Species (single lineages) of two clades (extended lineages) of the polycystine radiolarian subfamily Artiscinae were studied to determine their pattern of evolutionary change in Miocene tropical pelagic sediments. Morphometric measurements were made on specimens from 43 closely-spaced samples from a continuous stratigraphic sequence in DSDP core 9-77B, eastern equatorial Pacific. The samples are at approximately 165,000 year intervals through a 7.1 m.y. period. For this study, a new method of biosystematic analysis was formulated to deal with fossil organisms that are suspected to exhibit gradual anagenetic change through time. Three types of lineages and three types of extinction are defined to enable unequivocal interpretation of evolutionary processes and patterns.
Evolution in the artiscins is both punctuated and gradual: the Ommatartus tetrathalamus clade and the O. hughesi clad both contain gradually-changing characters, but in both clades descendants are separated from ancestors by cladogenetic events and the appearance of apomorphic characters. Other characters reveal stasis. Previous authors arbitrarily subdivided single continuous characters undergoing anagenesis to delimit artiscin species, but this practice is typological and ignores species variability and important apomorphic characters. For these reasons, the species Cannartus laticonus and O. antepenultimus are synonymized with O. penultimus. Furthermore, the character of polar caps is not homologous between the two artiscin lineages; in the O. hugesi lineage, caps develop from the spongy column, not as an extension of the cortical shell. Since anagenetic phyletic change in the artiscins controls macroevolutionary change to an extent equal to cladogenetic punctuated change, there is no reason to adopt a punctuated model over a gradual one for this taxon. However, artiscin classification must be radically revised in light of the monophyletic lineages (clades) delimited by cladistic analysis. Furthermore, the implications of this study make clear the necessity to rely solely on material, non-evolutionary data for biostratigraphic purposes. Lineage-zones and phyletic-biohorizons are invalid because they require a specific evolutionary model and method of phylogenetic inference, neither of which can apriorily be assumed to be correct.
| Identifer | oai:union.ndltd.org:RICE/oai:scholarship.rice.edu:1911/15786 |
| Date | January 1983 |
| Creators | SCHAFERSMAN, STEVEN DALE |
| Source Sets | Rice University |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | application/pdf |
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