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Pilekiines are the earliest diverging members of the successful trilobite family Cheiruridae. The pilekiine genus Anacheirurus is characterized by sub-quadratic to sub-oval glabella, pitted genae, and a distinct trunk with elongated pleural spines in its posterior part. Anacheirurus adserai is a common component of the Fezouata Shale (Lower Ordovician, Morocco), where it was intially included into several species of the genus Lehua. This assignment and taxonomic over-splitting created confusion, overestimated cheirurid diversity at this locality, and simultaneously underestimated morphological variability within A. adserai. In this contribution we examine new material of A. adserai from the Fezouata Shale, clarifying its morphology and systematics. A detailed re-description of the species shows that Anacheirurus is distinct from Lehua, the latter being a more derived member of Cheiruridae. The comparison of Anacheirurus with other pilekiines shows that morphological variability within this subfamily is mostly constrained to the trunk region. Exceptionally preserved specimens of A. adserai from the Fezouata Shale show details of appendages, revealing the endopodite and exopodite morphologies in early members of Cheiruridae. The endopodite of A. adserai is unique among trilobites in possessing comparatively longer distal podomeres 5 and 6, but otherwise, it has the same general morphology as other described trilobite endopodites. The exopodite morphology of A. adserai shows characters typical of some Cambrian species but differs in several aspects from those known in post-Cambrian taxa. It is concluded that trilobite exopodite morphology was probably more variable than the endopodite morphology, which remains rather conservative across different taxa. Morphological diversity of trilobite exopodites in post-Cambrian taxa might be related to ecological escalations during the Ordovician biodiversification and the transition between Cambrian and Ordovician trilobite faunas. [ABSTRACT FROM AUTHOR]

Nektaspids are Palaeozoic non-biomineralized euarthropods that were at the peak of their diversity during the Cambrian Period. Post-Cambrian nektaspids are a low-diversity group with only a few species described so far. Here we describe Tariccoia tazagurtensis, a new species of small-bodied nektaspid from the Lower Ordovician Fezouata Shale of Morocco. The new species differs from the type (and only other known) species from the Ordovician strata of Sardinia (Italy), Tariccoia arrusensis, in possessing more pointed genal angles, a cephalon with marginal rim, a pygidium with anterior margin curved forwards, a rounded posterior margin, and longer and more curved thoracic tergites. The two specimens of T. tazagurtensis sp. nov. show remains of digestive glands that are comparable to those seen in the Cambrian nektaspid Naraoia. The rare occurrence of T. tazagurtensis sp. nov. in the Fezouata Shale and the distribution of other liwiids suggest that these liwiids were originally minor members of open-marine communities during the Cambrian Period, and migrated into colder brackish or restricted seas during the Ordovician Period. [ABSTRACT FROM AUTHOR]

The Lower Ordovician Fezouata Shale is renowned for its exceptionally-preserved euarthropod fossils including numerous species of trilobites, some of which show remains of appendages and traces of the digestive system. Herein, we describe the early developmental stages of at least nine trilobite species from the Tremadocian strata of the Fezouata Shale, namely Platypeltoides magrebiensis , Nileus deynouxi , Symphysurus ebbestadi , Asaphellus sp., Megistaspis (Ekeraspis) hammondi , Orometopus sp., Anacheirurus adserai , Bavarilla zemmourensis , Indiligens sp., and several specimens of undetermined protaspides. This study considerably expands our knowledge of the development of early Ordovician trilobites. The preservation of appendages in the early stages of N. deynouxi and S. ebbestadi , and remains of the digestive tract in the latter species, suggests that some immature trilobites had similar morphology and anatomy as the adult individuals. Early developmental stages of Indiligens sp. might have fed and/or hidden on graptolites and demosponges. The extraordinarily large size of the early post-embryonic stages of P. magrebiensis , S. ebbestadi , Orometopus sp., Asaphellus sp., and undetermined protaspides suggests that these trilobites might have hatched from yolk-rich eggs. The presence of several trilobite species with notably large post-embryonic stages in the Fezouata Shale might be explained by seasonal or low productivity in the high-latitude margin of Gondwana. [ABSTRACT FROM AUTHOR]

The Ordovician Radiation shaped Paleozoic marine ecosystems and led to an increase in the biodiversity of marine organisms. The onset of the Plankton Revolution in the late Cambrian is an important step within the Ordovician Radiation. During this revolution, various organisms invaded pelagic realms, which affected the marine trophic web through the introduction of phytoplankton, zooplankton, as well as plankton- and suspension-feeding animals. Trilobites, a main Paleozoic group of marine metazoans, played a major role during the Ordovician Radiation. Previous studies showed that some trilobites evolved planktic larvae during the Furongian and Early Ordovician. Herein, we quantify the appearance of trilobite planktic larvae by providing highly resolved Cambrian and Ordovician data on 144 trilobite species with well-known developmental sequences. We show that Cambrian trilobites were dominated by species and families with exclusively benthic early post-embryonic stages. On the contrary, Ordovician seas comprised a remarkable number of trilobites with one or more planktic stages. However, species and families with exclusively benthic early stages still constituted about half of the Ordovician trilobite diversity. The first trilobites with planktic larvae might have been present in the Miaolingian, but their earliest fossil record is from the mid-Furongian. The appearance of planktic forms was accentuated later in the Ordovician with many trilobite lineages adapting a multi-staged planktic mode of life – more than one developmental stage colonizing the water column during ontogeny. Between the Miaolingian and the Middle Ordovician, both species- and family-level data show a progressive increase in the number of taxa incorporating planktic larvae in their development, highlighting a gradual transition in the structure of marine ecosystems during the early Paleozoic. [Display omitted] • Early trilobites were characterized by exclusively benthic developmental stages and these dominated Cambrian ecosystems. • During the Miaolingian-Middle Ordovician interval several trilobite lineages independently evolved planktic larvae. • Trilobites with planktic larvae were a fundamental part of the Ordovician marine trophic web. [ABSTRACT FROM AUTHOR]