William A. Clemens

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

Dinosaurs on the North Slope, Alaska: High Latitude, Latest Cretaceous Environments

Abundant skeletal remains demonstrate that lambeosaurine hadrosaurid, tyrannosaurid, and troodontid dinosaurs lived on the Alaskan North Slope during late Campanian—early Maestrichtian time (about 66 to 76 million years ago) in a deltaic environment dominated by herbaceous vegetation. The high ground terrestrial plant community was a mild- to cold-temperate forest composed of coniferous and broad leaf trees. The high paleolatitude (about 70� to 85� North) implies extreme seasonal variation in solar insolation, temperature, and herbivore food supply. Great distances of migration to contemporaneous evergreen floras and the presence of both juvenile and adult hadrosaurs suggest that they remained at high latitudes year-round. This challenges the hypothesis that short-term periods of darkness and temperature decrease resulting from a bolide impact caused dinosaurian extinction.

Paleoecological implications of Alaskan terrestrial vertebrate fauna in latest Cretaceous time at high paleolatitudes

The latest Cretaceous cool temperate environment of the Alaskan North Slope included dinosaurs (some species represented by both juveniles and adults) and mammals, all probably endothermic, and freshwater fish among its terrestrial vertebrate fauna. No traces have been found of amphibians or nondinosaurian reptiles, ectothermic vertebrates that are abundantly represented in approximately contemporaneous faunas of North American middle paleolatitudes. A geologically brief period of intense cold and darkness has been hypothesized as the primary cause of terminal Cretaceous extinctions. However, the extinction of the lineages of Alaskan dinosaurs and mammals, in contrast to the survival of most lineages of amphibians and nondinosaurian reptiles into the Tertiary, contradicts this hypothesis.

Upper Cretaceous–Paleocene biostratigraphy and magnetostratigraphy, Hell Creek and Tullock Formations, northeastern Montana

Fossils from the Hell Creek and Tullock Formations in northeastern Montana provide detailed documentation of terrestrial faunal and floral evolution during latest Cretaceous (Lancian) and early Paleocene (Puercan) time. Here the replacement of Lancian faunas by those of Puercan age, most obviously signaled by the extinction of dinosaurs, and the changes in pollen floras sometimes used to mark the Cretaceous-Tertiary boundary occurred during a period of reversed magnetic polarity. Paleontological correlations suggest that dinosaur extinction and the change in pollen floras took place in the Red Deer Valley area, Alberta, during the same period of reversed polarity. Furthermore, also on the basis of paleontological correlations, the extinction of dinosaurs in the San Juan Basin, New Mexico, appears to have occurred either during the same period of reversed polarity or, possibly, during the preceding period of normal polarity.

Purgatorius, an Early Paromomyid Primate (Mammalia)

Fragmentary mandibles of Purgatorius unio Van Valen and Sloan from the Puercan (approximately early Paleocene), Garbani Locality, Montana, preserve associated postcanines. Their morphology indicates that this mammal was an early paromomyid primate and suggests that primate ancestry does not include currently known members of the palaeoryctid and leptictid Insectivora or of the Condylarthra.

Out with a whimper not a bang

The recurring debate over the causes of the massive extinctions of such groups as the dinosaurs, ammonites, and calcareous nannoplankton, and the crash of the distinctive Aquilapollenites pollen province, used by biostratigraphers to mark the close of the Cretaceous, has been considerably enlivened with the resurgence of hypotheses suggesting catastrophic events. The impact of an asteroid, explosion of a supernova, sudden changes in oceanic circulation and composition, or atmospheric perturbations, to mention a few, have been put forward as malefactors. But, just how sound is the evidence of a biotic catastrophe?

Mesozoic continental vertebrates with associated palynostratigraphic dates from the northwestern Ethiopian plateau

The East African Rift separates the northwestern and southeastern Ethiopian high plateaus, which are capped by massive Cenozoic volcanics overlying thick deposits of marine and nonmarine Mesozoic sediments. During geological mapping projects of the 1920s-1930s, a few Mesozoic vertebrate fossils were found on the southeastern plateau. in contrast, beginning in 1976, and then from 1993 to the present, paleontological field work in the Abay (Blue Nile) River gorge along the eastern edge of the northwestern plateau resulted in the discovery of fossil chondrichthyans (Priohybodus, Hybodus, Rhinobatos), osteichthyans (Lepidotes, cf. Pycnodus), dipnoans (Asiatoceratodus), chelonians (Pelomedusidae, Plesiochelyidae, Pleurosternidae), crocodylians (Goniopholis), dinosaurs, (cf. Acrocanthosaurus Hypsilophodontidae), pollen and other microfossils documenting a coastal biota in part, if not entirely, of latest Jurassic (Tithonian) age. These fossils include new biogeographic records for Africa and document biostratigraphic range extensions. The Ethiopian Mesozoic fauna adds to the growing evidence of limited interchange of vertebrates between Africa and Western Europe during the transition from the Jurassic into the Cretaceous.

THE SMALLEST KNOWN TRICERATOPS SKULL: NEW OBSERVATIONS ON CERATOPSID CRANIAL ANATOMY AND ONTOGENY

Abstract The discovery of the smallest Triceratops skull (UCMP 154452) provides a new ontogenetic end member for the earliest stage of ceratopsid (Centrosaurinae plus Chasmosaurinae) cranial development. The lack of co-ossification among the parietal, squamosals, postorbitals, quadratojugal arch, and the braincase preserves sutural contacts and bone surfaces that later become obscured in adults. The ability to document the early development and morphology of the horns and frill in Triceratops allows a reevaluation of their functional roles. UCMP 154452 shows that the cranial ornamentation of the frill and the postorbital horns were not restricted to adults, but began at an early age in this species. This evidence supports the hypothesis that the function of ceratopsid horns and frills was potentially important for visual communication and species recognition because in this young form it could not have functioned in sexual display. Although some features of UCMP 154452 anticipate or mimic the adult character states, some braincase characters recapitulate the juvenile and adult stages of more basal neoceratopsians.

A probable pseudo-tribosphenic upper molar from the Late Jurassic of China and the early radiation of the Holotheria

ABSTRACT A three-cusped, mammalian upper molar was recovered from the type locality of Shuotherium dongi Chow and Rich, 1982 in the Late Jurassic, Upper Shaximiao Formation Sichuan, China. This tooth is too large to be referable to S. dongi, which is known only from a fragment of a left mandible with teeth. Shared, derived (possibly autapomorphic), functionally related characters of the reconstructed dentition and occurrence at the same locality suggest that the new specimen probably is an upper molar of a closely related species, here named Shuotherium shilongi, sp. nov. Crown morphology and development of wear facets on the new specimen accord with the reconstruction of occlusal relationships of pseudo-tribosphenic molars proposed by Chow and Rich (1982). An alternative, less probable, interpretation is that the new specimen is an upper molar of a tribosphenic mammal and indicates a greater antiquity and diversity of the Tribosphenida than previously thought. This uncertainty highlights the problems in ...

High-resolution chronostratigraphy of the terrestrial Cretaceous-Paleogene transition and recovery interval in the Hell Creek region, Montana

Detailed understanding of ecosystem decline and recovery attending the Cretaceous-Paleogene boundary (KPB) mass extinctions is hindered by limited constraints on the pace and tempo of environmental events near the boundary. To mitigate this shortcoming, high-resolution 40Ar/39Ar geochronology was performed on tephras intercalated between fossiliferous terrestrial sediments of the upper Hell Creek and lower Fort Union Formations in the western Williston Basin of northeastern Montana (USA). Tephra samples were collected from 10 stratigraphic sections spanning an area of ∼5000 km2. Several distinctive tephras can be correlated between sections separated spatially by as much as ∼60 km. The tephras are thin distal deposits generally preserved only in lignite beds, which are interbedded with clastic deposits yielding vertebrate faunas of Lancian (late Maastrichtian) to Torrejonian (early Danian) North American Land Mammal Ages. Sanidine from 15 tephra samples was analyzed in 1649 total fusion experiments (1597 on single crystals) and 12 incremental heating analyses of multigrain aliquots. Ages were determined for 13 distinct tephras, ranging from 66.289 ± 0.051 to 64.866 ± 0.023 Ma, including only analytical uncertainties. This level of precision is sufficient to resolve the ages of all of the coal beds that have served as a basis for a regional stratigraphic framework. The data confirm that the Hell Creek–Fort Union formational contact is diachronous, and further support the age of the KPB impact layer at 66.043 ± 0.010 Ma (or ± 0.043 Ma considering systematic uncertainties). Application of the new results to previous magnetostratigraphic data indicates an appreciably compressed time interval between the base of chron C29r and the top of chron C28r, with a maximum duration estimate of 1.421 ± 0.066 Ma. Most notable is the implied brevity of chron C29r, with a maximum estimate of 457 ± 54 ka, and possibly as brief as 345 ± 38 ka, compared to the 710 ka estimate from the Geologic Time Scale 2012 (GTS2012). Further, application of new results to terrestrial biostratigraphy adds higher precision to the timing and tempo of biotic change before and after the KPB. Our results indicate that the timing of pre-KPB ecological decline is constrained to the last ∼200 ka of the Cretaceous, adding further support to the press-pulse extinction hypothesis. Additionally, the duration of the depauperate basal Paleogene Puercan 1 disaster fauna is confined to a 70 ka interval. Faunal recovery in this region, indicated by the appearance of primitive members of the placental mammal radiation and the restoration of taxonomic richness and evenness, occurred within ∼900 ka after the KPB. These results show that biotic recovery after the mass extinction in the terrestrial realm was more rapid than in the marine.