Catastrophic Events Conference 3118.pdf

DEVASTATION OF TERRESTRIAL ECOSYSTEMS AT THE K-T BOUNDARY IN NORTH AMERICA: THE FIRST CALIBRATED RECORD OF PLANT AND ANIMAL RESPONSE TO THE CHICXULUB IMPACT. Kirk Johnson1, Douglas Nichols2, Conrad Labandeira3, and Dean Pearson4, 1Denver Museum of Natu- ral History, 2001 Colorado Blvd., Denver, Colorado 80205, [email protected], 2U.S. Geological Survey, MS 939, Box 25046 DFC, Denver, Colorado 80225, [email protected], 3Paleobiology Department, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, [email protected], 4Pioneer Trails Regional Museum, Bowman, ND 58623, [email protected].

Introduction: Extensive collection of leaves, many and corresponds approximately to the beginning of with insect damage; palynomorphs; and vertebrates magnetic polarity subchron C29R. This flora is sig- from the uppermost Cretaceous (upper Maastrichtian) nificantly more diverse than that of the lower floral and lowermost Paleocene (Puercan) formations in zones and is characterized by leaf physiognomy that western North America has resulted in a combined suggests a significant climate warming in the final database of unparalleled density. These data are de- 350,000 years of the Cretaceous. Floral extinction at rived from sites from New Mexico to Canada with a the K-T boundary is extensive, effectively eliminating strong focus on the excellent record of the northern all dominant plant taxa in non-mire facies. The over- Great Plains of the United States. These latter sites lying depauperate Paleocene flora, preserved in a vari- and samples have been correlated within a ety of facies, is dominated by taxa that were previously lithostratigraphic framework calibrated by magne- restricted to Cretaceous mires. We interpret this to be tostratigraphy, radiometric dates, and the geochemical a post-catastrophe recovery flora that spread onto the and physical indicators of the Cretaceous-Tertiary (K- early Paleocene landscape from mires and wetlands T) boundary. This integrated record documents major that, for unknown reasons, served as refugia during devastation of terrestrial ecosystems as a direct conse- the terminal Cretaceous event. Recent work on more quence of the Chicxulub impact. than 70 quarries spanning the K-T in the more south- Plants: Plant fossils are studied as megafossils or erly Denver Basin in Colorado is still in progress but palynomorphs. While both are derived from the same preliminary results also suggest major floral extirpa- original vegetation, they have different taphonomic tion at the K-T boundary. pathways and taxonomic resolution. We use both types The record based on palynology. Palynomorphs of fossils to take advantage of their relative strengths. from 23 K-T boundary or near-boundary sections in Leaf megafossils are suitable for extinction studies southwestern North Dakota indicate 30 percent ex- because of their high taxonomic resolution and low tinction at the boundary. The palynological database potential for reworking. Palynomorphs have great for this area includes 110 palynomorph taxa for which abundance and broad distribution allowing high stra- relative abundance or presence/absence data were re- tigraphic resolution and the recognition of continent- corded in more than 230 samples based on surveys of wide patterns. more than 450,000 specimens. With a single excep- The plant megafossil record. The most densely tion, extinction in this area occurred exclusively sampled K-T leaf flora in the world exists in our Great among angiosperm taxa. By their nature, palyno- Plains research area in the Bighorn, Powder River, morphs represent higher-level taxa, so this extinction and Williston Basins (parts of Wyoming, Montana, involved not just species but genera or families of and North and South Dakota) where more than 30,000 plants. Additional data originate from detailed inves- leaves from 306 localities demonstrate an 80 percent tigations in five other research areas in the western extinction across the K-T boundary. The 180-m-thick United States and adjacent southern Canada (27 addi- section that spans the K-T boundary (K = 100 m, T = tional sections). Palynostratigraphy at all these locali- 80 m) in southwestern North Dakota has yielded a ties is calibrated by the iridium anomaly and shocked diverse megaflora (based on 13,500 voucher speci- minerals. Results from these areas show similar effects mens) of more than 385 morphotypes from 171 quarry on palynofloras of somewhat differing compositions. sites. The uppermost Cretaceous flora is dominated by On a continent-wide basis (16 study areas; 225 paly- angiosperms, with ferns, fern allies, cycads, ginkgos, nomorph taxa), about 45 percent of the late Maas- and conifers representing less than ten percent of total trichtian palynoflora was destroyed by the terminal taxa and specimens. The Cretaceous sequence can be Cretaceous event. This regional database covers some divided into three distinct floral zones. The uppermost 5000 km from north to south; results reflect strong zone begins roughly 20 m below the K-T boundary variation with latest Maastrichtian and earliest Paleo- Catastrophic Events Conference 3118.pdf

DEVASTATION OF TERRESTRIAL ECOSYSTEMS: K. R. Johnson et al.

cene floras that was related to paleolatitude. Extinc- until pronounced vegetational shifts in the Western tion was varied within major groups of plants conti- Interior of North America during the Early Cenozoic nent-wide: 51 percent among angiosperms, 36 percent Thermal Maximum in latest Paleocene to early Eo- among gymnosperms, and 25 percent among ferns cene time. and fern allies. Because total continent-wide extinc- Vertebrates: Recent surveys of the uppermost tion levels are larger than local extinction levels, we Cretaceous Hell Creek Formation in southwestern suggest that locally endemic taxa were more affected North Dakota have yielded a total of 10,034 identified by the terminal Cretaceous event than were wide- vertebrate fossils from 42 sites and dinosaur skulls or spread taxa. Earliest Tertiary palynofloras are charac- skeletons from 41 additional sites. This Lancian fauna terized not by new appearances but by loss of the is composed of 67 taxa of fish (including sharks and dominant elements of the Cretaceous vegetation. rays), amphibians, lizards, turtles, crocodilians, Unique palynomorph assemblages (such as fern-spore champsosaurs, dinosaurs, pterosaurs, birds, and abundance anomalies) in close proximity to ejecta mammals. The 83 sites span the entire 100-meter deposits show regional effects of impact-related dev- thickness of the formation. Most vertebrate sites in the astation of terrestrial ecosystems. These short-lived lower part of the Hell Creek Formation are in channel plant communities consisted almost exclusively of deposits; those in the upper part of the formation are opportunistic survivor species of ferns in most areas more common in floodplain deposits. Dinosaurs are investigated. Repopulation of post-impact landscapes found in the highest vertebrate sites in the Hell Creek evidently was from local refugia. Formation and one ceratopsian partial skeleton was Insects: Previous studies have indicated no evi- found in the overlying Fort Union Formation (1.4 m dence for mass extinction among insects at K-T below the K-T boundary). Other significant finds in- boundary. These studies were based on global, family- clude pterosaur fossils 8.4 m below the boundary, and level compilations of body-fossil data that indicated no sharks within 4 m of the boundary. The former is the deviation from background rates of extinction. In world’s youngest occurrence and the latter suggests contrast, the palynologic and megafloral evidence that the Western Interior seaway was still present in cited above demonstrates a major extinction of plant the area near the end of the Cretaceous. Sample size, taxa at the K-T boundary. To test these opposite con- rather than stratigraphic position, appears to be the clusions, we examined insect-mediated plant damage primary factor that determines the presence of taxa at on 6,000 leaves in a succession of 78 of our megaflo- these sites. All dinosaur taxa that occur at more than ral localities in southwestern North Dakota from a two sites also occur at the highest well-sampled (>500 section that spans a 2.0 m.y. interval of the late specimens) site, 8.4 m below the K-T boundary. Dino- Maastrichtian Hell Creek Formation and earliest Pu- saurs also occur in the highest sites that preserve ver- ercan Ludlow Member of the Fort Union Formation. tebrate fossils. Common vertebrate taxa range All characterizable leaf hosts from these floras were throughout the Hell Creek Formation and Lancian assigned to morphotypes and/or Linnean taxa, pro- faunal extinction appears to be coincident with plant viding explicit recognition of plant hosts. We identi- extinctions at the end of the Cretaceous. fied 42 types of insect damage, representing insect Summary: In general, this multidisciplinary ex- herbivory from four functional feeding groups: exter- amination of K-T ecosystem change documents a nal foliage feeders, leaf miners, gallers, and piercer- dramatic loss of Cretaceous species richness. The and-suckers. For the Hell Creek megaflora, 41 types of ecological landscape of North America was funda- damage were identified, including several conspicuous mentally altered by the disappearance of major groups examples of galls and scales on primary veins, ser- of plants and animals and the early Paleocene terres- pentine mines, and diagnostic slot-hole feeding, each trial biota was markedly depauperate. Twenty years occurring monospecifically and abundantly on sepa- ago, the Alvarez hypothesis posed the unexpectedly rate species of Lauraceae, Platanaceae, and Cannaba- difficult challenge for paleontologists to evaluate ceae. For the Fort Union megaflora, only 17 types of abrupt extinction in the terrestrial realm. The chal- damage were identified; abundance levels were found lenge has been met for western North America using to be significantly lower; and the associations were multidisciplinary methods that provide a standard for more generalized, lacking host-specific associations. future work. A complete understanding of the global These data demonstrate that at local scales insect her- effects of the Chicxulub impact on terrestrial ecosys- bivory suffered dramatically during the terminal Cre- tems awaits similar research on other continents. taceous event, with no evidence of subsequent recov- ery. Levels of herbivore diversity remained depressed