Study of conulariid and related phosphatic conical exoskeletons from the Prague Basin (Czech Republic)
Zdenka Brabcova1 and Petr Kraft2
1 Museum of West Bohemia, Department of Palaeontology, Kopeckeho sady 2, 301 36 Plzen, Czech Republic. E–mail:firstname.lastname@example.org
2 Charles University in Prague, Faculty of Science, Institute of Geology and Palaeontology, Albertov 6, 128 43 Praha 2, Czech Republic. E–mail: email@example.com
Key words: Conulariids. Sphenothallus. Exoskeletons. Microstructure. Paleoecology.
Families Conulariidae Walcott, 1887, Conulariellidae Kiderlen, 1937, and genus Sphenothallus Hall, 1847 are taxa of unclear systematic position. They are usually considered to be relatives due to the similarities in shape and chemical composition of their exoskeletons. They are common in sediments of Ordovician to Devonian age in the Prague Basin (Barrandian area, Czech Republic). In case of conulariids it is even one of the richest sections in the world.
First complex study of conulariids from the Prague Basin was given by Boucek (1928). Boucek and Ulrich (1929) published the only observations of microstructures of the Bohemian conulariids. Some 70 years long gap of low or almost no interest in conulariids followed Boucek’s activities. New stage of research has started recently. Stratigraphic distributions of Bohemian conulariid species were revised by Brabcova (1999). Macromorphological characteristics of Bohemian species were summarized by Brabcova (2000). Kraft and Brabcova (in prep.) have studied changes of the shape of the cross–sections and inferred a model of main trends for exoskeleton morphotypes. However, Sphenothallus and related taxa have never been systematically investigated in the Prague Basin yet. They are only sporadically referred in the lists of fauna.
As other interesting aspects can be studied in rich material of conulariids and related taxa from the Prague Basin the new project Nş 205/03/0170, supported by the Grant Agency of the Czech Republic, has started since this year. It is focused on macromorphological and micromorphological features of conulariids exoskeletons and complex study of members of genus Sphenothallus and related taxa. This is a report on first preliminary results of that project.
The studied material is housed in public collections of the Museum of West Bohemia in Pilsen, Faculty of Sciences Charles University in Prague, National Museum in Prague, Czech Geological Survey and Museum of Dr. B. Horak in Rokycany. It yields the sufficient number of specimens, which enable to isolate the original exoskeleton material (periderm) for SEM investigation and chemical analyses.
Family Conulariidae Walcott, 1887 is assigned to the suborder Conulariina Miller et Gurley, 1896 together with family Conulariellidae Kiderlen, 1937. Fossil representatives possess phosphatic or chitinophosphatic pyramid to conical exoskeletons. Various opinions on systematic position of this group were presented during the 19th century. It was placed among cephalopods, "worms", hemichordates or conodonts. It was advocated an idea about conulariids close relations to Cnidaria at the first half of the 20th century (Kiderlen, 1937; Knight, 1937). This idea had recently been widely supported. Stratigraphic range of conulariids is from the Middle Cambrian to the Lower Triassic.
Family Conulariidae Walcott, 1887 is represented by 26 species in the Prague Basin. They range from Lower Ordovician (Arenigian) to Middle Devonian (Chotec Formation). Family Conulariellidae Kiderlen, 1937 is represented by 3 species in the studied area. They are known from Arenigian to Dobrotivian.
Boucek and Ulrich (1929) observed lamellar structure of the exoskeletons, and chemical and physical disproportions between individual lamellae. Similar studies were published by Kowalski (1935) on French material from the Upper Ordovician, Kozlowski (1968) on Polish Ordovician conulariids, Bischoff (1978) on Australian taxa and Van Iten (1992) on the North American material. Jerre (1993) and Brood (1995) made this type of analyses in the Scandinavian conulariids.
The most recent study of conulariids exoskeletons, realized together with Dr. Van Iten, brought some new data on Bohemian species. Set of 99 samples taken from 13 Bohemian species was analyzed by SEM. The similar structures described by Kozlowski (1968) as choanophymes and pores and by Bischoff (1978) as pores were observed on the following species: Pseudoconularia grandissima (Barrande, 1867), Metaconualria imperialis (Barrande, 1867), Conulariella robusta (Barrande, 1867), Conulariella sulca (Zelizko, 1921). All objects called pores were observed on external (ornamentary) layer, which bears transversal and longitudinal ribs and/or rods. The pores are circular or elliptical depressions, steeply narrowed outwardly of the exoskeleton. The diameter of the pores ranges from 4–10 µm, their depths vary between 5–10 µm. Pores usually go through 5–10 most external laminae, individual thickness of which is approximately 1 µm.
During the study three main morphological types of pores were observed:
1) pores with very thin (1 µm) and slightly elevated rim, 2) pure pores, and 3) pores filled with sediment. The distribution of pores is usually random. However, two regular types of distribution were found on several samples, in both cases in frame of one lamina: 1) in lines, 2) in triangles.
Another interesting feature of the distribution of pores is considerable alternation of the pore–bearing set of lamellae with the non–pore or sparse pore set of lamellae in structure of exoskeleton. They are approximately of the same thickness.
The pores were observed only on facial parts of the exoskeletons in the studied samples. Never they have been found in the apical or apertural area, on corner grooves and midlines. The pore density varies between individual species and genera but also between particular specimens of one species. No characteristic pore densities for individual taxa were able to determine in this stage of study. However, Bohemian material displays lower numbers of pore density (50 pores per 1 mm2 ) in comparison with material from other regions.
Pores can be considered as primary structure of conulariid exoskeleton due to their identical or very similar distribution patterns and sizes in all studied specimens. Their function is not understood yet.
Sphenothallus and related fossils
Genus Sphenothallus Hall, 1847 and some related forms represent phosphatic, narrow conical shells of fossil animals. In the past, they were assigned to various groups: "worms", hyolithids etc. The opinion that these are thecae of cnidarians is generally accepted in the present days and most of authors support their close relationships to conulariids (e.g., Van Iten et al., 1996). They range from the Lower Cambrian to the Permian. Recently these fossils have been intensively studied not only for their unsolved taxonomical position and phylogenetic relationships but also for their local abundance in marine associations. Sphenothallus shows a good potential for paleoecologic interpretations (e.g., Mason and Yochelson, 1985, assume representatives of the genus Sphenothallus to be indicators of marine environment).
Sphenothallus and similar fossils occur in sediments of Ordovician to Devonian age in the Prague Basin of the Barrandian area. They are represented both by narrowly conical shells and by attaching discs on the smooth parts of trilobite carapaces, shells of gastropods and cephalopods. Preliminary knowledge brings interesting information on stratigraphically non–continual occurrence, which is characterized by the high abundance on certain levels. These stratigraphically dependent occurrences are specified by special morphotypes. It was also recorded that in formations differentiated into variable facies Sphenothallus and related fossils are restricted only to particular facies. The Klabava Formation (Arenigian) is such an example: Sphenothallus sp. is common in shales (Myto Shale sensu Kraft and Kraft 2003, herein) and it is only rare in rewashed tuffs (Ejpovice Member sensu Kraft and Kraft 2003, herein). It has not been recorded in Olesna Member and other facies yet. Biostratigraphically it is most abundant in the uppermost Azygograptus ellesi–Tetragraptus reclinatus abbreviatus Biozone. These facts are interpreted to be caused by different physical characters of environment (salinity, dynamics of water, etc.) and/or by distribution of suitable bases for attachment (especially fragments of shells or some living species; see below).
The only paleoecologic analysis of the genus Sphenothallus in the Prague Basin was made by Kraft (1988) who studied the distribution patterns of attaching discs on shells in the Klabava Formation. The present study confirmed his unpublished results. The discs occur exclusively on larger smooth surfaces of solid shells of invertebrates. Such as conulariids, gastropods, trilobites (Symphysurus rouvillei, both species of genus Euloma described from this stratigraphic level) and huge phyllocarid crustacean Mytocaris klouceki. These observations are in good agreement with interpretation of Kraft (1988) who supposed that epifaunal Sphenothallus sp. sought for the remains of withered organisms on the sea–floor or could live on the living, slowly moving epibenthic species which did not even temporarily lived covered with substrate (for example gastropods).
Microstructure study of conulariid genera Metaconularia, Pseudoconularia and Conulariella from the Prague Basin proved a presence of microscopic circular pores. Groups of pores concentrated in lines on ribs are supposed to be related with a sensorial function. A distribution pattern and size similarities were observed. However, their role as a taxonomic feature is still uncertain.
The stratigraphic and spatial distribution of Sphenothallus and related taxa show distinct patterns. The preliminary results proved their paleoecologic causes and illustrate their importance for community analyses.
We thank to Dr. Heyo Van Iten (Hanover College, Indiana, USA) for providing Hitachi S–3000N digital scanning electron microscope equipped with a Noran (460A 1SSS) energy dispersive elemental analyzer for conulariids analysis and for his discussions to the problem. This contribution and its presentation was prepared with the financial support of the Grant Agency of the Czech Republic, project Nş 205/03/0170.
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Received: February 15, 2003
Accepted: June 15, 2003