Eskov K.Yu.1, Vorontsov D.D.1,2, Selden P.A.3,4, Wang Bo5, Marusik Yu.M.6,7* 2026. Chimerarachne is the ‘arachnid platypus’: cuticular microstructures confirm the transitional position of a controversial amber fossil (Arachnida: Tetrapulmonata: Chimerarachnidae) // Arthropoda Selecta. Vol.35. No.3: 331–380 [in English].
1 Borissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya Str., 123, Moscow 117647 Russia.
2 Koltzov Institute of Developmental Biology, Russian Academy of Sciences, Vavilova Str., 26, Moscow 119334 Russia.
3 University of Kansas, 1414 Naismith Drive, Lawrence, KS 66045, USA.
4 Natural History Museum, Cromwell Road, London SW7 5BD, UK.
5 State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, No. 39 East Beijing Rd, Nanjing 210008, P. R. China.
6 Institute of Biological Problems of the North, Far Eastern Branch, Russian Academy of Sciences, Portovaya Str., 18, Magadan 685000 Russia.
7 Department of Zoology and Entomology, University of the Free State, Bloemfontein 9300, South Africa.
* Corresponding author
Kirill Eskov: afranius999@gmail.com ; https://orcid.org/0000-0002-7953-0746
Dmitry Vorontsov: colupaika@gmail.com ; https://orcid.org/0000-0002-1701-1748
Paul Selden: selden@ku.edu ; https://orcid.org/0000-0001-7454-4260
Bo Wang: bowang@nigpas.ac.cn ; https://orcid.org/0000-0002-8001-9937
Yuri Marusik: yurmar@mail.ru ; https://orcid.org/0000-0002-4499-5148
doi: 10.15298/arthsel.35.3.06
ABSTRACT. The mid-Cretaceous genus Chimerarachne Wang et al., 2018 occupies a phylogenetic position relative to ‘true spiders’ (Aranei) analogous to that of the monotreme platypus relative to ‘true mammals’ (Theria): it is a sister lineage that retains a mosaic of ancestral and derived characters. Both ancestral features (such as a completely segmented opisthosoma and retention of the flagellum) and derived features (such as spinnerets and the male palpal organ) are well documented in Chimerarachne. We analyzed 55 morphological characters of chimerarachnids, including a number of previously overlooked microstructural features, revealing additional plesiomorphic and apomorphic characters. For example, the trichobothrial pattern and bothrial morphology represent plesiomorphies shared with Devonian trigonotarbids, whereas epiandrous spigots associated with sperm-web construction indicate a spider-like mechanism of indirect sperm transfer. No synapomorphies were identified linking Chimerarachne with the classical uraraneids, nor were any found supporting a monophyletic Uraraneida comprising Attercopus Selden et Shear in Selden et al., 1991 and Permarachne Eskov et Selden, 2005. Although the inclusion of Chimerarachne within Aranei can be justified by synapomorphies, its numerous plesiomorphic characters would obscure the diagnosis of such an expanded order. These results suggest that uraraneids and chimerarachnids represent successive grades along the spider stem lineage rather than a monophyletic clade.
KEY WORDS: chimerarachnid phylogeny, microstructures, uraraneids, origin of spiders, ‘araneization of tetrapulmonates’.