Of all the stories in vertebrate evolution, the transition from dinosaur to bird is one that never ceases to capture the imagination of the public and academic community alike…
If you didn’t know it already (and you should), all birds are, technically speaking, dinosaurs.
The rules of taxonomic hierarchy mean that, by virtue of directly descending from animals that we would call dinosaurs, birds fall within the clade Dinosauria. Hence, by extension, it is fair to say that dinosaurs still represent the most speciose group of terrestrial vertebrates alive, with over 10,000 species known to still be around today.
Obviously modern day birds appear in many ways to be very different creatures from some of the non-avian dinosaurs that populate childrens’ books and museum corridors. A considerable amount of time has elapsed since the crown-group of birds (= the least inclusive group that contains all living members) first appeared. A lot of changes have occurred in this elapsed time and many of these changes are now, thanks to work by palaeontologists, quite well understood in terms of the order and tempo of their appearance.
Recently, there has been a number of papers appear in the scientific literature that shed further light on this transition. While I am usually not that interested in the theropod lineage (= meat eating dinosaurs), I thought it might be worth at least presenting the finds of a few of these recent papers as they are, admittedly, quite cool.
First up… Ichthyornis
A month or so ago, new work was published on the animal we call Ichthyornis (which means ‘fish bird’). This ancient seagull like bird has been known to palaeontologists since its discovery and naming by Othniel Charles Marsh in the late 1800s. However, with cool new CT scanning tech and a new specimen, more about this bizarre, extinct sea bird has been revealed. You can read the article, which was published in Nature, here.
The paper, written by Field et al., illuminates the mosaic assembly of the avian head by detailing this new anatomical information. The long and short of it is, perhaps unsurprisingly, that this ancient and ‘primitive’ bird has some features of modern birds whilst also retaining a number of features more traditionally associated with non-avian dinosaurs including… wait for it… TEETH!
Yes, as theropod dinosaurs, birds would, ancestrally, have had teeth; only later on in their evolutionary story do they abandon this feature entirely and rely on the beak that we so easily recognise as a bird-like feature these days.
Anyway, I for one wouldn’t want to encounter this toothy hell-gull sitting on a Late Cretaceous beach while trying to enjoy my fish’n’chips, or whatever. Thank god for asteroids!
Another couple of colleagues also recently published a paper on the acquisition of the beak in the bird line, based upon new evidence of dinosaur incubation periods. You can read this article by Yang & Sander in Biology Letters, here.
In this article, the authors suggest that election for tooth loss was a side effect of selection for fast embryo growth and thus shorter incubation, above any other explanations for such a loss. Quite an interesting idea, but one that certainly needs more testing.
Another colleague, Dr Cau, has published a massive work that covers the assembly of the avian body plan over 160 million years of theropod evolution. He has kindly placed it on Researchgate, where anyone can read it for free, here.
This is a cool work for many reasons, and really hammers home the arguments about how many of the features that were once assumed to be bird features related to bird-like behaviours, such as flight, actually have their origins well before the origin of birds. Many of these features e.g., feathers, actually appear before true birds and before flight, and were later exapted* for other uses.
* Exaptation = a term used in evolutionary biology to describe a trait that has been co-opted for a use other than the one for which natural selection has built it.
I also like this paper for a selfish reason… notice anything right down at the base of the tree?
In this tree, Ornithischia is placed closer to Theropoda than Sauropodomorpha is… So this paper also represents the first independent recovery of Ornithoscelida in a phylogenetic analysis of dinosaurs since our publication last year.
However, I would argue that, without the same broad sample of non-theropod dinosaurs that Paul and I had, this dataset is not entirely fit for purpose when trying to solve that particular mystery. If we could maybe work my dataset and Cau’s into one coherent dataset, then maybe we might get something different. Who knows?
Anyway, that’s not really related to the evolution of birds in any way that really matters, so I’ll end it there.
Dr Matthew Grant Baron