HR: 1340h
AN: B33B-0265    [Abstracts]
TI: Testing the Molecular Clock Using the Best Fossil Record: Case Studies from the Planktic Foraminifera
AU: * Steel, B A
EM: b.steel@gl.rhul.ac.uk
AF: Dept of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX United Kingdom
AU: Kucera, M
EM: m.kucera@gl.rhul.ac.uk
AF: Dept of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX United Kingdom
AU: Darling, K
EM: Kate.Darling@ed.ac.uk
AF: Grant Institute of Geology, University of Edinburgh, Edinburgh, EH8 9BE United Kingdom
AB: Criticism of molecular clock studies often centres on inadequate calibration and a perceived lack of correlation between reproductive isolation and recognisable morphological evolution. Since many major groups (e.g. birds, mammals, reptiles) have a poor fossil record, it is often difficult to test and refute these limitations. Planktic foraminifera represent an exception to this rule. Deep-sea sediments are super-abundant in foraminifera, and large numbers of specimens and occurrences are easily garnered from Ocean Drilling Programme cores. Planktic foraminifera therefore represent an ideal model group with which to test and refine molecular clock studies. Since the 1990A�A�A��_sA�A��_zA�s, genetic sequences (principally 18S r-RNA) have been extracted from living planktic foraminifera, and a large genetic library has developed. Our study attempts to contextualise and test molecular data, particularly A�A�A��_sA�A<�"molecular clockA�A�A��_sA�A��_zA� dates, utilising material from two ODP cores (Site 926A (Atlantic) and 806 (Pacific), to examine the evolutionary history of two sibling-species complexes (Globigerinella siphonifera and Globigerinoides ruber, both common shallow-water species and both of considerable palaeoceanographic utility). Recent genetic studies have suggested that these two A�A�A��_sA�A<�"super-speciesA�A�A��_sA�A��_zA� in fact consist of a number of isolated forms, with contrasting ecologies and longevities, which in Recent and sub-Recent sediments can be distinguished either on the basis of pore ultrastructure (Gl. siphonifera) or test colouration (Gs. ruber). In both cases, molecular clock estimates are indicative of ancient (7-11 Ma) intra-species cryptic divergences, which seem to be considerably older than fossil dates. In particular, the calculated molecular split between the two forms of Gs. ruber (A�A�A��_sA�A<�"whiteA�A�A��_sA�A��_zA� and A�A�A��_sA�A<�"pinkA�A�A��_sA�A��_zA�) of around 11 Ma is considerably discordant with the fossil date of around 0.7 Ma. At first glance, this may appear to be a classic case of molecular over-estimation, often a feature of clock models, especially where, as in the foraminifera, substitution rates may vary widely. However, there is good reason to suspect that fossil range of the derived A�A�A��_sA�A<�"pinkA�A�A��_sA�A��_zA� form may have been artificially truncated by diagenetic degradation of the meta-stable test pigmentation. The deep molecular splits for Gl. siphonifera (around 7 Ma for the two main morphologically distinguishable sub-types), whilst not so obviously at odds with the fossil record, still belie the very small amount of morphological evolution observed within the plexus. We have used morphometric methods on a large (over 2000 pooled specimens) dataset in an effort to independently test the molecular clock, using SEM-based measurement of pore metrics (for Gl. siphonifera) and a multivariate analysis of whole-test characteristics (for Gs. ruber). Comparison of results for the two species suggests interesting patterns; whilst the two cryptic sub-types of Gl. siphonifera seemingly can be traced through time and seem to respond to external oceanographic forcing, the sub-types of Gs. ruber appear to be truly cryptic, and cannot be distinguished in the fossil record beyond 0.7 Ma. This raises two important points; firstly, the molecular clock (at least for foraminifera) bears considerable scrutiny, appears to be relatively robust to substitution bias and is seemingly broadly in accordance with morphological data; and secondly, the relationship between form and function in planktic foraminifera appears to be ill-defined, raising important questions for functional morphology.
DE: 4840 Microbiology
DE: 4855 Plankton
DE: 4267 Paleoceanography
DE: 3030 Micropaleontology
DE: 1040 Isotopic composition/chemistry
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting