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A remarkable new fossil horseshoe crab, Lunataspis aurora gen. et sp. nov., from recently discovered Upper Ordovician (c. 445 Ma) shallow marine Konservat-Lagerstät-ten deposits in Manitoba (Canada), is characterized by fusion of opisthosomal tergites into two sclerites. A broad mesosoma of six or seven fused segments, followed by a narrow metasoma of three reduced segments, represents an advanced transitional condition in the development of the xiphosurid thoracetron. Lunataspis further possesses a large crescentic prosomal shield bearing lateral compound eyes on weak ophthalmic ridges that flank a low cardiac lobe, and a keeled lanceolate telson. Lunataspis is much older than the proposed 'synziphosurine' stem lineage of Carboniferous and post-Palaeozoic Xiphosurida, yet is strikingly similar to crown group limuline horseshoe crabs, indicating that major features of the distinctive and highly conserved xiphosurid Bauplan evolved considerably earlier in the Palaeozoic than was previously suspected. In addition to establishing a new temporal benchmark for assessing hypotheses of early chelicerate relationships, the discovery of horseshoe crabs in a Late Ordovician marginal marine setting marks the earliest definitive record of this persistent ecological association.

MOTS CLÉSChordata, Craniata, animal-conodonte, codage des caractères, analyse cladistique, paléohistologie. KEY WORDSChordata, Craniata, conodont animals/ elements, character coding, cladistic analysis, palaeo-histology. ABSTRACTAn evidence-based reassessment of the phylogenetic relationships of conodonts shows that they are not "stem" gnathostomes, nor vertebrates, and not even craniates. A signifi cant group of conodont workers have proposed or accepted a craniate designation for the conodont animal, an interpretation that is increasingly becoming established as accepted "fact". Against this prevailing trend, our conclusion is based on a revised analysis of traditional morphological features of both discrete conodont elements and apparatuses, histological investigation and a revised cladistic analysis modifying that used in the keystone publication promoted as proof of the hypothesis that conodonts are vertebrates. Our study suggests that conodonts possibly were not even chordates but demonstration of this is beyond the scope of this paper. To summarize, in conodonts there is low cephalization; presence of simple V-shaped trunk musculature and unique large-crystal albid material in the elements; lack of a dermal skeleton including characteristic vertebrate hard tissues of bone, dentine and enamel; lack of odontodes with bone of attachment and a unique pulp system; lack of segmentally-arranged paraxial elements and dermal elements in median fi ns, all of which supports neither a vertebrate nor a craniate relationship for conodonts. RÉSUMÉ Des pseudo-dents : une réévaluation des relations phylogénétiques des conodontes et des vertébrés.Une réévaluation des relations phylogénétiques des conodontes est fondée sur de nouvelles preuves. Elle montre que les conodontes ne sont ni des gnathostomes-souches, ni des vertébrés, ni même des crâniates. Un groupe signifi catif de spécialistes des conodontes a proposé, ou accepté, que ces organismes soient considérés comme des crâniates, une interprétation qui est en train de s'installer en tant que fait avéré. Notre conclusion va à l'encontre de cette tendance ; elle est fondée sur une révision des traits morphologiques traditionnels à la fois des éléments isolés et des assemblages de conodontes, sur les données histologiques et sur une analyse cladistique révisée, ce qui modifi e les conclusions de la publication principale qui a promu l'hypothèse selon laquelle les conodontes seraient des vertébrés. Notre étude suggère même que les conodontes n'aient pas été des chordés, mais la démonstration de cette hypothèse va au-delà de l'objectif de cet article. En résumé, chez les conodontes, le degré de céphalisation est faible ; la musculature du tronc a une forme simple en V ; les éléments isolés montrent un tissu blanc avec des cristaux de grande taille, uniques pour ce tissu ; il n'y a pas de squelette dermique incluant les tissus durs caractéristiques des vertébrés tels que l'os, la dentine et l'émail ; il n'y a pas d'odontodes avec leur os et leur système pulpaire unique ; il ...

Trilobites of the Callavia Zone s.1. occurring together with phosphatic fossils in the Little Hollow Formation are the first known Early Cambrian faunas from mainland Nova Scotia. The tectono-stratigraphic affinity of the northern Antigonish Highlands with less intensely deformed portions of the Avalonian Platform is indicated by: (1) the Avalonian character of the trilobites and Early Ordovician brachiopods, (2) the nature of the lithologic sequence of the Iron Brook Group, and (3) local evidence for a Hadrynian orogeny.Rhombocorniculum cancellatum (Cobbold) is here informally designated a "pseudoconodont" based on its unique histology and is regarded as a conodontophorid-like organism. Eccentrotheca kanesia n. gen., n. sp. is a multiplated organism with apparent affinities to groups regarded as mitrosagophorans. Sunnaginia imbricata Missarzhevsky and possibly Turcutheca are demonstrated to occur in strata younger than Tommotian (lowest Lower Cambrian).

In the past an ‘explosion’ in diversity and abundance of small shelly fossils and of trace fossils has served to mark the base of the Cambrian. However, no evidence has been presented to prove that the ‘explosions’ of the two groups were synchronous. We describe small shelly fossils and trace fossils from the same phosphatic limestone beds that indicate that the two events were separate in time. The small shelly fossils are Anabarites trisulcatus, Hyolithellus cf. H. isiticus, Microcornus? sp., Protohertzina anabarica, P. unguliformis, P. sp. A, Pseudorthotheca sp. A, Rushtonia? sp. A, four types of tuberculate plates and one type of reticulate plate. These fossils represent a restricted, ‘pre‐explosion’ fauna and are assigned to the Anabarites‐Circotheca‐Protohertzina Assemblage Zone, an uppermost Precambrian zone in the Meishucun Stage, Yunnan Province, China. A point at the top of this zone has received strong international endorsement for future designation as the base of the Cambrian. Associated with the small shelly fossils are the trace fossils Cruziana sp. A, Cruziana? sp. B, Rusophycus sp. A, Palaeophycus rubdark and arthropod scratch marks. If found in isolation, this trace fossil assemblage would be considered as post‐Precambrian because it includes large, highly organized arthropod traces that are traditionally accepted as occurring above the trace fossil ‘explosion’. We therefore conclude that the trace fossil ‘explosion’ predates the small shelly fossil ‘explosion’. If the proposed location of the base of the Cambrian in Yunnan is accepted, the small shelly fossil ‘explosion’ concept and its relationship to the boundary would not be greatly modified. The trace fossil ‘explosion’, however, would no longer indicate the base of the Cambrian and the ranges of some trace fossils would be extended into the Precambrian.