Calibrating the zenith of dinosaur diversity in the Campanian of the Western Interior Basin by CA-ID-TIMS U-Pb geochronology

Abstract

The spectacular fossil fauna and flora preserved in the Upper Cretaceous terrestrial strata of North America's Western Interior Basin record an exceptional peak in the diversification of fossil vertebrates in the Campanian, which has been termed the 'zenith of dinosaur diversity'. The wide latitudinal distribution of rocks and fossils that represent this episode, spanning from northern Mexico to the northern slopes of Alaska, provides a unique opportunity to gain insights into dinosaur paleoecology and to address outstanding questions regarding faunal provinciality in connection to paleogeography and climate. Whereas reliable basin-wide correlations are fundamental to investigations of this sort, three decades of radioisotope geochronology of various vintages and limited compatibility has complicated correlation of distant fossil-bearing successions and given rise to contradictory paleobiogeographic and evolutionary hypotheses. Here we present new U-Pb geochronology by the CA-ID-TIMS method for 16 stratigraphically well constrained bentonite beds, ranging in age from 82.419 ± 0.074 Ma to 73.496 ± 0.039 Ma (2σ internal uncertainties), and the resulting Bayesian age models for six key fossil-bearing formations over a 1600 km latitudinal distance from northwest New Mexico, USA to southern Alberta, Canada. Our high-resolution chronostratigraphic framework for the upper Campanian of the Western Interior Basin reveals that despite their contrasting depositional settings and basin evolution histories, significant age overlap exists between the main fossil-bearing intervals of the Kaiparowits Formation (southern Utah), Judith River Formation (central Montana), Two Medicine Formation (western Montana) and Dinosaur Park Formation (southern Alberta). Pending more extensive paleontologic collecting that would allow more rigorous faunal analyses, our results support a first-order connection between paleoecologic and fossil diversities and help overcome the chronostratigraphic ambiguities that have impeded the testing of proposed models of latitudinal provinciality of dinosaur taxa during the Campanian.

Figure 1

Paleogeographic reconstruction of the North American continent in the late Campanian illustrating the areal expanse of the Western Interior Seaway separating the Laramidia and Appalachia landmasses, after Blakey. Our study locations are marked by stars. Potential centers of coeval arc volcanism are also indicated. Source map © 2022 Colorado Plateau Geosystems Inc.

Figure 2

Correlation chart of rock formations and principal vertebrate-bearing continental deposits of Campanian age in the Western Interior Basin, modified from Roberts et al. with new data^,, and the results of this study. Shown in red are the calibrated temporal ranges of principal fossil-bearing intervals (excluding uncertainties). Fm, Formation; Mbr, Member; NALM, North American Land Mammal; Sh, shale; Sst, sandstone.

Figure 3

Ranked age plot of analyzed zircons from Campanian ash beds of the Western Interior Basin. (A) Dinosaur Provincial Park, southern Alberta, Canada; (B) Two Medicine and Judith River formations, Montana; (C) Kaiparowits Formation, southern Utah; (D) Kirtland and Fruitland formation, San Juan Basin, New Mexico. Vertical bars are individual zircon analyses with their 2σ analytical uncertainty; black bars are analyses used in age calculation. Arrows represent the analyses that plot outside the diagram. Blue band signifies the weighted mean age with its 95% confidence level (2σ) internal uncertainty (X). See Table 1 for bentonite and age information, and Supplementary Table S2 for complete U–Pb isotopic data. JC, Jackson Coulee; LCZ, Lethbridge Coal Zone; PPF, Power Plant Ferry; SF, Stafford Ferry.

Figure 4

Lithostratigraphy, U–Pb geochronology and Bayesian age-stratigraphic models for the Dinosaur Park (top) and Kaiparowits (middle) formations. Dinosaur Park and Kaiparowits stratigraphy after Eberth and Hamblin and Roberts et al., respectively. Red shading corresponds to the principal fossil-bearing intervals and their model ages, dotted lines incorporate the model age uncertainties. Bottom panel illustrates the interformational correlation of fossil intervals based on this study. ^*Boundary age from Beveridge et al..

Figure 5

Lithostratigraphy (after Rogers et al.), U–Pb geochronology and Bayesian age-stratigraphic model for the Judith River Formation in its type area. Pm Ss Mbr, Parkman Sandstone Member. Red shading corresponds to the principal fossil-bearing interval and its model age, dotted lines incorporate the model age uncertainties. Note that the single U–Pb age from Kennedy Coulee (KC051517-1) has been projected onto the type stratigraphy (see text for explanation).

Figure 6

Temporal correlations among five principal Campanian fossil-bearing formations of the Western Interior Basin, based on the age models of this study. Stratigraphic columns are drawn to the same scale. Black is coal bed. Red dots mark ash beds with U–Pb ages connected by color-coded temporal correlation lines; dashed lines signify age projection in the absence of a quantitative age-stratigraphic model. Lithostratigraphy after: (1) Eberth and Hamblin, (2) Rogers, (3) Rogers et al., (4) Roberts et al., (5) Fassett and Heizler, Sullivan and Lucas.