Sören Jensen

The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution

The earliest evolution of the animals remains a taxing biological problem, as all extant clades are highly derived and the fossil record is not usually considered to be helpful. The rise of the bilaterian animals recorded in the fossil record, commonly known as the ‘Cambrian explosion’, is one of the most significant moments in evolutionary history, and was an event that transformed first marine and then terrestrial environments. We review the phylogeny of early animals and other opisthokonts, and the affinities of the earliest large complex fossils, the so‐called ‘Ediacaran’ taxa. We conclude, based on a variety of lines of evidence, that their affinities most likely lie in various stem groups to large metazoan groupings; a new grouping, the Apoikozoa, is erected to encompass Metazoa and Choanoflagellata. The earliest reasonable fossil evidence for total‐group bilaterians comes from undisputed complex trace fossils that are younger than about 560 Ma, and these diversify greatly as the Ediacaran–Cambrian boundary is crossed a few million years later. It is generally considered that as the bilaterians diversified after this time, their burrowing behaviour destroyed the cyanobacterial mat‐dominated substrates that the enigmatic Ediacaran taxa were associated with, the so‐called ‘Cambrian substrate revolution’, leading to the loss of almost all Ediacara‐aspect diversity in the Cambrian. Why, though, did the energetically expensive and functionally complex burrowing mode of life so typical of later bilaterians arise? Here we propose a much more positive relationship between late‐Ediacaran ecologies and the rise of the bilaterians, with the largely static Ediacaran taxa acting as points of concentration of organic matter both above and below the sediment surface. The breaking of the uniformity of organic carbon availability would have signalled a decisive shift away from the essentially static and monotonous earlier Ediacaran world into the dynamic and burrowing world of the Cambrian. The Ediacaran biota thus played an enabling role in bilaterian evolution similar to that proposed for the Savannah environment for human evolution and bipedality. Rather than being obliterated by the rise of the bilaterians, the subtle remnants of Ediacara‐style taxa within the Cambrian suggest that they remained significant components of Phanerozoic communities, even though at some point their enabling role for bilaterian evolution was presumably taken over by bilaterians or other metazoans. Bilaterian evolution was thus an essentially benthic event that only later impacted the planktonic environment and the style of organic export to the sea floor.

A complex trace fossil from the Spitskop Member (terminal Ediacaran–? Lower Cambrian) of southern Namibia

Streptichnus narbonnei igen. et isp. nov., a new trace fossil from the upper part of the Spitskop Member of the Urusis Formation, southern Namibia, consists of clusters of unidirectionally curved radial elements, in which individual elements typically are composed of imbricated sickle-shaped segments somewhat comparable to those of Treptichnus pedum . Such complex trace fossils generally are found only in Cambrian or younger strata. This opens to question the position of the Ediacaran–Cambrian boundary in the Nama Group, suggesting that it may locally be within the uppermost part of the Urusis Formation, rather than at the base of the Nomtsas Formation.

Vendian-Cambrian subsidence of the passive margin of western Baltica - application of new stratigraphic data from the Scandinavian Caledonian margin

Stratigraphic information from the Neoproterozoic to Cambrian cover sequences at the Caledonian margin in central Scandinavia has been compiled from the literature and our own data. Four flooding events can be recognized in the autochthonous cover rocks and parts of the eastern Caledonian Lower Allochthon: one, at the base of the Vendian at 590 Ma, two Early Cambrian events (540 and 530 Ma), and the fourth at the base of the Mid Cambrian and the alum shales (518 Ma). Stratigraphic successions of the western Baltica margin from northern Sweden to southern Norway are correlated using these flooding events. Based on these correlations, depth and time sections are constructed and subsidence curves calculated. Although Early Cambrian flooding events lead to temporarily higher sedimentation rates, the subsidence appears to have decreased through time. Such a decrease is consistent with models of lithospheric stretching and subsequent thermal subsidence. A review of available age data on tectonic events suggests...

Development of early Palaeozoic ichnofabrics: evidence from shallow marine siliciclastics

Abstract Earliest Cambrian fine-grained sediments appear to have been firm close to the sediment-water interface. As a result there was a high preservational potential of shallow tiers. There is limited evidence for a mixed layer at this time; rather, most of the preserved trace fossils were open burrows. Later in the Cambrian, depth of sediment mixing increased but firmground conditions are still found relatively close to the sediment-water interface. The development of the mixed layer and properties of Cambrian muddy sediments have numerous stratigraphic and ichnological consequences. These include secular trends in the preservation of event beds and shallow-tier trace fossils including Rusophycus and Cruziana.

Revised biochronology of the Lower Cambrian of the Central Iberian zone, southern Iberian massif, Spain

Trilobites from the upper unit of the Lower Cambrian Pusa Formation (south-central Spain) substantially antedate other Iberian trilobites, according to widely published correlation schemes, and arguably would be among the earliest trilobites globally. These trilobites, previously only briefly mentioned in texts, are here described and illustrated, and their biochronological context examined. The Pusa Formation trilobites are treated in open nomenclature but with suggested affinity to the genus Abadiella. They are associated with small shelly fossils, including Pelagiella sp., chancellorid spicules and Cupitheca sp., and unidentfied archaeocyathans. Trace fossils from the upper unit of the Pusa Formation, down-section of the trilobites, include Dactyloidites isp. and Rusophycus isp., the latter representing the lowest occurrence of this ichnogenus in the region. This biostratigraphical context demonstrates that the Pusa Formation trilobites are substantially younger than had traditionally been thought. In terms of Iberian regional stages they are Ovetian, not Corduban as previously thought. As a consequence of the data presented here, the definitions of Iberian Lower Cambrian regional stages are discussed and a substantially revised correlation between key Lower Cambrian strata of the Central Iberian and Ossa Morena zones is proposed.

A brief review of the fossil record of the Ediacaran–Cambrian transition in the area of Montes de Toledo–Guadalupe, Spain

Abstract The area of western Montes de Toledo to Guadalupe boasts a thick succession of mainly siliciclastic sediment spanning terminal Ediacaran to lower Cambrian strata as indicated by a relatively sparse but diverse palaeontological record. A terminal Ediacaran age is based on the occurrence of Cloudina in platform carbonates of the lower part of the Ibor group and in correlative levels of olistostromes at the base of the Río Huso group. Higher in the Río Huso group are found trace fossils which indicate a Cambrian age, notably Treptichnus bifurcus, which overlaps the local stratigraphic range of macroscopic carbonaceous disc-shaped fossils identified as Beltanelliformis. Strata underlying the Río Huso group contain treptichnids. The fossil record of the terminal Ediacaran–Lower Cambrian of this area is in part comparable to Cloudina-bearing sediments from other regions, in particular Namibia, where treptichnid trace fossils also overlap the range of Cloudina. The possibility of a wider biostratigraphic significance of this should be further tested, including its relation to the base of the Cambrian.

Cambrian–Ordovician acritarchs in the Meguma terrane, Nova Scotia, Canada: Resolution of early Paleozoic stratigraphy and implications for paleogeography

Organic-walled microfossils (acritarchs) provide age constraints for the previously poorly dated Cambrian and Lower Ordovician turbiditic Goldenville and Halifax Groups in the northern Appalachian Meguma terrane of Nova Scotia and address controversies about stratigraphy and provenance. The oldest exposed formation of the ∼8-km-thick Goldenville Group contains the trace fossil Oldhamia , which suggests an age of late Early Cambrian, whereas the coticule-bearing uppermost formation yielded an acritarch species consistent with Middle Cambrian (Epoch 3) age. The conformably overlying ∼5-km-thick Halifax Group includes basal pyritiferous units that yielded a Late Cambrian (Furongian) assemblage of acritarch species, providing further confirmation that the underlying manganese-bearing formations are of Cambrian Epoch 3 age, and not Ordovician as recently claimed. Overlying nonpyritiferous formations contain the Early Ordovician graptolite Rhabdinopora flabelliformis flabelliformis and acritarchs of similar age. Samples collected up section from the graptolite occurrence yielded acritarch species that are indicative of the later Tremadocian and Floian. The new fossil data confirm a gap in age of ∼30 m.y. between the Halifax Group and the overlying Silurian to Lower Devonian Rockville Notch Group. The new ages are consistent with stratigraphic units defined in regional mapping and support recent interpretation of fundamental differences in depositional and tectonic environments between the northwestern and southeastern parts of the Meguma terrane. They also provide new constraints on paleogeography of the terrane by confirming age similarities to stratigraphy in north Wales, with which correlation has been proposed previously based mainly on lithological similarities.

Trace fossils from the Dividalen Group of northern Finland with remarks on early Cambrian trace fossil provincialism

Abstract The Dividalen Group is exposed in a narrow zone along the eastern Caledonian front of northern Fennoscandia. Trace fossils provide important age constraints in these largely siliciclastic strata and demonstrate that in Sweden the entire Dividalen Group is of Cambrian age. Here we provide the first detailed description of trace fossils from the Dividalen Group in Finland, based on new material from the Kilpisjärvi area. This material includes the vertical spiral Gyrolithes polonicus and trace fossils with a three-lobed lower surface, here informally described as the Bure ichnocomplex. These trace fossils have a wide distribution in Baltica and probably represent a rare case of trace fossil provincialism.

Cheiichnus gothicus igen. et isp. n., a new Bergaueria-like arthropod trace fossil from the Lower Cambrian of Västergötland, Sweden

Abstract A new arthropod-type trace fossil from the Lower Cambrian Mickwitzia sandstone of Västergötland, Sweden, shows an enigmatic rotational behaviour in an arthropod, perhaps an olenellid trilobite. Cheiichnus gothicus igen. et isp. n., is hemispherical to plug-shaped with two types of scratch traces. It shares characters with both Rusophycus and Bergaueria. Indeed, poorly preserved specimens of Cheiichnus could be identified as the actinian-type trace fossil Bergaueria.

Chapter 5 Testing for palaeogeographical patterns in the distribution of Cambrian trace fossils

Abstract We examined the palaeogeographical distribution of selected Cambrian trace fossils. Astropolichnus hispanicus, Climactichnites, Syringomorpha nilssoni and early examples of Paleodictyon all have a restricted palaeogeographical distribution, probably representing that of their producers. A cosmopolitan distribution is seen in Didymaulichnus miettensis and in early examples of Rusophycus and Dactyloidites. Oldhamia shows a wide distribution in Lower Cambrian deep-sea sediments although that of Oldhamia geniculata is restricted.