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

Life cycle evolution in the trilobites Balangia and Duyunaspis from the Cambrian Series 2 (Stage 4) of South China

The evolution process can be reconstructed by tracking the changes in the dynamic characters of life cycles. A number of related trilobites from the Cambrian of South China provide additional information for the study of trilobite evolutionary patterns, which has been hampered by previous incomplete fossil record though. Here, Balangia and Duyunaspis represent related Cambrian oryctocephalid trilobites from South China, are comprehensively discussed over the ontogeny, and the results show that, from B. balangensis via D. duyunensis to D. jianheensis, their exoskeletal morphology shows a directional evolution. Based on the direction of evolutionary changes in the development of Balangia and Duyunaspis, we speculate that Duyunaspis likely evolved from Balangia instead of Balangia evolved from Duyunaspis, as was previously assumed. This inference is also supported by the phylogenetic tree. This research provides not only a better understanding of the mechanisms of evolution in trilobites, but also new insights for the relationship between developmental evolutionary changes and phylogeny in trilobites.

Developing an integrated understanding of the evolution of arthropod segmentation using fossils and evo-devo

Segmentation is fundamental to the arthropod body plan. Understanding the evolutionary steps by which arthropods became segmented is being transformed by the integration of data from evolutionary developmental biology (evo-devo), Cambrian fossils that allow the stepwise acquisition of segmental characters to be traced in the arthropod stem-group, and the incorporation of fossils into an increasingly well-supported phylogenetic framework for extant arthropods based on genomic-scale datasets. Both evo-devo and palaeontology make novel predictions about the evolution of segmentation that serve as testable hypotheses for the other, complementary data source. Fossils underpin such hypotheses as arthropodization originating in a frontal appendage and then being co-opted into other segments, and segmentation of the endodermal midgut in the arthropod stem-group. Insights from development, such as tagmatization being associated with different modes of segment generation in different body regions, and a distinct patterning of the anterior head segments, are complemented by palaeontological evidence for the pattern of tagmatization during ontogeny of exceptionally preserved fossils. Fossil and developmental data together provide evidence for a short head in stem-group arthropods and the mechanism of its formation and retention. Future breakthroughs are expected from identification of molecular signatures of developmental innovations within a phylogenetic framework, and from a focus on later developmental stages to identify the differentiation of repeated units of different systems within segmental precursors.

Anamorphic development and extended parental care in a 520 million-year-old stem-group euarthropod from China

Background

Extended parental care is a complex reproductive strategy in which progenitors actively look after their offspring up to – or beyond – the first juvenile stage in order to maximize their fitness. Although the euarthropod fossil record has produced several examples of brood-care, the appearance of extended parental care within this phylum remains poorly constrained given the scarcity of developmental data for Palaeozoic stem-group representatives that would link juvenile and adult forms in an ontogenetic sequence.

Results

Here, we describe the post-embryonic growth of Fuxianhuia protensa from the early Cambrian Chengjiang Lagerstätte in South China. Our data demonstrate anamorphic post-embryonic development for F. protensa, in which new tergites were sequentially added from a posterior growth zone, the number of tergites varies from eight to 30. The growth of F. protensa is typified by the alternation between segment addition, followed by the depletion of the anteriormost abdominal segment into the thoracic region. The transformation of abdominal into thoracic tergite is demarcated by the development of laterally tergopleurae, and biramous walking legs. The new ontogeny data leads to the recognition of the rare Chengjiang euarthropod Pisinnocaris subconigera as a junior synonym of Fuxianhuia. Comparisons between different species of Fuxianhuia and with other genera within Fuxianhuiida suggest that heterochrony played a prominent role in the morphological diversification of fuxianhuiids. Functional analogy with the flexible trunk ontogeny of Cambrian and Silurian olenimorphic trilobites suggests an adaptation to sporadic low oxygen conditions in Chengjiang deposits for F. protensa. Finally, understanding the growth of F. protensa allows for the interpretation of an exceptional life assemblage consisting of a sexually mature adult alongside four ontogenetically coeval juveniles, which constitutes the oldest occurrence of extended parental care by prolonged cohabitation in the panarthropod fossil record.

Conclusions

Our findings constitute the most detailed characterization of the post-embryonic development in a soft-bodied upper stem-group euarthropod available to date. The new ontogeny data illuminates the systematics, trunk segmentation and palaeoecology of F. protensa, offers insights on the macroevolutionary processes involved in the diversification of this clade, and contributes towards an improved understanding of complex post-embryonic reproductive ecology in Cambrian euarthropods.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1262-6) contains supplementary material, which is available to authorized users.

From egg to "no-body": an overview and revision of developmental pathways in the ancient arthropod lineage Pycnogonida

BACKGROUND

Arthropod diversity is unparalleled in the animal kingdom. The study of ontogeny is pivotal to understand which developmental processes underlie the incredible morphological disparity of arthropods and thus to eventually unravel evolutionary transformations leading to their success. Work on laboratory model organisms has yielded in-depth data on numerous developmental mechanisms in arthropods. Yet, although the range of studied taxa has increased noticeably since the advent of comparative evolutionary developmental biology (evo-devo), several smaller groups remain understudied. This includes the bizarre Pycnogonida (sea spiders) or "no-bodies", a taxon occupying a crucial phylogenetic position for the interpretation of arthropod development and evolution.

RESULTS

Pycnogonid development is variable at familial and generic levels and sometimes even congeneric species exhibit different developmental modes. Here, we summarize the available data since the late 19th century. We clarify and resolve terminological issues persisting in the pycnogonid literature and distinguish five developmental pathways, based on (1) type of the hatching stage, (2) developmental-morphological features during postembryonic development and (3) selected life history characteristics. Based on phylogenetic analyses and the fossil record, we discuss plausible plesiomorphic features of pycnogonid development that allow comparison to other arthropods. These features include (1) a holoblastic, irregular cleavage with equal-sized blastomeres, (2) initiation of gastrulation by a single bottle-shaped cell, (3) the lack of a morphologically distinct germ band during embryogenesis, (4) a parasitic free-living protonymphon larva as hatching stage and (5) a hemianamorphic development during the postlarval and juvenile phases. Further, we propose evolutionary developmental trajectories within crown-group Pycnogonida.

CONCLUSIONS

A resurgence of studies on pycnogonid postembryonic development has provided various new insights in the last decades. However, the scarcity of modern-day embryonic data - including the virtual lack of gene expression and functional studies - needs to be addressed in future investigations to strengthen comparisons to other arthropods and arthropod outgroups in the framework of evo-devo. Our review may serve as a basis for an informed choice of target species for such studies, which will not only shed light on chelicerate development and evolution but furthermore hold the potential to contribute important insights into the anamorphic development of the arthropod ancestor.

Three-dimensionally preserved minute larva of a great-appendage arthropod from the early Cambrian Chengjiang biota

A three-dimensionally preserved 2-mm-long larva of the arthropod Leanchoilia illecebrosa from the 520-million-year-old early Cambrian Chengjiang biota of China represents the first evidence, to our knowledge, of such an early developmental stage in a short-great-appendage (SGA) arthropod. The larva possesses a pair of three-fingered great appendages, a hypostome, and four pairs of well-developed biramous appendages. More posteriorly, a series of rudimentary limb Anlagen revealed by X-ray microcomputed tomography shows a gradient of decreasing differentiation toward the rear. This, and postembryonic segment addition at the putative growth zone, are features of late-stage metanauplii of eucrustaceans. L. illecebrosa and other SGA arthropods, however, are considered representative of early chelicerates or part of the stem lineage of all euarthropods. The larva of an early Cambrian SGA arthropod with a small number of anterior segments and their respective appendages suggests that posthatching segment addition occurred in the ancestor of Euarthropoda.

Ostracod (Ostracoda, Crustacea) genomics - Promises and challenges

Ostracods are well-suited model organisms for evolutionary research. Classic genetic techniques have mostly been used for phylogenetic studies on Ostracoda and were somewhat affected by the lack of large numbers of suitable markers. Genomic methods with their huge potential have so far rarely been applied to this group of crustaceans. We provide relevant examples of genomic studies on other organisms to propose future avenues of genomic ostracod research. At the same time, we suggest solutions to the potential problems in ostracods that the application of genomic techniques might present.

The fossil record of ecdysis, and trends in the moulting behaviour of trilobites

Ecdysis, the process of moulting an exoskeleton, is one of the key characters uniting arthropods, nematodes and a number of smaller phyla into Ecdysozoa. The arthropod fossil record, particularly trilobites, eurypterids and decapod crustaceans, yields information on moulting, although the current focus is predominantly descriptive and lacks a broader evolutionary perspective. We here review literature on the fossil record of ecdysis, synthesising research on the behaviour, evolutionary trends, and phylogenetic significance of moulting throughout the Phanerozoic. Approaches vary widely between taxonomic groups, but an overall theme uniting these works suggests that identifying moults in the palaeontological record must take into account the morphology, taphonomy and depositional environment of fossils. We also quantitatively analyse trends in trilobite ecdysis based on a newly generated database of published incidences of moulting behaviour. This preliminary work reveals significant taxonomic and temporal signal in the trilobite moulting fossil record, with free cheek moulting being prevalent across all Orders and throughout the Phanerozoic, and peaks of cephalic moulting in Phacopida during the Ordovician and rostral plate moulting in Redlichiida during the Cambrian. This study and a review of the literature suggest that it is feasible to extract large-scale evolutionary information from the fossil record of moulting.

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

BACKGROUND

Cuticular specialisations such as joints and membranes play an important role in the function of arthropod limbs. This includes sclerotisations and mineral incrustations of cuticular areas to achieve either more rigidity or flexibility. The anterior eight thoracopods of Malacostraca have limb stems comprising a coxa and a basipod, which carries the two rami. Their pleopods, the limbs of the posterior trunk part, have for long been regarded to lack a coxa. Several calcitic sclerites occur in the area between ventral body and limb stem. This raises the questions: do these elements represent specialisations of the membrane due to functional requirements, and do they originate from an originally larger limb portion, i.e., the coxa, or in fact represent it.

RESULTS

We investigated 16 species of selected malacostracan taxa from all major in-groups. Calcitic sclerites occur in constant numbers and position within a species (no individual variation, and independent of specific modification such as in genital appendages). These are even constant within a supra-specific taxon, which facilitates comparisons. In general the sclerites connect via two pivot joints to the sternite medially and the tergopleura laterally, and two more to the limb stem. Based on this, we reconstructed putative ground-pattern conditions for the sclerites of the examined taxa of Malacostraca.

CONCLUSIONS

The pattern of sclerites is characteristic for each monophyletic malacostracan taxon. The highest number of sclerites most likely represents the plesiomorphic state. Reduction of sclerite numbers occurs in Caridoida and its in-groups. Sclerite arrangement in these taxa provides an important character complex for phylogenetic studies. The presence of pivot joints to the body proximally and basipod distally demonstrates the existence of a coxa, which is just slightly less sclerotised, particularly on its posterior side. This can be explained by enhanced flexibility of the pleopods evolved in the course to their major role as swimming devices. Both the pivot joints and the proximal and distal extension of the calcitic sclerites demarcate the minimum area of the coxa. With this, sclerites appear as very valuable also in shedding more light on the putative relationships between Malacostraca, Myriapoda, Insecta, and Remipedia.

Evolution of eumalacostracan development-new insights into loss and reacquisition of larval stages revealed by heterochrony analysis

BACKGROUND

Within Malacostraca (Crustacea), direct development and development through diverse forms of larvae are found. Recent investigations suggest that larva-related developmental features have undergone heterochronic evolution in Malacostraca. In the light of current phylogenetic hypotheses, the free-swimming nauplius larva was lost in the lineage leading to Malacostraca and evolved convergently in the malacostracan groups Dendrobranchiata and Euphausiacea. Here we reconstruct the evolutionary history of eumalacostracan (Malacostraca without Phyllocarida) development with regard to early appendage morphogenesis, muscle and central nervous system development, and determine the heterochronic transformations involved in changes of ontogenetic mode.

RESULTS

Timing of 33 developmental events from the different tissues was analyzed for six eumalacostracan species (material for Euphausiacea was not available) and one outgroup, using a modified version of Parsimov-based genetic inference (PGi). Our results confirm previous suggestions that the event sequence of nauplius larva development is partly retained in embryogenesis of those species which do not develop such a larva. The ontogenetic mode involving a nauplius larva was likely replaced by direct development in the malacostracan stem lineage. Secondary evolution of the nauplius larva of Dendrobranchiata from this ancestral condition, involved only a very small number of heterochronies, despite the drastic change of life history. In the lineage leading to Peracarida, timing patterns of nauplius-related development were lost. Throughout eumalacostracan evolution, events related to epidermal and neural tissue development were clearly less affected by heterochrony than events related to muscle development.

CONCLUSIONS

Weak integration between mesodermal and ectodermal development may have allowed timing in muscle formation to be altered independently of ectodermal development. We conclude that heterochrony in muscle development played a crucial role in evolutionary loss and secondary evolution of a nauplius larva in Malacostraca.

A 520 million-year-old chelicerate larva

An important survival strategy for animal species is the so-called niche differentiation between larva and adult. Different developmental stages of the same animal occupy different ecological niches to avoid competing for food or other essential resources. Here, we describe an exceptionally preserved larval stage of the short great appendage (SGA) arthropod (megacheiran) Leanchoilia illecebrosa from the early Cambrian Chengjiang biota of China. The larval specimen preserves fine details of the main feeding limb, the SGA, which are unknown in the adult of the same species. This discovery demonstrates that niche differentiation during ontogeny was developed in this species of megacheiran--a group of fossil arthropods that has been considered to be early representatives of Chelicerata, which includes horseshoe crabs and arachnids. Hence, this type of niche differentiation, which is common today, originated from the early Cambrian.

Babes in the wood--a unique window into sea scorpion ontogeny

BACKGROUND

Few studies on eurypterids have taken into account morphological changes that occur throughout postembryonic development. Here two species of eurypterid are described from the Pragian Beartooth Butte Formation of Cottonwood Canyon in Wyoming and included in a phylogenetic analysis. Both species comprise individuals from a number of instars, and this allows for changes that occur throughout their ontogeny to be documented, and how ontogenetically variable characters can influence phylogenetic analysis to be tested.

RESULTS

The two species of eurypterid are described as Jaekelopterus howelli (Kjellesvig-Waering and Størmer, 1952) and Strobilopterus proteus sp. nov. Phylogenetic analysis places them within the Pterygotidae and Strobilopteridae respectively, both families within the Eurypterina. Jaekelopterus howelli shows positive allometry of the cheliceral denticles throughout ontogeny, while a number of characteristics including prosomal appendage length, carapace shape, lateral eye position, and relative breadth all vary during the growth of Strobilopterus proteus.

CONCLUSIONS

The ontogeny of Strobilopterus proteus shares much in common with that of modern xiphosurans, however certain characteristics including apparent true direct development suggest a closer affinity to arachnids. The ontogenetic development of the genital appendage also supports the hypothesis that the structure is homologous to the endopods of the trunk limbs of other arthropods. Including earlier instars in the phylogenetic analysis is shown to destabilise the retrieved topology. Therefore, coding juveniles as individual taxa in an analysis is shown to be actively detrimental and alternative ways of coding ontogenetic data into phylogenetic analyses should be explored.

Nervous system development in Spinicaudata and Cyclestherida (Crustacea, Branchiopoda)--comparing two different modes of indirect development by using an event pairing approach

Cladocera are the ecologically most important group within the Branchiopoda. They are unquestionably branchiopods but their evolutionary origin remains unclear. One favored explanation of their origin is that they evolved from a reproductive larva of a clam shrimp-like ancestor. To reveal a transformation and identify (potential) changes in chronology (heterochrony), we investigated and compared the development of representatives of two clam shrimp taxa, one of the Spinicaudata (Leptestheria dahalacensis) and one of the Cyclestherida (Cyclestheria hislopi), the sister group of Cladocera. Both taxa develop indirectly although the exact modes are quite different. The development of the nervous system, labeled and analyzed using immunohistochemical techniques and confocal microscopy, and that of the external morphology, scanned with an electron microscope, was investigated. L. dahalacensis hatch as a free-swimming nauplius and the nervous system and external morphology develop gradually. In C. hislopi, on the other hand, several internal and external structures develop before the hatching of a nonswimming embryo-like larva which is still carried in a dorsal brood pouch. The development in L. dahalacensis is directed from anterior to posterior, whereas in C. hislopi a more synchronous anterior and posterior differentiation is present. A comparison of both developmental sequences gives us the first indications of the evolutionary transformation which the Cladocera may have undergone from a clam shrimp-like ancestor.

Reconstruction of cell lineage and spatiotemporal pattern formation of the mesoderm in the amphipod crustacean Orchestia cavimana

BACKGROUND

Cell lineage studies in amphipods have revealed an early restriction of blastomere fate. The mesendodermal cell lineage is specified with the third cleavage of the egg. We took advantage of this stereotyped mode of development by fluorescently labeling the mesodermal precursors in embryos of Orchestia cavimana and followed the morphogenesis of the mesodermal cell layer through embryonic development.

RESULTS

The mesoderm of the trunk segments is formed by a very regular and stereotypic cell division pattern of the mesoteloblasts and their segmental daughters. The head mesoderm in contrast is generated by cell movements and divisions out of a mesendodermal cell mass. Our reconstructions reveal the presence of three different domains within the trunk mesoderm of the later embryo. We distinguish a cell group median to the limbs, a major central population from which the limb mesoderm arises and a dorsolateral branch of mesodermal cells.

CONCLUSIONS

Our detailed description of mesodermal development relates different precursor cell groups to distinct muscle groups of the embryo. A dorsoventral subdivision of mesoderm is prepatterned within the longitudinal mesodermal columns of the germ-band stage. This makes amphipods excellent crustacean models for studying mesodermal differentiation on a cellular and molecular level.

Multi-host model-based identification of Armillifer agkistrodontis (Pentastomida), a new zoonotic parasite from China

BACKGROUND

Pentastomiasis is a rare parasitic infection of humans. Pentastomids are dioecious obligate parasites requiring multiple hosts to complete their lifecycle. Despite their worm-like appearance, they are commonly placed into a separate sub-class of the subphylum Crustacea, phylum Arthropoda. However, their systematic position is not uncontested and historically, they have been considered as a separate phylum.

METHODOLOGY/PRINCIPAL FINDINGS

An appraisal of Armillifer agkistrodontis was performed in terms of morphology and genetic identification after its lifecycle had been established in a multi-host model, i.e., mice and rats as intermediate hosts, and snakes (Agkistrodon acutus and Python molurus) as definitive hosts. Different stages of the parasite, including eggs, larvae and adults, were isolated and examined morphologically using light and electron microscopes. Phylogenetic and cluster analysis were also undertaken, focusing on the 18S rRNA and the Cox1 gene. The time for lifecycle completion was about 14 months, including 4 months for the development of eggs to infectious larvae in the intermediate host and 10 months for infectious larvae to mature in the final host. The main morphological difference between A. armillatus and Linguatula serrata is the number of abdominal annuli. Based on the 18S rRNA sequence, the shortest hereditary distance was found between A. agkistrodontis and Raillietiella spp. The highest degree of homology in the Cox 1 nucleic acid sequences and predicted amino acid sequences was found between A. agkistrodontis and A. armillatus.

CONCLUSION

This is the first time that a multi-host model of the entire lifecycle of A. agkistrodontis has been established. Morphologic and genetic analyses supported the notion that pentastomids should be placed into the phylum Arthropoda.

Soft-part anatomy of the Early Cambrian bivalved arthropods Kunyangella and Kunmingella: significance for the phylogenetic relationships of Bradoriida

Bradoriids are small bivalved marine arthropods that are widespread in rocks of Cambrian to Early Ordovician age. They comprise seven families and about 70 genera based on shield ('carapace') morphology. New bradoriid specimens with preserved soft-part anatomy of Kunmingella douvillei (Kunmingellidae) are reported from the Early Cambrian Chengjiang Lagerstätte of China together with, for the first time to our knowledge, a second bradoriid species with preserved soft parts, Kunyangella cheni (Comptalutidae). Kunmingella douvillei has a 10-segmented limb-bearing body with uniramous ninth and tenth appendages and a series of homogeneous, apparently (proximal parts not preserved) unspecialized post-antennal biramous limbs with setose leaf-shaped exopods. Each endopod consists of five podomeres. A presumed penultimate instar of Ky. cheni preserves remnants of three head and two trunk appendages, and the adult is reconstructed as having four head appendages. This material allows testing of the affinity of the Bradoriida. Kunmingella is identified as a stem crustacean in character-based analyses, through both morphological comparisons and cladistic reconstructions. Global parsimony analysis recovers a monophyletic Bradoriida as the sister group to crown crustaceans.

Development of the caudal exoskeleton of the pliomerid trilobite Hintzeia plicamarginis new species

The later juvenile ontogeny of the caudal plate of the early Ordovician pliomerid trilobite Hintzeia plicamarginis new species likely comprised an initial phase during which the rate of appearance of new segments subterminally exceeded that of segment release into the thorax, a short phase of constant segment numbers, and a later phase during which release occurred but in which no new segments appeared. A distinct terminal region became manifest in the second phase. During the second and third phases growth coefficients for individual segments were about 1.1--1.2 per instar. Although the shapes of segments varied during growth, the pattern of ontogenetic shape change appears to have been broadly similar among segments. This suggests an homonomous trunk segment morphology regardless of thoracic or caudal identity in maturity. These results imply that control of trunk exoskeletal segment appearance and articulation were decoupled in this trilobite, and that the terminal region had a distinct mature morphology. H. plicamarginis is described as a new species.