After a long history of taxonomic isolation, recent studies show that Hymenophyllopsis K.I.Goebel is nested within the tree fern genus Cyathea Sm. s.s.. We compare developmental stages of typical Cyathea with an anatomical survey of Hymenophyllopsis. Such comparisons should clarify how the distinctive features of Hymenophyllopsis are related to or derived from those of typical Cyathea. Hymenophyllopsis and Cyathea samples from field-preserved materials and herbarium specimens were examined using standard light microscopy techniques. Hymenophyllopsis species have simple stem anatomy: all have solenosteles or simple dictyosteles, and sclerenchyma and secretory strands are lacking. Only H. superba has as many as four vascular bundles in the petiole base. Leaf blades lack stomata, the epidermal cells contain chloroplasts and there are only one or two mesophyll layers. Typical adult Cyathea species have complex stelar anatomy with medullary bundles (some with cortical bundles) and complex stem histology. There are numerous vascular bundles in the petiole. Leaf epidermal cells other than guard cells lack chloroplasts and the mesophyll has multiple layers. In contrast to their differences from adult Cyathea plants, Hymenophyllopsis anatomy corresponds closely to early stages of young Cyathea sporophyte development. The origin of “Hymenophyllopsis” from typical Cyathea ancestors involved a drastic reduction in size and anatomical complexity, with the precocious production of spores. This is a clear example of paedomorphosis among pteridophytes, where Hymenophyllopsis adult plants represent precociously fertile, permanent “young sporophytes” of tree ferns.