Megagametophyte Development in the Chaloneriaceae Fam. Nov., Permineralized Paleozoic Isoetales (Lycopsida)

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Abstract
Megaspores and megagametophytes that exhibit a wide range of variability were discovered within sporangia of the Pennsylvanian fossil lycophyte, Chaloneria cormosa. The smallest megaspores occur in tetrahedral tetrads, with larger specimens preserved individually. Full-sized megaspores are often devoid of contents, but some show cellular megagametophytes entirely within the spore, or megagametophytes that protrude out of the trilete and display several archegonia. The range of variability is compared with developmental variations in extant and other fossil lycophytes to interpret megaspore and megagametophyte ontogeny and several aspects of reproductive biology in Chaloneria. Based on numerous similarities to Isoetes and to the Mesozoic Pleuromeia complex, Chaloneria and several other Paleozoic lycophytes are grouped into the Chaloneriaceae fam. nov and placed within the Isoetales.