867), IPUM−Sic.5. SEM micrographs of transverse, polished and etched corallum surfaces. A. Corallum fragment with septal spines (arrows); rectangle indicate position of enlargement on B. B–D. Dark, radiating structures shown in Fig. 7E are composed of crystals with pockmarked surfaces (arrows in B, C). Specimen from Sinemurian Black Limestones, Longi, Sicily.

Fig. 6. Bivariate biometric plot of calicular diameter (CD) versus number of septa (S) for Stylophyllopsis sp. cf. S. rugosa (Duncan and Wright, 1867), and Stylophyllopsis sp. A. Note different shapes of ontogenetic trajectories of both species.