Palaeoecology

Palaeoecology is a diverse field. Some of it relies on clever analogies with living creatures, cool theories, and quantitative/computational techniques. In this website, we'll traverse the discipline, from small scales to big. I hope you enjoy it.

Introduction

We're going to cover:

Some ecology basics – Section 1
Niches – Section 2
Palaeoautoecology – Section 3
Gradients – Section 4
Palaeosynecology – Section 5
Large scale trends – Section 6

Palaeoecology a really important field – it allows us to understand ecosystems in the past, and thus what was going on in the biosphere at the time. This, coupled with our knowledge of past climates, lets us better understand how life responds to a climate change. The field also incorporates skills which help geologists work out the environment a rock was deposited in. All in all, it's useful stuff. Let's jump right in.

1 – Some ecology basics

Let's start by defining terms, and go over some of the ecological concepts than underpin our thinking about ecosystems – both today, and in the past!

Summary

We can think of palaeoecology as comprising two subfields – palaeoautoecology, described in the video above, and palaeosynecology (the study of ecosystems in the past).
We can split the biosphere into a series of hierarchical categories, and there are ecospaces associated with these.
Communities occupy ecosystems; the breadth of interactions in ecosystems make them really complex.
Species occupy a niche, which we can split into their fundamental and realised niche – the latter is the portion of ecospace in which a species actually lives.
The principle of competitive exclusion states that two species cannot coexist within a niche.

2 – On niches

The thing about niches – well, a thing about niches – is that organisms can help define their niche through their behaviour. This means that there are lots of ways in which niches can be differentiated. This is called niche partitioning – let's learn about it!

Summary

There are many ways in which niches can be differentiated.
We've met four examples:

Resource partitioning: species specialise in the resources they require (e.g. space, food) allowing them to avoid direct competition.
Predator partitioning: this relies on the differential impact of predators/pathogens that are prey/host specific.
Conditional differentiation: occurs when competing species differ in their ability to consume/employ a resource through varying environmental conditions.
Temporal partitioning: niches are partitioned based on timings of their activity.
(Of course, if time leads to conditions changing, this could be a form of conditional differentiation, but if you consider time a resource, it is a form of resource partitioning!)

3 – Palaeoautoecology

This is great. We understand the basics of ecology, and have learned a little more about niches. Let's now look into how adaptations to niches can help us understand the ecology of fossil species: how individual species in deep time lived their lives, and what role they played in their ecosystem. This is the world of palaeoautoecology.

Summary

Palaeoautecology will often focus broadly on mode of life and life traits, through comparisons to the morphology of organisms alive today (=functional morphology).
The inferences of functional morphology are derived from adaptations of organisms to the environment in which they live.
Comparisons can be qualitative (e.g. we draw out analogies through observation), or use quantitative techniques.

4 – On gradients

In the real world, conditions vary in space. When they do so, this often occurs along gradients – gradual transitions across an environment. Here we meet these and see how they interact with ecological niches.

Summary

The spatial distribution of organisms and their associations are controlled by physical, chemical and biological factors which often change along gradients.
Along a gradient, within a niche, we might expect organisms to have optimal conditions under which a species is most successful.
This is formalised as the Abundant-Centre Hypothesis (ACH); there is debate regarding how prevalent this pattern is.
The interaction of gradients and niches defines community distributions, but the nature of this interaction isn't clear cut.

5 – Palaeosynecology

Let's look now at palaeosynecology – how we can reconstruct past ecosystems, and also they ways in which fossil ecosystems have traditionally helped geologist understand the environment rocks were deposited.

Summary

When trying to infer how past ecosystems functioned, we have to consider taphonomy and the biases created through fossilisation.
A number of studies have quantified biases through analysis of extant ecosystems.
Fossils allow geologists to identify the depositional environment of a rock.
In case you want more detail on benthic assemblages, I've created an overview table in the bonus material below.

6 – Large scale trends

Let's finish by digging a tiny bit further into some cutting edge studies that use a quantitative approach to try and counter the biases in the fossil record, and look at biodiversity and ecology in deep time.

Summary

We shouldn't assume that number of fossils species is strongly related to the true biodiversity at the time those fossils were deposited.
Large databases allow us to correct for biases.
As an example, doing so for marine environments suggests that biodiversity does not continually increase to the present (it may rather reflect the prevalence of reef ecosystems).
We can also use these approaches to understand how species respond to changes in climate.
In the latest Carboniferous, aridification reduced diversity, but increased cosmopolitanism (i.e. species became more widespread).
These insights help us understand the impact that current environmental changes might have.

Bonus stuff!

As I have said, I think palaeoecology, especially the computational analyses I have highlighted in Section 6, is a fast developing and super exciting field. Here are some bonus materials in case you want to read more about the latest in palaeoecology!

Those super cool papers

Let's start with links to the two papers that I used as examples. The marine biodiversity study – from a team of super talented palaeoecologists – can be found here:.

Close, R.A., Benson, R.B., Saupe, E.E., Clapham, M.E. and Butler, R.J., 2020. The spatial structure of Phanerozoic marine animal diversity. Science, 368(6489), 420-424.

And here is the second, crazy cool, study. Also from a gifted bunch of palaeo people!

Dunne, E.M., Close, R.A., Button, D.J., Brocklehurst, N., Cashmore, D.D., Lloyd, G.T. and Butler, R.J., 2018. Diversity change during the rise of tetrapods and the impact of the 'Carboniferous rainforest collapse'. Proceedings of the Royal Society B: Biological Sciences, 285(1872), p.20172730.

Want to do some analysis yourself?

Computational palaeoecology is developing fast. And there's nothing like actually doing an analysis to get a feel for a field. If, like me, you like fiddling with code, you may want to give an analysis a shot.

If you do, then look no further. Below I link to a workshop for the Progressive Palaeontology 2020 student conference that was developed and then kindly released to the public by three colleagues, Bethany Allen, Alex Dunhill and Graeme Lloyd from the University of Leeds:

Sampling Bias in the Fossil Record: A workshop developed for Progressive Palaeontology 2020.

The above link includes all the code you need (you'll need to install R). I've put the introductory video below, so you can watch this first if this is all of interest to you. It covers the sources of sampling bias in the fossil record.

And this, second video, provides an overview of what sampling bias means for palaeontologists.

As a general note, I am not just very grateful for my colleagues who made this workshop and released it for free, but am very jealous of... you! Back when I was training as a palaeontologist, resources like this were rather harder to come by. I think it's fair to say that fewer palaeontologists, even a decade ago, used computer-based approaches. That you can do this sort of analysis on your own PC from anywhere in the world is... just super cool!

If you do try this, and get stuck, please feel free to ask for help and – whilst this is outside my wheelhouse – I will very happily try and help out.

Benthic assemblages

In section 4, I introduced biofacies and the associated benthic assemblages. These are summarised in the super handy table below for you. It was published in the following textbook:

Brenchley, P.J., Brenchley, P. and Harper, D., 1998. Palaeoecology: Ecosystems, environments and evolution. CRC Press.

You don't need to learn this, but if you want to get a better idea of the differences between these assemblages, look no further.

Brackish Lagoonal Environments

Context	Commonly associated with terrestrial, barrier bar, estuarine, marsh or shallow marine shoreface facies.
Lithology	Typically shales though siltstone, sandstones and less commonly coarser grained sediments may be introduced by tidal or storm processes in flood-tidal deltas and washover fans.
Taphonomy	Dispersed in situ or disturbed neighbourhood assemblages, typically as thin shell concentrations. Tidal and storm processes may introduce allochthonous assemblages from shoreface and inner shelf environments.
Taxonomy/Diversity	In situ faunas typically of low diversity; usually ostracodes, bivalves and gastropods.
Abundance	Variable, commonly low in thin shell accumulations, but high in thick, typically monospecific, shell beds.
Ichnology	Imperfectly described; low diversity assemblages of the Psilonichnus, Glossifungites, Cruziana and Zoophycos ichnofacies may each occur depending on energy and salinity levels, grain size, substrate consistency and specific depositional environment within a lagoon.

Shoreface Environments (BA1)

Context	Nearshore and associated beach facies, distinguished from inner shelf deposits by generally having only rare mudstones interbedded with sandstones. Shoreface facies commonly cap upward-coarsening shallow marine cycles.
Lithology	Typically sandstones with locally developed mudstones. Sandstones exhibit planar- or cross-stratification formed by wave, tidal or wind-forced currents, or hummocky cross-stratification formed by oscillatory or combined flow.
Taphonomy	Allochthonous shell concentrations that may occur as coquinas, on bedding planes or cross-stratified foresets.
Taxonomy/Diversity	Palaeozoic assemblages characterized mainly by brachiopods with more rarely bivalves and gastropods. Mesozoic/Cenozoic assemblages characterized by varied bivalves (including deep burrowers and attached forms) and gastropods. Taxa affected by substrate mobility (e.g. Corals) are rare. In situ faunas rare. Diversity, though still generally low, can be elevated by introduction of allochthonous faunas by storm, tidal and wave activity.
Abundance	Generally low in Palaeozoic rocks, but Cenozoic strata typically possess more abundant remains. In more subdued shoreface environments shells may be relatively common in coquinas.
Ichnology	Typically Skolithos, more rarely Psilonichnus and Cruziana ichnofacies depending on substrate mobility and subtle gradients in hydrologic sedimentologic and ecologic parameters. Diversity typically low.

Inner (shallow) Shelf Environments (BA2)

Context	Facies subject to frequent storm waves and tidal processes, commonly occurring below shoreface sandstones and above mid-shelf sequences in coarsening-upward cycles.
Lithology	Sandstones dominate, but may occur in equal abundance with interbedded shales. Storm-influenced facies exhibit hummocky cross-stratification; tidal facies may exhibit herringbone cross-stratification, sigmoidal cross-stratification or mud drapes.
Taphonomy	Allochthonous or disturbed neighbourhood assemblages at the base of storm sandstones, typically as coquinas. Tidally influenced sandstones commonly exhibit transported assemblages or shell lags distributed along foresets. Shales possess disarticulated and dispersed shells and bedding plane assemblages. In situ faunas rare or absent.
Taxonomy/Diversity	Palaeozoic strata dominated by moderately but variably diverse assemblages of brachiopods, particularly orthoids and strophomenoids: Mesozoic/Cenozoic strata typically possess variable infaunal and epifaunal bivalves. Representatives of other benthic macroinvertebrates may occur, sometimes in abundance.
Abundance	Generally moderate, but high in coquinas. Faunas can, however, be generally sparse where a freshwater influence is established, as, for example, in inner shelf environments in front of deltas.
Ichnology	Typically moderate to high diversity assemblages of the Cruziana and/or Skolithos ichnofacies. Mudstones commonly intensely bioturbated and possess trace fossils typical of the Cruziana ichnofacies: associated sandstones contain representatives of the Skolithos ichnofacies providing substrate mobility does not result in their destruction.

Middle Shelf Environments (BA3 and BA4)

Context	Commonly form portions of upward-coarsening sequences, but may also form part of a predominantly mudstone sequence. Below fair weather wave base, above storm wave base.
Lithology	Mudstones, typically intensely bioturbated, and interbedded sandstone tempestites with hummocky cross-stratification are common.
Taphonomy	Mudstones may contain in situ clumped or dispersed shells that are still commonly articulated, or as thin stringers or locally reworked disturbed neighbourhood assemblages. Sandstones commonly contain allochthonous lag assemblages or coquinas that are taxonomically similar or dissimilar to those assemblages within the associated mudstones.
Taxonomy/Diversity	Palaeozoic strata possess moderate to high diversity assemblages dominated by brachiopods represented by several orders. Crinoids, bryozoans, corals, trilobites and gastropods also common. Mesozoic/Cenozoic strata dominated by varied infaunal and epifaunal bivalve assemblages with gastropods, echinoderms, bryozoans and less commonly brachiopods also present.
Abundance	Generally high, decreasing with depth.
Ichnology	Similar to inner shelf environments with high diversity assemblages typical of the Arenicolites ichnofacies in sandstones, typically as opportunists following storm activity, and intense bioturbation by members of the Cruziana ichnofacies in associated mudstones.

Outer (deep) shelf environments (BA5)

Context	Mudrock-dominated sequences in association with shelf or slope/basin environments. Mainly below effective storm wave base, though extreme storms have disturbed shells.
Lithology	Bioturbated mudstones with uncommon, and where present typically thin. distal siltstone or sandstone tempestites.
Taphonomy	In situ dispersed, articulated faunas predominate. Thin stringers and bedding plane assemblages also common: disturbed neighbourhood assemblages may occur. Distal tempestites may contain allochthonous shells. though not commonly.
Taxonomy/Diversity	Palaeozoic strata dominated by brachiopods and trilobites: Mesozoic/Cenozoic strata by bivalves, particularly infaunal species - nuculoids are very common. Other groups also occur but in lower numbers. Pelagic forms eg. graptolites (Palaeozoic), ammonites, belemnites (Mesozoic) also commonly present. Diversity variable depending on parameters such as nutrients, oxygen levels, etc., but can be high, then typically markedly decreases at the shelf edge.
Abundance	Typically low.
Ichnology	Discrete ichnotaxa commonly difficult to recognise as a result of intense bioturbation. Representatives of Cruziana, Zoophycos or rarely Nereites ichnofacies may predominate depending on specific environmental conditions. Bioturbation intense though diversity commonly low to moderate.

Slope and Basin (BA6 and below)

Context	Mudrock-dominated sequences with associated sediment gravity flows particularly, but not exclusively, turbidites. Submarine canyons and associated fans commonly occur in association with slope and continental rise sequences. Contourites also common on slope and continental rise environments.
Lithology	Pelagic and hemipelagic mudstones, turbiditic sandstones, and siltstones. Canyons and fans typically possess coarser grained lithofacies.
Taphonomy	Autochthonous faunas, typically dispersed and articulated, in pelagic/hemipelagic mudstones. Allochthonous faunas introduced particularly by turbidity currents.
Taxonomy/Diversity	Dominated by pelagic macroinvertebrates (eg. Palaeozoic - trilobites, graptolites, nautiloids; Mesozoic - ammonites, belemnites). In situ forms include Palaeozoic trilobites, hyolithids, etc. and Mesozoic/Cenozoic bivalves, gastropods and crinoids. Other groups may occur but are uncommon. Diversity is low.
Abundance	Very low, but can be higher in sediment gravity flow deposits.
Ichnology	Variable, with representatives of the Skolithos, Arenicolites and/or Glossifungites ichnofacies commonly occurring in association with well-oxygenated submarine canyons and fans; Zoophycos ichnofacies with slopes with restricted circulation and low oxygen levels and the Nereites ichnofacies in more stable, classical flysch-like settings in generally quiet, but oxygenated waters.