Evenness and diversity in Upper Cambrian – Lower Ordovician trilobite communities from the Central Andean Basin (Cordillera Oriental, Argentina)

Abstract

Community evenness has recently received much attention, either because it is related to ecosystem functioning or because it may affect estimation of diversity. Temporal and environmental trends in diversity and evenness of trilobite communities during the Late Cambrian – Early Ordovician of the Cordillera Oriental (north‐western Argentina) are here analysed. Richness and evenness increase through time in both deep subtidal (between fair‐weather and storm wave base) and offshore (below storm wave base) communities. Two significant patterns are superimposed on this general trend: (1) the magnitude of the increase in evenness is much more pronounced in deep than in shallower settings, and (2) richness and evenness trajectories are decoupled (while a significant rise in evenness is recorded in the middle Tremadocian (Tr2), an increase in richness is delayed until the late Tremadocian (Tr3)). In contrast to expectations, a single family (Olenidae) is dominant in samples associated with this earlier rise in evenness relative to richness. Hence, this trend is explained neither by the number of families present in the communities nor by the familial identity of the most abundant taxon. Large‐scale comparisons of the timing and geographical components of these trends are restricted to the patterns recognized in Laurentian North American studies. Results from the Cordillera Oriental mirror those of Laurentia regarding the rise in both metrics in deep marine settings. Nevertheless, the timing of this increase in richness and evenness is delayed in the Cordillera Oriental, supporting the idea that palaeogeographical regions differed in the nature and timing of ecological changes. Finally, the rise in trilobite alpha‐diversity through the Late Cambrian – Early Ordovician of the Cordillera Oriental supports the idea that trilobite alpha‐diversity did not decline worldwide, suggesting that the relative decline in trilobite alpha‐diversity is most probably caused by the dilution effect.

Radiation, diversity and environmental expansion of Early Ordovician ostracods: a view from the Southern Hemisphere

“…Benedetto ; Benedetto et al . , ; Salas ; Balseiro & Waisfeld ; Cichowolski et al . ; Benedetto & Muñoz , ; Toro et al .…”

“…A wealth of palaeontological information, including the biostratigraphical context, systematics of different invertebrate groups, and palaeobiogeographical and palaeoecological analyses, is available from ostracod-bearing units (e.g. Benedetto 2003;Benedetto et al 2009Salas 2011;Balseiro & Waisfeld 2014;Cichowolski et al 2014;Benedetto & Muñoz 2015Toro et al 2015;Waisfeld & Balseiro 2016;Albanesi & Ortega 2016;Muñoz & Benedetto 2016;Meroi Arcerito et al 2017;Tortello et al 2016, among others).…”

“…O/T: the number of samples with ostracods vs. the total number of samples derived from systematic collections made for palaeoecological/taphonomic studies in the basin (e.g. Marengo 2011;Balseiro & Waisfeld 2014;Waisfeld & Balseiro 2016;Meroi Arcerito et al 2017); G/S: generic richness vs. specific richness; a: Fisher's alpha diversity.…”

Diversity patterns in upper Cambrian to Lower Ordovician trilobite communities of north‐western Argentina

“…In this context, trilobites are considered an ideal group for the study of early Palaeozoic faunas, fundamental for unravelling the processes underlying ecological changes across this critical interval (Foote ; Adrain et al . , ; Finnegan & Droser ; Balseiro & Waisfeld ; Waisfeld & Balseiro ).…”

“…During this interval, local diversity and evenness rose diachronically in different regions and this rise was more pronounced in deep marine environments relative to shallow ones (Adrain et al . ; Peters ; Balseiro & Waisfeld ). In a recent analysis, Waisfeld & Balseiro () described a change in the ecological structure of Cambro‐Ordovician trilobite assemblages from the Argentine Cordillera Oriental towards less dominated assemblages, first at the local scale (early late Tremadocian, Tr2) and later at the regional scale (middle late Floian, Fl2–3), explaining the previously recognized diachronic change in evenness and richness for these local communities (Balseiro & Waisfeld ).…”