ISSN 00310301, Paleontological Journal, 2014, Vol. 48, No. 1, pp. 74–78. © Pleiades Publishing, Ltd., 2014. Original Russian Text © O.B. Afanassieva, T. Märss, 2014, published in Paleontologicheskii Zhurnal, 2014, No. 1, pp. 75–79.
New Data on the Exoskeleton of the Osteostracan Genus Aestiaspis (Agnatha) from the Silurian of Saaremaa Island (Estonia) and the Severnaya Zemlya Archipelago (Russia) O. B. Afanassievaa and T. Märssb a
Borissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117997 Russia email:
[email protected] b Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, Tallinn, 19086 Estonia email:
[email protected] Received December 18, 2012
Abstract—The histological structure of exoskeleton of the osteostracan genus Aestiaspis Janvier et Lelievre, 1994 from the Silurian of Saaremaa Island is examined for the first time. The preservation of the material enables the fine exoskeleton structure to be described in detail. The study of Aestiaspis from Saaremaa Island and the Severnaya Zemlya Archipelago has shown structural details of exoskeleton sculpture. The formation of consolidated cephalothoracic shield in the phylogeny of Tremataspidoidei is discussed. The taxonomic position of the genus Aestiaspis in the Osteostraci system is analyzed. Keywords: Agnatha, osteostracans, Aestiaspis, morphology, Silurian, Saaremaa Island, Estonia, Severnaya Zemlya Archipelago, Russia DOI: 10.1134/S003103011401002X
no. 3257/607), which was well preserved and was not distorted. The extremely small cephalothoracic shield (about 10 mm long) suggested that this was probably a juvenile. This age stage is extremely scarce among osteostracan remains and, by the moment of descrip tion of this material, only presumably juvenile Param eteoraspis oberon (Janvier, 1985) of the family Param eteoraspididae Afanassieva, 1991 had been known. A thorough examination of the specimen from the Schmidt’s collection showed that it represents a defin itive ontogenetic stage of Aestiaspis viitaensis Janvier et Lelievre, 1994 and provided a more reliable recon struction of its exoskeleton (Afanassieva, 1996).
INTRODUCTION Remains of Osteostraci have been recorded in the Paleozoic beds of Saaremaa Island for more than 150 years. All described Estonian osteostracans belong to the tremataspid group (group Thyestidians after Janvier, 1981, 1985; suborder Tremataspidoidei after Afanassieva, 1991, 2004; order Thyestiida after Jan vier, 1996), which is one of the most thoroughly inves tigated taxa of osteostracan agnathans. The last form from this region described based on macroremains (an almost complete wellpreserved cephalothoracic shield) is a very small osteostracan, Aestiaspis viitaensis Janvier et Lelievre, 1994 referred to the family Trem ataspididae Woodward, 1891. The holotype of Aes tiaspis viitaensis (Pi, no. 7279 = GIT, no. 2471) is an almost complete cephalothoracic shield extracted from enclosing rock by dissolution in 10% formic acid (Janvier and Lelievre, 1994). This preparation technique provided preservation and subsequent examination of fine structural details of the individ ual, primarily its exoskeleton. However, a special study of shield sculpture and histological structure of the exoskeleton of Aestiaspis viitaensis has not been performed. At approximately the same time, the study of the collection of academician F.B. Schmidt, stored in the Borissiak Paleontological Institute of the Russian Academy of Sciences, Moscow (PIN) revealed a spec imen of a small longarmored osteostracan (PIN,
As a result of dissolution of rock samples from a number of Silurian localities of Saaremaa Island, including the type locality (Viita trench), many small exoskeleton fragments of various osteostracans have been obtained. They included specimens with distinc tive sculpture, which we referred more or less confi dently to A. viitaensis (Afanassieva and Märss, 1997). In the diagnosis of this species and genus, Janvier and Lelievre indicated that its exoskeleton sculpture con sists of parallel longitudinal dentin ridges (Janvier and Lelievre, 1994). A detailed study of the exoskeleton surface of the holotype and other specimens assigned to this species showed that, in addition to ridges, there are abundant small tubercles (Afanassieva and Märss, 1997, pl. V). Small tubercles in the exoskeleton of Aes tiaspis are positioned both between ridges and on their 74
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extensions, filling the exoskeleton surface in sites with complex sculpture (depressions, projections). This sculpture (ridges and tubercles) is observed on both fused scales (tesserae) on the dorsal and ventral sides of the cephalothoracic shield and isolated scales on the mobile part of the trunk. Our thorough SEM study of the exoskeleton of specimen PIN, no. 3257/607 revealed the presence of perforated septa in the exosk eleton of this specimen of A. viitaensis, which confirms the assignment of this form to the tremataspidoid group distinguished by this feature. Among small bone fragments from the Upper Sil urian beds of the Severnaya Zemlya Archipelago (a locality on the Ushakov River, October Revolution Island), which were obtained as a result of dissolution of rock samples, a specimen referred to the genus Aestiaspis and probably the same species A. viitaensis has recently been determined and described (Afanassieva, 2000). The specimen was examined under SEM, which allowed the description of its structure and showed the presence of wellpreserved perforated septa (Afanass ieva, 2000, pl. II, fig. 1). A joint project of the Estonian and Russian acade mies of sciences intended for the study of osteostracan vertebrates from the Paleozoic of Saaremaa Island and the Severnaya Zemlya Archipelago allowed us to con tinue the study of various osteostracans, including the genus Aestiaspis. To study the histological structure of their exoskeleton, we prepared thin sections of exosk eleton fragments of the cephalothoracic shield and isolated scales from various localities of Saaremaa Island (using a standard technique, with the Canada balsam). These thin sections were examined under a Nikon eclipse 50I light microscope. When studying histological sections, we revealed that the exoskeleton of Aestiaspis is well developed. It is built of three layers characteristic of this vertebrate group: superficial (dentin) and two bony (spongy mid dle and laminate basal) layers. These layers vary in development in different sites of the shield and tho racic scales. Superficial layer. It is shown that small ridges and tubercles on the surface of the cephalothoracic shield and scales of A. viitaensis are composed of dense den tinlike tissue. The dentin network is poorly devel oped, dentin tubules are short and widely spaced; they pierce the relatively dense tissue of the superficial layer. The lower part of some tubercles contains small cavities, which possibly correspond to pulp cavities. The lower part of tubercles and ridges gradually passes into the middle (bony) part of the exoskeleton. The superficial layer is well developed only in relatively large tubercles and ridges. Thin section of the thoracic scale (Pl. 13, figs. 1, 2) distinctly shows relative posi tions of cavities and canals in the exoskeleton layers (Pl. 13, fig. 3). The network of narrow dentin tubules is well developed in large tubercles (Pl. 13, fig. 4); the tubules repeatedly branch, anastomose (compare with Oeselaspis pustulata (Patten, 1931): Ørvig, 1967, text PALEONTOLOGICAL JOURNAL
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fig. 29; Timanaspis kossovoii Obruchev in Kossovoi and Obruchev, 1962: Afanassieva and Karataj u t e· Talimaa, 2009, textfigs. 3B, 3D, 4A), and reach the tubercle surface. Connections between dentin tubules and cavities of vascular canals inside large tubercles are distinct. The mesodentin layer is particularly strongly developed in the largest tubercles and ridges located on fused scales, extending along ribs of rigidity of the shield. Middle (spongy) layer. This layer is composed of bone tissue, which is rather thin in all specimens under study (Pl. 13, fig. 5). Bone tissue is compact, cavities of bone cells are relatively widely spaced, typical in shape (compare with Tremataspis milleri Patten, 1931 and Timanaspis kossovoii: Afanassieva and Karataj u t e· Talimaa, 2009). This layer is pierced by many canals varying in size and having dense walls. In our opinion, it is possible to divide all canals of this layer into two types, large canals 10–30 µm in diame ter located in the horizontal plane at one level and nar row canals less than 10 µm (usually 2–5 µm) in diam eter positioned more or less vertically and usually ris ing upwards from horizontal canals. Vertical thin sections distinctly show that the foramina of the widest canals coming onto the exoskeleton surface are closed by perforated septa. This arrangement of canals rela tive to perforated septa is evidence that these vascular canals belong to the lower vascular plexus. Thus, we believe that the sensory canals located in tremataspid osteostracans above the perforated septa were located in Aestiaspis in soft tissues between tubercles on the exoskeleton surface. In A. viitaensis, the position of the main sensory canals is marked by large tubercles arranged in line along the canals (compare with Thyestes verrucosus Eichwald, 1854: Afanassieva, 1985). In the holotype, it is possible to recognize posi tions of the infraorbital, postorbital, main lateral, and, probably, transverse sensory lines (Afanassieva and Märss, 1997, textfig. 2b). The horizontal canals of the middle layer of the exoskeleton of Aestiaspis, which in our opinion corre spond to the radiating canals of other osteostracans (including those possessing tesserae), are positioned according to the longitudinal arrangement of ridges and tubercles along the medial body axis rather than strictly radially, as in some other tremataspid osteost racans (Dartmuthia: cephalothoracic shield, dorsal side; Thyestes; Procephalaspis). In particular, in the bony part of the exoskeleton, subparallel canals pass between longitudinal ridges, whereas under large tubercles, the arrangement of canals is closer to a radial pattern (Pl. 13, fig. 7). Thus, the upper vascular plexus is only present in the exoskeleton of Aestiaspis viitaensis in relatively large ridges and tubercles and gives rise to narrow den tin tubules of the mesodentin layer. The lower vascular plexus is well developed under sculptural elements in bone tissues throughout the exoskeleton; it provided trophic processes in exoskeleton tissues.
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0.5 mm
100 µm
10 µm
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Basal layer. The basal layer is observed in all exosk eleton fragments examined in Aestiaspis. It is very well developed, usually rather thick, multilayered, shows a typical laminate structure (Pl. 13, fig. 6), contains large basal cavities, which sometimes give rise to ascending canals. In our opinion, the irregular basal cavities are evidence of resorption, which occurred in hard exoskeleton tissues of osteostracans. Denison (1952) was the first to describe a similar phenomenon in the basal layer of osteostracans, i.e., in Tremataspis mammillata Patten, 1931. When studying the exoskeleton of the holotype under a SEM, we recognized thin ribbing on the sur face of relatively large tubercles of a scalelike unit from the posterolateral margin of the shield (Afanassieva and Märss, 1997, pl. VI, fig. 6). Similar thin ribbing (after Afanassieva, 2004; longitudinal striation after Märss, 2004, 2006, textfig. 11C), which is character istic of this ancient vertebrate group, was also recog nized in other scales of Aestiaspis from various locali ties of Saaremaa Island, which we referred to this spe cies. It should be noted that, in the exoskeleton fragment of Aestiaspis from the Severnaya Zemlya Archipelago, similar ribbing on the tubercle surface has not been recognized in a SEM study (Afanassieva, 2000, pl. II, figs. 1b, 1c). A recent study of another Silurian osteostracan, Timanaspis kossovoii from the Upper Silurian of northern Timan (Afanassieva and Karataj u t e· Tali maa, 2009), has shown significant histological differ ences of this form from typical tremataspidids. The exoskeleton of T. kossovoii is composed of dense bone tissue without typical cavities of bone cells, special mesodentin type, porous fields or perforated septa in the middle layer, polygonal system in exoskeleton tis sues (Pl. 13, fig. 8). These data confirm the point of view of Obruchev (in Kossovoi and Obruchev, 1962) about a unique position of this genus among trematas pidids (subfamily Timanaspidinae Obruchev in Koss ovoi et Obruchev, 1962) and suggest that it should be assigned to a separate family, the Timanaspididae. In contrast to Timanaspis, the osteostracan genus Aes tiaspis shows a set of characters typical for tremataspi doid forms with a long consolidated shield: the pres ence of porous fields, welldeveloped canal system, typical mesodentin, cellular bone tissue and, hence,
belongs to the family Tremataspididae. At the same time, certain characters, such as a very small absolute size, one pair of lateral fields, segmentation of the cephalothoracic shield, convex ventral side of the shield, and special sculpture type, suggest that they should be ranked as a separate subfamily, Aestiaspidi nae Afanassieva, 1996, of the family Tremataspididae. Aestiaspis viitaensis is one of the most ancient osteostracan species, which is recorded in the Lower Silurian, beginning from the Upper Wenlockian Viita Beds of the Rootsiküla Stage. In a brief diagnosis of this species and genus, Janvier and Lelievre (1994, p. 123) characterized it as “a very small and primitive Tremataspididae.” We believe that the exoskeleton of A. viitaensis is a rare example of the transitional state between discrete scales to consolidated shield. The elongated shield of A. viitaensis is consolidated on the dorsal side of the cephalic part. Traces of transverse segmentation are distinct in the postcephalic part of the shield. The segments are represented by rows of fused scales, with a better pronounced segmentation towards the thoracic shield part. The ribs of rigidity of the shield have crests strengthened by groups of rela tively large elongated tubercles (usually one central and two lateral) arranged as scales. The space between them is completely filled with smaller elongated tuber cles, which are also grouped in places as separate scales. These scalelike units mark the positions of fused plates of the shield. At the histological level, this is supported by the arrangement pattern of canals, which is homologous from our point of view to the radial pattern. Thus, the armor structure of Aestiaspis is very interesting and provides information on proba ble phylogenetic development of the exoskeleton in tremataspidids and osteostracans as a whole, confirm ing the statement of Obruchev (1964) that armors of the most ancient vertebrates were formed by fusion of small initial elements. The material described is stored in the Institute of Geology at Tallinn University of Technology (IG TUT), Tallinn (collection nos. GIT 247, 299, 502) and the Borissiak Paleontological Institute of the Russian Academy of Sciences (PIN), Moscow (col lection nos. PIN 1934, 3257).
Explanation of Plate 13 Figs. 1–7. Aestiaspis viitaensis Janvier et Lelievre, 1994; Estonia, Saaremaa Island, Vesiku Brook; Vesiku Beds of the Rootsiküla Stage, Homerian, Upper Wenlockian, Lower Silurian: (1–6) specimen GIT, no. 50274, thoracic scale: (1) dorsal and (2) ventral views; (3–6) specimen GIT, no. 502742, vertical thin section through the middle part of the same scale: (3) general appearance, showing tissues and structures of all exoskeleton layers; (4) superficial layer of the exoskeleton, with branching mesodentin tubules in a large tubercle; (5) middle layer of the exoskeleton, with cavities at the base of some small tubercles and sections of perforated septa; (6) basal layer of the exoskeleton; (7) specimen GIT, no. 50267, horizontal thin section of the thoracic scale, with arrange ment of horizontal canals of the lower vascular plexus. Fig. 8. Timanaspis kossovoii Obruchev in Kossovoi et Obruchev, 1962; specimen PIN, no. 1934/23, vertical thin section of the exoskeleton through a large tubercle, dorsal side of the cephalothoracic shield (after Afanassieva and Karataj u te· Talimaa, 2009, fragment of textfig. 3C); Russia, northern Timan, locality on the Velikaya River; Eptarma Formation, Grebenskoi Stage, Upper Pridolian, Upper Silurian. Scale bars: (1, 2) 0.5 mm; (3, 7, 8) 100 µm; (4, 5) 10 µm; (6) 50 µm. PALEONTOLOGICAL JOURNAL
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ACKNOWLEDGMENTS We are sincerely grateful to G. Baranov (IG TUT), A.V. Mazin, and E.V. Karasev (PIN) for help in taking photographs of small bone fragments and thin sections and to all workers of IG TUT and PIN who contrib uted to our study. The study performed by T. Märss was supported by the Estonian Ministry of Education and Research, project no. SF0140020s08. REFERENCES Afanassieva, O.B., Exoskeleton features of thyestins (Agnatha), Paleontol. Zh., 1985, no. 4, pp. 70–75. Afanassieva, O.B., Tsefalaspidy Sovetskogo Soyuza (Agnatha) (Cephalaspids of the Soviet Union (Agnatha)), Moscow: Nauka, 1991. Afanassieva, O.B., On the morphology and systematic posi tion of the tremataspid osteostracan Aestiaspis viitaensis (Agnatha), Paleontol. Zh., 1996, no. 4, pp. 68–72. Afanassieva, O.B., New osteostracans from the Silurian of Severnaya Zemlya Archipelago (Russia) and some problems relating to the parataxonomy of armored agnathans, Paleontol. J., 2000, vol. 34, suppl. 2, pp. 138–146. Afanassieva, O.B., Osteostracans (Osteostraci), in Iskopae mye pozvonochnye Rossii i sopredel’nykh stran. Beschelyust nye i drevnie ryby (Fossil vertebrates of Russia and adjacent countries: Agnathans and ancient fishes), Novitskaya, L.I. and Afanassieva, O.B., Eds., Moscow: GEOS, 2004a, pp. 210–268. Afanassieva, O.B., Microrelief on the exoskeleton of some early osteostracans (Agnatha): Preliminary analysis of its sig nificance, in The Gross Symposium 2: Advances in Palaeoich thyology, Lukševic s, E., Ed., Riga, Latvia: Univ. Latviensis, 2004b, pp. 14–21. Afanassieva, O.B. and Karataj u t e· Talimaa, V.N., The his tology of the Upper Silurian osteostracan Timanaspis koss ovoii Obruchev (Agnatha) from north Timan, Russia, Acta Zool., 2009, vol. 90, suppl. 1, pp. 38–43.
Afanassieva, O.B. and Märss, T., Exoskeleton structure and distribution of Aestiaspis viitaensis (Agnatha) from the Sil urian of Estonia, Paleontol. Zh., 1997, no. 6, pp. 75–80. Denison, R., Early Devonian fishes from Utah: Part 1. Osteostraci, Fieldiana: Geol., 1952, vol. 11, no. 6, pp. 265– 287. Janvier, P., Norselaspis gracialis n. g., n. sp. et les relations phylogénétiques entre les Kiaeraspidiens (Osteostraci) du Dévonien inférieur du Spitsberg, Palaeovertebrata, 1981, vol. 11, nos. 2/3, pp. 19–131. Janvier, P., Les Céphalaspides du Spitsberg. Anatomie, phy logénie et systématique des Ostéostracés siluro–dévoniens. Révision des Ostéostracés de la Formation de Wood Bay (Dévonian inférieur du Spitsberg), Paris: Cahiers de Paléon tologie CNRS, 1985. Janvier, P., Early Vertebrates: Oxford Monographs on Geology and Geophysics, Oxford: Clarendon Press, 1996. Janvier, P. and Lelievre, H., A new tremataspid osteostra can, Aestiaspis viitaensis n. g., n. sp., from the Silurian of Saaremaa, Estonia, Proc. Eston. Acad. Sci. Geol., 1994, vol. 43, no. 3, pp. 122–128. Kossovoi, L.S. and Obruchev, D.V., On the Lower Devo nian of northern Timan, Dokl. Akad. Nauk SSSR, 1962, vol. 147, no. 5, pp. 1147–1150. Märss, T., Ultrasculpture pattern on the exoskeleton of lower vertebrates, in 10th International Symposium on Early Vertebrates/Lower Vertebrates, Gramado, 24–28th May 2004: Program and Abstracts, Richter, M., Ed., Gramado: Univ. Fed. Rio Grande Sul., 2004, pp. 25–26. Märss, T., Exoskeletal ultrasculpture of early vertebrates, J. Vertebr. Paleontol., 2006, vol. 26, no. 2, pp. 235–252. Obruchev, D.V., Subclass Osteostraci (Cephalaspides), in Osnovy paleontologii. Beschelyustnye, ryby (Fundamentals of Paleontology: Agnathans and Fishes), Moscow: Nauka, 1964, pp. 84–107. Ørvig, T., Histologic studies of placoderms and fossil elas mobranchs: 1. The endoskeleton, with remarks on the hard tissues of lower vertebrates in general, Arkiv Zool., 1967, vol. 2, no. 2, pp. 321–454.
Translated by G. Rautian
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