Stephen M. Rowland

Were there framework reefs in the Cambrian

It is commonly thought that framework reefs did not exist prior to Middle Ordovician time. Lower Cambrian archaeocyathan-algal limestones from several localities suggest that this view may be incorrect. In particular, limestones in the Poleta Formation of western Nevada and eastern California display a complex branched framework and a biological zonation similar to that described from Ordovician and younger rocks. These archaeocyathan-rich limestones are associated with coarse-grained oolite, contain tidal channels, and are flat-topped and laterally continuous rather than biohermal. I interpret them to have been wave-resistant, environment-dominating, framework reefs.

ARCHAEOCYATHS—A HISTORY OF PHYLOGENETIC INTERPRETATION

Abstract Archaeocyaths are calcareous, conical, Cambrian fossils with a long history of phylogenetic uncertainty and changing interpretations. The history of phylogenetic interpretation of archaeocyaths reveals five distinct schools of thought: the coelenterate school, the sponge school, the algae school, the Phylum Archaeocyatha school, and the Kingdom Archaeata school. Late nineteenth century and early twentieth century paleontologists worked within a paradigm of inexorably increasing diversity through time, and they did not believe in the concept of extinct phyla. Consequently, prior to about 1950, archaeocyaths were bounced around from coelenterates to sponges, to algae. By the 1930s, after considerable study, all workers agreed that archaeocyaths were sponges of one type or another. In the mid-twentieth century a significant paradigm shift occurred in paleontology, allowing the viability of the concept of a phylum with no extant species. Correspondingly, two new schools of thought emerged regarding archaeocyathan taxonomy. The Phylum Archaeocyatha school placed them in their own phylum, which was inferred to be closely related to Phylum Porifera within Subkingdom Parazoa. A second new school removed archaeocyaths and some other Paleozoic problematica from the animal kingdom and placed them in Kingdom Archaeata (later Kingdom Inferibionta). The Phylum Archaeocyatha school was the mainstream interpretation from the 1950s through the 1980s. However, the widespread use of SCUBA beginning in the 1960s ultimately led to the rejection of the interpretation that archaeocyaths belong in a separate phylum. SCUBA allowed biologists to study deep fore-reef and submarine cave environments, leading to the discovery of living calcareous sponges, including one aspiculate species that is morphologically similar to archaeocyaths. These discoveries in the 1960s and 1970s stimulated a re-examination of sponge phylogeny generally, and comparisons between archaeocyaths and sponges in particular. The result was the abandonment of the Phylum Archaeocyatha school in the 1990s. Present consensus is that archaeocyaths represent both a clade and a grade—Class Archaeocyatha and the archaeocyathan morphological grade—within Phylum Porifera.

Lower Cambrian bioconstructions in Northwestern Mexico (Sonora). Depositional setting, paleoecology and systematics of Archaeocyaths

Abstract Investigations on Lower Cambrian carbonates of Caborca (NW Mexico) indicate that in the Puerto Blanco Formation the following two types of archaeocyathan-algal buildups occur: o I -Framework reefs occurring at the base of the carbonate interval of the Puerto Blanco Formation. These consist of an archaeocyathan framework with encrusting renalcids and abundant fibrous cements. Cavities are filled with mudstone and contain rare sponge spicules. Cements are well developed upon internal sediments. The archaeocyathan fauna is dominated by colonial Paranacyathus along with Batenevia and Claruscoscinus with taeniae and curved tabulae. A few regular archaeocyaths are present, including Palmericyatbus and Aulocricus. Lateral to the reef facies the lime mud sediment contains abundant skeletal fragments of trilobites, echinoderms, Chancelloria, hyoliths, and archaeocyaths. These buildups are interpreted to have formed as wave-resistant, framework reefs in a tidally dominated, shelf-margin setting seaward of a carbonate barrier. II -Girvanella-dominated buildups located in the upper part of the carbonate interval of the Puerto Blanco Formation and associated with oolites. These are built by stacked Girvanella crusts, with an important contribution of Renalcis and a less important presence of Epiphyton. Shelter cavities are filled by mudstone often rich in siltsize quartz. Cements are poorly developed and confined to the intraskeletal cavities. Archaeocyaths are scattered in the Girvanella frame either as isolated individuals or as clusters of solitary or colonial forms. Regular archaeocyaths are here abundant, represented by several taxa with an ethmophylloid inner wall, including Palmericyatbus and Cordilleracyathus. The dominant genus is Aulocricus. Colonial species of Arcbaeocyathus and Arrythmocricus are well developed. These latter two genera are now recorded from Mexico to northern Canada. The internal and lateral sediment contains a rich

A Multicellular Alga With Exceptional Preservation From the Ediacaran of Nevada

Abstract Elainabella deepspringensis new genus new species is a one-mm-wide, non-biomineralized, three-dimensionally preserved fossil with segmented branches and apparent cellular structure. A single specimen was recovered from an interval of black shale within the Ediacaran portion of the Esmeralda Member of the Deep Spring Formation at Mt. Dunfee in Esmeralda County, Nevada. We interpret the fossil to be the thallus of a multicellular alga of uncertain division. EDS spectral analysis indicates that the exceptional preservation is not due to phosphatization or pyritization. Rather, it appears to be a case of Burgess Shale-type preservation, involving the kerogenization of non-mineralizing organisms. The fossil-bearing shale is closely associated with stromatolites, and we suggest that E. deepspringensis may have been an epibiont on stromatolites or other firm substrates. This is the first multicellular alga and the first occurrence of Burgess Shale-type preservation reported from the Ediacaran of Laurentia.

Trackways of a gregarious, dunefield-dwelling, Early Jurassic therapsid in the Aztec Sandstone of southern Nevada

ABSTRACT We describe and interpret a tracksite in the Lower Jurassic Aztec Sandstone in Valley of Fire State Park, southern Nevada. The site contains approximately one hundred tracks of the ichnogenus Brasilichnium, arranged in twelve, subparallel trackways, all on the same foreset bedding plane. The Brasilichnium trackmaker was most probably a fossorial, tritylodontid therapsid. Sedimentologicial analyses indicate that the trackway surface is a wind-ripple horizon with a primary dip of about 25°, and that the animals climbed straight up the slip face of the dune. A combination of features leads us to conclude that the footprints were impressed into a crust of moist, cohesive sand, leaving two modes of preserved tracks: (1) shallow, well-defined tracks without associated sand crescents, and (2) deeper, less well defined tracks with associated sand crescents. We interpret this assemblage of tracks to record gregarious behavior in a mixed-age group of tritylodontid therapsids. In the correlative Navajo Sandstone, other researchers have documented the presence of complex networks of burrows concentrated in elevated mounds, reminiscent of colonies of North American prairie dogs. The Brasilichnium trackmaker is a good candidate to have excavated the burrows. Although we cannot directly associate the Brasilichnium trackmaker with the burrow complexes, we hypothesize that these gregarious, fossorial animals lived in prairie-dog–town–like colonies. This study supports the aridity food-distribution hypothesis, which posits that the patchy distribution of food resources in arid environments creates selective pressure for colonial behavior.