First elasmobranch egg capsules from freshwater lake deposits of the Madygen Formation (Middle to Late Triassic, Kyrgyzstan, Central Asia)
by Jan Fischer, Sebastian Voigt & Michael Buchwitz, Freiberg
FISCHER, J., VOIGT, S. & BUCHWITZ, M. (2007): First elasmobranch egg capsules from freshwater lake deposits of the Madygen Formation (Middle to Late Triassic, Kyrgyzstan, Central Asia). - Paläontologie, Stratigraphie, Fazies (15), Freiberger Forschungshefte, C 524: 41-46; Freiberg.
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Field work in 2006 and 2007 in the non-marine Triassic of the Madygen fossil locality at the northern rim of the Turkestan Mountains in southwest Kyrgyzstan, Central Asia, revealed two distinct types of elasmobranch egg capsules (FISCHER et al., 2007). This is the first record of elasmobranch egg capsules from the Mesozoic of Central Asia. The fluvio-lacustrine deposits of the Madygen Formation belong to one of only a few occurrences of continental Triassic beds in Central Asia. During the 1960s Russian palaeobiologists recovered an unusually rich fossil content in the stratum typicum area of the Madygen Formation, including abundant macrophytes, more than 15,000 insect remains and unique small reptiles with well preserved soft tissue (SHAROV, 1970; SHAROV, 1971; VOIGT et al., 2006). According to the number, diversity and preservation of the findings, the locality represents a fossil lagerstaette, which is dated as Middle to Late Triassic (Ladinian-Carnian) in age, based on macrophytes (DOBRUSKINA, 1995). Four decades after the initial Russian work a comprehensive research approach is carried out on the Madygen ecosystem including a detailed litho- and biofacial analysis (VOIGT et al., 2006; VOIGT et al., 2007).
The Madygen Formation is a several-hundred-meter-thick succession of predominantly siliciclastic rocks accumulated in a tectonically induced basin. Deposition has been started on the Palaeozoic basement with local Permo-Triassic molasse sediments. The section consists of mudstones, sandstones, conglomerates and fanglomerates. This wide variety of siliciclastic rocks reflects the complex spatial and temporal pattern of depositional sub-environments including alluvial fans, sandflats, swamps, back-swamp areas, and littoral to profundal lake zones (VOIGT et al., 2007).
Several specimens of each elasmobranch egg type have been found either compressed or as nearly uncompressed body fossils. Specimens of the first egg capsule type (type no. 1) are from the uppermost part of the Madygen Formation in the southwestern Madygen outcrop area (Urochishche Madygen). The fossils come from a massive light grey mudstone presumably representing fluvial input into a shallowing freshwater lake. The second egg capsule type (type no. 2) comes from the northwestern Madygen outcrop area (Urochishche Dzhaylyau-Cho). Here the fossils are preserved in laminated brownish mudstones interpreted as near-shore deposits of a rather deep lake environment.
Palaeoxyris sp. (type no. 1)
The material of egg capsule type no. 1 includes six compressed fragments and two uncompressed fragment. The body (corpus) of this capsule type is broadly fusiform, 30 mm long and 13 mm wide, and gradually tapers towards each end. The slender pedicle is minimum 30 mm long, invariably traversed by parallel arranged ribbing structures. The fragments indicate a maximum total length of the beak (rostellum) of 20 mm. The beak is traversed by roughly parallel arranged ribs. The body is sectioned by six spiral bands averaging 3 mm in width, showing a double fissure line between them. These segments apparently show a rhythmic variation in breadth: broad-broad-narrow-broad-broad-narrow. The surface shows a secondary, characteristic transverse rhomboidal pattern resulted from compression during fossilization. The uncompressed specimens show the original spiral ribbing structure.
Because of size, shape and banding characteristics the specimens of this egg capsule type no. 1 belong clearly to Palaeoxyris BRONGNIART, 1828 (RENAULT & ZEILLER, 1884; CROOKALL, 1930; MÜLLER, 1978), probably to Palaeoxyris regularis BRONGNIART, 1828 and Palaeoxyris münsteri PRESL, 1838 (CROOKALL, 1930; MÜLLER, 1978) which represent the most common Palaeoxyris species in Triassic sediments of Northern, Western and Central Europe.
Egg capsule indet. (type no. 2)
The egg capsule type no. 2 is preserved in three uncompressed and four compressed specimens. The body of this capsule type is cone-like, averaging 34 mm in length and 8 mm in width. Due to compression the flattened fossils show an egg-like outline and a width up to 18 mm. A beak is not visible. A slender pedicle, twisted in spirals, with a length of at least 13 mm and a maximum width of 2 mm, is occasionally preserved. The body is sectioned by two spiral bands averaging 10 mm in width and bearing a broad flange (collerette). Compression of the spiral band produced a rhomboidal surface pattern in flattened specimens. A fine longitudinal striation of the two bands is running parallel to the intersecting collerettes.
One specimen of type no. 2 collected during the 2006 fieldwork season by DMITRI SHCHERBAKOV, Moscow, shows two egg capsules agglutinated by their pointed beaks. This kind of attachment, which is known from a number of fossil egg capsule types (e.g. Palaeoxyris (= Spirangium) ETTINGSHAUSEN, 1852; RENAULT & ZEILLER, 1888; CROOKALL, 1932; WATERLOT, 1934; MÜLLER, 1978), can also be observed in modern oviparous forms like the heterodontid Port Jackson shark (Heterodontus), (MOJETTA, 2004). Egg capsule type no. 2 cannot surely be referred to any known taxon. This unique assembly probably indicates a new taxon combining features of the wide-spread species Palaeoxyris (RENAULT & ZEILLER, 1884; CROOKALL, 1930; MÜLLER, 1978) and Fayolia (RENAULT & ZEILLER, 1888; CROOKALL, 1928; SCHNEIDER & REICHEL, 1989; RÖSSLER & SCHNEIDER, 1997).
Oviparous behaviour in recent elasmobranchs
Embryos of recent oviparous elasmobranchs (sharks and rays) are encased by a tough horny capsule attached to plants or other structures on the surface or on substrates in the course of oviposition (COMPAGNO, 1990; CARRIER et al., 2004; MOJETTA, 2004). After deposition, no further care for the eggs is provided by the parent animals. Thus, the tough horny egg case is the embryo's only protection against predators. Egg cases are preyed by numerous predators, e.g. carnivorous gastropods and sharks, and so their mortality rate is rather high. A few weeks after deposition a small slit opens on each side of the egg capsule for gas exchange. Given that embryos are nourished only by the yolk store of the yolk sac, hatchlings are small sized because of limited nutrients (CARRIER et al., 2004). The whole incubation period of an elasmobranch may need a few months up to more than a year (MOJETTA, 2004). The number of eggs produced by living elasmobranchs averages less than 20 per year (COMPAGNO, 1990).
Oviparity is restricted to three modern shark families (Heterodontidae, Scyliorhinidae, Orectolobidae) and in rays (Rajiformes) (CARRIER et al., 2004), which account circa 43% of all living elasmobranch species (COMPAGNO, 1990). All these forms have a primarily benthic habit (KLAUSEWITZ, 2000) and usually a small body size. The young hatch is fully developed and equals the miniature copy of the adult animal. Oviparity is probably the ancestral type of reproduction in elasmobranchs (COMPAGNO, 1990; CARRIER et al., 2004). The potential for fossilization of the capsules is given by the toughness of the egg cases. In fact, a variety of different fossil egg capsule types referable to a number of distinct parataxa (e.g. Palaeoxyris, Fayolia, Vetacapsula) has already been found (MOYSEY, 1910; CROOKALL, 1930; VAN DER HEIDE, 1943; MÜLLER, 1978; SCHNEIDER & REICHEL, 1989).
The egg capsules described above represent the first evidence of elasmobranch eggs from the Mesozoic of Central Asia. The occurrence of these two egg capsule types within the Madygen Formation implies the presence of at least two different elasmobranch species using these Triassic freshwater environments for spawning. Both species apparently preferred the vegetated shallow water zones along the shoreline of a lake and along stream channels within the adjacent alluvial plain. Hybodont sharks are regarded as the most likely producers of Palaeoxyris (CROOKALL, 1932; BROWN, 1950; ZIDEK, 1976; MÜLLER, 1978). Especially, the small hybodont shark Lissodus, predominantly known from brackish and freshwater environments (FISCHER, 2005; FISCHER & SCHNEIDER, 2007), has been correlated with Palaeoxyris (SCHNEIDER & REICHEL, 1989). Regarding these correlations the Palaeoxyris egg capsule type no. 1 is referred here to a small Lissodus- or Lonchidion-like hybodont shark. For the undetermined egg type no. 2 the producer is still unknown.
The spatial and temporal distribution of the Madygen elasmobranchs, their way of living and their salinity tolerance cannot be inferred from data available yet. The new findings indicate the presence of at least one small Lissodus- or Lonchidion-like hybodont shark in an environment so far assumed as an internally drained upland basin (VOIGT et al., 2006). They question the current hypothesis of basin development and suggest drainage to lowland basins, or even to the Palaeo-Tethys.