Niche: Two Examples From Deep Time

The term 'niche' can very simply mean an ecologic space which a particular type of organism exploits. Scientists are a pedantic lot though. They need more rigorous definitions to work with. This has spawned many different ideas about what a niche means and how it can best be described and measured. There is the environmental niche concept which focuses on the physical and chemical attributes of an available space that may or may not be filled by organisms. In this idea, there may be vacant niches, which opportunistic organisms may come to exploit. On the other hand, there is the population niche concept where the niche itself is an attribute of the population. Organism-specific use of its resources, uniquely shaped by the organism's physiology, community structure, and behavior, defines the niche. 

Similar organisms may co-exist in a particular space. This may result in niche overlap and niche partitioning as different species vie for the available resources. Ideas of competitive exclusion (competition theory) derive from such co-habitation of space.

I am just giving a flavor of the arguments here and not diving into a discussion of the many niche concepts. For the purpose of this short post I will use the term niche to mean the actual utilization of a space and of available resources by a species/population.

Palaios is of my of my favorite science journals. It published papers on themes intersecting palaeontology, ecology, sedimentology and stratigraphy. Unfortunately, most of the papers are behind a paywall, so I have to make do with reading the abstracts and finding the occasional open access paper on Research Gate and Academia. Browsing through it last week, I came across two interesting examples of very specific fossil niches, one from the Cretaceous and another from the earliest Triassic. 

In the March 2020 issue, Alison J. Rowe and colleagues ( Late Cretaceous Methane Seeps As Habitats For Newly Hatched Ammonites) describe a community of ammonites (a type of mollusc) living in the vicinity of cold methane seeps. These fossils are preserved in the sedimentary rocks of the Late Cretaceous Western Interior Seaway, a time when sea levels were high and the mid North American continent was under the sea. Methane seeps often occurred along faults which provided the pathways for the gas to rise from the subsurface and be released on the sea floor. Ancient methane seeps are recognized by the presence of typical communities of clams, fossilized worm tubes, ammonites and bacterial microstructures. There is often the development of cone shaped sediment mounds known as Teppe Buttes, made up of calcium carbonate mud and skeletal remains of organisms. The carbon in the calcium carbonate is enriched in the lighter isotope C12, suggesting its derivation from a hydrocarbon source (the hydrocarbon itself is transformed organic matter which is richer in C12). 

Anaerobic oxidation of methane (bacteria stripped electrons and protons from the hydrogen in the methane to drive respiration) provided the energy to build a food web that formed the base of this chemosynthetic ecosystem. These ammonites were small, implying that they were born in this habitat, and their geochemistry indicated that they were incorporating the carbon released from methane to build their shells. What an utterly fascinating mode of life!

The second example is from the Latest Permian-Earliest Triassic (Dwelling In The Dead Zone- Vertebrate Burrows Immediately Succeeding The End-Permian Extinction Event in Australia), preserved in strata deposited just after the biggest mass extinction in earth history. Stephen McLoughlin and colleagues find and describe burrow structures made by small tetrapods. The sediments which contain these burrows are poor in other organic traces suggesting a terrestrial ecosystem which has been stripped bare due to prolonged debilitating environmental conditions.  Paleogeographic reconstruction indicates that at this time the Sydney Basin  occupied a high paleo-latitude. A burrowing lifestyle, coupled with relatively cooler climate of higher paleo-latitudes may have provided these tetrapods protection from the otherwise harsh post extinction conditions. A nice example of the ecology of post mass extinction survivor fauna.

If you want to explore the history of ideas on the niche concept I can recommend two essays. Niche: Historical Perspectives by James R. Griesmer, and, Niche: A Bifurcation in the Conceptual Lineage of the Term by Robert K. Colwell. Both have been published in the book Keywords in Evolutionary Biology, edited by Evelyn Fox Keller and Elisabeth A. Lloyd.

We have so much more to learn about life.

Magmas And Mass Extinction: Late Triassic

A new study on the synchronicity of igneous activity and the Late Triassic mass extinction which occurred around 201.5 million years ago.

Large-scale sill emplacement in Brazil as a trigger for the end-Triassic crisis- Thea H. Heimdal, Henrik. H. Svensen, Jahandar Ramezani, Karthik Iyer, Egberto Pereira, René Rodrigues, Morgan T. Jones & Sara Callegaro. The article is open access.

Magma intruded a thick pile of sediments in Brazil. The thermal reactions in the sediment would have resulted in the release of 88 trillion tons of CO2 from the degassing of sediments!

Abstract:

The end-Triassic is characterized by one of the largest mass extinctions in the Phanerozoic, coinciding with major carbon cycle perturbations and global warming. It has been suggested that the environmental crisis is linked to widespread sill intrusions during magmatism associated with the Central Atlantic Magmatic Province (CAMP). Sub-volcanic sills are abundant in two of the largest onshore sedimentary basins in Brazil, the Amazonas and Solimões basins, where they comprise up to 20% of the stratigraphy. These basins contain extensive deposits of carbonate and evaporite, in addition to organic-rich shales and major hydrocarbon reservoirs. Here we show that large scale volatile generation followed sill emplacement in these lithologies. Thermal modeling demonstrates that contact metamorphism in the two basins could have generated 88,000 Gt CO2. In order to constrain the timing of gas generation, zircon from two sills has been dated by the U-Pb CA-ID-TIMS method, resulting in 206Pb/238U dates of 201.477 ± 0.062 Ma and 201.470 ± 0.089 Ma. Our findings demonstrate synchronicity between the intrusive phase and the end-Triassic mass extinction, and provide a quantified degassing scenario for one of the most dramatic time periods in the history of Earth.

This prolonged phase of igneous activity resulted in the formation of the Central Atlantic Magmatic Province. Its connection to the mass extinction was hard to pin down due to a lack of accurate dates of the oldest igneous activity. This and some other work now show that phases of this magmatic episode were synchronous with the mass extinction.

A similar problem of lack of accurate dating of events had limited our understanding of the role of Deccan Volcanism in the mass extinction that took place at 66.04 million year ago. New geochronology work (summarized by Kale et. al. 2019)  is showing that volcanism spanned this mass extinction. Significant amount of lava eruptions took place before the mass extinction and would have played a role in the deterioration of environmental conditions. And volcanism continued well after the mass extinction delaying biotic recovery for hundreds of thousand of years.

Large injections of magma as laterally extensive intrusions (sills) into sediment has also been thought to have been the trigger for the end-Permian mass extinction that took place around 252  million years ago. Interestingly, like the end-Triassic, it was not emissions of carbon dioxide and methane directly from lava eruptions that is thought to be the driver of environmental change. Rather, it was the subsurface emplacement of sills and the thermal reaction (contact metamorphism) in buried sediment in contact with this hot magma that resulted in volumetric degassing from sediments. Limestones when heated this way would have released carbon dioxide upon breakdown of the mineral calcite. And organic matter would have released methane.

The long trajectory of evolution on earth has been disrupted and reoriented many times from deep within.

Post Permian Mass Extinction Size Reduction In Glossopteris Plant Lineages

Mass extinctions prune away branches of the tree of life. The late Permian-Triassic mass extinction event affected marine and terrestrial animal life severely, perhaps more than it affected terrestrial plant life. One common observation is that survivor species tend to be smaller bodied representatives of groups. This is known as the Lilliput effect.

A lot of attention has been paid to evolutionary trends in size during and post mass extinction in various animal groups. An interesting paper in Current Science (open access)  documents the Lilliput effect in one of the iconic terrestrial plant groups  of Late Permian -Early Triassic times; the seed fern Glossopteris.

From the paper:

Modern day high-resolution regional palaeoecological studies have proved the myth wrong, which stated that the mass extinction event had little macroecological or evolutionary consequence for terrestrial plants The early Triassic (Panchet Formation) in India witnessed more arid or semi-arid climate in comparison to the Permian. The early Triassic experienced greenhouse conditions with a warmer phase (Figure 6) due to global rise of temperature coupled with episodes of intense volcanism. The reduced size of the lamina is one of the strategies adapted by the plants during the adverse condition in early Triassic when there was indeed a shortage of essential nutrients in the soil, in addition to seasonal dry climate, irregular rainfall and widespread aridity. The leaves possessed cuticles with sunken stomata since the atmosphere during the early Triassic had higher levels of CO2 and lower O2levels (Figure 7). The Glossopteris flora of late Permian was adapted to temperate, cool and moist environments. The phenomenon of dwarfism as evidenced in Glossopteris has been observed only up to th e early Triassic, which is represented by the Panchet Formation. During the middle to late Triassic, typical Dicroidium flora associated with Lepidopteris takes over the preceding Glossopteris flora.

Records of plant fossils substantiate the evidence that numerous physiological, reproductive and behavioural traits enabled the smaller sized plant species to persist in extreme climatic conditions.

I didn't know there was a myth that mass extinctions have little macro-ecological or evolutionary consequences for terrestrial plants. Although, I do get the impression that studies of plant evolution get less attention in the media than animal evolution.