Evidence for ammonite survival into the Danian (Paleogene) from the Cerithium Limestone at Stevns Klint, Denmark MARCIN MACHALSKI & CLAUS HEINBERG Machalski, M. & Heinberg, C. 2005–12–31. Evidence for ammonite survival into the Danian (Paleo- gene) from the Cerithium Limestone at Stevns Klint, Denmark. Bulletin of the Geological Society of Denmark, Vol. 52, pp. 97–111. © 2005 by Geological Society of Denmark. ISSN 0011–6297. Two ammonite species, Baculites vertebralis and Hoploscaphites constrictus, are recorded from the Cerithium Limestone (lower Danian, Paleogene) at Stevns Klint, Denmark. Most of the specimens represent internal and external moulds of shell fragments, the sole exception being a complete, well preserved internal mould of H. constrictus. The infilling of the body chamber and phragmocone of this specimen is indistinguishable from the surrounding limestone. The ð13C value for the phragmo- cone fill is 1.382, matching Danian values much better than the more positive Maastrichtian values for the fine-fraction ð13C curve of the Stevns Klint succession. Additionally, the phragmocone infill contains Operculodinella operculata, a calcareous dinocyst typical of the Danian at high latitudes. The specimen is thus interpreted to be an indigenous early Danian ammonite. In view of the mass occur- rence of calcitic bryozoans, brachiopods, and bivalves in the topmost Maastrichtian chalk at Stevns Klint, the total absence of brachiopods and near-absence of bryozoans and calcitic bivalves in the Cerithium Limestone provide evidence against significant redeposition of Maastrichtian fossils into the latter unit. Consequently, all ammonites from the Cerithium Limestone studied are interpreted as early Danian survivors. Key words: Extinction, survival, ammonites, reworking, calcareous dinocysts, stable isotopes, K–Pg boundary, Danian, Maastrichtian, Denmark. Marcin Machalski [[email protected]] Instytut Paleobiologii PAN, ul. Twarda 51/55, PL - 00-818 Warszawa, Poland. Claus Heinberg [[email protected]] Institut for Miljø, Teknologi og Samfund. Roskilde Universitetscenter, Box 260, DK 4000 Roskilde, Denmark. The hypothesis that an asteroid impact caused a mass isotopic data, postulated that some foraminifera pre- extinction at the Cretaceous–Paleogene boundary served in the lowest Danian strata at Brazos, Texas, (Alvarez et al. 1980; Alvarez et al. 1984; Smit 1999) are survivors rather than reworked Maastrichtian revived the old dispute between catastrophists and specimens. gradualists on the nature of biotic turnovers across The problem also concerns the ammonites, which major geological boundaries. However, the record are a text-book example of victims of the end-Creta- of such turnovers is commonly deformed by tapho- ceous crisis. Rare specimens of baculitid and nomic processes, e.g. the piping down of remains of scaphitid ammonites have been known for a long post-boundary biota into pre-boundary sediments by time from the lower Danian Cerithium Limestone at burrowing organisms, or exhumation of pre-bound- Stevns Klint, south of Copenhagen, Denmark ary biota and their redeposition into post-boundary (Rosenkrantz 1924; Rasmussen 1971; Birkelund 1979, sediments. The latter process is often invoked to ex- 1993; Surlyk & Nielsen 1999). Most authors have re- plain the presence of typically pre-boundary biota in garded these to have been derived from the upper- younger sediments. However, after more detailed most Maastrichtian chalk (Rosenkrantz 1924; Ras- investigations, many of the supposedly reworked mussen 1971; Birkelund 1979, 1993). However, Sur- fossils have instead been suggested to be remains of lyk & Nielsen (1999) suggested that the ammonite short-term survivors (‘holdover taxa’ of Hallam & remains in the Cerithium Limestone might well rep- Wignall 1997 or ‘failed survivors’ of Korn et al. 2004). resent short-term Danian survivors, a view subse- For example, Barrera & Keller (1990), using stable- quently challenged by Machalski (2002). Machalski & Heinberg: Evidence for ammonite survival into the Danian (Paleogene) · 97 Fig. 1. Details of the K–Pg (Cretaceous–Paleogene) boundary interval at Stevns Klint. K: Cretaceous. Pg: Paleogene. Hg: hard- ground truncating the crests of the Maastrichtian bryozoan bioherms and the Danian Cerithium Limestone. FC, Fish Clay. Cer. Lst., Cerithium Limestone. Brz.Lst.: bryozoan limestone (modified after Heinberg 1999). All previous discussions on ammonites from the The Cerithium Limestone itself is a hard yellow Cerithium Limestone have suffered from a lack of limestone unit, penetrated by numerous Thalassi- illustrations and descriptions of these fossils in the noides burrows filled with skeletal hash piped down literature. The aim of the present paper is to fill this from the bryozoan limestone (Rasmussen 1971; gap. It presents a description and illustrations of an Bromley 1975; Ekdale & Bromley 1984; Surlyk & ammonite faunule collected in situ from the Ceri- Håkansson 1999). The Cerithium Limestone is a mi- thium Limestone, and discusses this in the context crosparite which may have formed as a direct pre- of reworking and survival hypotheses. cipitate from sea water (Hansen 1990). In terms of the standard foraminiferal biostratigraphy, the Ceri- thium Limestone ranges from the P0 to the P1C zones (Schmitz et al. 1992; Heinberg et al. 2001). However, The Cerithium Limestone the base of the unit is older in southern parts of the Stevns Klint and younger to the north as demonstrat- The Cerithium Limestone (Cerithiumkalk) is the ed in a quantitative biostratigraphic study based on lowermost carbonate unit of Danian age at Stevns planktonic foraminifera (Rasmussen at al. 2005). Klint. Together with the underlying iridium-bearing The Cerithium Limestone is highly fossiliferous Fish Clay (Fiskeler), of earliest Danian age, it forms and therefore important for studying the K–Pg turn- infillings of small basins between the crests of the over (Rosenkrantz 1939, 1960; Heinberg 1999, 2005). uppermost Maastrichtian bryozoan bioherms The bulk of its fauna is composed of originally arag- (Rosenkrantz 1924; Surlyk 1997; Kunzendorf et al. onite-shelled gastropods and bivalves; calcitic bivalve 1990; Surlyk & Håkansson 1999; Hart et al. 2004). The shells and other calcitic fossils, e.g. bryozans, being basins range from less than ten metres to some tens extremely rare (Heinberg 1999, 2005). of metres in width, depending on the direction of section, and the maximum thickness of the Cerithium Limestone is 90 cm. The Cerithium Limestone ba- sins are exposed along the almost entire 12 km long Material cliff at Stevns Klint. The Cerithium Limestone and the crests of the The complex stratigraphy across the K–Pg bounda- Maastrichtian bioherms are truncated by an intra- ry at Stevns Klint was deciphered by Rosenkrantz Danian hardground, overlain by the lower Danian (1924, 1939). Earlier, the name ‘Cerithiumkalk’ (or bryozoan limestone (Fig. 1). Clasts of the Maastrich- ‘Faxoelaget’) was applied to the whole indurated tian chalk and the Cerithium Limestone, phos- zone, including both the Cerithium Limestone sensu phatised or glauconite-coated, are sporadically stricto and the crests of the intervening Maastrich- present in the basal part of the bryozoan limestone tian bioherms (see Gravesen 1979 for a historic re- (Figs 1, 4C; see Rasmussen 1971; Surlyk & Håkans- view of stratigraphic terminology). Thus, it is possi- son 1999; Heinberg 1999). ble that many specimens from older collections, la- 98 · Bulletin of the Geological Society of Denmark belled ‘Cerithiumkalk’ or ‘Faxoelaget’, were actual- preserved to be identified at the specific level, as B. ly collected in situ from the indurated portions of vertebralis Lamarck, 1801 and H. constrictus (J. Sow- the Maastrichtian chalk. Material collected prior to erby, 1817). As no other species of Baculites and Ho- 1924 is accordingly not included in the present study. ploscaphites have so far been found in the uppermost The material studied, eleven specimens in total, is Maastrichtian of Denmark (Birkelund 1993, text-fig. deposited at the Geological Museum, Copenhagen 3), the remaining material is tentatively assigned ei- (abbreviation MGUH). It comprises the following ther to B. vertebralis or to H. constrictus. specimens: Six specimens, including four Baculites and two Hoploscaphites (MGUH 27359–27364), coming from Baculites vertebralis Lamarck, 1801 various locations along the cliff (Højerup, Eskesti, Mandehoved, Holtug). All these specimens are frag- All specimens of Baculites from the Cerithium Lime- mentary; some of them are preserved as part and stone are fragmentary (Figs 2A, B, D–H). The largest counterpart. piece, MGUH 27369, is 16 mm long (Fig. 2E). In their The specimens were collected by the second au- dimensions and proportions all specimens corre- thor, along the three other Baculites individuals not spond to those of Baculites vertebralis, which is the included in the present study, during processing of most abundant ammonite species in the uppermost 64 bulk samples of the Cerithium Limestone from Maastrichtian chalk at Stevns Klint (Birkelund 1993). various locations. These bulk samples totalled more Only specimen MGUH 27369 (Fig. 2E) is well enough than 258 kg in weight (not all samples were weighed). preserved to show the egg-shaped cross-section, di- Occurrence of nine ammonites in this relatively small agnostic of the species (Birkelund 1993). amount of rock suggests that these cephalopods are not so uncommon in the Cerithium Limestone as