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.     

Ontogeny of Agnostida

Evidence about ontogeny of the miomerid trilobite groups Eodiscina and Agnostina is summarizied and unified in terminology. There appear to be strong similarities in the juvenile development of both suborders. They both have prolonged meraspid degree 0 and 1 stages, including two to six molts. The degree 0 stage tends to shorten from six to four molts in Eodiscina, and probably from three to two in Agnostina, while degree 1 tends to lengthen whithin Eodiscina. During degree 0 and 1 moltings, none of the segments are liberated into the thorax, and one segment is added into the transitory pygidium. The liberation of thoracic segments marks the beginning of the meraspid degrees and the holaspid period. This developmental scenario contrasts with that of the classical polymerid—in the latter case, the liberation of thoracic segments is accompanied by insertion of new segments in the transitory pygidium. In the Eodiscina, during meraspid degree 2, the polymerid scheme is present.     

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.     

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.     

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。     

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.     

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.     

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.     

A Cambrian corynexochoid trilobite with only two thoracic segments

Discovery of articulated specimens of Vistoia? minuta Resser shows that species to be an isopygous corynexochoid trilobite with only two thoracic segments in the holaspid period. A new genus, Thoracocure, is proposed to include Vistoia? minuta Resser and Tonkinella idahoensis Resser, and concepts of those species are revised. Neoteny appears to be responsible for several holaspid characters of T. minuta. its holaspid size may be the smallest known for trilobites, and a pelagic mode of life is suggested. The combination of characters possessed by T. minuta requires revision of current concepts of the orders Agnostida and Corynexochida. As a related matter, it is proposed that the Miomera of Jaekel (1909) be suppressed as a subjective junior synonym of the Agnostida (nomen translatum Kobayashi, 1935) of Salter (1864).     

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.     

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.     

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.     

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.     

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.     

Lack of spermatogenic variation in a polymorphic lizard,Sceloporus aeneus (Squamata: Phrynosomatidae)

Although different mechanisms exist to explain the presence of polymorphism in lizards, one model suggests that multiple morphotypes display the same level of fitness. Three male morphs (grey, yellow and orange) coexist in Sceloporus aeneus, a Mexican endemic oviparous lizard. Using a histological perspective, we test the hypothesis that spermatogenic output does not vary across morphotypes of S. aeneus during its maximum testicular activity. Males of S. aeneus (five grey, five yellow and five orange) were collected in Calimaya, Estado de México, Mexico. Snout-vent length (SVL), testis mass, diameter and epithelial heights for the seminiferous tubules and epididymis, and the number of layers of germ cells did not vary among morphs; moreover, according to principal component analysis, a high overlap among lateral colour morphs exists. Our results suggest strongly that the lateral colour morphs in S. aeneus have the same spermatogenic output, and natural selection may be a stronger driving force than sexual selection within this species. Further studies into other lizard species with multiple morphotypes are required to determine whether the lack of variation in spermatogenic output observed in this endemic lizard is consistent across polymorphic species which will provide a greater understanding of the selective mechanisms acting on an individual’s fitness.     

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.     

Late ontogeny of the trilobite Tsinania shanxiensis (Zhang and Wang, 1985) from the Cambrian (Furongian) of Anhui,China and its systematic implications

A new occurrence of Tsinania shanxiensis (Zhang and Wang, 1985) n. comb., is reported herein from northern Anhui, China. This species is here transferred from Guluheia to Tsinania as the differences between the two genera are not enough to support the generic difference. The late ontogeny of T. shanxiensis is documented herein, which provides important evidence for the evolutionary origin of the Family Tsinaniidae. The late ontogeny of the pygidia of T. shanxiensis is divided into four stages: PSS+2 stage, PSS+1 stage, early holaspid stage, and late holaspid stage. The immature morphology of T. shanxiensis retains long pygidial spines, reminicent of Mansuyia. Therefore, the morphology of T. shanxiensis can be regarded as the transitional type between kaolishaniids (e.g., Mansuyia orientalis, M. tani) and tsinaniids (e.g., T. canens, Shergoldia laevigata) during the middle Furongian.     

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.