Kirk R. Johnson

Habitat-related error in estimating temperatures from leaf margins in a humid tropical forest

Leaf margin characters are strong predictors of mean annual temperature (MAT) in modern plant communities and widely used tools for reconstructing paleoclimates from fossil floras. However, the frequency of nonentire-margined species may vary dramatically between different habitats of the same forest. In this paper we explore the potential for this habitat variation to introduce error into temperature reconstructions, based on field data from a modern lowland forest in Amazonian Ecuador.The data show that the provenance of leaves can influence temperature estimates to an important degree and in a consistent direction. Woody plants growing along lakes and rivers underestimated MAT by 2.5°-5°C, while those in closed-canopy forest provided very accurate predictions. The high proportion of liana species with toothed leaves in lakeside and riverside samples appears to be responsible for a large part of the bias. Samples from closed-canopy forest that included both lianas and trees, however, were more accurate than tree-only or liana-only samples.We conclude that paleotemperature reconstructions based on leaf margin characters will be misleading to the extent that fossilization provides a better record of certain habitats than others. The preponderance of lake and river deposits in the angiosperm fossil record suggests that underestimation of mean annual paleotemperature may be common.

Leaf-fossil evidence for extensive floral extinction at the Cretaceous-Tertiary boundary, North Dakota, USA

Abstract Forty-seven collections of plant macrofossils (megaflora) from the upper 17 m of the Hell Creek Formation (late Maastrichtian) and the lower 80 m of the Ludlow Member of the Fort Union Formation (Paleocene; Puercan) in south-western North Dakota document floral change across the Cretaceous-Tertiary (K-T) boundary. These collections were made within a detailed stratigraphic framework based on 37 measured sections in eight areas along a 60 km transect and were supplemented with a microstratigraphic palynological analysis of the K-T boundary. The boundary is defined by the extinction of 30% of the late Maastrichtian Wodehouseia spinata Assemblage Zone palynomorphs and occurs within a few meters above or below the Hell Creek-Fort Union formational contact. The boundary is documented palynologically in seven sections and, in one section, coincides with an iridium anomaly and shocked mineral grains. The megaflora of the uppermost 17 m of the Hell Creek Formation (megafloral zone HC III) is a diverse (65 taxa) angiosperm-dominated assemblage that is taxonomically distinct from the megaflora of the lower Hell Creek. The HC III megaflora suffers 80% extinction across the K-T boundary and is replaced by the lower Fort Union megaflora (FU I) which, with 73 taxa, appears more diverse than HC III but has fewer taxa per locality. The Paleocene flora, with abundant aquatic angiosperms and taxodiaceous conifers, reflects the increased abundance of pond and swamp environments. This indicates that some of the floral change near the boundary was due to sedimentological facies change. The number, abruptness and regional extent of extinctions combined with the presence of iridium and shocked minerals at the extinction horizon support an extinction scenario compatible with an extraterrestrial impact. The mechanisms of that extinction, however, remain unclear.

Preliminary assessment of insect herbivory across the Cretaceous-Tertiary boundary: Major extinction and minimum rebound

Forty-nine insect damage types morphologically characterize plant-insect associations spanning a 136 m composite boundary interval from the uppermost Cretaceous Hell Creek Formation to the lowest Paleocene Fort Union Formation. The time duration of this interval is ∼2.2 m.y., 1.4 m.y. of which is latest Cretaceous. These data originate from 80 localities from the Williston Basin of southwestern North Dakota and are allocated to four assemblage zones consisting of 385 megafloral morphotypes of bryophytes, ferns, conifers, cycads, Ginkgo, monocots, and 333 dicots. All 49 foliar damage types were produced by external foliage feeding, mining, galling, and piercing and sucking insects, each of which was assigned one of the following host specificity levels: generalized, intermediate, or specialized. A distinctive pattern emerged when these damage types were plotted stratigraphically: of the 22 damage types that survive the Cretaceous-Tertiary boundary (K-T), 55% are generalized; of the 15 damage types that are extinguished immediately below the boundary, all are specialized or intermediate, as are the 12 damage types that disappear in stepped fashion somewhat before the boundary. Within this interval only two damage types are unique to the Fort Union Formation, occurring during the lowest Paleocene. These data indicate that highly and moderately specialized associations were preferentially culled at the K-T boundary, disproportionately enriching early Paleocene floras in generalized herbivores as dominant colonizers. These data are local, and the Signor-Lipps effect may partially obscure the true pattern, thus restricting extrapolation to the Western Interior of North America. Nevertheless, (1) the selective extinction of specialist herbivores at the boundary, (2) the absence of significant evidence for immigrant colonization, (3) the prolonged occurrence of species-depauperate floras during the Paleocene, and (4) the high percentage of last occurrences at and not before the boundary, despite the Signor-Lipps effect, suggest a role for end-Cretaceous abiotic perturbation and consequent biotic response in the evolution of modern plant and insect associations.

Intercontinental dispersal of giant thermophilic ants across the Arctic during early Eocene hyperthermals

Early Eocene land bridges allowed numerous plant and animal species to cross between Europe and North America via the Arctic. While many species suited to prevailing cool Arctic climates would have been able to cross throughout much of this period, others would have found dispersal opportunities only during limited intervals when their requirements for higher temperatures were met. Here, we present Titanomyrma lubei gen. et sp. nov. from Wyoming, USA, a new giant (greater than 5 cm long) formiciine ant from the early Eocene (approx. 49.5 Ma) Green River Formation. We show that the extinct ant subfamily Formiciinae is only known from localities with an estimated mean annual temperature of about 20°C or greater, consistent with the tropical ranges of almost all of the largest living ant species. This is, to our knowledge, the first known formiciine of gigantic size in the Western Hemisphere and the first reported cross-Arctic dispersal by a thermophilic insect group. This implies intercontinental migration during one or more brief high-temperature episodes (hyperthermals) sometime between the latest Palaeocene establishment of intercontinental land connections and the presence of giant formiciines in Europe and North America by the early middle Eocene.

First South American Agathis (Araucariaceae), Eocene of Patagonia

PREMISE OF THE STUDY Agathis is an iconic genus of large, ecologically important, and economically valuable conifers that range over lowland to upper montane rainforests from New Zealand to Sumatra. Exploitation of its timber and copal has greatly reduced the genuss numbers. The early fossil record of Agathis comes entirely from Australia, often presumed to be its area of origin. Agathis has no previous record from South America. METHODS We describe abundant macrofossils of Agathis vegetative and reproductive organs, from early and middle Eocene rainforest paleofloras of Patagonia, Argentina. The leaves were formerly assigned to the New World cycad genus Zamia. KEY RESULTS Agathis zamunerae sp. nov. is the first South American occurrence and the most complete representation of Agathis in the fossil record. Its morphological features are fully consistent with the living genus. The most similar living species is A. lenticula, endemic to lower montane rainforests of northern Borneo. CONCLUSIONS Agathis zamunerae sp. nov. demonstrates the presence of modern-aspect Agathis by 52.2 mya and vastly increases the early range and possible areas of origin of the genus. The revision from Zamia breaks another link between the Eocene and living floras of South America. Agathis was a dominant, keystone element of the Patagonian Eocene floras, alongside numerous other plant taxa that still associate with it in Australasia and Southeast Asia. Agathis extinction in South America was an integral part of the transformation of Patagonian biomes over millions of years, but the living species are disappearing from their ranges at a far greater rate.

Paleoecology of a late Paleocene (Tiffanian) megaflora from the northern Great Divide Basin, Wyoming

To reconstruct a late Paleocene (Tiffanian) vegetation, we intensively sampled and censused plant megafossils of the Fort Union Formation from Bison Basin, Wyoming. Extensive sampling of this deposit at the local scale was undertaken to assess the species richness and spatial distribution of leaf megafossils in a well-dated Paleocene assemblage. The megafossils were recovered from a series of ten quarries in a single fossiliferous horizon composed of fine-grained claystone that represents deposition in a shallow floodplain pond. Early and middle Paleocene leaf localities from the northern Great Plains are typically depauperate; this deposit was no exception, yielding only 28 distinct leaf morphotypes from a collection of 5,650 specimens (average: 10.6 morphotypes/quarry). Significant differences in the numbers of specimens per morphotype were observed. Four species, Corylites sp. (37.66%), Archeampelos acerifolia (33.93%), Metasequoia occidentalis (18.35%), and Fortuna cf. F. marsilioides (5.29%) comprised >95% of the specimens recovered. Megafloral dominants were fairly consistent from quarry to quarry indicating that this vegetation was relatively homogeneous at the scale of the local community. Detrended Correspondence Analysis indicates that there is no environmental gradient apparent within this community. This extremely well-sampled and laterally extensive leaf site supports the view that the late Paleocene (Tiffanian) flora of the northern Rockies is depauperate compared to megafloras of the Late Cretaceous and late Paleocene (Clarkforkian).

Opinion: Specimen collections should have a much bigger role in infectious disease research and response

When public health officials become aware of the first signs of a disease outbreak, they need to determine a few critical things as quickly as possible. What’s the disease agent? How did it get here? How does it spread and how can it be contained? Has it been seen before? If so, what was the approach and how well did it work?

Plant Ecological Strategies Shift Across the Cretaceous–Paleogene Boundary

The End-Cretaceous Impact Winter Killed Off Slow-Growing Plants The end-Cretaceous mass extinction caused the selective extinction of plant species with slow-growth strategies, consistent with an impact winter hypothesis.

First record of Todea (Osmundaceae) in South America, from the early Eocene paleorainforests of Laguna del Hunco (Patagonia, Argentina)

UNLABELLED PREMISE OF THE STUDY The early Eocene Laguna del Hunco caldera-lake paleoflora (ca. 52 Ma) from Chubut Province, Argentina, is notably diverse and includes many conifer and angiosperm lineages that are extinct in South America but extant in Australasian rainforests. No ferns have been previously described from Laguna del Hunco. We describe and interpret a new species of fossil Osmundaceae based on fertile and sterile pinnae. • METHODS The fossil specimens were compared with other extant and fossil Osmundaceae based on living and herbarium material and published descriptions. A morphological matrix based on 29 characters was constructed for 17 living species in Osmundaceae, four species assigned to the fossil genus Todites, and the new fossil species. Phylogenetic analyses were conducted under parsimony using morphology and total evidence matrices. • KEY RESULTS Both the new fossil and the Todites species were consistently resolved within the leptopteroid clade of Osmundaceae, and the new species resolved in a clade with the two living Todea species, which are now restricted to Australia, New Guinea, New Zealand, and southern Africa. • CONCLUSIONS Todea amissa sp. nov. is the first record of Todea, living or fossil, in South America and only the second fossil record worldwide. The distribution of extant Todea on Gondwanan continents other than South America is broadly shared with other taxa from Laguna del Hunco, further indicating that a large component of this flora represents a Gondwanic biome that is no longer found on the South American continent.

Novel Insect Leaf-Mining after the End-Cretaceous Extinction and the Demise of Cretaceous Leaf Miners, Great Plains, USA

Plant and associated insect-damage diversity in the western U.S.A. decreased significantly at the Cretaceous-Paleogene (K-Pg) boundary and remained low until the late Paleocene. However, the Mexican Hat locality (ca. 65 Ma) in southeastern Montana, with a typical, low-diversity flora, uniquely exhibits high damage diversity on nearly all its host plants, when compared to all known local and regional early Paleocene sites. The same plant species show minimal damage elsewhere during the early Paleocene. We asked whether the high insect damage diversity at Mexican Hat was more likely related to the survival of Cretaceous insects from refugia or to an influx of novel Paleocene taxa. We compared damage on 1073 leaf fossils from Mexican Hat to over 9000 terminal Cretaceous leaf fossils from the Hell Creek Formation of nearby southwestern North Dakota and to over 9000 Paleocene leaf fossils from the Fort Union Formation in North Dakota, Montana, and Wyoming. We described the entire insect-feeding ichnofauna at Mexican Hat and focused our analysis on leaf mines because they are typically host-specialized and preserve a number of diagnostic morphological characters. Nine mine damage types attributable to three of the four orders of leaf-mining insects are found at Mexican Hat, six of them so far unique to the site. We found no evidence linking any of the diverse Hell Creek mines with those found at Mexican Hat, nor for the survival of any Cretaceous leaf miners over the K-Pg boundary regionally, even on well-sampled, surviving plant families. Overall, our results strongly relate the high damage diversity on the depauperate Mexican Hat flora to an influx of novel insect herbivores during the early Paleocene, possibly caused by a transient warming event and range expansion, and indicate drastic extinction rather than survivorship of Cretaceous insect taxa from refugia.