The Genus Obolus (Brachiopoda) As an Example Christian C

Tools for linguloid taxonomy: the genus Obolus (Brachiopoda) as an example Christian C. Emig

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Christian C. Emig. Tools for linguloid taxonomy: the genus Obolus (Brachiopoda) as an example. Carnets de Geologie, Carnets de Geologie, 2002, CG2002 (A01), pp.1-9. hal-00138295

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Tools for linguloid taxonomy: the genus Obolus (Brachiopoda) as an example

Christian C. EMIG1

Abstract: This study points out some basic problems of linguloid systematics and proposes solutions for them. A taxonomic examination of the unique species of the genus Obolus found in the Upper Cambrian of Estonia and Russia, O. apollinis (= O. ruchini, O. transversus, O. rebrovi and Ungula convexa) is used as an example of a methodology employing all of the characters valid for distinguishing species of both extant and fossil Lingulidae. These characters are: - umbonal region; - body musculature; - septa or ridges; - main mantle canals - as established and figured by EMIG (1982, 1983) and BIERNAT and EMIG (1993). All of them have been determined to be taxonomically stable and have been studied and compared to take into account intraspecific variability; they should be used to describe or to redescribe any taxon of the superfamily Linguloidea. Characters of the shell and valves, such as shape, size, and dimensional ratios have no taxonomic value. Key Words: Taxonomy; Obolus; Brachiopoda; Cambrian - Ordovician; Estonia

Citation: EMIG 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_CCE) Résumé : Des outils pour la taxinomie des Lingulidoida : le genre Obolus (Brachiopoda) pris comme exemple.- Cette étude met en exergue des problèmes fondamentaux de la systématique des Linguloïdes et propose des solutions méthodologiques basées sur les caractères utilisés pour identifier les espèces de Lingulides actuelles et fossiles. L'unique espèce du genre Obolus, O. apollinis (= O. ruchini, O. transversus, O. rebrovi and Ungula convexa), récoltée dans divers gisements du Cambrien moyen et supérieur en Estonie et Russie, sert ici d'exemple. Les caractères sont : - la région umbonale ; - la musculature du corps ; - les septums et crêtes internes ; - les canaux du manteau ; selon la description et la représentation faites par EMIG (1982, 1983) et BIERNAT et EMIG (1993). Tous ces caractères se sont révélés taxinomiquement stables tout en présentant une relative variabilité intraspécifique ; ils sont donc à être utilisés pour décrire ou redécrire tous les taxons de la superfamille des Linguloidea. Il convient de souligner que les caractères de la coquille ou des valves, tels que formes, tailles, rapports dimensionnels n'ont aucune valeur taxinomique. Mots-Clefs : Taxinomie ; Obolus ; Brachiopoda ; Cambrien - Ordovicien ; Estonie strata were classified as belonging to the genera Introduction Ungula PANDER, 1830, Oepikites KHAZANOVITCH et POPOV, 1984, and Schmidtites SCHUCHERT et As stated by EMIG (1977) diagnoses of the LEVENE, 1929, all representatives of the family extant species of the genera Lingula BRUGUIÈRE, Obolidae (Lingulida: Linguloidea). Other 1797, and Glottidia DALL, 1870, are too poor to specimens studied are in the collections of the permit discrimination between species. Museum of Geology in the Institute of Geology Consequently, all species referred to these of the University of Tartu, Estonia; they were genera have been redescribed by EMIG (1982, collected by L. POPOV from: 1983) using a new taxonomy based on morpho- (1) localities along the Sarya and anatomical characters. Recently, these Volkhov rivers (Ingra, Russia: Middle Cambrian) characters have been used successfully by and identified as Obolus apollinis EICHW ALD, BIERNAT and EMIG (1993), GAGNIER et alii (1996) 1829, and MÁRQUEZ-ALIAGA et alii (1999) to revise the (2) the locality Sarya River (Ingria taxonomy of Palaeozoic and Mesozoic Russia, Middle Cambrian) identified as O. ruchini brachiopod genera and species. KHAZANOVITCH et POPOV, 1984, During recent field trips to Estonia hundreds of well preserved specimens were collected from the Upper Cambrian (PUURA, 1996) near Tallinn. Previously, brachiopods from these

1 CNRS UMR 6540, Centre d'Océanologie, Rue de la Batterie-des-Lions, 13007 Marseille (France) e-mail: [email protected]

Manuscript online since November 27, 2002

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(3) the Sayas River locality identified as superfamily Linguloidea. O. transversus (PANDER, 1830), (4) from Ladoga and Suma River Main taxonomic characters localities (Ingria Russia: Upper Cambrian) identified as Ungula convexa PANDER, 1830. The main taxonomic criteria used to discriminate between linguloid taxa were It is of fundamental importance that the established and figured by EMIG (1982, 1983) taxonomic criteria used here to define the and BIERNAT and EMIG (1993). They are listed Cambrian genus Obolus are applied in the below, illustrated for the genus Obolus on Fig. revision of the systematics of and within the 1 and described in Tables 1 and 2.

Figure 1: Diagrams of the main taxonomic characters of Obolus apollinis. A. Umbonal regions (internal view), see also Fig. 2, 3; B. Average disposition of muscles and main mantle canals (drawn from a real specimen); C. Variability in musculature (established from more than 20 specimens) and main mantle canals (on 3 specimens). Muscle terminology as established by EMIG (1982): 1 Anterior Oblique; 2 Anterior Lateral Oblique; 3 Median Lateral Oblique; 4 Anterior Internal Oblique; 4' Median Internal Oblique; AA Anterior Adductor; PA Posterior Adductor.

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Umbonal region of each valve: internal its systematics (see POPOV et alii, 1989; PUURA, view, and when needed external view and 1996 for reviews), because many species have profile; been assigned to this genus without regard to its taxonomic characteristics stated above; Body muscle imprint or scars on the internal side of each valve: arrangement The genus Obolus has been redefined based and variability; on a study of four species: i.e. O. apollinis, O. ruchini, O. transversus (= O. rebrovi Septa or ridges on the valves: when KHAZANOVITCH et POPOV, 1984), and Ungula present convexa (Table 2). Consequently, three of Main mantle canals (vascula lateralia), these species must be considered junior and secondary ones (vascula media) when synonyms of O. apollinis, the type-species, present: disposition and variability. originally described by EICHWALD (1829). Most of these features are used to define All of the specimens of the above cited species, but some appear to have generic species show so many similarities in their value also, for example the septa in the extant characters that they are described and genus Glottidia. Some serve as criteria to discussed herein under the name Obolus define higher taxa among the Linguloidea, for apollinis. Furthermore their original example the ridge in the dorsal valve between descriptions (EICHWALD, 1829; PANDER, 1830; the anterior oblique muscles, a characteristic KHAZANOVITCH and POPOV, 1984; POPOV et alii, perhaps of the entire superfamily. Symmetrical 1989) do not establish valid distinctions musculature is a defining characteristic of the between them. family Obolidae and asymmetrical musculature The shell ranges from subcircular to a serves to distinguish the Lingulidae. All of these subtriangular in outline, the former being most criteria exhibit great variability, so when the common (Fig. 1). Externally, the valves are erection of a new linguloid genus or species is smooth with fine growth lines, some of which contemplated its characteristics must be are stronger. Sometimes weak radial marks are measured and compared not only with a single present. The insides of both valves shows a related taxon but also with all possible thickened visceral area, ranging from a very relatives. The description of fossil taxa should slight elevation to a very well developed be based on no less than 20 to 30 well- thickened area, common in large specimens. In preserved specimens, none fragmentary. To the ventral valve there is a heart-like attain this requisite number, several hundred depression between the anterior and posterior specimens may have to be collected, prepared adductor muscles. POPOV et alii (1989) stated and studied in detail. Furthermore a species that Obolus differs from Ungula in having a should be defined only on the basis of its subcircular, thinner and flatter shell and lacking occurrence in at least three discrete geographic a heart-like depression in the ventral visceral populations, as suggested by WILEY (1981). area: However these characteristics have no In addition, it is highly desirable that the full taxonomic value because they occur in both range and limits of variability of diagnostic Obolus and Ungula. characters be figured, and that the figures be Umbonal regions: integrated into the definition of the species as has been done here in Fig. 1 for Obolus The pseudointerarea always overhangs the apollinis. internal valve surface, except at the level of the pedicle groove, which is generally External features, such as the shape, size, continuous with the internal side of the valve. and dimensional ratios of the valves, have Laterally it is limited by well-marked and been demonstrated to have no taxonomic elevated flexure lines, which are aligned with value (EMIG, 1982, 1983; BIERNAT and EMIG, the Anterior (or Median) Internal Oblique - 1993). Consequently, they cannot be used to Anterior Oblique muscles. The flexure line is define either genus or species but may be sometimes accentuated by a narrow groove. given additional to the diagnosis. Only Furthermore, in the ventral valve the exceptionally is shell form diagnostic of a pseudointerarea, triangular and slightly species, for example the quadrangular shape of concave, extends laterally over about 30 to 60 the shell of Lingula adamsi when compared to per cent of the umbonal region of which the those of the other species of Lingula (EMIG, beak is rather rounded. A rather narrow 1982). subparallel pedicle groove medially divides the ventral pseudointerarea (Fig. 1A, 2). The The Obolus example length of the pedicle groove ranges between 1.3 and 3.8 mm (mean = 2.2 mm; n = 31). In Among the lingulid taxa of the Middle larger specimens the groove sometimes is less Cambrian to Ordovician (Tremadocian) in prominent or unimpressed anteriorly over half Estonia and Russia, the genus Obolus (Table 1) to one-third of the length of the has been the subject of controversy regarding pseudointerarea.

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Figure 2: Obolus apollinis: Internal umbonal region of the ventral valve: profile view from the beak and a frontal view. On the dorsal valve, the subtriangular On each side of the pseudointerarea is a pseudointerarea is slightly concave flat, broad lateral umbonal plate, which largely androunded; it has an extent similar to that of overhangs the Oblique muscle scars. These the ventral pseudointerarea, i.e. it occupies plates are commonly as wide as the length of from 30 to 60 per cent of the umbonal region. the pedicle groove. Nevertheless, such plates The median length of the pseudointerarea appear rather fragile and are partly broken in ranges from 0.6 and 3.4 mm (mean = 1.6 mm; many specimens. Pseudointerarea and lateral n = 34). The flexure lines are well-marked and umbonal plates overhang the scars of the elevated: they are more or less aligned with Anterior and Internal Oblique muscles the three postero-lateral muscles, i.e. the sometimes so far that they are hidden Median Lateral Oblique muscle, the Anterior (or completely. These features may vary in their Median) Internal Oblique muscle and the expression in relation to the degree of internal Anterior Lateral Oblique muscle. Anteriorly, the thickening of the valve. flexure lines on either side extend towards the Arrangement and variability of the Anterior Lateral Oblique muscle (Fig. 1B, 3). musculature: The clearly established existence of flexure lines on the dorsal valve (Fig. 3) is cited here The muscle system of Obolus was first for the first time as regards the genus Obolus, described by BULMAN (1939). The absence in because their lack was used by POPOV et alii the obolids of the Internal Posterior Oblique (1989) to distinguish the genus Obolus from muscle, considered by this author as an the genus Ungula. Consequently, this character undivided Median Internal Oblique muscle, is cannot be used to separate the genera. an important feature because the muscle

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Figure 3: Obolus apollinis: Internal umbonal region of the dorsal valve: frontal view and postero-lateral view with well-marked flexure lines. posterior adductor muscles (Fig. 1, 2). This arrangement is bi-symmetrical in the obolids ridge was first cited as the "Seitensepta" by and asymmetrical in the lingulides. This MICKWITZ (1896). On the dorsal valve, a median phylogenetic character was overlooked in the ridge at the level of the anterior oblique muscle overlooked in the diagnosis of the family scars is narrow and poorly developed. One may Obolidae by HOLMER and POPOV (2000). infer that, as in extant lingulides, the However, it is a character which should importance of this ridge generally increases perhaps be applied at superfamily level, thus with the size of the specimen. implying a revision of the higher taxa in the Main mantle canals: order Lingulida. The vascula lateralia are described as New characteristics of musculature in arcuate, submarginal by POPOV et alii (1989). Obolus are established: muscle scars "deeply" No observation is given on the vascula media. impressed into the inner surface of the valves, in particular the postero-lateral scars; in the As a result of this study, a new diagnosis for ventral valve, the muscle succession Anterior Obolus EICHWALD, 1829, is provided (Table 1) as Oblique - Internal Oblique is more or less well as for the unique species Obolus apollinis aligned with the flexure lines of the EICHWALD, 1829, remaining in the genus (Table pseudointerarea, covered by the lateral 2). Furthermore, Ungula convexa described by umbonal plates until about the middle of the PANDER (1830) was considered by subsequent Anterior Oblique muscle, and less extended designation (ROWELL, 1965: see HOLMER and antero-laterally than in Ungula; in the POPOV, 2000) as the type species of Ungula but composite muscle formed by the Anterior this species is now assigned to Obolus and Adductor and the Anterior Lateral Oblique, the considered a synonym of Obolus apollinis. latter is located in the posterior part of the Thus, a new type species for Ungula must be scar; in the inner side of the dorsal valve the proposed (paper in preparation): that is Ungula lateral umbonal plates overhang the alignment ingrica (EICHWALD, 1829), originally described of muscles: Median Lateral Oblique - Anterior under Obolus ingricus, and new diagnoses will Lateral Oblique - Anterior Internal Oblique (or be provided for the species and for Ungula. Median Internal Oblique) until about the middle of the Anterior Lateral Oblique muscle; the The genus Obolus now represented by only Anterior Adductor muscles are elongate and one species is an example of what occurs when slightly convergent posteriorly as observed also the taxonomic characters discussed above are in the obolid Schmidtites celatus (unpublished applied. Furthermore, this genus along with the data) (Fig. 1, 2, 3). genus Lingula, both of which have provided a name for the family, share an occurrence Septa or ridges: unique among the inarticulated brachiopods: that is many more or less complete fossils On the ventral valve a narrow to well- specimens have been assigned to one or the developed median ridge extends over several other genus based solely on the shape of the millimetres at the level of and between the shell.

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Previous diagnosis New diagnosis from HOLMER and POPOV (2000)

Bi-symmetrical muscle arrangement (*)

Ventral valve: Triangular umbonal region; the pseudointerarea reduced, slightly concave, Ventral propareas with deep, narrow with elevated flexure lines.

pedicle groove. Lateral umbonal plates, overhanging the internal side. Posterior adductor muscle paired, separated by short median septum.

Dorsal valve: Rounded umbonal region with reduced, Dorsal pseudointerarea lacking flexure slightly concave, pseudointerarea with lines. elevated flexure lines. Dorsal median ridge vestigial or absent. Lateral umbonal plates flat and large overhanging the internal surface. Posterior adductor muscle unpaired.

Shell circular to rounded triangular, (*) Nota: The Posterior Internal Oblique dorsibiconvex to subequally biconvex. muscle (numbered 4": EMIG 1982) is at Visceral area of both valves weakly present only known in Lingula, Glottidia and thickened, extending to midvalve. Vascula Lingularia, which have an asymmetrical lateralia of both valves submarginal, arcuate. muscle arrangement.

Table 1: Obolus EICHWALD, 1829: the previous diagnosis and the proposed one emended (in italics the non- taxonomic characters, without generic significance).

Because Obolus was selected as the A diagnosis cannot be based on features genotype at a family level (see HOLMER and without taxonomic value, or even more POPOV, 2000) the taxonomic criteria used here significantly on chacters without phylogenetic should be applied to the genera and their value, or include only a part of the characters species in all the Obolidae. The same remark is that define a taxon. A species should not be valid for all the Lingulidae (see Lingulidae in identified by any of the characters that define http://paleopolis.rediris.es/BrachNet/). the genus and vice versa, so that the diagnosis for each hierarchical taxon is clearly differentiated (Recommendation 13A). Recommendations Taxonomy is a tool based primarily on a Taxonomy is a tool based primarily on a diagnosis of each taxon. When comparing the diagnosis of each taxon. When comparing the diagnoses in Tables 1 and 2, remember that diagnoses in Tables 1 and 2, remember that the definition of the term diagnosis as given in the definition of the term diagnosis as given in ICZN (1999), differs slightly in the French and ICZN (1999), differs slightly in the French and English versions: "Énoncé écrit établissant English versions: "Énoncé écrit établissant l'ensemble des caractères d'un taxon qui l'ensemble des caractères d'un taxon qui suffisent à le distinguer des autres taxons suffisent à le distinguer des autres taxons auxquels il peut être utilement comparé" and auxquels il peut être utilement comparé" and "A statement in words that purposes to give "A statement in words that purposes to give those characters which differentiate the taxon those characters which differentiate the taxon from other taxa with which it is likely to be from other taxa with which it is likely to be confused". A diagnosis should also comply with confused". A diagnosis should also comply with Recommendation 13A (ICZN, 1999) as well as Recommendation 13A (ICZN, 1999) as well as recommendation 13B that concerns the recommendation 13B that concerns the languages in which a diagnosis should be languages in which a diagnosis should be given. given.

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Previous diagnosis New diagnosis from PUURA (1996)

including Fig. 1 Synonyms:

O. ruchini, O. transversus, O. rebrovi, Ungula convexa Muscle scars deeply impressed, in particular the postero-lateral ones.

Ventral valve: Umbonal region with pseudointerarea forming a beak; reduced subtriangular pseudointerarea, slightly concave, with laterally elevated flexure lines and medially a narrow subparallel pedicle groove. Pseudointerarea generally slightly Ventral pseudointerarea narrow and overhanging the visceral area, but pedicle triangular, with well-defined flexure lines and groove continuous with the internal valve narrow and deep pedicle groove. surface. Lateral umbonal plates overhanging the internal surface until about the middle of the Anterior Oblique muscle. Anterior Oblique and Internal Oblique muscles more or less aligned with the flexure lines of the pseudointerarea. Dorsal valve: Rounded umbonal region; pseudointerarea reduced, triangular, slightly concave, with elevated flexure lines. Lateral umbonal plates overhanging the internal side until about the middle of the Dorsal pseudointerarea narrow, with wide, Internal Oblique muscle. slightly concave median groove; propareas More or less extended median ridge at the high, reduced, without flexure lines; with level of the anterior oblique muscle scars. narrow median ridge. Anterior adductor muscles subparallel to slightly convergent posteriorly. Median Lateral Oblique muscle, Anterior Internal Oblique (or Median Internal Oblique) muscle and Anterior Lateral Oblique muscle separated and placed in a line.

Vascula lateralia on both sides arcuate, peripherally placed (submarginal). Vascula media (dorsal) - no data. Shell biconvex to slightly dorsibiconvex, Shell flattened, slightly dorsibiconvex, subcircular to subtriangular in outline, smooth

subcircular in outline. with some concentric growth lines well marked.

Ventral visceral area slightly elevated Visceral area in both valves varying from extending to about mid-valve. slightly elevated to very strongly thickened, Dorsal visceral area large, strongly the ventral valve shows a heart-like impressed elongate oval Anterior Adductors depression between the anterior and posterior muscle scars. adductor muscles.

Table 2: The new diagnosis of Obolus apollinis EICHWALD, 1829, is compared to the previous diagnosis as translated and modified by PUURA (1996) from the paper of POPOV et alii (1989), concerning O. apollinis, O. ruchini, O. transversus, and Ungula convexa. Non-taxonomic characters are in italics.

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A diagnosis cannot be based on features brachiopods.- Acta Palaeontologica Polonica, without taxonomic value, or even more Warsaw, Vol. 38, N° 1/2, pp. 1-20. significantly on chacters without phylogenetic BRUGUIÈRE J.G. (1797).- Vers, coquilles, value, or include only a part of the characters mollusques et polypiers. Tableau that define a taxon. A species should not be encyclopédique et méthodique des trois identified by any of the characters that define règnes de la nature, vol. 2. Agasse, Paris, the genus and vice versa, so that the diagnosis Pl. 96-314. for each hierarchical taxon is clearly BULMAN O.M. (1939).- Muscle systems of some differentiated (Recommendation 13A). inarticulate brachiopods.- Geological Magazine, Cambridge, Vol. 76, pp. 434-444. Information on the sites where types are DALL W.H. (1870).- A revision of the deposited and their accessibility for study or terebratulidae and lingulidae.- American revision is sometimes rather difficult to obtain Journal of Conchology, Vol. 6, pp. 88-168. for linguloid taxa, contrary to Recommendation EICHWALD E. (1829).- Zoologia specialis, quam 72F of the ICZN (1999). None of the types of expositis animalibus tum vivis, tum Obolus species are available from the fossilibus potissimum Rossiae in universum Geological Museum in St-Petersburg (Russia) et Poloniae in specie, in usum lectionum and the only specimens of Obolus that can be publicarum in Universitate Caesarea studied currently are in Tartu (Estonia). Vilnensi. Volume 1.- Josephi Zawadski, Finally, a scientific approach to systematics Vilnae, 314 p. should be based on a good knowledge of the EMIG C.C. (1977).- Réflexions sur la taxonomie taxa, comparative morphology and anatomy. des espèces du genre Lingula Ecological requirements may also be criteria in (Brachiopodes, Inarticulés).- Comptes systematics (ARNAUD and EMIG, 1987). Rendus de l'Académie des Sciences, Paris, Systematics and taxonomy must propose new Vol. 285, pp. 523-525 ideas and test hypotheses. Systematics as a EMIG C.C. (1982).- Taxonomie du genre Lingula tool to identify a taxon is but a technical (Brachiopodes, Inarticulés).- Bulletin du consequence. This paper is a first step in better Muséum National d'Histoire Naturelle, Paris, understanding in linguloid taxonomy, as well as Sér. 4, Vol. 4, N° 3/4, pp. 337-367 in suggesting new hypotheses and cladistic EMIG C.C. (1983).- Taxonomie du genre analyses. In these latter, a plesiomorphous or Glottidia (Brachiopodes Inarticulés).- apomorphous condition has to be proved and Bulletin du Muséum National d'Histoire discussed; that such a state exists cannot be Naturelle, Paris, Sér. 4, Vol. 5 (Sect. 4), N° based only on a simple computer calculation. 2, pp. 469-489. GAGNIER P.Y., BLIECK A., EMIG C.C., SEMPERE T., VACHARD D. and VANGUESTAINE M. (1996).- Acknowledgements New paleontological and geological data on This study was supported by a CNRS- the Ordovician and Silurian of Bolivia.- Estonia grant for cooperation between France Journal of South America Earth Sciences, and Estonia and through a grant by the French Vol. 9, N° 5/6, pp. 329-347. HOLMER L.E. and POPOV L.E. (2000).- Lingulida. Embassy in Tallinn. I thank Ivar PUURA and the Institute of Geology staff (University of Tartu, In: KAESLER R.L. (Ed.), Revised Estonia) for laboratory facilities and for help in Brachiopoda.- Treatise on Invertebrate Paleontology, Geological Society of America, collecting specimens, and Chantal BEZAC (Centre d'Océanologie, Marseille) for SEM New York, and University of Kansas, preparations. I am most grateful to Nestor J. Lawrence, Part H, Vol. 2, pp. 32-95. INTERNATIONAL COMMISSION ON ZOOLOGICAL SANDER (USA) for comments and English improvement of the earlier draft, as well as to NOMENCLATURE (1999).- International code of zoological nomenclature, Fourth Edition, the two referees, Art BOUCOT (OSU, Corvallis, adopted by the International Union of USA) and Fernando ALVAREZ (Universidad de Oviedo, Spain). Biological Sciences, The International Trust for Zoological Nomenclature / The Natural History Museum, London, xxix + 306 pp. References KHAZANOVITCH K.K. and POPOV L.E., In: KHAZANOVITCH K.K., POPOV L.E. and MELNIKOVA ARNAUD P.M. and EMIG C.C. (1987).- La L.M. (1984).- Inarticulate brachiopods, population, unité fonctionnelle de la ostracodes (bradoriids) and hyolithelminths biocoenose. In: Biologie des Populations.- from the Sablinka Formation of the Actes du Colloque National du Centre Leningrad District (in Russian).- National de la Recherche Scientifique, Lyon Paleontologicheskiy Zhurnal, Moscow, Vol. 1986, pp. 69-72. 1984, N° 4, pp. 33-47. BIERNAT G. and EMIG C.C. (1993).- Anatomical MÁRQUEZ-ALIAGA A., EMIG C.C. and BRITO J.M. distinctions of the Mesozoic lingulide (1999).- Triassic lingulide brachiopods from

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the Iberian Range (Spain).- Géobios, Lyon, PUURA I. (1996).- Lingulate brachiopods and Vol. 32, pp. 815-821. biostratigraphy of the Cambrian-Ordovician MICKWITZ A. (1896).- Über die boundary beds in Baltoscandia.- Dr. Thesis, Brachiopodengattung Obolus EICHWALD.- Univ. Uppsala, 136 pp. Mémoires de l'Académie Impériale des ROWELL A.J. (1965).- Inarticulata. In: MOORE Sciences de St Pétersbourg, Classe physico- R.C. (Ed.), Brachiopoda.- Treatise on mathématique, St-Péterbourg, Vol. 4, N° 2, Invertebrate Paleontology, Geological pp. 1-216. Society of America, New York, and PANDER C.H. (1830).- Beiträge zur Geognosie University of Kansas, Lawrence, Part H, p. des Russischen Reiches, St. Petersburg, 165 260-296. p. SCHUCHERT C. and LEVENE C.M. (1929).- POPOV L., KHAZANOVITCH K.K., BOROVKO N.G., Brachiopoda (generum et genotyporum SERGEYEVA S.P. and SOBOLEVSKAYA R.F. index et bibliographia). In: Fossilium (1989).- The Key sections and stratigraphy Catalogues, Vol. 1, Animalia, Pars 42. Junk, of the phosphate bearing Obolus beds of the Berlin 140 p. North-East of the Russian platform (in WILEY E. O. (1981).- Phylogenetics. The theory Russian). Nauka Trudy, Leningrad, Vol. 18, and practice of phylogenetic systematics.- pp. 1-222. Wiley-Interscience, New-York, 439 p.