Late Quaternary environmental changes recorded in the Danish marine molluscan faunas

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Danish sites with marine sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 The Late Pleistocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 The Holocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 The recent fauna of shell-bearing molluscs compared to the subfossil fauna . . . . . . . . . . . 12 Molluscan finds within the seven regions during the Holocene . . . . . . . . . . . . . . . . . . . . . 15 The Bælt Sea area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 The Baltic area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 The Kattegat area with fjords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 The Limfjord area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 The North Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 The Vendsyssel area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 The Skagen Well area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 The Danish Late Quaternary marine molluscs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Class Polyplacophora . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Order Neoloricata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Class Gastropoda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Subclass Prosobranchia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Order Archaeogastropoda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Order Mesogastropoda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Order Heterogastropoda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Order Neogastropoda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Subclass Heterobranchia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Order Heterostropha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Subclass Opisthobranchia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Order Bullomorpha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Order Anaspidea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Order Thecosomata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Order Gymnosomata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Subclass Pulmonata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Order Basommatophora . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Class Scaphopoda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Order Siphonodentalioida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Order Dentalioida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Class Bivalvia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Subclass Palaeotaxodonta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Order Nuculoida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Subclass Pteriomorphia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Order Arcoida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Order Mytiloida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Order Pteroida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Subclass Heterodonta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Order Veneroida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Order Myoida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Subclass Anomalodesmata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Order Pholadomyoida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 GEUS Bulletin no 3.pmd 28-06-2004, 08:45 3 4 The Skagen Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 The Skagen Well – perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 The pre-Late Quaternary deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 The Late Pleistocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Eemian deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 The Early/Middle Weichselian, marine and glacigene deposits . . . . . . . . . . . . . . . . . . 100 The Late Weichselian marine and glacigene deposits . . . . . . . . . . . . . . . . . . . . . . . . . 101 The Holocene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 The Preboreal–Boreal 10 000 – 8000 C years B.P. . . . . . . . . . . . . . . . . . . . . . . . . . . 103 The Atlantic 8000–5000 C years B.P. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 The Subboreal 5000–2500 C years B.P. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 The Subatlantic 2500– C years B.P. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 The older Subatlantic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 The younger Subatlantic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Conclusive remarks on the Skagen Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 The environmental changes through time in the seven sectors based on the molluscan records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Eemian species sorted after climatic affinities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 The Bælt Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 The Baltic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 The Kattegat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 The North Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Vendsyssel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 Skagen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Early/Middle Weichselian species sorted after climatic affinities . . . . . . . . . . . . . . . . . . . . 126 The Kattegat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Vendsyssel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Skagen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Late Weichselian species sorted after climatic affinities . . . . . . . . . . . . . . . . . . . . . . . . . 130 Vendsyssel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 Skagen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Holocene species sorted after climatic affinities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 The Bælt Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 The Baltic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 The Kattegat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 The Limfjord . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 The North Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Vendsyssel . . . . . . . . . . . . . . . . . . . . .

Late Quaternary, marine deposits in Denmark have yielded 247 subfossil species of molluscs. The sites are presented, and comparisons are made between the subfossil mollusc assemblages and the 278 shell-bearing mollusc species presently living in the Danish seas. 184 species are common to the two groups. The 63 species no longer occurring around Denmark are used as indicators of changing environmental conditions, including temperature, salinity and depth, throughout the last 130 000 years. Seven modern faunal regional units are defined and considered: the Baelt, the Baltic, the Kattegat, the Limfjord, the North Sea and the Vendsyssel regions, and the Skagen area based on the Skagen III Well DGU File No. 1.287. The Late Quaternary, marine, shell-bearing molluscs, comprising 341 subfossil and recent species, are characterised from the point of view of climatic (i.e. Arctic, Subarctic, Boreal and Lusitanian) affinities and animal-sediment relationships. On this background the faunal and environmental evolution recorded in the 217 m long Skagen Well core is analysed and described. The mollusc assemblages in the Skagen sequence indicate a deeper-water facies during the Eemian, the Weichselian and the older Holocene in contrast to what hitherto was known in other parts of the Danish area during the Late Quaternary. For the Skagen Well the chronozones Preboreal/Boreal, Atlantic, Subboreal and Subatlantic can be identified by 14 C dating. The environmental changes within the seven regions through the Late Quaternary are evaluated by depicting the molluscan communities encountered in the seven Late Quaternary stages together with remarks on studies of the neighbouring areas. By following the marine communities through the Late Quaternary in the light of the classical bottom communities sensu C.G.J. Petersen, it is demonstrated how facies have changed both through time and space within the Danish marine realm. The wellestablished, more temperate Eemian marine fauna was closely associated with shallow-water environments. The inferred climatic changes reflect an interglacial-glacial cycle. However, the climatically induced changes during the Holocene in the marine environment were small and overshadowed by the facies changes. Out of the 341 species recorded in this study, 140 occur in the Eemian, 36 in the Early/Middle Weichselian and 41 in the Late Weichselian. The Holocene fauna is represented by 183 species of shell-bearing molluscs, of which the first recorded occurrence of 148 species has been radiocarbon-dated.

Introduction
In the middle of 19th century, Denmark had its first 'Geology of Denmark' published by G. Forchhammer, in 1835. However, as Forchhammer expressed it in 1851 when making some notes on the work by the malacologist O. A.L. Mørch (1828A.L. Mørch ( -1878 at the Mineralogical Museum of Copenhagen. It has hitherto been enough for the geognost to establish formations using the characteristic fossils, but in the future we have to give a closer description from a zoological point of view (Petersen 1997, p. 5). Considering only the younger deposits, the efforts of the zoologist in geological works are highly significant and became important already in the 19th century. C.G.J. Petersen (1860Petersen ( -1928 is an outstanding example of such an influence with his work on the extent of shell-bearing molluscs in the Danish seas inside the Skagen (Petersen 1888, 1893). Here he points to the faunal conditions also in the Pleistocene and Holocene marine deposits compared to the recent distribution. In the description accompanying the geological map sheets of Vendsyssel (Jessen 1899), Jessen gives full credit to C.G.J. Petersen andA. Jensen (1866-1953) for their studies on the molluscan species recorded from that part of the country. Later both Petersen and Jensen contributed further to our knowledge of the marine molluscan fauna. Petersen formed the concept of the bottom communities (Petersen & Jensen 1911;Petersen 1913Petersen , 1914Petersen , 1915Petersen , 1918) that has been the tool for further work, not only within the Danish waters but all over the world with the so-called parallel bottom communities (Thorson 1957). Though the concept of parallel molluscan communities in the sense of Thorson (1957) has been considerably modified in the last 30 years (Erwin 1983), there remains a recognition that particular molluscan assemblages are associated with various types of habitat. In 1899 the zoologist V. Nordmann  was engaged by the Geological Survey of Denmark to study the molluscs from the Quaternary deposits. Part of this work was already reflected in the next geological map sheet covering the southern part of Vendsyssel (Jessen 1905).
Here Nordmann has identified the molluscs and given the faunal remarks on the Holocene marine fauna in the north-eastern part of the Limfjord (Fig. 1). In his work, the zoological considerations are given, elucidating the Holocene palaeoenvironments. However, from the beginning of the century Nordmann touched upon many other aspects within the Late Quaternary marine environments which form the most important base for the present study covering marine deposits from the Eemian, the Weichselian and the Holocene. In the following chapter the presentation of some observed sites with marine sediments will be given as an introduction to an answer to the question raised by Petersen (1910, p. 29): "What I have often missed in the geological studies is a thorough or detailed comparison between the fossil faunas and the molluscan faunas now living before our eyes".
The aim of this work is to characterise the changing environments in the Danish waters through time as seen in the macrofaunas and bottom communities mainly based on molluscs.

Danish sites with marine sediments
Initially, the findings and descriptions of the Danish marine localities shown in Fig. 1 were part of the university studies pioneered by G. Forchhammer. However, since the start of the Geological Survey of Denmark in 1888, much of the information has come from the systematic mapping of Denmark, and the results have been published in the descriptions to the geological map sheets of Denmark (Fig. 2).
As seen from the plan for the geological mapping of Denmark (e.g. Sørensen & Nielsen 1978) it was decided to do the mapping first in the northern parts of Jylland and Sjaelland and to present a record of the marine deposits from the areas mapped. Today, up to 80 per cent of the country has been mapped and descriptions for many map sheets have been published. The main information on the Holocene marine mol-luscs is available in these publications and is used in the present description supplemented by specific molluscan studies within the areas.
Consequently, the frame will be the transition area between the North Sea and the Baltic and the descrip-

The Late Pleistocene
In 1841   The aminostratigraphic investigations of the Danish Late Pleistocene deposits as published by Miller & Mangerud (1985) sustain only to some extent the abovementioned correlations: "None of the sites regarded here as Eemian (Strandegaards Dyrehave) gave ratios as high as in Holsteinian deposits or as low as in Middle Weichselian deposits" (Miller & Mangerud 1985, p. 261). In the case of the Holmstrup Weichselian site, only three out of eleven individuals of Macoma calcarea gave Weichselian ratios (Miller & Mangerud 1985, p. 264).
The marine molluscan fauna of the Late Weichselian has been studied intensively only from the Vend-

The Holocene
Forchhammer participated in the work of the so-called 'Lejrekomité', an interdisciplinary committee studying human remains along the shore. This commission gave the first -and now famous -description of the 'køkkenmødding' (kitchen midden), a mound consisting of shells of edible molluscs and other refuse, marking the site of a prehistoric human habitation (Hanks 1971). 'Køkkenmødding' is one of the few Danish international terms (Forchhammer et al. 1851). The work of the 'Lejrekomité' was concentrated on the marine molluscs in order to establish out whether the shell deposits were naturally based -oyster banks -or whether they were formed as waste deposits produced by men living at coastal sites. The other members of the commission were J. Worsaae and J. Steenstrup, representing archaeology and zoology respectively. Consistently, the study of the molluscan elements was based mainly on Steenstrup's work. However, while working in the commis-sion, Forchhammer continued his studies on the sea levels (Forchhammer 1838(Forchhammer , 1840. This was essential for the discussion of whether the molluscs found belonged to raised marine deposits or were gathered by man. Forchhammer's study led to the concept of raised marine deposits north of a line from Nissum Fjord to south of Korsør in the Storebaelt area ( Fig. 1). This line still carries the name of Forchhammer and divides the country into two parts, with the raised marine areas to the north-east, and to the south-west the area where the land has been sinking. Together with the study of the Holocene molluscan fauna by Johnstrup (1882b), such observations on shorelines were also collected. It became one of the points specially mentioned in the instructions for the autographic geologists when the systematic geological mapping of Denmark was started in 1888 by the Geological Survey of Denmark (Sørensen & Nielsen 1978).

The recent fauna of shell-bearing molluscs compared to the subfossil fauna
The record of recent Danish shell-bearing molluscs has been taken from the annotated check list of recent marine molluscs of Danish waters (Jensen & Knudsen 1995). In Appendix 1 the species are presented taxonomically following Jensen & Knudsen (1995). Late immigrants from the last centuries -transferred by man --have been omitted from the list, because the aim of the present study is to present the development in the subfossil Late Quaternary molluscan fauna also in Ap-pendix 1 compared to the natural fauna of today. According to Fredén (1986), subfossil means that the weight of the object when found does not exceed its original weight, which is obviously the case for younger deposits seen geologically as shells from the Late Quaternary.
In all, 278 recent species of shell-bearing molluscs are recorded from the Danish waters: The Class Polyplacophora is represented by seven 13 species forming 2.5% of the total number of known species. The Class Gastropoda is represented by 151 species forming 54.3% of the total number of known species.
The Class Scaphopoda is represented by three species forming 1.1% of the total number of known species.
The Class Bivalvia is represented by 117 species forming 42.1% of the total number of known species.
The list of known finds of subfossil species amounts to 247 species. With regard to the classes, it appears that Polyplacophora is now represented by only one species, which formed 0.4% of the total subfossil molluscan species.
Within the Class Gastropoda 125 species occur, forming 50.6% of the total number of subfossil species, a figure which is nearly 5% lower than that for recent gastropods.
The Class Scaphopoda is represented by five fossil species which form 2.0% of the subfossil shell-bearing species which is a little higher than the ratio for the recent fauna.
The Class Bivalvia is represented by 116 species forming 47.0% of the total, which is a little more than 7% above the recent ratio.
The low number of subfossil Polyplacophora can be explained by the fact that the shells from those species are nearly always broken, and this excludes identification to species level, so to say, following the statement made by Knudsen (1970, p. 1): "Isolated and worn plates were neglected altogether".
Among the Gastropods, the subclasses and orders, except the Order Heterostropha within the subclass Heterobranchia, have a lower representation of subfossil finds than of recent ones. The Heterostropha, which has a 2.5% higher representation among the subfossil finds than among the recent ones, is a group of mostly tiny specimens which might be more looked for in the geological samples than in the recent bottom samples often used in the more practical work of evaluation benthos introduced by C.G.J. Petersen. However, many of these small species should be considered with the utmost care, with respect to the difficulty of identifying them to species level within subfossil material.
The reason why the Class Scaphopoda has a twice as great a representation within the subfossil material cannot be given, although it is tempting to regard the different palaeoenvironment back in the Late Quaternary as the explanation of the higher frequency. The greater variety of palaeoenvironment and different climate back in time is clearly the reason why the Bivalvia within all subclasses has a higher percentage than in the recent fauna.
However, as an overview, the total subfossil species could be compared to the recent ones arranged also after their climatic affinities, as will be thoroughly discussed in one of the following chapters. With respect to distribution of molluscan species within the North Atlantic -West European realm, four zones may be distinguished, viz.: the Arctic = a, the Subarctic = s, the Boreal = b and the Lusitanian = l (Figs 4, 5).
It appears from the comparison between subfossil species and recent species sorted after climatic affinity (Appendix 1) that the subfossil species have their dominance in the extreme groups, i.e. Arctic = a; Arctic/ Subarctic = as; Arctic, Subarctic and Boreal = asb and Subarctic/Boreal, while the species with a wide tolerance -Arctic, Subarctic, Boreal and Lusitanian = asblhave a higher representation within the recent fauna.
Also the middle group, which is represented by faunal element from the Subarctic, Boreal and Lusitanian, the Boreal and Lusitanian (which is the most numerous group with 140 subfossil species) has a higher representation in the recent fauna. But the group of purely Lusitanian species has a clearly better representation among the subfossil species, as seen by the percentage figure 6.2% compared to 0.7% for the purely Lusitanian faunal elements among the subfossil and recent faunas respectively.
These observations reveal that the Late Quaternary fauna covers a period of 130 000 years with changing climatic conditions both with colder and warmer periods than at present.
So considering the totals of subfossil and recent species one has to discuss the difference not only quantitatively but qualitatively; because only 184 species are shared between the Late Quaternary and the recent finds, while 63 species have to be considered as particular ones occurring within the Late Quaternary during the Eemian, the Weichselian or the Holocene, in one, two or in all three groups but not the recent one.
Within the Bivalvia, the highest amount of subfossil species (31) found only in the Late Quaternary occur. Such species are also the species which must be focused on in the evaluation of the changing environment through time.

Molluscan finds within the seven regions during the Holocene
The molluscan finds within each region (see Fig. 2) from the Holocene, as appearing mainly from the descriptions accompanying the geological map sheets of Denmark, are presented.

The Baelt Sea area
From the Baelt Sea area the information on the occurrences of molluscs has been taken from the following map sheets: Madsen (1902) and Jessen (1907(contributions by V. Nordmann), 1935, 1945; V. Milthers (1940) and K. Milthers (1959). Nordmann (1906) has a record of molluscs found in Skaelskør Nor (SW Sjaelland) and Petersen records from the areas south of Fyn, Storebaelt andLillebaelt (1985c, 1989

The Baltic area
The Baltic area is here restricted to the area east of Darss and south of Øresund at Saltholm, which must be considered the Baltic sensu stricto when regarding the present distribution of the marine fauna and also taking into consideration the subfossil Holocene molluscan fauna, as will be demonstrated by a following comparison with the other areas. The main map sheet published is by V. Milthers from 1908 with contributions by V. Nordmann on the Holocene molluscan fauna. The subfossil Holocene fauna has also been studied later in the western part by Petersen (1994b

The Limfjord area
From the Limfjord area (western part), excluding the part which falls within Vendsyssel, only one description for a map sheet has been published (Gry 1979). However, the molluscs are recorded in publications by Petersen (1976Petersen ( , 1981Petersen ( , 1985aPetersen ( , 1986a

The Vendsyssel area
The Vendsyssel area includes the description accompanying the map sheets over the northern, central and southern parts, all by Jessen (1899,1905), but with a contribution by V. Nordmann, who wrote the part on the Holocene molluscan fauna in the latter publication. In this description by Nordmann he presents the different faunal communities as discovered in the subfossil assemblages. It was Nordmann's intention to continue the work further west into the western Limfjord area, but his first investigations were not used in the systematic geological mapping. They were, however, of great importance for the understanding of the development of the Holocene molluscan fauna (Nordmann 1910(Nordmann , 1918. In 1928 in connection with the International Congress in Copenhagen a final overview by Nordmann of the Quaternary marine deposits in Denmark was given in the Summary of the Geology of Denmark (Madsen et al. 1928). Here Nordmann points to the Dosinia layers first described at the beginning of the century from Vendsyssel (Nordmann 1904), with a record of a fauna not found in the older Tapes beds originally demonstrated by Petersen (1888).