Cambrian Trilobites from the Parahio and Zanskar Valleys, Indian Himalaya

Abstract

New collections of trilobites from the type section of the Parahio Formation in the Parahio Valley, Spiti, and from the Parahio, Karsha, and Kurgiakh formations in the Zanskar Valley, permit biozonation based on material precisely located within measured stratigraphic sections. Specimens preserved in limestone with mild tectonic deformation clarify the features of several Himalayan taxa known previously only from severely deformed specimens preserved in shale. A total of 75 trilobite taxa from the Cambrian of Spiti and Zanskar can be referred, questionably at least, at the generic level or below, and 61 of these are present in our new collections. This new material is assigned with confidence to 29 existing species, and to 12 new species. Three new genera,Haydenaspis, Bhargavia, andHimalisania, are established; new species includeHaydenaspis parvatya, Prozacanthoides lahiri, Probowmania bhatti, Xingrenaspis parthiva, X. shyamalae, Bhargavia prakritika, Kaotaia prachina, Gunnia smithi, Sudanamonocarina sinindica, Proasaphiscus simoni, Koldinia odelli, andTorifera jelli. Ten additional Himalayan forms are assigned at the generic level only, and another 11 are questionably assigned to genera or species. The zonation proposed includes 6 zones and 3 levels, including theHaydenaspis parvatyalevel, theOryctocephalus indicuslevel, theKaotaia prachinaZone, theParamecephalus defossusZone, theOryctocephalus salteriZone, theIranoleesia buteslevel, theSudanomocarina sinindicaZone, theLejopyge acanthaZone, and theProagnostus bulbusZone. The sections span from the upper part of the informal Stage 4, Series 2 of the Cambrian System, about 511 Ma old, to theProagnostus bulbuszone of the Guzhangian Stage near the top of Series 3, dated at about 501 Ma. This time interval is represented by about 2000 m of section, which is thick compared to similar intervals elsewhere and is consistent with high rates of sedimentation along the Himalayan margin at the time. The fauna resembles others from equatorial peri-Gondwanaland, with closest similarity to that of South China. It also bears strong affinity to the North China fauna. Juvenile trilobites are described for the first time from India. A new Chinese species,Monanocephalus liquani, is also described.

Detailed trilobite biostratigraphy across the proposed GSSP for Stage 5 (“Middle Cambrian” boundary) at the Wuliu-Zengjiayan section, Guizhou, China

“…If this shape of the glabella is reliable in the tectonically distorted specimens, then S. himalaica is a different species. Peng et al (2009) Lu et al, 1974a. If the specimens preserved in limestone illustrated by Peng • E -cranidium, covered with calcite layer, GK B3 0187, FZX35.…”

“…Other specimens with more upturned anterior borders have furrows that are very shallow. Peng et al (2009) synonymized X. xingrenensis with X. hoboi, however the two species differ. Based on the illustration of the type material in Zhang & Jell (1987, pl.…”

“…Xingrenaspis shyamalae and X. parthiva, both named by Peng et al (2009), are two closely related species, but are not Xingrenaspis based on the pygidia having a short bluntly rounded axis with only two segments, stronger interpleural furrows [similar to Alokistocaridae as revised by Sundberg 1999(= Ehmaniellidae Sundberg, 1994], and narrow margins and matching narrow doublure.…”

Paleobiogeography, Paleoecology and Paleoenvironmental Significance of the Cambrian Trilobites from the Zanskar Region (Zanskar-Spiti-Kinnaur Basin), Northwest Himalaya

“…The overlying Karsha Formation includes the Thidsi and Teta members (Myrow et al 2006). The top part of the Karsha Formation has yielded well preserved agnostids and polymerids trilobites of Lejopyge laeivigata and Proagnostus bulbus zones of the latest Middle Cambrian Hughes et al 2008) and Lejopyge acantha and Proagnostus bulbus zones of the latest Middle Cambrian (Peng et al 2009). The Karsha Formation is interpreted to have been deposited under shallow water carbonate platform condition Myrow et al 2006).…”

“…The shale (Surichun member) and sandstone (Kuru member) units of the Kurgiakh Formation overlie the Karsha Formation. The latest Middle Cambrian trilobites have been reported from the basal part of the Surichun member (Whittington, 1986;Jell and Hughes, 1997;Hughes and Jell, 1999;Peng et al 2009). The top part of the Kurgiakh Formation is truncated by the regional angular unconformity (Cambro-Ordovician) and unconformably overlain by the sandstone-conglomeratic successions of Thango (Thaple) Formation dated as early Middle Ordovician.…”