ABSTRACT This field guide synthesizes more than a half century of research by many geologists and paleontologists on Frenchman Mountain and Rainbow Gardens, southern Nevada, USA. The field-trip consists of seven stops to be visited in one day. The guide was written not only for field-trip participants on the occasion of the 2022 ­Cordilleran/Rocky Mountain Geological Society of America Joint Section Meeting in Las Vegas, but also with future users in mind. The Frenchman Mountain/Rainbow Gardens block of crust exposes an extraordinary sample of Earth history. The geologic features include (1) Proterozoic crystalline rocks, (2) the Great Unconformity, (2) a Paleozoic interval that is essentially a ­tilted section of the western Grand Canyon, (3) a Mesozoic interval that preserves strata that were eroded off the southern Colorado Plateau during the Miocene “Great Denudation” episode, and (4) a Cenozoic section that records a wealth of paleo­climatic, paleontological, and tectonic data. Among the many stories that are recorded in the rocks of the Frenchman Mountain/Rainbow Gardens block, I have chosen three to emphasize in this field guide: (1) the history recorded in the Proterozoic Vishnu Basement Complex and the Great Unconformity; (2) the stratigraphy, sedimentology, paleontology, and geochronology of the Cambrian Tonto Group [with a focus on (a) trilobite biostratigraphy in the Bright Angel Formation, (b) the significance of the abundance of glauconite in the Tonto Group, and (c) the Frenchman Mountain Dolostone]; and (3) the tectonic story recorded in the Thumb Member of the Horse Spring Formation. The basement rocks record events and processes associated with the assembly of supercontinents Nuna and Rodinia. The Great Unconformity records the breakup of Rodinia and the associated denudation interval that played a role in triggering the Cambrian explosion. The Tonto Group, which was recently expanded to include the Frenchman Mountain Dolostone, records the Sauk Transgression. And the Thumb Member of the Horse Spring Formation contains rock avalanche deposits that have played a key role in sorting out the tectonic history of the southern Nevada region and the translational history of the Frenchman Mountain/Rainbow Gardens block.

ABSTRACT Here I rescue from obscurity a mid-twentieth-century sequence of ten paintings representing biotas and ecosystems present in different periods of geologic time. They were used to illustrate a 1955 book titled The History of Life on Earth by University of Vienna paleontologist Erich Thenius. The paintings were also mass produced as classroom teaching aids in the form of wall chart roll-ups. Thenius collaborated with Viennese landscape artist Fritz Zerritsch to produce these scenes from Deep Time. In terms of the selection and arrangement of animals in some of the scenes, Thenius and Zerritsch were probably influenced by well-known paleoartists Rudolph Zallinger and Charles R. Knight. I corresponded with Professor Thenius concerning his collaboration with Zerritsch, and his answers to my questions illuminate some of the choices he made. The Zerritsch/Thenius collection of paleo-scenes is a good example of the pageant-of-life-through-time genre of paleontological art. I use this sequence of prehistoric tableaux to examine artistic conventions within this genre.

ABSTRACT Benjamin Waterhouse Hawkins (1807–1894) was a British scientific illustrator and sculptor who illustrated many British exploration reports in the 1830s and 1840s. In the early 1850s, Hawkins was commissioned to create life-size, concrete sculptures of Iguanodon , ichthyosaurs, and other extinct animals for a permanent exhibition in south London. They were the first large sculptures of extinct vertebrates ever made, and they are still on view today. Inspired by his success in England, Hawkins launched a lecture tour and working trip to North America in 1868. Soon after his arrival, he was commissioned to “undertake the resuscitation of a group of animals of the former periods of the American continent” for public display in New York City. Had it been built, this would have been the first paleontological museum in the world. As part of this ambitious project, with the assistance of the American paleontologist Joseph Leidy, Hawkins cast the bones of a recently discovered Hadrosaurus specimen and used them to construct the first articulated dinosaur skeleton ever put on display in a museum. It was unveiled at the Academy of Natural Sciences of Philadelphia in November 1868. Hawkins worked tirelessly on New York’s proposed “Paleozoic Museum” for two years, until his funding was cut by William “Boss” Tweed, the corrupt leader of the Tammany Hall political machine, who grew hostile to the project and abolished the Central Park Commission that had made it possible. When Hawkins defiantly continued to work, without funding, Tweed dispatched a gang of thugs to break into his studio and smash all of the sculptures and molds. Although Hawkins would create several copies of his articulated Hadrosaurus skeleton for other institutions, the prospect of building a grand museum of paleontology in America was forever destroyed by Tweed’s actions.

In the eighteenth century, many Europeans and Americans embraced a world-view in which the natural world was seen as complete, full, and perfect, as created by God. Within this worldview, no species ever became extinct because such an event would destroy the perfection of nature. Toward the end of the eighteenth century, the concept that no species had ever become extinct was increasingly challenged by evidence from the fossil record. By the early nineteenth century, a new paradigm, the “former-worlds” view of Earth history, began to emerge. Buffon had argued that New World quadrupeds were degenerate varieties of Old World species, and that at least one of them had gone extinct. The idea of New World degeneracy thus became connected with the concept of extinction. Thomas Jefferson conducted a long, personal campaign to discredit these ideas of Buffon’s, arguing against them in the early 1780s in Notes on the State of Virginia and also in his 1797 Megalonyx memoir. Jefferson resisted the concept of extinction for a very long time, and he was never able to let go of his “completeness-of-nature” worldview. I suggest that several factors contributed to Jefferson’s inability to relinquish his worldview, in spite of the fact that there was considerable empirical evidence showing that it was not valid. The most influential factors were (1) Jefferson’s emotional and public commitment to the completeness-of-nature worldview, and (2) Jefferson’s personality traits, which were acquired in part through his experiences as an eldest son.

Abstract Esmeralda County, Nevada, is extraordinary for the presence of Ediacaran and early Cambrian reefs at several stratigraphic positions. In this road log and field guide we present descriptions and interpretations of the most instructive exposures of three of these reef-rich intervals: (1) the Mount Dunfee section of the Middle Member of the Deep Spring Formation (Ediacaran in age), (2) the Stewart's Mill exposure of the Lower Member of the Poleta Formation (mid-early Cambrian), and (3) an exposure on the north flank of Slate Ridge of reefs near the top of the Harkless Formation (latest early Cambrian). We introduce the term “congruent ecosystems” for ecosystems of different age that occupied similar environments. The Ediacaran reefs of the Deep Spring Formation and the early Cambrian reefs of the Lower Member of the Poleta Formation occupied similar environments but exhibit distinctively different ecological structure. Thus we propose these two reef complexes as our premier example of non-congruent communities within congruent ecosystems.

Abstract The oldest Phanerozoic reefs were built in the Early Cambrian (Sauk I) by a consortium of calcimicrobes and archaeocyaths, with the calcimicrobes being the critical reef-building organisms. Archaeocyath-Kena/c/s reefs first occur at the base of the Tommotian Stage on the Siberian Platform; these are cavernous, framework pinnacle reefs about two meters in diameter, with constructor, baffler, and dweller guilds already present. Throughout the rest of the Early Cambrian, archaeocyath-calcimicrobe reefs spread around the world into a variety of low-latitude, mostly shallow-water settings, including shelf margin-oolite shoal environments and inner-shelf settings, often with considerable siliciclastic sediment present. Many Early Cambrian reefs are meter-scale in size and lenticular in shape, often with individual lenses (kalyptrae) stacked one upon the other to form a compound buildup. Coralomorphs and radiocyaths sometimes occur with archaeocyaths in these reefs. According to the Kiessling-Flügel database that accompanies this book, about one-third of Early Cambrian reefs are less than ten meters thick, and two-thirds are thicker than ten meters. Some were quite massive, ecologically zoned reefs with a rigid framework of branched archaeocyaths and encrusting calcimicrobes. Archaeocyaths reached a diversity of about 170 genera in the Botomian Stage, decreased precipitously in the Toyonian Stage, and became virtually extinct at the end of the Early Cambrian, thus ending the first episode of reef-building by metazoans. Bioerosion was apparently not as important a process in Early Cambrian reefs as in Mesozoic and Cenozoic reefs, although evidence of microboring is common. During the Sauk II interval (Middle Cambrian through early Late Cambrian), reefs were common in subtidal and intertidal facies on low-latitude continents. These are microbialite reefs, which occur in three basic mesostrucrural types: dendrolites, stromatolites, and thrombolites. Many of the calcimicrobe taxa that are found in Sauk I reefs also occur in Sauk II reefs, most conspicuously in dendrolitic reefs, a distinctive type of Sauk II reef that is not known to occur in Sauk III. Stromatolitic reefs are common in Sauk II rocks, occurring in a wide range of morphologies. Thrombolitic Sauk II reefs are rare, but non-reefal columnar thrombolites are fairly common. The only known Sauk II reef with a conspicuous metazoan component is in Iran; it contains a framework of anthaspidellid-grade sponges encrusted by filamentous microbes. Echinoderm plates occur fairly commonly with reefs in this interval, especially eocrinoid columnals in the latest Sauk II reefs. During the Sauk III interval (mid-Late Cambrian through earliest Ordovician) sea level was high and thrombolitic and stromatolitic reefs became very widespread. In some areas (e.g., the Great Basin) thrombohtes are the dominant reef type, while in others stromato Utes are more common. On most continents thrombolites tend to dominate in the earliest Ordovician. Calcimicrobes belonging to the Renalcis, Epiphyton, and Girvanella groups can often be identified in Sauk III reefs. Some endemism occurs among Sauk III stromatolite morphologies, which indicates the presence of geographically restricted microbial floras and/or unique environments. As in the Sauk II interval, one case is known of metazoan-built Sauk III reefs. The Sauk III metazoan reefs are in the Wilberns Formation of central Texas, in which anthaspidellid sponges constitute up to 25% of the reef boundstone. The most common invertebrates found in association with Sauk III microbialite reefs are eocrinoids, trilobites, hyoliths, and grazing molluscs. Toward the end of Sauk III, the diversity of reef-dwelling metazoans began to increase, leading up to the diversity explosion at the end of the Early Ordovician. We examine six factors that may have influenced reef development during the Cambrian and earliest Ordovician: (1) biotic evolution, (2) plate motions, (3) eustatic sea-level change, (4) nutrient availability, (5) paleoclimate, and (6) technically forced shifts in seawater chemistry. All of these factors probably influenced reef building to some extent. The most conspicuous change in reef building during the Cambrian and Early Ordovician was the collapse of the archaeocyath-calcimicrobe reef ecosystem at the end of the Early Cambrian, and the corresponding resurgence of microbialite reefs from the Middle Cambrian through the Early Ordovician. This anomalous interval of microbialite-dominated reefs, which lasted approximately forty million years, has not been satisfactorily explained. We explore seven hypotheses: (1) the post-extinction lag hypothesis, (2) the photosymbiosis recovery hypothesis, (3) the reduced grazing hypothesis, (4) the nutrient deficiency hypotheses, (5) the high levels of atmospheric CO 2 hypothesis, (6) the global warming hypothesis, and (7) the Mg/Ca seawater chemistry hypothesis. We reject the first three hypotheses because they do not account for the range of phenomena associated with the microbialite resurgence. Some or all of the remaining four hypotheses probably operated synergistically, possibly with other processes not yet discovered, to inhibit metazoan reef building and promote microbialite reef building from the Middle Cambrian through the Early Ordovician. We address the question of photosymbiosis in Early Cambrian reefs. Although photosymbiosis is very important in modern reefs, and presumably in Mesozoic and Cenozoic reefs, during the 1990s several papers were published in which it was concluded or inferred that photosymbiosis was probably not present in Early Cambrian and other Paleozoic reefs. This implies that the energetics of Cambrian reef communities were fundamentally different than those of Mesozoic and younger reefs. We find the arguments in support of this conclusion to be unpersuasive. Sponges are well known to form symbioses quite easily, and many modern reef-dwelling sponges harbor photosymbiotic cyanobacteria. As a preliminary test of the hypothesis that Early Cambrian reefs did not have photosymbiosis and therefore could not live in nutrient-poor waters, we use the Kiessling-Flügelreef database to compare depositional environments of Early Cambrian reefs with reefs of the mid-Early Cretaceous. There are no significant differences between the environments of the early Paleozoic and the late Mesozoic reefs, which suggests that Paleozoic reefs are not fundamentally different from Mesozoic reefs. This simple comparison does not settle the question of photosymbiosis in Paleozoic reefs, but it suggests a new approach using the Kiessling-Flügel database. Other than the occasional building-stone quarry, Cambrian and Lower Ordovician reefs are not particularly important economically. They are not known to be significant hydrocarbon reservoirs, which is probably due to one or more of the following factors: a small amount of reef-derived debris, small size (in the case of many Sauk I reefs), and an abundance of microbialite (in the case of Sauk II and Sauk III reefs).

The Frenchman Mountain structural block lies near the intersection of a right-lateral strike-slip fault, a left-lateral strike-slip fault, and a regionally significant low-angle normal fault. It has commonly been presumed that this block was translated several tens of kilometers northwestward or southwestward during Basin and Range extension, but the details of this translation have not been rigorously examined. Al-though it is not yet possible to reconstruct the detailed histories of the individual faults that were involved in the translation of the Frenchman Mountain block, the determination of the net translational and rotational history of this block can be used to evaluate the relative importance of various types of faulting during extension of the Lake Mead region. The Frenchman Mountain block contains a thick section of Paleozoic, Mesozoic, and Miocene strata, including the syntectonic Miocene Horse Spring Formation. Comparisons of clast compositions in the breccias of the Thumb Member of the Horse Spring Formation with various areas of exposed Precambrian basement indicate that the Gold Butte granite complex, 65 km to the east, is the only viable presently exposed source area for these breccias. Sedimentology of the Thumb Member indicates that this unit was deposited in proximal and medial alluvial fan settings. Channel orientation and facies relations indicate a transport direction of N60W ± 30°. Two-meter clasts and the presence of distinctly channelized, matrix-supported breccia indicate that these sediments were deposited no farther than 5 km from their source. Large blocks up to 100 m long and 20 m thick in southern Rainbow Gardens were translated no farther than a few hundred meters. We conclude that the pre-extension position of the Frenchman Mountain block was probably on the western or northwestern margin of the Gold Butte granite complex, directly adjacent to the Gold Butte fault. This reconstruction requires that a fault exists on the southern boundary of Rainbow Gardens roughly beneath Las Vegas Wash. Pennsylvanian eolian cross-bed orientations at Frenchman Mountain are identical to those in the Gold Butte area. This suggests that no significant tectonic rotation accompanied the westward translation of the Frenchman Mountain block. As an independent test of Frenchman Mountain’s preextension position we have compared the thicknesses and facies of Cambrian and Devonian strata in the Frenchman Mountain block with those in the eastern Lake Mead region. These Paleozoic data indicate a preextension position in the northeastern part of the Lake Mead area, thus supporting the interpretation that the block originally lay adjacent to the Gold Butte granite complex. Considering all available Paleozoic and Miocene sedimentologic and stratigraphic data, and assuming that the Gold Butte granite complex itself experienced about 10 km of westward translation, our restoration vector for the Frenchman Mountain block relative to the Colorado Plateau has a magnitude of 80 ± 8 km and a bearing of N80E + 5°. The documentation of 80 ± 8 km of translation and associated stratal tilting, with no significant tectonic rotation, indicates that detachment faulting has been the dominant Neogene deformational process in the Lake Mead region.