Fern spike

In paleontology, a fern spike is the occurrence of unusually high fern spore abundance in the fossil record, usually immediately (in a geological sense) after an extinction event. The spikes are believed to represent a large, temporary increase in the number of ferns relative to other terrestrial plants after the extinction or thinning of the latter. Fern spikes are strongly associated with the Cretaceous–Paleogene extinction event,[1][2] although they have been found in other points of time and space such as at the Triassic-Jurassic boundary.[3][4] Outside the fossil record, fern spikes have been observed to occur in response to local extinction events, such as the 1980 Mount St. Helens eruption.[5]

Causes

Extinction events have historically been caused by massive environmental disturbances, such as meteor strikes. Volcanic eruptions can also wipe out local ecosystems through pyroclastic flows and landslides, leaving the ground bare for new colonization.[6] For a population to recover and thrive after such an event, it must be able to tolerate the conditions of the disturbed environment. Ferns have multiple characteristics which predispose them to grow in those environments.

Spore Characteristics

Plants generally reproduce with spores or seeds, meaning those will be what germinates in a disaster's aftermath. But spores have advantages over seeds in the environmental conditions produced by a disaster. They're generally produced in higher numbers than seeds, and are smaller, aiding wind dispersal.[6] While many wind-dispersed pollens of seed plants are smaller and farther dispersed than spores,[7] pollen cannot germinate into a plant and must land in a receptive flower. Some seed plants also require animals to disperse their seeds, which may not be present after a disaster. These characteristics allow ferns to rapidly colonize an area with their spores.

Fern spores require light to germinate.[8] Following major disturbances that clear or reduce plant life, the ground would receive ample sunlight that may promote spore germination. Some species' spores contain chlorophyll, which hastens germination and may aid rapid colonization of clear ground.[9]

Environmental tolerance

After the eruption of El Chichón, the fern Pityrogramma calomelanos was observed to regenerate from rhizomes buried by ash, even though the plants' leaves were destroyed.[6] The rhizomes tolerated exposure to heat and sulfur from the volcanic matter. Their survival suggests resilience of ferns to the harsh environmental conditions imposed by certain kinds of disasters, and rhizome regeneration may have been a factor in fern recovery after other events.

Ecology

Fern spikes follow the pattern of ecological succession. In the past and in modern times, ferns have been observed to act as pioneer species.[5] Eventually, their abundance at a site decreases as other plants such as gymnosperms begin to grow.[2]

Spore availability

Fern spikes cannot not occur without ferns already existing in the area, so spikes occur primarily in regions where ferns are already a prominent part of the ecosystem. At the Cretaceous-Paleogene extinction event, a fern spike occurred in the New Zealand area, where ferns made up 25% of plant abundance pre-extinction. After the event, fern abundance increased to 90%.[2]

Detection

Prehistoric fern spikes can be detected by sampling sediment. Sources include sediment that has been accumulating in a lake since the event of interest and sedimentary rocks such as sandstone.[5] Because sediment accumulates over time and thus shows superposition, layers can be assigned to certain times. Spore concentration in a layer can be compared to the concentration at different times, and concentration of other particles such a pollen grains. A fern spike is characterized by a suddenly higher abundance of fern spores following a disaster, generally accompanied by a decrease in other plant species as indicated by their pollen. Eventually fern abundance will decrease, hence the term "spike" describing the pattern.

Modern fern spikes can simply be directly observed, and allow for observation of factors contributing to the spike that may not be detectable otherwise, such as rhizomes persisting in ash.[6]

Significance

Because fern spikes generally coincide with certain disasters such as meteorite strikes and volcanic eruptions, their presence in the fossil record can indicate those events. A fern spike is believed to support a meteorite impact as cause of the Triassic-Jurassic extinction event, similar to the one later causing extinction at the end of the Cretaceous period.[3]

Known Events

A fern spike followed a fungal spike after the Permian-Triassic extinction event. It has been observed in Australia.[10]

After the Triassic-Jurassic extinction event, ferns drastically increased in abundance while seed plants became scarce. The spike has been detected in eastern North America and Europe.[3][11]

A very widespread fern spike occurred after the Cretaceous–Paleogene extinction event.[2] The spike has been predominantly observed in North America, with just one observance outside the continent in Japan.[1]

Fern spikes today are often observed after volcanic eruptions. The areas affected by the eruptions of Mount St. Helens and El Chichón exhibited such a pattern.[5][6]

See also

Fern spike succession
Succession of an ecosystem following disturbance, in the form of a fern spike.

References

  1. ^ a b Schultz, P.H.; D'Hondt, S. (1996). "Cretaceous-Tertiary (Chicxulub) impact angle and its consequences". Geology. 24 (11): 963–967. Bibcode:1996Geo....24..963S. doi:10.1130/0091-7613(1996)024<0963:CTCIAA>2.3.CO;2.
  2. ^ a b c d Vajda, V.; Raine, J.I.; Hollis, C.J. (2001). "Indication of global deforestation at the Cretaceous-Tertiary boundary by New Zealand fern spike". Science. 294 (5547): 1700–1702. Bibcode:2001Sci...294.1700V. doi:10.1126/science.1064706. PMID 11721051.
  3. ^ a b c Fowell, S.J.; Olsen, P.E. (1993). "Time calibration of Triassic-Jurassic microfloral turnover, eastern North-America". Tectonophysics. 222 (3–4): 361–369. Bibcode:1993Tectp.222..361F. doi:10.1016/0040-1951(93)90359-R.
  4. ^ Olsen, P. E.; Kent, D.V.; Sues, H.D.; Koeberl, C.; Huber, H.; Montanari, A.; Rainforth, E.C.; Fowell, S.J.; et al. (2002). "Ascent of dinosaurs linked to an iridium anomaly at the Triassic-Jurassic boundary". Science. 296 (5571): 1305–1307. Bibcode:2002Sci...296.1305O. doi:10.1126/science.1065522. PMID 12016313.
  5. ^ a b c d Adams, Jonathan (2009). Species Richness: Patterns in the Diversity of Life. Springer-Verlag Berlin Heidelberg. p. 125. ISBN 9783540742784.
  6. ^ a b c d e Spicer, Robert A.; Burnham, Robyn J.; Grant, Paul; Glicken, Harry (1985). "Pityrogramma calomelanos, the Primary, Post-Eruption Colonizer of Volcán Chichonal, Chiapas, Mexico". American Fern Journal. 75 (1): 1–5. doi:10.2307/1546571.
  7. ^ Raynor, Gilbert S.; Ogden, Eugene C.; Hayes, Janet V. (July 1976). "Dispersion of Fern Spores Into and Within a Forest". Rhodora. 78: 473–487 – via JSTOR.
  8. ^ Gantt, Elisabeth; Arnott, Howard J. (January 1965). "Spore Germination and Development of the Young Gametophyte of the Ostrich Fern (Matteuccia struthiopteris)". American Journal of Botany. 52 (1): 82. doi:10.2307/2439978. ISSN 0002-9122.
  9. ^ Lloyd, Robert M.; Klekowski, Edward J. (1970). "Spore Germination and Viability in Pteridophyta: Evolutionary Significance of Chlorophyllous Spores". Biotropica. 2 (2): 129–137. doi:10.2307/2989770.
  10. ^ Retallack, G. J. (January 1995). "Permian-Triassic Life Crisis on Land". Science. American Association for the Advancement of Science. 267: 77–80 – via JSTOR.
  11. ^ Falkowski, P. G.; Rosenthal, Y.; Richoz, S.; Röhling, H.-G.; Petschick, R.; Fiebig, J.; Pross, J.; Heunisch, C.; Püttmann, W. (August 2009). "Floral changes across the Triassic/Jurassic boundary linked to flood basalt volcanism". Nature Geoscience. 2 (8): 589–594. doi:10.1038/ngeo577. ISSN 1752-0908.
Cretaceous–Paleogene extinction event

The Cretaceous–Paleogene (K–Pg) extinction event, also known as the Cretaceous–Tertiary (K–T) extinction, was a sudden mass extinction of some three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the exception of some ectothermic species such as the leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kilograms (55 lb) survived. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era that continues today.

In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth's crust, but abundant in asteroids.As originally proposed in 1980 by a team of scientists led by Luis Alvarez and his son Walter Alvarez, it is now generally thought that the K–Pg extinction was caused by the impact of a massive comet or asteroid 10 to 15 km (6 to 9 mi) wide, 66 million years ago, which devastated the global environment, mainly through a lingering impact winter which halted photosynthesis in plants and plankton. The impact hypothesis, also known as the Alvarez hypothesis, was bolstered by the discovery of the 180-kilometre-wide (112 mi) Chicxulub crater in the Gulf of Mexico in the early 1990s, which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact. The fact that the extinctions occurred simultaneously provides strong evidence that they were caused by the asteroid. A 2016 drilling project into the Chicxulub peak ring, confirmed that the peak ring comprised granite ejected within minutes from deep in the earth, but contained hardly any gypsum, the usual sulfate-containing sea floor rock in the region: the gypsum would have vaporized and dispersed as an aerosol into the atmosphere, causing longer term effects on the climate and food chain.

Other causal or contributing factors to the extinction may have been the Deccan Traps and other volcanic eruptions, climate change, and sea level change.

A wide range of species perished in the K–Pg extinction, the best-known being the non-avian dinosaurs. It also destroyed a plethora of other terrestrial organisms, including certain mammals, pterosaurs, birds, lizards, insects, and plants. In the oceans, the K–Pg extinction killed off plesiosaurs and the giant marine lizards (Mosasauridae) and devastated fish, sharks, mollusks (especially ammonites, which became extinct), and many species of plankton. It is estimated that 75% or more of all species on Earth vanished. Yet the extinction also provided evolutionary opportunities: in its wake, many groups underwent remarkable adaptive radiation—sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches. Mammals in particular diversified in the Paleogene, evolving new forms such as horses, whales, bats, and primates. Birds, fish, and perhaps lizards also radiated.

Fern

A fern is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissues that conduct water and nutrients, in having branched stems and in having life cycles in which the sporophyte is the dominant phase. Like other vascular plants, ferns have complex leaves called megaphylls, that are more complex than the microphylls of clubmosses. Most ferns are leptosporangiate ferns, sometimes referred to as true ferns. They produce coiled fiddleheads that uncoil and expand into fronds. The group includes about 10,560 known extant species.Ferns are defined here in the broad sense, being all of the Polypodiopsida, comprising both the leptosporangiate (Polypodiidae) and eusporangiate ferns, the latter itself comprising ferns other than those denominated true ferns, including horsetails or scouring rushes, whisk ferns, marattioid ferns, and ophioglossoid ferns.

Ferns first appear in the fossil record about 360 million years ago in the late Devonian period, but many of the current families and species did not appear until roughly 145 million years ago in the early Cretaceous, after flowering plants came to dominate many environments. The fern Osmunda claytoniana is a paramount example of evolutionary stasis; paleontological evidence indicates it has remained unchanged, even at the level of fossilized nuclei and chromosomes, for at least 180 million years.Ferns are not of major economic importance, but some are used for food, medicine, as biofertilizer, as ornamental plants and for remediating contaminated soil. They have been the subject of research for their ability to remove some chemical pollutants from the atmosphere. Some fern species are significant weeds. They also play certain roles in mythology and art.

Hell Creek Formation

The Hell Creek Formation is an intensively-studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions of Montana, North Dakota, South Dakota, and Wyoming. In Montana, the Hell Creek Formation overlies the Fox Hills Formation. The site of Pompeys Pillar National Monument is a small isolated section of the Hell Creek Formation.

It is a series of fresh and brackish-water clays, mudstones, and sandstones deposited during the Maastrichtian and Danian (respectively, the end of the Cretaceous period and the beginning of the Paleogene) by fluvial activity in fluctuating river channels and deltas and very occasional peaty swamp deposits along the low-lying eastern continental margin fronting the late Cretaceous Western Interior Seaway. The climate was mild, and the presence of crocodilians suggests a sub-tropical climate, with no prolonged annual cold. The famous iridium-enriched Cretaceous–Paleogene boundary, which separates the Cretaceous from the Cenozoic, occurs as a discontinuous but distinct thin marker bedding above and occasionally within the formation, near its boundary with the overlying Fort Union Formation.

The world's largest collection of Hell Creek fossils is housed and exhibited at the Museum of the Rockies, in Bozeman, Montana. The specimens displayed are the result of the museum's Hell Creek Project, a joint effort between the museum, Montana State University, the University of Washington, the University of California, Berkeley, the University of North Dakota, and the University of North Carolina which began in 1998.

Local extinction

Local extinction or extirpation is the condition of a species (or other taxon) that ceases to exist in the chosen geographic area of study, though it still exists elsewhere. Local extinctions are contrasted with global extinctions.

Local extinctions may be followed by a replacement of the species taken from other locations; wolf reintroduction is an example of this.

Paleocene

The Paleocene ( ) or Palaeocene, the "old recent", is a geological epoch that lasted from about 66 to 56 million years ago. It is the first epoch of the Paleogene Period in the modern Cenozoic Era. As with many geologic periods, the strata that define the epoch's beginning and end are well identified, but the exact ages remain uncertain.

The Paleocene Epoch is bracketed by two major events in Earth's history. It started with the mass extinction event at the end of the Cretaceous, known as the Cretaceous–Paleogene (K–Pg) boundary. This was a time marked by the demise of non-avian dinosaurs, giant marine reptiles and much other fauna and flora. The die-off of the dinosaurs left unfilled ecological niches worldwide. The Paleocene ended with the Paleocene–Eocene Thermal Maximum, a geologically brief (~0.2 million year) interval characterized by extreme changes in climate and carbon cycling.

The name "Paleocene" comes from Ancient Greek and refers to the "old(er)" (παλαιός, palaios) "new" (καινός, kainos) fauna that arose during the epoch.

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