Wish me luck!
Here are some definitions:
Unfortunately, the meaning of the word ``cladistics'' is somewhat muddied by the fact that it seems to carry a philosophy with it as well as methodolody. That philosophy is that the only groupings to be discussed are clades - that is, groups consisting of an ancestor together with all its descendents. So, for example, cladists do not accept the old concept of ``reptiles'', since it omits dinosaurs and birds which are descendents of the reptiles as commonly understood. Whether this is a reasonable stance is a separate issue (see ``What do terms like phylum, order and family mean?'' ) but quite what it has to do with the cladistic method is anyone's guess.
From this ``cladistic philosophy'' come phrases like ``the strict cladistic meaning of Reptilia'' as opposed to ``the traditional meaning of Reptilia''. What's meant by this?
When the class Reptilia was first postulated, it consisted of several groups of animals that were obviously related because of features such as their scaly skin: lizards and snakes, turtles, crocodiles, etc. When the dinosaurs were discovered, they were added to the Reptilia, since they have many skeletal features in common with other reptiles.
Since then, increasingly sophisticated analyses have shown that the most recent common ancestor of these groups is also the ancestor of the mammals (which are synapsid reptiles) and birds (which are dinosaurs.) It can be said, then, that the group consisting of lizards, snakes, turtles, crocodiles, dinosaurs, etc. but not mammals and birds is unnatural (specifically, it is paraphyletic -- see ``What do terms like monophyletic, paraphyletic and polyphyletic mean?'' ). Some people argue that such unnatural groups should not be used in scientific writing, and that the meaning of the word Reptilia should be changed to include the mammals and birds. This new interpretation of the Reptilia is sometimes referred to as the ``cladistic meaning''.
Analogously, now that it is more or less established that birds are dinosaurs (but see ``Is there any remaining doubt that birds are descended from dinosaurs?'' ), most scientists prefer to use the ``cladistic'' interpretation of the class Dinosauria, which includes the birds.
Once again, this dispute about ``cladistic'' terminology has little or nothing to do with the process of cladistic analysis and the derivation of putative phylogenies.
What are the differences between all these names?
Each of these names describes a taxonomic group, or ``taxon'' (plural ``taxa''). They are progressively less inclusive - that is, the groups at the bottom of the list include fewer specimens than those at the top. At the lowest level listed here, Tyrannosaurus is a genus - the type genus of all the other taxa.
We'll start by dealing with the most familiar of these terms, then move onto the more obscure ones.
The dinosaur names that we're all familiar with - Diplodocus, Tyrannosaurus, Velociraptor, etc. - are all genus names. A genus (the plural is ``genera'') is a group of one or more species. Genus names are always written with an initial capital letter (Diplodocus, never diplodocus), and are set in italics for valid genera (those which have been properly described.)
Species names always appear together with their genus name (or its one-letter abbreviation) - a species name alone is not meaningful. Like genus names, valid species names are set in italics (Diplodocus carnegiei or D. carnegiei, never Diplodocus Carnegiei or D. Carnegiei.)
In non-technical writing, dinosaurs are nearly always referred to at the genus level - everyone talks about Velociraptor, no-one about Velociraptor mongoliensis. For some reason, there's one big exception to this: there's something irresistible about the name T. rex!
To go with the tyrannosaur example, there is a single genus, Tyrannosaurus, which is generally considered to include three species: Tyrannosaurus rex, Tyrannosaurus bataar and Tyrannosaurus efremovi. (Although some people place T. bataar in its own genus, Tarbosaurus; and some think that T. efremovi also belongs there, while others don't think it's valid at all.)
You won't see these too often, but some people like to have a classification level intermediate between genus and species (that's a subgenus) and one even more specific than species (that's subspecies.) For example, some workers consider Giraffatitan to be subgenus of Brachiosaurus (while, predictably, others consider it a genus in its own right, and others consider it a mistake, referring the species brancai to Brachiosaurus. But that's neither here nor there.)
When a subgenus name is used, it appears in parentheses after the genus name; and like a genus name, it is both italicised (when valid) and capitalised. It looks like this: Brachiosaurus (Giraffatitan) brancai.
Subspecies names are not generally used in dinosaur palaeontology; but it's a given that someone will come out with one the moment I put this statement up on the web :-)
Subspecies names, when they are used, appear immediately after the species name, but not in parentheses. (You want consistency? Go study pure maths instead!) Like species names, they are not capitalised, but are set in italics when valid. The best-known example of subspecies is probably that of humans: orthodoxy changes, of course, but there was a time when Neanderthal man and modern man were considered subspecies of Homo sapiens - they were known as Homo sapiens neanderthalensis and Homo sapiens sapiens respectively. And a good one to irritate people with at parties is the case of the two subspecies of gorilla: the type subspecies is Gorilla gorilla gorilla (Yes! Really!), the lowland gorilla; and there also Gorilla gorilla beringei, the mountain gorilla.
We sometimes wish to talk about a group of related dinosaurs - a family. Families are named after their ``type genus'' - usually either the first one to be named, or ``the most important'' genus, in some sense. The family name is made replacing the genus name's ending with the suffix ``idae''. For example, the family containing Tyrannosaurus, together with similar genera such as Alectrosaurus, Gorgosaurus and Daspletosaurus, is called the ``Tyrannosauridae''.
Like genus names - and indeed all other group names - family names are capitalised. Unlike genus names, but again like all other group names, they are not set in italics. So it's ``Tyrannosauridae'', not ``tyrannosauridae'' or ``Tyrannosauridae''.
In some cases, it's useful to break a family down further, into sub-families. Subfamily names are made by replacing the ending of the type genus name with the suffix ``inae'' (so that they are distressingly similar to family names.)
For example, the subfamily Tyrannosaurinae contains Tyrannosaurus itself along with Gorgosaurus and Daspletosaurus (and others). Some workers consider that the family Tyrannosauridae also contains another subfamily, Aublysodontidae, which contains Aublysodon and Alectrosaurus.
And sure enough, sometimes we want to talk about a group larger than a family: for example, the group containing the family Tyrannosauridae together with some other related genera. Such a grouping is called a superfamily, and its name is made by replacing the ending of the type genus name by the suffix ``oidea'' (which, yes, is also painfully easy to misread as a family name.)
Unfortunately, the superfamily Tyrannosauroidea is not a very interesting example, since no genus outside of the Tyrannosauridae is known to be inside it - although several, such as Stokesosaurus and Siamotyrannus are candidates for this classification. (A better example of a superfamily is the Hadrosauroidea, which contains the genera Ouranosaurus, Altirhinus and Nanyangosaurus as well as the Hadrosauridae.)
Of course, this isn't the end of the story. On occasion people like to talk about smaller-than-subfamily groupings such as ``tribes''. An example tribe would be the Tyrannosaurini, which has never been formally defined but might include Tyrannosaurus and Tarbosaurus, if the latter is indeed a separate genus. There are also such nonsensical groupings as Hyperfamilies, Grandfamilies, Megafamilies and even Gigafamilies. I am not making these groupings up.
Fortunately, you can ignore these. Pretty much no-one uses them any more.
Even the individual grouping names are not used that often. Consider the case of everyone's favourite duck-bill, Parasaurolophus. It's a member of the tribe Parasaurolophini, which is contained in the subfamily Lambeosaurinae, which in turn is contained in the Euhadrosauria, then the Hadrosauridae, Hadrosauroidea, Iguanodontoidea, Styracosterna, Ankylopollexia, Dryomorpha, Euiguanodontia, Iguanodontia and Euornithopoda. Which is, of course, contained in the sub-order Ornithopoda.
Who needs all that? No-one. Don't let it bother you. Once you're down beyond the sub-order level, families, subfamilies and superfamilies are 99% of the game.
When we want to say that an animal is a member of a superfamily Somethingidae, we describe it as being a somethingid (not capitalised.) Similar rules exist for the other groups, as follows:
| Rank | Suffix | Example | Individual | Example |
|---|---|---|---|---|
| Family | -idae | Tyrannosauridae | -id | tyrannosaurid |
| Subfamily | -inae | Tyrannosaurinae | -ine | tyrannosaurine |
| Superfamily | -oidea | Tyrannosauroidea | -oid | tyrannosauroid |
| Tribe | -ini | Tyrannosaurini | -in | tyrannosaurin |
So, for example, we can say that an Alectrosaurus is a tyrannosauroid and also a tyrannosaurid, but not a tyrannosaurine nor a tyrannosaurin.
The group types genus, species, family, subfamily etc. are called ``ranks''. They come from the old-fashioned Linean system of taxonomy. When the more modern cladistic system is used strictly, ranks other than genus and species are not used (although the groups themselves - Tyrannosauridae etc - are.)
For a much more complete and scholarly treatment of tyrannosaur relationships, see Thomas R. Holtz Jr.'s Tyrannosauroidea page in the the Tree Of Life: ag.arizona.edu/tree/eukaryotes/animals/chordata/dinosauria/tyrannosauroidea/tyrannosauroidea.html
1. heat, head, held, hold, cold.
To understand why, we need to take a look at the history of biological classification.
The science of classification really started with Carl Linnaeus (1707-1778), a medical doctor from Sweden who is also known by the variant names Carl von Linné and Carolus Linnaeus. At the age of 28, he published the first edition of Systema Naturae, his ground-breaking classification of animals and plants - initially a single thin volume, although subsequent editions were to mushroom in size.
Before Linnaeus, classification was a haphazard business. For example, the common wild briar rose was referred to by different botanists as Rosa sylvestris inodora seu canina and as Rosa sylvestris alba cum rubore, folio glabro! There was little organisation of names: organisms were assigned to species, and species grouped into genera (for example, lions and tigers were recognised as separate species, but joined in a single genus, Panthera), but there were no higher level groupings.
As Systema Naturae progressed through its editions, it introduced two important innovations. The first of these was the so-called ``binomial system'', whereby each species is referred to by a two-word name, consisting of the generic name followed by the specific name - as in Panthera leo, or indeed Tyrannosaurus rex. This approach was in consistent use by the tenth edition of 1758, and remains more or less unchanged today.
The second and more far-reaching innovation was the introduction of higher-level groupings than the genus. In Linnaeus's original system, genera (such as Panthera and Canis, the great cats and the dogs) were joined in orders (such as Carnivora, the carnivorous mammals); orders (such as Carnivora and Chiroptera, the bats) were joined into classes (such as Mammalia, the mammals); and classes (such as Mammalia and Reptilia, the reptiles) into kingdoms (such as Animalia, which I shall not insult your intelligence by translating!) The key insight here was that life can be arranged into a branching tree-like hierarchy. The level-names order, class, etc. were known as ranks.
This simple five-level system (kingdom > class > order > genus > species) was soon augmented by intermediate levels such as the family, intermediate in level between the genus and order - for example, the family Felidae consists of all cats, whether great (Panthera) or small (Felis). As people found the need for more precision in locating how inclusive a proposed grouping was, along came superfamilies, subfamilies, tribes, superorders, infraorders, hyperfamilies, and ... well, the list goes on. Among these additional ranks is the phylum, intermediate in level between the kingdom and order. For example, the phylum Chordata, (animals with spinal a cord, including the vertebrates), is a part of the animal kingdom.
It doesn't take too much of this sort of thing before the system becomes, if not actually unworkable, then at least obscene. For example, in a Dinosaur Mailing List message, George Olshevsky has listed a hierarchy of thirteen ranks just between order and family!
For the more common of the lower ranks (superfamily, family, subfamily, etc.), taxon names have conventional endings - for example, family names end with ``-idae'' and subfamily names with ``-inae''. However, above this level, standardisation is at best partial, and varies across disciplines. For example, while bird orders all have an ``-iformes'' ending (e.g. the Galliformes), there is no such uniformity in the names of mammalian orders (which include Edentata, Pholidata, Lagomorpha, Rodentia and Macroscelidea among others.) In the same way, while bony fish have suborders ending in ``-oidei'', chondrichtyians (sharks and rays) have suborders ending in ``-oidea'' (which ending is conventionally used for superfamilies in the dinosaur world.) As if this isn't confusing enough, the rank names are used rather differently in different disciplines: for example, a ``division'' in plant taxonomy are broadly equivalent to a ``phylum'' in animal taxonomy, where ``division'' (when it's used at all) is more specific than class.
In the face of this maze of similar-sounding ranks, semi-standardised endings and contradictory ``rules'' (Quick! Which is more inclusive? A grandfamily or a hyperfamily?), one reaction has been a move to abandon named ranks altogether, simply naming nodes in the tree and making no judgement about how high or low level they are - and it is a matter of judgement rather than of fact; one man's parvorder is another man's nanorder. This minimal approach is particularly popular in the cladistics community, perhaps in part because the trees generated by cladistic methods have far too many nodes, and change far too often, to be amenable to labelling with ranks.
Proponents of this ``cladistic'' approach to taxonomy, then, would like to abolish all ranks above the level of species, which they argue is the only rank with an objective definition (but see ``When is a new dinosaur erected as a new species or genus?'' ). Under this scheme, even the rank of genus would be abolished. And therefore, of course, so would Linnaeus's binomial: every species would have to be given a new, unique name. Not surprisingly, these radical proposals have not met with unanimous enthusiasm - see ``Is the PhyloCode a good thing?'' for more discussion of pros and cons of this approach.
It remains to be seen how nomenclatural practices will change, but what seems to be emerging by default is an approach in which some of the more ``important'' nodes in the large, deep trees generated by cladistic methods get quietly labelled as families, orders, etc. according to a blend of historical precedent and what seems to be useful at the time. For example, most historical dinosaur families seem to have survived the transition into the cladistic age, and many new dinosaurs which do not fit into existing families have new families named after them - or at least, new groupings whose names end with ``-idae'', which is at least very suggestive!
For example, when a specimen is called ``Diplodocus sp.'', the person referring to it is satisfied that it belongs to Diplodocus; but it might be Diplodocus longus, Diplodocus carnegiei, Diplodocus hayi, or perhaps a completely new Diplodocus species.
The fact, is there is no real definition of ``genus'', any more than there is of ``family'', ``order'', etc. (See ``What do terms like phylum, order and family mean?'' .) There's no scientific way to decide whether a given difference in anatomy is sufficent that two species differ ``at the genus level'' because the idea of a genus is so artificial: designating certain groupings as genera is really just an arbitrary but convenient way for us to partition up what would otherwise be an uncomfortably large set of taxon names.
As Thomas Holtz says, ``If you are more interested in book-keeping than science, then `genus' might be of great importance; otherwise, however, it doesn't have any particular biological significance.''
This is just as true in classifying extant animals as it is with dinosaurs. For example, the blue whale is variously known as Balaenoptera musculus and Sibbaldus musculus, according to the beliefs and preference of individual workers. Similarly, the American black bear may be Ursus americanus or Euarctos americanus, and the lion Felis leo or Panthera leo or even its own genus, Leo leo.
The only hard and fast rule here is that of precendence: that is, when someone considers that two or more genera are actually the same, then the older of the genus names must be used. For example, if Saurornitholestes langstoni and Velociraptor mongoliensis turn out to be species of the same genus, then that genus would be called Velociraptor (established in 1924) rather than Saurornitholestes (established in 1978); and the species would accordingly be called V. langstoni and V. mongoliensis rather than S. langstoni and S. mongoliensis.
In summary, there is no consensus even within dinosaur palaeontology - let alone across biological disciplines - on the what ``genus'' should mean and how inclusive or exclusive it should be in terms of morphological, behavioral or molecular diversity.
A someone once wrote, ``Linnean taxonomy is as much an art form as a science.''
Who wrote that, exactly?
Moving further back down the tree, no-one who is remotely familiar with extant mammals would ever confuse, say, a lion with a wolf, even though they are both members of the order Carnivora. Would it be similarly true that no-one would ever confuse the theropods Tyrannosaurus and Allosaurus?
Tarbosaurus Tyrannosaurus
\ /
\ /
Gorgosaurus \/
\ /
\ /
Allosaurus \/
\ /
\ /
\/
The differences between Tyrannosaurus and its very close relative Tarbosaurus would be very hard to spot based on our knowledge of their bones - the skulls are shaped all but identically, for example. But, like lions and tigers, they might have had completely different skin colouring or patterns.
The differences between Tyrannosaurus and Gorgosaurus would be rather more apparent: apart from being somewhat smaller, the latter is generally more slender, has a narrower snout, a proportionally longer tail and other differences that would be immediately apparent to anyone who was as familiar with these animals as we are with, say, lions and cheetahs.
Going further down the tree, despite the gross similarities between them, Tyrannosaurus and Allosarus are different enough, both in head shape and body proportions, that even at a distance the differences would be obvious to anyone at all familiar with them.
More generally, though, you can't answer a question about how similar members of the same genus, family or whatever are to each other, because there is no general definition of what a genus or a family is - nor indeed of what a dinosaur species is (although most zoologists have definitions of species that they're happy with for living animals.) See ``When is a new dinosaur erected as a new species or genus?'' .)
For example, there is much more variation between the eight species of Psittacosaurus than between the two genera Tyrannosaurus and Tarbosaurus. The whole process of assigning dinosaur species to genera, and genera to families and so on, is arbitrary and inconsistent. Better just get used to it.
Aves
/
/
Crocodilia /
Mammalia \ Dinosauria
\ \ /
\ \ /
\ \ /
Synapsida Reptilia
\ /
\ /
\ /
Amniota
Here are examples of all three types of group:
Monophyletic groups are also called clades, and are generally considered as the only ``natural'' kind of group. They are very important in phylogenetic classification.
The ``non-avian dinosaurs'' make up a singly paraphyletic group because only one clade need be omitted from its base definition. Groups may also be doubly paraphyletic, thrice paraphyletic, etc., depending on how many sub-clades they omit.
So far, so straightforward. The only wrinkle in this scheme is that some workers use the word ``monophyletic'' in a sense that includes what we have described here as paraphyletic groups. These people then use ``holophyletic'' to describe what are usually called monophyletic groups. It's tempting in the face of this ambiguity just to abandon the word ``monophyletic'' and use a holophyletic/paraphyletic dichotomy, but this terminological abuse is probably not widespread enough to merit such extreme measures. It's just something to be on the watch for.
Because clades are so important, there is common notation for specifying them (taken from the Phylocode: see note 9.4.1 in http://www.ohiou.edu/phylocode/art9.html).
For example, Neosauropoda is defined as Clade(Saltasaurus + Diplodocus), that is, the most recent common ancestor of Saltasaurus and Diplodocus together with all its descendants. And Eutitanosauria, the ``true titanosaurs'' can be defined as Clade(Saltasaurus + Argyrosaurus + Lirainosaurus).
For example, Coelurosauria is often defined as Clade(Neornithes <-- Allosaurus), that is, modern birds and everything sharing a more recent common ancestor with them than with Allosaurus. And Eusauropoda, the group of ``true sauropods'', is defined essentially by listing a lot of taxa that are not included in it, as Clade(Saltasaurus <-- Barapasaurus, Ohmdenosaurus, Vulcanodon, Zizhongosaurus).
Stem-based clades are useful for neatly partitioning a node-based clade. For example, within the Avetheropoda, which is defined as Clade(Neornithes + Allosaurus), the two subgroups are the Carnosauria, defined as Clade(Allosaurus <-- Neornithes), and the Coelurosauria, defined as its complement: Clade(Neornithes <-- Allosaurus).
Whatever the hell that means.
These notations are not standard in formal technical literature, but appear frequently on the Dinosaur Mailing List.
It's unfortunate that this notation is so clumsy. The following, more concise, alternative notation is sometimes used (notably in Mike Keesey's admirable Dinosauricon):
As examples of this last, we might describe the informal grouping ``non-avian dinosaurs'' as {Dinosauria-Aves}, and the ``traditional reptiles'' as {Reptilia-Aves}.
Cladistics is a method of deriving possible family trees, or cladograms, from a set of specimens. It works by measuring the states that a selected set of characters take in each of the specimens, and finding the trees in which the fewest transitions occur between different states. (The total number of state transitions in a tree is called its length.)
The process is related to, but not directly bound up with, both the ``cladistic philosophy'' that the only legitimate groupings are clades (see ``So can we say that birds are dinosaurs?'' ) and the ``cladistic taxonomy'' which eschews Linnaean ranks (see ``What do terms like phylum, order and family mean?'' )
A trivial worked example will make this process much clearer.
Suppose we want to work out the relationships between three genera: Apatosaurus Brachiosaurus, and Camarasaurus (which conveniently happen to begin with A, B and C.) The chances that any of these is directly descended from any of the others is vanishingly small, so we do not consider trees in which the taxa under consideration appear at the branch points. Accordingly, we have three possible trees, corresponding to which of the three genera was first to branch off the lineage that eventually gave rise to both of the others:
tree #1 tree #2 tree #3
B C A C A B
\ / \ / \ /
A \/ B \/ C \/
\ / \ / \ /
\/ \/ \/
(Only the topology of these trees is significant, not the geometry: so, for example, if B and C were swapped in tree #1, the resulting tree would be equivalent to #1.)
In order to determine which of these candidate trees is the most likely, we need to select a set of characters that we are going to analyse for each taxon, and make the appropriate measurements on our specimens. The results are entered in a matrix.
For this example, we choose the following characters:
This gives us the following matrix:
| Character number | ||||
| 1 | 2 | 3 | 4 | |
| Apatosaurus | no | yes | no | no |
| Brachiosaurus | yes | no | no | yes |
| Camarasaurus | yes | yes | yes | yes |
Now consider the three putative trees above. Assuming that the common ancestor of all three taxa lacks all four characters (see below), we find that:
The principle of parsimony says that, other things being equal, we should expect as few transitions as possible - that is, we assume the simplest evolutionary sequence that accounts for the observable facts. So we consider tree #1 as the most likely of the three alternatives presented here - and indeed, this is the tree that most workers consider to be correct, with both Brachiosaurus and Camarasaurus falling within the group called Macronaria, and Apatosaurus outside that group.
For this tiny example, we've been able to work out the alternatives ``by hand'', but as we start to consider more taxa, then number of possible trees grows frighteningly fast. Three taxa give rise to three possible trees, and four to fifteen; but as few as seven taxa yield ten thousand trees, and ten taxa can be arranged into more than 34 million trees!
It's clear that computers are required to analyse large data-sets; and the availability these days of powerful computers is part of the reason that cladistics is a relatively new approach to systematics. Typically the data matrices - perhaps of dozens of taxa and hundreds of characters - are fed to a computer program such as PAUP (Phylogenetic Analysis Using Parsimony) or MacClade, and a lot of magic goes on behind the scenes: heuristics are required to slice off chunks of the search space, since for large data-sets, the total number of theoretically possible trees is too great even for computers to consider.
In addition to the difficulties presented by this trivial example, more problems arise when doing cladistic analysis on real data sets: for example, due to the incomplete nature of the fossil record, we rarely have all the data: many characters have to be coded as ``don't know'' for some taxa. Programs such as PAUP need to be able to deal with this, as well as multi-state characters such as ``End of tail unspecialised (0), whiplash (1) or club (2)''.
Sophisticated cladistic analysis programs should also allow characters to be weighted by importance, so that (for example) when taxa share the same value for a character x, this can be considered twice as likely to imply that they are related as if they shared the less significant character y.
However, like any computerised method, cladistics suffers from the problem that the quality of the output it produces is entirely dependent on the quality of the input it is given - or, to put it another way, ``garbage in, garbage out''. There are several choices that the worker must make before the computer, in all its objectivity, can be put to work, and some further issues may also cast doubt over the results of a cladistic analysis.
Poorly chosen characters can yield incorrect results. In the example above, if character 4 were replaced with ``hind legs longer than forelegs'' - not an unreasonable choice - the analysis would choose tree #2 as most parsimonious, with A and B more closely related to each other than to C.
To take an even more extreme example, if we applied the cladistic method to three taxa, lions, tigers and zebras, using only the single character ``has stripes'', we would obviously get the nonsensical result that tigers and zebras are more closely related to one another than either is to lions.
This highlights the need to choose an appropriate set of characters for a cladistic analysis. How can this be done?
Another way in which a worker's preconceived ideas can affect the result of a cladistic analysis is in the assumptions about the primitive states of the characters. Since we can't know what taxon is actually the most recent common ancestor of those being analysed, we use a well-understood outgroup as a proxy for that ancestor - that is, a taxon outside of, but as close as possible to, the clade containing the taxa to be analysed. For example, in the analysis above, if we think that the phylogeny looks like this:
A, B and C (in some combination)
\ | /
\|/
V Haplocanthosaurus
\/
\ Jobaria
\/
Then we might perform the analysis on the assumption that A, B and C's common ancestor had the same character states as Haplocanthosaurus; or, if we decided that its remains are too fragmentary to be used in this way, we might use the less closely related but better represented Jobaria. In practice, several different outgroups are typically analysed to help determine the most likely ancestral state of characters.
However, the choice of taxa to use as outgroups is clearly a subjective one: it is chosen on the basis of how the phylogeny is likely to look. To pick an extreme example, consider an analysis of the relationship between various deinonychosaurs and cretaceous birds. Most workers (believing that birds are dinosaur descendants) would use something like Oviraptor; but someone who believes that birds evolved from a non-dinosaurian ancestor such as Megalancosaurus would have to choose a much more primitive outgroup such as a basal archosauromorph. This would obviously affect the results of the analysis significantly.
Classically, cladistics works only with fossil morphology, but of course we have other knowledge which may contradict the results of cladistic analysis. For example, we should be wary of any analysis which suggests that a Triassic taxon is more derived than a Cretaceous one; or that a Laurasian taxon evolved from a Gondwanan lineage during the time that the two supercontinents were separate.
Some work is being done on modifying cladistics algorithms to take specimen age into account, but it's too early to say whether this will have much effect on how things are done.
In choosing the most likely candidate tree, cladistics programs rely on the principla of parsimony; but nature is not always parsimonious! As John Jackson points out, ``Lineages of animals have a way of evolving a feature, then removing it, and then re-evolving it again, in a way they have often had to be spoken to about.''
This is called a reversal. For example, sufficiently basal ancestors of birds were flightless, and birds evolved the feature of flight; but flightless birds such as ostriches and penguins have lost that feature. An analysis using the character ``can fly'' would be fooled into misreading this as evidence that penguins are more closely related to bird-ancestors than to other birds.
Finally, there is an implicit assumption that similarity of form - which is what cladistic analysis discovers and measures - implies commonality of descent. This is usually a good assumption, but not always. See Jonathan R. Wagner's article ``What is a cladogram anyway?'' at www.dinosauria.com/jdp/misc/cladogram.html for more discussion of this distinction.
Given these problems, why use cladistics at all? For all its limitations, it does offer the following advantages over older methods of hypothesising phylogeny:
(A person more cynical than myself might speculate that it's for this very reason that BAND (Birds Are Not Dinosaurs) proponents are among the few groups unconvinced by the merits of the cladistic method.)
In other words, the primary benefit to cladistic analysis may not be that the results are ``better'' so much as that systematists have to ``show their working'', which makes it much easier for others to improve upon it in the future.
As an example of this kind of openness, see Paul Sereno's data matrix for the whole of the dinosauria at www.sciencemag.org/feature/data/1041760.shl - a matrix of 146 characters scored against thirteen dinosaurian subgroups. As new data is discovered, other workers can fill in some of the question marks in this matrix and re-run the analysis, hopefully yielding improved results.
In summary, while cladistic analysis is a powerful tool, it is not a ``silver bullet''. Human interpretation is still vital in the business of systematics. For this reason and because of the constant discovery of new specimens, the results of every phylogenetic analysis are provisional, subject to change in future research.
For a much more complete description and discussion of cladistics, see www.ucmp.berkeley.edu/clad/clad1.html and the linked pages.
Isn't there some kind of international standards bodies that has to pass names as acceptable?
In 1903, Elmer Riggs' re-examination of Marsh's specimens led him to conclude that they represented the same genus (although see below), meaning that the names were synonyms. In such cases, the ICZN (International Commission on Zoological Nomenclature, see www.iczn.org) mandates that the oldest name has priority - which means that the rather dull Apatosaurus (``deceptive lizard'') wins out over the much more resonant ``Brontosaurus'' (``thunder lizard'').
So why does the world still talk about ``Brontosaurus'' all the time? The paper in which Riggs established the synonymy was published in the Geological Series of the Field Columbian Museum - a relatively obscure journal, so the findings were not as widely known as they should have been. Also, the sexier invalid name received a lot of public exposure from non-scientific sources: for example, the Sinclair oil company used a ``Brontosaurus'' as its logo for many years. (Rather inappropriately, as it turns out, since oil is formed from plant matter, not animals. Never mind.)
So the world continued and continues to use ``Brontosaurus''; but Apatosaurus should be used in all serious writing.
I said that Riggs established that Apatosaurus and ``Brontosaurus'' were from the same genus. But the two Marsh specimens are still considered to represent separate species: the older specimen is Apatosaurus ajax and the newer Apatosaurus (nee ``Brontosaurus'') excelsus. However, since it's always a judgement call whether any species belong in the same genus (see ``When is a new dinosaur erected as a new species or genus?'' ), there are palaeontologists - notably Robert Bakker - who feel that the two species are sufficiently distinct that excelsus merits a separate genus. Under this scheme, the old genus name is still perfectly good, so Bakker still uses the formal name Brontosaurus excelsus (but never Brontosaurus ajax.)
Finally: for many years, Apatosaurus was believed to have a head similar to that of Camarasaurus - a mistake that was rectified in the 1970s with the discovery of a specimen with associated cranial remains closely resembling the head of Diplodocus. This has led to a misapprehension in some quarters that the name ``Brontosaurus'' refers to the combination of an Apatosaurus body with a Camarasaurus head. No so: the naming confusion is quite separate from this issue.
So why has everyone's favourite dinosaur name survived?
The truth is, it's hard to avoid the conclusion that T. rex was just too damned cool to change, so everyone sort of looked the other way when the idea of synonymy came up, and changed the subject.
Fortunately, the problem has now gone away forever. As of 1st January 2000, a new ICZN ruling has come into effect, saying that a name that's been considered valid for fifty [1] years can't now be replaced by one that's been considered invalid during that time. So we can shout ``Manospondylus is Tyrannosaurus'' all we want, and the name will still remain safe for democracy.
Phew.
Notes
[1] Or it might be a hundred years, I'm not sure. I made a genuine attempt to find out the duration, but it was stymied by the rather odd ICZN policy which considers its decisions to be trade secrets which you're only allowed to know if you pay them. See http://www.museum.unl.edu/research/systematics/Orti/ICZN-changes.html for more information. [back]
When Richard Owen first named the Dinosauria in 1842, based on the genera Iguanodon, Megalosaurus and Hylaeosaurus, he defined them as a group of large, extinct reptiles characterised by various anatomical features, primarily a fused sacrum: that is, the five or six vertebrae in the hip region are fused together.
In her excellent historical overview The Dinosaur Hunters, Deborah Cadbury writes:
[...] the 'Lacertian' division had key defining characteristics. They were reptiles, and had scaly skin and laid eggs, but they possessed mammal-like characteristics in the shape and alignment of the limb bones and the sacrum. They did not sprawl like a crocodile, but moved on upright, pillar-like legs: these were reptiles designed for walking on land. They could be defined as a distinctive group of land-dwelling reptiles that walked with straight legs tucked under their bodies.
[...]
[Quote from Owen's 1842 paper:]The combination of such characters, some, as the sacral ones, altogether peculiar among Reptiles, others borrowed, as it were, from groups now distinct from each other, and all manifested by creatures far surpassing in size the largest of living reptiles, will, it is presumed, be deemed sufficient grounds for establishing a distinct tribe or suborder or Saurian Reptiles for which I would propose the name of 'Dinosauria'.[...]
Glorying in his new creation, he proclaimed: 'No reptile now exists which combines a complicated ... dentition with limbs so proportionately large and strong, having such well-developed marrow bones, and sustaining the weight of the trunk by ... so long and complicated a sacrum, as in the order Dinosauria.'
Owen's definition was fine when only a few dinosaurian genera were known, but as more and more dinosaurs and related animals were discovered, it because apparent that this was not sufficiently rigorous to allow animals to be classified as dinosaurian or not.
The Dinosauria were defined cladistically by Padian and May in 1993 as the most recent common ancestor of Passer (the sparrow) and Triceratops, together with all its descendants; or, more tersely, as {Passer + Triceratops}. This definition now seems to be commonly accepted. (See ``What do terms like monophyletic, paraphyletic and polyphyletic mean?'' )
This means that the following animals (among others) are dinosaurs: Passer, Archaeopteryx, Velociraptor, Troodon, Oviraptor, Therizinosaurus, Ornithomimus, Tyrannosaurus, Allosaurus, Megalosaurus, Carnotaurus, Diplodocus (from closest to furthest from Passer); and Triceratops, Chasmosaurus, Centrosaurus, Protoceratops, Psittacosaurus, Pachycephalosaurus, Iguanodon, Stegosaurus (from closest to furthest from Triceratops).
It also means that the following animals are not dinosaurs: crocodiles, lizards, turtles, Dimetrodon (which is more closely related to mammals) and mammals themselves, including mammoths, Smilodon (``sabre-tooth cats''), etc. (The notion that any big extinct animal is a dinosaur is of course complete nonsense.)
(For historical reasons, some people prefer the alternative and equivalent cladistic definition {Megalosaurus + Iguanodon}. This doesn't seem to have caught on, but you can use it if you like, since the group which it defines contains exactly the same animals as {Passer + Triceratops}.)
How can we tell what is a dinosaur and what isn't? We have a good definition, but we also need a diagnosis - that is, a set of characters unique to dinosaurs, whose presence in an animals tells us that it is probably a dinosaur, and whose absence tells us that it is probably not. (Such a character is called an synapomorphy.)
Different workers have proposed different sets of characters for the Dinosauria, since putative synapomorphies are matters of observation and interpretation rather than of fact. However, the following list, (from The Dinosauria, ed. Weishampel, Dodson & Osmolska, 1990) is fairly representative:
The following list - more complete, more up to date, but also more incomprehensible (:-) is from Sereno's 1999 article in Science 284:2137-2147:
It's nice to see that Owen's fused sacrum is still there, number two on Weishampel et al.'s character list and number six on Sereno's - even if it's been somewhat diluted by recent discoveries (e.g. Sellosaurus has only two sacral vertebrae). But it's important to understand that this character is now only a part of the diagnosis, not the definition. In other words, a reptile with five fused sacral vertebrae is only a dinosaur if it's descended from the ancestor of Passer and Triceratops; and if we found a descendent of that ancestor in which the sacrum was not fused, that animal would still be a dinosaur - as is Sellosaurus with its puny two-vert sacrum.
This diagnosis is very helpful. Unfortunately, however, even so comprehensive a character list is not sufficient for us to be certain whether certain genera are dinosaurs or merely close relations.
For example, consider the case of Eoraptor, often considered the earliest dinosaur. (To be precise, it's often considered the earliest known dinosaur: no-one suggests it is actually the most recent common ancestor of Passer and Triceratops.)
It's clear that Eoraptor branches off the evolutionary line that leads from non-dinosaurian archosaurs to the theropods; and the ornithischians and sauropodomorphs also branch off that line (in that order - theropods and sauropodomorphs are more closely related to each other than to ornithischians.) But exactly where in that branching sequence the Eoraptor branch falls is open to debate.
Consider the following cladogram:
Sauropodomorpha
\
Ornithischia \ C (Advanced theropods)
\ \ \ /
Crurotarsi \ \ \/
(crocodiles) \ \ /=Theropoda (stem-based)
\ \ B \/
\ \ \ /
\ \ \/
\ \ /=Saurischia (stem-based)
\ A \/=Dinosauria (node-based)
\ \ /
\ \/
\ /
\/=Archosauria (node-based)
/
/
We are not certain where Eoraptor branched off from the line that led to advanced theropods: at point A, B or C. Although this may seem a fine distinction - these points on the tree are very ``close'' - it has a profound effect on poor old Eoraptor's identity:
So it's possible for Eoraptor to be non-dinosaurian or a basal saurischian, but not a basal dinosaur - neither saurischian nor ornithischian. Why is this? Because Saurischia and Ornithischia are defined as sister stem taxa {Passer > Triceratops} (meaning Passer and everything that has a more recent common ancestor with it than with Triceratops) and {Triceratops > Passer} respectively. These definitions exactly partition the dinosauria, so that there is no such thing as a ``basal dinosaur'' (except the common ancestor itself - and the chances of that ancestor actually being Eoraptor are vanishingly small.)
There are three aspects to the identity of any taxon, including the Dinosauria:
That said, here is a brief survey of the major dinosaurian groups. The following table of contents can also be read as a basic family tree, with derived groups below and to the right of their parents.
1. Saurischia
1.1. Sauropodomorpha
1.1.1. ``Prosauropods''
1.1.2. Sauropoda
1.1.2.1. Diplodocidae
1.1.2.2. Macronaria
1.1.2.2.1. Brachiosauridae
1.1.2.2.2. Titanosauria
1.2. Theropoda
1.2.1. Ceratosauria
1.2.1.1. Coelophysoidea
1.2.1.2. Neoceratosauria
1.2.2. Tetanurae
1.2.2.1. Spinosauria
1.2.2.2. Carnosauria
1.2.2.3. Coelurosauria
1.2.2.3.1. Tyrannosauria
1.2.2.3.2. Ornithomimosauria
1.2.2.3.3. Oviraptorosauria
1.2.2.3.4. Therizinosauria
1.2.2.3.5. Alvarezsauria
1.2.2.3.6. Troodontidae
1.2.2.3.7. Deinonychosauria
1.2.2.3.8. Avialae
2. Ornithischia
2.1. Thyreophora
2.1.1. Stegosauria
2.1.2. Ankylosauria
2.2. Marginocephalia
2.2.1. Ceratopsia
2.2.1.1. Centrosaurinae
2.2.1.2. Chasmosaurinae
2.2.2. Pachycephalosauria
2.3. Ornithopoda
2.3.1. Iguanodontia
2.3.1.1. Hadrosauridae
2.3.1.1.1. Hadrosaurinae
2.3.1.1.2. Lambeosaurinae
Since the earliest days of dinosaur palaeontology, dinosaurs have been split into two very broad groups - the Saurischia and Ornithischia (meaning lizard-hipped and bird-hipped respectively.) Indeed for some time it was believed that these two groups had no common ancestor, so that the term ``dinosaur'' was not a taxonomically sound one. However, most palaeontologists now believe that there was a single common ancestor of all dinosaurs, and the grouping Dinosauria is restored.
In recent times, though, even the basic Saurischia/Ornithischia division - like almost everything - has been questioned. Some people now hold that the two main divisions within Saurischia - the Sauropoda and Theropoda - are actually separate suborders, yielding a three-fold division; while others would now classify the sauropods together with the Ornithischia in a grouping called Phytodinosauria, meaning ``plant(-eating) dinosaurs''. The jury seems still to be out on this one.
Rather unnecessarily, life is made yet more confusing by those who wish to rename the Ornithischia as the Predentata (named after the predentary, a bone at the front of the lower jaw which is unique to this group of dinosaurs.) In this document, we use the older, better established term. If we were in the business of assigning new names to existing taxa, there are plenty with more inappropriate names than the Ornithischia (for example, the theropods and ornithopods seem to have been given each other's names.) My feeling is that since there's so much scope for getting legitimately confused when studying dinosaurs, it's not necessary to create artificial new confusion. Or as Henry Spencer has put it, ``Your creativity is better used in solving problems than in creating beautiful new impediments to understanding.''
New theories notwithstanding, current orthodoxy has two main groupings in the Saurischia: the sauropods are the largest of all dinosaurs, with heavy bodies and long necks and tails; and the theropods are the bipedal meat-eaters. The Therizinosauria (previously known as Segnosauria) have sometimes been considered as a candidate third group within the Saurischia (and have sometimes been considered as ornithischians!) but current consensus buries them deep within the theropods. More on these beasties later.
In one of the naming foul-ups that so persistently characterise dinosaurian classification, it turns out that birds are saurischian dinosaurs - that is, they are ``lizard-hipped'' rather than ``bird-hipped''. This just goes to show that it's a dangerous thing to name groups of animals before you know much about them. But then what alternative did the early palaeontologists have? They could hardly wait around for a century or so to see what would be discovered subsequently before naming their groups. The long and short of it is that we're stuck with the name Saurischia, and the best thing to do is just shrug and accept it.
The sauropods together with the ``prosauropods'' and few more basal animals are collectively known as the Sauropodomorpha.
The name ``prosauropod'' means ``before sauropods'', reflecting the old belief that they were the ancestors of the sauropods. However, current orthodoxy holds that prosauropods were a branch off the evolutionary line that led to the sauropods; or even that they are the aggregate of several such branches, and thus not a valid phylogenetic grouping at all. Typified by Plateosaurus, they were broadly similar in body shape to the sauropods, but rather smaller. Some genera may have been primarily or exclusively bipedal. The consensus is that they were herbivorous, but some argue for an omnivorous or carnivorous diet.
The two main groups within the sauropods are the Diplodocidae and Macronaria; in addition to these groups are numerous genera which don't fall into either camp, among them Cetiosaurus, the first sauropod to be discovered, and Mamenchisaurus, the dinosaur with the proportionally longest neck.
This grouping contains Diplodocus itself along with larger close relatives such as Supersaurus and Seismosaurus, as well as slightly more distant relatives including Apatosaurus (``Brotosaurus''.)
Apart from some of the larger Diplodocidae, all the largest (in the senses of longest and heaviest) dinosaurs fall into the Macronaria. In addition to basal genera such as Camarasaurus, the two major groupings within the Macronaria are the Brachiosauridae and the Titanosauria.
These are tall, heavily built sauropods characterised by their front legs being longer than their back legs, and therefore sloping upwards from the hips to the shoulders (hence the name Brachiosaurus, meaning ``arm lizard''.)
There is some controversy over how the various ``Brachiosaur'' genera are related. Some people place Brachiosaurus itself, Giraffatitan (if it's valid), Sauroposeidon, Cedarosaurus and Sonorasaurus in the group; others exclude the latter two (some of them considing Sonorasaurus to be a late-surviving species of Brachiosaurus), and yet others think that only Brachiosaurus belongs there.
For those who don't recognise the family Brachiosauridae at all, all these genera, along with a grab-bag of others, are left lurking guiltily around in the Titanosauriformes, a sub-macronarian group which also contains the Titanosauria proper. Perhaps we can look forward to clarification as newer genera like Sauroposeidon are more fully explored.
Unusually among Sauropods, at least some of the titanosaurs were armoured with bony plates embedded in the skin, in a somewhat similar manner to ankylosaurs. Recent work in the southern hemisphere, particularly Argentina, seems to be turning up new, massive, titanosaurs every five minutes or so. As I write, the titanosaur Argentinosaurus is widely considered the largest properly-described dinosaur, at perhaps 45m in length and 100 metric tonnes in weight; but already, new and as yet unnamed Argentinian titanosaurs have been discovered for which greater size is claimed in every dimension.
Traditionally, the theropods have been divided into just two groups: the big ones were called carnosaurs, and the small ones coelurosaurs. This grouping is appealing in its simplicity, but a moment's thought shows that it makes no sense - it's like classifying wolves together with leopards because they're both big, and chihuahuas together with Siamese cats because they're both small.
In recent times, this scheme has been thrown up in the air, although the names of both the old groups live on with new definitions. Amusingly, Tyrannosaurus rex, the quintessential carnosaur, is now classified as a coelurosaur. (You can't help thinking that the film Carnosaur! wouldn't have sounded quite so scary had it been named Coelurosaur!)
The basic division now is into two groups: the Ceratosauria and Tetanurae (the latter group containing both the new carnosaurs and coelurosaurs). Outside of these two major groups fall a few primitive theropods, notably Eoraptor (generally considered the oldest known dinosaur) and probably the Herrerasauria - although some consider them not to be dinosaurs at all, but to be the sister group to the Dinosauria.
This group is broken in two: the older Coelophysoidea and the more recent Neoceratosauria (from the early Jurassic and mid Cretaceous respectively.)
These are early predators such as Coelophysis (the first dinosaur in space[1]) and Dilophosaurus - the latter restored rather idiosyncratically in the film version of Jurassic Park, with a neck frill and poison-spitting ability neither of which are so much as hinted at in the fossil record.
This group consists of a few genera such as Ceratosaurus together with an enigmatic group called the Abelisauroidea, which contains, among others, the weird Carnotaurus. Many neoceratosaurs, including these two genera, had crests, horns and other such displays on their skulls.
Some palaeontologists have considered the massive carcharodontosaurines to be abelisauroid, but current consensus has them in with the allosaurs.
The name means ``stiffened tail'', which is among the characteristics of this very varied group. It is generally more advanced than its sister group the Ceratosauria.
This group contains Megalosaurus, the first dinosaur ever described, together with three main sub-groups: the spinosaurs, the carnosaurs and the coelurosaurs. The two latter groups (in the new senses of these terms) together with a few stray genera make up the Neotetanurae.
These are large animals, conjecturally piscivorous and characterised by long, crocodile-like snouts and sails on their backs. They include Spinosaurus (the rumoured star of Jurassic Park III), the charmingly named Irritator, and Baryonyx (which I have a soft spot for as it was the last exciting dinosaur found in England. STOP PRESS: that's not true now - we have Eotyrannus!)
Some people consider the spinosaurs to be advanced coelophysids, but this is a minority view.
This group now excludes many genera which it used to include, such as Carnotaurus (now a ceratosaur) and Tyrannosaurus (now a coelurosaur). In its new sense, it has been stripped down to basically the allosaurs plus a few bin-ends.
Of course, that's enough to be getting on with. As well as Allosarus itself, the archetypal Jurassic predator, this group is generally considered to contain the Carcharodontosaurines, a group represented by genera such as Giganotosaurus, perhaps the largest predator of all time. (Some people think they belong with the Abelisaurids, but this is a minority view.)
This is an extremely varied grouping, containing a vast selection of very different carnivores. Among the more basal coelurosauria are Ornitholestes, Compsognathus and Dryptosaurus (the most likely identity of the specimens named as ``Laelaps'' in the early decades of dinosaur palaeontology.)
The more advanced coelurosaurs are known as Maniraptoriformes. Below this level, classification is open to a great deal of interpretation, and orthodoxy seems to be in constant flux. However, for now, we can postulate the following relationships:
Coelurosauria
|--Tyrannosauria
`--Maniraptoriformes
|--Ornithomimosauria
`--Maniraptora
|--Alvarezsauria
|--+--Oviraptorosauria
| `--Therizinosauria
`--Paraves
|--Avialae
`--+--Troodontidae
`--Dromaeosauridae
If this phylogeny is correct, then the advanced coelurosaurs are the Tyrannosauria and Maniraptoriformes, with the latter consisting of Ornithomimosauria and Maniraptora; the Maniraptora consist of the Alvarezsauria an Oviraptorosauria-and-Therizinosauria clade known as the enigmosauria, plus Paraves; Paraves consists of the Avialae (including modern birds), and a clade consisting of Troodontidae and Dromaeosauridae
But remember that all of this is subject to change.
Tyrannosaurus rex and his buddies were the most advanced group of large carnivores, with specialisations including much larger and more robust skulls than other theropods, and tiny vestigial forelimbs with only two fingers on each hand.
T. rex is perhaps the most studied of all dinosaur species, yet controversy still abounds in almost every possible area of its study. Was it a hunter or a scavenger? Could it run fast, or was it restricted to slower speeds by its size? Were the males larger or smaller than the females? We still don't know, though study continues unabated.
This group contains the ``bird-mimics'' such as Ornithomimus, Struthiomimus and the mysterious Deinocheirus, of which only a gigantic pair of arms is known. The ornithomimosaurs had long necks, small heads with toothless, beak-like mouths, and long legs, with the distal parts much longer than the femur - an adaptation for fast running. The result of these characteristics is that they somewhat resemble large ostriches with tails, hence the name of the group.
Unlike other theropods, these animals may have been omnivorous or perhaps even herbivorous.
These specialised, medium-sized animals are characterised by a peculiar skull with a toothless beak and a nasal crest. Some adaptations of the skull suggest an egg diet, hence the name of the type genus, meaning ``egg stealer''. (Although ironically, the eggs that the type specimen of Oviraptor was thought to be stealing turn out to be its own, which it was actually brooding.)
The most notable feature of these weirdos is the very long claws on their hands - of the order of a meter in the case of Therizinosaurus. They also depart from theropod normality in many other ways: for example, they have four toes on each foot rather than usual three; and the pubis points backwards rather than forwards. These and other oddities have meant that in the past they have have been classified with the ``prosauropods'' and even within the Ornithischia!
These are very odd smallish dinosaurs, with long, slender hind legs and short, powerful arms. The latter suggest that the alvarezsaurs may have been strong diggers, but this seems to be at odds with the cursorial lifestyle suggested by the hind legs.
Troodon and its kin were small, agile dinosaurs with large forward-facing eyes (hence, probably, steroscopic vision) and grasping claws. They also had the largest brains (relative to body size) of all the dinosaurs, and can thus be considered to have been the most intelligent dinosaurs. Troodontids, like deinonychosaurs, are equipped with a single, large, retractable claw on each foot; however, this is considered to be an example of convergence, rather than a trait derived from a shared ancestor.
Troodontids have in the past been considered to belong inside Deinonychosauria as a sister group to the dromaeosaurids, and in several other positions in the family tree. Like so much, the exact postioning is still open to interpretation; however, pretty much everyone agrees that they are at least Maniraptoriformes.
Mostly smallish predators with grasping hands, stiff tails, and relatively large brains. The ``Velociraptors'' (actually more like Deinonychus) of Jurassic Park are typical of this group. Their most obvious feature is an enlarged recurved claw on the innermost toe of each foot, which was carried clear of the ground to keep it sharp.
Birds (including modern birds) and their close relatives. The line between advanced non-birds and primitive birds is so blurred that there are plenty of genera which may or may not be birds.
Some workers invert pretty much all of the coelurosaurian family tree, considering that birds came first (the so-called BCF hypothesis), and that many or all of the coelurosaurs were actually secondarily flightless birds - like penguins.
The Ornithischia are so named because their hip bones superficially resemble those of modern birds, in that the public bone points downwards and backwards, rather than down and forwards as in most (though not all) saurischians.
With no exceptions so far as we know, the ornithischians were exclusively herbivorous, and the more derived members of the group are characterised in part by successively more efficient chewing apparatus for dealing with tough plant material.
The ornithischians can be divided into three major sub-groups: the Thyreophora or ``armoured dinosaurs'', the Marginocephalia (``margin heads'') and the Ornithopoda (``boring dinosaurs''[2], [3]).
All of the Thyreophora are believed to derive from an animal like Scelidosaurus, a relatively unexciting quadropod with small bony plates embedded in its back. From these unpromising beginnings came two of the more spectacular dinosaur lineages.
These are the plated dinosaurs typified by Stegosaurus itself, the largest of the group, which had a series of eighteen plates arranged along its spine in two alternating rows, together with four spikes on the tail - two on each side. Other members of the group had different configurations of the same basic plan - for example Kentrosaurus had spikes along its spine instead of plates, and various members of the group also had spikes on their shoulders.
These are very heavily armoured dinosaurs with low, wide bodies, and consequently an unusually wide gait. In these animals, the small bony plates of the ancestral forms had developed into a solid, thick covering for the whole of the upper body, and in some cases the lower body too.
Within the Anylosauria are two major sub-groups: the Ankylosauridae have a bony ``club'' on the end of the tail (which they somehow managed nevertheless to carry clear of the ground!), which the Nodosauridae lack.
This grouping is characterised by a shelf (or ``margin'') on the top of the back of the skull, which overhangs the point on connection to the spinal column. This is much more obvious in the Ceratopsia, but is also clearly present in the Pachycephalosauria, along with other morphological indications that they are more closely related to the Ceratopsia than to the Ornithopods, as had previously been thought.
The name means ``horned face'', a reference to the most prominent feature of the more advanced members of the group - the Centrosaurinae and Chasmosaurinae, collectively known as the Ceratopsidae. All the members of this subgroup were quadropedal, and tended to be large.
All of the Ceratopsia share with this subgroup a substantial frill projecting backwards from the top of the back of the skull - an extreme form of the ``margin'' in Marginocephalia. Some of the non-ceratopsid Ceratopsia - such as Protoceratops - were quadropedal; other, more basal, members - such as Psittacosaurus were bipedal.
Of the two major ceratopsian subgroups, the Centrosaurinae are characterised by a large horn on the nose with the horns above the eyes being smaller or completely absent. Centrosaurines have short, deep snouts, and smaller frills than chasmosaurines.
By contrast, the chasmosaurines have larger brow horns than nasal horns, and long, shallow, narrow, snouts. In general, their frills are larger than those of the Centrosaurinae, and tend to have larger openings in the frill bones (although these openings would have been covered in tough skin in life). The exception to this generalisation is Triceratops which, uniquely among the Chasmosaurinae, has a completely solid frill.
The name means thick-headed lizards, which is an appropriate description. These quadropedal animals had thickly reinforced skulls, which must have been effective weapons. Because of their bipedal stance, they were once considered more closely allied to the ornithopods than the ceratopsians, but closer analysis reveals their true affinities.
The other unusual characteristic of the pachycephalosaurs is their very long and wide gut, extending as far back as the base of the tail. Pelvic modifications allowed the gut to pass right though the hip area.
Basal ornithopods include small to medium bipedal forms such as Thescelosaurus and Hypsilophodon. More advanced ornithopods, collectively known as the Iguanodontia, tend to be larger, and facultatively (that is, optionally) quadropedal.
Iguanodon and its relatives are characterised by a hand in which the first finger (thumb) has become a spike - presumably for defense - and the fifth finger is opposable. Iguanodon itself has the dubious distinction of having been restored in the bewildering variety of different stances, from Owen's original fat quadroped, through the first bipedal stance of the Bernissart skeletons and the all-but-vertical posture of Knight's paintings to the current, presumably correct, bipedal stance with the spine held horizontally.
The so-called ``duck-bills'' were very abundant by the late Cretaceous, so we have excellent fossil records including complete growth series for some genera. They are notable for their well-developed chewing mechanisms, including batteries of up to two thousand teeth in some genera.
In the past, the hadrosaurs were considered to be aquatic because their deep tails were considered good for swimming. It's now recognised that the tails were far far too stiff to be used in this way, and that the hadrosaurs were primarily terrestrial.
These hadrosaurs did not in general have crests on their heads. The group includes the original Hadrosaurus, the first dinosaur described from America, and the massive Shantungosaurus - the largest known non-sauropod dinosaur.
Finally we come to the crested duck-bills, which sported a variety of hollow bony crests, now considered most likely to be primarily display structures. (In the past, they were interpreted as snorkels!)
For more detailed and scientifically rigorous classification overview, Thomas Holtz's excellent series of cladograms at www.geol.umd.edu/~tholtz/G104/10417what.htm (and following pages) strike a fine balance between detail and exposition/comprehensibility; and Mike Keesey's dinosauricon website does a great job of tracking current thought in more detail: dinosauricon.com/taxa/index.html
Note 2. OK, I admit that Ornithopoda does not really mean ``boring dinosaurs''. It means ``bird-foot'', so named because ornithopods' feet look like those of ``beasts'', or mammals. No, that can't be right. That should mean that they're called ``theropods''. Oh no, wait - theropods are carnivores, so named because their feet resemble those of birds. Oh dear. I think this is where we came in ... [Back]
Note 3. Someone (well, several people) on the Dinosaur Mailing list objected to my non-literal translation of the term ``Ornithopoda'', and claimed that ducks are much more boring. To which Darren Naish <darren.naish@port.ac.uk> replied:
Errm... the evolution of carpal spurs and knobs, extreme pugnacity and territoriality, nest parasitism, creching behaviour, parental carrying of young both in the water and (!) in the air, monogamous pair-bonding, underwater copulation and the (?)reinvention of the penis, major sexual variation in tracheal structure, grass-eating and 20-minute gut carrying time, niche partitioning according to intestine size, carrion feeding on Subantarctic islands, the evolution of fern-eating, island giantism, island dwarfism, crepuscularity, serrated bill margins, filter feeding with buccal lamellae, deep-diving, species where males are flightless but females flighted, coevolution of browsing forms with spiky lobelioideaens, repeated increases and decreases in body size during phylogeny, the annual transportation of TONNES of sand... and, pant pant pant, quacking[4].Which just goes to show that everything is interesting if you take the time to learn about it. [Back]How *ON EARTH* can ducks be boring?????
Note 4. All that and he forgot to mention hoi-sin sauce and very thin pancakes! [Back]
This issue is clouded by the fact that the meaning of the word ``dinosaur'' has changed over time - which may or may not be a good thing.
When the first dinosaurs were discovered, it never crossed anyone's mind for a moment that they might be ancestral to birds, so the original definition of ``dinosaur'' certainly didn't include them. Even with the discovery of Archaeopteryx in 1861, the dinosaur origin of birds was far from being accepted as fact. It's only really since Ostrom demonstrated the similarities between Archaeopteryx and Deinonychus in the late 60s that the theory has gained any respectability - and so, the idea that the dinosaurs even might include birds dates back only a few decades.
Since then, a fundamental shift in nomenclature practices has meant that this bird-inclusive definition of the Dinosauria is now accepted as orthodox. This shift has been towards a viewpoint often called the ``cladistic'' view (related to, but separate from, the issue of cladistic analysis), which is that the only groupings which may validly by given names are monophyletic ones - that is, those consisting of a single animal together with all of its ancestors.
This spells trouble for the old definition of the Dinosauria, which is essentially the common ancestor of the Saurischia and Ornithischia with all of its descendants except the birds. Such a grouping is described as paraphyletic and is considered by cladists to be an unnatural group. So as cladistic nomenclature has taken hold, so the newer, monophyletic and more inclusive definition of the Dinosauria has become accepted.
Is this change a good thing? Yes, because it means that Dinosauria are now defined in a way that's in keeping with widespread taxonomic practice. And no, because the change itself is disruptive, leaving what was once a clear meaning unclear. A similar situation has arisen with regard to the class Reptilia, which is considered to include the dinosaurs and hence also the birds. (So, yes, birds are reptiles!)
Some people, notably Ken Kinman, argue that it would have been better to make up completely new names for the inclusive monophyletic groups rather than re-assigning the old names. In his paper Origin of Birds: The Final Solution? (American Zoologist: Vol. 40, No. 4, pp. 504-512), Peter Dodson is particularly forthright:
For example, the word dinosaur was not previously problematic - it was universally understood. Within cladistics it has now been redefined to include birds [...] and then a new and cumbersome phrase, non-avian dinosaur, has been substituted. This is not progress; this is semantic obfuscation not enlightened communication.
In the alternative approach, the monophyletic group consisting of all dinosaurs including birds could have been given a new name - Eudinosauria, say - and reptiles including dinosaurs and birds given a new name such as Eureptilia. Whether or not this would have been a good idea, it seems that the moment has passed: it's not going to happen.
With all that said, I'm still going to continue hedging, because there is a big difference between technical nomenclature and everyday language. When we talk to non-scientists, they will understand the term ``dinosaur'' to include the likes of T. rex and Triceratops but not swans and sparrows. OK then, there's no need to rock that boat - let's use the word in accordance with people's expectations - in everyday life, ``dinosaurs'' generally means ``non-avian dinosaurs'' - or what I like to think of as ``Real Dinosaurs'' :-)
More than that, most of the time scientists also use the word ``dinosaur'' in the informal sense that excludes birds. For example, the Dinosaur Mailing List's administrative message describes the list's purpose as ``to give people a forum for the scientific discussion of dinosaurs.'' Yet everyone implicitly understands that this means non-avian dinosaurs: the list only ever discusses birds in as much as they are relevant to Real Dinosaurs.
So even scientists don't need scientific nomenclature all the time. Generally, though, the feeling is that it is better to stick with formal nomenclature for most technical discussion; and certainly in formal publications, in which misunderstanding would be disastrous.
As a rough rule of thumb, when people use latinate terms like Dinosauria and Reptilia, they often intend them to be understood in the monophyletic, inclusive sense; whereas informal terms such as ``dinosaurs'' and ``reptiles'' tend to carry their historical meanings. But this is by no means hard and fast.
In many technical contexts, the informal notion of Real Dinosaurs simply isn't rigorous enough for many uses. When an author wants to refer to dinosaurs other than birds, exactly which animals does she with to exclude? All of the Aves? Just the extant groups? The whole of the Avialae, or even all the Maniraptora? Or just the Pygostylia? In many contexts, it's necessary use an unambiguously precise term such as ``non-avialan Dinosauria''.
But we shouldn't allow that to blind us to the obvious - so that when we read that Spielberg is making a film about dinosaurs, we can be pretty sure it's not going to Sparrow Park.