Xianguang Hou

Mechanism for Burgess Shale-type preservation

Exceptionally preserved fossil biotas of the Burgess Shale and a handful of other similar Cambrian deposits provide rare but critical insights into the early diversification of animals. The extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft-bodied fossil assemblages has remained enigmatic since Walcott’s initial discovery in 1909. Here, we demonstrate the mechanism of Burgess Shale-type preservation using sedimentologic and geochemical data from the Chengjiang, Burgess Shale, and five other principal Burgess Shale-type deposits. Sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation. Low sulfate concentrations in the global ocean and low-oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability. Subsequently, rapid entombment of fossils in fine-grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments. A permeability barrier, provided by bed-capping cements that were emplaced at the seafloor, is a feature that is shared among Burgess Shale-type deposits, and resulted from the unusually high alkalinity of Cambrian oceans. Thus, Burgess Shale-type preservation of soft-bodied fossil assemblages worldwide was promoted by unique aspects of early Paleozoic seawater chemistry that strongly impacted sediment diagenesis, providing a fundamentally unique record of the immediate aftermath of the “Cambrian explosion.”

Complex brain and optic lobes in an early Cambrian arthropod

The nervous system provides a fundamental source of data for understanding the evolutionary relationships between major arthropod groups. Fossil arthropods rarely preserve neural tissue. As a result, inferring sensory and motor attributes of Cambrian taxa has been limited to interpreting external features, such as compound eyes or sensilla decorating appendages, and early-diverging arthropods have scarcely been analysed in the context of nervous system evolution. Here we report exceptional preservation of the brain and optic lobes of a stem-group arthropod from 520 million years ago (Myr ago), Fuxianhuia protensa, exhibiting the most compelling neuroanatomy known from the Cambrian. The protocerebrum of Fuxianhuia is supplied by optic lobes evidencing traces of three nested optic centres serving forward-viewing eyes. Nerves from uniramous antennae define the deutocerebrum, and a stout pair of more caudal nerves indicates a contiguous tritocerebral component. Fuxianhuia shares a tripartite pre-stomodeal brain and nested optic neuropils with extant Malacostraca and Insecta, demonstrating that these characters were present in some of the earliest derived arthropods. The brain of Fuxianhuia impacts molecular analyses that advocate either a branchiopod-like ancestor of Hexapoda or remipedes and possibly cephalocarids as sister groups of Hexapoda. Resolving arguments about whether the simple brain of a branchiopod approximates an ancestral insect brain or whether it is the result of secondary simplification has until now been hindered by lack of fossil evidence. The complex brain of Fuxianhuia accords with cladistic analyses on the basis of neural characters, suggesting that Branchiopoda derive from a malacostracan-like ancestor but underwent evolutionary reduction and character reversal of brain centres that are common to hexapods and malacostracans. The early origin of sophisticated brains provides a probable driver for versatile visual behaviours, a view that accords with compound eyes from the early Cambrian that were, in size and resolution, equal to those of modern insects and malacostracans.

Chelicerate neural ground pattern in a Cambrian great appendage arthropod

Preservation of neural tissue in early Cambrian arthropods has recently been demonstrated, to a degree that segmental structures of the head can be associated with individual brain neuromeres. This association provides novel data for addressing long-standing controversies about the segmental identities of specialized head appendages in fossil taxa. Here we document neuroanatomy in the head and trunk of a ‘great appendage’ arthropod, Alalcomenaeus sp., from the Chengjiang biota, southwest China, providing the most complete neuroanatomical profile known from a Cambrian animal. Micro-computed tomography reveals a configuration of one optic neuropil separate from a protocerebrum contiguous with four head ganglia, succeeded by eight contiguous ganglia in an eleven-segment trunk. Arrangements of optic neuropils, the brain and ganglia correspond most closely to the nervous system of Chelicerata of all extant arthropods, supporting the assignment of ‘great appendage’ arthropods to the chelicerate total group. The position of the deutocerebral neuromere aligns with the insertion of the great appendage, indicating its deutocerebral innervation and corroborating a homology between the ‘great appendage’ and chelicera indicated by morphological similarities. Alalcomenaeus and Fuxianhuia protensa demonstrate that the two main configurations of the brain observed in modern arthropods, those of Chelicerata and Mandibulata, respectively, had evolved by the early Cambrian.

Brain structure resolves the segmental affinity of anomalocaridid appendages

Despite being among the most celebrated taxa from Cambrian biotas, anomalocaridids (order Radiodonta) have provoked intense debate about their affinities within the moulting-animal clade that includes Arthropoda. Current alternatives identify anomalocaridids as either stem-group euarthropods, crown-group euarthropods near the ancestry of chelicerates, or a segmented ecdysozoan lineage with convergent similarity to arthropods in appendage construction. Determining unambiguous affinities has been impeded by uncertainties about the segmental affiliation of anomalocaridid frontal appendages. These structures are variably homologized with jointed appendages of the second (deutocerebral) head segment, including antennae and ‘great appendages’ of Cambrian arthropods, or with the paired antenniform frontal appendages of living Onychophora and some Cambrian lobopodians. Here we describe Lyrarapax unguispinus, a new anomalocaridid from the early Cambrian Chengjiang biota, southwest China, nearly complete specimens of which preserve traces of muscles, digestive tract and brain. The traces of brain provide the first direct evidence for the segmental composition of the anomalocaridid head and its appendicular organization. Carbon-rich areas in the head resolve paired pre-protocerebral ganglia at the origin of paired frontal appendages. The ganglia connect to areas indicative of a bilateral pre-oral brain that receives projections from the eyestalk neuropils and compound retina. The dorsal, segmented brain of L. unguispinus reinforces an alliance between anomalocaridids and arthropods rather than cycloneuralians. Correspondences in brain organization between anomalocaridids and Onychophora resolve pre-protocerebral ganglia, associated with pre-ocular frontal appendages, as characters of the last common ancestor of euarthropods and onychophorans. A position of Radiodonta on the euarthropod stem-lineage implies the transformation of frontal appendages to another structure in crown-group euarthropods, with gene expression and neuroanatomy providing strong evidence that the paired, pre-oral labrum is the remnant of paired frontal appendages.

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.

A New View of the Cambrian Arthropod Fuxianhuia

Abstract Fuxianhuia from the Chengjiang fauna is one of the least derived Cambrian arthropods. A supposed pair of hook-shaped post-antennal appendages (Chen et al. 1995) was later reinterpreted as a pair of gut diverticula situated in a carapace fold (Waloszek et al. 2005). The latter interpretation of the diverticular nature of this structure is verified, but we show that the diverticula are situated inside a head capsule. This capsule carries a pair of antennae. It is shown that the head has also a pair of ordinary locomotory limbs. It is also demonstrated that a number of body segments have more than one pair of legs. There is indication of a possible pair of small eyes in addition to the large pair. The gut may include sediment from the substrate, which indicates that sediment was ingested. Fuxianhuiids share with many lobopodians characters such as a lack of specialisation between and within the ventral appendages. The exopod rises as a lateral fold on a straight limb axis, indicating an origin as a secondary addition to a uniramous limb. The terms Arthropoda s.l. (or pan-Arthropoda) and Arthropoda s.s. are replaced by Aiolopoda Hou & Bergström, 2006, and Arthropoda (in the original sense). The origination of the arthropods is discussed.

Chengjiang arthropod Leanchoilia illecebrosa (Hou, 1987) reconsidered

Abstract The common Chengjiang arthropod Leanchoilia illecebrosa (Hou, 1987) has been restudied on the basis of more than 500 new specimens. Important news concern the eyes: (1) There is a pair of kidney-shaped compound eyes, or possibly two confluent eyes; (2) optical lobes appear to reach the eyes, leaving no space for any eye stalks. Important news on the ventral appendages include the presence of: (3) an ample, annulated body-basipod articulation membrane with two sclerotized folds; (4) a prominent basipod with a spinous medial edge; (5) an articulation between the two rami extending to the middle of the 3rd endopod podomere; (6) up to nine endopod podomeres; (7) a long spine forming the limb tip; and (8) an exopod composed of a sub-triangular proximal portion and a leaf-shaped distal portion, both with marginal setae. New reconstructions are presented. There is a notable intraspecific variability. We conclude that none of the additional proposed species of Chengjiang Leanchoilia can be positively identified as separate from L. illecebrosa (Hou, 1987). In the possession of pointed anterior and posterior body ends, all species of Leanchoilia differ from Alalcomenaeus Simonetta, 1970.

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.

The lobopodian Paucipodia inermis from the Lower Cambrian Chengjiang fauna, Yunnan, China

New specimens of Paucipodia inermis Chen, Zhou & Ramskold, 1995, are described from the Lower Cambrian Chengjiang Lagerstatte in Haikou, Kunming. Details not previously seen in the Chengjiang material appear to be caused by early diagenetic processes. Some features not previously observed in Palaeozoic lobopodians include details of the dermomuscular sac, body cavities, contents of the gut, possible paired ventral nerve ganglia, and a rasping or biting apparatus with teeth. The latter implies a fundamental difference from onychophorans and rules out an ancestral position for Palaeozoic lobopodians. The supposed tail is shown to be the head, and it is shown that this animal possessed nine pairs of lobopods rather than six, as originally stated. The family Paucipodiidae n. fam. is introduced.

Cambrian anemones with preserved soft tissue from the Chengjiang biota, China

Hou, X.-G., Stanley Jr, G.D., Zhao, J. & Ma, X.-Y. 2005 09 12: Cambrian anemones withpreserved soft tissue from the Chengjiang biota, China. Lethaia, Vol. 38, pp. 193–203.Oslo. ISSN 0024-1164.The group Cnidaria includes ‘jellyfish’, soft-bodied anemone and anemone-like formsand calcified corals. These diploblastic organisms have a fossil record extending back tothe earliest metazoans of the Neoproterozoic; however certain cnidarians of the subclassZoantharia, characterized by soft-bodied anemone-like forms, are absent or poorlyrepresented in the fossil record. Despite the paucity of fossils, it is thought that calcifi-cation by soft anemone-like animals was responsible for producing the skeleton thatallowed the preservation of the first corals. We report discovery of an abundant assem-blage of in situ soft-bodied polyps with tissues. They are preserved in exquisite detail andcome from the well-known Lower Cambrian Chengjiang biota of Yunnan, China. Thesoft-bodied polyps display a simple anatomy that is comparable to some extant anemonesof the order Actinaria. The new fossils are assigned to Archisaccophyllia kunmingensis n.gen. et n. sp. Their simple and conservative form suggests that these fossils may representsome kind of ancestral rootstock. The preserved life assemblage provides a uniquesnapshot of Lower Cambrian anemone life and provides clues for relationships withextant actiniarians as well as calcified corals. & Actiniaria, anemone, Chengjiang biota,China, Lower Cambrian, soft-bodied fossils.Xian-guang Hou (corresponding author) [xghou@ynu.edu.cn], Jie Zhao [biofeature@hotmail.com] and Xiao-ya Ma [littleya@vip.sina.com], Yunnan Key Laboratory forPalaeobiology, Yunnan University, Kunming 650091, China; George D. Stanley, Jr.[george.stanley@umontana.edu], Department of Geology, The University of Montana,Missoula, MT 59812, USA; 19th August 2004, revised 11th April 2005.