Oxygen as a Driver of Early Arthropod Micro-Benthos Evolution

Abstract

BackgroundWe examine the physiological and lifestyle adaptations which facilitated the emergence of ostracods as the numerically dominant Phanerozoic bivalve arthropod micro-benthos.Methodology/Principal FindingsThe PO2 of modern normoxic seawater is 21 kPa (air-equilibrated water), a level that would cause cellular damage if found in the tissues of ostracods and much other marine fauna. The PO2 of most aquatic breathers at the cellular level is much lower, between 1 and 3 kPa. Ostracods avoid oxygen toxicity by migrating to waters which are hypoxic, or by developing metabolisms which generate high consumption of O2. Interrogation of the Cambrian record of bivalve arthropod micro-benthos suggests a strong control on ecosystem evolution exerted by changing seawater O2 levels. The PO2 of air-equilibrated Cambrian-seawater is predicted to have varied between 10 and 30 kPa. Three groups of marine shelf-dwelling bivalve arthropods adopted different responses to Cambrian seawater O2. Bradoriida evolved cardiovascular systems that favoured colonization of oxygenated marine waters. Their biodiversity declined during intervals associated with black shale deposition and marine shelf anoxia and their diversity may also have been curtailed by elevated late Cambrian (Furongian) oxygen-levels that increased the PO2 gradient between seawater and bradoriid tissues. Phosphatocopida responded to Cambrian anoxia differently, reaching their peak during widespread seabed dysoxia of the SPICE event. They lacked a cardiovascular system and appear to have been adapted to seawater hypoxia. As latest Cambrian marine shelf waters became well oxygenated, phosphatocopids went extinct. Changing seawater oxygen-levels and the demise of much of the seabed bradoriid micro-benthos favoured a third group of arthropod micro-benthos, the ostracods. These animals adopted lifestyles that made them tolerant of changes in seawater O2. Ostracods became the numerically dominant arthropod micro-benthos of the Phanerozoic.Conclusions/SignificanceOur work has implications from an evolutionary context for understanding how oxygen-level in marine ecosystems drives behaviour.

Origin of ecdysis: fossil evidence from 535-million-year-old scalidophoran worms

“…B 286: 20190791 epidermal structure compatible with ecdysis, and close correlations between body plans and the moulting process (table 1). Ecdysis seems to have appeared in the context of the profound environmental changes that characterized the Precambrian-Cambrian transition, including increasing oxygen levels [87][88][89] and interactions among animals [90][91][92], and thus the onset of more complex marine ecosystems (e.g. food webs and colonization of infaunal niches [90,91]).…”

Baring it all: undressing Cambrian ‘Orsten’ phosphatocopine crustaceans using synchrotron radiation X-ray tomographic microscopy

“…They first appeared in Cambrian Epoch 2 and became diverse and abundant in the latest Guzhangian and Furongian and disappeared from the fossil record in provisional Age 10 of the Furongian Epoch (Williams et al . ). The taxon Phosphatocopina has been demonstrated to represent the sister‐group of the Eucrustacea within the Labrophora sensu Siveter et al .…”

“…) and appear to have been adapted to seawater hypoxia (Williams et al . ). In their monograph, Maas et al .…”

“…; Williams et al . ). However, a muscle system that was able to open and close the valves – as present in ostracod crustaceans – is lacking.…”

Late Eifelian and Early Givetian ostracod assemblages from Wellin, Hotton and On-Jemelle (Ardenne, Dinant Synclinorium, Belgium). Paleoenvironmental implications

“…Ostracods are small aquatic bivalve micro-crustaceans, known with certainty from the Ordovician (Williams et al, 2008), but probably present as early as the Middle Cambrian (Williams et al, 2011). If Paleozoic ostracods are essentially marine and brackish-water taxa, these small crustaceans also colonized freshwater, at least since the Carboniferous (Bennett, 2008); they can also be found nowadays in more extreme environments (e.g.…”