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Hormiga, Gustavo

The limits and the interfamilial relationships of the minute orb-weaving symphytognathoid spiders have remained contentious and poorly understood. The circumscription and diagnosis of the symphytognathoid family Mysmenidae have always been elusive, and its monophyly has never been thoroughly tested. We combine sequence data from six genes with a morphological dataset in a total-evidence phylogenetic analysis (ca. 6100 characters, 109 taxa: 74 mysmenids), and explore the phylogenetic signal of the combined dataset, individual genes, and gene combinations with different parsimony methods and model-based approaches. Several support values and parameter-sensitivity schemes are explored to assess stability of clades. Mysmenidae monophyly is supported by ca. 20 morphological and ca. 420 molecular synapomorphies. Mysmenidae is monophyletic under all combined analyses that include morphology. Almost no gene or gene combination supports Mysmenidae monophyly. Symphytognathoids are delimited to include: (Theridiosomatidae (Mysmenidae (Synaphridae (Anapidae + Symphytognathidae)))). Micropholcommatids are a lineage nested within the anapid clade and thus are synonymized with Anapidae (Micropholcommatinae New Rank). We provide morphological diagnoses for all symphytognathoid families and discuss the behavioural evolutionary implications of our hypotheses of relationships. The planar orb web evolved independently twice from three-dimensional webs. The orb web was modified into sheet or cobwebs three times independently. The spherical mysmenine web has a single origin. Kleptoparasitism evolved once in mysmenids. We comment on the discrepancies and lack of resolving power of the molecular datasets relative to the morphological signal, and discuss the relevance of morphology in inferring the total-evidence phylogenetic pattern of relationships.

Understanding the genealogical relationships among the arachnid orders is an onerous task, but fossils have aided in anchoring some branches of the arachnid tree of life. The discovery of Palaeozoic fossils with characters found in both extant spiders and other arachnids provided evidence for a series of extinctions of what was thought to be a grade, Uraraneida, that led to modern spiders. Here, we report two extraordinarily well-preserved Mesozoic members of Uraraneida with a segmented abdomen, multi-articulate spinnerets with well-defined spigots, modified male palps, spider-like chelicerae and a uropygid-like telson. The new fossils, belonging to the species Chimerarachne yingi, were analysed phylogenetically in a large data matrix of extant and extinct arachnids under a diverse regime of analytical conditions, most of which resulted in placing Uraraneida as the sister clade of Araneae (spiders). The phylogenetic placement of this arachnid fossil extends the presence of spinnerets and modified palps more basally in the arachnid tree than was previously thought. Ecologically, the new fossil extends the record of Uraraneida 170 million years towards the present, thus showing that uraraneids and spiders co-existed for a large fraction of their evolutionary history.