Randall B. Irmis’s research while affiliated with University of Utah and other places

The upper Paleozoic Cutler Group of southern Utah, USA, is a key sedimentary archive for understanding the Earth-life effects of the planet’s last pre-Quaternary icehouse–hothouse state change: the Carboniferous–Permian (C–P) transition, between 304 and 290 million years ago. Within the near paleoequatorial Cutler Group, this transition corresponds to a large-scale aridification trend, loss of aquatic habitats, and ecological shifts toward more terrestrial biota as recorded by its fossil assemblages. However, fundamental questions persist. (1) Did continental drift or shorter-term changes in glacio-eustasy, potentially driven by orbital (Milankovitch) cycles, influence environmental change at near-equatorial latitudes during the C–P climatic transition? (2) What influence did the C–P climatic transition have on the evolution of terrestrial ecosystems and on the diversity and trophic structures of terrestrial vertebrate communities? The Paleozoic Equatorial Records of Melting Ice Ages (PERMIA) project seeks to resolve these issues in part by studying the Elk Ridge no. 1 (ER-1) core, complemented by outcrop studies. This legacy core, collected in 1981 within what is now Bears Ears National Monument, recovered a significant portion of the Hermosa Group and the overlying lower Cutler Group, making it an ideal archive for studying paleoenvironmental change during the C–P transition. As part of this project, the uppermost ∼ 450 m of the core were temporarily transferred from the Austin Core Repository Center to the Continental Scientific Drilling Facility at the University of Minnesota for splitting, imaging, and scanning for geophysical properties and spectrophotometry. Here we (1) review the history of this legacy core, (2) introduce recently obtained geophysical and lithologic datasets based on newly split and imaged core segments to provide a sedimentological and stratigraphic overview of the Elk Ridge no. 1 core that aligns more accurately with the currently recognized regional lithostratigraphic framework, (3) establish the position of the boundary between the lower Cutler beds and the overlying Cedar Mesa Sandstone in the core, and (4) outline our ongoing research goals for the core. In-progress work on the core aims to refine biostratigraphic and chemostratigraphic age constraints, retrieve the polarity stratigraphy, interrogate preserved cyclostratigraphy, analyze sedimentary structures and paleosol facies, investigate stable isotope geochemistry, and evaluate elemental abundance data from X-ray fluorescence (XRF) scanning. Together with outcrop studies throughout Bears Ears National Monument and its vicinity, these cores will allow the rich paleontological and paleoenvironmental archives recorded in the continental Carboniferous–Permian transition of western North America to be confidently placed in a robust chronologic context that will help test hypotheses relating ecosystem evolution to the Carboniferous rainforest collapse, initial decline of the Late Paleozoic Ice Age, and long-wavelength astronomical cycles pacing global environmental change.

Triassic non-marine strata from Gondwana are dated almost exclusively by biostratigraphic means, given the rarity of precise radioisotopic and magnetostratigraphic datasets. A primary method for constraining the age of these sedimentary sequences is palynomorph biostratigraphy, which implicitly assumes that differences in the presence/absence of taxa between two or more assemblages reflect a difference in geological ages. But without biostratigraphically-independent age data, this assumption often remains untested. The Triassic El Peñasco Group of the Santa Clara sub-basin in the Cuyana Basin of central-west Argentina is an excellent study system to help test these assumptions, because the upper part of the sequence comprises a fluvio-deltaic-lacustrine succession that preserves volcaniclastic horizons and extensive palynomorph assemblages. Previous palynostratigraphic analyses had inferred a Late Triassic (Carnian-Norian) age for the Santa Clara Abajo and overlying Santa Clara Arriba formations, but we present new precise U-Pb CA-TIMS zircon ages that suggest an upper Anisian (Middle Triassic) age for both formations. This makes it the only non-marine Anisian Gondwanan sequence, and one of only a handful globally, to be precisely dated geochronologically. These new ages also constrain the depo-sition of the >700 m-thick succession of the Santa Clara Abajo and Santa Clara Arriba formations to no more than 2.1 my of time, suggesting it represents a synrift phase of the sub-basin. Combining these and existing geochronologic data with a newly assembled comprehensive presence/absence dataset of palynomorphs from the Anisian-Norian of Gondwana, we demonstrate that paleogeography (paleolati-tude) has a significantly stronger correlation with taxonomic composition of assemblages than does geo-logic time. These results imply that geography is an important null hypothesis in explaining differences in early Mesozoic Gondwanan palynomorph assemblages, and that precise geochronologic age constraints are important for refining the accuracy of Triassic palynomorph biochronology.

Assemblages of mammal skeletal remains provide a powerful tool for censusing wildlife populations to establish zoological baselines required for evaluating biogeographic trends over varying timescales. Caves provide an ideal depositional setting to preserve these skeletal remains despite potential time averaging and taphonomic filtering. We describe a Holocene paleontological assemblage from Boomerang Cave in the Bear River Range of Cache County, northern Utah, United States, at an elevation of 2,231 m, and at the boundary between the Great Basin and Rocky Mountain biogeographic provinces. We analyzed 1,228 surface-collected specimens from six areas within the cave, and identified a minimum of 22 nonoverlapping mammalian taxa, comprising all size classes present in the region. Compared to museum records for mammals from the Bear River Range and individuals trapped or observed in the vicinity of the cave, specimen-based rarefaction demonstrates that our assemblage captures most of the mammalian diversity expected in the area. This is particularly apparent for carnivorans and soricids, which are particularly well-represented in the Boomerang Cave assemblage, with the former clade represented by at least nine taxa. This high level of diversity can be attributed to the relatively random nature of natural trap cave deposition, reducing accumulation biases due to size or diet. We also record the first occurrence of Merriam’s Shrew (Sorex merriami) from the Bear River Range. Our analysis does not indicate any mammalian changes between late Holocene and present-day communities, but these data establish a new zoological baseline for an alpine community at the interface between two key biogeographic provinces in western North America. Our work highlights the value of collecting skeletal remains from cave assemblages as a convenient and fast method for censusing terrestrial mammalian communities.

Ichthyosaurs achieved their maximum size during the Late Triassic Period. The largest described genus, Shonisaurus, appeared in the latest Carnian and persisted into the middle Norian, though the exact timing of its extinction is not completely clear. In Europe, a series of recent publications have described disarticulated material that demonstrates that giant shastasaurid ichthyosaurs continued into the Rhaetian, and suggests that, at least in the Tethys, large-bodied ichthyosaurs may have persisted up to the end-Triassic extinction (ETE). This material implies that the absence of late Norian and Rhaetian occurrences in North America may result from sampling bias rather than a true ecological signal. A recent discovery in New York Canyon (NYC) in the Gabbs Valley Range of Nevada, U.S.A., provides clear evidence of the persistence of giant ichthyosaurs in Panthalassa during the Rhaetian. Previous studies have reported isolated ichthyosaur elements from NYC, but these have never been adequately studied or described. Here we report new in situ vertebrate fossils from the late Rhaetian of the Gabbs Formation in NYC. The specimen comprises at least 17 semi-articulated ribs and two centra from a giant ichthyosaur, comparable in size and shape to the largest known examples of Shonisaurus. NYC is a well-studied fossiliferous marine reference section for the ETE and Triassic-Jurassic boundary with excellent ammonite biostratigraphic and δ13Corg geochemical controls. The bone-bearing horizon falls within the latest Rhaetian ammonite biozone for Panthalassa, the Choristoceras crikmayi Zone, and is just 1.7 meters below the negative δ13Corg excursion that marks the beginning of the ETE in this section. This specimen is the youngest shastasaurid ichthyosaur and indicates that these giant ichthyosaurs did not go extinct during the Norian in Panthalassa. Instead, they likely persisted until the ETE, perishing as a casualty of the mass extinction event.

Two recently discovered vertebrae collections from the Gabbs Formation in New York Canyon, Nevada, USA are among the first Late Triassic pistosauroid fossils reported from North America. Pistosauroidea were a group of long-necked secondarily aquatic reptiles that belong to the clade sauropterygia. Pistosauroids first evolved in the Triassic Period but later became an integral part of the Mesozoic marine ecosystem as the iconic plesiosaurs during the Jurassic and Cretaceous Periods. Our findings include a single small centrum from near the Triassic/Jurassic boundary and a block of similarly sized associated vertebrae from the early Rhaetian. The vertebrae exhibit a uniquely pistosauroid external morphology. The centrum is antero-posteriorly narrow but dorso-ventrally tall with gently amphicoelous faces. The histology revealed through micro-CT scanning, is also diagnostically pistosauroid. In sagittal cross-section, a denser layer of bone is visible along the faces of the centrum, while a unique V-shaped texture extending from the base of the neural canal is visible in transverse cross-section. The New York Canyon locality has long been renowned as a reference section for the Late Triassic and Triassic/Jurassic boundary but has only recently become a focus for vertebrate research. The Gabbs Formation at this locality is a relatively shallow marine environment and ranges from mid ramp to inner ramp. Vertebrate material has been noted throughout New York Canyon, nearly all of which has been identified as ichthyosaur, but these discoveries have shown the potential for the area as an important site for Late Triassic sauropterygians which have a poor record globally and were previously unknown from North America. Despite the limited nature of this new material, it is significant in providing evidence of the presence of pistosauroids in the Late Triassic of Eastern Panthalassa and helps fill in the exceptionally sparse history of sauropterygians in the Triassic of cordilleran North America.

The femora of diapsids have undergone morphological changes related to shifts in postural and locomotor modes, such as the transition from plesiomorphic amniote and diapsid taxa to the apomorphic conditions related to a more erect posture within Archosauriformes. One remarkable clade of Triassic diapsids is the chameleon-like Drepanosauromorpha. This group is known from numerous articulated but heavily compressed skeletons that have the potential to further inform early reptile femoral evolution. For the first time, we describe the three-dimensional osteology of the femora of Drepanosauromorpha, based on undistorted fossils from the Upper Triassic Chinle Formation and Dockum Group of North America. We identify apomorphies and a combination of character states that link these femora to those in crushed specimens of drepanosauromorphs and compare our sample with a range of amniote taxa. Several characteristics of drepanosauromorph femora, including a hemispherical proximal articular surface, prominent asymmetry in the proximodistal length of the tibial condyles, and a deep intercondylar sulcus, are plesiomorphies shared with early diapsids. The femora contrast with those of most diapsids in lacking a crest-like, distally tapering internal trochanter. They bear a ventrolaterally positioned tuberosity on the femoral shaft, resembling the fourth trochanter in Archosauriformes. The reduction of an internal trochanter parallels independent reductions in therapsids and archosauriforms. The presence of a ventrolaterally positioned trochanter is also similar to that of chameleonid squamates. Collectively, these features demonstrate a unique femoral morphology for drepanosauromorphs, and suggest an increased capacity for femoral adduction and protraction relative to most other Permo-Triassic diapsids.