Trilobite body patterning and the evolution of arthropod tagmosis

Preservation permitting patterns of developmental evolution can be reconstructed within long extinct clades, and the rich fossil record of trilobite ontogeny and phylogeny provides an unparalleled opportunity for doing so. Furthermore, knowledge of Hox gene expression patterns among living arthropods permit inferences about possible Hox gene deployment in trilobites. The trilobite anteroposterior body plan is consistent with recent suggestions that basal euarthropods had a relatively low degree of tagmosis among cephalic limbs, possibly related to overlapping expression domains of cephalic Hox genes. Trilobite trunk segments appeared sequentially at a subterminal generative zone, and were exchanged between regions of fused and freely articulating segments during growth. Homonomous trunk segment shape and gradual size transition were apparently phylogenetically basal conditions and suggest a single trunk tagma. Several derived clades independently evolved functionally distinct tagmata within the trunk, apparently exchanging flexible segment numbers for greater regionally autonomy. The trilobite trunk chronicles how different aspects of arthropod segmentation coevolved as the degree of tagmosis increased.     

A new aglaspidid arthropod from the Upper Cambrian of Tasmania

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

New arthropods from the Lower Devonian Hunsru¨ck Slate (Lower Emsian, Rhenish Massif, western Germany)

Four new genera and species of arthropod, Cambronatus brasseli Wingertshellicus backesi Eschenbachiellus wuttkensis and Magnoculus blindi , are described from the Hunsru¨ck Slate (Lower Emsian) of Germany. All four occur in the Wingertshell Member in the vicinity of Bundenbach. They preserve remarkable details of the ventral morphology, including the appendages, as a result of pyritization. In each case the body consists of just two tagmata, a cephalon and a large number of similar trunk somites. Both Cambronatus Wingertshellicus have fluke-like appendages making up a tail fan. The affinities of these arthropods lie with the Crustacea, apart from Magnoculus , which is an arachnomorph, but they do not fall within those clades with modern representatives. They show that morphologies other than those represented by trilobites and modern arthropod groups persisted long after the Cambrian, at least in muddy bottom marine settings.     

The Arthropod Alalcomenaeus cambricus Simonetta, from the Middle Cambrian Burgess Shale of British Columbia

More than 300 specimens of the previously rare arthropod Alalcomenaeus cambricus Simonetta have been collected from a new Burgess Shale locality in the Glossopleura Zone on Mount Stephen, British Columbia. This new material provides much more complete information on its morphology. The cephalon was covered by a shield. A pair of pedunculate eyes and three median eyes were followed by a large anterior appendage, the 'great appendage', bearing three long flagella. The two posterior head appendages, like those of the trunk, were biramous. They consisted of a segmented, inner branch, and a flap-like outer branch, fringed with long filaments. The trunk consisted of 11 somites, each protected by a tergite and bearing a pair of biramous limbs. The telson was paddle-like and fringed posteriorly with wide flat spines. Alalcomenaeus was probably a predator, moving mainly by swimming. It is now known to be one of the more abundant, widely distributed and longest ranging of Burgess Shale arthropod genera. Its affinities lie with the Arachnomorpha.     

A new probable stem lineage crustacean with three-dimensionally preserved soft parts from the Herefordshire (Silurian) Lagerstätte, UK

A new arthropod with three-dimensionally preserved soft parts, Tanazios dokeron, is described from the Wenlock Series (Silurian) of Herefordshire, England, UK. Serial grinding, digital photographic and computer rendering techniques yielded 'virtual fossils' in the round for study. The body tagmata of T. dokeron comprise a head shield and a long trunk. The head shield bears six pairs of horn-like spines and the head bears five pairs of appendages. The antennule, antenna and mandible are all uniramous, and the mandible includes a gnathobasic coxa. Appendages four and five are biramous and similar to those of the trunk: each comprises a limb base with an endite, an enditic membrane, and two epipodites, plus an endopod and exopod. The hypostome bears a large cone-like projection centrally, and there may be a short labrum. The trunk has some 64 segments and at least 60 appendage pairs. A very small telson has the anus sited ventrally in its posterior part and also bears a caudal furca. Comparative morphological and cladistic analyses of T. dokeron indicate a crustacean affinity, with a probable position in the eucrustacean stem group. As such the epipodites in T. dokeron are the first recorded in a eucrustacean stem taxon. The new species is interpreted as a benthic or nektobenthic scavenger.     

Trace fossils of the arthropod Camptophyllia from the Westphalian (Carboniferous) rocks of Lancashire, UK and their palaeoenvironmental context

Two arthropod trace fossils are described and analysed from the Carboniferous Lower Westphalian (C. communis and basal A. modiolaris chronozones) coal-bearing strata of Lancashire. The biserial trackway Diplichnites triassicus consists of five overlapping en echelon sets of 7–9 tracks preserved as epichnia and hypichnia in lacustrine siltstones. The trackway suggests subaqueous in-phase walking by a multi-segmented producer with a body length of 35–40 mm, width 17–22 mm, and 7–9 appendages. Curved, clustered, or laterally repeated, hypichnial lobes with transverse striations on the base of ripple cross-laminated sandstone are identified as Rusophycus versans. This trace fossil is interpreted as shallow resting or furrowing burrows of a homopodous arthropod, 30–60 mm long, 15–30 mm wide, and probably the same kind of arthropod as produced D. triassicus.A review of contemporary arthropod body fossils from Lagerstätten in Lancashire favours the onisciform, or Arthropleura like arthropod Camptophyllia as a potential producer of both of these trace fossils in a lacustrine palaeoenvironment.This study integrates the analysis of sediments, trace fossils and body fossils for reconstructing the arthropod biota and ecology in Westphalian lacustrine and crevasse splay fluvial palaeoenvironments.     

The cuticle of the enigmatic arthropod Phytophilaspis and biomineralization in Cambrian arthropods

Lin, J.‐P., Ivantsov, A.Y. & Briggs, D.E.G. 2011: The cuticle of the enigmatic arthropod Phytophilaspis and biomineralization in Cambrian arthropods. Lethaia, Vol. 44, pp. 344–349. Many non‐trilobite arthropods occur in Cambrian Burgess Shale‐type (BST) biotas, but most of these are preserved in fine‐grained siliciclastics. Only one important occurrence of Cambrian non‐trilobite arthropods, the Sinsk biota (lower Sinsk Formation, Botomian) from the Siberian Platform, has been discovered in carbonates. The chemical compositions of samples of the enigmatic arthropod Phytophilaspis pergamena Ivantsov, 1999 and the co‐occurring trilobite Jakutus primigenius Ivantsov in Ponomarenko, 2005 from this deposit were analysed. The cuticle of P. pergamena is composed of mainly calcium phosphate and differs from the cuticle of J. primigenius, which contains only calcium carbonate. Phosphatized cuticles are rare among large Cambrian arthropods, except for aglaspidids and a few trilobites. Based on recent phylogenetic studies, phosphatization of arthropod cuticle is likely to have evolved several times. □arthropod cuticle, Burgess Shale‐type preservation, fossil‐diagenesis, phosphatization.     

Lower Jurassic Arthropod Resting Trace from the Hartford Basin of Massachusetts,USA

Cheliceratichnus lockleyi ichnogen. nov. et ichnosp. nov. is a new ichnotaxon of arthropod resting trace (cubichnium) from the Lower Jurassic (Hettangian) East Berlin Formation in Holyoke, Massachusetts, USA. The trace fossil is preserved as showing many of the external anatomical features of the exoskeleton, which resemble those of some chelicerates, notably sun spiders (Solifugae). The resting trace is directly associated with a trackway of the ichnospecies Acanthichnus cursorius Hitchcock. This is the first described fossil resting trace of a solifugan-like arthropod, and the first direct evidence of a trackmaker of A. cursorius.     

Martinssonia elongata gen. et sp.n., a crustacean-like euarthropod from the Upper Cambrian 'Orsten' of Sweden

A tiny arthropod, with five growth stages, is described. Three of the instars are metanauplius-like larvae, having unsegmented bodies and four pairs of appendages. The largest stage, with a length of about 1.5 mm, may still be immature. Its body is divided into three tagmata. The cephalon, including five appendiculate segments, h a projecting forehead with a rostral spine and a small shield with a joint between fourth and fifth segments. Eyes are absent. The trunk is composed of seven annular segments, the anterior two with appendages. The caudal end is a long pleotelson-like segment with the anus on its ventral surface. There are seven pairs of appendages: uniramous antennulae, composed of few tubular podomeres; four pairs of biramous postantennular, almost homeomorphic cephalic appendages; two pairs on the trunk, the anterior pair being similar to the cephalic appendages except for the exopodite, the posterior being much smaller, uniramous and apparently rudimentary. Martinssonia was probably benthic, feeding on detritic particles which it stirred up from the bottom. Besides various crustacean-like features, the new form reveals structures different from Crustacea as well as from all other known arthropodan groups. Martinssonia presumably is a descendant of an euarthropodan group, originating from the crustacean branch long before reaching the eucrustacean level of evolution.     

Redefining the head-trunk interface for the neural crest

The head-trunk interface lies at the occipito-cervical boundary, which corresponds to the somite 5/6 level. Previous studies have demonstrated that neural crest cells also behave differently either side of this boundary and that this may be due to intrinsic differences between cranial and trunk crest. However, it is also possible that some of the observed differences between cranial and trunk crest are assigned by environmental cues. We have therefore scrutinised the behaviour of the neural crest cells generated either side of the occipito-cervical boundary in chick and, interestingly, find that both behave in a truncal fashion by traversing the anterior half of their adjacent somites. Furthermore, although not previously described, we find that transient DRGs form opposite somites 4 and 5. Crest cells produced anterior of the somite 3/4 boundary avoid the somites and behave in a non-truncal fashion; these cells populate the pharyngeal arches, and thus contribute to the developing head. We have further shown, via somite transplantations, that differential behaviour of the posterior versus anterior occipital crest is assigned by the somites. If somites 1 to 3 are replaced by trunk somites, then the anterior occipital crest will behave in a truncal fashion by invading the somites. Correspondingly, if these anterior occipital somites are transplanted in place of trunk somites, they perturb the migration of trunk crest. Thus, for the neural crest, the head-trunk interface does not lie at the occipito-cervical boundary, but rather lies at the somite 3/4 level and is defined by the somites. The fact that this boundary lies at the somite 3/4 level in chick is significant as it reflects the more ancient posterior occipital boundary; in fish, only the first three somites contribute to the occipital bone.     

Anatomy and lifestyle of Kunmingella (Arthropoda, Bradoriida) from the Chengjiang fossil Lagerstätte (lower Cambrian; Southwest China)

An updated reconstruction of the body plan, functional anatomy and life attitude of the bradoriid arthropod Kunmingella is proposed, based on new fossil specimens with preserved soft parts found in the lower Cambrian of Chengjiang and Haikou (Yunnan, SW China) and on previous evidence. The animal has a single pair of short antennae pointing towards the front (a setal pattern indicates a possible sensory function). The following set of seven appendages (each composed of a 5-segmented endopod and a leaf-like exopod fringed with setae) is poorly differentiated, except the first three pairs (with possible rake-like endopodial outgrowths, smaller exopods) and the last pair of appendages (endopod with longer and more slender podomeres). The endopods are interpreted as walking legs with a possible role in handling food particles (marginal outgrowth with setae). The leaf-like exopods may have had a respiratory function. The trunk end is short, pointed, flanked with furcal-like rami and projects beyond the posterior margin of the carapace. The attachment of the body to the exoskeleton is probably cephalic and apparently lacks any well-developed adductor muscle system. The inferred life attitude of Kunmingella (e.g. crawling on the surface of the sediment) was that of a dorsoventrally flattened arthropod capped by a folded dorsal shield (ventral gape at least 120°), thus resembling the living ostracode Manawa. The animal was also probably able to close its carapace as a response to environmental stress or to survive unfavourable conditions (e.g. buried in sediment). The anterior lobes of the valves are likely to have accommodated visual organs (possibly lensless receptors perceiving ambient light through the translucent head shield). Preserved eggs or embryos suggest a possible ventral brood care. The presence of Kunmingella in coprolites and its numerical abundance in Chengjiang sediment indicate that bradoriids constituted an important source of food for larger predators. Kunmingella differs markedly from the representatives of the crown group Crustacea (extant and Cambrian taxa) and from the stem group derivatives of Crustacea (exemplified by phosphatocopids and some ‘Orsten’ taxa) in showing no major sign of limb specialization (e.g. related to feeding strategies). Although it resembles other Chengjiang euarthropods in important aspects of its body plan (e.g. uniramous antennae, endopod/exopod configuration), Kunmingella possesses several features (e.g. antennal morphology, post-antennular appendages with 5-segmented endopods) which support the view that bradoriids may be very early derivatives of the stem line Crustacea.     

Head patterning and Hox gene expression in an onychophoran and its implications for the arthropod head problem

The arthropod head problem has puzzled zoologists for more than a century. The head of adult arthropods is a complex structure resulting from the modification, fusion and migration of an uncertain number of segments. In contrast, onychophorans, which are the probable sister group to the arthropods, have a rather simple head comprising three segments that are well defined during development, and give rise to the adult head with three pairs of appendages specialised for sensory and food capture/manipulative purposes. Based on the expression pattern of the anterior Hox genes labial, proboscipedia, Hox3 and Deformed, we show that the third of these onychophoran segments, bearing the slime papillae, can be correlated to the tritocerebrum, the most anterior Hox-expressing arthropod segment. This implies that both the onychophoran antennae and jaws are derived from a more anterior, Hox-free region corresponding to the proto and deutocerebrum of arthropods. Our data provide molecular support for the proposal that the onychophoran head possesses a well-developed appendage that corresponds to the anterior, apparently appendage-less region of the arthropod head.     

Middle Pennsylvanian Ichnofauna from Eastern Oklahoma,USA

The Keota site, in the Middle Pennsylvanian (Desmoinesian) Keota Sandstone Member, McAlester Formation (Krebs Group) of Haskell County, Oklahoma, yields an extensive ichnofossil assemblage of arthropod trackways, insect resting traces, arthropod feeding and grazing traces, fish swimming traces, and tetrapod footprints. This ichnofossil assemblage occurs in a 0.8 to 1.5-m-thick unit of thinly laminated and ripple-laminated sandstone at the base of the Keota Sandstone Member that is interpreted as a tidal flat sandstone. The arthropod traces are assigned to the ichnotaxa Diplichnites gouldi Gevers types A and B, Diplopodichnus biformis Brady, Paleohelcura tridactyla Gilmore, Tonganoxichnus buildexensis Mángano, Buatois, Maples and Lanier, Gordia marina Emmons, cf. Cochlichnus sp. and Treptichnus bifurcus Miller. The fish swimming traces are assigned to Undichna britannica Higgs, and the tetrapod tracks to Notalacerta ichnosp. and Pseudobradypus ichnosp. The Keota ichnofossil assemblage thus documents the first North American Pennsylvanian record of Paleohelcura tridactyla and the first record of Pseudobradypus from the western United States. This ichnofossil assemblage also provides compelling evidence of a freshwater habitat in a tidal flat setting during deposition of the fossil-bearing sandstone at the Keota site. Indeed, the Keota ichnofossil assemblage is characteristic of a Carboniferous-Permian Tonganoxichnus assemblage that identifies nonmarine tidal flat settings.     

Cellular and molecular investigations into the development of the pectoral girdle

The forelimbs of higher vertebrates are composed of two portions: the appendicular region (stylopod, zeugopod and autopod) and the less prominent proximal girdle elements (scapula and clavicle) that brace the limb to the main trunk axis.We show that the formation of the muscles of the proximal limb occurs through two distinct mechanisms. The more superficial girdle muscles (pectoral and latissimus dorsi) develop by the “In–Out” mechanism whereby migration of myogenic cells from the somites into the limb bud is followed by their extension from the proximal limb bud out onto the thorax. In contrast, the deeper girdle muscles (e.g. rhomboideus profundus and serratus anterior) are induced by the forelimb field which promotes myotomal extension directly from the somites. Tbx5 inactivation demonstrated its requirement for the development of all forelimb elements which include the skeletal elements, proximal and distal muscles as well as the sternum in mammals and the cleithrum of fish. Intriguingly, the formation of the diaphragm musculature is also dependent on the Tbx5 programme. These observations challenge our classical views of the boundary between limb and trunk tissues. We suggest that significant structures located in the body should be considered as components of the forelimb.     

A new cajaro catfish (Siluriformes: Pimelodidae:Phractocephalus) from the Late Miocene of southwestern Amazonia and its relationship to °Phractocephalus nassi of the Urumaco Formation

A new fossil species of the living catfish genusPhractocephalus is described from fluvial Sediments of late Miocene age in Acre State, Brazil and the Madre de Dios region, Peru. †Phractocephalus acreornatus n. sp., is known from a complete neurocranium with associated Weberian complex vertebrae and posttemporal-supracleithra, and several isolated skeletal elements. °Phractocephalus acreornatus is diagnosed by the following combination of characters: 1) anterior half of supraoccipital, frontals and sometimes mesethmoid with coarse parallel ridges and sulci in addition to reticulating ridges and subcircular pits; 2) mesethmoid very broad and coarsely ornamented; 3) lateral ethmoid projecting anteriorly above palatine condyle into olfactory capsule, its anterolateral margin convex and fluted, and its orbital notch reduced; 4) supraoccipital process long, concealing Weberian vertebrae in dorsal view; 5) opercle covered with rough, reticulating ridges and pits; 6) interopercle relatively deep, its outer side coarsely ornamented; 7) pectoral spine shaft ornamented with reticulating ridges and pits. Variability of the form of the much expanded supraoccipital process inPhractocephalus is evaluated for its taxonomic significance. Although °P.acreornatus is distinct from both modernP. hemloliopterus and fossil °P. nassi, like most Miocene fossil fishes from South America, this extinct species is closely similar to its relatives. Available characters provide evidence for a close relationship between °P. acreornatus and °P. nassi from the Urumaco Formation, Venezuela. The provenance of these late Miocene catfishes in lowland western Amazonia and north-central South America is congruent with paleogeographic modeis positing a large, north flowing “Paleo-Amazon-Orinoco” river System in the Andean foreland basin during the Neogene.      

Cephalic and Limb Anatomy of a New Isoxyid from the Burgess Shale and the Role of “Stem Bivalved Arthropods” in the Disparity of the Frontalmost Appendage

We herein describe Surusicaris elegans gen. et sp. nov. (in Isoxyidae, amended), a middle (Series 3, Stage 5) Cambrian bivalved arthropod from the new Burgess Shale deposit of Marble Canyon (Kootenay National Park, British Columbia). Surusicaris exhibits 12 simple, partly undivided biramous trunk limbs with long tripartite caeca, which may illustrate a plesiomorphic “fused” condition of exopod and endopod. We construe also that the head is made of five somites (= four segments), including two eyes, one pair of anomalocaridid-like frontalmost appendages, and three pairs of poorly sclerotized uniramous limbs. This fossil may therefore be a candidate for illustrating the origin of the plesiomorphic head condition in euarthropods, and questions the significance of the “two-segmented head” in, e.g., fuxianhuiids. The frontalmost appendage in isoxyids is intriguingly disparate, bearing similarities with both dinocaridids and euarthropods. In order to evaluate the relative importance of bivalved arthropods, such as Surusicaris, in the hypothetical structuro-functional transition between the dinocaridid frontal appendage and the pre-oral—arguably deutocerebral—appendage of euarthropods, we chose a phenetic approach and computed morphospace occupancy for the frontalmost appendages of 36 stem and crown taxa. Results show different levels of evolutionary decoupling between frontalmost appendage disparity and body plans. Variance is greatest in dinocaridids and “stem bivalved” arthropods, but these groups do not occupy the morphospace homogeneously. Rather, the diversity of frontalmost appendages in “stem bivalved” arthropods, distinct in its absence of clear clustering, is found to link the morphologies of “short great appendages,” chelicerae and antennules. This find fits the hypothesis of an increase in disparity of the deutocerebral appendage prior to its diversification in euarthropods, and possibly corresponds to its original time of development. The analysis of this pattern, however, is sensitive to the—still unclear—extent of polyphyly of the “stem bivalved” taxa.     

Distribution and migration pathways of HNK-1-immunoreactive neural crest cells in teleost fish embryos.

Whole mounts and cross-sections of embryos from three species of teleost fish were immunostained with the HNK-1 monoclonal antibody, which recognizes an epitope on migrating neural crest cells. A similar distribution and migration was found in all three species. The crest cells in the head express the HNK-1 epitope after they have segregated from the neural keel. The truncal neural crest cells begin to express the epitope while they still reside in the dorsal region of the neural keel; this has not been observed in other vertebrates. The cephalic and anterior truncal neural crest cells migrate under the ectoderm; the cephalic cells then enter into the gill arches and the anterior truncal cells into the mesentery of the digestive tract where they cease migration. These cephalic and anterior trunk pathways are similar to those described in Xenopus and chick. The neural crest cells of the trunk, after segregation, accumulate in the dorsal wedges between the somites, however, unlike in chick and rat, they do not migrate in the anterior halves of the somites but predominantly between the neural tube and the somites, the major pathway observed in carp and amphibians; some cells migrate over the somites. The HNK-1 staining of whole-mount embryos revealed a structure resembling the Rohon-Beard and extramedullary cells, the primary sensory system in amphibians. Such a system has not been described in fish.     

Onychophoran‐like myoanatomy of the Cambrian gilled lobopodian Pambdelurion whittingtoni

Arthropods are characterized by a rigid, articulating, exoskeleton operated by a lever‐like system of segmentally arranged, antagonistic muscles. This skeletomuscular system evolved from an unsegmented body wall musculature acting on a hydrostatic skeleton, similar to that of the arthropods’ close relatives, the soft‐bodied onychophorans. Unfortunately, fossil evidence documenting this transition is scarce. Exceptionally‐preserved panarthropods from the Cambrian Lagerstätte of Sirius Passet, Greenland, including the soft‐bodied stem‐arthropod Pambdelurion whittingtoni and the hard‐bodied arthropods Kiisortoqia soperi and Campanamuta mantonae, are unique in preserving extensive musculature. Here we show that Pambdelurion's myoanatomy conforms closely to that of extant onychophorans, with unsegmented dorsal, ventral and longitudinal muscle groups in the trunk, and extrinsic and intrinsic muscles controlling the legs. Pambdelurion also possesses oblique musculature, which has previously been interpreted as an arthropodan characteristic. However, this oblique musculature appears to be confined to the cephalic region and first few body segments, and does not represent a shift towards arthropodan myoanatomy. The Sirius Passet arthropods, Kiisortoqia and Campanamuta, also possess large longitudinal muscles in the trunk, although, unlike Pambdelurion, they are segmentally divided at the tergal boundaries. Thus, the transition towards an arthropodan myoanatomy from a lobopodian ancestor probably involved the division of the peripheral longitudinal muscle into segmented units.     

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

The evolution of segmentation in Crustacea, that is, the formation of sclerotized and jointed body somites and arrangement of somites into tagmata, is viewed in light of historical traits and functional constraints. The set of Early to Late Cambrian 'Orsten' arthropods have informed our current views of crustacean evolution considerably. These three-dimensionally preserved fossils document ancient morphologies, as opposed to purely hypothetical models and, because of the unusual preservation of larval stages, provide us with unparalleled insight into the morphogenesis of body somites and their structural equipment. The variety of evolutionary levels represented in the 'Orsten' including lobopodians, tardigrades, and pentastomids also allows phylogenetic interpretations far beyond the Crustacea. The 'Orsten' evidence and data from representatives of the Lower Cambrian Chengjiang biota in southwestern China, including phylogenetically earlier forms, form the major source of our morphology-based review of structural and functional developments that led toward the Crustacea. The principal strategy of arthropods is the simultaneous development of head somites, as expressed in a basal "head larva," and a successive addition of postcephalic somites from a preterminal budding zone with progressive maturation of metameric structures. This can be recognized in the developmental patterns of extant and fossil representatives of several euarthropod taxa, particularly crustaceans, trilobites, and chelicerates (at least basally). The development of these taxa points to an early somite-poor and free-living hatching stage. Embryonic development to a late stage within an egg, as occurring in recent onychophorans and certain in-group euarthropods, is regarded as achieved several times convergently.     

Transitions in functional morphology from “large branchiopods” to Cladocera: Video and confocal microscopic studies of Cyclestheria hislopi (Cyclestherida) and Sida crystallina (Cladocera: Ctenopoda)

Great diversity is found in morphology and functionality of arthropod appendages, both along the body axis of individual animals and between different life-cycle stages. Despite many branchiopod crustaceans being well known for displaying a relatively simple arrangement of many serially post-maxillary appendages (trunk limbs), this taxon also shows an often unappreciated large variation in appendage morphology. Diplostracan branchiopods exhibit generally a division of labor into locomotory antennae and feeding/filtratory post-maxillary appendages (trunk limbs). We here study the functionality and morphology of the swimming antennae and feeding appendages in clam shrimps and cladocerans and analyze the findings in an evolutionary context (e.g., possible progenetic origin of Cladocera). We focus on Cyclestheria hislopi (Cyclestherida), sister species to Cladocera and exhibiting many “large” branchiopod characters (e.g., many serially similar appendages), and Sida crystallina (Cladocera, Ctenopoda), which likely exhibits plesiomorphic cladoceran traits (e.g., six pairs of serially similar appendages). We combine (semi-)high-speed recordings of behavior with confocal laser scanning microscopy analyses of musculature to infer functionality and homologies of locomotory and filtratory appendages in the two groups. Our morphological study shows that the musculature in all trunk limbs (irrespective of limb size) of both C. hislopi and S. crystallina comprises overall similar muscle groups in largely corresponding arrangements. Some differences between C. hislopi and S. crystallina, such as fewer trunk limbs and antennal segments in the latter, may reflect a progenetic origin of Cladocera. Other differences seem related to the appearance of a specialized type of swimming and feeding in Cladocera, where the anterior locomotory system (antennae) and the posterior feeding system (trunk limbs) have become fully separated functionally from each other. This separation is likely one explanation for the omnipresence of cladocerans, which have conquered both freshwater and marine free water masses and a number of other habitats.