May 8 2006. I have been reading that the ammonites died out too soon to have died at the end of the Cretaceous defined by the Iridium concentration in the Danish Fish Clay and that the meteorite which hit Mexico was also too soon and merely reworked into that later time-plane (D. Eldred in p. 16-17 of Geoscientist 16 (5) May 2006). A couple of weeks ago I wrote reviews of papers by Marcian Machalski, a Polish Cretaceous ammonite specialist, for a German journal. His conclusion was that a redefinition of the sexually dimorphic subspecies of the ammonite Hoploscaphites constrictus (James Sowerby) produced a final subspecies only known from the shallow-water Grey Chalk in Denmark, some contemporaneous deposits in Poland and the Netherlands and the Cerithium Limestone grading up out of the Danish Fish Clay. This subspecies together with on or more species of the straight ammonite genus Baculites apparently lived after the Mexican impact and also the deposition of iridium plus unusual stable isotopes (M. Machalski, Acta Palaeontologica Polonica v.50 p653-696, 2005). Another of his papers with a Danish geologist Claus Heinberg supported by access to private collections of ammonites previously noted by F. Surlyk and J. M. Neilsen in Bulletin of the Geological Society of Denmark v. 46 p. 115-119 (1999) and appeared in that journal in 2005 (v.52 p.97-111). They make a good case of limestone matrix and microfossils inside the most intact Hoploscaphites being of post Cretaceous earliest Danian age. Since the Cerithium Limestone matrix was similar and graded down via the Fish Clay into both shallow-water and then older deep-water chalk beds, it was concluded that this ammonite was not merely reworked from the Chalk.

Above the Cerithium Limestone is an erosion surface capped by the Bryozoan Limestone and cutting through into the Cretaceous Chalk. The Bryozoan Limestone flints with a paler red-brown colour to the black Chalk flints yielded the probable lower jaw of Hoploscaphites constrictus found as a modern beach pebble by Surlyk and Nielsen (1999). In Poland there is an equivalent occurrence of this calcite part of the ammonites on a early post-Cretaceous (Danian) erosion surface resting on the penultimate and relatively deep-water layer of the Cretaceous Chalk. Neither of these last records of ammonites has to be due to reworking of these calcite plates; but it seems likely when they follow an interval of Chalk erosion this part of the ammonites apparently not preserved with the last Hoploscaphites subspecies. In my view one should use the existing formal Linnean name Pseudostriaptychus portlockli (Sharpe), described from the late Campanian deep-water chalk of Norfolk in England, for these calcite lower jaws of Hoploscaphites contrictus. Perhaps the other post-Cretaceous ammonite Baculites had a similar jaw structure as it did in late Cretaceous specimens reviewed by K. Tanabe and N.H. Landman in 2002 (AbhandlungenGeologischen Bundesanstalt v. 57, p.157-165). The type material is over five million years older than these final undoubted records from the deep-water chalk. But it is useful to have a name for this different type of fossil and to avoid having to write “originally aragonitic shell of” and “calcite lower jaw of” all through the literature on ammonites.


This debate about the last records of ammonites relates to a short period of the geological time-scale although a long one on the time-scale of modern climate cycles and glaciations. Probably it was within one million years five or ten percent of the time between the end of deep-water Chalk deposition and the return of deep-water deposits, with ammonite-like shells related to modern Nautilus (genus Aturoidea vredenburg), at the base of a broadly defined London Clay Formation at Harwich and Walton in Essex.

My previous comments on landslips at Southend omitted the stratigraphy of the local nautiloids discussed in my article in Tertiary Research volume 21, p 39-50 (Leiden 2002). Since that article was written it has become clear that the last record of the Nautilus-like shell Euciphoceras regale (J. Sowerby) does indeed match the sandy MacMurdo Road bed concretions, but that this bed probably lies around 122m up the London Clay in a syncline where the representative of the Wilson Road bed of concretions is at least 130 m above the base and the base of the Claygate Member above that. This type of location and correlation of chance finds is on the same time-scale as the ammonite debate and they are both related to the effect of water depth of habitats and shell preservation. If it is the case that the ammonites were not all killed by the direct effects of the meteorite on plankton etc, then there is no reason why rare specimens could not be found in the London Clay Formation. I have seen what appeared to be a pair of Pseudospinaptychus portlocki plates in a private collection from Butts Cliff in Essex. The specimen needs to be studied morphologically but whatever it was it was local to the London clay level approximating to the base of the cliffs at Southend and being preserved as a pair of plates in a septarian claystone concretion not reworked in from the Chalk in Eocene or modern times. One can collect real Cretaceous ammonites from phosphatic clay and chalky greensand matrices at Southend, and they help to explain why William Smith listed ammonites as index fossils from the London or blue Clay in his pioneer stratigraphic synthesis of England. However, experienced local geologists can solve that kind of distracting problem unlike visitors like William Smith.

Regardless of whether or not this London clay specimen enters the formal fossil record it raises some general issues; not least the need to have a separate formal nomenclature for the separated jaws/operculum of ammonites and nautiloids. One possibility, raised by Zev Lewy in numerous recent papers, is that scaphitid ammonites lost their aragonitic shell and lived on, either as modern naked octopus, or the octopus with a calcite shell (Argonauta). Another possibility is that the giant new and scarce nautili genus Aturoidea developed the ammonite type of jaw when it replaced the ecological position of the extinct ammonites. A third is that ammonites survived as rarely preserved shells.