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.     

THE LOWER CAMBRIAN EODISCOID TRILOBITE CALODISCUS LOBATUS FROM SWEDEN: MORPHOLOGY, ONTOGENY AND DISTRIBUTION

Abstract:  Several thousand disarticulated remains together with a few complete enrolled specimens of the lower Cambrian eodiscoid trilobite Calodiscus lobatus ( Hall, 1847 ) have been collected at two outcrop areas in Sweden. The material reveals new details of morphology and morphogenesis during ontogenetic development. Size-frequency analyses show that the material from the Fånån rivulet in Jämtland, central Sweden, represents a natural population dominated by juveniles, whereas the material from Gislövshammar in Scania, southern Sweden, has been sorted during postmortem transport. Three stages of protaspid development can be traced and defined as well as all subsequent ontogenetic stages for the cephalon, hypostome and pygidium. The early meraspid pygidium has a pronounced larval notch, which persists, though becoming progressively less distinct in later meraspides. The number of axial rings in the transitory pygidium increases throughout meraspid development until a third and final thoracic segment is liberated. During ontogeny the articulating half-rings are strongly developed, and both meraspides and holaspides were capable of full sphaeroidal enrollment and outstretched postures. The hypostome undergoes some dramatic modifications; in M0 the anterior margin is axe-shaped, by M1 the area of attachment greatly decreases and the hypostome becomes more elongated and pear-shaped, before attaining its adult form, which has an overall resemblance to that of polymerid trilobites. During ontogeny, the hypostome changes from a conterminant attachment to a natant condition, thereby mirroring hypostomal evolution within trilobites generally. The morphology, ontogeny, enrollment, hypostomal development and the presence of calcified protaspides suggest polymerid rather than agnostoid affinities of the eodiscoids.     

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.     

Trunk segmentation of Cambrian eodiscoid trilobites

SUMMARY Here we report, from Cambrian Series 2 of Chongqing, southern China, on some three-dimensionally phosphatized exoskeletons representing a series of instars of a specialized eodiscoid trilobite Badiscus spinosus gen. et sp. nov. The preservation is unusual and its protaspides (the youngest juveniles) belonging to different growth stages markedly exhibit some "embryonic trunk segments," which imply the specification of all body segments during embryogenesis. This previously unknown style of tagmosis, as being inconsistent with the conventional view of sequential segmentation during postembryonic anamorphosis, emphasizes the complexity of trilobite trunk segmentation during ontogeny and indicates for the first time that epimorphic development, like other known patterns during postembryonic development, may have already been adopted by some of the earliest trilobites.     

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.     

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.     

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.     

Ontogeny of the eodiscoid trilobite Tsunyidiscus acutus from the Lower Cambrian of South China

Abstract: The morphology and ontogeny of the eodiscid trilobite Tsunyidiscus acutus Sun is described on the basis of numerous calcified specimens collected from the Lower Cambrian Shuijingtuo Formation in Yichang and Changyang, Hubei Province, South China. An ontogenetic series is established based on the articulated material including the previously unknown protaspides and meraspides (degrees 0 and 1). The material, revealing some prominent morphological changes such as the number of pygidial axial rings, genal spines retained throughout ontogeny and pygidial pleurae from furrowed to unfurrowed, enables a discussion on the trunk segmentation schedule, indicating that somitogenesis and tagmosis occurred independently during the ontogenetic development of T. acutus.     

Post-embryonic development of the Furongian (late Cambrian) trilobite Tsinania canens: implications for life mode and phylogeny

SUMMARY The current concept of the order Asaphida was proposed to accommodate some Cambrian and Ordovician trilobite clades that are characterized by the possession of a ventral median suture. The family Tsinaniidae was recently suggested to be a member of the order Asaphida on the basis of its close morphological similarity to Asaphidae. Postembryonic development of the tsinaniid trilobite, Tsinania canens , from the Furongian (late Cambrian) Hwajeol Formation of Korea, reveals that this trilobite had an adult-like protaspis. Notable morphological changes with growth comprise the effacement of dorsal furrows, sudden degeneration of pygidial spines, regression of genal spines, and loss of a triangular rostral plate to form a ventral median suture. Programmed cell death may be responsible for degenerating the pygidial and genal spines during ontogeny. Morphological changes with growth, such as the loss of pygidial spines, modification of pleural tips, and effacement of dorsal furrows, suggest that T. canens changed its life mode during ontogeny from benthic crawling to infaunal. The protaspid morphology and the immature morphology of T. canens retaining genal and pygidial spines suggest that tsinaniids bear a close affinity to leiostegioids of the order Corynexochida. Accordingly, development of a ventral median suture in T. canens demonstrates that the ventral median suture could have evolved polyphyletically, and thus the current concept of the order Asaphida needs to be revised.     

Reassessment of the mode of life of Pagetia Walcott, 1916 (Trilobita: Eodiscidae) based on a cluster of intact exuviae from the Kaili Formation (Cambrian) of Guizhou, China

Pagetia is the most abundant trilobite in the Kaili Formation (lower to middle Cambrian). During the course of studying the museum collections of Kaili trilobites ( n  > 1000), a cluster containing 22 pagetiid individuals at various growth stages is noted. Specimens record the growth range from the degree 0 meraspid to late holaspid phases. Based on the relative completeness of the moults except for missing free cheeks, these specimens are interpreted as intact exuviae that had undergone minimal transport prior to burial. If Kaili pagetiids had pelagic or planktonic living habits as previously suggested, it would be difficult to explain the presence of intact exuviae in clusters. Therefore, Kaili pagetiids are interpreted here as having a benthic mode of life after the onset of the meraspid phase.     

Growth and variation in the Silurian proetide trilobite Aulacopleura konincki and its implications for trilobite palaeobiology

Morphometric analysis of growth in Aulacopleura konincki reveals several important features: (1) morphological variability is approximately constant throughout growth, increasing slightly in later ontogeny; (2) shape is more tightly constrained than the numbers of postcephalic segments, which can be quite variable; and (3) a major ontogenetic transition occurs at glabellar lengths of about 1.7 mm. This transition divides the ontogeny into two distinct growth phases, is smooth rather than abrupt, and is expressed as changes in growth trajectories, especially in the pygidium. The transition is not strictly correlated with the number of thoracic or pygidial segments. These results suggest a re-evaluation of the concepts of meraspid and holaspid growth stages in A. konincki , using growth trajectories rather than thoracic segment number to define the stages. Developmental flexibility in holaspid segment numbers in this phylogenetically advanced trilobite suggests that environmental factors, rather than phylogenetic constraint, may explain variations in segment production patterns within some advanced Trilobita. Morphometrics, trilobites, ontogeny, variability, segmentation, Silurian, Czech Republic.     

ONTOGENY AND HETEROCHRONY IN THE EARLY CAMBRIAN ORYCTOCEPHALID TRILOBITES CHANGASPIS, DUYUNASPIS AND BALANGIA FROM CHINA

Abstract:  Ontogenies are described for the first time for three species of Early Cambrian oryctocephalid trilobites: the oryctocephaline Changaspis elongata Lee, in Chien and the oryctocarines Duyunaspis duyunensis Chang and Chien, in Zhou et al . and Balangia balangensis Chien from the Balang Formation in eastern Ghuizhou Province, south-west China. The complete protaspid to holaspid ontogeny for Duyunaspis duyunensis and incomplete meraspid to holaspid ontogenies for Changaspis elongata and Balangia balangensis are described. The relative degree of ontogenetic variation within the lineage Duyunaspis duyunensis – Balangia balangensis is analysed and indicates that Balangia may have evolved from Duyunaspis by paedomorphosis. Decrease in thoracic segment number and concomitant increase in pygidial segment number in the later genus mirrors the pattern seen in the evolution of another oryctocephalid genus, Arthricocephalus . In both cases, selection was probably targeting an increase in cephalic convexity. This resulted in the evolution of a larger pygidium due to increased elevation of the axis above the substrate. The ontogenetic development of Changaspis provides evidence supporting the view that the Oryctocarinae may be considered as paedomorphic descendants of the Oryctocephalinae.     

Growth and development of agnostids Pentagnostus proanabarensis Fedoseev, 1999

Juvenile development of the Middle Cambrian agnostid trilobite Pentagnostus proanabarensis Fedoseev, 1999 is studied. A sequence of eight juvenile stages is defined based on changes in morphology and measurements. Three of them belong to meraspid degree 0, another three refer to meraspid degree 1, and two represent early holaspid stages. Neither the frequency distribution of the length of cephalons and pygidia nor the length-width scatter diagrams give clear clusters of molts; the number of juvenile molts can be estimated through the analysis of the posteroaxis length distribution. Thus, the measurements and morphology of some juvenile pygidia with vestiges of the pleural and postaxial furrows allow reconstruction of the number of segments during individual growth. At the initial stage of development, the pygidial axis of P. proanabarensis probably had four segments, the pygidial axis increased up to no less than six segments in adults.     

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.     

Ontogeny of the Furongian (late Cambrian) trilobite Proceratopyge cf. P. Lata Whitehouse from northern Victoria Land,Antarctica, and the evolution of metamorphosis in trilobites

There were multiple origins of metamorphosis‐undergoing protaspides in trilobite evolution: within the superfamilies Remopleuridioidea, Trinucleoidea, and within the Order Asaphida. Recent studies have revealed that the protaspides of the Cambrian representatives of the Remopleuridioidea and the Trinucleoidea did not undergo metamorphosis. However, ontogeny of the Cambrian members of the Order Asaphida has remained unknown. This study documents the ontogeny of the Furongian asaphoidean ceratopygid trilobite, Proceratopyge cf. P. lata Whitehouse, from northern Victoria Land, Antarctica. Two stages for the protaspid phase, five developmental stages for the post‐protaspid cranidia, and ten stages for the post‐protaspid pygidia have been identified. Interestingly, the protaspis directly developed into a meraspis without metamorphosis. A new cladistic analysis resulted in a single most parsimonious tree, according to which the presence of the bulbous commutavi protaspis turns out to be a synapomorphy for Asaphidae + Cyclopygoidea, not a synapomorphy for the Order Asaphida as previously suggested. In addition, it is inferred that there was convergent evolution of indirectly‐developing commutavi protaspides during the Furongian and Early Ordovician. Metamorphosis‐entailing planktonic larvae evolved in many different metazoan lineages near the Cambrian–Ordovician transition, due to the escalating ecological pressure of the Great Ordovician Biodiversification Event. Since the bulbous commutavi protaspid morphology is thought to be an adaptation for a planktonic life mode, the convergent evolution of the indirect development in the three trilobite lineages at this period might have been a result of adaptation to the early phase of the Great Ordovician Biodiversification Event.     

Ontogeny and Structure of a New, Miniaturised and Spiny Olenid Trilobite from Southern Sweden

Several hundred specimens of a tiny olenid trilobite, Ctenopyge ceciliae sp. nov., have been found in stinkstone nodules in the upper Cambrian Peltura scarabaeoides Zone in southern Sweden. This exceptionally spinose form is known only from disarticulated specimens, but is quite well preserved, and all growth stages are represented. The early ontogenetic stages are exceptionally small, the protaspis being only half the size of that of the associated Peltura species. There may have been no more than three thoracic segments. Thus the whole ontogeny was compressed, and this together with the very small size of the adult indicates a true miniaturisation. Whereas the likely control of the origin of the tiny C. ceciliae was basically progenesis, the extreme spinosity had a different origin; allometric growth or possibly peramorphosis. C. ceciliae is small enough for the spines to have appreciably retarded sinking through frictional effects, and this small trilobite is interpreted as a free-swimming or floating form.     

Comparative ontogeny of different species of agnostina

This study is focused on the growth pattern of agnostids, a group of organisms that were very abundant in the Cambrian. The developmental patterns of six agnostid species are compared. It is shown that, like other trilobites, agnostids have two developmental stages, i.e., meraspid degrees 0 and 1 and the holaspid stage. Meraspid degrees are named according to the number of released thoracic segments. Several molts took place during every meraspid stage resulting in successive instars with a unique set of features. Meraspid degree 0 includes two or three instars, the first and last of which have a distinctive morphology and have been encountered in most of the species studied. During meraspid degree 1, the animal molted from two to four times. The number of molt stages appears to be species-specific. The first and last instars are easily distinguished from other developmental forms. Early holaspids of the majority of agnostid species studied also have certain features in common.     

Developmental traits and life strategy of redlichiid trilobites

The developmental mode of four redlichiid trilobites is summarized, based upon exceptionally well-preserved, articulated specimens from Cambrian Series 2 (stages 3 and 4) strata in southwestern China and South Australia. These relatively complete developmental sequences indicate a balanced rate in segment increase and addition to the thorax during the meraspid phase, which might explain why most redlichiids possess micropygous body patterning during ontogeny. In addition, an analysis of the size distribution, developmental strategy, and distribution of specimen numbers at different growth stages reveals a distinct developmental strategy during the redlichiid life cycle. A relatively short pre-holaspid and a prolonged holaspid phase in these redlichiid taxa offers insight into the developmental control and life strategy in these primitive arthropods.     

贵州寒武纪杷榔组Changaspis elongata Chien,1961个体发育研究及意义

根据采自贵州剑河辣子寨杷榔组的张氏虫Changaspis 296块标本,采用传统形态测量方法,对头部及躯干特征进行度量;结合散点图、简化主轴回归分析及线性拟合对比分析后,确认这些标本均为长形张氏虫Chang-aspis elongata的标本,厘定出从分节6期至成虫期共计12个连续个体发育序列,揭示其形态变化特点。从分节6期至10期,背壳长、头鞍长、头盖长和宽的增长幅度相对小;随着体节的增多,头盖长度与背壳长度的比值从50%减少至34%;随着头鞍长度的增长,头鞍前部及后部宽均呈线性关系的连续增长趋势;个体发育序列确认该种成虫期有17节胸节,不同以往认为的16节胸节。对比分析及线性拟合的数据比较论证,确认原为C.micropyge和C.placenta 2个种的特征应为C.elongata在个体发育变化中的种内差异,确认先前命名的这2种应为C.elongata的晚出同义名。     

Structure, ontogeny, and moulting of the olenid trilobite Ctenopyge (Eoctenopyge) angusta Westergård, 1922 from the upper Cambrian of Västergötland, Sweden

The genus Ctenopyge is known mainly from disarticulated sclerites and from rare complete specimens flattened in shales. Hitherto, very few specimens have been found preserved intact and in three dimensions. In a recently discovered fauna, however, in the Peltura minor Subzone in Västergötland, central Sweden, there occur several species of Ctenopyge , of which many are complete and superbly preserved; moreover they occur at all stages of growth. Of these the abundant Ctenopyge ( Eoctenopyge ) angusta Westergård, 1922 is described and reconstructed here as an adult, and the entire ontogeny is documented for all post–protaspid growth stages. Many characters typical of the adult, such as the long genal spines and the caudal spine, develop very early in ontogeny, and the relative dimensions of the cranidium do not greatly change during growth. Macropleural spines, however, develop later. The transitory pygidium, relatively large and shield–shaped in the early meraspid, later becomes very small as the ten thoracic segments are liberated; a median spine develops on the last thoracic segment only at the holaspid stage. Instar groupings can be clearly distinguished for the early stages. Recurrent associations of sclerites are interpreted as moulting configurations. As reconstructed, the genal spines are horizontal and parallel with the extended thorax; an adaptation which presumably allowed the trilobite to rest on the sea floor.