James B. Jago

Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes

Until recently, intricate details of the optical design of non-biomineralized arthropod eyes remained elusive in Cambrian Burgess-Shale-type deposits, despite exceptional preservation of soft-part anatomy in such Konservat-Lagerstätten. The structure and development of ommatidia in arthropod compound eyes support a single origin some time before the latest common ancestor of crown-group arthropods, but the appearance of compound eyes in the arthropod stem group has been poorly constrained in the absence of adequate fossils. Here we report 2–3-cm paired eyes from the early Cambrian (approximately 515 million years old) Emu Bay Shale of South Australia, assigned to the Cambrian apex predator Anomalocaris. Their preserved visual surfaces are composed of at least 16,000 hexagonally packed ommatidial lenses (in a single eye), rivalling the most acute compound eyes in modern arthropods. The specimens show two distinct taphonomic modes, preserved as iron oxide (after pyrite) and calcium phosphate, demonstrating that disparate styles of early diagenetic mineralization can replicate the same type of extracellular tissue (that is, cuticle) within a single Burgess-Shale-type deposit. These fossils also provide compelling evidence for the arthropod affinities of anomalocaridids, push the origin of compound eyes deeper down the arthropod stem lineage, and indicate that the compound eye evolved before such features as a hardened exoskeleton. The inferred acuity of the anomalocaridid eye is consistent with other evidence that these animals were highly mobile visual predators in the water column. The existence of large, macrophagous nektonic predators possessing sharp vision—such as Anomalocaris—within the early Cambrian ecosystem probably helped to accelerate the escalatory ‘arms race’ that began over half a billion years ago.

Formation and reactivation of the Cambrian Kanmantoo Trough, SE Australia: implications for early Palaeozoic tectonics at eastern Gondwana’s plate margin

The Kanmantoo Group is a thick and largely metamorphosed sedimentary succession that filled an isolated arcuate Cambrian basin (Kanmantoo Trough) which formed within continental Gondwana, and now lies on the southern margin of the present Australian continent. Kanmantoo Group sediments unconformably overlie Neoproterozoic and older Cambrian rocks. We consider that the geometry of the southern part of this trough was influenced by strike-slip movement along an intra-continental tear fault. To the north, the basin changes to a style dominated by orthogonal extension and eventually tapers and dies out normal to the tear fault. Balanced sections show that the kinematic style and strain distribution developed during early Palaeozoic inversion was controlled by the specific architecture of the Kanmantoo Trough. Early Palaeozoic tear faulting could have linked contrasting subduction polarities along the then contiguous palaeo-Pacific margin of Gondwana. The Kanmantoo Trough is considered to have formed at a passive margin related to east-directed subduction in what is now the Australian continent. In contrast, west-directed subduction formed an active margin at contiguous parts of current Antarctica. Kanmantoo Group sediments were derived from the south by erosion of a Grenvillean province mixed with sediments eroded from the emergent active margin of Gondwana. The inception, localization and sedimentation in the Kanmantoo Trough reflects a complex interaction of tectonic processes along the encroaching Ross–Delamerian Orogen.

Modern optics in exceptionally preserved eyes of Early Cambrian arthropods from Australia.

Despite the status of the eye as an “organ of extreme perfection”, theory suggests that complex eyes can evolve very rapidly. The fossil record has, until now, been inadequate in providing insight into the early evolution of eyes during the initial radiation of many animal groups known as the Cambrian explosion. This is surprising because Cambrian Burgess-Shale-type deposits are replete with exquisitely preserved animals, especially arthropods, that possess eyes. However, with the exception of biomineralized trilobite eyes, virtually nothing is known about the details of their optical design. Here we report exceptionally preserved fossil eyes from the Early Cambrian (∼515 million years ago) Emu Bay Shale of South Australia, revealing that some of the earliest arthropods possessed highly advanced compound eyes, each with over 3,000 large ommatidial lenses and a specialized ‘bright zone’. These are the oldest non-biomineralized eyes known in such detail, with preservation quality exceeding that found in the Burgess Shale and Chengjiang deposits. Non-biomineralized eyes of similar complexity are otherwise unknown until about 85 million years later. The arrangement and size of the lenses indicate that these eyes belonged to an active predator that was capable of seeing in low light. The eyes are more complex than those known from contemporaneous trilobites and are as advanced as those of many living forms. They provide further evidence that the Cambrian explosion involved rapid innovation in fine-scale anatomy as well as gross morphology, and are consistent with the concept that the development of advanced vision helped to drive this great evolutionary event.

Cambrian trilobites from Northern Victoria Land, Antarctica, and their stratigraphic implications

Abstract Fifty‐four trilobite taxa from 15 new localities in the Bowers Terrane of Northern Victoria Land, including the first fossils from Molar Formation, are described. The fossils indicate an age range from late Middle Cambrian (Boomerangian or older) to mid Late Cambrian (late Idamean). At Reilly Ridge, the Spurs Formation crops out in a number of fault‐bounded slices; the new fossils indicate marked lateral facies contrasts between slices and suggest considerable lateral displacement along the bounding faults. At Houliston Glacier, trilobites of Mindyallan age in Molar Formation imply that the boundary between the Molar and Spurs Formations is strongly time transgressive. New species described in this paper are Reillopleura braddocki gen. et sp. nov. and Notoaphelaspis horizontalis sp. nov. Faunal affinities are mainly with the Ellsworth Mountains (West Antarctica), western Queensland, and China.

The geological context of the Lower Cambrian (Series 2) Emu Bay Shale Lagerstätte and adjacent stratigraphic units, Kangaroo Island, South Australia

The lower Cambrian (Cambrian Series 2, Stage 4) Emu Bay Shale Lagerstätte, which is by far the most important Burgess Shale-type (BST) deposit in Australia, occurs mainly in the bottom 10 m of the Emu Bay Shale at Big Gully on the north coast of Kangaroo Island, South Australia. In this area, the exposed Cambrian succession commences with the White Point Conglomerate, the bulk of which comprises a crudely cross-bedded cobble to boulder conglomerate with minor mudstone and sandstone facies. The conglomeratic horizons thin markedly to the south. The White Point Conglomerate was deposited as coalesced fan deltas derived from an uplifted tectonic margin immediately to the north of the present coastline. The White Point Conglomerate is overlain by the sandstone, siltstone and conglomerate beds of the Marsden Sandstone (new name), the basal 3 m of which is a distinctive fossiliferous argillaceous limestone and shale, the Rouge Mudstone Member (new name). Syndepositional folding and faulting affected both the White Point Conglomerate and Marsden Sandstone prior to the deposition of the Emu Bay Shale, the base of which represents a sequence boundary. The Lagerstätte occurs within dark grey to black laminated micaceous mudstone facies, some of which show evidence of syndepositional disturbance, and are interpreted to have been deposited in isolated stagnant, anoxic to oxic depressions on the sea floor, beneath a normally oxic water column, with a sharp redox boundary at the sediment–water interface; below this boundary the pore water was anoxic. Thin (up to 20 cm) structureless fine sandstone horizons within the mudstone are interpreted as either sediment gravity flow or storm deposits. The Lagerstätte-bearing mudstone beds thin southwards and disappear 500–600 m south of the coast. The Emu Bay Shale coarsens upwards; arthropod tracks are abundant in fine sandstone beds towards the top of the Emu Bay Shale. In coastal sections the sandstone facies of the Boxing Bay Formation rest conformably on the Emu Bay Shale; inland the contact is channelled.

Global Cambrian trilobite palaeobiogeography assessed using parsimony analysis of endemicity

Abstract Palaeobiogeographical data on Cambrian trilobites obtained during the twentieth century are combined in this paper to evaluate palaeoceanographic links through c. 30 myr, once these arthropods biomineralized. Worldwide major tectonostratigraphic units are characterized at series intervals of Cambrian time and datasets of trilobite genera (629 for Cambrian Series 2, 965 for Cambrian Series 3, and 866 for the Furongian Series) are analysed using parsimony analysis of endemicity. Special attention is given to the biogeographical observations made in microcontinents and exotic terranes. The same is done for platform-basinal transects of well-known continental margins. The parsimony analysis of endemicity analysis resulted in distinct palaeogeographical area groupings among the tectonostratigraphic units. With these groupings, several palaeobiogeographical units are distinguished, which do not necessarily fit the previously proposed biogeographical realms and provinces. Their development and spatial distributions are broadly controlled by Cambrian palaeoclimates, palaeogeographical conditions (e.g. carbonate productivity and anoxic conditions) and ocean current circulation. Supplementary material: Global dataset of Cambrian Epoch 2 (A), Cambrian Epoch 3 (B) and the Furongian Epoch (C) trilobite genera are provided at: http://www.geolsoc.org.uk/SUP18669

Turbidite deposition in the Cambrian Kanmantoo Group, South Australia

The Kanmantoo Group of South Australia is a thick (∼7–8 km) succession of predominantly clastic marine sedimentary and metasedimentary rocks that were deposited very rapidly in a localised basin (Kanmantoo Trough) during the Early Cambrian. Despite structural complexity and varying grades of metamorphism, a surprising amount of primary sedimentological information is still available. Although a variety of depositional facies are represented, the group is dominated by parallel, sharp‐based, mineralogically immature sandstone interbedded with mudstone. The sandstone beds are most commonly fine to medium grained, massive and lacking in obvious grading except at the top. Single beds often reach several metres in thickness and amalgamation of beds is not uncommon. We argue that these sandstone beds could be the products of sustained high‐density turbidity currents. Triggering mechanisms for such turbidity currents remain uncertain, but they may have been initiated as hyperpycnal flows during catastrophic flood events at the mouths of high‐load‐capacity rivers, or from the failure of unstable buildups of sediment on delta slopes. Palaeocurrent studies from sole marks suggest a southerly source, which was probably an active orogenic terrain in formerly contiguous Antarctica. It is likely that a major delta complex lay at the southern end of the basin.

Paleoredox status and thermal alteration of the lower Cambrian (Series 2) Emu Bay Shale Lagerstätte, South Australia

While exceptionally diverse fossil assemblages of non-biomineralised organisms (Lagerstätten) are rare, they are unusually common in marine sedimentary sequences of early and mid-Cambrian age. Their mode of preservation has been the subject of much debate. The lower Cambrian (Series 2) Emu Bay Shale biota, found at Big Gully on the north coast of Kangaroo Island, is by far the richest Burgess Shale-type (BST) fauna known in the southern hemisphere. Such fauna are preserved characteristically as two-dimensional compression fossils, comprising both carbonaceous and mineralised films on bedding surfaces of the host marine mudstones. The biotic diversity of the Big Gully assemblage suggests a habitat very favourable for life. Its preservation is exceptional, with gut remains and other soft parts quite common. Evidence of predation and scavenging is rare, and the finely laminated texture of the host mudstone attests to a lack of burrowing and bioturbation. Recent studies indicate that conservation of organic tissues, rather than authigenic mineralisation of their more labile components, is the principal taphonomic pathway responsible for BST deposits. In so far as such preservation requires suppression of the early diagenetic processes that normally result in the rapid decay of organic matter at or near the sea floor, the oxicity of the bottom waters, below which the Emu Bay Shale accumulated, becomes critically important. Here we determine the paleoredox status of the fossiliferous basal portion of the formation using selected trace element proxies, in combination with total organic carbon (TOC) concentrations and isotopic signatures (δ13Corg). We also establish its degree of thermal alteration as a datum for use in taphonomic comparisons with other Cambrian Lagerstätten. The Emu Bay Shale contains insufficient organic matter (TOC = 0.25–0.55%) to have accumulated under stable anoxic conditions. Even allowing for the inevitable loss of organic carbon during the oil- and gas-generation phases of thermal maturation, to a present rank equivalent to ∼1.5% vitrinite reflectance, its original TOC content was <1%. Measurement of a series of redox-sensitive elemental ratios (viz. U/Th, V/Cr, Ni/Co and V/Sc) across the lower 8 m-thick fossiliferous section of the Emu Bay Shale confirms that it was deposited beneath an oxic water column. In this respect it is similar to the archetypical Burgess Shale. In the absence of an exaerobic zone, benthic cyanobacterial mats are likely to have mantled recently dead fauna and helped maintain the integrity of a sharp redox boundary at the sediment–water interface.

The Emu Bay Shale Konservat-Lagerstätte: a view of Cambrian life from East Gondwana

Recent fossil discoveries from the lower Cambrian Emu Bay Shale (EBS) on Kangaroo Island, South Australia, have provided critical insights into the tempo of the Cambrian explosion of animals, such as the origin and seemingly rapid evolution of arthropod compound eyes, as well as extending the geographical ranges of several groups to the East Gondwanan margin, supporting close faunal affinities with South China. The EBS also holds great potential for broadening knowledge on taphonomic pathways involved in the exceptional preservation of fossils in Cambrian Konservat-Lagerstätten. EBS fossils display a range of taphonomic modes for a variety of soft tissues, especially phosphatization and pyritization, in some cases recording a level of anatomical detail that is absent from most Cambrian Konservat-Lagerstätten.

Stratigraphy of the Kanmantoo Group: a critical element of the Adelaide Fold Belt and the Palaeo‐Pacific plate margin, Eastern Gondwana

The Kanmantoo Group, occupying a key position along the Cambrian Palaeo‐Pacific margin of southern Australia Gondwana, comprises two distinct successions (from bottom to top): (i) the Keynes Subgroup comprising the Carrickalinga Head and Backstairs Passage Formations (lowstand systems tract, transgressive systems tract, highstand systems tract); and (ii) the unconformably overlying Bollaparudda Subgroup (new name) comprising the Talisker, Tapanappa, Tunkalilla, Balquhidder, Petrel Cove and Middleton Formations. Both conventional stratigraphy and recent isotopic work point to the base of the Kanmantoo Group as representing a major change in the depositional history of the Adelaide Geosyncline. The type section of the Kanmantoo Group, along the south coast of the Fleurieu Peninsula, is now recognised as being characterised by structural complexity (shear zones, folding), contrary to the essentially largely non‐faulted homoclinal structure described by most earlier authors. Many of the formation boundaries in the type section, previously described as conformable, are in fact faulted. As a result, subsidiary type sections are proposed for the Talisker and Tunkalilla Formations.