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Agnathan fish hold a key position in vertebrate evolution, especially regarding the origin of the head and neural-crest-derived tissue. In contrast to amphioxus, lampreys and other vertebrates possess a complex brain and placodes that contribute to well-developed eyes, as well as auditory and olfactory systems. These sensory sytems were arguably a trigger to subsequent vertebrate diversifications. However, although they are known from skeletal impressions in younger Palaeozoic agnathans, information about the earliest records of these systems has been largely wanting. Here we report numerous specimens of the Lower Cambrian vertebrate Haikouichthys ercaicunensis, until now only known from the holotype. Haikouichthys shows significant differences from other fossil agnathans: key features include a small lobate extension to the head, with eyes and possible nasal sacs, as well as what may be otic capsules. A notochord with separate vertebral elements is also identifiable. Phylogenetic analysis indicates that this fish lies within the stem-group craniates. Although Haikouichthys somewhat resembles the ammocoete larva of modern lampreys, this is because of shared general craniate characters; adult lampreys and hagfishes (the cyclostomes if monophyletic) are probably derived in many respects.

Cambrian fossil-Lagerstätten (sites of exceptional fossil preservation), such as those from Chengjiang (Lower Cambrian) and the Burgess Shale (Middle Cambrian), provide our best window into the Cambrian 'explosion'. Such faunas are known from about 40 localities, and have yielded a widely disparate series of taxa ranging from ctenophores to agnathan fish. Recent excavations of the Chengjiang fossil-Lagerstätte, known from a series of sites near Kunming in Yunnan, south China, have resulted in the discovery of several new forms. In conjunction with material described earlier, these provide evidence for a new group of metazoans, the vetulicolians. Several features, notably a series of gill slits, suggest that this group can throw light on an early stage of deuterostome diversification.

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Naraoiids, defined as lightly sclerotized arthropods with a dimidiate tergum of two sclerites separated by a single transverse articulation, have been found in the Cambrian and Silurian. During the Cambrian they had a wide distribution coinciding with trilobite realms. This pattern may be related to the breakup of a Neoproterozoic supercontinent, probably Pannotia, which implies that naraoiids originated before the Cambrian “explosion.” Based on new observations on the original material from the Burgess Shale (Middle Cambrian, British Columbia),Naraoia haliais reconsidered as a valid species. The validity is further confirmed by a new record of the occurrence of this species in the Chengjiang Lagerstätte (Lower Cambrian, China). In addition, some structures ofN. compactaof the Burgess Shale have been reinterpreted. Two more naraoiid species are redescribed in detail from the Chengjiang Lagerstätte on the basis of more than 1,000 well-preserved specimens.Naraoia spinosashows dimorphism andMisszhouia longicaudataexhibits geographical variation in the overall shape of the dorsal exoskeleton. Naraoiids may have a protaspis-like larva, but the previously assigned protaspis has proven to be a separate taxon,Primicaris. In dorsal view, naraoiids resemble a giant “degree 0” meraspis (i.e., without thorax), and could have originated from different heterochronic processes, neoteny or hypermorphosis. Naraoiids are generally accepted as vagrant benthos. A predatory/scavenging life mode is supported by functional morphology and recent analogues. A healed injury inM. longicaudatasuggests that they could be the prey of larger predators, most likely anomalocaridids. We suggest that differences in exopod composition might represent evolutionary changes through the Early–Middle Cambrian. The monophyly of the Naraoiidae is not firmly established. Similarity to liwiids, supposed to be the close relatives of naraoiids, is limited to overall shape. We exclude naraoiids from the Trilobita, though there do exist a number of similarities between them.

BackgroundExtant cubozoans are voracious predators characterized by their square shape, four evenly spaced outstretched tentacles and well-developed eyes. A few cubozoan fossils are known from the Middle Cambrian Marjum Formation of Utah and the well-known Carboniferous Mazon Creek Formation of Illinois. Undisputed cubozoan fossils were previously unknown from the early Cambrian; by that time probably all representatives of the living marine phyla, especially those of basal animals, should have evolved.MethodsMicroscopic fossils were recovered from a phosphatic limestone in the Lower Cambrian Kuanchuanpu Formation of South China using traditional acetic-acid maceration. Seven of the pre-hatched pentamerous cubozoan embryos, each of which bears five pairs of subumbrellar tentacle buds, were analyzed in detail through computed microtomography (Micro-CT) and scanning electron microscopy (SEM) without coating.ResultsThe figured microscopic fossils are unequivocal pre-hatching embryos based on their spherical fertilization envelope and the enclosed soft-tissue that has preserved key anatomical features arranged in perfect pentaradial symmetry, allowing detailed comparison with modern cnidarians, especially medusozoans. A combination of features, such as the claustrum, gonad-lamella, suspensorium and velarium suspended by the frenula, occur exclusively in the gastrovascular system of extant cubozoans, indicating a cubozoan affinity for these fossils. Additionally, the interior anatomy of these embryonic cubozoan fossils unprecedentedly exhibits the development of many new septum-derived lamellae and well-partitioned gastric pockets unknown in living cubozoans, implying that ancestral cubozoans had already evolved highly specialized structures displaying unexpected complexity at the dawn of the Cambrian. The well-developed endodermic lamellae and gastric pockets developed in the late embryonic stages of these cubozoan fossils are comparable with extant pelagic juvenile cubomedusae rather than sessile cubopolyps, whcih indicates a direct development in these fossil taxa, lacking characteristic stages of a typical cnidarian metagenesis such as planktonic planula and sessile polyps.

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