Furcophyllia septafindens ( Volz, 1896 )

Stolarski, Jarosław, Roniewicz, Ewa & Grycuk, Tomasz, 2004, A model for furcate septal increase in a Triassic scleractiniamorph, Acta Palaeontologica Polonica 49 (4), pp. 529-542 : 530-533

Furcophyllia septafindens ( Volz, 1896)

Figs. 1 , 2 .

Montlivaltia septafindens Volz, 1896: 44 , pl. 3: 22–25.

Neotype: IPUM−DOL/610; proximal part not preserved.

Type locality: St. Cassian Beds, Forcella di Sett Sass (descriptions in Ogilvie 1893: fig. 3).

Material.—The neotype (IPUM−DOL/610), thin−sectioned (IPUM−DOL/610−str, 610−slon) and with a series of peels (ZPAL H.24/1−peels/1–13). Also examined was Volz’s original longitudinal thin section (MGUWr 137sz), however, it may not represent the taxon here described (see Remarks).

Description.—Corallum ceratoid, laterally flattened. Calicular rim narrow, axial fossa long, radial elements thin and abundant. Proximal part obliquely broken: lower half of the section records earlier, whereas upper half records later phases of corallum ontogeny. Septa originate as single blades at the calicular perimeter.

In the more proximal part of corallum, internal borders of septa lying on the opposite sides of the calice meet along the fossa. The axial borders of the longest septa are clavate, whereas the borders of the shorter septa are sharp ( Fig. 1D ). In the more distal part of the corallum, septa differentiate into branching septal sets separated by non−branching septa ( Fig. 1A, G ). Branching septa bifurcate centripetally, repeatedly, and usually alternately; they form sets of numerous branches herein called septal brooms. Branching affects at least 52 septa that are composed of 3–10 blades ( Fig. 1B ). Each septal broom has a unique branching pattern however, in which further generations appear successively in a manner resembling sympodial growth of branches. Remarkably, distances between adjacent branches and septa are equal; interseptal spaces are very narrow, 0.2–0.3 mm across ( Fig. 1F ). The number and size of the septa that separate the branching sets are variable. In places there are only 2–3 separating septa, in other places there are as many as 7 septa ( Fig. 1A ). The longest septum reaches half the distance to the fossa; the next or− der septa are considerably shorter. The shortest septa are thin and regularly distributed. All septa and septal branches have approximately the same thickness in cross−section (usually ca. 0.2 mm in the thickest part). Remarkably thinner septa (ca. 0.08 mm thick) are ontogenetically younger i.e., their axial margins and the incipient septa inserted at the wall region. About 110 septa of all orders (including also incipient septa, only 0.5 mm in length) were counted in the thecal zone of the well preserved part of the calice (white line with arrows in Fig. 1A ). The estimated septal number in the thecal zone for the distal part of the corallum is ca. 300. The septa in transverse section show regular thickenings, ca. 250 µm in diameter. Short symmetrical and asymmetrical offsets form regularly distributed lateral septal granulations. The granulations are free or fuse into ridges (menianae), parallel to the distal septal border ( Fig. 1E ).

Endotheca dense, constructed of abundant, small or flat and extended dissepiments. Dissepiments are steeply inclined in an axial direction. The wall is constructed by peripheral borders of thin and densely crowded septa and, occasional dissepiments. In places, there are possible traces of thin epitheca.

Remarks.— Volz (1896: 45, pl. 3: 24) documented a narrow mid−septal zone ( RAF sensu Stolarski 2003) in the species, the “ Urseptum ”; the character is common in traditional caryophylliinans (clearly polyphyletic in molecular studies: Romano and Cairns 2000). Though the original microstructure of the newly acquired specimen is diagenetically altered, Volz’ observations are supported by the very thin axial borders of septa.

The species is very rare, as already noted by Volz (1896: 45), until now recognized only at the Forcella di Sett Sass locality. Of the two figured specimens ( Volz 1896: pl. 3: 22–25), one is presented in lateral view (pl. 3: 22) and in transverse sections (pl. 3: 23 and 24); the second is in longitudinal (pl. 3: 25) section. One of the transverse sections (pl. 3: 24) shows abundant, small dissepiments, whereas the longitudinal section shows vesicular, large and scanty dissepiments. The specimen investigated here, IPUM−DOL/610, with diagnostic branching septa, has endotheca corresponding to the first of the Volz’ illustrations ( Volz 1896: pl. 3: 24). It is thus very likely that the second illustration of Volz (pl. 3: 25) does not represent the same taxon.

Various aspects of the unique septal branching of Furcophyllia are discussed in the following sections. Here we focus on another distinct character of Furcophyllia , its very long and narrow calicular fossa. This feature correlates with lateral corallum compression and the lack of an axial structure. This is a relatively rare architectural combination in Recent (occurs e.g, in Flabellum ) and fossil corals. Among contemporaneous Triassic corals, calices with comparably long fossae occur in the Triassic coral Cuifia Melnikova, 1975 . For example, in a specimen of C. marmorea ( Frech, 1890) described by Roniewicz (1996) the GCD reaches 110 mm, and the fossa is 30 mm long.

Occurrence.—Carnian, St. Cassian Beds, Forcella di Sett Sass, Dolomites, Italy.

Figures

Fig. 1. Furcophyllia septafindens (Volz, 1896). IPUM−DOL/610 (thin−section figured in 1F, 1G is IPUM−DOL/610−str, and in 1E is IPUM−DOL/610−slon). Triassic (Middle Carnian), St. Cassian Beds, Forcella di Sett Sass, Dolomites (Italy). A, B. Transverse polished section through distal part of corallum (see Fig. 2A); with the brightness and contrast of picture; B adjusted. Septa of principal orders (52) branch repeatedly and centripetally and form septal brooms composed of 3–10 blades. Branching nodes of individual septal brooms are indicated as small circles in B and their total number are summed at the septal base (if given: number in bracket is the total number of nodes, the first number indicates branch generations of septal broom); nodes of the same generation (identically colored) connected with thin lines between brooms. About 110 septa of all orders were counted in a well preserved part of the corallum (white line with arrows); the estimated septal number for the section is ca. 300. C. Thin and dense dissepiments, easily visible, particularly in septal branching nodes (arrows). D. Transverse−oblique polished section in a more proximal part of corallum (see Fig. 2A); the upper half of the section is cut through a higher part of corallum and shows branching septa, whereas in the lower half of the section (and corallum) septa branch only rarely. Slightly expanded ends of the longest septa/branches (ca. 60) meet along a narrow and long axial fissure. E. Section with septa cut obliquely−longitudinally (white arrow) or perpendicularly (black arrow); note minute and dense menianae (white dots). F. Enlarged region of transverse thin section (G) in crossed polarized light (XPL) to accentuate shape of sediment−infilled spaces (dark) between recrystallized septa (light). G. Transverse thin section of the uppermost part of preserved corallum (note the wide sediment−infilled fossa).

Fig. 2. Reconstruction of Furcophyllia septafindens (Volz, 1896). A. Presumable position of polished sections (Fig. 1A, D) indicated on a simplified outline of the corallum. B. Distal−lateral view of a cornute corallum (see Volz 1896: fig. 22); septa very dense, probably not very exsert, fossa deep (IPUM−DOL/ 610), costae covered with bands of thin epitheca (speculation based on common occurrence of epitheca in Triassic scleractinians). C. Postulated section in the region indicated by the arrow: septa bifurcate rarely and extend to a narrow axial fissure (see Volz 1896: fig. 23, and herein Fig. 1D). D. Enlarged portion of the distal corallum showing centripetal septal bifurcations (septal flank menianae are omitted). Reconstructions based on sections of IPUM−DOL/610, and specimens of Montlivaltia septafindens illustrated by Volz (1896: pl. 3: 22–25).