Gerald F. Webers

Late Cambrian molluscan faunas and the origin of the Cephalopoda

Abstract Late Cambrian gastropod and monoplacophoran faunas from western Antarctica, eastern and mid-western North America, and northern China, are diverse and provide some insight into palaeogeography and faunal distribution. The oldest of these, a well-preserved trilobite-mollusc fauna from the Ellsworth Mountains, Antarctica is dated as latest Dresbachian (Idamean); the Antarctic fauna is not unlike a slightly younger one (Franconian) from east-central Minnesota, USA. Slightly younger Franconian-age rocks from north China contain the first authentic cephalopods. A Trempealeauan fauna from eastern New York, USA, is closely comparable to another Trempealeauan fauna from north China. Relatively high taxonomic diversity suggests tropical to sub-tropical marine environments for all these faunas. Plate tectonic position as interpreted from palaeomagnetism supports this interpretation. The occurrence of these molluscs on separate continents, and the presumed widely separated position of these continents in the Late Cambrian, makes it likely that the distribution mechanism for the molluscs was long-lived larval forms. A key member of the Antarctic faunule is the monoplacophoran Knightoconus which has been considered representative of a group directly ancestral to the oldest cephalopod Plectronoceras. This view is reconsidered in the light of criticisms and alternatives proposed during the last 15 years; these occupy the bulk of the paper. Reaffirmation of evolution from the Antarctic Knightoconus is supported both by refutation of the criticisms and by ontogenetic studies of new topotypical material of the genus.

A revision of Palaeacmaea (Upper Cambrian) (?Cnidaria)

Palaeacmaea typica, the type species of the genus, and P. irvingi, the only other Late Cambrian taxon considered congeneric are redescribed herein. Their morphology suggests that they are neither Monoplacophora, where they are currently assigned, nor are they Mollusca. Specimens of P. irvingi demonstrate considerable variation in shape, interpreted as distortion of a flexible integument, of essentially no thickness. A neotype is designated for P. irvingi ; it is interpreted as a medusiform fossil, possibly a cnidarian. Only the holotype of P. typica is known, but it shows comparable features. The genus and the family Palaeacmaeide are placed in Phylum Incertae Sedis. Four Ordovician species previously assigned to Palaeacmaea, show none of the characteristics noted, and they are tentatively reassigned to other genera.

Zur Fazies der Karbonatgesteine in den Ellsworth Mountains (Paläozoikum, Westantarktis)

ZusammenfassungAnstehende und umgelagerte Karbonate treten in verschiedenen Horizonten der Schichtenfolge in den Ellsworth Mountains vom Kambrium bis ins Permo-Karbon auf. Soweit sie datierbar sind, ergibt sich ein kambrisches Alter für die primäre Kalksedimentation.Die Faziesanalyse läßt folgende Sedimentationsräume erkennen:a)Unterkambrische Archaeocyathinenkalke, die sich mit Stromatolithen verzahnen. Nur durch umgelagerte Komponenten aus dem permokarbonen Diamiktit bekannt.b)Eine unter (?)-kambrische ausgedehnte Karbonatplattform; nur in umgelagerten Komponenten in einem unter- bis mittelkambrischen Konglomerathorizont beobachtet.c)Autochthone oolithische Kalke mit geringer Verbreitung aus dem Mittelkambrium.d)Eine ausgedehnte Karbonatsedimentation im Oberkambrium von wenigen Metern Mächtigkeit in der nördlichen Heritage Range (Webers Peaks) bis zu einigen 100 m im S (Marble Hills) mit nach S ansteigendem Metamorphosegrad (bis nahe der Grenze very low grade/low grade).SummaryDuring the Lower and/or Middle Cambrian a wide carbonate platform developed in or close to the Ellsworth Mts. Skeletal algae and archaeocyathid boundstones with sparry calcite or mud as matrix are typical sediments of an open marine environment with low to medium hydrodynamic energy. A high energy environment is required to form oosparites. The grains with a medium diameter between 4 to 7 mm are made up of concentric ooids with simple and complex structures. Nuclei are formed by abraded ooids and oolitic intraclasts. Half-moon ooids with collapsed internal structures are attributed to partial solution and indicate a hypersaline depositional environment (Ca sulfate?) for some parts of the oolitic bank facies.Laminites with fenestral structures are low-energy lagoonal deposits. Dome-shaped and LLH stromatolites are supposed to indicate an intertidal environment.Some areas of the carbonate platform were elevated and partly dissolved by fresh water influence. During the following period of subsidence voids were filled again by rhythmic cementation of fibrous carbonates. Renewed uplift resulted in erosion and destruction of the carbonate platform. Therefore no autochthonous Lower Cambrian carbonates are exposed in the Ellsworth Mts.; only reworked clasts are found in the Lower Heritage Group and the Whiteout Conglomerate.Autochthonous marly and oolitic carbonates are first known from the Middle Cambrian sequence north of Drake Icefall. The thickness of the Upper Cambrian Minaret Formation increases from 8 m in the northern Webers Peaks to several hundred meters in the Marble Hills area.Medium to high hydrodynamic energy prevailed in the Springer Peak section (Webers Pks.: biosparite, oncosparite, pelsparite). The high diversity of the fauna indicates an open marine environment. The section near Hurst Peak starts with a few meters of restricted quiet-water carbonates which are overlain by high-energy oolites and sandstones. Further to the south (south of Dolence Peak) facies fluctuated from medium (onkolite) to high (oolite) energy environments; fossils (brachiopods, trilobites) are rare. Near the top of the sequence a hypersalinary milieu is indicated by layers of early diagenetic dolomicrite. Similar facies types are exposed in the Liberty and Marble Hills. Fast subsidence in this area was compensated by rapid sedimentation of shallow-water carbonates.The Mn contents of the carbonate rocks show a decrease of silicoclastic influence from N to S. In contrast, Sr content and illite crystallinity indicate increasing deformation and metamorphism of the Minaret Formation from N to S in the Heritage Range of the Ellsworth Mts.