Reappraisal of the genus Dicroidium Gothan from the Triassic sediments of India

Abstract

The genus Dicroidium Gothan, belonging to Corystospermaceae, is characterised by pinnately compound leaves with proximally forked primary rachis. The genus was earlier included under the genus Thinnfeldia Ettingshausen. Dicroidium is the most consistent macrofloral element in the Triassic strata of Southern Hemisphere. The present reassessment deals with the morphotaxonomy and stratigraphic significance of the species of Dicroidium in India. A critical review of the literature reveals that the specimens of Dicroidium described so far from India require reassessment, because same morphotypes have often been placed under different species names and sometimes dissimilar elements have been assigned to the same species. In view of this, a thorough analysis of Indian Dicroidium was undertaken based on fresh collections along with the species described earlier by previous workers.
 The present reappraisal reveals that the genus in the Triassic of Peninsular India is represented by eight species. These are D. hughesii (Feistmantel) Lele, D. zuberi (Szajnocha) Archangelsky, Dicroidium sp. cf. D. crassinervis (Geintz) Anderson & Anderson, D. townrovii Retallack, D. nidpurensis Bose & Srivastava, D. gopadensis Bose & Srivastava, D. giarensis Pal and Dicroidium sp. In Indian peninsula, appearance of Dicroidium heralds the onset of Triassic sediments belonging to the early Triassic Panchet Formation (Induan). In the younger sequence of Triassic, i.e. during late Triassic Tiki and Parsora formations (Carnian to Norian–Rhaetian), the diversity and species richness of the genus gradually enhanced and Dicroidium foliages became the most predominant component of Triassic macroflora of India. The genus does not occur in strata younger than the Triassic and thereby the total extinction of Dicroidium signifies the end of the Triassic Period.

Mummified Dicroidium (Umkomasiales) leaves and reproductive organs from the Upper Triassic of South Australia

“…The genus becomes a common element of fossil assemblages in Spathian strata (RETALLACK 1977;HOLMES & ASH 1979) and dominates floras in succeeding strata up to theend-Triassic extinction horizon (BALME et al 1995). Dicroidium has its centre of distribution in the middle and high palaeolatitudes of Gondwana, with particularly large collections described from the Middle to Upper Triassic of South Africa (DU TOIT 1927;THOMAS 1933;ANDERSON & ANDERSON 1983), South America (FRENGUELLI 1943(FRENGUELLI , 1944bBORTOLUZZI & BARBERENA 1967;ARCHANGELSKY 1968;PETRIELLA 1981;GUERRA-SOMMER & CAZZULO KLEPZIG 2000), Antarctica (TOWNROW 1967;BOUCHER et al 1993;BOMFLEUR & KERP 2010;ESCAPA et al 2011;BOMFLEUR et al 2012), India (e.g., FEISTMANTEL 1879aPAL et al 2014;RAO & LELE 1962;SEWARD 1932), Australia (e.g., WALKOM 1917, 1925a, 1928JONES & DE JERSEY 1947;TOWNROW 1957;HOLMES & ANDERSON 2005;PATTEMORE 2016), and New Zealand (e.g., ARBER 1913RETALLACK 1980b).…”