Functional morphology, coaptation and palaeoecology of Hollardops (Trilobita, Acastidae), with descriptions of new species and two new genera from the Devonian of Morocco

A group of asteropygine trilobites with 10 thoracic segments from the lower Emsian to lowermost Eifelian of western Europe and northwest Africa is deemed monophyletic. Available names for this clade are Hollardops Morzadec, 1997, Philipsmithiana Lieberman & Kloc, 1997, Modellops Lieberman & Kloc, 1997 and Pennarbedops Bignon & Crônier, 2013, the first of which has priority. Well-preserved Hollardops specimens from southern Morocco have revealed previously undescribed details of the mineralised exoskeleton that are interpreted here as coaptative and sensory devices. It is proposed that Hollardops, like many other asteropygines, practised a dual mode of coaptation: fully locked enrolment was alternated with a retracted pygidium, allowing temporary influx of oxygenated seawater and excretion through slits between the pygidial lappets. This putative ‘breathing device’ enabled the trilobite to remain enrolled for a longer period of time while maintaining vital body functions. A poor understanding of the problematic type species of Hollardops, H. mesocristatus (Le Maître, 1952), has clouded actual diversity in Algeria and Morocco. Hollardops klugi sp. nov. is the oldest known member of the genus and extends its confirmed stratigraphic range into the lower Emsian. Two species, H. kyriarchos sp. nov. and H. multatuli sp. nov., are recorded from just above the base of the upper Emsian. Additionally, H. angustifrons sp. nov., H. luscus sp. nov. and the first well-preserved specimens of H. boudibensis Morzadec, 2001 are recorded from the upper Emsian. The types of H. hyfinkeli (Lieberman & Kloc, 1997) and H. burtandmimiae (Lieberman & Kloc, 1997) are refigured and original species concepts reiterated. Feruminops Haas, 1968, including its junior subjective synonym Morzadecops Bignon & Crônier, 2013, from the lower Emsian of Morocco and Türkiye, may comprise the sister group of Hollardops. Additionally, the enrolment strategies and systematics of several other members of Asteropyginae are discussed. Platykardiapyge gen. nov. (type species: Metacanthina maderensis Morzadec, 2001) is erected for a group of Pragian–early Emsian asteropygines from Morocco, Spain and Türkiye with a widely heart-shaped pygidium and comparatively many pygidial pleurae, among other features. Bignonops gen. nov. (type species: Kayserops tamnrhertus Chatterton et al., 2006) is erected for some species previously included in Gandlops Bignon & Crônier, 2013. Minicryphaeus suavius sp. nov. is described from the lower Emsian of Morocco. The identity and generic affinity of the oldest known asteropygine, Ganetops gdoumontensis (Asselberghs, 1930) from the Pridolian of Belgium, are discussed.