GEOL2301: Palaeoecosystems

The morphology of an organism is a product of fitness (its efficacy in converting biotic and abiotic resources into biomass) and evolutionary history (reflected in its genetic constitution).

In some cases (e.g. Tyrannosaurus rex teeth) there is a very clear link between morphological form and fitness.

In other cases (e.g. the ‘jaws’ of velvet worms / onychophorans) morphology is largely sculpted by evolutionary history and developmental constraints.

Plants and environment

Plants tend to be very closely tied to their environment – to the extent that some plant systematists feel that morphology is almost useless for systematics.

Leaves, for example, have a limited range of constraints. They need to maximise surface area (for photosynthesis) and minimise water loss (through stomata) whilst remaining attached to their branch. Plants in hot or arid environments (where evaporation is extreme) tend to have fewer stomata. Plants in wet environments have drip tips (so leaves don’t fall off, and water/algae doesn’t block photosynthesis). Plants in warm environments are more likely to have large, ‘leathery’ leaves with entire (smooth / non-serrated) leaf margins.

• How does the shape of leaf margins correspond to climate?
• The percentage of species with entire leaf margins is strongly correlated with mean annual temperature in living and extinct assemblages.

This close correspondence between environment and phenotype allows morphological form to act as a proxy for ancient environments.

• What mechanisms might account for this correspondence?
• Range change (via local extinction or immigration); phenotypic plasticity; evolutionary change

Handily, we don’t need to know which mechanism is most important in order to use the proxy.

Morphology can also reflect characteristics relevant to the sedimentary setting of an environment.

Here are two examples of the modern scleractinian coral Porites:

• Which colony is more likely to inhabit an energy with high sedimentary energy?
• The first
• The second
• The long delicate branches of the first would soon be broken off by high sedimentary energy – see the evidence of damage at the bottom of the colony? The round blob-shaped colony is far more rugged.
• Which is likely to live at the shallower water depth?
• The first
• The second
• Deeper water sees fewer storms: a branching morphology may survive the occasional brief storm, but is unlikely to last for long above fair weather wave base.

Here are two gastropod molluscs, Patella and Murex:

• Which gastropod is more likely to inhabit an energy with high sedimentary energy?
• Patella
• Murex
• Same principle again: fine spines don't survive strong currents. In fact, the low, humped profile and loss of coiling that characterizes the limpet Patella has arisen multiple times in different gastropod lineages, an example of convergent evolution.

Here's a bivalve and a brachiopod, each exhibiting a ridged / corrugated shell.

• Which is more likely to inhabit an energy with high sedimentary energy?
• The bivalve
• The brachiopod
• The bivalve ridges are classic structural reinforcements. The brachiopod's corrugated valve is a side-effect of its zig-zag commissure, reflecting feeding mode, not structural constraints.

We can also reconstruct ecological conditions from organism morphology.

Predators play a key role in establishing the structure of an ecosystem; they will crop back any particularly dominant K-strategists, creating new substrate for r-strategists.

At a large scale, this capacity to generating diverse ecosystems has been linked to two major diversifications in Earth history: the Cambrian ‘explosion’ and the Mesozoic Marine Revolution.

How can we infer the degree of predation intensity from:

• Direct evidence of predators / predation
• Direct evidence may include gut contents (coprolites?) or trace fossils (capturing predatory behaviour), but is likely to be rare.
• Predation damage
• Borings are common; their distribution and frequency can illuminate predation intesity, particularly when considered alongside shell size (i.e. age of prey). Other forms of damage – e.g. fractures from clab-claw crushing – become more abundant during the predation and can point to the rise of new predatory strategies. But not all predatory activity leaves evidence of its damage, and some may completely obliterate shell remains, so this evidence needs to be weighed carefully.
• Morphological defences
• Does the onset of widespread mineralization in the Cambrian reflect the inexorable rise of predation? The abundance and nature of mineralized defences is likely linked to the expected likelihood of predation, though environmental conditions may need to be taken into account (e.g. availability of calcium ions in freshwater).
• Predatory adaptations
• Taxa with obvious adaptations for predation are probably predators (though not always – e.g. the herbivorous panda has relict canines), and can potentially be linked to prey by signs of relevant damage. Predation and scavenging are not always easy to distinguish.
• Range distribution
• Taxa that cannot resist predation – such as sessile brachiopods, which lack the metabolic stature to secrete thick shells or expend energy keeping them closed – may be driven to exinction in settings where predation is a significant force.
• Ecosystem structure
• Predation intensity tends to correspond to diversity, as demonstrated by starfish-caging experiments; Ediacaran Cloudina reefs may thus be tentatively interpreted as free from significant predatory pressure.

Brachiopod functional morphology

Brachiopods offer a useful case study in adaptive morphology: their form often reflects the environment they inhabit. Brachidia (skeletal elements that support the lophophore) are good examples of convergent evolution. This raises a more general point: if there’s a best solution to a physical problem, biology will often find it.

What brachiopod morphologies might you expect to find in the following environments?

• Fine, soupy sediments
• Concavo-planar productid brachiopods can employ the 'iceberg' strategy and float in the soupy mud, keeping their commissure free of the sediment in order to feed and respire.
• Shallow ocean with high terrigenous input
• The water may be rich in suitable organic foodstuffs, but high energy and large clasts washed in may threaten to choke the lophophore. A zig-zag commissure could come in very handy here.
• Mid-to-high productivity, slightly stressed Palaeozoic outer shelf
• This sounds like good turf for brachiopods; the deeper water and stress may keep predators at bay, but the high productivity may support multiple tiers of suspension feeders. As such, we may expect to see a mix of pedunculate brachiopods, accessing higher tiers, and taxa with small pedicles (for anchorage only) or no pedicles, accessing lower tiers.
• Mid-to-high productivity inner shelf in recent oceans, with no major environmental fluctuations
• This sounds like the ideal setting for a high-diversity ecosystem in which predation pressure is likely high. That's bad news for brachiopods, which are unlikely to exist here in any number.