Alex Page

Ubiquitous Burgess Shale–style “clay templates” in low-grade metamorphic mudrocks

Despite the Burgess Shale’s (British Columbia, Canada) paleobiological importance, there is little consensus regarding its taphonomy. Its organic fossils are preserved as compressions associated with phyllosilicate fi lms (“clay templates”). Debate focuses on whether these templates were fundamental in exceptional preservation or if they formed in metamorphism, meaning that it is important to establish the timing of their formation relative to decay. An early diagenetic origin has been proposed based on anatomy-specifi c variations in their composition, purportedly refl ecting contrasts in decay. However, we demonstrate that these films bear a remarkable similarity to those that occur on organic fossils in graptolitic mudrocks and form as a normal product of low-grade metamorphism. Such phyllosilicates may also occur within voids created by volume loss in maturation, a process that may have aided their formation. In bedding-plane assemblages from graptolitic mudrocks, different taxa are associated with distinct phyllosilicates. This likely refl ects stepwise maturation of their constituent kerogens in an evolving hydrothermal fl uid, with different hyllosilicates forming as each taxon progressively underwent maturation. These observations provide an analogue for the distribution and composition of phyllosilicates on Burgess Shale fossils, which we interpret as refl ecting variations in the maturation of their constituent tissues. Thus, their clay templates seem unremarkable, forming too late to account for exceptional preservation.

Syntectonic monazite in low-grade mudrocks: a potential geochronometer for cleavage formation?

Monazite paragenesis during low-grade metamorphism is poorly understood. The first unequivocal instance of the mineral having crystallized synchronously with compressional cleavage formation is reported. Compositionally, these monazites resemble diagenetic forms but differ texturally by displaying syntectonic microfabrics. Their predominant association with organic-walled fossils implies that organic material may play an important role in controlling the behaviour of light rare earth elements (LREE) during low-grade metamorphism, probably by releasing diagenetically sequestered LREE and organically bound phosphorus during thermal maturation (devolatilization), and/or by promoting a favourable environment for monazite precipitation. Potentially, such monazites could provide a powerful geochronometer for cleavage formation.

Did lingering ice sheets moderate anoxia in the Early Palaeozoic of Libya

The Hirnantian glaciation of West Gondwana produced a glacially sculpted topography, which is draped by organic-rich latest Ordovician and early Silurian ‘hot shales’. Although these are the most important Early Palaeozoic source rock in North Africa, organic enrichment is distributed unevenly. For example, in Al Kufrah Basin, Libya, ‘hot shales’ are elusive, but outcrop analysis at the western basin demonstrates why this is the case. The topmost Mamuniyat Formation, of Hirnantian age, comprises glaciogenic sandstones, passing upward into mixed facies of the Tanezzuft Formation, which has a latest Ordovician–early Silurian age. The basal Tanezzuft Formation contains a shelly carbonate (cool-water deposits accumulated under oxygenating conditions) and bioturbated sandstone succession. Above, hummocky cross-bedded and graded sandstone intervals are intercalated with shale and siltstone (storm influx onto a muddy shelf). These are interrupted by several lonestone-bearing intervals (ice-rafted debris), a striated pavement (of subglacial origin), and manganese oxide crusts and concretions. The concretions and bioturbation imply oxygenation of the sea floor during transgression. These putative glacial deposits were deposited following the main phase of the Hirnantian glaciation, at the same stratigraphic level as ‘hot shales’ elsewhere in northern Gondwana. Lingering ice caps may have produced well-oxygenated marine waters precluding ‘hot shale’ deposition.

Exceptionally preserved ostracodes from a Middle Miocene palaeolake, California, USA

Abstract: Exceptionally well-preserved specimens of a new cypridid ostracode (Crustacea), Raepula ira sp. nov., are described from palaeolake sediments of the Middle Miocene Barstow Formation of the Mud Hills, southern California. This is only the second occurrence of exceptionally preserved ostracodes from the Miocene. Based on ages obtained from associated volcanic tuffs the palaeolake sediments were deposited between 16.3 and 15.8 Ma. The ostracodes form one element of a diverse lake community dominated by fairy shrimps, copepods, diatoms, larvae of diving beetles, flies and mosquitoes, and body fossils and ephippia of branchiopods and anomopods. The ostracodes are preserved three dimensionally with their soft anatomy replicated in microcrystalline silica. Submicron-scale details such as sensory setae are preserved, surpassing the resolution of most other ostracode-bearing lagerstätten and allowing their biology to be compared with extant taxa. Supplementary material: Red–blue anaglyphs of the exceptionally preserved ostracodes are available at http://www.geolsoc.org.uk/SUP18407.

Normalograptus kufraensis, a new species of graptolite from the western margin of the Kufra Basin, Libya

Normalograptus kufraensis sp. nov. occurs as monospecific assemblages in the Tanezzuft Formation at the western margin of the Kufra Basin (Jabal Eghei), southern Libya. These graptolites have parallel-sided rhabdosomes with long, straight virgellae, climacograptid thecae and a full straight median septum. N. kufraensis is intermediate between Ordovician graptolites from the N. angustus (Perner) lineage and the younger sister species N. ajjeri (Legrand) and N. arrikini Legrand. N. kufraensis differs from these taxa as follows: it is broader than N. angustus; it has greater thecal spacing than N. ajjeri or N. arrikini. A table comparing measurements of N. kufraensis with 44 other Normalograptus taxa differentiates it from other members of this morphologically conservative group. Even though N. angustus and N. ajjeri are very long-ranging graptolites, a stratophenetic approach suggests that the specimens from Jabal Eghei may be of late Hirnantian or younger age. The faunal composition and preservation suggests these graptolites occupied the ‘cratonic invader’ biotope. The stratigraphic succession records deglacial flooding and fluctuating of redox in the Tanezzuft Formation, with the graptolites indicating a short-lived interval of anoxia.

Polymorphic organization in a planktonic graptoloid (Hemichordata: Pterobranchia) colony of Late Ordovician age

Graptolites are common fossils in Early Palaeozoic strata, but little is known of their soft-part anatomy. However, we report a long-overlooked specimen of Dicranograptus aff. ramosus from Late Ordovician strata of southern Scotland that preserves a strongly polymorphic, recalcitrant, organic-walled network hitherto unseen in graptoloid graptolites. This network displays three morphologies: proximally, a strap-like pattern, likely of flattened tubes; these transform distally into isolated, hourglass-shaped structures; then, yet more distally, revert to a (simpler) strap-like pattern. The network most likely represents a stolon-like system, hitherto unknown in graptoloids, that connected individual zooids. Its alternative interpretation, as colonial xenobionts that infested a graptoloid colony and mimicked its architecture, is considered less likely on taphonomic and palaeobiological grounds. Such polymorphism is not known in non-graptolite pterobranchs, which are less diverse and morphologically more conservative: a division of labour between graptoloid zooids for such functions as feeding, breeding and rhabdosome construction may have been the key to their remarkable evolutionary success.

Soft-part preservation in a bivalved arthropod from the Late Ordovician of Wales

A new component of the Early Palaeozoic arthropod fauna is described from a monospecific accumulate of carapaces in a Late Ordovician (Katian) hemipelagic mudstone from the Cardigan district of southwest Wales (UK). Its non-biomineralized carapace is preserved as a carbonaceous residue, as is more labile anatomy (soft-parts) including the inner lamella and sub-ovate structures near its antero-dorsal margin, which we interpret to be putative eyes. The depositional context and associated fauna indicate that the arthropods inhabited an area of deep water and high primary productivity above a pronounced submarine topography. The preserved density of carapaces suggests the arthropods may have congregated into shoals or been transported post-mortem into depressions which acted as detritus traps. The accumulate provides a rare example of soft-part preservation in hemipelagic mudstones and highlights the role of organic material as a locus for authigenic mineralization during metamorphism.

Dawsonia Nicholson: linguliform brachiopods, crustacean tail-pieces and a problematicum rather than graptolite ovarian vesicles

Though little is known of the graptoloid reproductive mechanism, graptolites with putatively sac-like appendages, supposedly ovarian vesicles, have been known from the Moffat Shales Group, Southern Uplands, Scotland, for over 150 years. Locally, these co-occur with isolated, two-dimensional, discoidal or ovatotriangular fossils. In the 1870s, Nicholson interpreted these isolated fossils as being graptoloid ‘egg-sacs’, detached from their parent and existing as free-swimming bodies. He assigned them to the genus “Dawsonia”, though the name was preoccupied by a trilobite, and named four species: “D.” campanulata, “D.” acuminata, “D.” rotunda (sic.) and “D.” tenuistriata. A reassessment of Nicholson’s type material from the Silurian of Moffatdale, Scotland, and the Ordovician Levis Formation of Quebec, Canada, shows that Dawsonia Nicholson comprises the inarticulate brachiopods Acrosaccus? rotundus, Paterula? tenuistriata and Discotreta cf. levisensis, the tail-piece of the crustacean Caryocaris acuminata and the problematic fossil “D.” campanulata. Though “D.” campanulata resembles sac-like graptolite appendages, morphometric analysis reveals the similarity to be superficial and the systematic position of this taxon remains uncertain. There is no definite evidence of either “D.” campanulata or sac-like graptoloid appendages having had a reproductive function.

Dawsonia Nicholson: Linguliform brachiopods, crustacean tail-pieces and a problematicum rather than graptolite ovarian vesicles

Though little is known of the graptoloid reproductive mechanism, graptolites with putatively sac-like appendages, supposedly ovarian vesicles, have been known from the Moffat Shales Group, Southern Uplands, Scotland, for over 150 years. Locally, these co-occur with isolated, two-dimensional, discoidal or ovatotriangular fossils. In the 1870s, Nicholson interpreted these isolated fossils as being graptoloid ‘egg-sacs’, detached from their parent and existing as free-swimming bodies. He assigned them to the genus “Dawsonia”, though the name was preoccupied by a trilobite, and named four species: “D.” campanulata, “D.” acuminata, “D.” rotunda (sic.) and “D.” tenuistriata. A reassessment of Nicholson’s type material from the Silurian of Moffatdale, Scotland, and the Ordovician Levis Formation of Quebec, Canada, shows that Dawsonia Nicholson comprises the inarticulate brachiopods Acrosaccus? rotundus, Paterula? tenuistriata and Discotreta cf. levisensis, the tail-piece of the crustacean Caryocaris acuminata and the problematic fossil “D.” campanulata. Though “D.” campanulata resembles sac-like graptolite appendages, morphometric analysis reveals the similarity to be superficial and the systematic position of this taxon remains uncertain. There is no definite evidence of either “D.” campanulata or sac-like graptoloid appendages having had a reproductive function.

J. Zalasiewicz 2008. The Earth After Us. What Legacy Will Humans Leave in the Rocks? xv + 251 pp. Oxford, New York: Oxford University Press. Price £14.99 (hard covers). ISBN 978019 921497 6.

Though we all know that the present is the key to the past, The Earth After Us takes geological investigation one step further. By asking ‘What precisely will mankinds legacy be 100 million years hence?’, the book offers a unique and thought-provoking insight into anthropogenic climate change. The premise is simple: a future race of geological explorers chances upon a widespread and charismatic event stratum and this, the prologue tells us, relates to a planet-wide environmental crisis and a widespread mass extinction. But such hyperbolae are swiftly set aside. This is not some wild fantasy about alien geologists (or indeed a future …