Musculature of an Early Cambrian cycloneuralian animal.

Cycloneuralians are ecdysozoans with a fossil record extending to the Early Cambrian Fortunian Age and represented mostly by cuticular integuments. However, internal anatomies of Fortunian cycloneuralians are virtually unknown, hampering our understanding of their functional morphology and phylogenetic relationships. Here we report the exceptional preservation of cycloneuralian introvert musculature in Fortunian rocks of South China. The musculature consists of an introvert body-wall muscular grid of four circular and 36 radially arranged longitudinal muscle bundles, as well as an introvert circular muscle associated with 19 roughly radially arranged, short retractors. Collectively, these features support at least a scalidophoran affinity, and the absence of muscles associated with a mouth cone and scalids further indicates a priapulan affinity. As in modern scalidophorans, the fossil musculature, and particularly the introvert circular muscle retractors, may have controlled introvert inversion and facilitated locomotion and feeding. This work supports the evolution of scalidophoran-like or priapulan-like introvert musculature in cycloneuralians at the beginning of the Cambrian Period.

Calcitic sclerites at base of malacostracan pleopods (Crustacea)--part of a coxa.

BACKGROUND: Cuticular specialisations such as joints and membranes play an important role in the function of arthropod limbs. This includes sclerotisations and mineral incrustations of cuticular areas to achieve either more rigidity or flexibility. The anterior eight thoracopods of Malacostraca have limb stems comprising a coxa and a basipod, which carries the two rami. Their pleopods, the limbs of the posterior trunk part, have for long been regarded to lack a coxa. Several calcitic sclerites occur in the area between ventral body and limb stem. This raises the questions: do these elements represent specialisations of the membrane due to functional requirements, and do they originate from an originally larger limb portion, i.e., the coxa, or in fact represent it. RESULTS: We investigated 16 species of selected malacostracan taxa from all major in-groups. Calcitic sclerites occur in constant numbers and position within a species (no individual variation, and independent of specific modification such as in genital appendages). These are even constant within a supra-specific taxon, which facilitates comparisons. In general the sclerites connect via two pivot joints to the sternite medially and the tergopleura laterally, and two more to the limb stem. Based on this, we reconstructed putative ground-pattern conditions for the sclerites of the examined taxa of Malacostraca. CONCLUSIONS: The pattern of sclerites is characteristic for each monophyletic malacostracan taxon. The highest number of sclerites most likely represents the plesiomorphic state. Reduction of sclerite numbers occurs in Caridoida and its in-groups. Sclerite arrangement in these taxa provides an important character complex for phylogenetic studies. The presence of pivot joints to the body proximally and basipod distally demonstrates the existence of a coxa, which is just slightly less sclerotised, particularly on its posterior side. This can be explained by enhanced flexibility of the pleopods evolved in the course to their major role as swimming devices. Both the pivot joints and the proximal and distal extension of the calcitic sclerites demarcate the minimum area of the coxa. With this, sclerites appear as very valuable also in shedding more light on the putative relationships between Malacostraca, Myriapoda, Insecta, and Remipedia.

An epipodite-bearing crown-group crustacean from the Lower Cambrian.

Crown-group crustaceans (Eucrustacea) are common in the fossil record of the past 500 million years back to the early Ordovician period, and very rare representatives are also known from the late Middle and Late Cambrian periods. Finds in Lower Cambrian rocks of the Phosphatocopina, the fossil sister group to eucrustaceans, imply that members of the eucrustacean stem lineage co-occurred, but it remained unclear whether crown-group members were also present at that time. 'Orsten'-type fossils are typically tiny embryos and cuticle-bearing animals, of which the cuticle is phosphatized and the material is three-dimensional and complete with soft parts. Such fossils are found predominantly in the Cambrian and Ordovician and provide detailed morphological and phylogenetic information on the early evolution of metazoans. Here we report an Orsten-type Konservat-Lagerstätte from the Lower Cambrian of China that contains at least three new arthropod species, of which we describe the most abundant form on the basis of exceptionally well preserved material of several growth stages. The limb morphology and other details of this new species are markedly similar to those of living cephalocarids, branchiopods and copepods and it is assigned to the Eucrustacea, thus representing the first undoubted crown-group crustacean from the early Cambrian. Its stratigraphical position provides substantial support to the proposition that the main cladogenic event that gave rise to the Arthropoda was before the Cambrian. Small leaf-shaped structures on the outer limb base of the new species provide evidence on the long-debated issue of the origin of epipodites: they occur in a set of three, derive from setae and are a ground-pattern feature of Eucrustacea.

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.

Tagmatization in Stomatopoda - reconsidering functional units of modern-day mantis shrimps (Verunipeltata, Hoplocarida) and implications for the interpretation of fossils.

INTRODUCTION: We describe the tagmatization pattern of the anterior region of the extant stomatopod Erugosquilla massavensis. For documentation we used the autofluorescence capacities of the specimens, resulting in a significant contrast between sclerotized and membranous areas. RESULTS: The anterior body region of E. massavensis can be grouped into three tagmata. Tagma I, the sensorial unit, comprises the segments of the eyes, antennules and antennae. This unit is set-off anteriorly from the posterior head region. Ventrally this unit surrounds a large medial sclerite, interpreted as the anterior part of the hypostome. Dorsally the antennular and antennal segments each bear a well-developed tergite. The dorsal shield is part of tagma II, most of the ventral part of which is occupied in the midline by the large, partly sclerotized posterior part of a complex combining hypostome and labrum. Tagma II includes three more segments behind the labrum, the mandibular, maxillulary and maxillary segments. Tagma III includes the maxillipedal segments, bearing five pairs of sub-chelate appendages. The dorsal sclerite of the first of these tagma-III segments, the segment of the first maxillipeds, is not included in the shield, so this segment is not part of tagma II as generally thought. The second and third segments of tagma III form a unit dorsally and ventrally. The tergites of the segments of tagma III become progressively larger from the anterior to the posterior, possibly resulting from a paedomorphic effect during evolution, which caused this reversed enlargement. CONCLUSIONS: The described pattern of tagmosis differs from current textbook knowledge. Therefore, our re-description of the anterior body area of stomatopods is of considerable impact for understanding the head evolution of Stomatopoda. Likewise, it has a bearing upon any comparisons with fossil stomatopods, as mainly sclerotized areas are fossilized, and, on a wider scale, upon larger-scale comparisons with other malacostracans and eucrustaceans in general.

Autofluorescence imaging, an excellent tool for comparative morphology.

Here we present a set of methods for documenting (exo-)morphology by applying autofluorescence imaging. For arthropods, but also for other taxa, autofluorescence imaging combined with composite imaging is a fast documentation method with high-resolution capacities. Compared to conventional micro- and macrophotography, the illumination is much more homogenous, and structures are often better contrasted. Applying different wavelengths to the same object can additionally be used to enhance distinct structures. Autofluorescence imaging can be applied to dried and embedded specimens, but also directly on specimens within their storage liquid. This has an enormous potential for the documentation of rare specimens and especially type specimens without the need of preparation. Also for various fossils, autofluorescence can be used to enhance the contrast between the fossil and the matrix significantly, making even smallest details visible. 'Life-colour' fluorescence especially is identified as a technique with great potential. It provides additional information for which otherwise more complex methods would have to be applied. The complete range of differences and variations between fluorescence macrophotography and different types of fluorescence microscopy techniques are here explored and evaluated in detail. Also future improvements are suggested. In summary, autofluorescence imaging is a powerful, easy and fast-to-apply tool for morphological studies.

Evolution of mantis shrimps (Stomatopoda, Malacostraca) in the light of new Mesozoic fossils.

BACKGROUND: We describe new specimens of Mesozoic mantis shrimps (Stomatopoda, Malacostraca) that exhibit morphological and developmental information previously unknown. RESULTS: Specimens assigned to the taxon Sculda exhibit preserved pleopods, thoracopods including all four raptorial limbs as well as details of antennae and antennulae. The pleopods and the antennulae resemble those of the modern mantis shrimps, but the raptorial limbs are not as differentiated as in the modern species. In some specimens, the first raptorial limb (second thoracopod) is not significantly larger than the similar-sized posterior three pairs (as in extant species), but instead these appendages become progressively smaller along the series. In this respect they resemble certain Palaeozoic stomatopods. Another specimen, most likely belonging to another species, has one pair of large anterior raptorial thoracopods, a median-sized pair and two more pairs of small-sized raptorial appendages and, thus, shows a new, previously unknown type of morphology. A single specimen of Pseudosculda laevis also exhibits the size of the raptorial limbs; they are differentiated as in modern species, one large pair and three small pairs. Furthermore, we report additional larval specimens and show also post-larval changes, e.g., of the tail fan. CONCLUSIONS: These new data are used to reconsider the phylogeny of Stomatopoda. We still need a strict taxonomical revision of the Mesozoic mantis shrimps, but this first examination already demonstrates the importance of these fossils for understanding mantis shrimp evolution and the interpretation of evolutionary pathways of particular features.

External morphology of Lightiella monniotae (Crustacea, Cephalocarida) in the light of Cambrian 'Orsten' crustaceans.

The species-poor meiofaunal Cephalocarida have played an important role in discussions of the phylogeny and evolution of Crustacea since their discovery in 1955. One reason may be that the morphology of cephalocarids includes some aspects of putatively ancient appearance, such as the simple roof-shaped head shield, the anterior three head appendages resembling those of a nauplius larva, or the trunk-limb-like second maxilla. Cephalocarida have even been suggested to represent the sister taxon to all other Eucrustacea. Presence of possibly plesiomorphic characters, however, does not necessarily point to a basal position in the system. Growing evidence demonstrates that the modification of the fourth post-antennular cephalic appendage, the 'maxilla', into a "mouth part" may have occurred independently in the different eucrustacean lineages, so a trunk-limb-like maxilla is an ancient feature that does not hold only for cephalocarids. Retention of its plesiomorphic shape and function in the Cephalocarida remains, however, noteworthy. Cephalocarids are still little studied and incompletely known, especially their external morphology. By examining several adults and one young specimen of Lightiella monniotae Cals and Delamare Deboutteville, 1970 from New Caledonia, we aimed to a) document as many details as possible, and b) compare these data with other species of Cephalocarida. We also aimed to reconstruct aspects of the ground pattern of Cephalocarida, which is a pre-requisite for any comparisons in a broader perspective of crustacean phylogeny. Among the new findings or conclusions are: (1) Lightiella is in need of a revision since several assumed differences between the species are questionable or subject to intra-specific variability; (2) the cuticle of the trunk-limb basipod is sub-divided into a number of smaller sclerotized areas as in various exceptionally 3D preserved fossil crustaceans from Cambrian 'Orsten' faunal assemblages; (3) a small transitional portion on the post-maxillulary limbs in the area where the endopod and basipod connect is discussed as either a reduced, proximal endopod segment or as an evolutionary new joint of the basipod to enhance its flexibility; (4) the so-called pseud-epipod is interpreted as an outer branch of the exopod; (5) compared to 'Orsten' crustaceans many characters of the Cephalocarida are more modified than previously assumed, including the morphology of the trunk-limb basipod, and the unique, ring-shaped appearance of the abdominal segments. Also the development is not as plesiomorphic as sometimes assumed, at least not compared to that of the strictly anamorphic series of the 'Orsten' eucrustacean Rehbachiella kinnekullensis. The application of SEM techniques has again proved to be especially appropriate because of the small size of these animals, and because it permits direct comparisons with other similarly small crustaceans and the 'Orsten' crustaceans and their larvae.

Early embryonic development of the head region of Gryllus assimilis Fabricius, 1775 (Orthoptera, Insecta).

We report our investigations on the embryonic development of Gryllus assimilis, with particular attention to the head. Significant findings revealed with scanning electron microscopy (SEM) images include: (1) the pre-antennal lobes represent the anterior-most segment that does not bear any appendages; (2) each of the lobes consists of central and marginal regions; (3) the central region thereof develops into the protocerebrum and the optic lobes, whereas the marginal region thereof becomes the anterior portion of the head capsule; (4) the initial position of the antennal segment is posterior to the mouth region; (5) appendage anlagen are transitorily present in the intercalary segment, and they later vanish together with the segment itself; (6) a bulged sternum appears to develop from the ventral surface of the mandibular, maxillary and labial segments. Embryonic features are then compared across the Insecta and further extended to the embryos of a spider (Araneae, Chelicerata). Striking similarities shared by the anterior-most region of the insect and spider embryos lead the authors to conclude that such comparison should be further undertaken to cover the entire Euarthropoda. This will help us to understand the embryology and evolution of the arthropod head.

High-level phylogenetic analysis using developmental sequences: the Cambrian +Martinssonia elongata, +Musacaris gerdgeyeri gen. et sp. nov. and their position in early crustacean evolution.

The ontogenetic sequence of +Martinssonia elongata, a derivative of the stem lineage of Eucrustacea, has been re-investigated. Morphological and morphometric data provide a revision of the original description of this species. Specimens originally assigned to second and third developmental stages have been removed from the +M. elongata sequence and assigned, together with several larger specimens, to an entirely new species, +rMusacaris gerdgeyeri gen et. sp. nov having a completely unsegmented body, giving the appearance of a giant euarthropod head larva. This is interpreted either as a hypertrophied larva or a late developmental stage of a neotenic species. Only the earliest unsegmented larvae and segmented instars of the original sequence are ascribed to +M. elongata. The two species are apparently closely related and are closer to Labrophora (+Phosphatocopina + Eucrustacea) than to other Cambrian " 'Orsten' crustacean stem derivatives". The ontogenetic sequences of the two taxa and those of the other derivatives of the recently investigated labrophoran stem lineage indicate several heterochronic peramorphic ('adultising') events during early crustacean evolution. This is most evident in the development of the proximal parts of the appendages.

A eucrustacean metanauplius from the Lower Cambrian.

A new eucrustacean arthropod, Wujicaris muelleri gen. et sp. nov, is represented by a Lower Cambrian early metanauplius of strikingly modern morphology despite being the oldest known fossil of such an early immature crustacean larva. The morphology of the metanauplius closely mirrors that of corresponding developmental stages of living barnacles and copepods, and it is likely that its appendages had a similar function for feeding and locomotion. The metanauplius larva demonstrates remarkable stasis in morphology, life history, and lifestyle of (small) eucrustaceans over 525 million years, probably as a result of adaptation to a long-lasting physical niche and regime involving low Reynolds numbers and laminar current flow.

Early development of the anterior body region of the grey widow spider Latrodectus geometricus Koch, 1841 (Theridiidae, Araneae).

We document the early morphogenesis of Latrodectus geometricus, particularly of the anterior body region. Significant changes in the development of the external prosomal structures revealed with scanning electron microscopy (SEM) images include: (1) reorganisation of each pre-cheliceral lobe by subdivision and internalisation of its central area; (2) shortening of the ventro-median bridge connecting the pre-cheliceral lobes and its eventual disappearance; (3) appearance and expansion of a prospective mouth region between the pre-cheliceral lobes with a recessed median area surrounded by lip-like borders, the anterior lip-part developing into the hypostome; (4) reduction of the mouth region to an area around the hypostome and the lip-like latero-posterior border of the mouth opening; (5) change of the position of the mouth region from anterior to the insertions of the chelicerae to posterior to them; (6) eventual shortening of the mouth opening to a slit overhung by the hypostome; (7) origination of the prosomal shield from the anterior margin of the pre-cheliceral lobes and the tergal portions of the four posterior-most prosomal segments; and (8) expansion of a 'ventral sulcus' from the cheliceral to the fifth opisthosomal segment separating the sides of these segments. Embryonic features are compared across the Chelicerata and discussed briefly in a phylogenetic context.

The evolutionary history of crustacean segmentation: a fossil-based perspective.

The evolution of segmentation in Crustacea, that is, the formation of sclerotized and jointed body somites and arrangement of somites into tagmata, is viewed in light of historical traits and functional constraints. The set of Early to Late Cambrian 'Orsten' arthropods have informed our current views of crustacean evolution considerably. These three-dimensionally preserved fossils document ancient morphologies, as opposed to purely hypothetical models and, because of the unusual preservation of larval stages, provide us with unparalleled insight into the morphogenesis of body somites and their structural equipment. The variety of evolutionary levels represented in the 'Orsten' including lobopodians, tardigrades, and pentastomids also allows phylogenetic interpretations far beyond the Crustacea. The 'Orsten' evidence and data from representatives of the Lower Cambrian Chengjiang biota in southwestern China, including phylogenetically earlier forms, form the major source of our morphology-based review of structural and functional developments that led toward the Crustacea. The principal strategy of arthropods is the simultaneous development of head somites, as expressed in a basal "head larva," and a successive addition of postcephalic somites from a preterminal budding zone with progressive maturation of metameric structures. This can be recognized in the developmental patterns of extant and fossil representatives of several euarthropod taxa, particularly crustaceans, trilobites, and chelicerates (at least basally). The development of these taxa points to an early somite-poor and free-living hatching stage. Embryonic development to a late stage within an egg, as occurring in recent onychophorans and certain in-group euarthropods, is regarded as achieved several times convergently.