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Twelve species (nine of which are new taxa) of trissocyclid radiolarians are arranged in three evolutionary lineages that have been traced through upper Eocene and lower Oligocene sediments from the Caribbean and central equatorial Pacific. The history of these radiolarians at and directly following the "terminal Eocene event" is obscured by hiatuses, but fine-scale correlation of seven sections provides an almost complete record of their evolution. Although the timing, evolutionary rate, and structural modifications differ, each lineage exhibits a similar phylogenetic trend in the early Oligocene, such that the skeleton underwent a significant inflation and assumed an approximately discoidal shape. This trend was of only short duration, however, and all three lineages re erted to their original morphology before the close of Oligocene time. These lineages have long independent Paleogene histories that are not considered here, but the oldest species at the base of the studied interval are: lineage I, Phormospyris inferispina (Goll); lineage II, Trissocyclus geniculosus (Goll); lineage III, Phormospyris sp.
Cephalic inflation has its simplest expression in lineage I, where the trend has its earliest appearance and longest duration. At the base of the Oligocene, the simple lattice shell abruptly expanded to completely enclose the sagittal ring (Nephrospyris anthocyrtoides morphology). The first appearance of N. anthocyrtoides is a good secondary marker for the base of the Theocyrtis tuberosa Zone.
The discoidal lattice shell modification occurred only very briefly in lineage II, and apparently it had no permanent impact. Trissocyclus geniculosus has a compact skeleton characterized by massive, regularly disposed lattice bars. For a brief episode during the early Oligocene, a morph abruptly appeared with a ribbonlike concentric ring of distal lattice in the plane formed by the vertical and lateral axes.
This evolution commenced later and proceeded at a slower pace in lineage III, with each successive stage appearing sequentially. The most extreme development of lattice inflation occurred briefly approximately 2 m.y. after the first indication that cephalic inflation impacted the lineage. Additionally, the thorax underwent inflation as well in this lineage and eventually surrounded the cephalis as a discoidal "cortical" chamber. Subsequently, a reversal of the trend is apparent in the occurrence of a morph with a small supracephalic chamber that is homologous to the apex of the "cortical" chamber of its precursor.
Two possible explanations are proposed for the evolution of these lineages. The approximate synchroneity with which this skeletal modification occurred suggests that this evolution is interrelated genetically. The development of "cortical" morphology in lineages II and III is analogous to spumellarian skeletal structure. Alternatively, lattice inflation could have been an adaptive response to changing pelagic environments of the Oligocene equatorial oceans.
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