† phylonepionicadj.BiologyObsoleterare (in the theories of A. Hyatt and others) relating to or designating an early stage in the evolutionary development of a lineage of organisms, supposed to correspond to the nepionic stage of ontogeny (cf. phylogeronticadj.).
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1893 A. HyattCarboniferous Cephalopods in E. T. Dumble Fourth Ann. Rep. Geol. Surv. Texas 389 Those coming nearer in time and structure may correspond to the nepionic and be designated as phylonepionic.