Developmental trait evolution in trilobites

We performed a tree-based analysis of trilobite postembryonic development in a sample of 60 species for which quantitative data on segmentation and growth increments between putative successive instars are available, and that spans much of the temporal, phylogenetic, and habitat range of the group. Three developmental traits were investigated: the developmental mode of trunk segmentation, the average per-molt growth rate, and the conformity to a constant per-molt growth rate (Dyar's rule), for which an original metric was devised. Growth rates are within the normal range with respect to other arthropods and show overall conformity to Dyar's rule. Randomization tests indicate statistically significant phylogenetic signal for growth in early juveniles but not in later stages. Among five evolutionary models fit via maximum likelihood, one in which growth rates vary independently among species, analogous to Brownian motion on a star phylogeny, is the best supported in all ontogenetic stages, although a model with a single, stationary peak to which growth rates are attracted also garners nontrivial support. These results are not consistent with unbounded, Brownian-motion-like evolutionary dynamics, but instead suggest the influence of an adaptive zone. Our results suggest that developmental traits in trilobites were relatively labile during evolutionary history.     

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.     

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.     

Head segmentation of trilobites

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

Perinatal life history traits in New World monkeys

Gestation length, neonatal and maternal body weight, and neonatal and adult brain weight data were collected for New World monkeys in an attempt to establish typical patterns of perinatal life history. This study attempts to illuminate the most accurate values from the available data, which suggest that gestation length and prenatal growth rate are broadly conserved in relation to maternal size in New World monkeys. Exceptions to the patterns evident in the data point to derivations in life history strategies. In particular, this study suggests that the extended gestation length of callitrichines is a function of minimum viable neonate size and not exclusively energy minimization associated with simultaneous lactation. Cebus is shown to undergo more postnatal brain growth relative to other New World monkeys, but not as much as previously believed. Alouatta is shown to be relatively small brained at birth as well as in adulthood. Saimiri is shown to present the most unusual package of perinatal life history traits, in which precocial neonates are gestated for a relatively long time and at a slightly faster growth rate than is typical for New World monkeys. © 1996 Wiley-Liss, Inc.     

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.     

Asymmetry of Early Paleozoic trilobites

Asymmetry in fossils can arise through a variety of biological and geological mechanisms. If geological sources of asymmetry can be minimized or factored out, it might be possible to assess biological sources of asymmetry. Fluctuating asymmetry (FA), a general measure of developmental precision, is documented for nine species of lower Paleozoic trilobites. Taphonomic analyses suggest that the populations studied for each taxon span relatively short time intervals that are approximately equal in duration. Tectonic deformation may have affected the specimens studied, since deviations from normal distributions are common. Several measures of FA were applied to 3–5 homologous measures in each taxon. Measurement error was assessed by the analysis of variance (ANOVA) for repeated measurements of individual specimens and by analysis of the statistical moments of the distributions of asymmetry measures. Measurement error was significantly smaller than the difference between measures taken on each side of a specimen. However, the distribution of differences between sides often deviated from a mean of zero, or was skewed or kurtosic. Regression of levels of FA against geologic age revealed no statistically significant changes in levels of asymmetry through time. Geological and taphonomic effects make it difficult to identify asymmetry due to biological factors. Although fluctuating asymmetry is a function of both intrinsic and extrinsic factors, the results suggest that early Cambrian trilobites possessed genetic or developmental mechanisms used to maintain developmental stability comparable to those of younger trilobites. Although the measures are biased by time averaging and deviations from the normal distribution, these data do not lend strong support to 'genomic' hypotheses that have been suggested to control the tempo of the Cambrian radiation.     

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.     

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.     

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.     

Developmental constraints in cave beetles

In insects, whilst variations in life cycles are common, the basic patterns typical for particular groups remain generally conserved. One of the more extreme modifications is found in some subterranean beetles of the tribe Leptodirini, in which the number of larval instars is reduced from the ancestral three to two and ultimately one, which is not active and does not feed. We analysed all available data on the duration and size of the different developmental stages and compared them in a phylogenetic context. The total duration of development was found to be strongly conserved, irrespective of geographical location, habitat type, number of instars and feeding behaviour of the larvae, with a single alteration of the developmental pattern in a clade of cave species in southeast France. We also found a strong correlation of the size of the first instar larva with adult size, again regardless of geographical location, ecology and type of life cycle. Both results suggest the presence of deeply conserved constraints in the timing and energy requirements of larval development. Past focus on more apparent changes, such as the number of larval instars, may mask more deeply conserved ontogenetic patterns in developmental timing.     

Periodic shell decollation as an ecology-driven strategy in the early Cambrian Cupitheca

Shell decollation is a growth strategy that has been adopted by a number of invertebrate taxa to offset the metabolic and ecological disadvantages of shell growth. However, little is known about the origin and evolution of this process. We here describe well-preserved specimens of the hyolith Cupitheca decollata sp. nov. preserving the decollation process, from the early Cambrian Yu'anshan Formation (c. 518 Ma) of South China. Based on a large number of specimens collectively representing different developmental stages, we use high-resolution X-ray microtomography and scanning electronic microscopy to reconstruct the process of decollation in this taxon. Cupitheca is among the earliest known small shelly fossils, and thus our discovery confirms that periodic decollation had evolved by the onset of the Cambrian explosion, reflecting the high intensity of the predator–prey arms race in early Cambrian ecosystems. A comparison between the decollation processes of Cupitheca and other shelly invertebrates suggests that periodic decollation and the associated molecular mechanisms of calcium dissolution, uptake, allocation and deposition may have had multiple independent origins.     

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.     

Food limitation and body size in the life cycles of planktonic rotifers and cladocerans

This review considers what is known about the effects of food limitation upon the life cycle characteristics of rotifers and planktonic cladocerans. The characteristics considered in rotifers are the size of eggs, juveniles and adults and the durations of the juvenile phase and period of egg production. In cladocerans, the life history features dealt with are their length-weight relationships, the body size, instar stage, age and fecundity of the primiparous female and their fecundity-adult size relationship. The influence of limiting food conditions is demonstrated for these characteristics by comparison with the situation in non-limiting circumstances; the comparison is confined to experiments where food concentrations are quantified. A direct comparison is made between rotifers and cladocerans in conditions of defined food resource availability in terms of their length-weight relationships, the daily allocation of adults or near-adults to growth and reproduction and their threshold food concentrations. These comparisons are discussed in relation to the following topics: the high cost of cumulated respiration resulting from prolongation of the juvenile phase of body growth; the fundamentally different nature of growth in the two taxonomic groups; the body size of species and the size that must be attained for reproduction; the ecological implications of the very different threshold food concentrations.     

Developmental anatomy of seedlings of Indodalzellia gracilis (Podostemaceae)

In Tristichoideae, aquatic angiosperms in the family Podostemaceae, Terniopsis, Tristicha, Indotristicha and Cussetia have creeping roots with flanking (sub)cylindrical shoots, while Dalzellia is rootless and has crustose shoots. Indodalzellia gracilis, sister to a clade of Dalzellia zeylanica and Indotristicha ramosissima, has subcrustose shoots on the side of creeping roots, suggesting that I. gracilis may be a key species to reveal how saltational evolution of the body plan occurred in these three species. We investigated developmental morphology of I. gracilis seedlings grown in culture, using scanning electron microscopy and semi‐thin serial sections. As in D. zeylanica, the plumular apical meristem in the seedling gives rise to two shoot apical meristems, which develop into horizontal subcrustose shoots with dorsal and marginal leaves. Neither radicle nor adventitious root is produced from the hypocotyl, but an adventitious root arises endogenously from the juvenile shoot and from some shoots of adult plants. These results, together with the phylogenetic relationships, suggest that the Indodalzellia seedling evolved by loss of the adventitious root derived from the hypocotyl, appearance of shoots in the axil of cotyledons, and appearance of adventitious roots from adventitious shoots. The difference in place of origin of the root between Indodalzellia and I. ramosissima suggests differing evolutionary origin of the root in Tristichoideae.     

Phylogenetic support for the monophyly of proetide trilobites

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

Global Standard Stratotype-section and Point (GSSP) of the Furongian Series and Paibian Stage (Cambrian)

The Global Standard Stratotype-section and Point (GSSP) of the Furongian Series (uppermost series of the Cambrian System) and the Paibian Stage (lowermost stage of the Furongian Series), has been recently defined and ratified by the International Union of Geological Sciences (IUGS). The boundary stratotype is 369 metres above the base of the Huaqiao Formation in the Paibi section, northwestern Hunan Province, China. This point coincides with the first appearance of the cosmopolitan agnostoid trilobite Glyptagnostus reticulatus, and occurs near the base of a large positive carbon isotopic excursion (SPICE excursion).     

Burlingiid trilobites from Norway, with a discussion of their affinities and relationships

Four Middle and Upper Cambrian burlingiid trilobites from the Oslo Region, Norway, are described including Burlingia angusta sp. nov. from the Ptychagnostus punctuosus Zone and Schmalenseeia athrotryphe sp. nov. from the lower part of the Lejopyge laevigata Zone. New complete material previously attributed to Schmalenseeia jagoi Whittington is assigned to Burlingia . Cladistic analysis supports the genera Burlingia and Schmalenseeia as currently understood, including the placement of the controversial middle Middle Cambrian Schmalenseeia acutangula Westergård in Schmalenseeia , even though it lacks typical characters of the genus such as the median ridge on the preglabellar field. The analysis also supports burlingiid monophyly, and suggests that Schmalenseeia was derived from a broadly Burlingia –like ancestor, with S. acutangula displaying how the transition may have occurred. The broader relationships of Burlingia remain obscure, although similarities between burlingiids and the arthropod Kleptothule from the Early Cambrian Sirius Passet fauna are discussed: these include overall form, lack of functional hinges in the thorax, and details of the cephalic region. It is unclear whether these similarities represent general progenetic features, are functional convergences or, less likely, represent a genuine relationship.