Biometry and lifestyle of the Ordovician proetide trilobiteCyamella stensioeiOwens, 1979

Abstract

Biometric analyses of the trilobite Cyamella stensioei Owens, 1979, from carbonate mud mounds of the Upper Ordovician Boda Limestone of central Sweden, along with a comparison of closely allied rorringtoniid taxa from outside Baltoscandia, reveal important systematic and ecological information on this proetide taxon. Biometric analysis of the cephala of C. stensioei shows a number of instars, reflecting several ontogenetic stages, and a meraspis stage of this taxon is figured for the first time. C. stensioei is exclusively found in mass accumulations in the former, open cavities of the carbonate mud mound and was therefore most likely a cavity-dweller, possibly adapted to this lifestyle through chemosymbiosis. Some of the cavities are interpreted as former seepage conduits. Comparison of C. stensioei with other rorringtoniid trilobites suggests possible ancestors occurring in South China and Sibumasu terranes, determined earlier as Decoroproetus sp. and Cyamella sp., all differing from the genus Paracyamella, which is reassessed here. In wider terms of ancestry, they originate from Baltica or Laurentia, where the first rorringtoniids are known from the middle Darriwilian.

Recognising moulting behaviour in trilobites by examining morphology, development and preservation: Comment on Błażejowski et al. 2015

“…Synchronised moulting events are well established in modern arthropods, but identifying this behaviour in the fossil record is more difficult. Trilobites are one of the only fossil arthropod groups found in large enough numbers that we can access group behavioural information and examine synchronised moulting events in deep time.…”

“…Gregarious behaviour in trilobites has been documented in many assemblages for protection , moulting , burrowing , feeding and reproduction . Unlike in modern arthropods, where behaviour can be observed directly, trilobite fossil aggregations must be considered in context to try to assess whether they are the result of gregarious behaviour or abiotic accumulations.…”

Intraspecific bimodal variability in eye lenses of two Devonian trilobites

“…One can only speculate that some restricted populations of P . prokopi became adapted to such a protected environment as with other trilobite taxa in African Gondwana, Baltica and European Gondwana (Kaufmann 1997, 1998; Cavalazzi 2006; Popp and Pärnaste 2011; Bignon and Cronier 2014). The water depth at both localities was probably comparable (the muddy sea floor in the deeper shelf lay below the fair weather wave base, possibly also below the storm wave base but still within the photic zone, for discussion, see also Chlupáč et al 1998).…”

Terminal Ordovician stratigraphy of the Siljan district, Sweden

“…The resulting mound would have a complex growth history giving a broad, essentially flat base variable surface topography and a 40 m or higher relief above the sea floor. The growth history and isotopic signature of some karstic structures inhabited by minute trilobite species was described by Popp & Pärnaste (2011). ?…”