Crocodiles and turtles are not reptiles? CNAH thinks so.

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For all those playing the home game, here is the story thus far:

Reptilia, the group, was created back in the early days of taxonomy. Its coiner, Carolus Linneaus (upon whom we get the dominant form of classification today), created the group to house all the critters that were neither mammalian, nor avian. Reptilia was originally a wastebin that housed all extant reptiles, as well as spiders and sharks.

Over the decades, classification schemes became more refined and the definition of Reptilia became more restricted until it eventually resulted in the definition we have today. Namely that group that incorporates snakes, lizards, turtles, crocodylians, and tuataras. A group defined (or once defined depending on ones systematic leanings) as a collection of animals all sharing epidermal scales, being bradymetabolic (or more erroneously, ectothermic), and sharing a series of skeletal affinities such as a small, or absent tabular, a large post-temporal fenestra, a suborbital foramen and a supraoccipital plate that is narrow.

This definition worked and served herpetologists and paleontologists well for decades. Then in the 70’s a new classification scheme came along. Deemed cladistics, it focused less on shared characteristics and more on shared, derived characters.

For example: Humans have hair and five fingers. The five fingers are a shared character with all other tetrapods (terrestrial vertebrates and their secondarily aquatic descendants). Meanwhile the hair is a shared, derived character with mammals.

Obviously the terms shared and shared derived (or plesiomorphy and synapomorphy, in the technical sense) are going to depend on one’s frame of reference. For instance if one was going to look for a synapamorphic trait for humans compared to rats, then hair wouldn’t work. Fingernails and tailessness would. Compare humans to other apes and now these last two characters don’t work either, so one must look for something else.

So on and so on.

Cladistics had a rocky start, but was eventually accepted as the main means of determining evolutionary relationships. Though there are still a few staunch detractors, the overall view on cladistics is that it is the most true way of expressing evolution.

Since cladistics groups creatures by their shared derived characters, once one is on a branch of the cladistic tree, one stays there. Creatures can split from this branch, but they will always be retained.

See the following figure for an example:



Note how even though sharks, crocodiles and rabbits have all split from the vertebrate branch, they are still retained on it.? Since branches can infinitely split, there is no trouble with showing evolutionary relationships this way. It creates a view of evolution as a very thick bush; which is a fairly accurate representation of the results of this process.

In terms of phylogenetics, this is just fine.? Cladistics kicks butt.

Unfortunately, some ardent supporters of cladistics thought that this method might work well in terms of classification.

Now some of you might be shaking your head right now thinking that phylogeny and classification are the same thing. They are not.

Classification is the act of categorization. It is an arbitrary way for humans to order what they see in the world around them. We classify everything!

Cars are broken down into their manufacturer and their model. That’s classification.

Clothing is broken down into seasons, body type and general design. Once again, classification.

Google breaks search results into web, images, shopping, scholarly texts, etc. That is classification.

Now there are those liberal arts types out there that like to think that classification only limits our perceptions and creates unwanted stereotypes. While this is partly true, the alternative is a world without order. If our brains worked differently this might be fine, but our current neurological makeup is such that a chaotic hodgepodge of things without names and categories, only results in confusion.

Like it or not, we will always need to classify things. The trick is not to let the classification completely colour our perceptions.

Coming back on track, certain systematists felt that the all inclusive nature of cladistics would work well with classification. So new rules were implemented. From now on a group could no longer be defined by its characters. Rather, its definition would now be dependent on a completely arbitrary association of members.

For instance snakes are no longer classified based off of being limbless, and lacking both temporal bars among other things. Instead they are now defined as being the group that contains all members that evolved between boas and blindsnakes. To put it in a more exaggerated sense: boas are snakes because snakes include boas. This classification is completely circular and meaningless.

However it is also stable. 20 years from now, the definition of snake will remain the same. For some systematists the stability of the name outweighs its lack of substance.

Another rule enacted was that only groups that contain an ancestor and all its descendants would be considered a “natural” or “real group.”

On the outset this might not seem a problem. Humans are hominids. Hominidae includes us and a few other apes. No big deal. Birds, as neornithines, include every single bird you see flying around today. Again no problem.

But what about larger groups. Especially groups like Reptilia, that were originally believed to have given rise to numerous other groups (birds and mammals). What of Osteichthys, the group that gave rise to every land vertebrate today.

Starting to see the problem yet?

The old definition of Reptilia no longer held up. Reptiles excluded one of their descendants; the birds. This made Reptilia paraphyletic (ancestor and some of its descendants). In order to “fix” this alleged problem, birds would need to be incorporated into the meaning. The result: birds are now reptiles.

Well, in some circles.

This kind of all inclusive naming scheme has been met with intense resistance. So much so, in fact, that 30 years after its inception, dinosaur paleontology seems to be the only branch of biology that actually follows these rules. Every other field seems perfectly content with paraphyletic groups.

And hey, why not? Paraphyly makes perfect sense in terms of classification. It is much easier to grasp the concept that whales evolved from cows, rather than calling whales cows.

Alas this battle appears to be far from over. For whatever reason, Reptilia seems to be at the heart of the argument. Many herpetologists, ornithologists and paleontologists are perfectly happy with leaving birds out of reptiles. Other paleontologists are not, and continue to do away with the old definition. Some have even gone so far as to try and remove Reptilia altogether from classification.

So back and forth it goes. This continuous arguing has made things a little confusing for students of evolutionary theory. When it comes to classification the bickering between both sides can be enough to turn students away, or at least give them a headache.

So the Center for North American Herpetology decided to take matters into their own hands and reclassified Reptilia all on their own.

Idealistic to be sure (I like the idea of a crocodylian and chelonian class), but controversial. CNAH decided that the most accepted version of reptile is one that doesn’t include either turtles or crocodiles.

What the hell were they thinking?

Needless to say, I doubt that this will catch on.

~Jura

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4 Responses to Crocodiles and turtles are not reptiles? CNAH thinks so.

  1. This is indeed a silly idea, but not, I think, for the reasons you mention. One of the big problems is that we already *have* a perfectly good name for the taxon they are calling “Reptilia”, and that’s “Lepidosauria”. (Also, “Testudinata” is preferable to “Chelonia”, since the latter is also a genus.) “Reptilia” really should just be retired to the Pasture of Historic Names, alongside “Vermes”, “Pisces”, “Quadrumana”, etc. There’s no problem with using it informally, like “fish”, “worm”, “herptile”, etc., but why the need to formalize it.

    The real problem here is that they are trying to simultaneously use arbitrary ranks and phylogenetic nomenclature (what you are semi-erroneously calling “cladistics”–“cladistics” generally refers to the scientific process of analysis, not to a system of nomenclature). They feel a need to designate something a “class”, but this is a useless designation that tells us nothing. It doesn’t do any harm, though … at least, until you consider fossil taxa.

    Which class does _Ankylosaurus_ belong to? _Stagonolepis_? _Pareiasaurus_? _Ichthyosauurus_? The only options are:
    1) Expand the classes to be total groups: make _Ankylosaurus_ an avian, _Stagonolepis_ a eusuchian, etc.
    2) Make hundreds of impossibly small classes: Class Deinonychosauria, Class Lagosuchidae, Class Ophiacodontidae, etc.
    3) Shoehorn them into paraphyletic classes, thus defeating the whole purpose.

    Note that, even under #1, the ancestors of crown groups cannot be classified beyond a certain rank without creating paraphyletic taxa.

    So, it seems to me that we must lose ranks or clades. The decision is fortunately an easy one: ranks are almost completely uninformative and subjective, while clades reflect propinquity of descent, a useful bit of information if there ever was one.

    Also, it’s a mistake to say that advocates of phylogenetic nomenclature are confusing phylogeny with classification. Quite the contrary, we see phylogeny as a *substitute* for classification! Instead of spending time dithering over whether Eusuchia should be a class or a subclass or an order or a parvorder, we just say that it’s the last common ancestor of _Alligator_, _Crocodylus_, _Gavialis_, and _Hylaeochampsa_ plus all descendants thereof. Then the phylogeny does the work for us — no classification necessary. Under this system, when people argue about what belongs to which taxon, they are actually arguing about something scientific, not just being fussy.

    Kudos to CNAH for showing us how silly rank-based nomenclature can be!

  2. Despite the arguments from folks like David Marjanovic, the statement that phylogenetic nomenclature (PN) and cladistic classification are different, is basically a semantic one. PN always used cladistics as its basis for naming. So what is the point of removing the cladistic part from the name? I admit I could have specified that a little better in my original post, though the original concept remains.

    I don’t see how PN is a substitute for classification. It’s still a system that is used for categorizing and naming objects. It’s just that the criteria for naming has changed, and the definitions have become fuzzier.

    In the end, these are my two main problems with cladistic based classification. Under PN rules, if I go fossil hunting and come across a fossilized scute, I can’t say what it belongs to. At least not in the conceptual sense. Under the “old rules” of classification I could look at this hypothetical scute and see that it was oblong with lots of deep scarring. A character that would fall in as diagnostic for a member of Crocodylia (actually crocodylomorpha, but I’m trying not to get too bogged down in details here). So I could then say that this scute belongs to some kind of crocodylian because of these features.

    Under the rules of PN, where names are based off of whose related to who, and the character diagnoses get removed, I’m forced to look at this oblong scute with its deep scarring and say just that. It is an oblong scute with deep scarring. Short of putting these characters in a data matrix (and a fitting one at that. Not some invert data matrix, or one featuring just sharks) I am unable to say exactly what this scute belongs to.

    Of course in the real world, I would still classify this scute based off of my initial observation. All paleontologists and biologists do this regardless of what classification scheme they use. Even the cladistic programs get their results from analysis of the characters.

    So in the end, if the name still appears to be defined by the characters. Why bother removing them from the definition?

    I get that this way is less stable. As more specimens show up, the characters that diagnose a certain group are likely to change. Still this seems a small price to pay for the ability to say that taxon x is defined as those creatures that share the following morphological traits. Traits that one can actually see.

    The fuzziness of the names comes from the all inclusive nature of PN. For example, if one used Osteichthys (or Sarcoptergyii, or whatever) as an example. The original definition limited one to “bony fish.” Right off the bat there is a concept in place that helps one more quickly find what one is looking for. If one views osteichthys as being those aquatic critters with bony skeletons, then one is not going to go looking for an osteichthyan in a terrestrial setting. Obviously there are cases where concepts can get in the way. Reptilia and dinosaurs being a prime example. Still these should only serve as cautionary tales of what not to do with a conceptual name (i.e. concepts should help guide ideas, but they shouldn’t control them). In the case of Reptilia, the concept is in dire need of an overhaul. Removing it altogether due to baggage seems pointless. Especially given its inertia in the popular mindset, as well as a lack of any real replacement names (sorry but I don’t see sauropsida taking off anytime soon). Returning to my example, if one instead, pulls for a more phylogenetic based approach, then suddenly we have mammals, reptiles and birds getting shoved into the Osteichthys group. Now the concept associated with name is destroyed. The term becomes fuzzy and essentially meaningless. I can now find osteichthyans on land and in the air. Heck there are even a few that make sojourns into space. So what does the term mean? Well, nothing really. Not anymore.

    If you take away the concepts associated with the names, then don’t you remove the purpose of giving names at all? How can I talk to someone else about bony fish, if there is no name for these fish anymore? Okay, I could just use “bony fish” when talking to another person about bony fish, but then all I’ve done is take a common name and used it to replace a taxonomic name. Alternatively I could stick some unwieldy qualifier in front of it like non-tetrapodan osteichthyan (or non-tetrapodan sarcopterygian, etc) in order to bring the name closer back to its original meaning. So, then, what is the point of the taxonomic name at all?

    I believe it is this reason why we don’t see paraphyletic groups disappearing from biology. Despite your misgivings there are those of us out there that do find these groups interesting and worth talking about. Paraphyly only gets muddy at the ends, where creatures don’t easily fit in one group, or the other. These, though, will always be the exceptions rather than the rules. If necessary one could stick a qualifier on these guys like how the old term “mammal-like reptile” was once used (a term that would be just fine if it weren’t for the fact that mammals didn’t evolve from reptiles). It’s still an unwieldy and rather annoying qualifier, but at least it refers to fewer taxa.

    Another alternative is that one could come up with a specific name for that “transitional” group. Conceptually this could prove chaotic as one find more and more taxa that blur the lines, thus requiring more and more new names. Realistically, though, this would be unlikely to happen for most groups as this would require a fossil record with a resolution well and above the one we actually have.

    Okay, I think I’m going to stop now. This has been enough of a brain dump for one night. Besides I’m pretty certain this comment is now longer than the original post. 🙂

  3. Have you seen the Naturserve.com taxonomy? (used by all state natural heritage inventories I am aware of)