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 Introduction

 

"brighter than glass, and yet, as glass is, brittle"
(Shakespeare, as cited by John Gwynn Jeffreys, 1869) 

 

A living specimen of the pteropod Cavolinia tridentata (Niebuhr, 1777), photographed by scuba

diving near the Bahamas by Ronald W. Gilmer, c. 3 times enlarged, showing its transparant shell (centre)

with pointed protoconch, the two parapodia or 'wings' (top) and two lateral mantle appendages, penetrating

through slits in the shell. Copied from Lalli & Gilmer (1989).

 

General

Organisms living their entire life in the sea, floating in the water column between bottom and surface, are called holoplanktonic. They belong to a wide variety of plant and animal taxa, as for instance dinoflagellates, nannoplankton, Foraminifera, various types of crustaceans, Coelenterata, and Mollusca.

In the molluscan phylum the holoplanktonic species do not form a group on their own, but representatives of various taxonomic groups independently developed a holoplanktonic way of life. The most important among them are the so-called 'Pteropoda', an obsolete, but quite practical name for what is officially indicated as Mollusca, Thecosomata. Also the many species of the Heteropoda, nowadays better indicated as Pterotracheoidea, are holoplanktonic. Finally, a restricted number of species within the large group of the Nudibranchia have the same way of life.

Within these molluscan groups not all species have shells, several among them are so-called 'naked' species, although most of them (if not all) have a very small embryonic or larval shell during the first stages of ontogeny. In the 'naked' species this shell is shed shortly after hatching. In the fossil record, of course, only the shelled species, or the very small protoconchs of the shell-less species, can be expected.

Heteropoda are the oldest group, known since the Jurassic. The Pteropoda are considerably younger, and only known from the Cainozoic. The oldest known pteropod species, Heliconoides mercinensis, is from Late Paleocene deposits in Europe and the United States.

The purpose of this website is predominantly to give specialized information and literature references on Cainozoic holoplanktonic Mollusca. Publications on this subject appear with irregular intervals, but quite a lot of information is as yet unpublished and can be made available in this way. Therefore these pages are constantly under construction.


Biostratigraphical application

Application of holoplanktonic Mollusca in biostratigraphy has been effectuated for the North Sea (Janssen & King, 1988) and Aquitaine basins (Cahuzac & Janssen, 2010), but the potential of this fossil group has not yet been fully exploited. Gürs & Janssen (2004.) described biostratigraphically useful pteropod occurrences during the North Sea Basin's Oligocene (Rupelian) as 'sea-level related molluscan plankton events'. Gürs & Janssen (2002) refined the pteropod zonation for the later Miocene in the North Sea Basin. Cahuzac & Janssen (2010) correlated the Ypresian to Serravallian time interval in the Aquitaine Basin with the zonation scheme for the North Sea Basin. Within the Mediterranean area a pteropod zonation for the Miocene-Pleistocene is in print.

 

Long distance correlation

Possibilities for long distance correlation of stratigraphical units were indicated by Janssen (1990, 1995). First order correlations have been effectuated, e.g. between the Mediterranean area, Japan and Australia (based on species such as Heliconoides tertiaria and Vaginella depressa), between the Mediterranean (northern Italy, Sicily, Malta) and the Caribbean (by means of Sphaerocina formai) and between New Zealand, Japan, the Mediterranean and most probably the Caribbean (by Cavolinia grandis).


"Pteropod power",  an example from
Malta

The Maltese Oligo-Miocene (Chattian to Langhian) so-called 'Globigerina Limestone Formation' is divided into three sub-units, Lower, Middle and Upper Globigerina Limestone members, respectively. On the boundaries of the lower and middle, and of the middle and upper subunits hardgrounds are present, followed by a basal deposit with phosphoritic concretions, indicated C 1 and C 2. Furthermore usually less obvious phosphorite concentrations are seen in various levels of the Globigerina limestones. The thicknesses of the subunits are variable, as a result of synsedimentary tectonics and winnowing. In the easternmost part of the island of Gozo, near Qala and Daghlet Qorrot, the Middle Globigerina Limestone Member is reduced in thickness, even to such an extend that C 2 is found directly on top of C 1. Aragonite in these deposits is dissolved and most fossils are exclusively found as internal phosphoritic moulds. Calcitic fossils, like pectinids, ostreids and echinoderms are usually found in shell preservation.

In February 2004 I happened to find a temporary construction pit for buildings in my residence Xewkija, on the island of Gozo. Several meters of Globigerina Limestone were exposed, containing a well-developed phosphorite level. I decided to sample this level for pteropods, and collected 10 kg of sediment from the phosphorite bed. The sample location is indicated with X on the map.

 

 

Already during sampling abundant specimens of Gamopleura melitensis were seen, a species only known to occur around and below the C 1 main phosphorite level, of Chattian (Late Oligocene) age. This is how the outcrop looked:

 

 

After formic acid treatment of the sample the residue was dried and fractionated. The fractions >5 mm indeed contained numerous specimens of Gamopleura melitensis, acknowledging the initial idea that this was phosphorite level C 1, but surprisingly also a number of other species was present, only known from much higher (Langhian) levels in the Globigerina Limestone, viz. Cuvierina paronai, Cavolinia cookei, Diacrolinia aurita, Vaginella acutissima and V. austriaca.

 

 

A closer look at the above photograph shows that the concretion level in fact is composed of two levels: a darker brown lower and a greyish upper one, separated by a very thin, light coloured limestone level. Fortunately the construction pit was still accessible when this was found out, and the two levels could be sampled separately on February 26, 2004.

 

 

The lower sample yielded exclusively pteropod species know from around the C 1 level, whereas the upper one contained species known from C 2 and highter levels, as seen in the next table:

 

 

The conclusion seemed to be obvious: this is a case of C 2 directly overlying C 1, a situation as yet only known from the easternmost part of Gozo. But even this is not true: the species Cavolinia cookei does not yet occur in C 2, and Vaginella acutissima (very common in the upper sample) is rare in C 2 and becomes only abundant in a higher level of the Upper Globigerina. Therefore we must conclude that this is a very exceptional section, in which the entire Middle Globigerina and the lowermost part of the Upper Globigerina members are missing.