Problematic phosphatic plates from the Silurian–Early Devonian of Bohemia, Czech Republic

Problematic phosphatic elements are reported for the first time from Bohemia, Czech Republic, and are attributed to Eurytholia bohemica n. sp. Similar mineralized elements, interpreted as sclerites, were known only in a very narrow interval from Middle-Late Ordovician beds bordering the Iapetus Ocean. This new report comes from the Silurian and Early Devonian and provides a significant range extension for these Problematica as well as an enlargement of their geographic extent. Comments open new perspectives in the interpretation of these elements.


INTRODUCTION T HE PRINCIPAL biostratigraphic subdivision of the Silurian and
Lochkovian strata in Bohemia is based upon increasingly elaborate graptolite and conodont biozonation. A joint project is underway for the precise calibration of conodont and graptolite biozonation in specific intervals of the Silurian (responsibles, ES and PS). Within this study, detailed resampling of some definite levels in famous sections of the Prague Basin (Beroun, Butovice, Hyskov, Kosov Quarry, Amerika Quarry, Lodenice-Cernidla, Muslovka Quarry, Pozary, U Topolu, Vseradice), where both co nodont-and graptolite-rich layers are present, has been recently performed in several field seasons. Samples have been processed with the standard conodont preparation technique using formic or acetic acid and residues concentrated with sodium politungstate. The enigmatic phosphatic elements described below were picked from the heavy fraction of several levels, either from Silurian or Lower Devonian beds. In addition, a collection of these plates, made by one of us (ES) in the Barrandian in 1983, has been included in this study.
Similar small "hat-like" phosphatic plates of unknown origin were reported by Sutton et al. (2001) from the Ordovician of the Iapetus Ocean, specifically from South Wales, United Kingdom (12 specimens), Alabama, USA (nine specimens), Dalarna, Swe den (two specimens), and Estonia (one specimen). All the ele ments came from a very narrow stratigraphic interval of the Mid dle-Late Ordovician (Pygodus serra and P. anserinus conodont zones). The plates were attributed to two species of Eurytholia Sutton et al., 2001, and interpreted as dorsal dermal sclerites be longing to an animal of uncertain affinity. A provisional recon struction of the scleritome was attempted, as a dorsoventrally flat tened animal having sclerites arranged in sublongitudinal rows (Sutton et al., 2001, fig. 4).

GEOLOGICAL SETTING
Several-kilometers-thick Cambrian to Middle Devonian depos its, unmetamorphosed and weakly tectonized by the Variscan cy cle, unconformably overlie Neoproterozoic basement in the Bar randian area of the Bohemian Massif (Chlupac et al., 1998).
In the Early Ordovician, a northeast-southwest-trending Prague Basin began to subside in axial parts of the Barrandian (Havlicek, 1981;Chlupac et al., 1998). In the earliest Silurian, the Ordovi cian shelfal, pelitic to psammitic sedimentation was replaced by hemipelagic black graptolitic shales. Depositional settings varied from foreslope to deep shelf/basin. In the middle Wenlock and Ludlow, rarely in the middle Llandovery, several volcanic centers produced basalt lavas, hyaloclastites, and tuffs. Basaltic volcanoes were elevated high above the anoxic basin floor and were fringed by shallow-water biodetrital limestones (Havlicek and Storch, 1990). The Ludlow Series of the Prague Basin (Fig. 1) (Chlupac et al., 1972). In the Early Devonian, crinoidal limestones, calciturbidites, nodular, and also reefal types gained prevalence.

MATERIAL
About 400 problematic phosphatic plates were recovered from six Bohemian sections (Butovice, Kosov Quarry, Amerika Quar ry, Muslovka Quarry, Pozary, and U Topolu; Fig. 1). All samples were precisely constrained biostratigraphically using conodonts. Five distinct Silurian conodont zones (Ludlow) and one Devonian (Lochkovian) conodont zone were documented (Fig. 2). All strati graphic intervals sampled for conodonts within the project are in Ludlow and Lochkovian sediments and produced phosphatic plates. It is therefore probable that other Silurian and Devonian beds might be discovered in Bohemia or elsewhere, thus giving a continuous stratigraphic record of these elements.
Phosphatic plates recovered from Bohemia occur in variable quantities, apparently not strictly related to the weight of pro cessed material. The number of plates recovered from the same section in the Kosov Quarry (Storch, 1995) ranges from a sole specimen (sample L: 5.6 kg of processed material) to three ele ments (sample J: 6.7 kg) or 14 plates (sample 3: 4.9 kg) and reaches up to 284 elements in the sample Muslovka Quarry A (5.4 kg) (Fig. 2).
Plate dimensions range between 0.3 mm and 2 mm and their color varies from amber to black in massive specimens. If com pared with conodonts, plates share the same C.A.I. 5 (Color Al teration Index) except of Kosov, where conodont elements have a slightly lower C.A.I. (3-4). Preservation of Silurian material is good. Early Devonian elements are, on the contrary, poorly pre served. Detailed SEM observation of the outer surface revealed no significant ornamentation patterns or growth lines. Peripheral striae were observed along the margin of a single specimen. Rare angular pores (2 (xm in diameter) are present distally and apically (Fig. 3.8c).
In addition, the electron microscope investigation confirmed the remarks given by Sutton

Diagnosis.-Elliptical plates having a linear or slightly curved median, submedian, or marginal ridge. A well-defined girdle with an inner furrow runs all along the margins. Wide basal cavity expands to the apex of the ridge.
Description.-Great variability in shape, size, and other main morphological features typifies our material (even within the same sample). Plate margins linear to curved. General outline transversally and longitudinally symmetrical to asymmetrical. The most common plate type (Fig. 3.8a, 3.8b)   ( Fig. 4.1, 4.6). Height and width of median ridge strongly variable and independent from general dimensions of the plate. The ridge may be, in fact, large (Fig. 3.4) or slim (Fig. 3.2), rounded or sharp, or having a small apical depression (Fig. 4.11b). The ridgeis sometimes apically worn (Fig. 3.11b) but no true abrasions, scratches, or cuts have been observed along the ridge that might be convincingly attributed to in vivo damage. Lateral slopes of ridge symmetrical to asymmetrical in transverse view. Plate halves often dorsally bent (Fig. 4.2). Plate girdle is delimited by two well-defined thickened margins (Figs. 3.3, 3.5, 4.8). A linear inner furrow runs in between all along the plate. The upper margin of the girdle may protrude outside the basal cavity margin (Fig. 3.9).
Rare pores (2 [im in diameter) with a geometric outline (Fig.  3.8c) may occur with no preferential arrangement either in the apical or distal part of the plates.
The wide basal cavity expands deeply below the ridge, and in plates having a marginal ridge the cavity is edged on one side by the ridge wall and on the other by a wide platform (Fig. 3.10).
A more rounded aspect characterizes some subcircular plates (Fig. 3.7), longitudinally and transversally symmetrical, having nonlinear anterior and posterior margins and lateral margins not easily detectable from the former. Others have, however, a subrectangular profile (Fig. 3.1, 3.2).
One specimen reveals a hole with circular outline on one side of the plate, and with perpendicular walls (Fig. 4.4). Excavations and predatory perforations are reported in various phosphatic groups such as Cambrian shells (Conway Morris and Bengtson, 1994), Ordovician brachiopods (Holmer, 1989), and conodonts (Muller and Nogami, 1972).
Two plates (Fig. 4.12a) were found fused together. Similar clus ters are common in conodonts and represent elements fused to gether by diagenetic minerals which may (e.g., Nicoll, 1985;Nicoll and Rexroad, 1987)  Discussion.-The Bohemian specimens appear bigger than the older reports, having a maximum width of 2 mm (compared to 1.2 mm in E. prattensis and E. elibata Sutton, Holmer, and Cherns, 2001). The Ordovician material bears anterior and pos terior margins often thickened in "rolls," extending possibly also to lateral margins. The Bohemian material reveals, on the con trary, a well-defined girdle, bordered by two margins, running all along the entire plate border with a more or less regular thickness. E. bohemica differs from E. prattensis in lacking the median indentation ("waist") in anterior and posterior margins, having median length often representing maximum plate length, some times even expanding outwards. E. bohemica differs from E. prat tensis in bearing a more rounded and less angular general aspect. E. bohemica differs from E. elibata in having no median arching of posterior and anterior margins above the resting plane. CONCLUSION Sutton et al. (2001) carefully analyzed possible affinities of these Problematica and regarded the phosphatic plates as disartic ulated skeletal elements of a scleritome possessing only one sclerite morphotype. Phosphatic elements were possibly arranged in soft tissue (presumably chitinous) and exerted a protective func tion.
Our observations of the Bohemian material confirmed many of the morphological and structural features already pointed out by previous authors. In addition, small pores have been observed. Moreover, compared to the simple thickening of the anterior and posterior margins of earlier reports, Eurytholia bohemica n. sp. bears a peculiar girdle along all margins which appears to be a more sophisticated site for attachment or insertion into the ani mal's soft tissue.
The possible evidence of predation in the Bohemian material, even if from a sole specimen, could support a protective role for these plates. In spite of that, a predator could have more easily attacked in unprotected parts of the body (such as plate inter spaces).
The Bohemian material considerably enlarges the geographic extent and significantly extends the range of these Problematica, which appear to be a common constituent of the late Silurian-Early Devonian fauna in Bohemia and possibly elsewhere. A re cent finding of E. bohemica from the late Silurian of the Austrian Carnic Alps (AF and ES personal obs.) corroborates the idea. The stratigraphical range of Eurytholia was a period when the earliest vertebrates were evolving. The possible affinity with the verte brates needs to be fully explored, mainly by histological analysis.