◄   Carnets Geol. 6 (A04)   ►

Contents

[Introduction] [Geological setting and material] [Systematics]
[Discussion] [References] [Figures] and ... [Table]

Schmidtites celatus (Obolida, Brachiopoda) from the "Obolus sands" (Upper Cambrian - Lower Ordovician) of Estonia

Christian C. Emig

 

CNRS - Centre d'Océanologie, Rue de la Batterie-des-Lions, 13007 Marseille and BrachNet, 20, Rue Chaix, 13007 Marseille (France)

Manuscript online since September 21, 2006

Abstract

Large collections of the brachiopod obolid Schmidtites celatus have been gathered from Upper Cambrian-Lower Ordovician strata in four northern Estonian localities. The morphological features and the taxonomic characters of the genus and of the single species representing it are re-described and illustrated. New diagnoses are proposed based on characters of the shell and morphological traits that permit Schmidtites celatus to be compared with and distinguished from the other obolid genera occurring in the same samples or areas, i.e. Ungula ingrica, Oepikites, and Obolus apollinis which now includes specimens formerly described as Ungula convexa. Schmidtites celatus differs from them mainly in the arrangement of its musculature.

Key Words

Schmidtites; Brachiopoda; Cambrian - Ordovician; Estonia; Obolid musculature.

Citation

Emig C.C. (2006).- Schmidtites celatus (Obolida, Brachiopoda) from the "Obolus sands" (Upper Cambrian - Lower Ordovician) of Estonia.- Carnets de Géologie / Notebooks on Geology, Brest, Article 2006/04 (CG2006_A04)

Résumé

Schmidtites celatus (Obolida, Brachiopoda) des grès à "Obolus" (Cambrien supérieur- Ordovicien inférieur) en Estonie.- De très nombreux exemplaires du brachiopode obolide Schmidtites celatus ont été récoltés dans quatre localités du Nord de l'Estonie, dans des couches appartenant au Cambrien supérieur et à l'Ordovicien inférieur. L'étude des caractères morphologiques et taxinomiques du genre et de l'unique espèce a permis de redécrire et d'illustrer Schmidtites celatus ; de nouvelles diagnoses sont proposées pour le genre et l'espèce. Les caractères ont été comparés avec ceux des autres genres d'obolides présents dans les mêmes strates ou dans la région, i.e. Ungula ingrica, Oepikites, et Obolus apollinis à laquelle est rattachée Ungula convexa. Schmidtites celatus s'en distingue notamment par l'arrangement musculaire.

Mots-Clefs

Schmidtites; Brachiopoda; Cambrien - Ordovicien; Estonie; musculature; Obolides.

Introduction

During several field trips to northern Estonia, hundreds of well-preserved specimens were collected from strata dated Upper Cambrian to the Lowest Ordovician. They comprise the genera Schmidtites Schuchert et LeVene, 1929, Ungula Pander, 1830, Obolus Eichwald, 1829, and Oepikites Khazanovitch et Popov, 1984, all representatives of the family Obolidae (Lingulida: Linguloidea). Other specimens studied are in the collections of the Museum of Geology in the Institute of Geology of the University of Tartu, Estonia. Because of the controversy regarding the systematics of the lingulid taxa of the Middle Cambrian to Lower Ordovician (Tremadocian) in Estonia and Russia, the tools described by Emig (2002) to discriminate between obolid taxa are used in the redescription of Schmidtites.

When comparing Schmidtites with the other obolid genera cited, significant differences are manifest in several characters involving the arrangement of the musculature. The statement of Williams & Hurst (1977) and Popov (1992) that the Obolidae have an almost identical pattern is not corroborated by the arrangements of the musculature. Consequently, an emended diagnosis is proposed for the genus and a new one, including figures, is offered for the species.

Geological setting and material

Along the northwestern part of the East European Platform, the Upper Cambrian-Lower Ordovician sands and sandstones are weakly cemented and alternate with black shales and clays. They are overlain by Lower Ordovician carbonate rocks cropping out along a slope oriented N to S extending from northern Poland to Lake Ladoga in northwestern Russia: these deposits represented a shallow epicontinental basin with abundant obolid brachiopods, and are traditionally known as "Obolus sands" or "Obolus phosphorites". In Estonia, the obolid coquinas (Fig. 1 ; Table 1 ) form well-known deposits in the famous shelly phosphorite layers (see Öpik, 1929; Kaljo et alii, 1986; Heinsalu et alii, 1987; Mens et alii, 1989; Nemliher & Puura, 1996; Puura, 1996; Mens et alii, 1996). In the Upper Cambrian and lowermost Ordovician strata of these coquinas Schmidtites celatus occurs with Ungula ingrica and U. convexa; this last taxon is now placed in the synonymy of Obolus apollinis (see Emig, 2002). Coeval beds with obolids are known to be present in Sweden (Holmer & Popov, 1990; Puura & Holmer, 1993), Poland (Biernat, 1964; Bednarczyk, 1998), Lithuania (Mens et alii, 1990; Lashkov et alii, 1992), and Russia (Popov et alii, 1989).

Schmidtites celatus has been collected in northern Estonia from 4 outcrops, the stratigraphy of which is represented graphically on Figure 1 . The phosphorite brachiopod fauna consists mainly of Schmidtites celatus and/or Ungula ingrica (Table 1 ). Several hundred specimens have been sampled and have been deposited in the Muséum National d'Histoire Naturelle de Paris (France). Additional material collected in Ingria (Russia) was made available by the Museum of Geology, University of Tartu (Estonia). Unfortunately, no specimens for study were available from the CNIGR museum (VSEGEI, St Petersburg, Russia) where, however, the neotype is housed.

Systematics

The Brachiopoda are classified here in accordance with the schema of the Treatise of Invertebrate Paleontology (2000).

Phylum Brachiopoda

Subphylum Linguliformea Williams, Carlson, Brunton, Holmer et Popov, 1996

Class Lingulata Gorjansky et Popov, 1985

Order Lingulida Waagen, 1885

Superfamily Linguloidea Menke, 1955

Family Obolidae King, 1846

Subfamily Obolinae King, 1846

Schmidtites Schuchert et LeVene, 1929

Synonymy

1869 nom. subst. pro Schmidtia Volborth, 1869, p. 208 (non Balsamo-Crivelli, 1863).

1877 Schmidtia: Dall, p. 62.

1892 Schmidtia: Hall & Clarke, p. 83.

1896 Obolus (Schmidtia): Mickwitz, p. 158

1908 Obolus (Schmidtia): Walcott, p. 142.

1912 Obolus (Schmidtia): Walcott, p. 441.

1929 Schmidtites: nom. subst. - Schuchert & LeVene, p. 121.

1965 Schmidtites: Rowell, p. H266.

1969 Obolus (Schmidtites): Gorjansky, p. 24.

1989 Schmidtites: Popov & Khazanovitch, p. 114.

1993 Schmidtites: Puura & Holmer, p. 219.

1996 Schmidtites: Puura, p. 68.

1998 Schmidtites: Bednarczyk, p. 50.

2000 Schmidtites: Holmer & Popov, p. 52.

Currently, the genus is known to occur in the Upper Cambrian and Lower Ordovician (Lower Tremadocian) of Estonia, Russia (Ingria), Poland, and Sweden. The type-species is Schmidtites celatus (Volborth, 1869), originally described under the name Schmidtia celata Volborth, 1869.

Diagnosis

The emended diagnosis of Schmidtites Schuchert et LeVene, 1929 is compared to the one published previously by Holmer & Popov (2000). Here the International Code of Zoological Nomenclature (ICZN, 1999) is applied to the descriptions of the genus and of the species in accordance with Art. 12 and Recommendations 13 A-C a, which state that every new name must be accompanied by a description or a definition of the taxon that it denotes. Consequently, genus and species have discrete diagnoses.

The genus Schmidtites, to which only a single species is assigned, was originally named by Volborth (1869) as Schmidtia celata with a type-locality near Aseri (Estonia; Fig. 1 ) in the Kallavere Formation. Mickwitz (1896) considered it a subgenus, made up of 4 species with a number of varieties. Gorjansky (1969) re-evaluated the infraspecific variation of Schmidtites celatus and placed most of Mickwitz's species, i.e. Obolus (Schmidtia) celatus, O. (S.) acuminatus, O. (S.) crassus, in synonymy with it. However, Obolus (Schmidtites) obtusus was accepted and considered to be congeneric by Gorjansky (1969). The first three were considered by Popov & Khazanovitch (1989) as synonyms of the type species, with Schmidtites thus becoming a monospecific genus, for Obolus (Schmidtia) obtusus Mickwitz, 1896, was assigned to the genus Oepikites Khazanovitch et Popov, 1984 by Popov & Khazanovitch (1989).

Schuchert & LeVene (1929) pointed out that Schmidtia is a junior synonym of a sponge genus described by Balsamo-Crivelli (1863). Consequently they proposed the name Schmidtites be adopted for the sub-genus. Rowell (1965) restored Schmidtites to generic rank.

The juvenile obolids assigned to Schmidtites by Biernat (1973) are described too poorly to allow identification. One can contend that juvenile specimens of the lingulids are generally impossible to identify because they do not express the required taxonomic features. Williams (1974) described Schmidtites (?) simplex, which Lockley & Williams (1981) considered synonymous with Siphonotreta micula M'Coy, 1851, and called it Schmidtites? micula (M'Coy) This form occurs in the Lower Ordovician of Wales, UK but was reassigned to Apotobolus by Nazarov & Popov (1980). Disregarding this assignment Botting & Thomas (1999) referred a pseudoplanktonic form from the Middle Ordovician to Schmidtites? micula. However, these authors pointed out that the only element implying a pelagic life for the brachiopods is their attachment to a graptolite by means of a filamentous alga. The identification of Schmidtites (?) sp. by Mergl & Elicki (2004) in Sardinia (Italy) must remain unconfirmed pending description of its soft-bodied characteristics.

Schmidtites celatus (Volborth, 1869)

Synonymy

1869 Schmidtia celata n. sp.: Volborth, p. 209-212, Pl. 27: 1-6.

1892 Schmidtia celata: Hall & Clarke, p. 83, Figs. 37-38.

1896 Obolus celatus: Mickwitz, p. 159, Pl. 2: 19-20.

1896 Obolus celatus var. orbiculatus n. var.: Mickwitz, p. 163, Pl. 2: 21-22.

1896 Obolus celatus var. praecisus n. var.: Mickwitz, p. 166, Pl. 2: 37-38.

1896 Obolus acuminatus n. sp.: Mickwitz, p. 179, Pl. 2: 39-40.

1896 Obolus acuminatus var. alatus n. var.: Mickwitz, p. 183, Pl. 2: 41-42.

1896 Obolus acuminatus var. humeratus n. var.: Mickwitz, p. 194, Pl. 2: 43-44.

1896 Obolus acuminatus var. subtriangularis n. var.: Mickwitz, p. 186, Pl. 2: 45-46,

1896 Obolus crassus n. sp.: Mickwitz, p. 187, Pl. 2: 47-49, 52-55.

1896 Obolus crassus var. angulatus n. var.: Mickwitz, p. 193, Pl. 2: 50-51.

1898 Obolus (Lingulella) celatus: Walcott, Pl. 26: 1-2.

1912 Obolus (Schmidtia) celatus: Walcott, p. 444, Pl. 14: 1, 1a-c.

1965 Schmidtites celatus: Rowell, p. H266, Figs. 159: 7a-b.

1969 Obolus (Schmidtites) celatus: Gorjansky, p. 26, Pl. 2: 4 & 8-9 (not Pl. 2: 5-7 & 10).

1977 Schmidtites celatus: Rowell, p. 260-296, Figs. 2-3.

1989 Schmidtites celatus: Popov & Khazanovitch, p. 115, Pl. 3: 12-14; Pl. 4: 5-14; Pl. 8: 1.

1993 Schmidtites celatus: Puura & Holmer, p. 219, Figs. 21C & 3A-D.

1996 Schmidtites celatus: Puura, p. 69, Figs. 56A-O.

2000 Schmidtites celatus: Holmer & Popov, p. 52, Figs. 20: 4a-cA-O.

Selected by Popov & Khazanovitch (in: Popov et alii, 1989, p. 115) a neotype has been deposited under n° 97/12348 (Toolse River; Upper Cambrian, Maardu Member) at the CNIGR Museum in St-Petersburg, Russia. Our specimens collected in Pakri, Iru, Ülgase, and Saka (Estonia) are deposited in the Muséum National d'Histoire Naturelle de Paris (France) under MNHN A25534. Saka is located about 25 Km from the Aseri (Fig. 1 ), the original type-locality of Schmidtites celatus, indicated by Volborth (1869).

Description

The shell of Schmidtites celatus is ventri-biconvex (Fig. 2 ): the ventral valves (mean height = 1 mm; n = 42) are slightly higher that of the dorsal valves (mean height = 0.8 mm; n=54). The maximum height and width occur at the mid-length of the shell.

Ventral valve and body area. Length: 4.0 - 6.8 mm, mean = 5.3 mm, Width: 3.2-5.6 mm, mean = 4.7 mm, Width/Length ratio: 0.77 - 1.01, mean = 0.89 (n=48); Height: 0.8 - 1.4 mm, mean = 1.0 mm, Height/Length ratio: 0.14 - 0.25, mean = 0.19 (n = 42) (Fig. 2  - Fig. 3 ).

The umbonal region is rounded and continuous with the posterolateral margins (Fig. 4  - Fig. 5 ). The pseudointerarea, slightly concave on the internal side of the valve, is divided by a narrow pedicle groove and limited laterally by well-expressed and slightly elevated flexure lines (Fig. 5 ). The width of the pseudointerarea occupies about half of the internal umbonal portion (Fig. 4  - Fig. 5 ).

A pair of narrow, V-shaped grooves extends from the anterior adductor muscles to unite at the level of the posterior adductor muscle. These grooves are impressions of the pedicle nerves and are not visible on all specimens.

At the level of the visceral area the valve is commonly thickened. The main mantle canals (vascula lateralia) are subparallel to slightly curved (Fig. 6  - Fig. 7 ). The distance between the tip of the vascula lateralia and the anterior margin of the valve represents about 21 to 33 % (mean = 26 %; n = 9) of the length of the whole valve length, and the lophophoral cavity occupies about 42 to 57 % (mean = 52 %; n = 11) of the internal shell space (Fig. 6  - Fig. 7 ).

The muscle arrangement is well defined, and the posterior adductor muscle is unpaired (Fig. 6  & Fig. 8 ). The perimial line is not visible.

Dorsal valve and body area. Length: 3.3 - 6.8 mm, mean = 5.2 mm, Width: 2.8 - 5.9 mm, mean = 4.6 mm, W/L ratio: 0.76 - 1.01, mean = 0.90 (n = 62); Height: 0.3 - 1.1 mm, mean = 0.8 mm; H/L ratio: 0.10 - 0.22, mean = 0.16 (n = 54) (Fig. 2  - Fig. 3 ).

The umbonal region is rounded and continuous with the posterolateral margins (Fig. 4  - Fig. 5 ). The pseudointerarea, slightly concave on the valve interior (Fig. 4 ), occupies in width about one third of the umbonal region; it is limited laterally by well-expressed and slightly elevated flexure lines (Fig. 4  - Fig. 5 ).

At the level of the visceral area the valve is thickened. It is bisected anteriorly by a weak median ridge that extends between 0.6 to 1.5 millimetres at the level of the anterior oblique muscle scars (about 14 to 30 % of the total valve length, mean = 22 %) (Fig. 6 ). The main mantle canals (vascula lateralia) are subparallel to slightly curved (Fig. 6  - Fig. 7 ). The distance between the tip of the vascula lateralia and the anterior margin of the valve ranges between 17 and 30 % (mean = 25 %; n = 8) of the total length of the valve, and the lophophoral cavity occupies 19 to 36 % (mean = 26 %; n = 14) of the internal shell space (Fig. 6  - Fig. 7 ).

The muscle arrangement is shown on Figure 6  & Figure 8 . The anterior adductor muscles are slightly divergent anteriorly. The scar of the anterior oblique muscles cannot be defined exactly; perhaps they extended over the main length of the median ridge. The "combinierte Muskeln" that Mickwitz (1896) described are Median Lateral Oblique muscles (3) - Anterior Lateral Oblique muscle (2), while our observations suggest that the composite muscle is 3 + 4 (Anterior Internal Oblique muscle) on the left side and 3 + 4' (Median Internal Oblique muscle) on the right side (Fig. 6  & Fig. 8 ). The posterior adductor muscle is unpaired. The perimial line is well marked in the anterior body part.

Diagnosis

The new diagnosis of Schmidtites celatus (Volborth, 1869) is compared to the previous diagnosis from Popov et alii (1989). The composite figure A  (Fig. 6  & Fig. 4 ) is an integral part of the diagnosis, and consequently should be reproduced with the diagnosis.

Occurrence

Upper Cambrian - Lower Ordovician.

Estonia: Tsitre and Kallavere Formations (Iru, Ülgase, Maardu quarries, Jägala River, Turjekelder, Valkla River, Vihula, Toolse River, Saka, subsurface south of Maardu, subsurface of Rakvere Phosphorite Deposit, subsurface in southern Estonia, Värska-6 core) (see Puura, 1996).

Russia: Lomashka and Tosna formations in Ingria (Luga River, subsurface of Kingisepp Phosphorite Deposit, Lomashka River, Solka River, Sarya River, Suma River, Tosna River, Putilovo Quarry, Volkhov River, Syas River) (Popov et alii, 1989).

Sweden: S. celatus occurs in the 'Obolus' beds in the Finngrundet core as well as in erratic boulders from Uppland (Puura & Holmer, 1993).

Poland: Krzyze Formation in Podlasie depression (Bednarczyk, 1998).

Discussion

The statement of Williams & Hurst (1977) that the Obolidae have an almost identical pattern is no longer corroborated by the arrangement of the musculature, at least in those of the obolid genera Schmidtites, Obolus, Ungula and Oepikites (see Emig, 2002, 2003a, 2003b; and unpublished data).

The posterior adductor muscle is unpaired on both dorsal and ventral sides in Schmidtites celatus; paired muscles have been established on the ventral side and unpaired on the dorsal side of Ungula; paired on both sides in Obolus, and according to Holmer & Popov (2000) in Oepikites too. Such variations in the posterior adductor (also named umbonal) musculature are contrary to the recent diagnosis by Holmer & Popov (2000) of the family Obolidae, in which all the genera have a "muscle system with paired umbonal muscle scars". Consequently, this diagnosis must be revised.

Not only in the Obolids but also in the Lingulides, the Anterior Oblique, Anterior Internal Oblique and Median Internal Oblique muscles on the ventral side, and the Anterior Lateral Oblique, Anterior Internal Oblique and Median Lateral Oblique muscles on the dorsal side are invariably located posterolaterally to the vascula lateralia, whereas the Anterior Oblique and the Anterior Adductor muscles on the ventral side and the Anterior oblique and the Anterior Adductor muscles on the dorsal side are located anterocentrally to the vascula lateralia.

The main difference in the muscle pattern of Schmidtites from those in Obolus, Ungula, and Oepikites is on the dorsal side and involves the disposition of the Median Lateral Oblique, Anterior or Median Internal Oblique and Anterior Lateral Oblique muscles, which are aligned in Obolus, Ungula, Oepikites (represented for Ungula on Figure 7 ; for Oepikites see Holmer & Popov, 2000; for Obolus see Emig, 2002), but join to form a composite muscle in Schmidtites (Fig. 6  - Fig. 7 ). This muscle arrangement was described by Mickwitz (1896) as "combinierte Muskeln" that is the Median Lateral Oblique muscle (3) + Anterior Lateral Oblique muscle (2). But our study shows that this composite muscle is formed on the right by the Anterior Internal Oblique muscle (4) + Anterior Lateral Oblique muscle (2) and on the left by the Median Internal Oblique muscle (4') + Anterior Lateral Oblique muscle (2) (Fig. 6  - Fig. 7 ). The location of the scar of the Anterior Lateral Oblique muscle (2) on the ventral side (Fig. 6  - Fig. 7 ) differs from that figured by Williams & Hurst (1977) and Popov (1992).

Consequently, the musculature of the obolids is neither identical nor entirely symmetrical, and in Schmidtites at least is remarkably different. It may be said that the arrangement in this genus is less conservative than in other obolids, or perhaps that it should not be referred to the family Obolidae.

Acknowledgments

This study was supported by a CNRS-Estonia grant for cooperation between France and Estonia and through a grant by the French Embassy in Tallinn. I thank Ivar Puura and the Institute of Geology staff (University of Tartu, Estonia) for laboratory facilities and for help in collecting, specimens, and Chantal Bezac (Centre d'Océanologie, Marseille) for SEM preparations. I am most grateful to Nestor Sander (Modesto, California, USA), Art Boucot (Oregon State University, Corvallis, USA), and Michal Mergl (University of West Bohemia, Pilsen, Czech Republic) for syntactical comments on and improvements in the readability of the earlier draft.

References

Balsamo-Crivelli G. (1863).- Di alcuni Spongiari del Golfo di Napoli.- Atti della Società Italiana di Scienze Naturali, Milano, vol. 5, p. 284-302, pls. 4-6.

Bednarczyk W.S. (1998).- Stratigraphical position of the "Obolus Sandstones" (Upper Cambrian-?Lowermost Ordovician) in Poland.- Bulletin of the Polish Academy of Sciences-Earth Science, Warzawa, vol. 46, n° 1, p. 43-54.

Biernat G. (1964).- Obolus apollinis Eichwald z otworu wiertniczego w Podborowisku.- Kwartalnik Geologiczny, Warzawa, vol. 8, p. 73-76.

Biernat G. (1973).- Ordovician brachiopods from Poland and Estonia.- Paleontologia Polonica, Warzawa, vol. 28, p. 1-116.

Biernat G. & Emig C.C. (1993).- Anatomical distinctions of the Mesozoic lingulide brachiopods.- Acta Palaeontologica Polonica, Warzawa, vol. 38, n° 1/2, p. 1-20.

Botting J.P. & Thomas A.T. (1999).- A pseudoplanktonic inarticulate brachiopod attached to graptolites and algae.- Acta Universitatis Carolinae Geologica, Praha, vol. 43, n° 1-2, p. 333-335.

Dall W.H. (1877).- Index to the names which have been applied to the subdivision of the class Brachiopoda.- Bulletin of the U.S. National Museum, Washington, vol. 8, p. 1-88.

Eichwald E. (1829).- Zoologia specialis, quam expositis animalibus tum vivis, tum fossilibus potissimum Rossiae in universum et Poloniae in specie, in usum lectionum publicarum in Universitate Caesarea Vilnensi. Volume 1.- Josephi Zawadski, Vilnae, 314 p.

Emig C.C. (1982).- Taxonomie du genre Lingula (Brachiopodes, Inarticulés).- Bulletin du Muséum National d'Histoire Naturelle, Paris, (sér. 4), vol. 4, n° 3/4, p. 337-367.

Emig C.C. (2002).- Tools for linguloid taxonomy: the genus Obolus (Brachiopoda) as an example.- Carnets de Géologie - Notebooks on Geology, Maintenon, Article 2002/01 (CG2002_A01), 9 p., 3 fig., 2 tabl.

Emig C.C. (2003a).- Proof that Lingula (Brachiopoda) is not a living-fossil, and emended diagnoses of the Family Lingulidae.- Carnets de Géologie - Notebooks on Geology, Maintenon, Letter 2003/01 (CG2003_L01), 8 p., 7 fig., 1 tabl.

Emig C.C. (2003b).- Reply to L.E. Popov and L.E. Holmer (2003): Obolid taxonomy.- Carnets de Géologie - Notebooks on Geology, Article 2003/06_Reply (CG2003_A06_Reply)

Gorjansky V.Yu. (1969).- Inarticulate brachiopods of the Cambrian and Ordovician of the northwest Russian Platform [in Russian].- Ministerstvo Geologii RSFSR, Severo-Zapadnoe Territorialnoe Geologicheskoe Upravlenie, Leningrad, 173 p.

Hall J. & Clarke J.M. (1892).- An introduction to the study of the genera of Palaeozoic Brachiopoda.- Geological Survey and Natural History of New York, Palaeontology, vol. VIII, Part I, 367 p.

Heinsalu H., Viira V., Mens K., Oja T. & Puura I. (1987).- The section of the Cambrian-Ordovician Boundary beds in Ülgase, Northern Estonia (In Russian).- Proceedings of the Estonian Academy of Sciences, Tallinn, (Geology), vol. 36, n° 4, p. 154-165.

Holmer L.E. & Popov L.E. (1990).- The acrotrecean brachiopod Ceratreta tanneri (Metzger) from the Upper Cambrian of Baltoscandia.- Geologiska Föreningens i Stockholm Förhandligar, Stockholm, vol. 112, p. 249-263.

Holmer L.E. & Popov L.E. (2000).- Lingulida. In: Kaesler R.L. (Ed.), Revised Brachiopoda.- Treatise on Invertebrate Paleontology, Geological Society of America, New York; University of Kansas, Lawrence, Part H, vol. 2, p. 32-95.

International Commission on Zoological Nomenclature (1999).- International code of zoological nomenclature, Fourth Edition, adopted by the International Union of Biological Sciences.- The International Trust for Zoological Nomenclature / The Natural History Museum, London, xxix + 306 p.

Kaljo D., Borovko N., Heinsalu H., Khazanovich K., Mens K., Popov L., Sergeyeva S., Sobolevskaya R. & Viira V. (1986).- The Cambrian-Ordovician boundary in the Baltic-Ladoga clint area (North Estonia and Leningrad region, USSR).- Proceedings of the Estonian Academy of Sciences, Tallinn, (Geology), vol. 35, n° 3, p. 97-108.

Khazanovitch K.K. & Popov L.E., In: Khazanovitch K.K., Popov L.E. & Melnikova L.M. (1984).- Inarticulate brachiopods, ostracodes (bradoriids) and hyolithelminths from the Sablinka Formation of the Leningrad District [in Russian].- Paleontologicheskiy Zhurnal, Moscow, vol. 1984, n° 4, p. 33-47.

Lashkov E.M., Lashkova L.N., Popov L.E. & Yankauskas T.V. (1993).- "Obolus" sandstones of the southeast of the east-baltic.- Geologija, Vilnius, vol. 14, p. 99-108.

Lockley M.G. & Williams A. (1981).- Lower Ordovician brachiopoda from mid and southwest Wales.- Bulletin of the British Museum (Natural History), London, (Geology), vol. 35, Part 1, p. 1-78.

M'Coy F. (1851).- On some new Cambrian-Silurian fossils.- Annals and Magazine of natural History, London, Vol. 2, n° 8, p. 387-409.

Mens K., Viira V., Paalits I. & Puura I. (1989).- Cambrian-Ordovician boundary beds at Mäekalda, Tallinn, North Estonia.- Proceedings of the Estonian Academy of Sciences, Tallinn, (Geology), vol. 38, n° 3, p. 101-111.

Mens K., Bergström J. & Lendzion K. (1990).- The Cambrian system on the East European platform. Correlation chart and explanatory notes.- IUGS Publication, vol. 25, 73 p.

Mens K., Heinsalu H., Jegonjan K., Kurvits T., Puura I. & Viira V. (1996).- Cambrian-Ordovician boundary beds in Pakri Cape Section, NW Estonian.- Proceedings of the Estonian Academy of Sciences, Tallinn, (Geology), vol. 45, p. 9-21.

Mergl M. & Elicki O. (2004).- Cambrian lingulid and acrotretid brachiopds from the Iglesiente area (Campo Pisano Formation, southwestern Sardinia).- Rivista Italiana di Paleontologia e Stratigrafia, Milano, vol. 110, n° 3, p. 581-589.

Mickwitz A. (1896).- Über die Brachiopodengattung Obolus Eichwald.- Mémoires de l'Académie Impériale des Sciences de St-Pétersbourg, St-Pétersbourg, (Classe physicomathématique), vol. 4, n° 2, 216 p.

Nazarov B.B. & Popov L.E. (1980).- Stratigraphy and fauna of Ordovician siliceous-carbonate deposits of Kazakhstan (in Russian).- Trudy - Geologicheskiy Institut, Akademiya Nauk SSSR, Moscow, vol. 5, p. 37-44.

Nemliher J. & Puura I. (1996).- Upper Cambrian basal conglomerate of the Kallavere formation on the Pakri Peninsula, NW Estonia.- Proceedings of the Estonian Academy of Sciences, Tallinn, (Geology), vol. 45, n° 1, p. 1-8.

Öpik A. (1929).- Der estländische Obolenphosphorit. - Verlage estländisches Handels- und Gewerbeministeriums, Tallinn, 49 p.

Pander C.H. (1830).- Beiträge zur Geognosie des Russischen Reiches.- St. Petersburg, 165 p.

Popov L., Khazanovitch K.K., Borovko N.G., Sergeyeva S.P. & Sobolevskaya R.F. (1989).- The Key sections and stratigraphy of the phosphate bearing Obolus beds of the North-East of the Russian platform [in Russian].- Nauka Trudy, Leningrad, vol. 18, 222 p.

Popov L.E. & Khazanovitch K.K. (1989).- Conditions for the formation of phosphorite bearing deposits of the Baltic Basin. Palaeoecology and taphonomy of lingulates of the 'Obolus sea' [in Russian].- Mezhvedomstvennyi Stratigraficheskii Komitet SSSR Trudy, vol. 18, p. 170-180.

Popov L.Ye. (1992).- The Cambrian radiation of brachiopods. In: Lipps J.H. & Signor P.W. (Eds), Origin and early evolution of the Metazoa.- Plenum Press, New-York, p. 399-423.

Puura I. & Holmer L.E. (1993).- Lingulate brachiopods from the Cambrian-Ordovician boundary beds in Sweden.- Geologiska Föreningens i Stockholm Förhandligar, Stockholm, vol. 115, Part 3, p. 215-237.

Puura I. (1996).- Lingulate brachiopods and biostratigraphy of the Cambrian-Ordovician boundary beds in Baltoscandia.- Dr. Thesis, Univ. Uppsala, 136 p.

Rowell A.J. (1965).- Inarticulata. In: Moore R.C. (Ed.), Brachiopoda.- Treatise on Invertebrate Paleontology, Geological Society of America, New York; University of Kansas, Lawrence, Part H, p. 260-296.

Rowell A.J. (1977).- Valve orientation and functional morphology of the foramen of some siphonotretacean and acrotretacean brachiopods.- Lethaia, Oslo, vol. 10, p. 43-50.

Schuchert C. & LeVene C.M. (1929).- Brachiopoda (generum et genotyporum index et bibliographia). In: Fossilium Catalogues, vol. 1, Animalia, Pars 42.- Junk, Berlin 140 p.

Volborth A.F. von (1869).- Über Schmidtia and Acritis, zwei neue Brachiopoden-Gattungen.- Verhandlungen (Zapiski) der Russisch-Kaiserlichen Mineralogischen Gesellschaft zu St. Petersburg, (Serie 2), vol. 4, p. 208-217.

Walcott C.D. (1898).- Cambrian Brachiopoda: Obolus and Lingulella, with description of new species.- Proceedings of the U.S. National Museum, Washington, vol. 21, p. 385-420.

Walcott C.D. (1908).- Cambrian Geology and Paleontology. 3.- Cambrian Brachiopoda, description of new genera and species.- Smithsonian miscellaneous Collections, vol. 53, p. 53-137.

Walcott C.D. (1912).- Cambrian Brachiopoda.- Monograph of the U.S. Geological Survey, Reston, vol. 51, 812 p.

Williams A. (1974).- Ordovician Brachiopoda from the Shelve District, Shropshire.- Bulletin of the British Museum (Natural History), London, (Geology), Suppl. 11, p. 3-163.

Williams A. & Hurst J.M. (1977).- Brachiopod evolution. In: Hallam A. ed, Patterns of Evolution.- Elsevier, Amsterdam, p. 79-121.

Figures

Click on thumbnail to enlarge the image.

Figure 1: Occurrences of Schmidtites celatus in the columns representing the stratigraphic successions of the sampled localities and their geographical location in Estonia.

Click on thumbnail to enlarge the image.

Figure 2: Regression curves of the Width/Length and Height/Length ratios of dorsal and ventral valves of Schmidtites celatus.

Click on thumbnail to enlarge the image.

Figure 3: Histograms of the valve dimensions (length, width, height) and ratios (height/length and width/length) of Schmidtites celatus.

Click on thumbnail to enlarge the image.

Figure 4: Umbonal region of Schmidtites celatus (internal view).

Click on thumbnail to enlarge the image.

Figure 5: Scanning electron micrographs of Schmidtites celatus. A, Internal umbonal region of ventral valves with the pedicle groove and muscle scars. B. Profile view of the umbonal beak, with pedicle groove and elevated flexure lines. C and D, internal umbonal region, more or less acute, of two dorsal valves, with some muscle scars. Flexure lines indicated by arrows.

Click on thumbnail to enlarge the image.

Figure 6: Muscle arrangement and disposition of the main mantle canals in Schmidtites celatus. A, their variability on the ventral body side and B, in a specimen. C, their variability in the studied specimens on the dorsal body side and D, in a specimen.

Click on thumbnail to enlarge the image.

Figure 7: Diagram of the extensions (minimum-maximum) of the lophophoral cavity and of the mean extension of the mantle canals based on average shell dimensions in the obolid Schmidtites celatus from the Upper Cambrian of Estonia, compared to Glottidia, Lingula and Lingularia (see also Biernat & Emig, 1993).

Click on thumbnail to enlarge the image.

Figure 8: Muscle arrangement in 3D on the ventral and dorsal sides in Schmidtites celatus and Ungula ingrica.

Table

Table 1: Distribution of the four brachiopod species in the outcrops investigated (see Fig. 1 ). Percentages are indicative. They were established on 200 to 300 hundred specimens at each level.
# few valves or fragments of valves
(*) Ungula convexa a synonym of Obolus apollinis (see Emig, 2002, 2003b)