Thoracic structure and enrolment style in middle Cambrian Eccaparadoxides pradoanus presages caudalization of the derived trilobite trunk

Abstract: The ability to enrol effectively evolved several times among trilobites. Here, we show that, unlike most redlichiid trilobites that could not enrol, both morphotypes of Eccaparadoxides pradoanus from the middle Cambrian of Spain enrolled so as to enclose most of the ventral surface beneath the exoskeleton and possessed specialized articulating devices that facilitated this behaviour. The holaspid thorax of all E. pradoanus was divided into two principal regions. The boundary between these marked a transition from anterior segments with short pleural spines, fulcra and ridge‐and‐groove inner pleural regions to posterior segments with longer, acuminate pleural spines that lack fulcra and inner pleural regions. Devices that aid articulation, such as fulcra with short articulating pleural surfaces, the petaloid articulating facet and long articulating half rings, are concentrated in the anterior region. These features, and the large number of specimens preserved in various degrees of enrolment, suggest an enrolment procedure in which the rear part of the trunk, containing both the posterior thorax and the pygidium, rotated as a single unit without significant internal flexure. As these posterior trunk articulations were apparently not required to permit enrolment, concentrating flexure in the anterior may have presaged the caudalized condition seen in many derived trilobite groups that encapsulated, in which a larger proportion of the trunk segments were allocated to the mature pygidium, and therefore unable to articulate.     

Extreme protomeric development in a burlingiid trilobite from Cambrian glacial erratics of Denmark

Pengia Geyer & Corbacho is a Cambrian burlingiid trilobite with fused trunk segments devoid of any articulation in the anamorphic and epimorphic phases of development. The type species is Pengia fusilis (Peng et al.) from the Wanshania wanshanensis Zone of China. Here we describe a second species, Pengia palsgaardia sp. nov., from the Lejopyge laevigata Zone of the Paradoxides forchhammeri Superzone. It comes from a glacial erratic in Denmark which probably originated in the Alum Shale Formation of Västergötland, Sweden. Pengia palsgaardia is a large burlingiid (~10 mm in length), with 14 fused segments in the trunk whose boundaries are marked by ridges. The axis is narrow, with the axial furrows faintly indicated or effaced across the median. Laterally along the axis and the tapering glabella, symmetrical globular lobes are developed that are pinched at their base. During ontogeny the glabellar furrows are pit‐like adaxially but shallow towards the axial furrow as the globular lobes develop. Their pit‐like appearance in Pengia palsgaardia and some other burlingiid species is not considered similar to the condition seen in oryctocephalid trilobites. A median preglabellar ridge resembling that of Schmalenseeia Moberg develops late in ontogeny but in early ontogeny the preglabellar field resembles that of Burlingia Walcott, Alumenella Geyer & Corbacho and Niordilobites Geyer & Corbacho. This gives Pengia a more basal position in the schmalenseeid lineage, outside the derived Schmalenseeia. In mature specimens the facial sutures in P. palsgaardia are fused, but an ocular suture may have been present. During ontogeny Pengia would have gone through the anamorphic and protomeric protaspid segmental conditions, but articulation between either the cephalon and pygidium, or pygidium and thoracic segments of the trunk never developed so it did not progress beyond the protaspid phase. This extreme protomeric development is considered to be a derived feature in Pengia.     

Moulting,ontogeny and sexual dimorphism in the Cambrian ptychopariid trilobite Strenuaeva inflata from the northern Swedish Caledonides

Abstract: Three thousand seven hundred disarticulated remains together with several articulated specimens of the Cambrian Series 2 ptychopariid trilobite Strenuaeva inflata Ahlberg and Bergström, 1978 have been collected from the Torneträsk area, northern Sweden. The material provides significant new data on the morphology, ontogeny, moulting and enrolment of the species. Two distinct morphotypes, possibly an expression of sexual dimorphism, are recognized. The morph with a pair of bulbs in the frontal area, interpreted as brood pouches, is considered to represent females. Statistical treatment of the length/width ratio in cranidia reveals isometric growth during ontogeny for both morphotypes. The transition from the meraspid to holaspid ontogenetic period has been established through recognition of the successive development of the number of thoracic segments in articulated late meraspides. Throughout its life cycle, S. inflata went through 11 meraspid degrees and at least 17 holaspid growth stages. Inferred moult ensembles and exuviae reveal the successive opening of cephalic sutures and the function of the rostral plate during exuviation. As in other ellipsocephalid trilobites in which enrolment is known, the pygidium and two or three thoracic segments of S. inflata are concealed beneath the cephalon (spiral enrolment) during complete enrolment.     

Palaeoecology and evolutionary implications of enrolled trilobites from the Kushan Formation,Guzhangian of North China

Damesella paronai is the earliest enrolled odontopleurid trilobite to be found in the fossil record. Whereas its interlocking devices were very poorly developed, its trunk shows advanced articulations, which only lack an articulating facet, though an anterior notch is present in the outer part of each pleura. Its body pattern and structures did not allow it an encapsulated enrolment style. On the other hand, it is coeval with the early asaphid Monkaspis daulis, found in the same beds and at the same level. This co-inhabiting trilobite had a more advanced structure that enabled it to achieve a fully enrolled, encapsulated style. These superior structures enhance the preservation of enrolled specimens of M. daulis, and it seems to be generally the case that more elegant structures actually improve preservation. The evolutionary trends of the lineages of these two trilobites show that while odontopleurids were very conservative, using the same enrolment style throughout their history, the asaphids developed different enrolment styles, which was one factor in giving them a capacity to occupy different ecological niches and a greater range of environments.     

Morphology and ontogeny of the eodiscoid trilobite Sinodiscus changyangensis from the lower Cambrian of South China

Abstract: A large number of complete specimens together with numerous disarticulated sclerites of the eodiscinid trilobite Sinodiscus changyangensis Zhang in Zhou et al., 1977 have been collected from the lower Cambrian Shuijingtuo Formation in Changyang, Hubei Province, South China. An ontogenetic series is established based on the immature and mature exoskeletons including the previously unknown protaspides and meraspides, in particular. No further substages can be differentiated in the protaspid specimens herein. Changes that took place during the meraspid period include the addition of postcephalic segments and prominent pygidial larval notches in early meraspid development which became progressively less distinct and disappeared in degree 2. Two holaspid stages are recognized based on the addition of a new pygidial segment, indicating that the start of the holaspid phase preceded the onset of the epimorphic phase and accordingly, its developmental mode is attributed to the protarthrous pattern. The trunk segmentation schedule of S. changyangensis is discussed, which is similar to other primitive eodiscoid trilobites, that is, as the boundary between the thorax and pygidium migrated posteriorly, there is no change in the number of the trunk segments. The processes of liberation of the thoracic segment and segment insertion into the pygidium are separated from one another, and the two different mechanisms, somitogenesis and tagmosis, progress independently during the ontogenetic development of the postcephalic region of these primitive eodiscinids.     

Sphaeroidal enrolment in middle Cambrian solenopleuropsine trilobites

Esteve, J., Zamora, S., Gozalo, R. & Liñán, E. 2010: Sphaeroidal enrolment in middle Cambrian solenopleuropsine trilobites. Lethaia, 10.1111/j.1502‐3931.2009.00205.x Fifty specimens belonging to species of Solenopleuropsis and Pardailhania from Spain and France demonstrate sphaeroidal enrolment in Cambrian trilobites for the first time. These solenopleuropsines show novel coaptative structures in different regions of the exoskeleton: in the cephalon there are vincular furrows and notches; in the thorax an articulating facet is developed at the pleural margins, with a ball and socket connection on the adaxial most portion, and an articulating half‐ring axially; the pygidium possesses an articulating facet. The interaction of these coaptative structures resulted in a sphaeroidal enrolment that was a progressive act from the first articulation between the occipital ring and the first segment to the pygidial articulating facet. A similar type of sphaeroidal enrolment is observed in the Devonian trilobite Phacops. Both Cambrian and Devonian trilobites developed a vincular furrow in the ventral surface of the cephalon to close their bodies tightly. In both cases, this is probably a convergent adaptation to protect against predators and obrution. Indeed, the enrolled trilobites are very common in obrution deposits restricted to shallow and soft muddy substrates. □Coaptative structures, convergence, Murero Formation, Pardailhania, Solenopleuropsinae, Solenopleuropsis.     

Modelling enrolment in Cambrian trilobites

Trilobites were capable of enroling in different ways based on the flexible articulation of thoracic segments and associated interlocking devices; the type of enrolment (spiral or sphaeroidal) is thought to have largely depended on the coaptative devices that each trilobite used to enclose the body. Based on X‐ray microtomography scans of complete enrolled specimens from the Cambrian, we created three‐dimensional (3D) computer models to assess the kinematics needed to achieve both enrolment types. We demonstrate that closely related trilobites with little morphological variation (Bailiaspis?, Conocoryphe and Parabailiella) developed different enrolment types as a result of small variations in the number of thoracic segments and the angle between adjacent segments. Moreover, our models indicate that sphaeroidal enrolment, which is associated with a smaller number of thoracic segments, enabled faster encapsulation. This supports the hypothesis that there was a trend in the evolution of trilobites towards reduction in the number of thoracic segments in phylogenetically derived taxa in order to enhance the efficiency of enrolment.     

辽东寒武纪三叶虫Redlichia (Pteroredlichia) murakamii Resser et Endo的蜕壳

华北辽东本溪寒武纪三叶虫Redlichia(Pteroredlichia)murakamii蜕壳标本完好地保存了头盖向下翻转的蜕壳状态,为寒武纪Redlichia的蜕壳模式提供了新的视角。此标本自由颊未保留,胸尾相连,头盖与胸部分离,头盖向下翻转,基本保持原位,为原地埋藏的蜕壳标本。蜕壳过程如下:面线裂开,头盖耸起卷曲并翻转,与胸部分离,胸部扭动,虫体摆脱老壳向前爬出。此蜕壳方式在McNamara(1986)报道的澳大利亚寒武纪Redlichia的蜕壳类型中并未出现。     

Intraspecific variation of trunk segmentation in the oryctocephalid trilobite Duyunaspis duyunensis from the Cambrian (Stage 4, Series 2) of South China

A detailed exploration of growth and trunk segmentation of the oryctocephalid trilobite, Duyunaspis duyunensis Chang & Chien in Zhou et al. 1977, from the lower Cambrian (Stage 4, Series 2) Balang Formation in western Hunan Province, South China, is presented. Because of the excellent preservation, the complete post‐protaspid ontogenetic series from merapsid degree 0 to the holaspid phase is described. The ontogenetic series reveals new information on morphological changes such as the migration of the posterior branch of the facial sutures (from proparian to opisthoparian) and contraction of the posteromedial notch in the pygidium. The abundance of articulated specimens available from a narrow stratigraphical interval makes this material singularly useful for studying the morphogenesis and post‐embryonic growth of D. duyunensis in comparison with other oryctocephalids. Strong evidence that multiple numbers of pygidial segment are recognized in each meraspid degree as well as in the holaspid period showed unusual intraspecific variability in the rate of trunk segmentation, providing insights into how Cambrian subisopygous trilobites controlled their body patterning, including size, shape and trunk segment number in both thorax and caudal plate during growth.     

A Parvancorina-like arthropod from the Cambrian of South China

Constraining the origin of animal groups is allowed, to some extent, by discoveries of Cambrian Lagerstätten that preserve both mineralizing and nonmineralizing organisms. A new species is reported here of the Cambrian arthropod Skania, which bears an exoskeleton that shares homologies with the Neoproterozoic (Ediacaran) organism Parvancorina and firmly establishes a Precambrian root for arthropods. A new monophyletic group, Parvancorinomorpha, is proposed as the first clade within the arthropod crown group demonstrably ranging across the Neoproterozoic–Paleozoic transition. The Parvancorinomorpha is interpreted to be the sister group of the Arachnomorpha. Incipient cephalization in Skania and related genera represents a step in the progression toward division of a cephalon from a large posterior trunk as shown in Cambrian arachnomorphs such as naraoiids and the addition of a pygidium and thoracic tergites as shown in the arachnomorph clade basal to trilobites. This evidence can serve as a new calibration point for estimating the divergence time for the last common ancestor of arthropods and priapulids based on molecular clock methods.     

Enrolment and trunk segmentation of a Cambrian eodiscoid trilobite

Completely enrolled, phosphatized, 3‐D specimens of the eodiscinid trilobite Tsunyidiscus yanjiazhaiensis from Cambrian Stage 3 of South China exhibit much morphological detail and show variation in delicate coaptative structures associated with their trunk segmentation. The relationship between enrolment mechanism and trunk segmentation during ontogeny confirms a unique developmental pattern among the early Cambrian eodiscoid trilobites, revealing how these animals controlled the rate of segment increase and release during post‐embryonic development in their life cycles.     

Functional integration for enrolment constrains evolutionary variation of phacopid trilobites despite developmental modularity

Modularity and integration are variational properties expressed at various levels of the biological hierarchy. Mismatches among these levels, for example developmental modules that are integrated in a functional unit, could be informative of how evolutionary processes and trade‐offs have shaped organismal morphologies as well as clade diversification. In the present study, we explored the full, integrated and modular spaces of two developmental modules in phacopid trilobites, the cephalon and the pygidium, and highlight some differences among them. Such contrasts reveal firstly that evolutionary processes operating in the modular spaces are stronger in the cephalon, probably due to a complex regime of selection related to the numerous functions ensured by this module. Secondly, we demonstrate that the same pattern of covariation is shared among species, which also differentiate along this common functional integration. This common pattern might be the result of stabilizing selection acting on the enrolment and implying a coordinate variation between the cephalon and the pygidium in a certain direction of the morphospace. Finally, we noticed that Austerops legrandi differs slightly from other species in that its integration is partly restructured in the way the two modules interact. Such a divergence can result from the involvement of the cephalon in several vital functions that may have constrained the response of the features involved in enrolment and reorganized the covariation of the pygidium with the cephalon. Therefore, it is possible that important evolutionary trade‐offs between enrolment and other functions on the cephalon might have partly shaped the diversification of trilobites.     

Nektaspid arthropods from the Lower Cambrian Emu Bay Shale Lagerstätte,South Australia,with a reassessment of lamellipedian relationships

Abstract: The lower Cambrian Emu Bay Shale on Kangaroo Island, South Australia, contains the only known Cambrian Burgess Shale‐type biota in Australia. Two new lamellipedian arthropods, Emucaris fava gen. et sp. nov. and Kangacaris zhangi gen. et sp. nov., from the Emu Bay Shale Lagerstätte are described as monotypic genera that are resolved cladistically as a monophyletic group that is sister to Naraoiidae + Liwiidae and classified within the Nektaspida as a new family Emucarididae. Shared derived characters of Emucarididae involve a bipartite, elongate hypostome and elongation of the pygidium relative to the cephalic shield and very short thorax. A monophyletic Liwiidae is composed of Liwia and the Ordovician Tariccoia + Soomaspis but excludes Buenaspis, and even the membership of Buenaspis in Nektaspida is contradicted amongst the shortest cladograms. New morphological interpretations favour affinities of Kwanyinaspis with Conciliterga rather than with Aglaspidida, and Phytophilaspis with Petalopleura.     

A new oryctocephalid trilobite from the Balang Formation (Cambrian Stage 4) of northwestern Hunan,South China,with remarks on the classification of oryctocephalids

A new trilobite, Longaspis paiwuensis n. gen. n. sp., from the Balang Formation (Cambrian Stage 4) in northwestern Hunan, South China, is described. This rare trilobite adds to an expanding taxonomic list of organisms recognized from the Balang Lagerstätte, a deposit of exceptional preservation from the Cambrian. Longaspis paiwuensis is an unusually large-sized, micropygous oryctocephalid trilobite that has proparian facial sutures, pit-like lateral glabellar furrows, up to 17 thoracic segments, and a distinct medial notch in the pygidium; it lacks marginal spines.The classification of the family Oryctocephalidae is reviewed. Three subfamilies are recognized, and Longaspis n. gen. is assigned to the subfamily Oryctocarinae.     

Ontogeny of the Upper Cambrian (Furongian) Olenid trilobite Protopeltura aciculate (Angelin, 1854) from Skåne and Västergötland,Sweden

Abstract: The ontogeny of the trilobite Protopeltura aciculata (Angelin, 1854) is described on the basis of material from the upper Cambrian (Furongian) of Andrarum (Skåne) and Hjelmsäter (Västergötland), Sweden. P. aciculata is present in the Parabolina brevispina and Parabolina spinulosa zones. Protopeltura aciculata is represented by all stages of growth, from early protaspides to holaspides, although most of the specimens are disarticulated and precise degrees are unknown. The cranidia have therefore been allocated to five morphological groups. Cuticular sculpture of the cranidia changes throughout ontogeny. Large tubercles are present in earlier stages, disappear gradually in middle meraspid stages and are replaced with a very faint granulation. The transitory pygidium, relatively large and shield‐shaped with upwardly and backwardly directed marginal spines in early meraspides, later becomes very small, triangular‐shaped and lacking spines as a late meraspid and holaspid. The development of hypostomes and librigenae is also described. Protopeltura aciculata shows major intraspecific variations throughout development, especially regarding the pygidium where variation is much less constrained than in many other olenids. This high developmental plasticity may be a survival strategy for a trilobite living in a stressed environment. Protopeltura inhabited a dysoxic environment, possibly unusually prone to localised spreading of anoxic or toxic water. Some morphs may have been less vulnerable than others to such stresses, surviving by chance and thus enabling the species to continue.     

寒武纪叶足动物链状心网虫躯干骨板功能形态学研究*

链状心网虫(Cardiodictyon catenulum)是寒武纪澄江生物群中较为常见的叶足动物, 其躯干骨板的形态一直存在较大争议。本项研究显示该叶足动物的躯干具有一纵列马鞍状骨板, 每个体节具有单一的骨板, 每个骨板从背部延伸至躯干两侧。因此该叶足动物的骨板与其余具有成对或环状分布的躯干骨板的叶足动物存在显著的区别。骨板中央存在一个圆锥形凸起, 骨板边缘显著增厚且呈锯齿状。链状心网虫的躯干骨板边缘增厚和鞍点的凸起提升了整体的刚性; 锯齿状边缘也指示了更优的保护及防御功能。本项研究还发现链状心网虫骨板表面具有显微筛状结构, 筛孔被充填并保存为众多等距离分布的黄铁矿化瘤突, 与微网虫、怪诞虫及爪网虫躯干骨板的显微网状结构类似, 可能为纤细乳突的出口。本文认为: 寒武纪叶足动物的躯干骨板普遍具有显微筛状结构; 这些骨板除了保护与防御, 可能还有感觉的功能。每个躯干体节对应骨板为寒武纪叶足动物的独有衍征, 可能为背骨板的早期形态, 也可能由成对骨板愈合而来。     

Morphology,ontogeny, and heterochrony in lower and middle Cambrian Gogiids (Eocrinoidea,Echinodermata) from Guizhou Province,China

Gogiid eocrinoids from the Lower Cambrian (Balang Formation — Guizhoueocrinus) and basal Middle Cambrian (Kaili Formation — Sinoeocrinus, Globoeocrinus) in Guizhou Province, China are found in great numbers and are extremely well preserved as high fidelity molds in shale/mud sized siliciclastics. Because of their numbers, complete ontogenetic growth sequences have been observed. Significant differences in growth patterns (heterochrony) are present between Lower and Middle Cambrian genera: thecal plates tend to be paedomorphic in development, while the sutural pores developed between them are peramorphic in their development. Because of the large surface area of theca and brachioles relative to the size of the attachment area, considerable drag in ambient currents would necessitate a strong attachment medium. Anchoring by “biogluing”, possibly by collagen, directly to the substrate or to biodetritus is proposed as the method of attachment.     

Biostratinomy and functional morphology of enrollment in two Middle Devonian trilobites

Although it is common knowledge that many trilobites enrolled, behavioral and functional aspects of enrollment are not at all well understood. Taphonomic details portrayed by enrolled trilobites in the Middle Devonian Hamilton Group (New York State) indicate that enrollment was a complex and morphologically constrained behavior. The trilobites Phacops rana (Green) and Greenops boothi (Green) are frequently enrolled in Hamilton strata; biostratinomic data indicate two very different enrollment postures. Interlocking morphologies (coaptative devices) and apodeme structure and disposition indicate that these postures reflect specific behaviors which involved interaction between tergal structures, inferred musculature, and the substratum. Phacops enrolled by burrowing forward and down into the sediment; dorsal muscles, attached to prominent articulating half-rings, imbricated the thorax such that each lappet overlapped the next posterior segment and locked into a posterior pleural facet. The pygidium was brought into place as the posterior segments of the thorax were placed into vincular notches along the lateral margin of the ventral cephalon. The pygidium locked with the cephalic vincular furrow to complete ‘perfect sphaeroidal’ closure. Greenops enrolled with the cephalon in an upright position at the sediment surface; a submarginal furrow on the ventral surface of the pygidium received the anterior rim of the cephalon. Relatively narrow articulating half-rings limited pleural rotation. Segments were loosely locked into narrow facets at the anterior margin of the next posterior lappet. In spite of rudimentary lappet and half-ring structures, Greenops displays an elaborate system of thoracopygidial muscles which involved dorsoventral and longitudinal attachments along the thorax and into the pygidium. Phacops, in contrast, displays very poorly developed apodemes which occur in the thorax only. Longitudinal muscle strength was likely less important during Phacops enrollment than is evident for the Greenops enrollment procedure. Conversely, Phacops clearly relied to a great degree upon competent closure devices which are poorly developed in Greenops. Biostratinomic data reveal different enrollment behaviors which reflect the function of different enrollment-related morphologies present in each species.     

Ontogeny of the redlichiid trilobite Eoredlichia intermediata from the Chengjiang Lagerstätte,lower Cambrian,southwest China

A detailed investigation of the morphology and ontogeny of the redlichioid trilobite Eoredlichia intermediata (Bulletin of the Geological Society of China, 3–4, 1940, 333) from the lower Cambrian Yu'anshan Member of the Heilinpu Formation, in Kunming, Yunnan Province, southwest China, is presented. The new material comprises a relatively complete ontogenetic series ranging from the early meraspid to the holaspid period, which reveals more details on morphological variation such as the appearance of bacculae in some holaspid specimens, contraction and disappearance of fixigenal spines, and macropleural spines of the first and second thoracic segments, which are all documented for the first time and can also be used as developmental markers defining ontogenetic phases. Two distinct morphotypes, possibly an expression of intraspecific variation or sexual dimorphism, are distinguished by the morphology of pleural spines of the second thoracic segment in meraspid degree 14 and holaspides. The trunk segmentation schedule of E. intermediata is also discussed and conforms to the protarthrous developmental mode. The distinction of the thoracic region into two portions can be observed during late meraspid and early holaspid periods, which might be considered as a reference for a better understanding on the relationship of tagmosis and growth segmentation among the Cambrian redlichiid trilobites.     

A new aglaspidid arthropod from the Upper Cambrian of Tasmania

Abstract: The aglaspidid arthropod, Australaglaspis stonyensis gen. et sp. nov., is described from the Idamean (Upper Cambrian) of Stony Point, Montagu, north‐western Tasmania. The dorsal exoskeleton comprises a semicircular cephalon with a well‐defined marginal rim, acute genal angles, oval eyes, subtriangular glabella area and a subtrapezoidal hypostome. The trunk bears eleven somites with well‐developed pleural spines that progressively curve backwards. Paired postventral plates cover the last trunk tergites and the base of the tailspine, which is long and characterized by a medial cleft. Bilobed ventral impressions indicate the presence of homopodous appendages, forming a food groove along the trunk. The fossil assemblage and its mode of preservation suggest that Australaglaspis originally possessed a phosphatic cuticle, but chemical traces of it have been lost because of regional metamorphism and weathering. Australaglaspis appears to be closely related to the North American aglaspidid Chraspedops modesta Raasch, 1939, based on its medially cleft tailspine.