Back in early 2015 I took a trip to LA, primarily to catch up with Mike Habib and look at some pterosaur and tyrannosaur material there, but I also took some time to see Andy Farke and Matt Wedel up in Claremont. We chatted about various ideas for things we could collaborate on and threw around a few ideas. Andy suggested something on ontogeny and this soon led to the issue of diagnosing life stages for dinosaurs – something that had been an issue for our Protoceratops paper – and within a few weeks I’d actually had an invitation to submit a review to Biology Letters, and so a plan was hatched.
That paper is now out and in it we look at the vexed issues of what are adult / subadult / juvenile / hatchling etc. dinosaurs. This is of course really quite fundamental to huge amounts of research, if it’s not clear how old an animal is, then issues like taxonomy, systematics and their position in an ecosystem are going to be hard to sort out. Comparing across specimens or species will also have their issues. None of this is a major surprise and yet looking though the literature it’s clear that although people recognise this, they don’t necessarily actually define the nature of the animals they are working on. Things are called ‘adult’ or ‘subadult’ without a definition, specific diagnosis or reference to papers or alternatively they do provide some kind of definition and reason for the assignment but it’s different from all the others out there. It doesn’t take long to find a bewildering and ever changing list of definitions, none of which can be aligned or compared easily between specimens or species.
There’s clearly nothing wrong in principal with diagnosing an animal by different means but not all specimens can be accessed in the same way or preserve things you want to look at. So something that can help bring them into alignment should help everyone. This is a key part of the paper as we try to come up with something close to a universal definition that should apply as widely as possible. We make it very clear that this should be only a starting point and that whatever works for people is fine, but that hopefully it helps, and even if people utterly ignore these definitions, in general we need to be much more careful about actually putting definitions into papers, even for things that are ‘obviously’ adults or juveniles.
Although short, we do cover a lot of ground in the paper and I hope there’s things in there that will resonate and be familiar and useful to many people (and of course lots of the points apply to other extinct clades too). There’s obviously a lot more to come here and more nuance and details than we could easily include but it’s one of he most contentious and important issues around at the moment and I really hope we have contributed meaningfully to it.
The paper currently seems to be available freely online and can be downloaded here.
Edit: here’s a bonus – Mat Wedel’s sauropod-centered take on the paper
Hone, D.W.E., Farke, A.A., & Wedel, M.J. 2016. Ontogeny and the fossil record: what if anything is an adult dinosaur? Biology Letters
Dave Hone's Archosaur Musings 
Might this settle the “Toroceratops” debate once and for all? I’m surprised you guys featured Zuniceratops instead.
Andrew – we don’t ‘feature’ Zuniceratops in any way. We wanted a nice skeletal for the figure and a ceratopsian made sense and Julius had that and kindly allowed us to use it. If you read the fig caption, there’s nothing there that is actually about this animal, it’s all representative / conceptual.