Abstract
The widespread assumption that the origin of polar marine faunas is linked to the onset of major global cooling in the Late Eocene – Early Oligocene is being increasingly challenged. The Antarctic fossil record in particular is suggesting that some modern Southern Ocean taxa may have Early Eocene or even Paleocene origins, i.e. well within the Early Cenozoic greenhouse world. A global analysis of one of the largest marine clades at the present day, the Neogastropoda, indicates that not only is there a decrease in the number of species from the tropics to the poles but also a decrease in the evenness of their distribution. A small number of neogastropod families with predominantly generalist trophic strategies at both poles points to the key role of seasonality in structuring the highest latitude marine assemblages. A distinct latitudinal gradient in seasonality is temperature-invariant and would have operated through periods of global warmth such as the Early Cenozoic. To test this concept a second global analysis was undertaken of earliest Cenozoic (Paleocene) neogastropods and this does indeed show a certain degree of faunal differentiation at both poles. The Buccinidae, s.l. is especially well developed at this time, and this is a major generalist taxon at the present day. There is an element of asymmetry associated with this development of Paleocene polar faunas in that those in the south are more strongly differentiated than their northern counterparts; this can in turn be linked to the already substantial isolation of the southern high latitudes. The key role of seasonality in the formation of polar marine faunas has implications for contemporary ecosystem structure and stability.
Highlights
It would seem only logical to place the origin of modern polar marine faunas largely within the major global cooling event that occurred between the late Middle Eocene and the Eocene – Oligocene boundary [1], [2]
It is the youngest clade, with a time of origin in the Early Cretaceous and major phase of radiation throughout the Cenozoic [28,29,30]. Widespread, it is ideal for regional scale biogeographical analyses and in this study a direct comparison will be made between the living neogastropod faunas of both polar regions and a composite tropical fauna to see the end-product of clade differentiation through the Cenozoic
To get a conservative estimate of tropical neogastropod diversity at the present day, a mean value was taken from the six selected localities within the 19 commonest families; these were compared directly with absolute values for both polar regions (Fig. 1)
Summary
It would seem only logical to place the origin of modern polar marine faunas largely within the major global cooling event that occurred between the late Middle Eocene and the Eocene – Oligocene boundary (i.e. approximately 34–41 m.y. ago) [1], [2]. There has been a small but persistent volume of evidence to suggest that Antarctic marine invertebrate faunas in particular may be of considerably greater antiquity This again comes from both paleontological and phylogenetic sources and is such as to suggest that some modern taxa are of Early Cenozoic, Cretaceous, or even greater age [12,13,14,15,16,17,18]. In a recent comprehensive taxonomic reassessment of Paleogene molluscan faunas from Antarctica, Beu [19] showed that more than 15% of the Paleocene taxa and 30% of the Early – Middle Eocene could be referred to modern genera When both the gastropod and bivalve components are considered at the family level, remarkably persistent compositional trends can be traced through much of the Cenozoic. The modern Antarctic molluscan fauna, at least, may have had its roots very firmly within the Early Cenozoic greenhouse world [19], [20]
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