Warm-water Dasycladaceae algae from the Late Ordovician of the Parahio Valley, Spiti, India

Warm-water Dasycladaceae algae Mastopora and Cyclocrinites were for the first time recorded from the Takche Formation (Upper Ordovician–lower Silurian), Parahio Valley, Spiti, India. They are preserved as external and internal moulds of the non-globular or possibly bulb-like cortical skeleton showing flattened thalli with a high degree of compaction. The occurrence of abundant cyclocrinitid remains in the Takche Formation indicates that the Spiti region of the northwestern Himalaya must have been located at about 30° palaeolatitude during the Late Ordovician and early Silurian. The cyclocrinitids were warm-water algae and their extinction at the end of the Ordovician is related to cooling and glaciations. The cyclocrinitids in the Ordovician are known from several localities in central and southern Asia, including Kazakhstan and western China.


GEOLOGICAL SETTING AND LITHOSTRATIGRAPHY
Rocks of Neoproterozoic to Early Cretaceous age are exposed in the Spiti region in the state of Himachal Pradesh (India) (Hayden 1904;Bhargava & Bassi 1998). In the northwest direction these strata extend further to the Baralacha La (Lahaul Valley) and to the Zanskar Valley (Jammu and Kashmir), to the southeast they are known from the Kinnaur and Kumaun regions (Bhargava & Bassi 1998). In the Spiti region the lower Palaeozoic successions are exposed in the Pin and Parahio valleys in the southeastern part of Spiti (Fig. 1A).
The lower Palaeozoic succession of the Spiti region has been known since the late nineteenth century when F. Stoliczka and C. L. Griesbach visited the area (Stoliczka 1866; Griesbach 1891). Later on, Hayden (1904) provided the first detailed descriptions of the Palaeozoic-Mesozoic succession. Reed (1912) provided the palaeontological record of the Hayden (1904) collection. Srikantia (1977Srikantia ( , 1981 and Srikantia et al. (1977) proposed the term 'Takche Formation' for the succession above the Thango Formation (Early-Middle Ordovician) and below the Muth Quartzite (Devonian). Goel & Nair (1977) classified the Ordovician-Silurian succession in the Spiti region as Shian Quartzite, Pin Limestone, Thannam limestone, Unnamed quartzite transitional and Unnamed siliceous and flaggy limestone. Bhargava et al. (1984) and Bhargava & Bassi (1998) grouped the Thango and Takche formations in the Sanugba Group (Fig. 1B). Suttner (2007) proposed the term 'Pin Formation' for the Takche Formation of Srikantia (1974Srikantia ( , 1977Srikantia ( , 1981 and Pin Limestone of Goel & Nair (1977); however, Bhargava (2008Bhargava ( , 2011 contested the name Pin Formation and preferred Takche because of easy accessibility of the Takche type section, welldefined lithostratigraphy with the top and bottom and its mappability from Zanskar to Kinnaur. Srikantia & Bhargava (2018) stated that the term 'Pin Formation' (Goel & Nair 1977;Suttner 2007) represents only a part of the Ordovician-Silurian siliciclastic carbonate sequence and is neither representative nor mappable. Hence they rejected the term 'Pin Formation' and upheld the status of the Takche Formation. The terms 'Takche Formation' and 'Pin Formation' are widely used in the literature (Srikantia 1974(Srikantia , 1977(Srikantia , 1981Goel & Nair 1977;Srikantia et al. 1977;Sinha 1989;Bhargava & Bassi 1998;Negi 1998;Suttner et al. 2005 The Takche Formation (Upper Ordovician-lower Silurian) has gradational contact with the underlying Thango (Shian Quartzite) Formation (?Early Ordovician) and is disconformably overlain by the ?Devonian Muth Quartzite (Bhargava & Bassi 1998;Draganits et al. 2001Draganits et al. , 2002Bhargava 2008Bhargava , 2011. The specimens of dasycladacean cyclocrinitid algae Mastopora and Cyclocrinites described herein were recovered from the Takche Formation exposed at the Gechang locality in the Parahio Valley (Fig. 1A). They occur abundantly in the lower part of the Takche Formation and particularly in the calcareous siltstone and sandstone units (Figs 2, 3).

GECHANG SECTION (PARAHIO VALLEY)
The Takche Formation is exposed along the Parahio River, but the exposure on the left bank of the Parahio River is more accessible near the Gechang village (Figs 1A, 2). The Gechang section (N 32°2′40.4″, E 077°59′31.11″) lies about 7 km WNW of the Sagnam and Ka Dogri villages on the left bank of the Parahio River in the Parahio Valley (Spiti). We measured ~226 m of the Takche Formation at the Gechang locality. It comprises fine to coarse sandstones, siltstones, argillaceous limestone, nodular limestone and fine to coarse crinoidal grainstone and marl (Fig. 3)

SYSTEMATIC DESCRIPTIONS
Order DASYCLADALES Pascher, 1931 Family CYCLOCRINACEA Maslov, 1956 Remarks. Nitecki (1970) regarded Mastopora as a synonym of Cyclocrinites (as are Nidulites Salter, Pasceolus Billings, Cerionites Meek & Worthen and Lunulites Owen), but due to the absence of 'covering plates' or 'membrane' and branching of laterals in Mastopora (Eichwald 1840;Maslov 1956;Korde 1963), we consider here Mastopora and Cyclocrinites as separate genera. These two genera also differ in the size of the thallus and in the degree of calcification (Maslov 1956;Korde 1963).   Genus Mastopora Eichwald, 1840 Mastopora sp. Figure 4 Description. Our material contains partially preserved external and internal moulds of the non-globular, possibly bulb-like cortical skeleton. No trace of the original skeletal material was found. The external surface of the original skeleton is preserved by the dark, finegrained mud which was moulded around it. The width of our partially preserved specimens is from 1.7 to 2.8 cm. The body is covered by relatively large polygonal, usually hexagonal cup-shaped facets. The exterior of each facet is moderately concave and marked off at the surface by sharp rims. The rims are 0.2 to 0.3 mm thick. The diameter of facets varies from 1.0 to 1.2 mm. The average diameter of the facets is 1 mm. The facets are on average 0.2 mm deep.
Remarks. The described specimens resemble most closely Mastopora concava Eichwald 1860 (p. 84, fig. VI 7) by the size and shape of facets. However, the studied specimens are too fragmentarily preserved to be assigned to any Mastopora species with certainty.
Genus Cyclocrinites Eichwald, 1840 Cyclocrinites sp. Figure 5 Description. Our material contains partially preserved external moulds of the globular cortical skeleton. The size and exact shape of the complete thallus are not known. No trace of the original skeletal material was found. The external surface of the original skeleton is preserved by the dark, fine-grained mud, which was moulded around it. The width of our specimens is from 0.9 to 1.1 cm. The body is covered by small polygonal cup-shaped facets having four to six sides. The exterior of each facet is deeply concave. The facets are surrounded at the surface by well-developed rims which are 0.05 to 0.1 mm thick. The diameter of facets varies from 1.1 to 1.2 mm, with the average diameter being 1.1 mm. The facets are about 0.2 mm deep.
Remarks. The described specimens resemble most closely Cyclocrinites mickwitzi Stolley 1896 (p. 49, fig. II 1-3) by the size and shape of facets. However, the studied specimens are too fragmentarily preserved and cannot be assigned to any Cyclocrinites species with certainty.

SIGNIFICANCE OF CYCLOCRINITIDS IN THE TAKCHE FORMATION
The cyclocrinitids (Middle Ordovician to early Silurian) are a small group of macrofossils and are usually regarded as an extinct tribe of dasycladacean algae (Beadle 1988). The tribe Cyclocriniteae was named by Pia (1920) and emended by Bassoullet et al. (1977). It includes Cyclocrinites Eichwald, Mastopora Eichwald, Coelosphaeridium Roemer and Apidium Stolley. Nitecki (1970) suggested that the cyclocrinitids are problematic algae related to receptaculitids. The cyclocrinitids preserved in the Takche Formation have flattened thalli and show a high degree of compaction. The cyclocrinitids in the Ordovician are known from several localities in central and southern Asia including Kazakhstan (Gnilovskaya 1972) and western China (Mu 1982a(Mu , 1982b. The oldest cyclocrinitids have been reported from the lower part of the Middle Ordovician of California (Nitecki 1970) as Cyclocrinites weller Nitecki. They were most abundant and diverse during the Caradoc, became less common in the Ashgill and declined throughout the Llandovery (Beadle & Johnson 1986) becoming extinct by the end of the Llandovery (Silurian). The decline and end of these warm-water algae in the Ordovician may correlate with the end Ordovician cooling and glaciations (Beadle 1988). The cyclocrinitids occur in normal marine waters to highly saline restricted water settings (Johnson & Campbell 1980). The palaeogeographic reconstruction shows that most and perhaps all cyclocrinitids lived within 30° of the palaeoequator (Beadle & Johnson 1986;Beadle 1988). They probably lived on soft substrate by attaching themselves to small solid objects; living dasycladaceans are often found on soft bottoms attached to pebbles, shells and coral fragments. Cyclocrinitid thalli were relatively fragile and largely restricted to quietwater environments, either below the wave base or in protected lagoons (Nitecki 1970;Beadle & Johnson 1986). The large accumulation of cyclocrinitids in the Takche Formation may be formed during a storm event which generated currents that penetrated into normally quiet water and swept thalli together (cf. Nitecki & Johnson 1978). The cyclocrinitids were most common at relatively shallow depths, below the wave base but within the photic zone. The comparison with living dasycladaceans suggests that they lived at a depth of less than 100 m (Beadle & Johnson 1986). The cyclocrinitids appear to be reliable indicators of low palaeolatitudes. The occurrence of abundant cyclocrinitid remains in the Takche Formation indicates that the Spiti region of the northwestern Himalaya must have been located in low palaeolatitudes during the Late Ordovician and early Silurian. The cyclocrinitids were warm-water algae and their extinction at the end of the Ordovician is related to the cooling and glaciations (Beadle 1988). They inhabited seas in low latitudes within 30º of the palaeoequator (Beadle & Johnson 1986;Beadle 1988). Ordovician cyclocrinitids occur at several localities in central and southern Asia, including eastern Kazakhstan (Gnilovskaya 1972) and western China (Mu 1982a(Mu , 1982b.