Defining Dinosauria

Richard Owen poses next to a Dinornis (moa) skeleton in 1879—but is it a dinosaur?

This group, which includes at least three well-established genera of Saurians, is characterized by a large sacrum composed of five anchylosed vertebrae of unusual construction, by the height and breadth and outward sculpturing of the neutral arch of the dorsal vertebrae, but the twofold articulation of the ribs to the vertebrae, viz. at the anterior part of the spine by a head and tubercle, and along the rest of the trunk by a tubercle attached to the transverse process only; by broad and sometimes complicated coracoids and long and slender clavicles, whereby Crocodilian characters of the vertebral organs also exhibit the same transitional or annectent characters in a greater or less degree. The bones of the extremities are of large proportional size, for Saurians; they are provided with large medullary cavities, and with well developed and unusual processes, and are terminated by metacarpal, metatarsal and phalangeal bones, which, with the exception of the ungual phalanges, more or less resemble those of the heavy pachydermal Mammals, and attest, with the hollow long-bones, the terrestrial habits of the species.

That’s how, in 1842, Richard Owen described “a distinct tribe or sub-order of Saurian Reptiles, for which I would propose the name of Dinosauria“, a group of organisms which needs no further introduction. But what, 170 years later, can be properly classified as a dinosaur? Former highschool science teacher Brian Thomas thinks he knows better than today’s scientists. He wrote on Wednesday, in “Four-Winged Dinosaur Definition Doesn’t Fly“:

What does it mean to be a dinosaur? Older generations understood dinosaurs as reptiles with hip structures that pointed their legs straight down, instead of out to the side to produce the sprawling waddle we see in crocodiles and lizards. Those old-timers probably never imagined that scientists would one day insist that dinosaurs grew feathers. Modern scientists seem to hold a broader, looser definition of dinosaurs, and a new fossil find has found its way into the often-blurred region between dinosaurs and birds. Was it really a dinosaur?

Presumably the “old-timers” mean the childhood dinosaur enthusiasts of yesteryear, whose curmudgeonly opposition to more recent discoveries is well known. Actual scientists, from the very beginning, have used much more robust classifications than a single imprecise character: whole lists of features that can be used to distinguish dinosaurs from other groups exist, while phylogenetic systems have been invented since Owen’s day in addition to these. But Brian Thomas, drawing on the arguments he uses in his quest to prove that mythical dragons were really dinosaurs that lived alongside humans, has come to fixate on the notion that dinosaurs’ legs point downwards, and that this is all you need. When last we addressed that subject I pointed to evidence showing crocodilians are perfectly capable of walking like that, but can’t be bothered most of the time. The definition fails further: even my legs, not terribly well behaved though they may be, fit this description, as I presume do Thomas’ and those of his colleagues at the ICR – but they are not dinosaurs, not in the literal sense. But most crucially so do birds, and with that Thomas’ argument here is dead in the water.

I have no idea why why this has not occurred to Thomas – perhaps it has, but this brings up the age-old question of deceit or (wilful) ignorance, which we wont contemplate further. Regardless, he takes his one-man one-character definition and uses it to attack the classification of the newly described four-winged Changyuraptor yangi as a (non-avian) dinosaur, rather than a bird. It appears that its more acceptable to Thomas that Aves include things with four wings, teeth, and a bony tail than any his precious dinosaurs have feathers. This is what happens when you let your obsessions run too far, kids. What do the birders at the Institute have to say about this, I wonder?

The paper, in Nature Communications, describes the species like so:

A microraptorine dromaeosaurid theropod characterized by having the unique combination of traits: furcula more robust than that of Sinornithosaurus millenii and much larger than that of Tianyuraptor ostromi; forelimb proportionally much longer when compared with hindlimb than in other microraptorines; humerus much longer (>20% longer) than ulna as opposed to Microraptor zhaoianus, in which these bones are more comparable in length; metacarpal I proportionally shorter than in Sinornithosaurus millenii (1/4–1/5 versus 1/3); well-developed semi-lunate carpal covering all of proximal ends of metacarpals I and II as opposed to the small semi-lunate carpal that covers about half of the base of metacarpals I and II in most other microraptorines; manual ungual phalanx of digit II is the largest, followed by that of digits I andt III, as opposed to Graciliraptor lujiatunensis in which the ungual of manual digit I is very small, and Sinornithosaurus millenii and Microraptor zhaoianus in which the unguals of manual digits I and II are comparable in size; ischium shorter than in Microraptor zhaoianus; midshaft of metatarsal IV significantly broader than that of metatarsal III or metatarsal II, as opposed to G. lujiatunensis in which metatarsal IV is the narrowest; mid-caudals roughly twice the length of dorsals as in Sinornithosaurus millenii as opposed to long caudal vertebrae in Microraptor zhaoianus; fewer caudal vertebrae (22 vertebrae) than Microraptor zhaoianus (25–26 vertebrae) and Tianyuraptor ostromi (28 vertebrae); rectrices significantly longer than in other microraptorines.

“Microraptorine” means that it is closely related to Microraptor, while dromaeosaurs are a group of (feathered) dinosaurs that includes Velociraptor, which the “old-timers” would definitely call a dinosaur even though this status would have to be sacrificed to have any hope of maintaining the purity that Thomas demands. Microraptor and related species can be pictured as quite closely related to birds, but represent an offshoot in a different direction, and their four- (or five- if you count the tail) winged mode of flying is useful for studying the origin of flight in true birds. Not having read the memo – or ignoring it – Thomas writes:

If this ancient creature really represents an evolutionary transition between dinosaurs and birds then details in its anatomy should look part-dinosaur and part-bird. For example, perhaps it should show feathery forelimbs that could not quite generate the lift required for flight. Ironically, however, the study authors insisted that Changyuraptor exemplified dino-to-bird transition, but on the other hand described it as a unique but fully functional flying creature.

I don’t actually claim to know the precise definition of “irony” in all its subtlety (few do), but I don’t think that’s it. Instead Thomas is trying to say that this is in some way a paradox, but that is not so. Not only is it not being claimed that Changyuraptor is not the direct line to birds, but the creationist refrain that a transitional form would be non-functional is simply wrong.

This is, however, all he has. He doesn’t actually return to the question of whether or not Changyuraptor fits his crude definition of dinosaur, but instead insists that it fits the description of a bird. But those categories are not mutually exclusive – that’s the whole point there. Finally, refering to an earlier article of his, he concludes:

Perhaps like the recent author who reverted back to calling a long-time bird a dinosaur after a second look at the evidence, these evolutionary researchers may one day regret having referred to Changyuraptor as a “four-winged dinosaur.” Changyuraptor had none of the transitional features required to morph a real dinosaur into a bird. True, it was unlike any of today’s familiar birds, but as the saying goes, if it quacks like a duck, has feathers like a duck, then it’s probably a duck. In this case, bird wings and feathers don’t make a dinosaur, but rather an exquisitely well-fashioned four-winged flying bird.

In my experience the saying is “if it looks like a duck and quacks like a duck” – and that ain’t no duck.