Developmental trait evolution in Trilobites

Fusco et al. Evolution 66(2): 314-329. DOI:10.1111/j.1558-5646.2011.01447.x. Developmental trait evolution in Trilobites

This is a guest post with Tim Astrop. Below, we give our first impressions of this article. Please comment below, or tweet TimWill or Lynsey (maybe use #pegejc). Think of this as a journal club discussion group!


Figure 1 from Fusco et al. – trilobite moult cycle

Tim Astrop

Tim Astrop

This paper is a wonderful example of how innovative thinking and an integrative approach to the fossil record continues to discover new keys with which to unlock the unknown biological information held within. The Trilobitomorpha is arguably the most diverse and abundant extinct arthropod subphylum, there is certainly no shortage of palaeontological research on these critters, that’s for sure. But despite such popularity and interest in the group we still know very little about them as biological organisms. In this respect it is very apparent that preserved remains are not representative of the living animal, there is much interpretation to be done (see the recent ‘spotted trilobite’ described by McRobets et al 2013).

By looking at different ontogenetic stages represented by the different instars (discrete developmental stage of arthropods ‘bookended’ by ecdysis, or molting) preserved for over 60 species of trilobite for which adequate data is available, the authors then derive new metrics to treat growth rate via average per molt growth increment (AGI) and conformity to Dyers rule (IDC), while the former metric is self explanatory, the latter is less intuitive. The IDC quantifies the fit of observed ontogeny to that of a constant growth rate, deviations from Dyers rule would be indicative of accelerated/decelerated growth at particular ontogenetic stages (readers with a thing for shape analyses may also have noted that allometry would upset this metric via modular differentiation of growth rates within an organism, well done, have a gold star).

What is particularly cool is that the authors use this info not just to look at the evolution of this group but also to shed light on early euarthropod development, as the Trilobitamorpha are basal arthropods. Euarthropod evolution is a very active area of research (see Yang et al 2013 for interesting recent discovery) and held by many as something of a rosetta stone for understanding arthropod origins and diversity.

The meat of this article has a little too much math for than I would normally elect to tackle, but after making it through the results the discussion highlighted the findings really well.

In a nutshell, it seems that trilobites conform to Dyers rule overall, with some smaller deviations occurring in early (protapsid) development. However, the particularly interesting thing, for me at least, is that there appears to be less phenotypic integration in later stages that enables very unique, differing morphologies to evolve that deviate from standard cut-and-paste metamerism (segment repetition) seen in many trilobites (but probably most evident in the familiar Myriapoda) and calls forward to the future of arthropod diversity in all it’s mis-matched-crazy-appendage glory. It also infers that that similar morphologies may be converged upon via different developmental pathways, to me this indicates that trilobite morphology is not restricted to an adaptive peak but provides a method to move between peeks via morphological lability, something the authors refer to as ‘evolutionary dynamism’, a trait that likely contributed to their success as a group.

In summary, I think this article is a wonderful example of how valuable palaeobiological approaches can be to understanding organismal evolution. Here we have a novel approach to an existing dataset using linear measurements (and complex stats) to derive new information about the evolution and biology of an extinct arthropod subphylum.

In my opinion, the true integration of biological and palaeontological principles allows fossils to be treated as the cryptic remains of biological entities that old-school paleontology can often overlook in favour of a ‘stamp-collecting’ mindset. Thankfully there appears to be something of movement among palaeobiologists at the moment back to a more synthesized ‘Simpsonian’ concept of the fossil record. Something I look forward to hearing more about, and to hopefully contribute to!

Will Pearse

Will Pearse

I normally hate people who loudly shout “yay science” on Facebook and the like, but this paper really was one of those moments for me. We can track the development of growing trilobites using fossils? We can robustly test hypotheses about the evolution of that development? Yay science!

The most parsimonious evolutionary model of these species’ traits is one where phylogeny doesn’t matter (a star phylogeny), so it’s hard to argue that there’s any constraint to trait evolution here, despite the phylogenetic signal of some of these traits. The authors suggest this reflects the influence of particular outlier clades that are biasing model fit; I completely agree. I spend much of my time doing analyses of ‘plants’, and it’s always struck me as rather strange that I lump all these diverse and divergent species under one label and then examine their evolution altogether. I imagine the same is true of trilobites – the devil is in the detail, and understanding exactly what clades are varying (and why) will probably give us a better handle on what drives the evolution of these traits.

In passing, I wonder what the consequences of doing morphological analyses on phylogenies of extinct species that have to be built from morphological data. What if some of these traits, or traits that are strongly coupled with these traits, were used to define the phylogeny of these species – after all, ‘ontology recapitulates phylogeny’, and information about how species develop would seem (to a person ignorant of paleontology) like useful data. Could this be why the authors find stronger phylogenetic signal in the earlier developmental stages, or does this actually reflect the biology of the system? Does more evolutionary constrain in juvenile growth suggest juveniles were competing more strongly than adults (mortality does tend to decrease as species get older)? The authors give very detailed supplementary information on the phylogeny’s construction, but I’m really nowhere near qualified to comment. Can anyone help me out?

Lynsey McInnes

Lynsey McInnes

You can’t deny that Will and I like to stretch ourselves in the diversity of papers that we discuss on this site. Who knew the coverage of the terms phylogeny, ecology, geography and evolution was so broad. This week we take another foray into paleo research, this time without the potential to incorporate any extant data, these things are extinct!

My first reading of this paper was definitely made while wearing a cynical hat, and I was unconvinced that the data were adequate to ask these questions, particularly regarding the patched together phylogeny. A bit more time with the paper though, and I was becoming more and more impressed with the authors’ ability to extract conclusions, all appropriately caveated given the antiquity and patchiness of the available data. Perhaps we are all becoming too obsessed with perfect datasets, Bayesianed up and with no room for interpretation based on slotting in genuinely new findings with an already amassed body of research. This paper is a great example of the latter approach. Neat!

I’m most acquainted with adaptive zones in terms of diversity dependent cladogenesis…clades diversify until a zone is full, diversification rate declines until some upstart clade pops off into a new zone to start the process again. Sadly, a lot of contemporary ‘adaptive zone’ research remains a numbers game with little detailed investigation in what adaptations a clade possesses to occupy their zone. In contrast, this paper makes a concerted attempt to actually think about the adaptations necessary to occupy different zones and finds strong evidence for these traits in the Trilobite lineages studied. Impressive given these things went extinct millions of years ago! Not only that but the authors also successfully distinguish between intrinsic and extrinsic drivers of the patterns that they find! I’m a bit obsessed with the landscape or environmental drivers of macroevolutionary patterns and was really pleased to see this angle tackled in this paper.

So, I’m no paleontologist or developmental biologist, and appreciated this paper mostly as it reminded me that science need not be all about the perfect dataset producing good model fit in a standalone clincal model, it can be messy and incomplete, but illuminating.


Macroevolutionary perspectives to environmental change

Condamine et al. 2013. Ecology Letters: early view. DOI:10.1111/ele.12062. Macroevolutionary perspectives to environmental change.

Below, we give our first impressions of this article. Please comment below, or tweet Will or Lynsey (maybe use #pegejc). Think of this as a journal club discussion group!

Lynsey McInnes

Lynsey McInnes

For our second post, we picked a monster by accident. This is a mammoth perspective paper on the potential of macroevolution to provide insights into expected environmental change. It’s good, it’s comprehensive, it’s kinda hard to get through in one reading. I commend anyone that did.

So, my commentary is going to be a bit haphazard and mostly just the thoughts that came to mind after my fourth skim. It should also be prefaced with the info that I’ve just turned my back a bit on macro-scale approaches because I’d become frustrated with all the patchings-over and arm waving necessary to get from pattern to process with data at this scale. Having said that, smarter people than me can probably make that leap and it be meaningful (i.e., a lot of the recent research cited here: note the prevalence of refs from 2011 and 2012 – this field is moving fast!).

I thought the authors did a great job of provided a measured perspective on the potential for macroevolution to provide practical insights into the effects of contemporary environmental change. They highlight that today our environment is changing extremely rapidly, potentially way more rapidly than in the past (although this may just be because we can’t resolve time to such narrow intervals in the past). They take great pains to highlight how dodgy extinction rate estimates from reconstructed phylogenies are (but indicate the sorts of conditions where estimates might be more reliable). They emphasise that extinction risk today might have different correlates to in the past (and outline neat ways to test for this). They collate and summarise a ton of (mostly very recent) paleo- and phylo- research in an accessible and intelligent way.

A tiny rant – so much on whales! Which demonstrates a point that I think the authors would also agree with: data availability remains an issue. The most robust contributions from macroevolution seem to be ones consisting of a mix of good paleo- and phylo- (here, reconstructed phylogenies of extant lineages) data. And this data is patchy or absent for most groups – except whales… How much are our inferences curtailed by lack of data versus lack of signal of past events in that data?

Cross-talk. One thing I did think the authors could have mentioned (in an otherwise comprehensive overview) is the potential lack of communication with researchers generating the data and researchers developing methods to analyse this data. Although there are (probably quite a few) exceptions, my view is that there is a small band of researchers developing ever more complex models that are then applied by another set of researchers on their painstakingly built phylogenetic dataset. A more fruitful method might be for more cross-talk between methodsy and data people so that data is collected and compiled explicitly to answer interesting questions with powerful methods. For instance, the authors end with a brief discussion of the impact of interaction networks and ecological traits on species’ responses to environmental change. What data would be most useful to start modelling these questions and who is best placed to generate it? THAT development would be exciting!

A random selection of other thoughts that came to mind:

Would macroevolutionary perspectives be most useful in conjunction with microevolutionary ones? As global change is so rapid, microevolutionary/ecological responses are the ones we are going to be able to measure. How do these translate into macroevolutionary change (i.e., what types of short-term responses are retained to be detected at the macro-scale – if we knew that, we could (maybe) look for such signals in existing phylogenies)?

Will we ever be able to confidently identify clades nested in larger phylogenies that have been diversifying according to some homogeneous process (or rather will we ever be able to identify higher-level units a la Barraclough 2010)? It seems like if we can do this we’ll be in a much stronger position to infer how past environmental change or past biotic interactions have influenced clade dynamics.

More generally, are we approaching the point where we’ve extracted all the information we can from macro-scale data, or are we just waiting on more sophisticated models/methods?

Finally, in 10 or 100 or 1000 years time, what will the tree of life look like?

Lynsey McInnes

Lynsey McInnes

Everything Lynsey said about this being a big paper is correct, but I think we’d both recommend you stick with it because it covers so much ground. One of the best things a paper can do is make you think, and I really enjoyed reading this paper with a beer in hand to fuel my thoughts!

Lynsey mentioned data availability, and while the authors mention foraminifera quite a few times, they only briefly mention Ezard et al. I like this paper for two reasons: firstly, it has some of my friends on it (…), and secondly, they assess extinction rates using a dataset where we can be almost certain that we caught most of the extinction and speciation events that mattered. Estimating extinction rates from molecular phylogenies is hard (the authors discuss this) – and sometimes it’s really hard to do. Should we (/could we) be shifting our efforts to systems like foraminifera where we have more precise data? This naturally leads me to wonder to what degree taxa differ in their extinction and speciation rates, and what impact this could have on the field…

I think there’s a weird disconnect between conservation biologists and evolutionary biologists, and (as someone who works on eco-evolutionary stuff) I really enjoyed their discussion of how conservation biologists could focus on areas that generate phylogenetic diversity. I think things like the EDGE list are a really good way of helping the general public place evolutionary dynamics in the wider context of conservation biology, but maybe we could do more to link these two areas. Conserving particular areas because they are evolutionary sources of biodiversity is one way, but could we start using information about the way in which species originated to help us better model how they are likely to respond in the future? For example, maybe species that radiated in ‘favourable’ conditions are more likely to go extinct when faced with difficult environmental conditions – a bit like cichlid species being lost when Lake Victoria becomes more polluted. Perhaps that’s stupid, but aren’t there other (fairly tractable) examples we could use?

A few small (probably increasingly silly) questions to finish off. There’s a lot of debate in ecology about spatial scaling, and the extent to which processes at the local (micro) scale apply at the global (macro) ecological scale. If conservation actions are more micro-scale in their application (we can’t make a protected area the size of Brazil, for example), does this reduce the utility of these kinds of historical analyses when trying to understand present-day change? Land-use change is distributed across the planet different to many previous drivers of extinction – does this matter for these kinds of studies?

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