Joseph H. Hartman

Stratigraphic Analysis of Upper Cretaceous Rocks in the Mahajanga Basin, Northwestern Madagascar: Implications for Ancient and Modern Faunas.

Upper Cretaceous strata of the Mahajanga Basin, northwestern Madagascar, yield some of the most significant and exquisitely preserved vertebrate fossils known from Gondwana. The sedimentology of these strata and their stratigraphic relations have been the focus of renewed geological investigations during the course of five expeditions since 1993. We here designate stratotypes and formalize the terrestrial Maevarano Formation, with three new members (Masorobe, Anembalemba, Miadana), and the overlying marine Berivotra Formation. The Maevarano Formation accumulated on a broad, semiarid alluvial plain bounded to the southeast by crystalline highlands and to the northwest by the Mozambique Channel. The Berivotra Formation was deposited in an open marine setting that evolved from a clastic‐ to a carbonate‐dominated shelf, resulting in deposition of the overlying Betsiboka limestone of Danian age. New stratigraphic data clearly indicate that the Maevarano Formation correlates, at least in part, with the Maastrichtian Berivotra Formation, and this in turn indicates that the most fossiliferous portions of the Maevarano Formation are Maastrichtian in age, rather than Campanian as previously reported. This revised age for the Maevarano vertebrate assemblage indicates that it is approximately contemporaneous with the vertebrate fauna recovered from the Deccan basalt volcano‐sedimentary sequence of India. The comparable age of these two faunas is significant because the faunas appear to be more similar to one another than either is to those from any other major Gondwanan landmass. The revised age of the Maevarano Formation, when considered in the light of our recent fossil discoveries, further indicates that the ancestral stocks of Madagascars overwhelmingly endemic modern vertebrate fauna arrived on the island in post‐Mesozoic times. The basal stocks of the modern vertebrate fauna are conspicuously absent in the Maevarano Formation. Finally, the revised age of the Maevarano Formation serves to expand our global perspective on the K/T event by clarifying the age of a diverse, and arguably the best preserved, sample of Gondwanan vertebrates from the terminal Cretaceous.

Cretaceous Extinctions: Multiple Causes

![Figure][1] Deccan plateau basalts. Lava from Deccan volcanism formed distinct layering. CREDIT: GSFC/NASA In the Review “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene boundary” (P. Schulte et al. , 5 March, p. [1214][2]), the terminal Cretaceous

Evidence for marine influence on a low-gradient coastal plain: Ichnology and invertebrate paleontology of the lower Tongue River Member (Fort Union Formation, middle Paleocene), western Williston Basin, U.S.A.

The Paleocene Tongue River Member of the Fort Union Formation contains trace-fossil associations indicative of marine influence in otherwise freshwater facies. The identified ichnogenera include: Arenicolites, Diplocraterion, Monocraterion, Ophiomorpha, Rhizocorallium, Skolithos linearis, Teichichnus, Thalassinoides , and one form of uncertain affinity. Two species of the marine diatom Coscinodiscus occur a few meters above the base of the member. The burrows occur in at least five discrete, thin, rippled, fine-grained sandstone beds within the lower 85 m of the member west of the Cedar Creek anticline (CCA) in the Signal Butte, Terry Badlands, and Pine Hills areas. Two discrete burrowed beds are found in the lower 10 m of the member east of the CCA in the little Missouri River area. Abundant freshwater ostracodes include Bisulcocypridea arvadensis, Candona, and Cypridopsis . Freshwater bivalves include Plesielliptio and Pachydon mactriformis . We recognize four fossil assemblages that represent fluvio-lacustrine, proximal estuarine, central estuarine, and distal estuarine environments. Biostratal alternations between fresh- and brackish-water assemblages indicate that the Tongue River Member was deposited along a low-gradient coastal plain that was repeatedly inundated from the east by the Cannonball Sea. The existence of marine-influenced beds in the Tongue River Member invalidates the basis for the Slope Formation.

Environmental change across a terrestrial Cretaceous-Paleogene boundary section in eastern Montana, USA, constrained by carbonate clumped isotope paleothermometry

The Cretaceous-Paleogene (K-Pg) mass extinction has been attributed to the impact of a large bolide at the end of the Cretaceous Period, although other potential causes have also been proposed, most notably climate change caused by Deccan Traps (India) flood volcanism. Reconstructing paleoclimate, particularly in terrestrial settings, has been hindered by a lack of reliable proxies. The recent development of carbonate clumped isotope paleothermometry has contributed to temperature reconstructions using geochemical proxies in terrestrial settings. We employ this method, along with new stratigraphic constraints, in the Hell Creek (Cretaceous) and overlying Fort Union (Paleogene) Formations (Montana, USA) to examine changes in temperature leading to and across the K-Pg boundary. We demonstrate that well-preserved ca. 66 Ma aragonitic bivalves serve as suitable paleoclimate archives. Although there are limitations in the stratigraphic availability of fossil bivalves for clumped isotope analysis, we record an apparent 8 °C decrease in summer temperatures over the last 300 k.y. of the Cretaceous that corresponds broadly with patterns observed in other paleotemperature proxies. This observed decrease plausibly could be explained by an absolute temperature decrease or by other environmental stresses on the organisms, but in either case suggests changing living conditions over the interval. Previously documented declines in vertebrate and invertebrate biodiversity occur over the same stratigraphic interval at this study location. These results are consistent with published models of the end-Cretaceous mass extinction in which destabilized ecosystems become more susceptible to an abrupt event like a bolide impact.

A phylogenetic and morphologic context for the radiation of an endemic fauna in a long-lived lake: Corbulidae (Bivalvia; Myoida) in the Miocene Pebas Formation of western Amazonia

Abstract The Corbulidae are one of a handful of a primarily marine bivalve clades that exhibit a remarkable radiation, marked by increased species richness and divergent morphologies, within a long-lived lake. For corbulids, this diversification occurred within the lower to middle Miocene Pebas Formation of western Amazonia. Only one taxon associated with this radiation (Anticorbula) remains extant. We conducted a series of phylogenetic analyses to characterize diversification of Corbulidae within the Pebas Formation and relate that diversification to geologically older freshwater corbulids from the Paleocene Fort Union Formation of the northern Great Plains (United States). We used these results, as well as a quantitative examination of morphospace occupation, to infer whether Pebasian corbulids represent a true species flock, and whether the lacustrine system represented by the Pebas Formation represents a cradle of, or reservoir for, freshwater corbulid diversity. We conducted two sets of phylogenetic analyses using shell morphology characters. A genus-level data set incorporated type species of freshwater corbulid genera, any Paleocene representatives of these genera, and selected brackish and marine corbulid genera. A species-level analysis added all described freshwater corbulid taxa to the genus-level matrix. Our results were highly resolved (few most-parsimonious trees), but not particularly robust (low branch support). For the genus-level matrix, we used a taxon jackknife procedure to explore the effects of taxon sampling on tree stability and topology. Jackknife results recover a subclade of freshwater taxa (including both Anticorbula and Pachydon species and the Paleocene Ostomya sp.) in 92.4% of trees, although placement of this subclade across the ingroup varies, as do the topologic positions of other freshwater species. Freshwater and marine corbulids also are morphologically distinct from each other, a factor that likely reduced the robustness of our phylogenetic results. By combining these results with paleoecologic, stratigraphic, and morphologic data, we infer that freshwater corbulids arose once within the family, prior to the Cenozoic, with three distinct freshwater lineages present at their first appearance in the late Paleocene of North America. Within the Miocene Pebas system of South America, we reconstruct supralimital morphologic evolution within three lineages as freshwater taxa became variously adapted to the fluid, dysoxic muds characterizing lake-bottom facies representative of the Pebas lacustrine system. In addition, corbulids apparently successfully coped with high predation pressures from co-occurring shell-crushing predators. Finally, we consider that freshwater Corbulidae were primarily fluvial taxa throughout their geologic history, with a relatively ephemeral radiation within the Pebasian lake system, thus making the Pebasian system a cradle of diversity for several corbulid lineages.

Unconformities and age relationships, Tongue River and older members of the Fort Union Formation (Paleocene), western Williston Basin, U.S.A.

An unconformable relationship is observed within the Paleocene Fort Union Formation in the western Williston Basin at the contact between the Tongue River Member and the underlying Lebo and Ludlow Members. Isotopic dates and pollen biozone data reported here are integrated with previously published data. A new correlation of these facies results in a revised history of localized depositional and tectonic events. One unconformity occurs at this lithological contact in the Pine Hills (PH), Terry Badlands (TB), and Ekalaka (E) areas west of the Cedar Creek anticline (CCA), and another unconformity occurs at the same lithological contact in the Little Missouri River (LMR) area east of the CCA. The two unconformities differ in age by about two million years. The older is the U 2 and the younger is the U 3 , which initially were recognized in the Ekalaka area of southeastern Montana (Belt et al., 2002). The U 2 crops out in the TB, PH, and E areas, where at least 85 m of Tongue River strata bearing palynomorphs characteristic of biozone P-3 are found above the unconformity. Radiometric dates from strata (bearing palynomorphs characteristic of biozone P-2) below the U 2 range in age from 64.0 to 64.73 Ma. The U 2 unconformity west of the CCA thus occurs in strata near the base of the lower P-3 biozone. The U 3 crops out in the LMR area (east of the CCA), where only 13 m of strata characterized by the P-3 pollen biozone occur above it. Radiometric dates from an ash <1 m above the U 3 in that area range in age from 61.03 to 61.23 Ma, and the P-3/P-4 pollen biozone boundary is located 13 m above the ashes. The U 3 thus occurs in strata characterized by upper parts of the P-3 pollen biozone east of the CCA. The U 3 is also identifiable in the middle of the ca. 200 m-thick Tongue River Member west of the CCA, where mammal sites 40 to 80 m above it are Tiffanian-3 in age. The strata below this unconformity are tilted gently to the northwest; strata above the unconformity are flat lying. This mid Tongue River unconformity probably correlates with the unconformity at the base of the Tongue River Member in the LMR area east of the CCA, where a Ti-2 mammal site (the “X-X” locality) occurs <10 m above it. Depositional and tectonic events can be summarized using North American Mammal Age nomenclature as a relative time scale. From latest Cretaceous through Puercan time, paleodrainage was toward the east or southeast, in the direction of the Cannonball Sea. The Black Hills did not serve as an obstruction at that time. During early Torrejonian time, the Miles City arch (MCA) and Black Hills were uplifted and partially eroded, leading to the U 2 unconformity. When deposition resumed, paleodrainages shifted to a northeasterly course. During middle and late Torrejonian time, facies of the lower Tongue River (“Dominy”) sequence and the Ekalaka Member of the Fort Union Formation were

Paleogene rim gravel of Arizona: Age and significance of the Music Mountain Formation

The early Tertiary exotic rim gravel scattered across the Colorado Plateau in Arizona (USA) provides the only widespread evidence concerning the nature of the regional Paleogene drainage system that preceded the emergence of the modern Colorado River. The term “rim gravel” includes a wide range of Laramide (herein ca. 85–40 Ma) and younger reworked quartzite-dominated, arkosic sediments with diverse origins and ages that have been clarified only recently. The parent arkoses, with subordinate gravel lenses, contain volcanic clasts with ages ranging from Late Jurassic to early Eocene. The Laramide-age arkoses were shed off uplifted Precambrian terranes to the south and west; they are separated from their reworked derivatives and younger sediments by a disconformity that is best preserved in the structurally isolated paleocanyons of the Hualapai Plateau. Younger generations of reworked gravels continued to evolve during and following the widespread eruption of basalts of late Oligocene to Pleistocene age. Basaltic clasts in the reworked gravels attest to their much younger ages. Combined paleontologic, stratigraphic, paleomagnetic, K-Ar, U-Th/He, and zircon studies of the parent arkoses indicate that the tectonic framework corresponds to the Laramide orogeny, the concurrent regional erosion of the Colorado Plateau margin, and ensuing widespread deposition. The overall time frame coincides with the similar tectonic history recorded in the early Tertiary strata of southern Utah. Comparable but unrelated quartzite-dominated gravels, reworked southward from the Utah source rocks, are widespread on strath terraces north of the Colorado River. The Utah-derived gravels are products of the much younger Neogene incision of the modern Grand Canyon drainage system. The name Music Mountain Formation redefines the Laramide-age suite of the Arizona rim gravel parent sediments, best preserved on the Hualapai and Coconino Plateaus of northern Arizona, and distinguishes them from younger reworked gravels occurring both north and south of the Colorado River. A lacustrine mollusk assemblage collected from thin limestones intertonguing with Music Mountain arkose constrains the age of one Coconino Plateau exposure to early Eocene, whereas the association with the Laramide orogeny indicates that a Late Cretaceous to middle Eocene time frame comprises the broader geologic setting.

Pushing the record of trematode parasitism of bivalves upstream and back to the Cretaceous

The Judith River Formation of Montana, USA, renowned for its preservation of Late Cretaceous dinosaurs, now yields the oldest-known evidence of trematode parasitism of bivalves. Highly distinctive igloo-shaped traces found on shells of the freshwater bivalve Sphaerium are virtually identical to igloo-shaped traces known from living marine bivalves infected by metacercaria larvae of gymnophallid trematodes (flatworms). This unique record of paleoparasitism provides key insights into the evolution of an important parasite group, reveals the inner workings of cryptic ecological associations, and enriches our understanding of ancient food webs. Our discovery extends the record of trematode–bivalve interaction back to the Late Cretaceous (ca. 76 Ma), and indicates that this parasite–host relationship was established in freshwater ecosystems much earlier than previously surmised. The complex multi-host lifecycles of modern trematodes and the general stability of parasite–host associations suggest that sphaeriid bivalves in the Judith River record likely served as the second intermediate host. Potential candidates for the definitive host range from molluscivorous fish to birds and non-avian predatory dinosaurs. With the history of trematode–bivalve interaction pushed back to the Late Cretaceous, patterns in trematode infection can now be interrogated across major episodes of global change, including the Cretaceous-Paleogene mass extinction and the Paleocene-Eocene thermal maximum.