The possibilities of unequivocally identifying migrational patterns in the fossil record vary considerably depending on such factors as the nature of the organism of interest and the degree of knowledge of the paleogeography and paleoecology of the time concerned. Brief accounts of examples involving assured migrational routes are given here for mammals, ammonites, and ostracods based on establishing a connexion in relation to water-bodies and Thiede's (1977) transient high-grounds. Also of importance is the passive transport of some species of marine ostracods over very long distances on floating "algal islands". The likelihood of enforced transport of living organisms by deep submarine currents is another feature of potential significance. Terrestrial migration is a more difficult problem and the identification of such events has not been attempted here. There are several varieties of spurious migration the best studied of which concerns mainly the nekroplantkonic dispersal of cephalopod shells. Another case of a well documented event involving crustal divergence arising from the effects of seafloor spreading result in simulating migration: a classical example of this, involving a distance of 67000 km, is the Krommelbein-Wenger analysis of the Neocomian (i.e Lower Cretaceous - Wealden) freshwater ostracod sequences of Gabon and Brazil.
The vexing question remains. How can one, unequivocally, distinguish the ad hoc kinds of "migration", spurious or genuine, in the fossil record? To be honest, I do not know. All I can hope to suggest is that each case must be approached with an open mind. Some years ago I had a certain success in applying quantitative statistical procedures to paleobiogeography (summary in Reyment 1991) and such methods form an integral part of my arsenal of techniques. I have not taken up this aspect in this short overview because of the far-reaching range of the subject. Examples of what I believe can open new fields for the study of migration in the fossil record are given in Reyment (1983c, 1993, 1995) for applications of multivariate quantitative genetics to paleontological problems in a broad sense. Useful paleobiogeographical information can be extracted from appropriately constructed statistical analyses of the shell-chemistry of marine invertebrates (Reyment 1996, 2000). The quantitative analysis of polyphenism in fossils has likewise proven to be a valuable utensil (Reyment 2004).
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