RECORD OF TITHOPELTOCERAS ARKELL (AMMONOIDEA) FROM THE LATE TITHONIAN OF KUTCH, INDIA: ITS STRATIGRAPHIC AND PALEOBIOGEOGRAPHIC SIGNIFICANCE

Abstract

in Kutch took place onan epicontinental platform which experienced many trans-gressive-regressive phases and became progressively shallow(Biswas, 1991). As a result, marine fossils, like ammonites andother stenohaline groups, are not ubiquitous and uniformly dis-tributed throughout the succession. From the Late Jurassic theKutch basin witnessed major sea-level fluctuations and during theTithonian the effects of transgressive-regressive couplets weremore frequently registered. The record of ammonites in manyareas, especially in the Indo–Madagascar Faunal Province, in-cluding Kutch (personal observation), are mainly associated withthe transgressive events (Riccardi, 1991 and references therein).In Kutch, the spatiotemporal distribution of the ammonite-bearingbeds is discrete in nature and these are not always regionallypersistent. Many previous works based on the study of museumcollections (e.g., Spath, 1927–1933) were plagued with strati-graphic imprecision and inadequate knowledge of the ammonite-rich localities, and generally lacked accurate lithologic descriptionof any section. This led to poor resolution of the regional bio-stratigraphy based on Tithonian ammonites.Major contributions to Tithonian paleobiogeography and revi-sions of faunas of different provinces have been recently carriedout (Fatmi, 1972; Leanza and Olo´riz, 1987; Riccardi, 1991; Enayand Cariou, 1997; Cecca, 1999; Olo´riz et al., 2000 and referencestherein). A reexamination of the Kutch fauna, the construction ofits regional chronostratigraphic standard, and its correlation withthose of the rest of the world are thus a matter of urgency. Manygenera described by Spath (1927–1933) from Kutch are cosmo-politan, long-ranging, and are inadequately understood in termsof modern taxonomy (see Cecca, 1999). For example, Virgato-sphinctes Uhlig, 1910, especially the densiplicatus group of spe-cies, ranges throughout the entire Late Tithonian and is found inboth intra- and interprovinces (Lemoine, 1911; Spath, 1927–1933;Fatmi, 1972; Verma and Westermann, 1973; Thomson, 1979,1983; Howlett, 1989; Fatmi and Zeiss, 1994). MicracanthocerasSpath, 1925, although confined to the Upper Tithonian in manyareas (Mexico–Imlay and Jones, 1970; Argentina–Leanza, 1945;Sicily–Warman and Arkell, 1954; Madagascar–Collignon, 1960;and Spain–Wiedmann, 1968; Olo´riz, 1976–1978; Tavera, 1985),has a stratigraphic position in Kutch whose precision is not wellunderstood. In many areas the Upper Tithonian rocks have beensuccessfully subdivided into several biostratigraphic zones on thebasis of many time-diagnostic genera or species and it is possibleto establish interprovincial correlation. For example, Cecca (1999)made correlations between the Mediterranean Tethys and East Af-rica; Riccardi (1991) did the same for the southeast Pacific andTethys; and Fatmi and Zeiss (1994) made correlations betweenthe Himalayas and western Tethys. In the majority of the casesthey were able to demarcate the Jurassic–Cretaceous Systemboundary, since the earliest Cretaceous marine beds containingammonite were also present. Olo´riz and Tavera (1989) correlatedMediterranean ammonites with the traditional Jurassic–Creta-ceous boundary. Unfortunately, in Kutch the Berriasian is markedby extreme shallowing due to global regression coupled with re-gional tectonism (Biswas, 1977), and consequently it lacks am-monites, except the single report by Krishna (1991).Our recent field investigations have yielded additional collec-tions of all previously described ammonite genera, as well asmuch new systematically collected Tithonian material with pre-cise stratigraphic and locality information (detailed taxonomic andbiostratigraphic studies in preparation). The collection includes asingle specimen of the rare genus Tithopeltoceras Arkell, 1953,which is described herein as Tithopeltoceras lakhaparensis newspecies. Previously Tithopeltoceras was recorded only from avery latitudinally narrow belt of the Mediterranean Province (Ar-kell, 1956; Arkell et al., 1957), where it is restricted to the Sub-betic Zone of the uppermost Tithonian (Enay, 1973; Olo´riz andTavera, 1982). This represents the first discovery of Tithopelto-ceras outside the Mediterranean Province. The significance of therestricted spatiotemporal distribution of Tithopeltoceras is high-lighted here. The possible existence of a migration route betweenKutch and the pelagic Mediterranean Province is examined andnew light is shed on the Jurassic–Cretaceous boundary in Kutch.