This is a topic I have been long meaning to cover on the musings as it is a pretty core part of modern palaeontology but exactly the kind of thing that never makes it from the research papers into newspapers of documentaries and thus I suspect many will not have even heard of it as a concept, let alone its application. As ever, this will be a reduced and simplified version so for those who do know better, please forgive the glossing over of details, but essentially the EPB runs as follows – if you have an unknown characteristic in an extinct species, but there are living relatives of that animal that phyologenetically are both more basal and more derived, then you can fairly safely conclude (with caveats, inevitably) that this was also a feature of your extinct organism.
To put this into practical perspective with an archosaurian example, crocodiles are related to dinosaurs but a phylogentically basal to them, and birds derived from dinosaurs and are thus more derived than them. However, while the non-avian dinosaurs are extinct, both crocs and birds are still alive today. Thus characteristics shared by birds and crocs were likely shared by dinosaurs. For example, even without the extensive evidence for dinosaur nests, if we had never found any eggs it would still have been quite a safe assumption that dinosaurs laid eggs since both crocodiles and birds do.
This as a concept obviously has enormous benefits as we can use it to try and reconstruct missing data from direct observation of living animals. We don’t have dinosaur muscles to work from (although muscle scars on bones give us a good idea) but with guidance from the EPB we can be much more confident in our restorations. Furthermore we can also use it to prune out bad ideas (or at least challenge them) if you want to argue that all dinosaurs had no lungs (to take an extreme example!) without any physical evidence it should be pretty obvious that since both birds and crocs have this (and yes, quite a few other things too), it’s going to be a very odd to assert that they are missing in dinosaurs.
There are of course problems with the method and I’ll elaborate on just a couple since in day-to-day archosaur work these are not huge issues normally (i.e. for the average person reading a dinosaur story, they don’t have much impact but are worth watching out for). First off there are obviously huge gaps of time and morphology going on between say the most basal theropods and living birds.
Similarly, one is also effectively trying to use these animals to reconstruct the condition of characteristics in dinosaurs, yet we know some things have changed which make these animals unsuitable. All living crocodiles are rather sprawling in posture, but not all of the extinct ones were and it’s unclear exactly what posture was adopted by animals around the time that crocs separated from the lineage that would become dinosaurs. As such just using them freely as a proxy is not always a good idea. In a similar vein, there are some things for which the EPB is wholly unsuited. For example if you want to (as I do) try and reconstruct the predatory behaviour of theropods the EPB is not a great way to go, modern crocs are largely specialised ambush or aquatic predators and most birds are not predatory (and those that are largely hunt in the air and the few that do not are rather derived and are certainly not basal taxa), so given the huge specialisations of these animals to their respective hunting methods it’s unlikely their behavioural adaptations can tell us much about an Allosaurus trying to take down a sauropod.
There are also times where the EPB is not so much problematic, as just not very helpful. Endothermy (or warm bloodedness to be less technical and less accurate) is an obvious example. Birds are endotherms, crocs are not – so how do you interpret dinosaurs? Were they endotherms or not, or only some and if so, which? Are the basal ones more likely to be ectotherms since they are closer to crocs and derived ones endotherms since they are closer to birds? Or were even basal birds ectotherms? And what about the orniothischians, they separated out from the saurischian (and ultimately bird) lineage over 50 million years before Archaeopteryx appeared so are they the same or different? Of a different note but equally an issue are things like pterosaurs – if these are indeed archosaurs then the EPB for them would be crocodiles and birds, though if they are actually more basal archosauromorphs then the EPB would be lizards and crocs – a very different consideration.
That in essence is the EPB, we can use living relatives to help reconstruct the missing in fossil data. These are inferences, not absolutes and you need to combine these suggestions with other information (is there a muscle scar present for where you think one should be, is that gait possible given the trackways we see, has this already changed in crocs making this inference problematic etc.) to come to a conclusion. It can be incredibly powerful and has been very influential in vertebrate palaeontology by providing a firm and reasoned foundation for making inferences about extinct animals based on real, observable evidence (living creatures).
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