Rhabdopleura compacta Hincks, 1880

Mierzejewski, Piotr & Kulicki, Cyprian, 2003, Cortical fibrils and secondary deposits in periderm of the hemichordate Rhabdopleura (Graptolithoidea), Acta Palaeontologica Polonica 48 (1), pp. 99-111 : 101-106

Rhabdopleura compacta Hincks, 1880

Figs. 2–6 .

Morphologically,thecoeneciaoftheyoungcoloniesunderinvestigation fit the descriptions by Stebbing (1970a, b) and Dilly(1986).Theyareabout0.8–2.9mmindiameterandform irregular, compact and encrusting mats of adhering repent tubes, from which arise 2, 3, or up to10 erect tubes ( Fig. 2 ). Afrayedmarginalmembranerunsaroundtheperipheryofthe attachedpartofeachcoenecium.Thecoeneciumiscomposed of a sicula, constructed by the primary zooid (oozooid), and a series of daughter tubes (i.e., equivalent to the thecae of other graptolites), made by blastozooids. There is no indication of the the so−called “embryonal ring”, recognized only by Schepotieff (1907) and Dilly (1985b) in Rh. normani . The coenecia are all devoid of dormant buds and their capsules. Underthelightmicroscopealmostallelementsofthecoenecia are nearly colourless and transparent or semitransparent; only the siculae and the oldest parts of creeping tubes are slightly brownish and rather opaque.

The sicula is easily identifiable in the central regions of thetwosmallercolonies( Figs.2A–C , 3A ).Thesicula’sgeneral appearance resembles those described by Stebbing (1970b), Dilly (1985 a, 1986) and Dilly in Urbanek (1986: fig. 4B). The prosicula is a subsphaerical vesicle, with a large, flat attachment surface and a convex upper surface. Unfortunately, only the upper surface is easily visible becauseitslateralslopesaretightlyovergrownandsurrounded byyoungerelementsofthecoenecium.Theuppersurfaceof theprosiculaiscoveredwithabundantforeignmaterialofan unknown(algal?inorganic?)nature,apparentlyincorporated partially into its structure ( Fig. 3 ). However, this enigmatic materialdoesnotcompletelymasktheprimarydetailsofthe dome. Its upper surface is covered by many small subcircular, polygonal, and irregular depressions or pits ( Fig. 3A,B )whicharedistributedinachaoticandunevenway.We have distinguished two types of these structures, based on their morphology and dimensions:

(1) Large pits ( Fig. 3B–F ), corresponding to those described first by Dilly (1985a, see also 1986). They are surrounded by prominent rings made of curved fibril−like elements.Ingeneral,theyareabout3.2–8.6µmindiameter,and the thickness of the fibril−like elements is ca. 0.35 µm. They are often tightly grouped into clusters. The bottom of each large pit is covered with several (usually 4–13) of the tiny pits, irregularly distributed, described next.

(2) Tiny pits ( Fig. 3C–G ), described herein for the first time. They are usually 0.54–1.27 µm in diameter, with a ratherindistinctmargin,andarebuiltofahomogenousmaterialidenticaltotheprosicularwall.Thesetinypitsareirregularly scattered both between and within the large pits; some appear to coalesce.

The metasicular part of the sicula (which is inhabited by the primary zooid after its metamorphosis) is sharply distinguished from the prosicula not only by its fusellar structure but also by a lack of pits ( Fig. 3A, B ). Both metasiculae closely resemble the material described by Stebbing (1970) and Dilly (1986).

The bulk of each coenecium is made of a mass of repent and erect tubes ( Fig. 2 ); their width is approximately 0.12–0.15 mm and 0.16–0.25 mm, respectively. The fuselli ofcreepingtubesaretypicallybuiltofhalfsegments,deposited alternately to left and right, which form a characteristic and very distinct median zigzag suture ( Figs. 2 , 4A ). The transitionfromcreepingtoerecttubeismarkedbyadiscontinuity in the fusellar structure. All erect tubes are made of completefusellarrings,witheachbeingintersectedbyasingle oblique suture, irregularly placed ( Figs. 1 , 5A, D). This suture marks the beginning and end of a fusellus. As a rule, the upper part of a fusellus takes the form of a collar (sensu Kulicki 1969) which is very characterictic of rhabdopleurid fusellar tubes ( Fig. 5). Fusellar heights varies within 45– 55µmincreepingtubes,and26–37µminerecttubes.Unlike Dilly and Ryland (1985), we failed to find any non−fusellar sections of tubes to match the nonfusellar periderm of fossil rhabdopleurids discussed by Mierzejewski (1986).

Ingeneral,theoutersurfacesofcoeneciaarenotsmooth. Even under low magnification, various irregular, plate−like or often filamentous structures cover the surface ( Figs. 2–5 ). We interpret this debris as foreign to the rhabdopleuran skeletal tissue, probably representing the remains of dense agglutinations of minute organic and inorganic particles (see Dilly 1975). The nature of these particles remainsobscure,buttheymayequatewithmaterialknownin some cephalodiscid skeletons, i.e. various epibionts, diatoms, sponge spicules, minute fragments of shells, and grains of sand (see Andersson 1907; Urbanek et al. 1980; Crowther 1981). Some plate−like particles on the outer surface of the coenecium produced on x−ray energy dispersive spectrum typical of layered silicates. Occasionally, where this foreign material has been removed mechanically from theprimaryoutersurfaceoftheperiderm( Fig.5A),thesurface appears smooth, with little relief at low magnification. There is no evidence for any secondary deposits on the primary fusellar layer similar to graptolite ectocortex or to the isolated thick fibres described by Dilly (1975). However, some fusellar surfaces are more or less distinctly wrinkled forming a longitudinal undulation. The outer surface of eachfusellusisarobustsheetfabricwhichusuallyhidesthe underlying fibril orientation. However, in some places at high magnification, fine parallel lineations cover the periderm and may indicate underlying fibrous material. In contrast, sheet fabric on the margins of collars displays distinct parallel lineations which unquestionably reflect more or less straight fibrils ( Figs. 4A , 5B, C). We conclude that fusellar collars are built of closely packed fibrils, arranged uniformlyandparalleltotheirmargins.Sometimes,moreor less distinct traces of periodicity can be observed along a single fibril.

A dimorphic arrangement of the fibrillar material becomesclearwheretheperidermhasbeenmechanicallydamaged ( Fig. 5C–H), i.e. fusellar fibrils proper and cortical fibrils( Fig.5C).Fusellarfibrilspredominateinfusellartissue; they are more or less wavy, produce a chaotic three−dimensional meshwork, and vary widely in diameter, from as little as 40 nm to nearly 300 nm. Strikingly, in some areas structuressimilartotheminglingoffusellarfibrilswithflakymaterial described by Mierzejewski and Kulicki (2001: fig. 4) from the Ordovician Kystodendron Kozłowski, 1959 and Rhabdopleurites Kozłowski, 1967 , were observed. Cortical fibrils are distinctly thicker, with a diameter of 150–520 nm. They are the basic component of the fusellar collars, with their characteristically solid, band−like construction, similar tothoseofOrdovicianrhabdopleurids.Theshapeofthesefibrilsisratherunstable;theycanbealmoststraightorslightly bent, smooth or annulated. The annulated fibrils display a distinctly variable periodicity. Morphological details of the cortical fibrils are best seen in the fissure between two adjacent fuselli ( Fig. 5E–H).

The inner surfaces of erect and creeping tubes are rather smooth,withonlyasmallamountofforeignmaterial.Sometimes, however, there are some areas of these surfaces covered distinctly with layers of secondary deposits. We have been able to recognize two different types of these deposits: (1) membranous inner secondary deposits, and (2) fibrillar inner secondary deposits.

Deposits of the first type, known since Kulicki’s (1971) and Andres’ (1980) light EM and TEM investigations, appear distinctly laminated ( Fig. 6B ). They are constructed of distinct and numerous (up to 10 or more) tightly packed membranes or sheets. No traces of any substructure have

c 50 µm s cf c cf A ff B 2 µm c c p cf 50 µm C 2 µm p D 2 µm cf c cf E F b 2 µm cf G 2 µm H cf 5 µm

beenrecognizedwithinthesesheets.Theintersheetmaterial, if present, is completely devoid of any discernible fibrous material. The membraneous inner secondary deposits were found only in the basal i.e. oldest part of erect tube, near the horizontal creeping tube.

Fibrillar inner secondary deposits, described herein for the the first time ( Fig. 6C, D ), take the form of a thin layer, onlyonefibrilthick.Fibrildiameterisgenerally 260–570nm and they can be identified as cortical fibrils. They are rather looselydistributed,straightandarrangedmoreorlessinparallel. There is little evidence that a membrane−like structure ever covered these fibrils.

Figures

Fig. 2. Rhabdopleura compacta Hincks, 1888, Recent. SEM micrographs of complete coenecia. A. Young coenecium composed of a sicula and two zooids (ZPAL Pb.5/1). B. Adult coenecium consisting of a sicula and a dozenorsozooids(ZPALPb.5/2). C.Youngcoeneciumbuiltofasiculaand three zooids (ZPALPb.5/3). Abbreviations: c, creeping tube; e, erect tube; m,metasicula;mm,marginalmembrane;p,prosicula;x,boundarybetween prosicula and metasicula.

Fig.3. Rhabdopleura compacta Hincks,1888,Recent(ZPALPb.5/1).Sur− face micromorphology of prosicula. SEM micrographs. A. Distal part of prosicula connected with proximal part of metasicula. B. Outer surface of prosicula near boundary with metasicula. C. Arrangement of prosicular large pits. D–E. Details of foreiign material covering the outer surface of prosicula.F.Arragementofprosiculatinypitsonlargepitbottoms. G.De− tails of tiny pits. Abbreviations: c, creeping tube; e, erect tube; fm, foreign material; lp, prosicular large pit; m, metasicula; p, prosicula; r, rim of tiny pit; tp, prosicular tiny pit.

Fig. 4. Rhabdopleura compacta Hincks, 1880, Recent (ZPALPb.5/1). SEM micrographs. Micromorphology of outer surface of creeping tubes. A. Fragmentofcreepingtubewithprominentzig−zagsuture. B–D.Corticalfibrilsinfusellarcollars.Abbreviations:c,fusellarcollar;cf,corticalfibril;fm,foreign material; p, fusellar pellicle.

Fig. 1. Inner secondary deposits in zooidal tubes of fossil Rhabdopleuroidea. A–C. Rhabdopleura kozlowskii Kulicki, 1969; Bathonian and Callovian of Poland. After Kulicki 1971, modified; A, ZPAL Pb.II/8; B, ZPAL Pb.II/9; C, ZPAL Pb.II/10. D. Rhabdopleurites primaevus Kozłowski, 1967; Lower Ordovician of Öland, Sweden. After Skevington 1965, modified.

Fig. 6. Rhabdopleura compacta Hincks, 1880, Recent (ZPALPb.5/3). SEM micrographs. Secondary inner deposits in erect tubes. A. Growing end of an erect tube with inner cortical deposits. B. Detail of Fig. 5A showing an arrangement of cortical fibrils. C. Broken erect tube near its base showing membraneousinnerdeposits. D.Multilayeredstructureofinnermembraneousdeposits.Abbreviations:cf,corticalfibril;i,interfusellarsuture;id,innerdeposit; l, last fusellus; n, next to the last fusellus; o, outer edge of an erect tube.