Reconstruction of the shape and optics of the lenses in the abathochroal‐eyed trilobite Neocobboldia chinlinica

Until now, the structure and optics of the calcite lenses in abathochroal trilobite eyes have not been investigated. So, the relationship of the abathochroal eye to other types of trilobite eyes has remained unclear. We have reconstructed the exact shape and optics of the lenses in the eodiscid trilobite Neocobboldia chinlinica to determine the mechanism of its abathochroal eye. The distal lens surface has a convex profile, while on the proximal lens surface there is a small central bulge, resulting in an undulating profile. Due to this bulge, the curvature and refractive power of the central region of the lens are greater than those of the peripheral zone. Consequently, the lens is bifocal. However, Neocobboldia could not take advantage of this bifocal property of its tiny lenses because of the diffraction of light and the infinite depth of field in object space. For the same reason, it is also sure that the undulating lower surface of the abathochroal lens did not evolve as a Huygensian profile, correcting for spherical aberration, as suggested earlier. This undulation is a result of the presence of the central bulge, the evolutionary significance of which remains enigmatic. On the basis of our results, we have outlined an evolutionary scenario for development of the optics of the lenses in trilobite eyes.     

A reconstruction of Telephina bicuspis, a pelagic trilobite from the Middle Ordovician of the Oslo Region, Norway

Well preserved parts of Telephina bicuspis described from dark limestone concretions of Llanvirn age, in the Oslo Region, Norway, exhibit many features associated with a pelagic life style for this trilobite. The free cheek has a huge holochroal eye with perfectly square lenses and a long genal spine which is unique in being directed vertically upwards rather than ventrally. Additional spines include an occipital spine, the macro spine on the rachis of the 6th thoracic segment and paired spines from each of the two rachial rings of the pygidium. These spines may have had a hydrodynamic and stabilizing function. The genal spine could have had a sensory function or been a dimorphic feature. The hypostoma is figured for the first time and a reconstruction is given for the complete exoskeleton.     

Primary and diagenetic microstructures in trilobites

McAllister, John E. & Brand, Uwe 1989 01 15: Primary and diagenetic microstructures in trilobites. Lethaia , Vol. 22, pp. 101–111. Oslo. ISSN 0024–1164.
Ordovician and Devonian trilobites were studied by scanning electron microscopy (SEM) to define primary and diagenetic microstructures. Primary structures such as Osmólska cavities and hexagonal surface openings are present in samples obtained from both shale and limestone lenses in shales. Generally, samples with primary microstructures are also geochemically well preserved. Diagenetic microstructures were found in samples from shales and limestones, but those from limestones are the most geochemically and microstructurally altered. 'Dendritic' patterns and fused matrices were found in the altered cuticles of trilobites of different species and ages and are therefore considered a normal feature of the diagenetic alteration of cuticular calcite. Primary and diagenetic microstructures of trilobite cuticles are independent of size, species and age of the specimens, but are largely controlled by lithology and thus diagenesis. * Trilobita, primary-, diagenetic microstructures, fossil diagenesis .     

Trilobites in Paleozoic carbonate buildups

The trilobite faunas found in carbonate buildups throughout much of the Paleozoic are remarkably similar in both composition and general morphological characteristics. This is primarily the result of the domination of these faunas by a limited number of trilobite families which are long-ranging in the buildup environment. Illaenids are the dominant trilobites in Ordovician-Devonian buildups and are generally accompanied by members of the Lichidae and Cheiruridae. Proetid trilobites are characteristic of Ordovician-Permian buildups but are seldom common. The number of trilobite families found in carbonate buildups is highest in the late Ordovician, with most of the important families persisting into the Devonian. After the Devonian only proetids are found in this environment. The conservative nature of these trilobite faunas suggests that the environmental characteristics of normal-marine Paleozoic carbonate buildups are similar. Trilobita, palaeoecology, faunistics, morphology. carbonate buildups. Palaeozoic .     

Geometric optics of trilobite eyes: a theoretical study of the shape of the aspherical interface in the cornea of schizochroal eyes of phacopid trilobites

A general method is given to determine theoretically the shape of the aspherical interface that eliminates spherical aberration in an optionally shaped thick lens. The theory is applied to trilobite eyes. On the basis of the geometric optical method presented, the refractive indices and focal length of the original corneal lenses of trilobites can be determined. The shape of the aspherical interface in the cornea of some phacopid trilobites with schizochroal eyes is investigated. The theoretical aspherical interfaces agree well with the real ones.     

The life habits of the Ordovician illaenine trilobite Bumastoides

The Ordovician illaenine Bumastoides exhibits a distinctive effaced and strongly convex morphology. Orientation of the visual field, the extreme convexity of the cephalon and the nature of the thoracic articulation support an interpretation of Bumastoides as an infaunal trilobite that was poorly suited to epifaunal crawling. The genus may have been sedentary; spending most of its post-larval life cycle within a burrow. Suspension feeding would be a viable existence for a sedentary trilobite such as Bumastoides. Maintenance of a burrow is essential for respiration and would require a firm, cohesive substrate. The infaunal niche had become occupied by trilobites by at least the Late Cambrian and continued to be exploited through the Ordovician, Silurian and. possibly, into the Devonian. Convergence has led to the appearance of the effaced, strongly convex morphotype in a number of unrelated families, including the Illaenidae, Asaphidac, Aulacopleuridae, Plethopeltidae and Scutelluidae. A high numerical abundance of illaenid trilobites, such as Illaenus and Bumastoides , is characteristic of the illaenid–chcirurid association, which persisted from the early Ordovician until at least the Late Silurian. This association has been recorded from shelf-edge and on-shelf carbonate buildups and shallow subtidal level bottom environments. It appears to be confined to firm substrates.     

Head segmentation of trilobites

Although trilobites have provided research subjects for more than two centuries, their head segmentation has remained unresolved. Four glabellar furrows (SO and S1–S3) marking the segmental boundaries are generally present in the cephalic axis, but there are trilobites with one more pair of furrows, the so‐called S4, in the cephalic axis, causing confusion in understanding trilobite head segmentation. Recent advances in developmental biology and palaeontology have shed light on the arthropod head problem, and thus, trilobite head segmentation can be reviewed in the light of this knowledge. Based on the information from the anatomy of exceptionally preserved trilobites and artiopodans closely related to trilobites, it is inferred that trilobite head contains five segments: the anteriormost ocular segment potentially associated with the hypostome, the antennal segment and the following three segments with walking legs. When present, the S4 furrows are situated where the eye ridges meet the cephalic axis of trilobites, indicating that the furrows are incised ‘within’ the anteriormost segment in trilobites with an anteriorly enlarged frontal lobe. Trilobites of the Order Redlichiida, the most primitive stock, show variable conditions in the frontal glabellar conditions, while in other more derived groups, the condition is rather constant. The frontal glabellar condition, therefore, could provide a clue to elucidate the unresolved Cambrian trilobite phylogeny and the Cambrian roots of the post‐Cambrian trilobites.     

The late Devonian trilobite crises

Mid‐Devonian to end‐Late Devonian trilobites of different taxonomic categories are updated as to their actual stratigraphical range with respect to the internationally defined stage boundaries. The main palaeogeographical and ecological occurrences are summarized. Numerical analyses emphasize the clear relationship between fluctuations in diversity and global eustatic events. Already declining in diversity from the early mid‐Devonian, shallow‐water communities became most restricted during the mid‐Givetian Taghanic transgression. After a phase of adaptive radiation, off‐shore trilobite communities were severely affected during the mid‐ and end‐Late Devonian crises. From an initial 5 orders 3 were lost at the end‐Frasnian Kellwasser crisis while only 1 from the remaining 2 orders survived the Devonian‐Carboniferous boundary Hangenberg event. In both cases extinction was preceded by a unidirectional evolutionary trend in eye reduction accompanied by impoverishment of lower rank taxa. This phenomenon is obviously a result of selective adaptation under constant long‐lasting environmental influences. Specialization to obligate epi‐ or even endo‐benthic life habit, however, led fatally to extinction when stable conditions became substantially perturbed. Sudden sea‐level changes with subsequent break in the REDOX‐equilibrium took place at the Kellwasser and Hangenberg events, which were most probably responsible for trilobite mass extinctions.     

Clustered trilobite assemblages in the Middle Devonian Hamilton Group

Clusters of well-preserved trilobites occur throughout lower and middle Paleozoic strata. This phenomenon, however, remains poorly understood. Several thin, regionally persistent beds in the Middle Devonian (Givetian) Hamilton Group of New York State display taphonomic and sedimentary features which indicate rapid burial by storm-generated clouds of fine-grained sediment. Furthermore, components in these units display little or no post-mortem disturbance and, therefore, reflect in faunal associations. These beds contain two types of clustered assemblages of the trilobites Phacops (Green), Greenops boothi (Green) and Dechenella rowi (Green). Body clusters comprise dense assemblages of three or more complete trilobite corpses. Moult clusters , in contrast, are composed of three more moult ensembles; i.e., articulated thoracopygidia in close proximity to corresponding cephala. Clusters are species-segregated and contain only a small portion of the total size range exhibited by trilobite species. Moreover, body clusters and moult clusters are morphometrically undifferentiated indicating that these two cluster modes represent related phenomena. Analogy with modern marine arthropods suggests that trilobites assembled into monospecific, age-segregated clusters and moulted prior to en masse copulation.     

THE EXTRAORDINARY TRILOBITE FENESTRASPIS (DALMANITIDAE, SYNPHORIINAE) FROM THE LOWER DEVONIAN OF BOLIVIA

Abstract:  The hitherto poorly known, monotypic trilobite genus Fenestraspis from the Lower Devonian of Bolivia is revised and its original assignment to the Synphoriinae supported. The thoracic morphology of the genus remains very poorly known. Fenestraspis is morphologically unusual because of the development of extensive fenestrae in the pleural region of the pygidium and apparently of the thorax; the presence of upwardly directed spines on the cephalon, thorax and pygidium; and the exceptionally large and highly elevated eyes with the palpebral rim projecting outwards above the visual surface. The function of the fenestrae remains uncertain. If they formed openings in the body of the trilobite in life they may have allowed circulation of oxygenated water to the limb exites so that respiration could have been maintained while the trilobite was enrolled. If they were covered with a flexible membrane, they may have been secondary respiratory structures or had a sensory function. The Synphoriinae is regarded as a subfamily of the Dalmanitidae rather than as an independent family of the Dalmanitoidea as proposed by some authors. The type species of the poorly known monotypic genus Dalmanitoides from the Lower Devonian of Argentina is illustrated photographically for the first time and compared with Fenestraspis .     

DEVONIAN TRILOBITES FROM THE FALKLAND ISLANDS

Abstract:  New trilobite material is described from the Fox Bay Formation (Lower Devonian; Pragian) of the Falkland Islands (South Atlantic). The fossils were collected from many localities in both East and West Falkland and represent the first significant collection of trilobites from East Falkland. The trilobites belong to three higher taxa: Proetida (the first proetid record in these islands); Homalonotidae, Burmeisteria Salter, 1865; and Calmoniidae, including Bainella Rennie, 1930 and Metacryphaeus Reed, 1907. All of the forms except proetids have now been found in both East and West Falkland.     

Phylogenetic support for the monophyly of proetide trilobites

The monophyly of the order Proetida, the only trilobite group to survive the end‐Devonian mass extinction, has been regularly questioned since its erection almost three decades ago. Through analysis of a novel phylogenetic data set comprising 114 characters coded for 55 taxa, including both traditional members of the Proetida along with a number of other trilobite groups, the monophyly of proetide trilobites is rigorously tested for the first time. Proetida is shown to be monophyletic, united by the initial compound eye formation in early protaspids occurring at the lateral margin rather than the anterior margin, and the form of the protaspid glabella being tapering with a pre‐glabellar field. A number of adult characters, including the possession of a quadrate or shield‐shaped hypostome with angular posterior margins, the hypostome median body being divided by a deep groove that entirely traverses the median body, the presence of an enlarged thoracic spine on the sixth tergite and a tergite count of between 7 and 10, also define the basal node. Hystricurid and dimeropygoid trilobites are shown to resolve at the base of the group, while the remaining proetide taxa are divided between large proetoid and aulacopleuroid clades. Some taxa previously allied with Aulacopleuroidea, such as rorringtoniids and scharyiids, are retrieved as basal members of the Proetoidea.     

南澳大利亚Officer盆地东部下寒武统Abadiella一新种

描述采用自南澳大利亚Officer盆地东部Manya-6号钻井的下寒武统Ouldburra组的Abadiella三叶虫的一新种－Abadiella officerensis,同时指出Parabadiella是Abadiella的同义名。     

Eine neueOtarion-Art (Trilobita, Arthropoda) aus dem Greifensteiner Kalk (unteres Mittel-Devon, Unter-Eifelium) von Greifenstein bei Herborn (Hessen, Deutschland)

en|This paper deals with a new species ofOtarion [Otarion (Otarionella) greifensteinen.se n. sp.] from the ‘Greifenstein Limestone’ of the ‘Wiege’ near Greifenstein, close to the village Greifenstein near Herborn (Lahn-Dill area, Hesse, Germany), which is described, figured and compared with all known species of this subgenus. It is a typical trilobite of the hercynian-bohemian shallow marine facies near the Lower-/Middle Devonian boundary (Upper Emsian / Lower Eifelian), which extends up into the lower part of the Middle Devonian (Lower Eifelian). The trilobites are transported for short distances only and, thus, are parautochthonous elements of the Greifenstein Facies.     

The sophisticated visual system of a tiny Cambrian crustacean: analysis of a stalked fossil compound eye

Fossilized compound eyes from the Cambrian, isolated and three-dimensionally preserved, provide remarkable insights into the lifestyle and habitat of their owners. The tiny stalked compound eyes described here probably possessed too few facets to form a proper image, but they represent a sophisticated system for detecting moving objects. The eyes are preserved as almost solid, mace-shaped blocks of phosphate, in which the original positions of the rhabdoms in one specimen are retained as deep cavities. Analysis of the optical axes reveals four visual areas, each with different properties in acuity of vision. They are surveyed by lenses directed forwards, laterally, backwards and inwards, respectively. The most intriguing of these is the putatively inwardly orientated zone, where the optical axes, like those orientated to the front, interfere with axes of the other eye of the contralateral side. The result is a three-dimensional visual net that covers not only the front, but extends also far laterally to either side. Thus, a moving object could be perceived by a two-dimensional coordinate (which is formed by two axes of those facets, one of the left and one of the right eye, which are orientated towards the moving object) in a wide three-dimensional space. This compound eye system enables small arthropods equipped with an eye of low acuity to estimate velocity, size or distance of possible food items efficiently. The eyes are interpreted as having been derived from individuals of the early crustacean Henningsmoenicaris scutula pointing to the existence of highly efficiently developed eyes in the early evolutionary lineage leading towards the modern Crustacea.     

Inferring internal anatomy from the trilobite exoskeleton: the relationship between frontal auxiliary impressions and the digestive system

Lerosey‐Aubril, R., Hegna, T.A. & Olive, S. 2011: Inferring internal anatomy from the trilobite exoskeleton: the relationship between frontal auxiliary impressions and the digestive system. Lethaia, Vol. 44, pp. 166–184. The digestive system of trilobites is rarely preserved. As a result, many aspects of its organization remain unknown. Fortunately, the exoskeleton sometimes preserves evidence of soft‐tissue attachment sites that can be used to infer internal anatomy. Among them are the frontal auxiliary impressions (FAIs), probable soft‐tissue insertion sites located on the fronto‐median glabellar lobe of some trilobites. FAIs are herein described in the Carboniferous trilobite Phillipsia belgica Osmólska 1970 – representing the only known example of such structures in the Proetida and their youngest occurrence. A taphonomic scenario is proposed to explain their variable preservation. Although particularly common in the Phacopina, FAIs or FAI‐like structures are also found in several orders that differ greatly. Comparisons with modern analogues suggest that FAIs might represent attachment sites for extrinsic muscles associated with a differentiated crop within the foregut. A review of purported remains of the trilobite digestive system indicates that it usually consisted of a tube‐like tract flanked by a variable number of metamerically paired diverticulae. Its anterior portion is not particularly individualized, except in a few specimens that might hint at the presence of a crop. This differentiation of a crop might have constituted a secondarily evolution of the foregut in trilobites, occurring independently in different clades. Accompanied by a strengthening of associated extrinsic muscles, this modification of the foregut might explain the presence of more conspicuous muscle insertion sites on the glabella. Study of FAIs might therefore provide new data on the anatomy of the foregut in trilobites and evidence of diverse feeding habits. □Arthropoda, digestive system, ecology, muscle scars, Proetida, Trilobita.     

A new species of trilobites from Sakmarian Reefs of Bashkortostan

The new trilobite species Cummingella (Cummingella) boikoi sp. nov. is described from the Lower Permian reef facies (Sakmarian Stage) of Bashkiria (Bashkortostan). The new findings extend the known range of the Permian genus Cummingella Reed, 1942 and provide the first record of trilobites in Sakmarian deposits of Bashkiria.     

A quantitative comparison of the ontogeny of two closely-related Upper Devonian phacopid trilobites

The best insight into the development of Devonian phacopids has been obtained from Trimerocephalus lelievrei Crônier & Feist, 1997, a Famennian phacopine from Morocco, where changes in size and shape have been quantified. In this study, a morphometric approach has been used: (1) to retrodeform and then establish patterns of morphological variation in a well preserved but tectonically deformed assemblage belonging to another phacopine species Weyerites ensae (Richter & Richter, 1926), a Famennian phacopine from Thuringia, and (2) to establish patterns of developmental and evolutionary changes within two closely related species: Weyerites ensae and Trimerocephalus lelievrei. The method of retrodeformation using a set of discrete points presumed to be homologous on all studied individuals, has demonstrated that the next analyses are possible on the retrodeformed material as compared to the undeformed material. Morphometric analysis based on outline analysis has permitted demonstration of progressive shape change in agreement with ontogenetic ordination and a comparison of changes in size and shape in Weyerites ensae. The main changes in shape appear to occur in the meraspid period, whereas increase in size takes place mainly in the holaspid period. This pattern, already reported for Trimerocephalus lelievrei, can be generalized for phacopine trilobites from the Late Devonian. Moreover, the comparison of the two ontogenetic trajectories has shown that most of the differences are related to 'structural' changes, probably linked to a relative pre- post-displacement. The results suggest that ecological adaptation may be studied by examining the changes in development that occur within species through time and space.     

A functional analysis of compound eye evolution

New data on the phylogenetic relationships of various arthropod groups have spurred interesting attempts to reconstruct the evolution of arthropod nervous and visual systems. Some of the relevant new data are cell identities and developmental processes in the nervous and sensory systems, which is particularly useful for reconstructing the evolution of these systems. Here, we focus on the structure of compound eye ommatidia, and make an evolutionary analysis with functional arguments. We investigate possible routes of evolution that can be understood in terms of selection for improved visual function, and arrive at a number of conclusions that are discussed in the light of recent phylogenetic hypotheses. On the basis of ommatidial focusing structures and the arrangement of receptor cells we show that the evolution of compound eyes proceeded largely independently along at least two lineages from very primitive ancestors. A common ancestor of insects and crustaceans is likely to have had ommatidia with focusing crystalline cones, and colour and/or polarization vision. In contrast, the compound eyes in myriapods and chelicerates are likely to date back to ancestors with corneal lenses and probably without the ability to discriminate colour and polarization.