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Cambrian trilobites mainly lived on the sea floor, and up till now few, if any, unequivocally planktonic trilo-bites have been reported from earlier than the Ordovician. The late Cambrian (Furongian) to late Ordovician olenids are a distinctive group of benthic (sea-floor dwelling) or nekto-benthic trilobites. Here we show, however, that one recent...
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Studies focused on the post-embryonic development of trilobites are important for our understanding of their palaeobiogeographic distribution, their survival during extinction events and their evolutionary relationships. This article briefly summarizes the current knowledge of the post-embryonic development of trilobites. Trilobites show a multista...
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... In the absence of further information, it was generally believed that a layer of sensory cells extended over the flat base and acted as a retina. When m-CT-scanning became generally available, it was possible to investigate trilobite eyes in a new way (Schoenemann et al. 2008b;Schoenemann & Clarkson 2012d. Specimens of the Middle Devonian phacopids Geesops schlotheimi (Bronn, 1825) (Fig. 4a), Phacops latifrons (Bronn, 1825) from the Ahrdorf Formation at a locality between Gees and Gerolstein, Germany and Barrandeops cf. ...
... The disadvantage for any model about the evolution of eyes, especially when considering those of extinct organisms, and the incomplete nature of the fossil record, is that one cannot be overly confident about what actually happened during the course of time; thus the conclusions remain purely hypothetical. A recent breakthrough came when the sensory structures of phacopid compound eyes were first made visible by m-CT and synchrotron analyses (Schoenemann et al. 2008bSchoenemann & Clarkson 2012d. Even if these structures are still not completely understood, this work opened the way for studying preserved soft tissues of eye structures to understand their function, internal complexity and evolutionary significance. ...
This paper presents a review of recent developments in the study of vision in fossil arthropods, beginning with a discussion of the origin of visual systems. A report of the eyes of Cambrian arthropods from different Lagerstätten, especially the compound and median arthropod eyes from the Chengjiang fauna of China, is given. Reference is made also to compound eyes from the lower Cambrian Emu Bay Shale fauna of Australia and the Sirius Passet fauna of Greenland; also to the three-dimensionally preserved ‘Orsten’ fauna of Sweden. An understanding of how these eyes functioned is possible by reference to living arthropods and by using physical tools developed by physiologists. The eyes of trilobites (lower Cambrian to Upper Permian) are often very well preserved, and the structure and physiology of their calcite lenses, and the eye as a whole, are summarised here, based upon recent literature. Two main kinds of trilobite eyes have been long known. Firstly, there is the holochroal type, in which the lenses are usually numerous, small and closely packed together; this represents the ancestral kind, first found in lowermost Cambrian trilobites. The second type is the schizochroal eye, in which the lenses are relatively much larger and each is separated from its neighbours. Such eyes are confined to the single suborder Phacopina (Lower Ordovician to Upper Devonian). This visual system has no real equivalents in the animal kingdom. In this present paper, the origin of schizochroal eyes, by paedomorphosis from holochroal precursors, is reviewed, together with subsequent evolutionary transitions in the Early Ordovician. A summary of new work on the structure and mineralogy of phacopid lenses is presented, as is a discussion of the recent discovery of sublensar sensory structures in Devonian phacopids, which has opened up new dimensions in the study of trilobite vision.
... Swimming trilobites are appropriate candidates for this type of study because the acquisition of swimming ability from a benthic lifestyle is a significant autecological innovation (Fortey 1985(Fortey , 2004. A few decades of research have demonstrated that "planktonic trilobites" commonly share certain morphological characteristics, such as a dwarfed body size and high spinosity (e.g., Fortey 1985Fortey , 2004Clarkson & Ahlberg 2003;Schoenemann et al. 2008Schoenemann et al. , 2010, which are consistent with those of modern planktonic arthropods (e.g., Marshall & Diebel 1995, Land 2000. ...
The mode of life of the remopleuridid trilobite Hypodicranotus striatulus Walcott (1875) was examined hydrodynamically with a special focus on the relationship between the autecological performances of swimming and feeding. To understand the effect of swimming height from the sea bottom on the hydrodynamic performance of the exoskeleton, we performed computational fluid dynamics simulations on four models at differing distances from the sea bottom. The results indicated that Hypodicranotus could launch itself from the sea bottom with a relatively strong hydrodynamic lift force from slow walking or swimming speeds. However, the lift force decreased as the swimming height increased at slow swimming speeds. Hence, Hypodicranotus would have had to increase its swimming speed to greater than 0.2 m/s and to obtain the most stable lift force at a swimming height equal to half of its own body height. Its exoskeletal morphology, with a forked hypostome, enabled it to launch itself at a slow velocity and swim at a modest distance, i.e., close to its own height, from the sea bottom. Feeding from the median vortex flows along the food groove between the two prongs of the hypostome may have been the best strategy near the sea bottom, where a large amount of food matter would have been available. Because arthropod musculature consists of striated muscles, which exhibit inferior endurance, Hypodicranotus most likely adapted to the near-bottom environment, where it could rest at times on the sea bottom as part of a nektobenthic mode of life.
... Swimming trilobites have persisted since the Late Cambrian, when they were represented by the olenid trilobite Ctenopyge ceciliae. This species is thought to have been a sluggish planktonic animal rather than an active swimmer (e.g., Schoenemann et al., 2008Schoenemann et al., , 2010. Beginning with planktonic behaviour in the seawater column, actively swimming trilobites evolved repeatedly from benthic groups throughout the Ordovician, including the telephinidid, cyclopygidid and remopleuridid phylogenetic lineages. ...
The sophisticated hydrodynamic performance achieved by the exoskeleton and the long, forked hypostome of the remopleuridid trilobite Hypodicranotus striatus was demonstrated using image-based modelling and computational fluid dynamics simulation techniques. To understand the function of the long, forked hypostome, we examined two types of exoskeletal models, one with and one without the hypostome. We simulated the flow structures around the exoskeletal models under several ambient flow velocities to evaluate the shapes of the streamlines, the values of the drag and lift forces and the relevant coefficients acting on the models. The simulation results showed that the long, forked hypostome prevents the formation of a ventral vortex; thus, it stabilises the flow structure under all of the ambient velocities tested. Moreover, the hypostome functions to create positive lift, with stable lift coefficients observed under a wide range of velocities, and to reduce the drag coefficient as velocity increases. These results imply that the hypostome can reduce viscous drag with a modest lift force, which is an essential requirement for actively swimming animals. We conclude that the long, forked hypostome evolved to provide an active and stable swimming system, and we therefore hypothesise that Hypodicranotus exoskeletal morphology resulted from the adaptation to be a high-performance swimmer.
... In horizontal section, the trabecula are arranged in thin lamellae radiating from the centre. They have also been shown recently by SEM in the holochroal eyes of Ctenopyge ceciliae (Clarkson & Ahlberg 2002) (Fig. 2B, C) (Schoenemann et al. 2008b), in early phacopine trilobites such as Zeliszkella oriens from the Ordovician of the Prague Basin (Budil et al. 2010) and in the Silurian Dalmanites sp. (Lee et al. 2007). ...
Three main types of eye have been defined in trilobites; holochroal, schizochroal and the rare abathochroal. At least in holochroal and schizochroal eyes, the lenses consist of oriented calcitic microcrystallites, the so-called trabecula, which run uninterruptedly throughout the lens from top to bottom. It is argued here that these are primary structures and not diagenetic. Holochroal eyes are commonly accepted to correspond to apposition eyes, which are the most common type of compound eyes in arthropods living today. Schizochroal eyes, present only in phacopine trilobites, are characterised by a doublet lens structure, with an aplanatic interface, correcting spherical aberration of the thick lenses to form a sharp focus for incident rays travelling parallel with the optical axis. This classic model seems to be functional for many phacopines. In these the trabecula are present, though all juxtaposed, forming a solid block. In the lenses of some phacopid species, however, the microcrystallites are separated from each other by gaps. If, during life, these gaps were filled with organic material, as the lens grew from the larval stages, or post-ecdysially, each trabeculum would be isolated from its neighbours by an organic sheath. A simple model is proposed here for the generation of the organic sheath surrounding each trabeculum. If the individual trabecula were isolated from each other, then the possibility of a new kind of visual system exists in these ‘derived’ phacopines. The differences between the refractive indexes inside and outside the trabeculum would ensure that each trabeculum acted as a light-guide, so that the whole ‘lens’ becomes a light guide bundle. This would result in a pixelled visual system, unique in the animal realm, but an archetype for modern technology of data transfer.
... Spiny gnathobases, as in Olenoides serratus and Naraoia compacta, have been cited to as evidence of predatory ⁄ scavenging habits, while sediment-infilled guts have been interpreted as indicative of deposit feeding (Bergström et al. 2007;but see Vannier & Chen 2002; for an alternative taphonomic interpretation of sediment-filled guts). Filterfeeding has also been suggested as a life mode for some trilobites on the basis of an inferred filter-chamber (Fortey & Owens 1999) or a planktonic lifestyle ( Schoenemann et al. 2008). For a review of trilobite feeding, see Fortey & Owens (1999) and Hughes (2001). ...
Hegna, T.A. 2010: The function of forks: Isotelus-type hypostomes and trilobite feeding. Lethaia, Vol. 43, pp. 411–419.
Despite previous investigations, the function of the forked morphology of asaphid trilobite hypostomes is enigmatic. The focus of this study is the large and robust forked hypostome of the largest known genus of trilobite, Isotelus, and the independently-derived forked hypostome of Hypodicranotus, the longest hypostome relative to body size of any trilobite. Although the trilobite hypostome is analogous to the labrum in other arthropods, forked hypostomes lack an obvious modern functional counterpart. The Isotelus hypostome is distinguished from other trilobite hypostomes by closely-spaced terrace ridges on a greatly thickened inner surface of the forked posterior margin, with the scarp of the terrace facing antero-ventrally. This is compatible with a grinding function, suggesting possible limb differentiation to complement this structure. The inner face of the tine (one of the two, prominent, sub-parallel posterior projections) is also unique in that it has a microstructure which is evident in section, running perpendicular to the surface. Macropredatory and filter-feeder roles are ruled out, and previous characterizations of the hypostome as knife-like or serrated are rejected. Its function is incompatible with that of other non-asaphid trilobites with forked hypostomes, like the remopleuridid Hypodicranotus, which lack similar terrace ridges and thickened inner-edge cuticle. □Arthropoda, Asaphida, ecology, functional morphology, Trilobita.
... Very rare in pre-Ordovician assemblages (Schoenemann et al., 2008), pelagic trilobites developed during the Ordovician and fairly diverse mesopelagic and epipelagic trilobite communities were established by the Tremadocian (e.g. Adrain et al., 2004;Racheboeuf et al., 2009). ...
... Setting aside the agnostids, whose mode of life has been greatly debated (see review by Fortey and Owens, 1999, pp. 455-458) but which might have been planktic in at least some taxa, few trilobites lived in the water column during the Cambrian (see Fortey and Rushton, 2007;Schoenemann et al., 2008). A substantial number of trilobite clades diversified during the Great Ordovician Biodiversification Event along similar trajectories to those of other invertebrate groups that typify the Paleozoic Evolutionary Fauna (Adrain et al., 2004). ...
The ‘Great Ordovician Biodiversification Event’ (GOBE) saw a spectacular increase in marine biodiversity at all taxonomic levels largely within the phyla established much earlier during the so-called ‘Cambrian Explosion’. The diversification was probably the result of a combination of several geological and biological processes and the positive feedbacks resulting from them. The present paper reviews the palaeoecological dimension of the GOBE. It involved major increases in α, ß and γ biodiversity largely associated with the rise of the Paleozoic Evolutionary Fauna dominated by suspension feeders and involving a greater occupation of ecospace and more complex ecological structures in the Ecological Evolutionary Units P1 and P2. In the benthos, these include more complex food webs than those of the Cambrian, greater tiering, especially above the sediment–water interface, and the development of guild structures indicating increased competition between taxa for particular resources. The Ordovician is characterized by a profound change in reef composition, with a switch from microbial-dominated reefs in the Early and Middle Ordovician to metazoan-dominated reefs in the Late Ordovician. Increases in complexity of deep-water trace fossil assemblages began in the Early Ordovician and mark the increasing exploitation in that environment and the development of hardgrounds permitted bioerosion and encrusting strategies together with the appearance of cryptic communities.
... Only few data exist for the Ordovician radiolarian diversity, not allowing precise diversity trends, although it appears that a major diversificiation took place during the Early and Middle Ordovician [69,70]. Occurrences of Cambrian planktonic trilobites are rare [71,72], but a number of cyclopygid, and telephinid trilobites, and caryocaridids, which are considered as midwater, free swimming arthropods [62,73] first appeared in the early Tremadocian and provide evidence for the widespread existence of complex pelagic food chains already in the Early Ordovician [62]. Additional data overwhelmingly support the existence of a strong Early Ordovician pulse of the invasion of the open marine realm. ...
During the Ordovician the global diversity increased dramatically at family, genus and species levels. Partially the diversification is explained by an increased nutrient, and phytoplankton availability in the open water. Cephalopods are among the top predators of today's open oceans. Their Ordovician occurrences, diversity evolution and abundance pattern potentially provides information on the evolution of the pelagic food chain.
We reconstructed the cephalopod departure from originally exclusively neritic habitats into the pelagic zone by the compilation of occurrence data in offshore paleoenvironments from the Paleobiology Database, and from own data, by evidence of the functional morphology, and the taphonomy of selected cephalopod faunas. The occurrence data show, that cephalopod associations in offshore depositional settings and black shales are characterized by a specific composition, often dominated by orthocerids and lituitids. The siphuncle and conch form of these cephalopods indicate a dominant lifestyle as pelagic, vertical migrants. The frequency distribution of conch sizes and the pattern of epibionts indicate an autochthonous origin of the majority of orthocerid and lituitid shells. The consistent concentration of these cephalopods in deep subtidal sediments, starting from the middle Tremadocian indicates the occupation of the pelagic zone early in the Early Ordovician and a subsequent diversification which peaked during the Darriwilian.
The exploitation of the pelagic realm started synchronously in several independent invertebrate clades during the latest Cambrian to Middle Ordovician. The initial rise and diversification of pelagic cephalopods during the Early and Middle Ordovician indicates the establishment of a pelagic food chain sustainable enough for the development of a diverse fauna of large predators. The earliest pelagic cephalopods were slowly swimming vertical migrants. The appearance and early diversification of pelagic cephalopods is interpreted as a consequence of the increased food availability in the open water since the latest Cambrian.
This article presents an overview of current research work, concerned with the analysis of fossilised eye systems. Besides the investigation of these sophisticated eyes in terms of the physical rules and optical principles they have to obey, it is possible by comparison with Recent systems to assign their owners to defined ecological conditions and life styles. This is demonstrated, exemplarily, in the tiny trilobite Ctenopyge ceciliae Clarkson & Ahlberg 2002, as well as in representatives of the Chengjiang Fauna from the lower Cambrian of China, surely one of the most important Lagerstätten to demonstrate the importance of vision in early evolutionary development.
Eyes other than those of trilobites are rarely preserved in the fossil record. We describe here a set of six tiny, isolated, three‐dimensionally preserved compound eyes. These secondarily phosphatized eyes were etched from ‘Orsten’ limestone nodules dated to the Agnostus pisiformis Biozone from the Cambrian Alum Shale Formation of Sweden. The ovoid eyes arise from an elongated stalk, their surface being covered by a mosaic of regular and hexagonal‐shaped facets representing the surface of ommatidia. Facet size and pattern change within the same specimen from the posterior to the anterior end. With regard to some morphological criteria, we grouped the material in two different morphotypes, type A and B, the first being represented by specimens of two different developmental stages. From stage to stage, mostly growth in overall size and addition of new ommatidia was noticed. Among the meiobenthic ‘Orsten’ arthropods, only the crustacean Henningsmoenicaris scutula has been described as possessing stalked eyes, but the eyes of the largest specimen with preserved eyes of this species are much smaller than the new eyes and do not display any kind of ommatidia on their visual surface. However, fragments of larger specimens of H. scutula and the co‐occurrence of this species with the new isolated eyes in the sieving residues make it likely that the latter belong to this species but belong to more advanced stages than those described previously of H. scutula. Ontogenetically, the eye stalks of this fossil crustacean elongate progressively, while the regular hexagonal facets, lacking in early stages, appear later on.
Cambrian trilobites mainly lived on the sea floor, and up till now few, if any, unequivocally planktonic trilobites have been reported from earlier than the Ordovician. The late Cambrian (Furongian) to late Ordovician olenids are a distinctive group of benthic (sea-floor dwelling) or nekto-benthic trilobites. Here we show, however, that one recently described, miniaturized and very spiny olenid species, Ctenopyge ceciliae must have been planktonic (passively drifting or feebly swimming in the upper waters of the sea). This interpretation is based not only upon body form but also on the analysis of its visual system and may be one of the earliest records of the planktonic realm being invaded by trilobites.
