The Spiti Shale Formation is a package of black shales with minor sandstone intercalations that stretches from northern Pakistan through northern India and Nepal to southern Tibet. These rocks represent outer shelf to upper slope deposits on the southern margin of the Neotethys, and their macroinvertebrate assemblages are dominated by ammonites and belemnites that commonly occur in claystone concretions. Benthic macroinvertebrates such as bivalves, gastropods and brachiopods are restricted to a few widely spaced layers, while large parts of the formation contain no macrobenthos at all. This patchy distribution reflects unsuitable conditions at the sea floor, in particular a lack of oxygen that was partly caused by upwelling of cold, nutrient-rich waters onto the shelf.

More than 240 bivalve specimens were collected from localities across the Spiti Valley, including sites near Chichim, Langza, Kaza, Kibber, Komic and Demul, as well as a single locality near Zangla in the Zanskar Valley. Most fossils come from the lower member of the Spiti Shale Formation, where they weather out of soft shales or occur in thin shell beds. Very few specimens were recovered from the underlying Ferruginous Oolite Formation, a unit of variable lithology containing sandstones, shelly limestones and iron-ooid bearing limestones that hosts a much richer benthic macrofauna, although one that is usually difficult to extract from the well-cemented rocks. The Spiti Shale Formation of the Zanskar Valley proved almost barren of benthic fossils.

Preservation is a central challenge in working with this fauna. With few exceptions, the bivalves are poorly preserved. Apart from some large and comparatively thick-shelled forms such as Pruvostiella, bivalves are well preserved only in concretions, where they occur as composite moulds. Shells in unconsolidated sediment are usually strongly crushed and fragmented by compaction and tectonic stress, which in some cases renders identification difficult. The style of distortion depends on the final burial position of the shells: specimens buried in growth position became dorso-ventrally shortened, while shells reoriented by burrowers or excavated by currents after death became laterally flattened. All intermediate preservational stages occur, and this compactional history has shaped how palaeontologists have interpreted the fauna for more than a century.

Among the most abundant taxa is Palaeonucula cuneiformis, a thick-shelled, elongated-oval protobranch bivalve that dominates the lower member together with Grammatodon (Indogrammatodon) egertonianus and Pruvostiella (P.) hermanni. Early workers including Herbert in 1831, Everest in 1833, Blanford in 1864 and Stoliczka in 1866 regarded the common nuculid as the species figured by J. de C. Sowerby in 1840, a highly variable form occurring from the Bathonian to the Kimmeridgian in the Kachchh Basin. Holdhaus, however, placed the Spiti specimens in two new species, Nucula spitiensis and N. hyomorpha, and he related the diagnostic features of these species to their preservational state, arguing that one was always dorso-ventrally compressed while the other was laterally compressed. Cox in 1940 regarded the two as synonyms, and the present study confirms that the Spiti material belongs to P. cuneiformis, a species widespread in the Ethiopian faunal province from Madagascar to the Arabian Peninsula.

Grammatodon (Indogrammatodon) egertonianus is a large arcoid bivalve with a rhomboidal to trapezoidal outline, much longer than high, and ornamented with radial ribs that are more numerous on the right valve than on the left. The species is closely related to G. (I.) virgatus, which is widespread within the Ethiopian faunal province, but it differs by being more elongated, having fewer radial ribs on both valves, and showing a less pronounced difference in ribbing between right and left valves. Nearly all specimens from the Spiti area are distorted by compaction, with the dorso-ventral axis commonly shortened, suggesting that the individuals are preserved in growth position.

The study also elevates Pruvostiella to generic rank and revises the astartid fauna of the Himalayas. The moderately-sized astartids from the Spiti Shale Formation have been accommodated in several species in the past, including Astarte spitiensis, A. unilateralis, A. hiemalis, A. sowerbyana, A. hermanni and A. scytalis. Because intermediate forms exist between these species and because two to four of these morphotypes repeatedly co-occur within individual horizons, the authors regard these forms as representing a single species, Pruvostiella (P.) hermanni, on ecological grounds. It is highly unlikely that several closely related species with a more or less identical mode of life would repeatedly co-occur within a community. The Kachchh specimens, although closely related, are more than three times the size of the Himalayan forms and are thus unlikely to be conspecific.

Several faunal elements in the Spiti Shale Formation, such as Australobuchia, Malayomaorica and Retroceramus, are also known from the southeastern margin of the Neotethys, including New Zealand and Indonesia. The occurrence of these high-latitude southern taxa at relatively low palaeolatitudes, between 20 and 30 degrees south according to the palaeolatitude calculator of van Hinsbergen and colleagues, can be explained by their restriction to outer shelf and slope environments where, due to upwelling, lower water temperatures prevailed. Isothermal submergence may explain the way polar faunas were able to cross the tropics. The genus Malayomaorica is documented from the Indian Himalayas for the first time in this study.

By contrast, other taxa, in particular Pruvostiella (P.) hermanni and Grammatodon (Indogrammatodon) egertonianus, are closely related but not identical to forms from the Kachchh Rift Basin situated at the western margin of the Indian Craton. This comparison between the poorly known deep shelf benthic bivalve fauna of the Himalayas and the shallow shelf Jurassic fauna of Kachchh is one of the most significant contributions of the present study, offering a rare opportunity to contrast benthic communities across a bathymetric gradient on the same continental margin.

The Spiti Shale Formation spans the time interval between the Callovian and the earliest Cretaceous, and its thickness is difficult to establish, but judging from the sections measured, the formation is at least 110 metres thick. The stratigraphic subdivision of the Jurassic strata of the Indian Himalayas has a long history, beginning with Stoliczka in 1866 and continuing through Diener in 1895, Krishna in 1983, Pathak in 1993 and Pandey and colleagues in 2013. The intensively folded and faulted succession has made it virtually impossible to measure a single continuous section, which nearly a hundred years later led to disputes over the correct lithostratigraphic composition of the formation. The present study follows the recognition of three informal members named the lower, middle and upper member.

In the lower member, the bivalve Bositra is abundant at the base, followed by a low-diversity fauna dominated by Palaeonucula, Pruvostiella and Indogrammatodon. Bositra buchii is a very widespread opportunistic byssate bivalve able to tolerate dysoxic conditions, and it occurs profusely in black silty shales at the base of the lower member at Langza, where it forms near-monospecific assemblages. In the middle member, bivalves are very scarce and usually found in mudrock concretions where they commonly form small clusters. In the upper member, bivalves locally form loosely packed concentrations in which the fossils are poorly preserved, composed of Australobuchia, Retroceramus and Malayomaorica.

The new taxonomic account revises the bivalve fauna recorded by earlier studies wherever possible. Stoliczka described 17 bivalve taxa from the two formations in 1866, of which only 13 remain valid after taxonomic revision. Holdhaus described 37 bivalve taxa from the Indian Himalayas in 1913, of which around 30 remain after sorting out synonymies. The present study describes 24 taxa from the Spiti Shale Formation and six additional ones from the Ferruginous Oolite Formation, providing a revised framework for understanding the benthic fauna that inhabited the mid to outer shelf regions of the southern Neotethys during the Jurassic.

The material has been deposited in the collections of the Bayerische Staatssammlung für Paläontologie und Geologie in Munich, ensuring that the specimens remain available for future research. As palaeoecological work on the Spiti Shale Formation continues, this revised taxonomy will serve as a foundation for reconstructing the structure and dynamics of benthic communities along the southern margin of the Neotethys, a region whose deep shelf faunas remain very poorly known compared with the shallow shelf faunas of the Kachchh Basin and other parts of the Ethiopian faunal province.

Subject of Research: Taxonomic revision of Jurassic bivalve fauna from the Spiti Shale and Ferruginous Oolite formations in the Indian Himalayas

Article Title: Jurassic bivalves from the Spiti area of the Himalayas, northern India

Article References: Fürsich, F. T., Alberti, M., Pandey, D. K., & Ayoub-Hannaa, W. S. (2022). Jurassic bivalves from the Spiti area of the Himalayas, northern India. Zitteliana, 96, 153-178. https://doi.org/10.3897/zitteliana.96.87253

Image Credits: AI Generated

DOI: 10.3897/zitteliana.96.87253

Keywords: Jurassic, bivalves, Himalayas, Spiti, palaeontology, Neotethys, Kachchh, palaeoecology, area, northern, India, scientific research