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.