Fungal Proliferation at the Cretaceous-Tertiary Boundary

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Science  05 Mar 2004:
Vol. 303, Issue 5663, pp. 1489
DOI: 10.1126/science.1093807

The evolution of life on Earth has been interrupted by several mass extinction events. The Cretaceous-Tertiary (K-T) extinction [65 million years ago (Ma)] is associated with the impact of a large bolide (1). On the basis of extensive data (24), the K-T boundary is characterized by a palynological extinction horizon coincident with a geochemical marker bed commonly succeeded by a bed rich in fern spores (25).

We have found that a fungal spike occurs between the diverse Late Cretaceous palynoflora and the low-diversity fern-dominated early Paleocene assemblages in a New Zealand section. The fungal layer is coincident with the Ir anomaly marking the extinction event.

The studied K-T boundary occurs within a 10-cm-thick coal seam bounded by mud-stones of the nonmarine Rewanui Coal Measures Member (upper Paparoa Coal Measures) at Moody Creek Mine, Greymouth Coalfield, New Zealand (171°16′40″E, 42°23′18″S). We identified palynomorph assemblages from consecutive 3- to 5- mm laminae through 20-cm coal and mud-stone succession. One half of each sample was analyzed for elemental abundance (6) (table S1).

The basal 8 cm of coal hosts Late Cretaceous spore-pollen assemblages incorporating around 80 conifer, fern, and flowering plant taxa typical of complex temperate forest communities (5). The succeeding ∼4-mm layer (Fig. 1) yielded only fungal spores and fragments of nonseptate hyphae belonging to previously undescribed species of fungi (7) (Fig. 1A). Overlying laminae contain low-diversity, fern-dominated assemblages. No significant macroscopic changes or clastic partings occur within the coal to suggest a change in sedimentation rate or depositional setting across the K-T boundary.

Fig. 1.

(A) Fungal spores, Monoporisporites spp., aff. Pluricellaesporites spp. and hyphae from the “fungal spike” layer at Moody Creek Mine. (B) Changes in Ir concentration and relative abundance of pollen and spores across the K-T boundary at Moody Creek Mine.

Analysis of a nearby section revealed an identical pattern of floristic turnover, indicating that the fungi-rich interval characterizes a distinct layer in the coal. The fungal acme is coincident with extinction of several miospore index taxa and an iridium abundance of ∼4 parts per billion (ppb) (Fig. 1B; table S1).

This fungi-rich interval implies wholesale dieback of photosynthetic vegetation at the K-T boundary in this region. The fungal peak is interpreted to represent a dramatic increase in the available substrates for saprophytic organisms (which are not dependent on photosynthesis) provided by global forest dieback after the Chixculub impact (5). Post-impact conditions of high humidity and reduced solar insolation due to increased atmospheric sulfur aerosols and dust (8) would have favored saprophyte activity, but this interval would have been short-lived because of rapid atmospheric settling.

Fungal dominance would have lasted a few years at most, because the recovery of ferns initiates within the maximum iridium anomaly layer (Fig. 1B). This suggests rapid reestablishment of pteridophyte communities following the impact event.

A global fungal or algal (Reduviasporonites) spike followed by a pteridophytegymnosperm recovery has also been reported from the Permian-Triassic (P-Tr) boundary (9). The K-T and P-Tr vegetation recoveries represent similar responses to terrestrial ecosystem destabilization and collapse, although the P-Tr biodiversity crisis was more prolonged (9).

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Materials and Methods

Table S1

References and Notes

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