Dating the fossil record updating mac os 10 3 9 to 10 4
Whilst the “Cambrian Explosion” continues to attract much attention from a wide range of earth and life scientists, the detailed patterns exhibited by the terminal Proterozoic–Early Cambrian biotas remain unclear, for reasons of systematics, biostratigraphy and biogeography.
In particular, recent changes in absolute dating of the Cambrian have refined the period of time that the fossil record might be of most help in revealing the dynamics of the undoubted radiation taking place at this time.
This view merely reflected the general Zeitgeist of the time, and the problems that systematists had had in trying to assign fossils to systematic positions.
The net effect was that, although fossils were generally thought of as providing the answer to the origins of major groups, in practice they tended to shed very little light on them.
It should be noted that this characteristic mix has been repeatedly noted in Cambrian fossils.
For example, Hughes (1975) said of the Cambrian arthropod did possess a mixture of characters …many of which are to be found in modern arthropods of various groups” (Hughes, 1975, p. Whilst the apparent oddness of Cambrian fossils is no different in kind from that of any other fossils, it was brought to prominence in two different ways.
Second, these taxa did seem to fall outside conventional taxonomic levels at a greater depth in the taxonomic hierarchy than later fossils tend to.
It is a matter of some irony therefore, that a key for this rethink was provided in the same volume on invertebrate origins.
Jefferies published here his critical paper that effectively introduced Hennig's views on the systematisation of fossils to the English-speaking world, with some important modifications of his own. A fossil can, for example, lie with in the grouping of extant echinoderms.
Nevertheless, this straightforward view has rightly been regarded sceptically, for several reasons.
The first of these is that the apparent very rapid appearance of taxa in the record must inevitably imply a period of cryptic evolution before this point (Dawkins, 1998; Cooper and Fortey, 1998).
For example, ammonites can be assigned, not just to the Mollusca, but also to the cephalopods, and indeed, are close relatives to the Coleoidea.