Search

The Great Basin of Nevada and Utah has been a classic region for Cambrian geology and paleontology for some 150 years. Studies in the late 19th century were prompted by the discovery of commercial ore deposits and led to the initial documentation of the trilobite faunas in the 1870s and 1880s. Further work on these faunas in the 1950s and early 1960s cemented the importance of the region, particularly as a standard succession for Cambrian trilobite biostratigraphy. However, many species are poorly documented by modern standards, and a revision is long overdue. This thesis focuses on the Dunderberg Formation in Nevada and coeval parts of the Orr Formation in Utah. With more than 100 trilobite species reported from the Dunderberg alone, a full revision of the fauna will not be attempted. Instead, the thesis deals with three genera of Family Elviniidae, Dunderbergia Walcott, 1924 and two close relatives, Dytremacephalus Palmer, 1954 and Elburgia, Palmer, 1960, and is based on new field collections from the Dunderberg and Orr formations. In addition, some species from the Riley Formation of central Texas and the Shallow Bay Formation of western Newfoundland are included in the study. A total of 18 species are documented, and eight of them are new. An analysis using computer- based parsimony methods places the species in a broader phylogenetic context. The results indicate that species of Dytremacephalus and Elburgia form monophyletic subclades that are nested within a set of species that have been assigned to Dunderbergia in previous studies; Dunderbergia is paraphyletic. An improved understanding of the relationships of these species must await a much larger analysis of the entire Family Elviniidae, a major undertaking that is beyond the scope of this thesis. As an interim step that preserves as much of the existing classification as possible, Dytremacephalus and Elburgia are treated as subgenera of Dunderbergia, with species of the latter referred to simply as Dunderbergia sensu lato. New species named in this thesis are: Dunderbergia (s.l.) astroplethes, Du. (s.l.) bovicephala, Du. (s.l.) multichauna, Du. (Dytremacephalus) aphropeca, Du. (Dy.) chondramma, Du. (Dy.) craniophysa, Du. (Dy.) pluviguttata, and Dunderbergia (Elburgia) magnatubercula. Species range data are used to develop a high- resolution zonation of four zones for the upper Candland Shale and correlatives in Nevada and Utah, in ascending order, the Du. (Dy.) pluviguttata, Prehousia alata, Dunderbergia (s.l.) anyta, and Dunderbergia (Elburgia) granulosa zones. They will allow detailed correlation of strata throughout the Great Basin region.

Most of the fossil record consists of "ordinary" deposits, those where only biomineralized parts such as teeth, shells, and bones are preserved. Rarely, when the proper conditions are met, nonbiomineralized tissues such as guts and muscles also can be preserved. These deposits can reveal new insights into the biology, ecology, and taphonomy of ancient organisms, so identifying and understanding them is critical to paleontology. Anoxia is one of the conditions thought to be necessary for exceptional preservation. Because of this, "ordinary" deposits from predominantly aerobic depositional settings have been largely disregarded as potential sites for exceptional preservation.The discovery of pyritized nonbiomineralized tissues in the Silica Shale (Devonian) of Ohio, Michigan, and Indiana seems to contradict historical assumptions about environments conducive to exceptional preservation. This study characterizes the extent of exceptional preservation in the Silica Shale, and provides a comparison with another similar "ordinary" deposit for exceptional preservation, the Cincinnatian (or 'Cincinnati Group'; Ordovician) of Ohio, Kentucky, and Indiana. Comparing these two occurrences provides insight into the mechanism for exceptional preservation in these deposits.Fossil specimens were examined using X-ray Computed Tomography (XCT) to non-destructively view internal structures. In total, 247 specimens from the Silica Shale and 102 specimens from the Cincinnatian were examined. They represent a variety of taxa including brachiopods, trilobites, corals, cephalopods, gastropods, bivalves, and echinoderms. More than 60% of Silica Shale specimens and 20% of Cincinnatian specimens contained pyritized nonbiomineralized tissues. XCT analysis reveals that one brachiopod specimen from the Silica Shale, assigned to Paraspirifer bownockeri, lived with situs ambiguous. In both the Silica Shale and the Cincinnatian, rapid burial during storm events likely created local, temporary areas of anoxia that allowed for exceptional preservation of nonbiomineralized tissues in a broadly well oxygenated environment. The results of this study open the possibility that other "ordinary" deposits also have been overlooked, and such deposits merit reexamination through this new lens.

Geochronologically calibrating the latest Cambrian and earliest Ordovician is critical for further investigations into the cycles of evolutionary diversifications and extinctions during this interval. The peri-Gondwanan terrane, Sibumasu (western Thailand, eastern Myanmar, northern Malaysia, and western Yunnan, China) is globally the only tectonic province known to have multiple zircon-bearing volcanic ash beds interbedded with fossiliferous strata from both the latest Cambrian and earliest Ordovician. The terrane thus has great potential for producing a geologically constrained biostratigraphic succession. Well-resolved taxonomy is foundational for reliable biostratigraphic correlations, without which absolute dates have limited value beyond the locality from which the dated material was collected. Trilobites are particularly useful fauna for correlations during this time interval as their diversity, environmental pervasiveness, and good preservation potential mean that they are found on most tectonic provinces and can be correlated with reasonably high precision. Hitherto the trilobites of Sibumasu have not been taxonomically well-resolved. This study uses field collections made during three excursions (2008, 2016, and 2018) to southern Shan State, Myanmar and Ko Tarutao, Thailand to revise Sibumasu’s Cambro-Ordovician trilobite fauna. It is the first description and illustration of trilobites from Cambrian Myanmar and substantially expands the trilobite record of the fauna of Thailand’s Tarutao Group. The latter, though the subject of three separate studies (1957–1988), had been left with most species in open nomenclature. Collections from both localities have tightly constrained stratigraphic context where possible. Detrital zircon and volcanic ash samples were collected at the same time as the fossils for use in dating and paleogeographic reconstructions. Three species, one of which is new, are recorded from Myanmar. 42 species were recovered from the Tarutao Group, 18 of which are new; following the most recent revision in 1988, only 18 species were recognized. Two new genera were also recovered from the Tarutao Group.

Trilobites were a diverse group of Paleozoic marine arthropods, whose growth is characterized by exoskeletal ecdysis; molting of the outer cuticle. If all sclerites from all growth stages were preserved in a fossil assemblage, the distribution would be expected to be strongly right skewed because individuals must have passed through smaller molt stages, but not all small individuals would have survived to large size. However, the overwhelming majority of observed trilobite size frequency distributions have normal size distributions. This thesis investigates this discrepancy using a modified version of the method used by Hartnoll and Bryant (1990) to model crab size frequency distributions. Here I apply this method to the trilobite Aulacopleura koninckii, a Silurian species whose growth is among the best known for any fossil, and for which size-specific assemblages of articulated individuals are observed. This required the use of parameters known for A. koninckii, in combination with other estimates based on living crab biology. Observed growth parameters from A. koninckii suggest that this trilobite underwent up to 33 post-protaspid instars separated by 32 post-protaspid molts. A mortality rate of 10% per instar was assumed based on evidence that this taxon occupied a low predation environment. Given the assumption of constant recruitment and crab-based estimates of X and Y, derived from the observation that intermolt period increased systematically as size increased, I was able to model predicted distributions of A. koninckii sizes that matched the largest individuals observed. Estimated life spans of A. koninckii according to these parameters lie between 1 and 10 years. As expected these distributions predicted far larger numbers of smaller specimens than larger ones, which is not what is observed. To explain the dearth of smaller individuals I accounted for the probability of preservation in order to explore how the size frequency distributions behaved. Simulations suggest that the size frequency distributions observed for A. koninckii could be the outcome of either a preservation bias against smaller trilobites, an event that killed off a live population, or the result of the fact that size frequency distributions have many variables that cannot be determined from the fossil record.

Remains of two trilobite species, Elrathia kingii from the Wheeler Formation (Cambrian Series 3), Utah, and Cornuproetus cornutus from the Hamar Laghdad Formation (Middle Devonian), Alnif, Morocco, were studied using computed tomographic (CT) and microtomographic (micro-CT) imaging techniques for evidence of nonbiomineralized alimentary structures. Specimens of E. kingii showing simple digestive tracts are complete dorsal exoskeletons preserved with cone-in-cone concretions on the ventral side. Inferred stomach and intestinal structures are preserved in framboidal pyrite, likely resulting from replication by a microbial biofilm. C. cornutus is preserved in non-concretionary limestone with calcite spar lining the stomach ventral to the glabella. Neither species shows sediment or macerated sclerites of any kind in the gut, which tends to rule out the possibilities that they were sediment deposit-feeders or sclerite-ingesting durophagous carnivores. Instead, the presence of early diagenetic minerals in the guts of E. kingii and C. cornutus favors an interpretation of a carnivorous feeding strategy involving separation of skeletal parts of prey prior to ingestion.

A concentration of well preserved, articulated juvenile and mature exoskeletons of the Czech middle Silurian trilobite Aulacopleura koninckii provides an unparalleled opportunity to explore the nature of exoskeletal growth and morphological variation during trilobite development. A new dataset selected via a comprehensive survey of the highest quality specimens offers improved resolution of original morphology by all measures considered. It confirms that meraspid size increase was exponential and the meraspid growth rate per instar was strikingly constant. The degree of variation in both size and shape among later meraspid instars was remarkably constant, and suggests that an active process channeled the meraspid development of both these attributes. The degree of precision of the growth control is remarkable given this species' well-known variance in holaspid trunk segment number. Size-related changes in the shape of the dorsal exoskeleton and of the segment-invariant cephalon were detected in the meraspid stage, but in the holaspid phase marked allometry was detected only in the trunk region, with the pygidium showing notable expansion in relative size. Meraspid cranidial allometry was subtle, with significant changes in instar form detectable only after several molts. Analyses suggest that this synarthromeric trilobite developed progressively without sharp transition throughout meraspid and holaspid ontogeny, except for the synchronous cessation of trunk segment appearance and release at the onset of the holaspid phase. We suggest that the remarkably tight control of the development of sclerite size and shape, and the gradual, "track-like" ontogeny of A. koninckii may have resulted from development in conditions of reduced oxygen availability in which physiological viability operated within narrow morphological limits.