Phanerozoic-cryptozoic and related transitions: new evidence

The fossil Pteridinium, a distinctive component of a worldwide early metazoan (Ediacaran)assemblage, is provisionally recorded from probable early Cambrian strata in eastern California. In context with other evidence, this finding implies a Cambrian age for the Ediacaran fauna and approximate coincidence of limits between Phanerozoic-Cryptozoic, Paleozoic-Precambrian, and Cambrian-Precambrian.     

The Cambrian conundrum: early divergence and later ecological success in the early history of animals

Diverse bilaterian clades emerged apparently within a few million years during the early Cambrian, and various environmental, developmental, and ecological causes have been proposed to explain this abrupt appearance. A compilation of the patterns of fossil and molecular diversification, comparative developmental data, and information on ecological feeding strategies indicate that the major animal clades diverged many tens of millions of years before their first appearance in the fossil record, demonstrating a macroevolutionary lag between the establishment of their developmental toolkits during the Cryogenian [(850 to 635 million years ago (Ma)], and the later ecological success of metazoans during the Ediacaran (635 to 541 Ma) and Cambrian (541 to 488 Ma) periods. We argue that this diversification involved new forms of developmental regulation, as well as innovations in networks of ecological interaction within the context of permissive environmental circumstances.     

Burgess shale faunas and the cambrian explosion

Soft-bodied marine faunas from the Lower and Middle Cambrian, exemplified by the Burgess Shale of British Columbia, are a key component in understanding the major adaptive radiations at the beginning of the Phanerozoic ("Cambrian explosion"). These faunas have a widespread distribution, and many taxa have pronounced longevity. Among the components appear to be survivors of the preceding Ediacaran assemblages and a suite of bizarre forms that give unexpected insights into morphological diversification. Microevolutionary processes, however, seem adequate to account for this radiation, and the macroevolutionary patterns that set the seal on Phanerozoic life are contingent on random extinctions. They weeded out the morphological spectrum and permitted rediversification among surviving clades. Although the predictability of which clades will play in successive acts of the Phanerozoic theater is low, at least the outlines of the underlying ecological plot are already clear from the opening of the drama.     

Ediacaran (precambrian) fossils from the wernecke mountains, northwestern Canada

Fossil medusoids identified as Cyclomedusa davidi?, Beltanelliformis brunsae, and cf. Sekwia excentrica are reported from Late Precambrian strata in the Wernecke Mountains. They are representatives of the Ediacaran fauna, the oldest assemblage of cosmopolitan metazoans, and are only the third such occurrence in Canada. In addition, specimens broadly resembling the problematic structure Rugoinfractus ovruchensis, previously known only from the Precambrian of the Ukraine, are reported from the Lower Cambrian of the nearby Mackenzie Mountains.     

Early cambrian ostracode larvae with a univalved carapace

Phosphatized univalves, recovered from the Lower Cambrian ( approximately 530 million years old) Qiongzhusi Formation in southern China, are recognized as early instars belonging to bradoriid ostracodes whose later instars are bivalved. The univalved form is the primitive larval character for shell-secreting crustaceans, although most post-Cambrian ostracodes bypassed this developmental phase. The univalved-bivalved transition during early on-togeny represents an important evolutionary event in ostracodes, with implications for crustacean classification, and implies that the ostracode ancestor achieved this bivalved capacity before the appearance of mineralized skeletons during the "Cambrian explosion."     

The oldest macroborers: lower cambrian of labrador

We have discovered numerous borings of Trypanites penetrating skeletons and synsedimentary cemented limestones in archaeocyathid reefs of the Forteau formation in southern Labrador. These are, to date, the oldest known macroborings. The discovery of these structures extends the record of large endolithic organisms 100 million years from the Lower Ordovician to the Lower Cambrian. This immediately postdates the appearance of metazoans with hard parts and confirms that endoliths have played a role in reef formation since the early Cambrian.     

Bilaterian burrows and grazing behavior at >585 million years ago

Based on molecular clocks and biomarker studies, it is possible that bilaterian life emerged early in the Ediacaran, but at present, no fossils or trace fossils from this time have been reported. Here we report the discovery of the oldest bilaterian burrows in shallow-water glaciomarine sediments from the Tacuarí Formation, Uruguay. Uranium-lead dating of zircons in cross-cutting granite dykes constrains the age of these burrows to be at least 585 million years old. Their features indicate infaunal grazing activity by early eumetazoans. Active backfill within the burrow, an ability to wander upward and downward to exploit shallowly situated sedimentary laminae, and sinuous meandering suggest advanced behavioral adaptations. These findings unite the paleontological and molecular data pertaining to the evolution of bilaterians, and link bilaterian origins to the environmental changes that took place during the Neoproterozoic glaciations.     

Collective behavior in an early Cambrian arthropod

Examples that indicate collective behavior in the fossil record are rare. A group association of specimens that belong to a previously unknown arthropod from the Chengjiang Lagerst?tte, China, provides evidence that such behavior was present in the early Cambrian (about 525 million years ago), coincident with the earliest extensive diversification of the Metazoa, the so-called Cambrian explosion event. The chainlike form of these specimens is unique for any arthropod, fossil or living, and most likely represents behavior associated with migration.     

Fossilized nuclei and germination structures identify Ediacaran "animal embryos" as encysting protists

Globular fossils showing palintomic cell cleavage in the Ediacaran Doushantuo Formation, China, are widely regarded as embryos of early metazoans, although metazoan synapomorphies, tissue differentiation, and associated juveniles or adults are lacking. We demonstrate using synchrotron-based x-ray tomographic microscopy that the fossils have features incompatible with multicellular metazoan embryos. The developmental pattern is comparable with nonmetazoan holozoans, including germination stages that preclude postcleavage embryology characteristic of metazoans. We conclude that these fossils are neither animals nor embryos. They belong outside crown-group Metazoa, within total-group Holozoa (the sister clade to Fungi that includes Metazoa, Choanoflagellata, and Mesomycetozoea) or perhaps on even more distant branches in the eukaryote tree. They represent an evolutionary grade in which palintomic cleavage served the function of producing propagules for dispersion.     

Halwaxiids and the early evolution of the lophotrochozoans

Halkieriids and wiwaxiids are cosmopolitan sclerite-bearing metazoans from the Lower and Middle Cambrian. Although they have similar scleritomes, their phylogenetic position is contested. A new scleritomous fossil from the Burgess Shale has the prominent anterior shell of the halkieriids but also bears wiwaxiid-like sclerites. This new fossil defines the monophyletic halwaxiids and indicates that they have a key place in early lophotrochozoan history.     

Morphological disparity in the cambrian

An analysis of the range of morphology among arthropods demonstrates that disparity among living arthropods is similar to that in Cambrian arthropods. The range of morphological design resulting from the Cambrian "explosion" has been overestimated, reflecting a tendency to separate as "problematic" taxa that cannot be accommodated in the classification on the basis of the living biota. Problematic taxa are largely an artifact of an inadequate taxonomy. Special evolutionary processes may not be necessary to explain the early radiation of the metazoans.     

The early radiation and relationships of the major arthropod groups

Cambrian arthropods are now well known, but there has been little agreement on how they contribute to an understanding of arthropod phylogeny. Fossils have either been lumped together as "trilobitomorphs" or, more recently, have been the subject of speculation invoking a multiple polyphyletic origin of arthropods. Cladistic analysis of characters of Cambrian and living representatives (excluding Uniramia) shows that trilobites and chelicerates are relatively advanced compared with "crustaceans," and there are doubts whether the latter constitute a national group. An undue emphasis on singular autapomorphies of problematic fossils has obscured these relationships in the past. "Trilobitomorphs" were simply an artificial taxon based on shared primitive characters. The arthropods that evolved during the Cambrian radiation show no more apparent morphological diversity than do the living groups. The evidence of wellpreserved problematica is critical to understanding the nature of this radiation and the affinities of the groups that remain today.     

Small bilaterian fossils from 40 to 55 million years before the cambrian

Ten phosphatized specimens of a small (<180 micrometers) animal displaying clear bilaterian features have been recovered from the Doushantuo Formation, China, dating from 40 to 55 million years before the Cambrian. Seen in sections, this animal (Vernanimalcula guizhouena gen. et sp. nov.) had paired coeloms extending the length of the gut; paired external pits that could be sense organs; bilateral, anterior-posterior organization; a ventrally directed anterior mouth with thick walled pharynx; and a triploblastic structure. The structural complexity is that of an adult rather than a larval form. These fossils provide the first evidence confirming the phylogenetic inference that Bilateria arose well before the Cambrian.     

Evidence for monophyly and arthropod affinity of cambrian giant predators

The Chinese Early Cambrian Chengjiang fauna includes three different anomalocaridids, a globally spread, extinct marine group including the largest known Cambrian animals. Anomalocaridids were active predators, and their presence implies that a complex ecosystem appeared abruptly in the earliest Phanerozoic. Complete specimens display several sets of characters shared only with some other exclusively Cambrian forms. This evidence indicates that anomalocaridids, Opabinia, and Kerygmachela form a monophyletic clade. Certain features indicate arthropod affinities of the lade, and for this group an unnamed (sub)phylum-level taxon within an arthropod (super)phylum is proposed.     

A fish from the upper cambrian of north america

Phosphatic dermal fragments of Anatolepis, interpreted as a heterostracan fish (class Agnatha), have been discovered in the Deadwood Formation of Late Cambrian age in northeastern Wyoming. This discovery extends back the age of the earliest known vertebrate fossils by approximately 40 million years. Other occurrences of Anatolepis in North America, Greenland, and Spitzbergen show that these fish had a widespread geographic distribution in Late Cambrian and Early Ordovician marine environments.     

Seawater chemistry and early carbonate biomineralization

The first appearances of aragonite and calcite skeletons in 18 animal clades that independently evolved mineralization during the late Ediacaran through the Ordovician (approximately 550 to 444 million years ago) correspond to intervals when seawater chemistry favored aragonite and calcite precipitation, respectively. Skeletal mineralogies rarely changed once skeletons evolved, despite subsequent changes in seawater chemistry. Thus, the selection of carbonate skeletal minerals appears to have been dictated by seawater chemistry at the time a clade first acquired its mineralized skeleton.     

Discoidal impressions and trace-like fossils more than 1200 million years old

The Stirling Range Formation of southwestern Australia contains discoidal impressions and trace-like fossils in tidal sandstones. The various disks have previously been linked to the Ediacaran biota, younger than 600 million years old. From this unit, we report U-Th-Pb geochronology of detrital zircon and monazite, as well as low-grade metamorphic monazite, constraining the depositional age to between 2016 +/- 6 and 1215 +/- 20 million years old. Although nonbiological origins for the discoidal impressions cannot be completely discounted, the structures resembling trace fossils clearly have a biological origin and suggest the presence of vermiform, mucus-producing, motile organisms.     

A Cambrian peak in morphological variation within trilobite species

Morphological variation within species is a raw material subject to natural selection. However, temporal change in morphological diversity has usually been studied in terms of variation among rather than within species. The distribution of polymorphic traits in cladistic character-taxon matrices reveals that the frequency and extent of morphological variation in 982 trilobite species are greatest early in the evolution of the group: Stratigraphically old and/or phylogenetically basal taxa are significantly more variable than younger and/or more derived taxa. Through its influence on evolutionary tempo, high intraspecific variation may have played a major role in the pronounced Cambrian diversification of trilobites.     

Authigenic potassium feldspar in cambrian carbonates: evidence of alleghanian brine migration

The shallow-water limestones and dolostones of the Conococheague Limestone (Upper Cambrian) of western Maryland contain large amounts of authigenic potassium feldspar. The presence of halite daughter crystals in breached fluid inclusions, low whole-rock ratios of chlorine to bromine, and thermochemical data suggest that the potassium feldspar formed at low temperature by the reaction of connate brines with intercalated siliciclastic debris. Analyses of argon age spectra indicate that the authigenic feldspar probably formed during Late Pennsylvanian to Early Permian time. These results may indicate mobilization and migration of connate brines brought about by Alleghanian folding. The widespread occurrence of authigenic potassium feldspar in Cambrian and Ordovician carbonate rocks throughout the Appalachians suggests that this may have occurred throughout the entire basin.     

Confirmation of the Carolina slate belt as an exotic terrane

An assemblage of Middle Cambrian Atlantic faunal province trilobites has been found in the rocks of the Carolina slate belt near Batesburg, South Carolina. Geologic and paleomagnetic data suggest that the Carolina slate belt and the adjacent Charlotte belt constitute an exotic terrane that was accreted to North America in early to middle Paleozoic time.