The tube ultrastructure of serpulids (Annelida, Polychaeta) Pentaditrupa subtorquata, Cretaceous, and Nogrobs cf. vertebralis, Jurassic, from Germany/Koja peenstruktuur serpuliididel (Annelida, Polychaeta) Pentaditrupa subtorquata Kriidist ja Nogrobs cf. vertebralis Juurast Saksamaalt.
The tube ultrastructure of recent calcifying polychaetes has recently been studied by many authors (ten Hove & Zibrowius 1986; Zibrowius & ten Hove 1987; ten Hove & Smith 1990; Nishi 1993; Pillai & ten Hove 1994; Weedon 1994; Sanfilippo 1996, 1998). Fine complex crossed lamellar (CCL) structure sensu Carter et al. (1990), ordered chevron structure sensu Weedon (1994, p. 5, fig. 2), and various prismatic structures have been described in serpulid polychaetes (ten Hove & Zibrowius 1986; Zibrowius & ten Hove 1987). The last authors hypothesize that the vitreous tube layer in serpulids is caused by the regular orientation of long crystals in the tube. Mesozoic serpulid polychaetes are taxonomically well studied (Regenhardt 1961; Lommerzheim 1979; Jager 1983). The ultrastructure of fossil polychaete tubes has been studied by Zibrowius & ten Hove (1987), Weedon (1994), Fischer et al. (1989), Fischer et al. (2000), and Schweitzer et al. (2005), but is still inadequately understood.
To affiliate fossil calcareous tubes of possible polychaetes with recent taxa such as Serpulidae, it is important to know the ultrastructural changes caused by fossilization. The goal of the paper is to find structural similarities in calcareous tubes between living and fossil polychaetes and describe the ultrastructure in Pentaditrupa subtorquata and Nogrobs cf. vertebralis for the first time.
MATERIAL AND METHODS
The tube of Nogrobs cf. vertebralis from the Middle Bathonian, Jurassic of Blumberg, SW Germany, and Pentaditrupa subtorquata from the Upper Campanian, Cretaceous of Hover by Hannover, Germany, were selected for SEM study. The examined tubes were ground in longitudinal direction, polished, and etched with 1% acetic acid for 1 min prior to SEM examination. Thereafter, the same tubes were re-polished and treated with a 1:1 mixture of 25% glutaraldehyde and 1% acetic acid, to which alcian blue was added (GA-solution), for 5-45 in before SEM study. Energy dispersive X-ray analysis was applied to distinguish calcite from aragonite by a high content of Mg or by a high content of Sr. All figured and measured specimens are deposited at the Zoological Museum, University of Amsterdam (ZMA) and Museum of Geology, University of Tartu (TUG).
TUBE STRUCTURE IN CRETACEOUS PENTADITRUPA SUBTORQUATA
This species has a tube composed of two calcitic layers separated by a distinct border. The external layer is 0.20-0.50 mm thick at a tube diameter of 3.2 mm and has a semiregular spherulitic prismatic ultrastructure similar to the molluscan spherulitic prismatic structure sensu Carter et al. (1990) (Pl. I, figs. 3, 4). The prisms are 80-160 [micro]m long and 6-20 [micro]m wide. The growth layers (growth increments), with an interval of 2-6 [micro]m, subperpendicular to the prisms, are partially preserved in the external tube layer (Pl. I, fig. 3). The 0.10 mm thick internal layer is composed of calcareous granules and has a homogeneous ultra-structure (Pl. I, fig. 8).
The original structure of the internal tube layer could not be identified in the sample etched with acetic acid and in one treated with the GA-solution. The homogeneous granular structure of P. subtorquata is possibly secondary, being very similar to the secondary homogeneous granular matrix in the fossilized Ditrupa internal layer (Zibrowius & ten Hove 1987, fig. 3E; Weedon 1994, p. 11). Such a structure is known in fossil material, but not in any recent serpulid tube (personal observations).
TUBE STRUCTURE IN JURASSIC NOGROBS CF. VERTEBRALIS
The tube of Nogrobs cf. vertebralis is composed of a single layer with a calcitic prismatic ultrastructure. The prisms are 100-150 [micro]m long and 3-4 [micro]m wide, having a spherulitic arrangement in some places (Pl. I, figs. 1, 2). The prisms appear partially as a negative relief on the polished cross- and longitudinal sections of the tube after treatment with the GA-solution (Pl. I, figs. 1, 2). Occasional growth layers perpendicular to the prisms are preserved in the tube wall (Pl. I, fig. 1). The structure of N. cf. vertebralis is somewhat similar to the molluscan irregular spherulitic prismatic structure sensu Carter et al. (1990).
The preservation of growth lines (growth increments) perpendicular to the prisms in the tube walls of fossil Pentaditrupa subtorquata (Pl. I, fig. 4) and Nogrobs cf. vertebralis (Pl. I, fig. 1) indicates that these taxa had originally a prismatic tube structure. The semiregular spherulitic prismatic external layer of P. subtorquata (Pl. I, figs. 3, 4) closely resembles the irregular spherulitic prismatic tube structure (sensu Carter et al. 1990) of recent serpulids Placostegus tridentatus (Pl. I, fig. 6) and Yitreotubus digeronimoi (Pl. I, fig. 5), giving further evidence of the ultrastructure of P. subtorquata being preserved in its original state. Also, the structure of N. cf. vertebralis slightly resembles that of Vitreotubus digeronimoi and Placostegus tridentatus, but has clearly longer prisms.
The tubes of recent Ditrupa arietina are composed of two layers (ten Hove & Smith 1990). The external hyaline layer (Pl. I, fig. 7) has a structure that resembles the irregular simple prismatic structure of molluscs (Carter et al. 1990), but differs from the semiregular and irregular spherulitic prismatic structure in fossil Pentaditrupa subtorquata and Nogrobs cf. vertebralis in a much larger size and more regular shape of prisms. From the aspect of tube ultrastructure fossil Pentaditrupa does not appear to be closely related to recent Ditrupa.
According to the present study prismatic structures seem to have good preservation potential and may help to link fossil serpulid taxa with recent ones. In contrast, non-prismatic structures, such as ordered chevron structure sensu Weedon (1994) in Pomatoceros sp. (Fig. 1.1) and complex crossed lamellar structures (Fig. 1.2) of the internal layer in Ditrupa arietina (Zibrowius & ten Hove 1987; ten Hove & Smith 1990), seem to be completely substituted by a homogeneous structure during fossilization. Thus, the serpulid fossils with a prismatic structure may yield more valuable information for the taxonomic and phylogenetic studies than the ones with a secondary homogeneous structure.
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I am grateful to H. A. ten Hove, Zoological Museum, Amsterdam, who helped with the literature and loaned identified material for the study, and to H. Mutvei and E. Dunca, Swedish Museum of Natural History, for their help with SEM work. I thank H. Mutvei and H. A. ten Hove for constructive review of the paper. I do acknowledge SYNTHESYS support made available by the European Community-Research Infrastructure Action under the FP6 Structuring the European Area Program to the project NL-TAF-111 and SE-TAF-113.
Received 2 September 2005, in revised form 8 November 2005
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Institute of Geology, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia; firstname.lastname@example.org