Ammonite eggs and ammonitellae from the Kimmeridge Clay Formation (Upper Jurassic) of Dorset, England

Abstract

Eight clusters of small spherical and subspherical objects, some isolated and some associated with shells of perisphinctid ammonites, have been recovered from the Lower and Upper Kimmeridge Clay (Upper Jurassic) of the Dorset coast, England. They have been interpreted as ammonite egg sacs and represent the freshest and best‐preserved examples known so far. Their structures and the ecological framework in which they occur are discussed. The parents are thought to be members of the two eudemic genera Aulacostephanus and Pectinatites that dominate the biostratigraphy of the ammonites in the range of the Kimmeridge Clay in which they occur. Isolated nuclei of ammonitellae have also been recovered.

Ammonoid Habitats and Life History

“…A benthic K-type strategy with few big eggs and a planktic r-type strategy with numerous small eggs. Most ammonoid eggs (Etches et al 2009;Tajika and Wani 2011;Stephen et al 2012;Laptikhovsky et al 2013;Yamada and Wani 2013;De Baets et al 2012, as inferred from the ammonitella, resembled those of the small to smallest eggs of present-day cephalopods. Environmental influences such as warm water and good oxygenation enhanced egg growth (Westermann 1996;Laptikhovsky et al 2013).…”

“…Recently the probable ecology of eggs was discussed by Etches et al (2009), Tajika and Wani (2011), Stephen et al (2012, and Laptikhovsky et al (2013). Etches et al (2009) reported eggs enclosed in egg sacks (see also Lehmann 1966Lehmann , 1976Müller 1969) from the Upper Jurassic (Kimmeridgian) of the UK.…”

“…The ammonitella is usually delimited by the primary (nepionic) constriction (Birkelund 1981;Landman et al 1996b, Klofak et al 1999Etches et al 2009). After that stage, a change in ornamentation and shape occured (Currie 1942(Currie , 1944Burnaby 1966, Lehmann 1966Kulicki 1974Kulicki , 1979Kulicki , 1996Bandel et al 1982;Ward and Bandel 1987;Landman 1987, Landman et al 1996b, 2007aMaeda 1993;Kulicki and Doguzhaeva 1994;Bucher et al 1996;Doguzhaeva 2002;Kulicki et al 2002;Sprey 2002;Korn and Klug 2007;Tanabe et al 2010b).…”

“…The terminology of growth stages used herein is based on ontogenetic stages defined by numerous authors (House 1985;Kant 1975;Kullmann and Scheuch 1970, Westermann 1954Landman 1987;Kullmann 1981;Kant and Kullmann 1988;Landman 1988;Bucher et al 1996;Klug 2001;Korn and Klug 2002Etches et al 2009;Lukeneder et al 2010;De Baets et al 2012Laptikhovsky et al 2013;. The exact definition of the ontogenetic stages is crucial for reconstructing life cycles with coeval changes of habitats.…”

Mature Modifications and Sexual Dimorphism

“…Ammonoid eggs have been reported by several authors (Lehmann 1966;Müller 1969;Zakharov 1969;Maeda 1991;Urlichs 2009;Etches et al 2009;Landman et al 2010;Klug et al 2012). In spite of these findings, there is as yet no report of a discovery of eggs within an ammonoid shell that is free of doubt (De Baets et al 2015b).…”