Reconsideration of the supposed naraoiid larva from the Early Cambrian Chengjiang Lagerstätte, South China

The Chengjiang Lagerstätte in the Lower Cambrian of South China yields a small, larva‐like arthropod, which was considered to be a protaspis of naraoiids by many authors. The discovery of a large number of well‐preserved specimens from many new localities has allowed the original study to be revised. The relatively large size, stable morphology and unusual structure of the appendages indicate that these specimens represent adults of a new arthropod, Primicaris larvaformis. The larva‐like outline is considered to have arisen by the heterochronic process of progenesis. In addition, this animal displays primitive aspects of bodyplan and limb morphology that suggest a basal position within arachnomorphs, or perhaps even arthropods, and the similarities to the Vendian arthropod‐like animal Parvancorina probably provide an evolutionary link between Vendian forms and Cambrian arthropods.     

The Collins’ monster,a spinous suspension-feeding lobopodian from the Cambrian Burgess Shale of British Columbia

Lobopodians, a paraphyletic group of rare but morphologically diverse Palaeozoic vermiform animals bearing metameric appendages, are key to the origin of extant panarthropods. First discovered in 1983 on Mount Stephen (Yoho National Park, British Columbia), the Cambrian (Wuliuan) Burgess Shale lobopodian nicknamed ‘Collins’ monster’ is formally described as Collinsovermis monstruosus gen. et sp. nov. A formal systematic treatment of the comparable and poorly known lobopodian Acinocricus stichus from Utah is also provided. The body of Collinsovermis is plump and compact but shows the diagnostic suspension-feeding characters of luolishaniid lobopodians. It possesses 14 contiguous pairs of lobopods, lacking space between them. The 6 anterior pairs are elongate, adorned with about 20 pairs of long and slightly curved ventral spinules arranged in a chevron-like pattern. These appendages terminate in a pair of thin claws and their dorsal surfaces are covered in minute spines or setae. The 8 posterior lobopod pairs, which attach to a truncated body termination, are stout and smooth, each terminated by a single strong recurved claw. Each somite bears a pair of dorsal spines; somites 4 and posteriad bear an additional median spine. The spines on somites 1–3 are much shorter than the spines on the remaining somites. The head is short, bears a terminal mouth and a pair of antenniform outgrowths, and is covered by an oblong sclerite. Collinsovermis, plus Collinsium and Acinocricus, are found to comprise a sub-group of stout luolishaniid lobopodians with remarkably long spinules on the front lobopods, interpreted here as a clade (Teratopodidae fam. nov.) This clade is distinct from both the comparatively slenderer Luolishania and a sub-group composed of Facivermis and Ovatiovermis lacking body sclerites. Luolishaniids were mostly sessile forerunners of arthropods that had coupled efficient suspension-feeding devices and, as in Collinsovermis, strong defensive or deterrent features.     

A new 'great-appendage' arthropod from the Lower Cambrian of China and homology of chelicerate chelicerae and raptorial antero-ventral appendages

The uniramous ‘great appendages’ of several arthropods from the Early to Middle Cambrian are a characteristic pair of pre‐oral limbs, which served for prey capture. It has been assumed that the morphological differences between the ‘great‐appendage’ arthropods indicate that raptorial antero‐ventral and anteriorly pointing appendages evolved more than once in arthropod phylogeny. One set of Cambrian ‘great‐appendage’ arthropods has, however, very similar short antero‐ventral appendages with a peduncle of two segments angled against each other (elbowed) and with stout distally or medio‐distally directed spines or long flexible flagellate spines on each of the four distal segments. Moreover, the head appendages of all these forms comprise the ‘great appendages’ and three pairs of biramous limbs. To this set of taxa we can add a new form from the Lower Cambrian Maotianshan Shale of southern China, Haikoucaris ercaiensis n. gen. and n. sp. It is known from three specimens, possibly being little abundant in the faunal community. It can be distinguished from all other taxa by the prominence of the proximal claw segment of its ‘great appendages’ and by only three distal spines (one on each of the distal segments). The similarity of the short, spiky ‘great appendages’ of Haikoucaris with the chelicera of the Chelicerata leads us to hypothesize that this particular type of ‘great appendages’ was the actual precursor of the chelicera. Homeobox gene and developmental data recently demonstrated the homology between the antenna of ateloceratans and the antennula of crustaceans on one side and the chelicera of chelicerates on the other. To this we add palaeontological evidence for the homology between the chelicerae of chelicerates and the ‘short great appendages’ of certain Cambrian arthropods, which leads us to hypothesize that the evolutionary path went from the ‘short great appendages’, by progressive compaction, toward the chelicera with only a two‐spined chela. The new form from China is regarded as the possible latest offshoot, whereas the other ‘great appendages’ arthropods with similar short grasping limbs were derivatives of the stem lineage of the crown‐group Chelicerata. Consequently, the chelicera with a chela with one fixed and one mobile finger is an autapomorphy of the crown group of Chelicerata, whereas a raptorial, but more limb‐like antenna, with more distal spine‐bearing segments, characterized the ground pattern of Chelicerata. Further taxa having ‘great appendages’, including the large Anomalocarididae, are also discussed in the light of their possible affinities to the Chelicerata and possible monophyly of all of these arthropods with raptorial anterior appendages.     

A new arthropod from the Early Cambrian of North Greenland,with a ‘great appendage’‐like antennula

Kiisortoqia soperi gen. et sp. nov. is an arthropod species from the Early Cambrian Sirius Passet Lagerstätte of North Greenland. A head, incorporating four appendiferous segments and biramous limbs, with an anteroposteriorly compressed basipod with a spine bearing median edge, support the euarthropod affinities of K. soperi gen. et sp. nov. Similarities with ‘short great appendage’ arthropods, or megacheirans, like the nine‐segmented endopod, and the flap‐ or paddle‐like exopod, may be symplesiomorphies. The antennula, however, resembles in composition and size the anteroventral raptorial appendage of anomalocaridids. Thus, the morphology of K. soperi gen. et sp. nov. provides additional support for the homologization of the anomalocaridid ‘great appendage’ with the appendage of the antennular or deutocerebral segment of extant Euarthropoda. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158 , 477–500.     

New Middle Cambrian bivalved arthropods from the Burgess Shale (British Columbia,Canada)

The morphology of two new bivalved arthropods, Loricicaris spinocaudatus gen. et sp. nov. and Nereocaris briggsi sp. nov. from the middle Cambrian (Series 3, Stage 5) Burgess Shale Formation (Collins Quarry locality on Mount Stephen, Yoho National Park, British Columbia, Canada), is described. The material was originally assigned to the genus Branchiocaris, but exhibits distinctive character combinations meriting its assignment to other taxa. Loricicaris spinocaudatus possesses an elongate and spinose abdomen comparable to the contemporaneous Perspicaris and Canadaspis, as well as chelate second head appendages and subtriangular exopods, comparable to Branchiocaris. Nereocaris briggsi possesses a laterally compressed carapace, elongate and delicate appendages and a medial eye located between a pair of lateral eyes on a rhomboidal eye stalk. Although undoubtedly congeneric with Nereocaris exilis from a slightly younger horizon of the Burgess Shale Formation, N. briggsi differs in overall proportions and segment number, warranting assignment to a new species. The newly described taxa were coded into an extensive cladistic analysis of 755 characters, and 312 extinct and extant panarthropods, including a variety of Cambrian bivalved arthropods from both the Burgess Shale and the Chengjiang Lagerstätten. Cambrian bivalved arthropods consistently resolved as a paraphyletic assemblage at the base of Arthropoda. Important innovations in arthropod history such as the specialization of the deutocerebral head appendages and a shift from a nekton‐benthic deposit feeding habit to a benthic scavenging/predatory habit, the symplesiomorphic feeding condition of Euarthropoda (crown‐group arthropods), were found to have occurred among basal bivalved arthropods.     

Reappraising the early evidence of durophagy and drilling predation in the fossil record: implications for escalation and the Cambrian Explosion

The Cambrian Explosion is arguably the most extreme example of a biological radiation preserved in the fossil record, and studies of Cambrian Lagerstätten have facilitated the exploration of many facets of this key evolutionary event. As predation was a major ecological driver behind the Explosion – particularly the radiation of biomineralising metazoans – the evidence for shell crushing (durophagy), drilling and puncturing predation in the Cambrian (and possibly the Ediacaran) is considered. Examples of durophagous predation on biomineralised taxa other than trilobites are apparently rare, reflecting predator preference, taphonomic and sampling biases, or simply lack of documentation. The oldest known example of durophagy is shell damage on the problematic taxon Mobergella holsti from the early Cambrian (possibly Terreneuvian) of Sweden. Using functional morphology to identify (or perhaps misidentify) durophagous predators is discussed, with emphasis on the toolkit used by Cambrian arthropods, specifically the radiodontan oral cone and the frontal and gnathobasic appendages of various taxa. Records of drill holes and possible puncture holes in Cambrian shells are mostly on brachiopods, but the lack of prey diversity may represent either a true biological signal or a result of various biases. The oldest drilled Cambrian shells occur in a variety of Terreneuvian‐aged taxa, but specimens of the ubiquitous Ediacaran shelly fossil Cloudina also show putative drilling traces. Knowledge on Cambrian shell drillers is sorely lacking and there is little evidence or consensus concerning the taxonomic groups that made the holes, which often leads to the suggestion of an unknown ‘soft bodied driller’. Useful methodologies for deciphering the identities and capabilities of shell drillers are outlined. Evidence for puncture holes in Cambrian shelly taxa is rare. Such holes are more jagged than drill holes and possibly made by a Cambrian ‘puncher’. The Cambrian arthropod Yohoia may have used its frontal appendages in a jack‐knifing manner, similar to Recent stomatopod crustaceans, to strike and puncture shells rapidly. Finally, Cambrian durophagous and shell‐drilling predation is considered in the context of escalation – an evolutionary process that, amongst other scenarios, involves predators (and other ‘enemies’) as the predominant agents of natural selection. The rapid increase in diversity and abundance of biomineralised shells during the early Cambrian is often attributed to escalation: enemies placed selective pressure on prey, forcing phenotypic responses in prey and, by extension, in predator groups over time. Unfortunately, few case studies illustrate long‐term patterns in shelly fossil morphologies that may reflect the influence of predation throughout the Cambrian. More studies on phenotypic change in hard‐shelled lineages are needed to convincingly illustrate escalation and the responses of prey during the Cambrian.     

Moulting in the lobopodian Onychodictyon from the lower Cambrian of Greenland

A number of lobopodian taxa from the Cambrian display pairs of sclerotized plates symmetrically positioned along the dorsum of the animal, predominantly above the walking appendages. Most genera were described from complete body fossils exquisitely preserved in the famous Cambrian Lagerstätten, but lobopodian phosphatized plates are found worldwide as typical components of Cambrian small shelly fossil assemblages (SSF). Details regarding intraspecific and ontogenetic variation in lobopod plates are elusive, and the lack of details of ornamentation in Lagerstätte specimens does not minimize the problem. We document here an assemblage of well‐preserved isolated plates of Onychodictyon sp. from the Lower Cambrian (Cambrian Series 2, Stage 4) of North Greenland. Two specimens exhibit perfectly conjoined plates from successive moults. Details of ornamentation and the outline and profile of the fixed plates are identical, but width and length of the underlying plate are 24% larger. These specimens boost the body of evidence that lobopodians moulted but also show that plate outline and ornamentation did not vary during ontogeny.     

A large new leanchoiliid from the Burgess Shale and the influence of inapplicable states on stem arthropod phylogeny

Characterized by atypical frontalmost appendages, leanchoiliids are early arthropods whose phylogenetic placement has been much debated. Morphological interpretations have differed, some of which concern critical characters such as the number of eyes and head appendages, but methodological approaches also have diverged. Here, we describe a new leanchoiliid, Yawunik kootenayi gen. et sp. nov., based on 42 specimens from the newly discovered Marble Canyon locality of the Burgess Shale (Kootenay National Park, British Columbia; middle Cambrian). This new morphotype demonstrates the presence of a four‐segmented head in leanchoiliids, along with two small antero‐median eyes in addition to lateral eyes. Yawunik is characterized by a 12‐segmented trunk and a carinate, lanceolate telson adorned with minute spines. The ‘great appendages’ of the animal bear teeth on their two distal rami, which would have enhanced their ability to grasp prey. Attitudes of specimens, resulting from burial at multiple aspects of bedding, suggest the ‘great appendages’ were flexible and capable of antero‐posterior rotation. We also discuss the nature of intersegmental tissues and filaments present within the ‘great appendages’. Our phylogenetic analyses extend the monophyly of leanchoiliids to include Haikoucaris and Yohoia in a new clade, the Cheiromorpha nom. nov. (within Heptopodomera nom. nov.). Other nodes are poorly resolved unless implied weights are used, and in this case, the topology is critically sensitive to the coding prerogative of inapplicable states (NAs). Both the traditional ‘Arachnomorpha’ hypothesis (NAs as additional states) and the more recently favoured ‘Artiopoda + Crustacea’ (NAs as uncertainties) were obtained using the same data set and outgroup. This result stresses, first, the historical importance of polarization over data content in scenarios of early arthropod evolution, and second, a pressing need to investigate the impacts of coding inapplicables, especially given the inflating effect of implied weights.     

A New Bivalved Arthropod from the Early Cambrian Sirius Passet Fauna, North Greenland

A new bivalved arthropod is described from the Lower Cambrian (?Upper Atdabanian) Buen Formation of North Greenland. Pauloterminus spinodorsalis gen. et sp. nov. possesses a bivalved carapace that covers the head, which has a single pair of antennae, and anteriormost thorax. No mouthparts are visible. The five‐segmented abdomen was limbless and terminated in a telson plus a pair of large, lobate uropods. A suite of at least six biramous thoracic limbs are present: the short endopods are made up of small, serial podomeres, while the exopods are lobate and may have functioned as gills as well as in swimming. Partially infilled guts are occasionally visible, suggesting that this animal may have been a sediment feeder. It is compared to other Cambrian bivalved arthropods, especially the waptiids Chuandianella ovata from the Lower Cambrian Chengjiang fauna (China) and Waptia fieldensis from the Middle Cambrian Burgess Shale (British Columbia). Of these three animals, the Greenland and Chinese taxa appear to be the most closely related. P. spinodorsalis possesses many typical arthropod features, but it also demonstrates more primitive characters that are more reminiscent of the lobopodians.     

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.     

Gut evolution in early Cambrian trilobites and the origin of predation on infaunal macroinvertebrates: evidence from muscle scars in Mesolenellus

Trilobites are particularly common Cambrian fossils, but their trophic impact on the rapidly evolving marine ecosystems of that time is difficult to assess, due to uncertainties on how diverse their feeding habits truly were. Gut anatomy might help to constrain inferences on trilobite feeding ecology, but preservation of digestive organs is exceedingly rare. Muscle scars on the glabella, known as ‘frontal auxiliary impressions’ (FAIs), have been interpreted as evidence of the evolution of a pouch‐like organ with powerful extrinsic muscles (i.e. a crop) in some trilobites. Here we describe FAIs in Mesolenellus hyperboreus from Cambrian Stage 4 strata of North Greenland, which represents the oldest example of such structures and their first report in the Suborder Olenellina. Mesolenellus FAIs suggest that the crop in trilobites was clearly differentiated from the rest of the digestive tract, and essentially located under a hypertrophied glabellar frontal lobe. Reviews of the digestive anatomy of trilobite sister‐taxa and the glabellar morphology of the oldest‐known trilobites suggest that the gut of the trilobite ancestor was an essentially simple tract (i.e. no well‐differentiated crop) flanked laterally by numerous midgut glands. A crop first evolved in the Cambrian in groups like olenelloids and (later) paradoxidoids. Using ichnological evidence, we hypothesize that the emergence of olenelloids yields evidence for the evolution of predatory inclinations in a group of arthropods originally dominated by surface‐deposit‐feeders. By allowing the exploitation of a rapidly developing food source, infaunal animals, the diversification of feeding strategies in trilobites might partially explain their unparalleled evolutionary success.     

Morphology of Luolishania longicruris (Lower Cambrian,Chengjiang Lagerstätte,SW China) and the phylogenetic relationships within lobopodians

New material of the lobopodian Luolishania longicruris has been recovered from the Lower Cambrian Chengjiang Lagerstätte, southwest China. The specimens throw new light on several morphological features of the species, including the paired antenniform outgrowths, eyes, head shield, setae and other cuticular projections, as well as the differentiated sclerites, appendages, claws, and lobopod interspaces. L. longicruris shows well developed tagmosis: a distinct head and a trunk divided into two sections. The new data allow a revised comparison with other lobopodians. Miraluolishania haikouensis Liu et al., 2004 is considered to be a junior synonym of L. longicruris Hou and Chen, 1989. Evidence from gut filling and specialized morphological characters indicates that L. longicruris may have had a filter feeding lifestyle. A new cladistic analysis suggests that fossil lobopodians are paraphyletic or even polyphyletic and L. longicruris may be an important representative of the stem lineage leading to arthropods.     

Exceptional fossil preservation and the cambrian explosion

Exceptionally preserved, non-biomineralizing fossils contributeimportantly to resolving details of the Cambrian explosion,but little to its overall patterns. Six distinct "types" ofexceptional preservation are identified for the terminal Proterozoic-Cambrianinterval, each of which is dependent on particular taphonomiccircumstances, typically restricted both in space and time.Taphonomic pathways yielding exceptional preservation were particularlyvariable through the Proterozoic-Cambrian transition, at leastin part a consequence of contemporaneous evolutionary innovations.Combined with the reasonably continuous record of "Doushantuo-typepreservation," and the fundamentally more robust records ofshelly fossils, phytoplankton cysts and trace fossils, thesetaphonomic perturbations contribute to the documentation ofmajor evolutionary and biogeochemical shifts through the terminalProterozoic and early Cambrian. Appreciation of the relationship between taphonomic pathwayand fossil expression serves as a useful tool for interpretingexceptionally preserved, often problematic, early Cambrian fossils.In shale facies, for example, flattened non-biomineralizingstructures typically represent the remains of degradation-resistantacellular and extracellular "tissues" such as chaetae and cuticles,whereas three-dimensional preservation represents labile cellulartissues with a propensity for attracting and precipitating earlydiagenetic minerals. Such distinction helps to identify theacuticular integument of hyolithids, the chaetae-like natureof Wiwaxia sclerites, the chaetognath-like integument of Amiskwia,the midgut glands of various Burgess Shale arthropods, and themisidentification of deposit-feeding arthropods in the Chengjiangbiota. By the same reasoning, putative lobopods in the SiriusPasset biota and putative deuterostomes in the Chengiang biotaare better interpreted as arthropods.     

The mouth apparatus of the Cambrian gilled lobopodian Pambdelurion whittingtoni

Omnidens is a large feeding apparatus composed of circlets of teeth, first documented from the early Cambrian of China. Originally interpreted as the oral cone of a radiodontan, it was later reinterpreted as the introvert of a priapulan. In both cases the Omnidens mouthparts underpinned estimates of gigantic (c. 2 m) body size. Recent evidence has been used to suggest that pharyngeal teeth and radially‐arranged oral plates in the stem‐group onychophoran Hallucigenia and the lower stem‐group euarthropod Jianshanopodia are homologous to structures of the introvert in priapulans and other scalidophorans, and are thus primitive characters for moulting animals (Ecdysozoa) as a whole. Here we show that the early Cambrian gilled lobopodian Pambdelurion whittingtoni from Sirius Passet, Greenland, possesses a mouth apparatus identical to Omnidens, being composed of the same three zones with detailed similarities of sclerites in each zone. An oral cone with rings of pharyngeal teeth, radial plates and outer scalid‐like plates are ecdysozoan characters retained in the euarthropod stem group. Omnidens from China probably belongs to an unrecognized Pambdelurion‐like animal rather than being part of a giant priapulan.     

A revision of brain composition in Onychophora (velvet worms) suggests that the tritocerebrum evolved in arthropods

Background  

The composition of the arthropod head is one of the most contentious issues in animal evolution. In particular, controversy surrounds the homology and innervation of segmental cephalic appendages by the brain. Onychophora (velvet worms) play a crucial role in understanding the evolution of the arthropod brain, because they are close relatives of arthropods and have apparently changed little since the Early Cambrian. However, the segmental origins of their brain neuropils and the number of cephalic appendages innervated by the brain - key issues in clarifying brain composition in the last common ancestor of Onychophora and Arthropoda - remain unclear.     

The soft anatomy of Isoxys minor from the Guanshan fauna,lower Cambrian of Southwest China

Isoxys is a very common Cambrian bivalved arthropod, specimens of which are normally preserved only as valves. The discovery of the soft anatomy of Isoxys may greatly assist understanding affinities and functional morphology. Isoxys minor Luo and Hu in Luo et al., 2008 is the most common representative of all animal species known from the lower Cambrian Guanshan fauna (Cambrian Series 2, Stage 4) at the Shitangshan Section, near Kunming, Yunnan Province, Southwest China. Here we describe and reconstruct the morphology of I. minor on the basis of newly illustrated fossils and a few new specimens that bear soft-parts including new discovery of frontal grasping appendages. Like the soft anatomy of other known Isoxys, it bears a pair of large stalked eyes, a pair of specialized frontal grasping appendages, approximately 12–14 paired biramous limbs, and a helm-like tail exposed outside the valves.     

Three-dimensional preservation of arthropod integument from the Middle Cambrian of Australia

Three-dimensional preservation of arthropod soft integument occurs in Middle Cambrian sediments of the Georgina Basin, western Queensland, Australia. The beds are referred to the Monastery Creek Phosphorite Formation, Gowers Formation, Inca Shale Formation and Devon-court Limestone Formation. The finds include arthropod type-A larvae previously described by Müller & Walossek and several complete appendages, possibly of early Palaeozoic 'ostracodes', as well as indeterminable cuticular remains. The Australian sediments were in part deposited under high water-energy conditions, whereas previously known occurrences of three-dimensional soft-integument preservation have been from environments of lower water energy. Such preservation may thus be more widespread in the Early Paleozoic than hitherto known. □ Phosphatization, arthropods, type-A larvae, appendages, Middle Cambrian, Geogina Basin, Australia, three-dimensional preservation.