San Juan Basin, New Mexico. Photo credit: Daniel J. Pepi
Asteroids, as Neil deGrasse Tyson explains, are ancient remnants from our early solar system, drifting through space, potential carriers of the ingredients of life or agents of apocalyptic death.
In 1980, in a landmark lecture, American physicist Luis Alvarez declared, “Lucifer’s hammer killed the dinosaurs,” presenting geochemical evidence he and his son had uncovered for a massive asteroid strike at the end of the Cretaceous period. The very next year, the Mexican oil company Pemex identified the Chexalub crater on the Yucatan Peninsula as the site of a large asteroid impact. The impact released the energy equivalent of 100 teratons of TNT. Three-quarters of all plant and animal species on Earth disappeared, including non-avian dinosaurs, pterosaurs, marine reptiles, ammonites and planktonic foraminifera.
Paleogeography of North America during the late Campanian stage of the Late Cretaceous (∼75 Ma). From Simpson et al., 2010
In recent decades, there has been a lively debate about whether the dinosaurs were in decline or were thriving until they were suddenly wiped out by the main pulse of the K/PG extinction, a sudden asteroid impact. This conflict is worsened by geographic bias, as most of the data comes from the Northern Hemisphere. The Nashobito Member (San Juan Basin, New Mexico) provides an important piece of the puzzle, capturing a snapshot of the continent’s last dinosaur communities and their diversity across the Cretaceous range.
The opening of the Western Interior Seaway (about 99.5 MYA) divided the North American continent into two distinct landmasses: Laramidia (a long, narrow strip of land from present-day Alaska to Mexico) and Appalachia (the eastern part of the continent). During the Campanian (83.6–72.1 million years ago), Laramidia had a high regional diversity, with northern and southern fauna diverging. In the Maastrichtian (72.1–66.0 Ma), that ecosystem became more homogeneous. The “synergy” of these ecosystems pushed the non-avian dinosaurs into a long decline, setting the stage for their eventual extinction. A new study provides a new look at the ecosystems dominated by the last surviving dinosaurs by presenting the Nachobiota member (contemporaneous with the Hell Creek fauna) to test whether there were changes in the faunal province during the Cretaceous Paleogene range.
Latest Cretaceous–early Paleogene terrestrial basins in western North America and their fauna. From Flynn et al., 2025
Ecological analysis shows that the Nashobito dinosaurs were extremely diverse, spanning many species, body sizes, and diets. This diversity is consistent with earlier periods and suggests that dinosaurs continued to thrive in New Mexico through the Cretaceous Paleogene. The new study further indicates that temperature was a key factor in the distribution of dinosaurs, with sauropods dominating warmer environments (southwestern North America), and hadrosaurs dominating cooler regions (the modern Great Plains). These findings emphasize the importance of abiotic factors in promoting eminence in dinosaur-dominated ecosystems prior to the Cretaceous-Pelagene extinction event.
References:
Flynn, A.G., Brosti, S.L., Chieranza, A.A., Garcia-Guerin, J., Davis, A.J., Fenley, C.W., Leslie, C.E., Secord, R., Shelley, S., Weil, A., Heisler, M.T., Williamson, T.E., & Pape, D.J. (2025). Late New Mexican dinosaur surveys illuminate high-end Cretaceous diversity and provinciality. Sciencefor , for , for , for , . 390(6771), 400–404. https://doi.org/10.1126/s ience- adw3282
TheDean, CD, Chatrenza, AA, Dozer, JW, Farnsworth, A, Jones, LA, Lester, SJ, Othwaite, CL, Valdes, PJ, Butler, RJ, & Menin, PD (2025). Structure of the Late Cretaceous Dinosaur Fossil Record in North America. Current Biology: CB. https://doi.org/10.1016/j.cub.2025.03.025
Simpson SD, Loewen MA, Frick AA, Roberts EM, Forster CA, Smith JA, et al. . PLoS One 5 (9): E12292. https://doi.org/10.1371/journal.pone.0012292
