Bruce Runnegar

Molluscan phylogeny: The paleontological viewpoint

Stasek (1) theorized that the extant mollusks are the progeny of three separate lineages that separated before the phylum was well established. He wrote that no known intermediate forms, fossil or living, bridge the enormous gaps between any two of the three lineages, and therefore treated each as a separate subphylum. These subphyla are (i) the subphylum Aculifera Hatscheck 1891, containing only the class Aplacophora, derived from the most primitive ancestors of the Mollusca; (ii) the subphylum Placophora von Jhering 1876, containing only the class Polyplacophora, and emphasizing the pseudometamerism of its more advanced premollusk ancestor; and (iii) the subphylum Conchifera Gegenbaur 1878, containing the Monoplacophora and the other classes derived from it. We point out that the Polyplacophora may be derived from the Monoplacophora instead of a more primitive ancestral stock. We also suggest that the Conchifera can be separated into two major lineages, each worthy of the rank of subphylum. The fossil record indicates that the Monoplacophora gave rise to the Gastropoda, Cephalopoda, Rostroconchia, and possibly Polyplacophora, and that the Pelecypoda and Scaphopoda are derived from the Rostroconchia. These last three classes thus form a lineage that diverged from the Monoplacophora in the Early Cambrian. They emphasized a shell form that in all groups is primitively open at both ends, allowing the gut to remain relatively straight, with an anterior mouth and posterior anus. They became burrowing (infaunal) deposit or filter feeders. We coin the term Diasoma (through-body) for the subphylum containing these three classes (Rostroconchia, Pelecypoda, and Scaphopoda). The remaining three classes (Monoplacophora, Gastropoda, and Cephalopoda) emphasize a conical univalved shell, usually twisted into a spiral. The relatively small single aperture forces the anus to lie close to the mouth, and the gut is bent into a U. Most are surface-dwelling (epifaunal) grazers or carnivores. We coin the name Cyrtosoma (hunchback-body) for the subphylum containing these three classes. Strictly speaking, the cyrtosomes are the ancestors of the diasomes but, in fact, both subphyla appeared and began to diversify within a few million years in the Early Cambrian. Note added in proof: After proofs were corrected we were informed that the new genus Opikella (40) is preoccupied by (�pikella = Oepikella) Thorslund 1940, an Ordovican ostracod. We rename the mollusk genus Oepikila.

Oxygen requirements, biology and phylogenetic significance of the late Precambrian worm Dickinsonia, and the evolution of the burrowing habit

Dickinsonia was a very thin (<3 mm) subcircular to elliptical worm that grew to about a metre in length. It appears to have been a common, conspicuous and widespread Late Precambrian animal. Dickinsonia probably had a hydrostatic skeleton (true coelom); metamerically arranged dorsoventral muscles; longitudinal, transverse and possibly diagonal muscles; an inextensible collagenous basement membrane; a well-developed circulatory system; and possibly a gut with lateral lobes. It was probably an annelid, but may have been neither a polychaete nor an oligochaete, and its geometry and ontogeny suggest that its similarity to the living discoidal polychaete Spinther is due to convergence. Calculations indicate that Dickinsonia could have respired in sea water containing about a tenth of the present amount of oxygen. Since it lived in well-aerated sublittoral environments, its peculiar shape may reflect the low level of oxygen in the Late Precambrian atmosphere. The geometry of coeval animals reinforces this view....

Australian Middle Cambrian molluscs and their bearing on early molluscan evolution

Twenty-eight species of fifteen genera of Middle Cambrian molluscs are described from tiny phosphatic moulds or silica replicas of the shells. The molluscs were etched from limestones at two sites: one in the earliest Middle Cambrian Coonigan Formation of the Mootwingee area, 130 km northeast of Broken Hill, New South Wales; and another in the middle Middle Cambrian Currant Bush Limestone of the Thorntonia area, 150 km northwest of Mt Isa, Queensland. These unusually diverse collections show that many different kinds of molluscs lived in the tropical Australian seas of the Middle Cambrian and provide new information on the way the molluscan classes Cephalopoda, Gastropoda, Rostro- conchia, and Pelecypoda evolved. In other sections, we discuss the problems of classifying and naming Cambrian molluscs; define a number of terms that can be used to describe shell form (including a new adjective, gyrogastric); reclassify the Class Monoplacophora after incorporating the helcionellacean and bellerophontacean “gas...

The Cambrian explosion: Animals or fossils?

Abstract The abrupt appearance of animal fossils at the end of the Precambrian can be explained in two ways—either as the initial explosion of animal life, or as the explosive appearance of fossilisable animals. Evidence from the oxygen‐binding proteins of living animals supports the latter view, since it points to a billion year history for the lower invertebrate phyla. It was the rise of large, muscular and/or mineralised animals that caused the Cambrian explosion, and the best explanation for their appearance seems to be a sharp increase in the oxygen content of the atmosphere.

Shell microstructures of Cambrian molluscs replicated by phosphate

The original microstructure of the aragonistic and calcite shells of Cambrian molluscs is frequently visible at high magnifications on the surfaces of phosphatic internal moulds. More rarely, the nature of the original microstructure may be determined by examining the internal surfaces of phosphate layers which coated the shells, or by examining the phosphate fillings of the tunnels made by endolithic algae within the shells. Because the original aragonitic microstructures of molluscan shells are almost invariably destroyed by recrystallisation in rocks older than the Carboniferous, the discovery of replicated shell microstructures has provided a new understanding of the evolution of the molluscan shell. Most of the common molluscan microstructures — spherulitic prismatic aragonite, tangentially arranged fibrous aragonite, crossed-lamellar aragonite, nacre and foliated calcite — had probably appeared by at least the beginning of the Middle Cambrian, and most are found in representatives of the Class Monop...

Rostroconchia: A New Class of Bivalved Mollusks

Four Paleozoic bivalved genera are assigned to the new molluscan class Rostroconchia: Eopteria, Euchasma, Conocardium, and Pseudoconocardium. These mollusks have ani uncoiled univalved larval shell; an untorted bivalved adult shell; no hinge teeth, ligament, or adductor muscles; and a fused, almost inflexible. hinge. Rostroconchianis developed separately from the pelecypods through the ribeirioids, but are regarded as more closely related to the Pelecypoda and Scaphopoda than to other known classes of mollusks.

Fordilla troyensis Barrande: The oldest known pelecypod

Specimens of the small bivalved animal Fordilla troyensis Barrande from New York State show that this fossil is the oldest known pelecypod mollusk and not a conchostracan arthropod. This finding extends the range of the class Pelecypoda backward in time from the Early Ordovician (about 495 million years ago) to the Early Cambrian (about 540 to 570 million years ago). The morphology of Fordilla troyensis suggests that it lived infaunally and that it was ancestral to the pelecypod subclasses Heteroconchia and Isofilibranchia.

Ecology of Eurydesma and the Eurydesma fauna, Permian of eastern Australia

Eurydesma is a large, equivalved, monomyarian bivalve found at more than 100 localities in eastern Australia and other parts of Gondwanaland. It has massively thickened umbones which maintained the shells in an upright posture analogous to that seen in the modern reef clam Tridacna. Eurydesma is preserved in this orientation in many localities in eastern Australia, and seems to have preferred hard clean sublittoral substrates; it was an opportunist which rapidly colonized sediments derived from rocky shorelines. With the possible exception of a specimen from the Late Carboniferous of Argentina, the oldest occurrences of Eurydesma are probably approximately coeval but its exit seems to be diachronous in different parts of eastern Australia; Eurydesma may have disappeared because of habitat restriction, ecological replacement, or global warming. Since its youngest occurrences can be correlated with palaeolatitude, the last is most likely.

Crystallography of the foliated calcite shell layers of bivalve molluscs

The foliated calcite of living scallops (Pectinacea), oysters (Ostreacea), window-pane and jingle shells (Anomiacea) consists of approximately 250 nm thick sheets (folia) formed of subparallel laths joined laterally. The surfaces of these very thin folia are not the basal pinacoid of calcite (0001), as has previously been stated, but in each species studied are one of the rhombohedral forms (1011), (1012) or (0112). One of these rhombohedra, (1012), is rarely found in non-biological calcite because it lies perpendicular to a direction of very fast crystal growth. Another form, (1011), is much more widespread; it occurs commonly in inorganic crystals, and it lies parallel to the secreting surface in the skeletal calcite of some scallops and oysters, in the pentaliths of some planktonic algae, in the tests of some forams, and in the outer layer of the hen eggshell. By contrast, the foliated calcite of some extinct articulate brachiopods appears to be composed of laths which are flattened parallel to (0001);...

The Permian faunas of northern New South Wales and the connection between the Sydney and Bowen basins

Abstract Permian sediments are continuous between the Sydney and Bowen Basins west of the Hunter‐Mooki fault system and its probable northern continuation, the Goondiwindi Fault. Both fault systems appear to have influenced sedimentation in Early Permian time. A disconformity between Lower Permian coal measures (dated by plant microfossils) and Upper Permian sandstones and shales (dated by marine macrofossils) is present in the northern extension of the Sydney Basin. This hiatus may be correlated with a similar break in sedimentation in the southeastern part of the Bowen Basin. It is probably related to a Mid‐Permian diastrophism which folded Lower Permian and older sediments east of the Mooki and Peel Faults. Marine connection between the Sydney and Bowen Basins appears to have been interrupted during the event so that the two basins may have been temporarily isolated. The difference in the fossil faunas of the Sydney and Bowen Basins may well reflect this isolation.