Adrian Currie writes...
This is our third post on conservation-paleobiology: the intersection of paleontology and conservation (this does not involve going back in time to save the Dodo, more's the pity...). To my mind, Derek and Leonard had different (but complimentary!) approaches in their posts. Derek’s point is largely epistemic – he argues in favour of what he calls ‘past-sourced’ modelling. This is the use of the deep past to inform models applicable to nowadays. Leonard looks at the conceptual connections between paleobiology and conservation biology by highlighting how the two conceive of extinction.
Using the deep past to inform our understanding of the present is potentially enormously powerful: consider extinction and the climate. Although the last two centuries have yielded pretty good records, this is a tiny snapshot of the climate's wildly diverse history. We can observe extinction’s occurrence in the present—in front of our very eyes in fact—but a wider scale is required to get a grip on the long term causes of extinction. To understand the complex natural systems which are being jeopardized by increasing human population, consumption and technology, we need analogues at the right scale—and the deep past is our best source.
Today, I’d like to pick up on an objection to the past’s capacity to inform models of the present, rooted in the notion of uniqueness (which Aviezer Tucker did a very nice job of unpacking back in 1998).
Here’s the quick version of the objection: the deep past and the Anthropocene are just too different. The cause of the extinctions, and of the increases in greenhouse gas, is unlike anything the Earth has seen before [hint: it’s us]. These differences undermine using paleo-analogies to inform the present.
Here’s the quick version of my answer. The Anthropocene's uniqueness doesn't stop conservation-paleobiology in its tracks. We can (1) conceive of the event on a continuum with other cases; (2) use a wide range of analogues, each informing our understanding of different aspects of our target.
Let’s begin by thinking about analogous reasoning. Imagine I want to understand some target extinction event. I might do this by constructing a model of extinctions—their dynamics, how they occur, etc… Such models require empirical data. Relevantly similar extinctions could fit the bill. To understand one event, I construct a model by looking at a bunch of similar events:
So far so good. To understand some big problems (climate change, extinction, etc…) we need to construct models which capture their dynamics. The deep past is a potential source of the evidence we need to do this: paleodata can help construct, calibrate and test our models. But. What happens if we just don’t have any analogues available? Then there isn’t enough evidence to construct our model. We seem to reach an epistemic dead-end.
However, getting epistemic traction on apparently unique events is something which paleobiologists do all the time. As I see it, they do this using two interconnected strategies, which I’ll call the 'exquisite corpse' method.
Exquisite Corpse is a game developed by surrealist artists (most notably Andre Breton) in the 1920’s. In its original version, each player writes a word, or a sequence of words, either following a rule (for instance, the first player writes an adjective, the next a noun, and so on) or by hiding much of the previous writing from the next player. The name comes from a sentence generated in this way "Le cadavre exquis boira le vin nouveau." ("The exquisite corpse shall drink the new wine."). A version of the game salient here involves drawing rather than writing. Together, players create an odd creature. The first player draws the feet, and then folds the paper over so the next player can only see the tops of the ankles. The new player draws the legs, folding the paper similarly and passing to the next player, and so forth until the creature is drawn. Here's an example (courtesy of Leonard and Shay!):
The critter generated by the game is often an amalgamation of different figures. Similarly, in exquisite corpse methodology the target is reconstructed on the basis of its similarity with a range of different analogues. Scientists navigate between various examples, which inform their models only insofar as the example's features match those of the target. The resulting model (or models) is a patchwork relying on many different analogues concerning different aspects of the target’s unique set of properties. Let’s see it in action.
Take a look at these two skulls.
On the left is our old friend Smilodon fatalis, a placental cat with impressive saber-teeth: the Pleistocene’s saber-toothed lion. On the right is… something a bit different.
This is Thylacoleo atrox: a South American predator, about leopard-sized, from the Sparassodonta family (a close relative of the marsupials). And she’s weird. Based on her size and them great-big chompers she’s presumably in contention for apex predator of the South American Cenozoic (well, other than Terror Birds). However, how hard mammals bite is typically related to the size of their prey—and T. atrox has a weak bite (see Wroe et al 2005 for comparisons). Modern-day panthers (70ish kilos) have a bite reaction force from the jaw of 484 Newtons, while the 80-100 kilo T. atrox managed a pathetic 38 N (Wroe et al 2013).
T. atrox posses a unique suite of traits. A large predator, sporting a ridiculous skull and gape with a tiny bite force (based on jaw musculature, at least). There are no perfect analogues here... But this doesn’t mean that analogues can’t be informative...
First, we can understand T. atrox as an extreme version of a general killing strategy. Let’s look at how scientists have drawn on both other saber-toothed analogues and a range of other animals to construct an exquisite corpse model of T. atrox.
The synapsids are identified by the ‘fused arch’ (the literal translation) formed by two holes behind the eyes in the skull (such is the excitement of large-scale taxa differentiation!). They’ve had two big moments in evolutionary history. One we’re pretty familiar with, given that it includes us. Over the last 55 million years, mammalian synapsids have dominated the world’s megafauna (well, except for giant leopard-like eagles in NZ, and the aforementioned terror birds in South America—oh, and let’s not forget Australia’s giant lizards!). But let's begin with the synapsid’s earlier—and more temporally impressive—time in the limelight. This covered around 90 million years, from the late Carboniferous to the early Triassic, and included such luminaries as the first land-based megacarnivore: Dimetrodon.
Synapsids evolved saber-teeth in at least 4 different lineages (van Valkenburg & Jenkins 2002 summarize synapsid history vis-à-vis saberteeth). Have a look at this gorgonopsid skull:
Early synapsid bite morphology follows a distinct pattern. Basal synapsids bit with a crocodile-like ‘snap’, but in many lineages this was swapped for a ‘static pressure’ bite-and-hold system. Saber-teeth only evolved in lineages once that system was adopted. This tells us that there’s a relationship between static pressure bites and saber-teeth: sabers aren't for snappy slashing, but for pressing deeply into prey.
Let’s turn to placental critters.
We know a lot about how cats kill. The basic cat killing bite is an extreme version of the bite-and-hold technique. Roughly, a cat kills by choking: static-pressure par excellence. Their dental morphology reflects this, with minimized molars providing neck-squishing space. In cats, bite power is a function of jaw musculature.
By comparing standard cats to Similodon fatalis, and using some funky modeling, we’ve developed a pretty good idea of how the saber-toothed lion operated. She also relied on the press-and-hold approach (as we’d expect, given analogies from earlier saber-teeth), but here bite force didn’t rely on jaw muscles alone—neck muscles also played a role.
How does this help with T. atrox? Well, if we include neck muscles in our reconstruction, its bite force is comparable with other cats (Wroe et al 2013)—thus, it could have been a megacarnivore after all. Does this make Similodon fatalis a good analogue of T. atrox? Not at all:
"… our findings suggest that in order to minimize stress on the canine teeth and resistance as the canines were inserted, T. atrox needed to move its head considerably further forward and downward relative to the positions of the jaw-joint than would S. fatalis" (Wroe et al, 2013, 6).
The two have very different bites. T.atrox bites using a kind of forceful, downwards head-butt, while S. fatalis still relies on jaw muscles. T. atrox can be understood as an extreme example of the saber-tooth lifestyle. We can imagine cat-style-biting on a continuum with your standard kitty relying on jaw muscles, saber-teeth like S. fatailis using a combination of jaw and neck, and T. atrox more completely adopting the saber-tooth technique by relying on its neck musculature almost entirely.
And so, although T. atrox is unique, analogues can still inform its reconstruction.
But its not just saber-teeth and cats that feed into the exquisite corpse model. Scientists reconstructing T. atrox draw on analogues from all over the place. For instance, T. atrox has remarkably strong forelimbs, and it’s been suggested that these are used to immobilize prey – thus mitigating their weak jaw muscles and comparatively brittle saber-teeth. This same behavior is seen in the (also weak-jawed) bear. Moreover, T. atrox may have had specially adapted musculature to mitigate the bite-force cost of its wide gape. This is seen in (of all things) pygmy marmosets, who open their jaws especially wide to gauge trees (Eng et al 2009).
So, in approaching this unique organism, scientists do two things: T. atrox is conceived of as an extreme example of a general strategy, and treated as possessing a combination of various traits seen in various animals. T atrox is an exquisite corpse.
So, uniqueness does not undermine past-sourced modelling. This is because events are typically 'unique' insofar as the combination of properties they instantiate are unique. Each individual property (at least usually) is not unique. This opens the door for exquisite-corpse modeling. First, we can unify current and future events with past events, but along a scale. A nice example is understanding the extent of anthropogenic extinctions in terms of mass-extinctions—see here, for instance.
Second, different aspects of the past can be drawn upon to construct exquisite corpse models. For instance, the Paleocene-Eocene thermal maxima is certainly not a perfect analogue for our present-day situation, but nonetheless it could (for example) provide important information about what biological patterns could emerge in reaction to increased global temperatures.
So, as for the climate, undoubtedly, as Gavin Schmidt said in 2010,
“There are no true palaeoclimate analogues for the global changes projected for the 21st century… “
But! He goes on:
“However, many of the uncertainties highlighted in that report do involve aspects of the climate that have certainly changed in the past.”
In the study of extinction, or the study of climate, the deep past is a rich treasure-house of epistemic resources we must look at in order to understand many of the challenges facing us today. But we shouldn't go looking for 'perfect' analogues. In fact, sensitivity to the uniqueness of the Anthropocene should motivate us to consider more instances in the past, to develop models drawing on long-term patterns and a wide variety of instances. That is, we should adopt the exquisite corpse method.
Many of the ideas of this post were drawn from my book which maybe perhaps might be finished one day?
For discussion of exquisite corpse:
Brotchie, Alastair; Mel Gooding (1991). Surrealist Games. London: Redstone Press. pp. 143–144. ISBN 1-870003-21-7.
Here’s the rest:
Eng CM, Ward SR, Vinyard CJ, Taylor AB (2009) The morphology of the masticatory apparatus facilitates muscle force production at wide jaw gapes in tree-gouging common marmosets (Callithrix jacchus). The Journal of Experimental Biology 212, 4040–4055 212: 4040–4055.
Schmidt, G.A, 2010: Enhancing the relevance of palaeoclimate model/data comparisons for assessments of future climate change. J. Quat. Sci., 25, 79-87.
Tucker, A. (1998). Unique events: The underdetermination of explanation. Erkenntnis, 48(1), 61-83.
Van Valkenburgh B, Jenkins I (2002) Evolutionary patterns in the history of Permo-Triassic and Cenozoic synapsid predators. Paleontological Society Papers 8: 267–288.
Wroe, S., Chamoli, U., Parr, W. C., Clausen, P., Ridgely, R., & Witmer, L. (2013). Comparative biomechanical modeling of metatherian and placental saber-tooths: a different kind of bite for an extreme pouched predator. PloS one, 8(6), e66888.
Wroe, S. McHenry, C & Thomason, J (2005). Bite club: comparative bite force in big biting mammals and the prediction of predatory behaviour in fossil taxa Proc. R. Soc. B (2005) 272, 619–625.