Why Pterosaurs aren't Dinosaurs

So I decided to take a trip to Cecil’s “storehouse of knowledge”, and re-discovered the article, Pterosaurs aren’t considered true dinosaurs. Why not?

Reading through the article, it occurred to me that it could probably use an update. So, being a) bored and b) a geek, here we go:


When the term “dinosaur” was first coined by Sir Richard Owen in 1842, it was for a group of three(!) then-known genera of large reptiles: Megalosaurus, Iguanadon and Hylaeosaurus (Cetiosaurus and Streptospondylus where also known at the time, but were not considered by Owen to be related to these three. Two other genera, Plateosuarus and Poekilopleuron, where also known at the time, but since Owen had not examined them, he did not include them within the group). The primary character which Owen drew upon to determine that these animals were related was the fusion of their sacral vertebrae; all three genera had this character, while other reptiles did not. This indicated, to Owen, common descent from a reptilian ancestor (obviously, they shared many other features with reptiles in general).

Owen therefore classified his “Dinosauria” as reptiles, granting Dinosauria the Linnaean rank of Suborder within Class Reptilia (incidently, he also classified them within Lacertilia, as he considered them to be simply really big lizards; thus his coining of the word “dinosaur”, or “terrible lizard” – he meant that literally!).

As more dinosaurs were discovered (largely as a result of the ‘bone wars’ between Othniel C. Marsh and Edward D. Cope during the mid-1800s), other classification schemes were advanced; perhaps the most notable was Marsh’s proposed scheme of four separate Orders within Reptilia: sauropods, theropods, stegosaurs and ornithopods. This scheme, like many of the others proposed, was based on such characters as foot and tooth structure. However, Harry G. Seeley, in a paper written in 1887 and published in 1888, noted that all of these reptiles could be separated into two rather distinct groups, on the basis of hip structure: those with a forward-pointing pubis (the sauropods and theropods), and those with a backward-pointing pubis (the stegosaurs and ornithopods). He proposed the name of Saurischia for the former group, as this was the typical “reptilian” hip structure, and Ornithischia for the latter group, as this hip structure was reminiscent of that of modern birds (note that other characters, such as braincase and vertebrae structure, also served to distinguish the two groups in Seeley’s scheme; however, primacy was given to the character of hip structure). Both groups were given the rank of Order within Reptilia.

Seeley also felt that Saurischia and Ornithischia were distinct, unrelated groups within Reptilia; Dinosauria proper was thus dismantled, and the term “dinosaur” became an informal one within scientific literature.

As a side note, Cecil mentions in his article that “until the 1970s paleontologists believed, in their heart of hearts, that there weren’t any true dinosaurs. ‘Dinosaur’ was an informal term used to describe two distinct groups of animals, the Saurischia and the Ornithischia.” An updated revision of this statement, then, would read, “between 1888 and the 1970s paleontologists believed…”, since between 1842 and 1888, “Dinosauria” was accepted as a formal term.

Which brings us to:


Around the mid 1970s, the concept of “dinosaur” underwent a bit of an overhaul. First up were Bakker and Galton, in 1974, who argued that Dinosauria be reunited and elevated to its own Class, with Aves subsumed within. A suite of features associated with upright stance, bipedality and increased metabolism were used to justify this new version of Dinosauria. Unfortunately, the premise of a new vertebrate class was generally not accepted, nor was the reiteration that dinosaurs were indeed monophyletic. Seeds of dissent had been sown, however. Argentinian paleontologist J. F. Bonaparte met similar resistance in 1976 when he likewise proposed that dinosaurs were monophyletic.

In 1993, Kevin Padian and Julian(?) May formally defined the various groups using phylogenetic systematics (aka cladistics). Jacques Gauthier had given Archosauria (the group containing dinosaurs, pterosaurs, crocodilians, and, eventually, birds) a cladistic overhaul around 1986, and had confirmed, based on his analyses of numerous characters, that a) dinosaurs were, as suspected, monophyletic – that is, they represented a true group of animals descended from a common ancestor – and b) that under strict cladistic defintions, Aves belonged within Dinosauria, as they, too, were descended from the same common ancestor as all other non-avian dinosaurs. (As a result, one encounters informal mention of “non-avian dinosaurs” in some literature.)

Anyway, Padian’s & May’s formal definitions for the groups were as follows:
[li] Dinosauria = all descendants of the most recent common ancestor of Passer (Swallows) and Triceratops.[/li] [li] Ornithischia = all dinosaurs closer to Triceratops than to Passer (that is, eveything sharing a more recent common ancestor with Triceratops than with Passer).[/li] [li] Saurischia = all dinosaurs closer to Passer than to Ornithischia (that is, eveything sharing a more recent common ancestor with Passer than with Triceratops).[/li][/ul]

It was later proposed that Megalosaurus and Iguanadon should have been used instead of Passer and Triceratops, respectively, to define the groups, as the group thus described would be exactly the same as that proposed by Padian and May. This would have served primarily as an homage to Owen’s original definition; regrettably, it was too late to change the definitions because of nomenclature priority rules (the suggestion occurred after Padian and May had formally published their deifnitions). This latter definition has gained popularity as an informal one, however.

So, now we have a (phylogenetic) definition of ‘dinosaur’: The most recent common ancestor of Passer and Triceratops (or Megalosaurus and Iguanadon, if you prefer), and all its descendants.

“But Finch,” you ask, “what is a dinosaur?” And that is indeed a good question (and the point of this post, after all!).

The phylogenetic definition tells us who belongs to the dinosaur club. What we want, in order to determine how one gains entry into this club in the first place, is a diagnosis, or a set of characters which are unique to the members of a group (in cladistics terminology, such characters are termed synapomorphies, or shared derived characters, indicating a suite of novel characters shared by all, or most, members of the group) - in this case, of course, dinosaurs.

Such characters are largely a matter of observation and interpretation. However, a representative (and, if one is not versed in bone/skeletal terminology, possibly incomprehensible) list of such characters might be as follows:

  1. Enlongate vomers that reach caudally at least to the level of the antorbital fenestra
  2. Three or more sacral vertebrae
  3. Scapulocoracoidal glenoid facing fully backward
  4. Low deltopectoral crest that runs one-third or one-half of the way down the shaft of the humerus
  5. Three or fewer phalanges in the fourth digit of the hand
  6. Largely to fully open acetabulum
  7. Fully offset proximal head of femur with a distinct neck and ball
  8. Greatly reduced fibula
  9. Well-developed ascending process of astragalus

(Modified from The Dinosauria, ed. Weishampel, Dodson & Osmolska, 1990)

A further-refined (and extended) list of characters, from various authors (post The Dinosauria, mentioned above), might be as follows:

  1. Ectopterygoid lateral to transverse flange of pterygoid
  2. Postfrontal absent
  3. Temporal muscles extend anteriorly onto skull roof
  4. Quadrate head exposed laterally
  5. S-shaped neck
  6. At least 3 fully incorporated sacral vertebrae (with 3rd incorporated from dorsal vertebrae)
  7. Forelimb < 50% length of rear (reversals in several groups)
  8. Humerus with elongate deltopectoral crest
  9. Manus 4 with < 4 phalanges
  10. Claws on 1-3 only
  11. Semi-perforate (usually fully perforated) acetabulum with buttress
  12. Brevis shelf on ilium
  13. Ischium with obturator process restricted to anterior 1/3rd
  14. Femur with ball-like head
  15. Shaft of femur straight or bowed anteriorly
  16. Femur vertical
  17. Femur has greater, lesser & 4th trochanters
  18. Tibia with cnemial crest
  19. Well-developed ascending process of astragalus on anterior face of tibia
  20. Calcaneum with concave surface for articulation of fibula
  21. Metatarsals elongate and function as part of pes

(Note that not all authors regard all of the above characters as being diagnostic of Dinosauria. And some, such as the 3 sacral vertebrae [#6] and partially/fully perforated acetabulum [#11], carry more weight than others.)

So, what all this means is that 1) you are a dinosaur if you are descended from the most recent common ancestor of Passer and Triceratops, and 2) we can tell if you are, in fact, part of that group based on whether you possess the above characteristics (or, at least, the bulk of them).

Which brings us to pterosaurs.

Things are a little less clear where pterosaurs are involved. The general consensus is that pterosaurs are archosaurs (like dinosaurs) and belong to the clade Ornithodira (also like dinosaurs). Other ornithodires are Lagosuchus and Lagerpeton, which are believed to be early dinosaurian relatives (with Lagosuchus being the more dino-like of the two).

Not unlike dinosaurs, there are two main groups of pterosaurs: the small, long-tailed Rhamphorhynchoidea and the larger, short-tailed Pterodactyloidea. Both groups sport an enormous (relative to body size) digit IV (the equivalent of our ring finger), which supports the famous wing membrane. Some of the few uniting characters that have been agreed upon by various authors include large eyes, big brains, hollow bones, the presence of a keeled sternum (much like birds), and a bird-like scapula. In addition to the whole “giant finger” thing, of course. The aforementioned Dr. Padian put forth a number of synapomorphies for Pterosauria in 1997; however, he also admits that many of these characters are shared by Dinosauromorphs (the group which includes Dinosauria, Lagerpeton and Lagosuchus) in general.

So, the real difficulty, then, is in defining what pterosaurs are, as opposed to what dinosaurs are. Clearly pterosaurs do sport novel features (e.g., the structure of the wing). They also lack some of the aforementioned (and important) dinosaurian features, most notably those which pertain to the hip structure (dinosaurs are unique among reptiles in holding their limbs directly beneath the body, as opposed to them splaying out to the side; pterosaurs are thought to be splay-footed, though Dr. Padian disagrees). Pterosaurs also lack the “perforated acetabulum” [a hole where the three hip bones – the ilium, ischium and pubis – meet] that all dinosaurs, including birds, possess.

Arriving at a proper definition is further complicated by the fact that pterosaurs are represented rather poorly within the fossil record – particularly early pterosaurs which might better allow researchers to determine their proper relationships to other archosaurs & archosaur relatives.

So, as things stand now, pterosaurs are not dinosaurs by virtue of descent: they are not believed to have descended from the most recent common ancestor of Passer and Triceratops. This has been determined, to the best ability given currently-known fossils, by the lack of a number of truly dinosaurian features, primarily those with respect to the structure of the hip. Future finds could, of course, place pterosaurs within Dinosauria; however, the current thought is that the primeval pterosaur is more likely to be either an offshoot of the eosuchians (early reptiles from the Permian which later gave rise to the archosaurs) or the same (or similar) archosaurian ancestors which gave rise separately to dinosaurs and crocodiles.

MODERATOR NOTE: please be aware that this thread is from 2002-03 until revived near end of 2nd page, in post #66, in April 2014. That’s OK, I just don’t want someone getting worked up about responding to a post that’s over ten years old, and expecting anyone to remember what they said (if they’re even still here) – CKDH

Thanks. Very well done, if so technical I really didn’t follow the details much except to understand there are sets of features that differ enough to suggest the points of common ancestry split at different times.

Because I walked out of Cecil’s Column still not really understanding why Pterosaurs weren’t considered dinosaurs from the beginning. Oh, it said they were defined not to be dinosaurs, but didn’t really explain why. Your answer kinda does.

Thanks :slight_smile:

Are there any particular concepts / definitions / whatever which might need some clarification? Since it was already rather long, I did leave out a lot of such explanations. I do plan to “simplify” the characters list a bit (to make it a bit more comprehensible), but that wll have to wait until morning.

Like I said, the technical terminology is thick for the uninitiated, but you did a good job conveying the gist.

One slight correction. Passer is the genus to which house sparrows belong (Passer domesticus).

Swallows are (usually) in the genus Hirundo as in the barn swallow ( the common European swallow).

D’oh! You’re correct. I really meant sparrow, honest!

Thanks for catching that.

Thank you, Darwin’s Finch, for an excellent presentation of this commonly glossed over detail–the matter of definition–in a popular and vivid subject. Of course, the common public’s definition of a dinosaur would be something like “any over-size reptile, now extinct” but that’s hopelessly vague to any student starting a serious study.

But your definition–all descendents of the most recent common ancester etc.–seems to state one thing that raises my eyebrows: According to taxonomists, all avians are dinosaurs. Do you mean to say that, by the official definition, all birds are true, living, in-the-flesh dinosaurs? This must be news to millions of schoolchildren. :slight_smile:

I’m picturing a trivia question: Which of the following is a true dinosaur?

  1. Pteranosaurus
  2. Plesiosaurus
  3. Pseudosuchus
  4. Kimodo Dragon
  5. common sparrow

Now THAT’S misdirection. :slight_smile:

Bravo, Darwin’s Finch!

Satyagrahi, birds are not dinosaurs any more than humans are shrews. Sure, they evolved from dinosaurs the way humans evolved from a shrew-like ancestor, but they no longer are dinosaurs.

Well, that depends on whether you go by traditional or cladistic terminology. In traditional use, one group can split off from another group (e.g., the mammals split off from the reptiles). But in cladistics, a group is forever a member of every ancestral group (e.g., mammals are still amniotes – which is the closest term to “reptile” in cladistic use).

What I am *not/i] sure of is the current state of affairs in this overall debate. Is all traditional Linnaean classification above the level of genus or so being replaced by a cladistic system, or are the two going to coexist forever?

No, the two cases are not analogous. Birds descend from dinosaurs, and therefore cladistically speaking they are dinosaurs. Humans and modern shrews share a common ancestor.

This whole cladistic classification thing keeps spawning a raft of related questions: Are humans apes? Are birds dinosaurs? Are birds reptiles? Are early tetrapods properly described as “amphibians”? Are early amniotes properly described as “reptiles”? The whole thing is worthy of a GD thread, but I’m afraid to start one–all they talk about over there are religion and politics.

My ex was a shrew, but that’s another story.

Laypeople like me have trouble understanding how and why the greatest creatures on Earth became tiny sparrows. I understand basic evolution, and that drastic changes can take place when tens of millions of years are given. However, the the dinosaur to bird process is still puzzling. Of course, up until about ten thousand years ago we had flightless birds on the order of twelve feet tall, which is a link a little easier for us non-scientists to grasp. However, it’s all so much more complicated!

If the Komodo Dragon is the largest living lizard, what is the largest crocodilian?

Ahhh, I think I discern the root of my confusion here. I learned the traditional, Linnaean system in school back several decades ago and I was unaware that ‘cladistics’ was a competing system of classification rather than a methodology within the Linnaean system.

OK. Since Darwin’s Finch seems to be on vacation, I’ll go google “cladistics” and become enlightened.

Thanks, everyone!

Why? Why would you assume that bigger is always better? There are gazillions of examples of small critters evolving from large ones. I imagine, for example, that guppies have done quite a bit of downsizing over the millennia. And think of the size difference between tigers and pussycats, which are a lot more closely related than sparrows and tyrannosaurs.

jklann, you misunderstand me. I was not saying that humans are descended from modern shrews. I was saying humans are descended from a shrew-like ancestor. That ancestor would also be a shrew, of sorts. I abbreviated, because it is tedious typing “shrew-like ancestor” all the time.

Or to put it on a more similar note, sense I feel other objections coming. The shrew-like ancestor was a mammal, just as humans are mammals. The shrew-like ancestor was a rodent. Humans are not rodents. A bird is not a dinosaur, even though it descended from dinosaurs.

But apparently I’m not up on cladistics.

Breathe Exhaust, kill off all dinosaurs bigger than a chicken. Now is it easier to see how they became sparrows?

No, see, that’s the key point. Actually, even modern shrews aren’t rodents–they’re insectivores, more closely related to moles than hamsters. But the important thing is that there’s no special relationship between the “shrew-like ancestor” and modern shrews. The resemblance is superficial; we describe the extinct ancestor as “shrew-like” just because we need some living animal to compare it to so that we can picture what it looked like. The “shrew-like ancestor” is neither rodent nor insectivore; it can’t belong to any order of mammals because it lived before the mammal clade divided into orders. It is the progenitor of every order.

Whereas with birds, they don’t descend from “dinosaur-like ancestors”; they actually descend from dinosaurs.

Breathe Exhaust, kill off all dinosaurs bigger than a chicken. Now is it easier to see how they became sparrows? **

Oh, I assumed it was the end of the line for the “non-avian” dinosaurs when the asteroid slammed into the planet 65 million years ago. How then could triceretops (sp?) “evolve” into sparrow if its entire order was extinct?

I never assumed “bigger is better,” it’s just hard to fathom the amzaing process of evolution whereby enormous creatures become tiny ones. Speaking of pussy cats, I’ve heard house-cat size cats preceded lion-size cats in evolution, whereas the saber-tooth cats split off into another line and became extinct.:confused:

A) Not all dinosaurs were large – ever. The immediate ancestors of birds were about chicken-sized.

B) But in any case, evolution produces smaller creatures in a situation where small creatures survive and reproduce a lot. (They don’t even have to be “better adapted” than the large ones; they just have to be successful on their own terms.)

I can tell you one thing… they’re all related.

I’m just going to throw a bunch of comments into one post since this site is loading a bit slowly for me for some reason:

  1. The fact that birds come from the same ancestors as dinosaurs shouldn’t be a surprise to any of us. This is why the discovery of the archaopteryx was so important – it showed us the “missing link” between birds and reptiles.

  2. There was a comment made that dinosaurs were unique to other reptiles by having legs underneath them for support, rather than to the side. This is correct, but one must not forget that many dinosaurs also had legs at the side for support. I just don’t want people to mistakenly believe that ALL dinos had their legs beneath them, as do modern mammals.

  3. Cladistics is the older way of showing links between animals, and is not generally being done anymore. The new way of showing ancestry in science is phylogeny.

cladogram - a branching diagram showing the pattern of sharing of evolutionarily derived characteristics amoing species and higher taxa.

phylogeny - the origin and diversification of any taxon, or the evolutionary history of its origin and diversification, usually presented in the form of a dendrogram.

(definitions from:
Hickman, Roberts and Larson. 1998. Biology of Animals. New York: McGraw-Hill.)

  1. As far as my 2nd year university text is concerned (the same book referenced just above), pterosaurs are decended from archosaurs, but are not listed as being grouped directly with the other dinosaurs: ornithischians, sauropods and theropods. The Archaeopteryx is also listed on its own, apart from other dinosaurs, but descended from archosaurs. Birds are shown as being directly descended from the archaeopteryx. It should be noted that archosauria also includes the crocodilians. Thus, crocs, dinos and birds are all intimitely related to one another.