Reptilia? Sauropsida? Diapsida? Sauria?

Komodo Dragon and Hoatzin. Photo by Thore Noernberg. 

During my sophomore year in college I was fortunate enough to have two professors that had their preference for their “ideal” taxonomy and their reasonings behind it. My vertebrate professor loved cladistics. She explained to the class in a lecture once that cladistics is ideal as life is one, big evolutionary saga where lineages (even species) are potentially one and the same. It is all about the last common ancestor. My anatomy professor prefer the traditional Linnaean system, where organisms were classified based on shared morphological traits and there is ranking involved. To her, cladistics can be "rather clunky” and potentially confusing but my vertebrate professor found ranking of organisms is rather silly. They did both agree that it was important to categorize organisms based on relationships - the monophyletic clade the ideal group which biologists love. While most of my peers find taxonomy and systematics to be trivial and a nightmare to study, I find them to be quite enjoyable and also important in understanding not just how organism lineages came to be but also how nature had selected certain behaviors and anatomical features that are best fit for reproduction for a species. I strongly agree we need to categorize organisms into monophyletic clades and I can understand why both of my professors have chosen their prefer taxonomy system. Of course then you have people like Colin Tudge (2000) and Robert Bakker (1986) that tries to combine the best of cladistics and Linnaean taxonomy (known as “Neo-Linnaean”), but that is a whole other animal and one that can be very flawed. More so - in my opinion - than both cladistics and the Linnaean system! Indeed the chose of the taxonomic system that scientists use can affect the way we see the relationship of species. A classic group that beautifully illustrates the point are the reptiles and the birds.

Taxonomic Segregation

Despite the overwhelmingly amount of evidence and the obvious that birds are reptiles, majority of major herpetological and ornithological organizations (as well as some books, particularly field guides) still place reptiles and birds into two separate clades Class Reptilia and Class Aves respectfully. I have yet to see a field guide to birds and reptiles of North America being placed in the same clade together. Even more so there are still people that cannot quite see how birds are reptiles, even though feathers and reptile scales are homologous structures made of keratin, same type of blood, and most important of all molecular and morphological work has shown a close relationship between birds and crocodilians. The problem stems from the fact that in the Linnaean system, a class cannot be in a class. Classes can be sister taxa but that is it and nothing else. It is a paradox for it is not the fact that the world’s ornithologists and herpetologists do not acknowledge this relationship. On the contrary, these majority of these scientists do support the recognition of birds being “glorified reptiles” it is just they prefer the less clunky route. If they are so focused on updating evolutionary relationships (and they do. Taxonomy changes amongst scientists is a vicious, political game. But here is not to discuss that.), then they would lump. General biologists and paleontologists do a good job in placing birds and reptiles in the same clade that could regard as a class. If one were to simply follow the cladistic approach you are truly not only finally acknowledging the evolutionary relationship on paper at these meetings, but you are also helping eliminate “how birds are reptiles” from the public and educate them to understand the closeness. This will end taxonomic segregation.

Divide Reptilia! Wait, No!

This class, according to the cladistic approach, would be the most speciose and successful of all the terrestrial vertebrates, colonizing every single corner on the planet. It would have about 20,808 species worldwide, from the Tuatara to passerines. But what should the name of this class be? You would think that the name for the class would be Reptilia, given the argument that name of ancestral lineages has priority over daughter lineages (a classic example would be when the then Order Pinnipedia was found out to be nested in the Order Carnivora. Pinnipeds are now seen as members of the Order Carnivora). But there is a problem. Some cladistic scientists have argued that the general concept of Class Reptilia is extremely invalid for it is pretty much a synonym of Amniota (Tudge, 2000); stem-mammals (“synapsids”) were originally classified as members of Class Reptilia, but majority of work today suggests they have diverge before true reptiles, adding more fuel to the invalidity of Reptilia. Lastly the most common argument is Class Reptilia did not intended on including the clades Class Aves and Class Mammalia. This left two options. One option by the Neo-Linnaeanists was to make Lepidosauria, Testudinata and Archosauria as classes, which Baker (1986) advocated very much so in his book The Dinosaur Heresies. Other scientists and myself disagree with this. It is redundant and would create confusion in regards to the many fossil reptiles that do not fall within any of these three groups. The second option was of course giving a new name. The name cladistic taxonomists use is Sauropsida.

Class Sauropsida? Or How About Class Diapsida?

This diagram is Reptilia vs. Sauropsida. Diagram by Petter Bøckman

Sauropsida does seem to be a good name. It is define to include the last common ancestor of reptiles, including birds which they most closely related to crocodilians. So not only it is good, but it is also valid as well. However I do have to detest. I never truly see Sauropsida as something as class worthy. Rather I see it as more of a stem-branching that contains stem and crowned animals just as how most scientists view Synapsida. Synapsids contain both stem-mammals and crowned-mammals. I can make an argument that Sauropsida is a stem-branching as it contain both stem-reptiles and crowned-reptiles. Indeed this thought probably never occur as for a while there was a possibility that turtles might be the last, surviving anapsids on the planet. It has become very clear that turtles are definitely diapsids and the definition of a crowned-group is it must contain extant members. Therefore, the anapsids of Parareptilia are not crowned-reptiles as well as the earliest sauropsids and even the early eureptiles in my opinion. Not to mention that all my professors prefer Reptilia over Sauropsida as they feel you can simply redefine the definition and its membership - see Modesto & Anderson (2004) as well for more reading. An old name are better to keep than make a new one, as new names can be redundant to make. Of course this would not eliminate the whole Amniota issue and I would still think Reptilia as a stem-branching group. So if not Reptilia or Sauropsida, then what would be a prefer name for a class of reptiles and birds in a field guide concerning them? Diapsida? Possible, since molecular data has shown all reptiles and birds are descended from a diapsid ancestor (Crawford et. al, 2015). However there are diapsids that fall outside a lineage consisting the lepidosaurs, turtles and archosaurs. They would be stem-reptiles still. So scratch that one off the list. All might seem to be lost. There is, however, one I have found and I actually prefer.

Class Sauria?

Sauria was originally intended as a suborder containing the lizards in Order Squamata. Of course, the name has fallen out of use and is almost obsolete. However I have seen the name a couple of times in some papers, with at least one paper using Sauria as a clade containing the lepidosaurs, turtles and archosaurs and their stem representatives (Crawford et. al, 2014). Sauria would be/is the ideal name for a class. Sauria can be defined as a crown-based grouping for it contains extant members which are our modern day reptiles and birds. The interrelationships among reptiles and birds is still the same, is not at risk of being the same thing as Amniota and it can be use for the designation of class (we would still have the problem of making classes for stem-reptiles, but we have to look at the context for our modern species. Not to mention I feel as if we are coming to a point where “crowned” can equal to a Linnaean ranking, such as crowned-mammals being in the clade Class Mammalia). Lastly we can collectively call reptiles and birds as “saurians”. In short the name of a single clade containing reptiles and birds is ideally Class Sauria. I don’t have high hopes for this catching on, but I do hope someone would also come to the same conclusion as I have. I would love it in the near future to see book stores selling field guides of saurians of North America. Such a thought might cause herpetology to fully divide into batrachology (the science and study of amphibians) and a new field that might merge with ornithology. Perhaps “saurology” and the birth of major saurological organizations and parties?

Birds being a Single Clade Order

Self explanatory; crocs and Tuatara make up 0.02%! Based on the Reptile Database and TiF.

Despite not being the main focus of the article, there is something that also needs to be address as well. How many orders would there be in Sauria? The number of reptile orders is not the issue, it is the number of bird orders. Should Sauria have a total number of at least 50 orders? In my mind that is a bit excessive. Not to mention birds as a group are just as diverse as the squamates, yet I find nobody arguing or suggesting splitting the squamates up into several different orders. One could make an argument that birds are actually a pretty much a homogenized group with feathers, a beaked head and bipedal. Of course bird species do vary in these attributes, but so do various insect orders such as beetles. And for a single order to contain 10,000 species would not be an issue. After all, squamates approach at least 9,000 species. What would the name of the order be? Since birds are descended from theropods and defined as such, it would be Order Saurischia. Under this argument, crocodilians might have a similar predicament as they are descended from rauisuchians (Nesbitt, 2011) so therefore the clade is Order Rauisuchia, not Order Crocodylia. This does not mean we have to call them “saurischians and rauisuchians” from now. We can still call them "birds and crocodylians” as they used much more in everyday usage in regards to modern species. But that is only a suggestion of mine. Still, we need future scientists to finally end the coffin to Reptilia and Aves and bring out Sauria.

References

• Bakker, R. T. (1986). The Dinosaur Heresies: New Theories Unlocking The Mystery of the Dinosaurs and Their Extinction. William Morrow, 165 p.
• Crawford, N. G., et. al. (2015). A phylogenomic analysis of turtles. Molecular phylogenetics and evolution, 83, 250-257.
• Modesto, S. P., & Anderson, J. S. (2004). The phylogenetic definition of Reptilia. Systematic biology, 53(5), 815-821.
• Nesbitt, S. J. (2011). The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History, 1-292.
• Tudge, C. (2000). The Variety of Life. Oxford: Oxford University Press, 407-410.