Eukaryotic organisms radiated in Proterozoic oceans with oxygenated surface waters, but, commonly, anoxia at depth. Exceptionally preserved fossils of red algae favor crown group emergence more than 1200 million years ago, but older (up to 1600–1800 million years) microfossils could record stem group eukaryotes. Major eukaryotic diversification 800 million years ago is documented by the increase in the taxonomic richness of complex, organic-walled microfossils, including simple coenocytic and multicellular forms, as well as widespread tests comparable to those of extant testate amoebae and simple foraminiferans and diverse scales comparable to organic and siliceous scales formed today by protists in several clades. Mid-Neoproterozoic establishment or expansion of eukaryophagy provides a possible mechanism for accelerating eukaryotic diversification long after the origin of the domain. Protists continued to diversify along with animals in the more pervasively oxygenated oceans of the Phanerozoic Eon.

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Paleobiological Perspectives on Early Eukaryotic Evolution

The Great Ordovician Biodiversification Event (GOBE): The palaeoecological dimension

Of all the components of biogeochemical cycles, few attract more attention than the waste product of oxygenic photosynthesis. Chemically unstable and biosynthetically dangerous, diatomic oxygen is the key ingredient in aerobic metabolism, and a prerequisite for the evolution of large complex organisms that define the modern biosphere. Exactly how much they require is only loosely constrained, but Catling et al . (2005) suggest something in the order of 10 3 –10 4 pascals [Pa] ( ≈ 1–10% atmospheric partial pressure [ p O 2 ]; ≈ 5–50% present atmospheric level [PAL]). Moreover, geochemical modelling indicates that p O 2 has fluctuated substantially over the course of the Phanerozoic: up to c . 35% in the late Palaeozoic, and down to perhaps 10% in the early Mesozoic (Falkowski et al ., 2005; Berner, 2006). Such shifts have been considered instrumental in directing the course of Phanerozoic evolution and extinction (Falkowski et al ., 2005; Huey & Ward, 2005; Ward et al ., 2006; Berner et al ., 2007), including the Ediacaran–Cambrian radiations of macroscopic life (Berkner & Marshall, 1965; Anbar & Knoll, 2002; Catling et al ., 2005; Holland, 2006; Canfield et al ., 2007). This is certainly a worthy hypothesis, and one that has gained an exceptionally wide following over the past decade – but a popular paradigm still needs to be tested and weighed against competing scenarios. In this editorial, I take critical look at the oxygen-evolution connection and discuss an alternative, biological explanation for geochemical signatures through the terminal Proterozoic. In the absence of direct measurements, the oxygen content of ancient atmospheres is inferred from a combination of geochemical proxies and modelling. These values, however, are accompanied by substantial error (see Berner et al ., 2007: fig. 1; Kump, 2008: fig. 2), and differing modelling assumptions yield conspicuously different results. For example, where Bergman et al . (2004) estimate the oxygen content of Mesozoic atmospheres to be continuously at or above PAL, Berner (2006) puts late Triassic/early Jurassic levels close to 50% PAL, followed by an early Cretaceous rise to more or less modern levels. Falkowski et al . (2005) also recognize an early Jurassic minimum, but with PAL not reached until the Eocene. All models are of course limited by the runaway wildfire induced at p O 2 > 35%, and lack of fire at p O 2 < 15% (Belcher & McElwain, 2008), neither of which have obtained for the past 420 million years (with the possible exception of a ‘charcoal gap’ in the late Devonian (Scott & Glasspool, 2006)). Atmospheric oxygen concentrations are more difficult to constrain in the absence of a land plant record, though the appearance of iron-retaining palaeosols at or around the 2.45 Ga ‘great oxygenation event’ marks the onset of p O 2 > 0.2% (1% PAL), and the rise of conspicuously macroscopic fossils from c . 575 Ma sets a Phanerozoic lower limit of c . 1% p O 2 or 5% PAL (Canfield, 2005; Catling et al ., 2005; Holland, 2006; Kump, 2008). Deep-sea geochemical proxies and modelling have been used to estimate an upper limit of c . 8% p O 2 (40% PAL) for most of the Proterozoic.

Oxygen, animals and oceanic ventilation: an alternative view.

A phylogenetic analysis of the arachnid orders based on morphological characters

The geochemical carbon cycle is strongly influenced by life on land, principally through the effects of carbon sequestration and the weathering of calcium and magnesium silicates in surface rocks and soils. Knowing the time of origin of land plants and animals and also of key organ systems (e.g. plant vasculature, roots, wood) is crucial to understand the development of the carbon cycle and its effects on other Earth systems. Here, we compare evidence from fossils with calibrated molecular phylogenetic trees (timetrees) of living plants and arthropods. We show that different perspectives conflict in terms of the relative timing of events, the organisms involved and the pattern of diversification of various groups. Focusing on the fossil record, we highlight a number of key biases that underpin some of these conflicts, the most pervasive and far-reaching being the extent and nature of major facies changes in the rock record. These effects probably mask an earlier origin of life on land than is evident from certain classes of fossil data. If correct, this would have major implications in understanding the carbon cycle during the Early Palaeozoic.

https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.2011.0271

A timeline for terrestrialization: consequences for the carbon cycle in the Palaeozoic

Distribution and dispersal history of Eurypterida (Chelicerata)

The fossil record has yielded various gigantic arthropods, in contrast to their diminutive proportions today. The recent discovery of a 46 cm long claw (chelicera) of the pterygotid eurypterid (‘sea scorpion’) Jaekelopterus rhenaniae , from the Early Devonian Willwerath Lagerstatte of Germany, reveals that this form attained a body length of approximately 2.5 m—almost half a metre longer than previous estimates of the group, and the largest arthropod ever to have evolved. Gigantism in Late Palaeozoic arthropods is generally attributed to elevated atmospheric oxygen levels, but while this may be applicable to Carboniferous terrestrial taxa, gigantism among aquatic taxa is much more widespread and may be attributed to other extrinsic factors, including environmental resources, predation and competition. A phylogenetic analysis of the pterygotid clade reveals that Jaekelopterus is sister-taxon to the genus Acutiramus , and is among the most derived members of the pterygotids, in contrast to earlier suggestions.

/pdf/giant-claw-reveals-the-largest-ever-arthropod-1evuf0eg47.pdf

Giant claw reveals the largest ever arthropod.

The morphology of the Late Silurian (Přidoli) scorpion Proscorpius osborni (Whitfield, 1885a) (Arachnida: Scorpiones), from the Phelps Member of the Fiddlers Green Formation of New York, the ‘Bertie Waterlime’ of earlier stratigraphic schemes, is revised based on studies of new and existing material (a total of 32 specimens). Previous reports of four cheliceral articles, gnathobasic coxae, a labium and gill slits in P. osborni can be dismissed. However, we confirm the presence of both median and compound lateral eyes, a pair of tarsal claws, albeit on a more digitigrade foot compared to that of modern scorpions, more than five ventral mesosomal sclerites and a fairly modern pattern of metasomal (i.e. tail) carinae. The co-occurring Archaeophonus eurypteroidesKjellesvig-Waering, 1966 and Stoermeroscorpio delicatusKjellesvig-Waering, 1986 are regarded as junior synonyms of P. osborni. Fossil scorpion higher systematics is plagued by a plethora of unnecessary and largely monotypic higher taxa and we draw on the results of Jeram’s cladistic analysis from 1998 to synonymize formally a series of families and superfamilies with Proscorpiidae Scudder, 1885.

https://research.amnh.org/users/lorenzo/PDF/Dunlop.2008.Paleo.Proscorpius.pdf

Reinterpretation of the Silurian scorpion Proscorpius osborni (Whitfield): integrating data from Palaeozoic and Recent scorpions

Ecdysis in sea scorpions (Chelicerata: Eurypterida)

The fossil remains of eurypterid cuticles in this study yield long-chain (<C9 to C22) aliphatic components similar to type II kerogen during pyrolysis–gas chromatography/mass spectrometry, in contrast to the chitin and protein that constitute the bulk of modern analogs. Structural analysis (thermochemolysis) of eurypterid cuticles reveals fatty acyl moieties (derived from lipids) of chain lengths C7 to C18, with C16 and C18 components being the most abundant. The residue is immune to base hydrolysis, indicating a highly recalcitrant nature and suggesting that if ester linkages are present in the macromolecule, they are sterically protected. Some samples yield phenols and polyaromatic compounds, indicating a greater degree of aromatization, which correlates with higher thermal maturity as demonstrated by Raman spectroscopy. Analysis (including thermochemolysis) of the cuticle of modern scorpions and horseshoe crabs, living relatives of the eurypterids, shows that C16 and C18 fatty acyl moieties likewise dominate. If we assume that the original composition of the eurypterid cuticle is similar to that of living chelicerates, fossilization likely involves the incorporation of such lipids into an aliphatic polymer. Such a process of in situ polymerization accounts for the fossil record of eurypterids.

https://bioone.org/journals/palaios/volume-22/issue-4/palo.2006.p06-057r/THE-FOSSILIZATION-OF-EURYPTERIDS-A-RESULT-OF-MOLECULAR-TRANSFORMATION/10.2110/palo.2006.p06-057r.pdf

O. Erik Tetlie

Papers

Distribution and dispersal history of Eurypterida (Chelicerata)

Giant claw reveals the largest ever arthropod.

Reinterpretation of the Silurian scorpion Proscorpius osborni (Whitfield): integrating data from Palaeozoic and Recent scorpions

Ecdysis in sea scorpions (Chelicerata: Eurypterida)

The fossilization of eurypterids: a result of molecular transformation