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Ricinulei Thorell, 1876 is an order of Arachnida currently represented in the New and Old Worlds by 103 living species. The order is also represented in the fossil record from the Carboniferous (ca. 305–319 Ma) and the Cretaceous (ca. 99 Ma) periods. In the present contribution, Hirsutisoma grimaldii sp. nov., a new extinct species of the suborder Primoricinulei Wunderlich, 2015, is described from a specimen preserved in Cretaceous Burmese amber. The specimen is a well-preserved adult male in which several taxonomically informative structures are visible, allowing the new species to be differentiated from Hirsutisoma bruckschi Wunderlich, 2017, the only other congener for which a male is known. This description raises the number of Cretaceous Ricinulei species to six. A comparative table documents morphological differences among the various species of this lineage. Hypotheses concerning the paleoecology and functional morphology of this species and, by extrapolation, other primoricinuleids, are presented. The evidence suggests that Primoricinulei were corticolous, scansorial predators. 

Abstract Spiders are unique in having a dual respiratory system with book lungs and tracheae, and most araneomorph spiders breathe simultaneously via book lungs and tracheae, or tracheae alone. The respiratory organs of spiders are diverse but relatively conserved within families. The small araneoid spiders of the symphytognathoid clade exhibit a remarkably high diversity of respiratory organs and arrangements, unparalleled by any other group of ecribellate orb weavers. In the present study, we explore and review the diversity of symphytognathoid respiratory organs. Using a phylogenetic comparative approach, we reconstruct the evolution of the respiratory system of symphytognathoids based on the most comprehensive phylogenetic frameworks to date. There are no less than 22 different respiratory system configurations in symphytognathoids. The phylogenetic reconstructions suggest that the anterior tracheal system evolved from fully developed book lungs and, conversely, reduced book lungs have originated independently at least twice from its homologous tracheal conformation. Our hypothesis suggests that structurally similar book lungs might have originated through different processes of tracheal transformation in different families. In symphytognathoids, the posterior tracheal system has either evolved into a highly branched and complex system or it is completely lost. No evident morphological or behavioral features satisfactorily explains the exceptional variation of the symphytognathoid respiratory organs.