ISSN 0031-0301, Paleontological Journal, 2009, Vol. 43, No. 3, pp. 306–310. © Pleiades Publishing, Ltd., 2009. Original Russian Text © S.V. Moloshnikov, 2009, published in Paleontologicheskii Zhurnal, 2009, No. 3, pp. 67–70.
A New Species of Psammosteiform Heterostracan (Agnatha) from the Givetian (Middle Devonian) of the Voronezh Region S. V. Moloshnikov Earth Science Museum, Moscow State University, Moscow, 119899 Russia Borissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117997 Russia e-mail:
[email protected] Received May 19, 2008
Abstract—A new pycnosteid species, Schizosteus shkurlatensis sp. nov. (Agnatha, Psammosteiformes), from the Staryi Oskol beds (Givetian, Middle Devonian) of the Voronezh Region is described. This is the first species of the genus Schizosteus Obruchev from the Central Devonian Field. The similarity between species of this genus and early species of Psammolepis is discussed. DOI: 10.1134/S0031030109030095 Key words: Agnatha, Psammosteiformes, Middle Devonian, Central Devonian Field, Russia.
INTRODUCTION Remains of psammosteiform heterostracans are infrequent in the Devonian of southern European Russia (Central Devonian Field). To date, only two large localities of these agnathans have been discovered in the Central Devonian Field: the Mikhailovskii mine in the Kursk Region (Utekhin, 1972; Obrucheva and Obrucheva, 1977; Vorobyeva et al., 1997; Mansurov, 2004; Moloshnikov, 2007, 2008) and the Pavlovskii mine (Shkurlat) in the Voronezh Region (Raskatova, 2004). In these localities, Psammosteus praecursor Obruchev (P. cf. P. praecursor Obruchev), Psammolepis sp., and Tartuosteus (?) zheleznogorskensis Moloshnikov have been recorded and described. In 2001, O.A. Lebedev found a clayey–siltstone concretion that enclosed several bones and scales of new species of the genus Schizosteus Obruchev in the Staryi Oskol beds (Vorob’evka (?) Horizon) of the Pavlovskii mine. This genus has previously been recorded only in the Middle Devonian of the Main Devonian Field (Obruchev, 1940; Tarlo, 1964; 1965; Obruchev and Mark-Kurik, 1965) and, probably, Czech Republic and North America (Tarlo, 1965; Novitskaya, 2004). The species recorded beyond the Main Devonian Field were described based on bone fragments and tentatively assigned to this genus (Novitskaya, 2004). The material from the Pavlovskii mine includes the first reliable specimens of the genus Schizosteus that come from the localities beyond the Main Devonian Field. The purpose of the present study is the description of a new species of the genus Schizosteus Obruchev. The description is accompanied by the measurements, which are performed according to Tarlo (1964, text-fig.
9). The material studied is housed in Borissiak Paleontological Institute of the Russian Academy of Sciences, Moscow (PIN), collection no. 5163. SYSTEMATIC PALEONTOLOGY Order Psammosteiformes Family Pycnosteidae Tarlo, 1962 Genus Schizosteus Obruchev, 1940 Schizosteus shkurlatensis Moloshnikov, sp. nov. Plate 8, figs. 1–8
E t y m o l o g y. From the village of the 3rd Shkurlat, situated close to the locality. H o l o t y p e. PIN, no. 5163/21, dorsale; Voronezh Region, vicinity of Pavlovsk, left bank of the Gavrilo River, Pavlovskii mine (Shkurlat); Middle Devonian, Givetian, Staryi Oskol Superhorizon, Vorob’evka (?) Horizon. D e s c r i p t i o n (Fig. 1). A small-sized member of the genus, the cephalothorax was approximately 15– 20 cm long. The branchiale (Figs. 1a, 1b; Pl. 8, fig. 1) is long and narrow, extended triangular, almost rectangular. The lateral margin of the bone is slightly convex. On the dorsal side, a free surface covered with dentin tubercles is very narrow. In the middle of the bone, it is outlined anteriorly by a distinct boundary (Fig. 1b; Pl. 8, fig. 1b). In this place, the branchiale was probably overlain by the postorbitale (Fig. 1d). The lateral angle of the bone is about 80°. On the ventral side, the growth lines are well-pronounced (Pl. 8, fig. 1a). Judging from the arrangement of the growth lines, the young of this species had relatively wider branchialia; in adults, these bones became relatively more extended.
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Explanation of Plate 8 Figs. 1–8. Schizosteus shkurlatensis sp. nov.: (1) specimen PIN, no. 5163/20, right branchiale, ×1.1; (1a) ventral and (1b) dorsal views; (2) specimen no. 5163/23, scale, ×3.1; (3) specimen PIN, no. 5163/22, scale, ×4.25; (4) specimen PIN, no. 5163/24, scale, ×4.5; (5) specimen PIN, no. 5163/20, dentin tubercles on the ventral side of the branchiale, ×12; (6–8) holotype PIN, no. 5163/21, dorsale: (6) external side, ×1.3, (7) dentin tubercles, ×3.8, and (8) tessera in the posterior part of the dorsale, ×2.5.
The dorsale is flat; judging from the arrangement of the growth lines, it was almost circular (Fig. 1c; Pl. 8, fig. 6), with a relatively wide bordering zone without sculpture (up to 22 mm wide). The margin of the sculptured surface is undulating. The growth center was probably in the bone center, surrounded by distinct growth lines. The posterior part of the sculptured surface has tesserae, ranging in shape from rounded quadrangular to polygonal (Pl. 8, fig. 8). The scales are flat, varying in shape: rounded quadrangular, polygonal, rhombic, expanded transversely or longitudinally (Pl. 8, figs. 2–4). The anterior margin of scales has a border without dentin tubercles, up to 2−3 mm wide. Dentin tubercles (Pl. 8, figs. 5, 7) are small and low, on average about 22 per 1 cm of bone length. At the lateral margin of the branchiale, dentin tubercles are larger, up to 15 per 1 cm of bone length. As in all species of this genus, tubercles are densely spaced, forming the so-called herringbone pattern. They vary in shape, ranging from circular, hexagon, irregularly polygonal, to squamate (Pl. 8, fig. 5), longitudinally extended or isometric. Tubercles occasionally have widely spaced and very low denticles, which do not branch. M e a s u r e m e n t s in mm. Specimen PIN, no. 5163/20, right branchiale: reconstructed length, 105; width, ca. 50; thickness, up to 3.5; holotype PIN, no. 5163/21, dorsale: reconstructed length, ca. 100; width, ca. 90; thickness, up to 3; specimen PIN, no. 5163/22, scale: length, 4.5; width, 6.4; thickness, 1.2; specimen PIN, no. 5163/23, scale: length, 6; width, 7.1; thickness, 1.1; specimen PIN, no. 5163/24, scale: length, 5.6; width, 5; thickness, 1.1. C o m p a r i s o n. The new species differs from all known congeners in the narrow free surface on the dorsal side of the branchiale. In addition, it differs from S. striatus (Gross) and S. asatkini Obruchev in the presence of tesserae in the marginal part of the dorsale, the shorter denticles on the dentin tubercles, and in the presence of tubercles varying in shape on the bones (in S. striatus and S. asatkini, dentin tubercles are usually isometric and elongated tubercles are absent). It differs from S. striatus in the absence of dentin ridges and ribs on the branchiale; in S. striatus, they are formed of fused tubercles. The new species sharply differs from S. heterolepis (Preobrazhensky) in the smaller tuberPALEONTOLOGICAL JOURNAL
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cles (in S. heterolepis, 5–14 per 1 cm; in the new species, up to 22 per 1 cm) and in the smaller size of bones. S. heterolepis, has additional small dentin tubercles between large tubercles; in S. shkurlatensis sp. nov., they are absent. S. shkurlatensis sp. nov. resembles S. splendens (Eichwald) in the presence of tesserae on the dorsale and differs from it in the narrow and long branchialia (in S. splendens, they are wide, resembling in shape the branchialia of the genus Pycnosteus) and in the smaller size. M a t e r i a l. In addition to the holotype, one almost complete branchiale, one uncertain bone fragment, and three scales from the type locality. DISCUSSION The new species and other representatives of the genus Schizosteus share a number of morphological characters with early members of Psammolepis. One of the earliest psammolepidids, Psammolepis toriensis (Mark-Kurik), comes from the Pärnu Horizon (Eifelian) of the Main Devonian Field (Obruchev and MarkKurik, 1965, 1968; Tarlo, 1965; Mark-Kurik, 1968; Novitskaya, 2004). The branchiale of the last species is similar to that of the majority of species of Schizosteus Obruchev. Therefore, there is no surprise that this species was originally assigned to Schizosteus (Obruchev and Mark-Kurik, 1965; Tarlo, 1965). Subsequently, the dorsal plate of P. toriensis was described (Mark-Kurik, 1968); this provided a more precise identification of the position of this species among psammosteiforms and assignment to the genus Psammolepis Agassiz. S. shkurlatensis sp. nov. and P. toriensis share some features, such as the elongated triangular shape of the branchialia, rounded dorsalia, with a sculptured field around the bone center. The only difference is the presence of tesserae on the dorsalia. In species of the genus Schizosteus (S. shkurlatensis sp. nov. and S. splendens), only isolated tesserae in the posterior part of the dorsale are present, whereas, in Psammolepis, tesserae cover most of this bone. This similarity confirms the assumption of close phylogenetic relationship between Schizosteus and Psammolepis, such that the first is ancestral to the second (Tarlo, 1962; Obruchev and Mark-Kurik, 1965).
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MOLOSHNIKOV Plate 8
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4 2
5 6
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(b)
2 cm (c) (d) ?PO
Br D 2 cm Fig. 1. Schizosteus shkurlatensis sp. nov.: (a, b) specimen PIN, no. 5163/20, right branchiale: (a) ventral and (b) dorsal views; (c) holotype PIN, no. 5163/21, dorsale, external side; (d) reconstruction of the dorsal body side. Designations: (Br) branchiale, (D) dorsale, and (PO) postorbitale.
ACKNOWLEDGMENTS I am grateful to O.A. Lebedev for providing me with the specimens studied. REFERENCES 1. R. Kh. Mansurov, “Species Characteristics of Devonian Fishes from the Phosphorite-bearing Beds of the Mikhailovskii Iron Ore Locality (KMA),” in Metallogeny of Ancient and Recent Oceans—2004: Progress at the Boundary of Centuries, Vol. 2 Deposits of Noble Metals, Problems of Mineralogical–Geochemical Studies, Ed. by V. V. Zaikov PALEONTOLOGICAL JOURNAL
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5. L. I. Novitskaya, “The Subclass Heterostraci,” in Fossil Vertebrates of Russia and Adjacent Countries: Agnathans and Ancient Fishes: Handbook for Paleontologists, Biologists, and Geologists, Ed. by L. I. Novitskaya (GEOS, Moscow, 2004), pp. 69–207 [in Russian]. 6. D. V. Obruchev, “On Some Psammosteids from the Middle Devonian of the Leningrad and Baltic Regions,” Dokl. Akad. Nauk SSSR 28 (8), 766–768 (1940). 7. D. V. Obruchev and E. Yu. Mark-Kurik, Psammosteids (Agnatha, Psammosteidae) from the Devonian of the Soviet Union (Inst. Geol. Akad. Nauk Est. SSR, Tallinn, 1965) [in Russian]. 8. D. Obruchev and E. Mark-Kurik, “On the Evolution of the Psammosteids (Heterostraci),” Izv. Akad. Nauk Est. SSR, Ser. Khim. Geol. 17 (5), 279–284 (1968). 9. O. P. Obrucheva and E. D. Obrucheva, “Fishes of the Central Devonian Field,” in Sketches on the Phylogeny and Systematics of Fossil Fishes and Agnathans (Nauka, Moscow, 1977), pp. 24–28 [in Russian]. 10. M. G. Raskatova, “Myospore Zonation of the Middle– Upper Devonian Beds of the Southeast Part of the Voronezh
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