So after the original Anchiornis paper and now the follow up things should be pretty clear about the identity of this animal – it’s a troodontid that looks an awful lot like a basal bird. This is no surprise given that birds and troodontids are very close relatives and that Anchiornis is an especially basal troodontid and thus is probably closer in appearance to other basal birds than many others of its kind, and of course the original specimen in lacking a head did not have all the characters that might have helped us solve this earlier. So now onto the issue of time and evolution which is nicely demonstrated here by the new find – the oft quoted and very misleading temporal paradox.
If you have not met ghost lineages before then you can check out this nice little post here that I did on the subject as an introduction. As a micro-recap these are situations where we have a taxon in the fossil record which according to our understanding of its relationships should be pretty old, but we only find fossils of it in much more recent rocks, the ‘gap’ being the ghost lineage where we believe it to exist but cannot find it.
In the case of the earliest birds we do of course have Archaeopteryx in the later Late Jurassic rocks of Germany and then various other birds appearing in the Early Cretaceous and more recently than this. However, the closest relatives to birds according to our analyses are dromaeosaurs and troodontids, and a little further down the tree are things like oviraptorosaurs and alvarezsaurs (among others). But these taxa only currently appear in the latest Jurassic or the Early Cretaceous, significantly after Archaeopteryx first appears. Thus we have a ghost lineage with these various theropods having (we infer) appeared sometime in the Middle or early part of the Late Jurassic and then birds appearing but the only record (apart form a few hard to identify teeth) being Archaeopteryx itself. In short, if our hypothesis of bird ancestry is correct we really should see some of these other lineages appearing well before Archaeopteryx and until now, did not. This problem has been termed the ‘temporal paradox’ by the BAND group who maintain that this is evidence against a dinosaur origin for birds (despite the obvious caveat that this is true they then have to account for an absence of birds going back to the Triassic which is tens of millions of years a bigger gap – oh).
Until now of course, because since Anchiornis is now identified as a troodontid and the specimens that have been recovered (the original and the newly described one) from the early Late Jurassic and thus predate Archaeopteryx. This of course does not entirely eliminate the problem (we would still really like some old oviraptorosaurs and dromaeosaurs for starters) but it does demonstrate that the gap is far smaller than stated and does show a definitive troodontid that predates birds. As such hopefully the already unstable and rather unreasonable temporal paradox can finally leave these shores and not rear its head again (unless someone finds an Early Jurassic bird of course in which case we get to start again).