Abstract
The recent re-interpretation of the Lower Palaeozoic euarthropod group Mollisonia as belonging to Chelicerata has triggered a renewed interest for the poorly known family Mollisoniidae. In this contribution, we revise the anatomy, taxonomic diversity, and systematics of Thelxiope, the sister-taxon of Mollisonia. This mollisoniid genus comprises four species, and is characterized by the presence of one cephalic, seven thoracic (one per tergite), and three pygidial long sagittal spines. The type species, T. palaeothalassia Simonetta & Delle Cave, is a rare taxon in the Wuliuan Burgess Shale Formation of Canada, which can be recognized by the hypertrophy of a single of its sagittal spines, the posteriomost one. T. spinosa (Conway Morris & Robison)–a species originally assigned to a distinct genus ‘Ecnomocaris’ herein synonymised with Thelxiope–is known from a single specimen found in the Drumian Wheeler Formation of the House Range of Utah. It differs from the type-species in the hypertrophy of both the anteriormost (cephalic) and the posteriormost (third pygidial) sagittal spines. The same Wheeler strata have also yielded a single specimen of a new taxon, T. holmani sp. nov., which lacks hypertrophied sagittal spines and features blunt thoracic tergopleural tips. A putative fourth species, referred to Thelxiope sp. nov. A, extends the stratigraphical range of Thelxiope to the Lower Ordovician (Tremadocian), and its palaeographic range to West Gondwana. Currently under study, this relatively common component of the lower Fezouata Shale fauna is only briefly discussed. Features characterizing the genus Thelxiope and its components almost exclusively pertain to the sagittal spines, for the scarcity and inconsistent preservation of the Cambrian materials as-yet available preclude a confident assessment of the variability of other morphological features. The pygidium in Thelxiope and Mollisonia is not composed of four, but three tergites essentially similar to thoracic ones, except for the lack of articulations.
Highlights
The family Mollisoniidae is a poorly studied group of Lower Palaeozoic non-biomineralizing euarthropods, with a subparallel-sided body composed of sized cephalic and pygidial shields and a seven-segmented articulated thorax
Thelxiope primarily differs from other mollisoniids by the presence of well-developed sagittal spines all along the dorsal exoskeleton
Three of them represent rare components of Miaolingian (Wuliuan–Drumian) exceptionally-preserved faunas from Laurentia, whereas the fourth appears to be relatively common in the Tremadocian lower Fezouata Shale of Morocco (West Gondwana)
Summary
The family Mollisoniidae (sensu Lerosey-Aubril et al, in press) is a poorly studied group of Lower Palaeozoic non-biomineralizing euarthropods, with a subparallel-sided body composed of sized cephalic and pygidial shields and a seven-segmented articulated thorax. Mollisoniids (i.e., Mollisoniidae, throughout the text) are best known from the Miaolingian strata of Laurentia, with occurrences in the Wuliuan Burgess Shale of Canada (various localities; Walcott, 1912; Walcott, 1931; Simonetta, 1964; Simonetta & Delle Cave, 1975; Caron et al, 2010; Caron et al, 2014; Aria & Caron, 2019), the Wuliuan Spence Shale of northern Utah (Briggs et al, 2008), and the Drumian Wheeler Shale of the House Range and Drum Mountains of western Utah (Robison, 1991; Briggs et al, 2008; Lerosey-Aubril et al, in press) These Laurentian occurrences mostly concern Mollisonia, the type genus of the family, in which chelicerae-looking appendages were recently described (Aria & Caron, 2019). If corroborated by further findings, mollisoniids would represent the oldest representatives of the group Chelicerata and as such, would be key to our understanding of the origin of this subphylum of euarthropods
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