Ben Creisler
Dino-related papers:
Insect cocoons comprise a depauperate Celliforma ichnofacies at Egg Mountain.
Pervasive traces and lack of bedding imply relatively slow, continuous deposition.
Insect traces throughout the section suggest persistent, workable soil conditions.
Well-drained, workable soils may explain the abundance of vertebrate nesting.
Egg Mountain offers unique view of a Cretaceous terrestrial ecosystem.
Abstract
We describe the diversity and abundance of insect (specifically hymenopterans and coleopterans) pupation structures in the Upper Cretaceous (Campanian) Two Medicine Formation at the Egg Mountain locality, western Montana, U.S.A., an important dinosaur nesting site. The study interval comprises a massive calcareous siltstone and indurated silty limestone horizons interpreted as the product of cumulative paleosols. A 7âm by 11âm area was quarried with a jackhammer at intervals of 12.5âcm thickness for a 1.5âm thick stratigraphic section. The ichnoassemblage comprises four morphotypes (small, medium, large, and wide) assigned to Fictovichnus sciuttoi, of which three represent wasp (hymenopteran) cocoons while the fourth (wide) type potentially was produced by a coleopteran. Medium and small F. sciuttoi are dominant while large and wide Fictovichnus are less common and absent in some sample intervals. Other probable insect traces include partial perforations in cocoons (Tombownichnus), isolated burrows, and an enigmatic hemispherical trace. Material is representative of a depauperate Celliforma ichnofacies. Pervasive cocoons and other traces throughout the sequence suggest persistent soil conditions suitable for insect nesting and pupation, and suggest an absence of sediment pulses of sufficient thickness to prohibit thorough colonization. Peaks in pupation chamber abundance may reflect episodes of reduced sedimentation rates otherwise unseen in the absence of primary bedding structures. Well-drained and friable soil conditions favorable for insect nesting also may help explain the abundance of dinosaur nests and other vertebrate nesting events in associated strata as well as the presence of small terrestrial forms.
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Paleoclimatic record from the Prince Creek Formation serves as high-latitude evidence for greenhouse conditions.
Î18O of meteoric water calculated from bentonitic smectite is ~â23.0â, indicating highly 18O-depleted precipitation.
Precipitation and temperature data suggest a warmer and moister greenhouse Earth between 72 and 69âMa.
Abstract
The Upper Cretaceous (Maastrichtian) Prince Creek Formation of Arctic Alaska (~82â85ÂN paleolatitude) preserves successions of alluvial paleosols that are used to estimate mean annual precipitation (MAP), mean annual temperature (MAT) and meteoric water oxygen isotope composition. For the Prince Creek Formation, the highest MAP estimates range between 1000 and 3900âmm/yr and the lowest range between 350 and 1200âmm/yr. The precipitation variability, derived from stable carbon isotope analysis, occurs on a time scale of 104âyears. MAP values agree with previous interpretations from fossil pollen, pedological features, soil types, vegetation composition, sedimentological indicators and CLAMP estimates that suggest high precipitation amounts and high humidity. Despite the considerable uncertainty with MAT proxies, our values are consistent with warm month mean temperatures obtained previously from paleobotanical data. The Î18O of smectite from bentonite beds clusters around ~+5.0â (VSMOW). The Î18O of meteoric water calculated from bentonitic smectite is ~â23â (VSMOW), assuming a mean annual temperature of 6.3âÂC, which is slightly more 18O-depleted than meteoric water calculated from pedogenic siderite in stratigraphically younger beds closer to the Brooks Range. Chronostratigraphic correlation with the lower Cantwell Formation, Denali National Park, Alaska (~64ÂN paleolatitude), suggests higher precipitation rates and highly 18O-depleted precipitation in the Late Cretaceous paleo-Arctic compared to central Alaska. These data are consistent with previous studies that suggest a warmer and moister greenhouse Earth and an intensified hydrological cycle that enhanced latent heat transport, resulting in increased rainout effects.
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