A recent thread on animal classification got me Wikipedia diving, and I found them using some new (for me) definitions. Last I knew, extant amniotes were divided into anapsids (turtles), synapsids (mammals), and diapsids (birds, crocs, lizards, etc.), and the term “reptiles” was just an archaic, pre-cladistic term for “non-mammalian, non-aviaian amniotes”. Wiki-P is dividing amniotes into synapsids (mammals) and reptiles (everything else).
Is this an accepted division now? When did that happen? Was I wrong to begin with?
Yes and no. “Reptiles” were cold-blooded, scaly animals that laid amniotic (hard- or leathery-shelled) eggs and that did not have fur or feathers. Since mammals and birds were known to have evolvd from reptiles, that made the definition cladistically invalid – it did not include all amniotes, just the ones that had not ‘progressed’ tp mammalhood or birdhood.
Reptiles were divided on the basis of the presence or absence of lateral fenestrae in the skull into anapsids (turtles and the extinct cotylosaurs) with no fenestrae, synapsids with lower, temporal fenestrae (pelycosaurs and ‘mammal-like reptiles’), euryapsids (aquatic forms such as placodonts, plesiosaurs, and ichthyosaurs) with upper, parietal fenestrae, and diapsids with both temporal and parietal fenestrae. The latter were divided, for reasons I’m not completely clear on, into the archosaurs (dinosaurs, pterosaurs, theocodonts, and crocodilians) and lepidosaurs (cbampsosaurs, protorosaurs, sphenodonts, snakes, and lizards). Snakes and lizards lost the posterior connection separating temporal fenestra from oral cavity, the snakes the upper posterior connection separating parietal fenestra from oral-temporal opening as well).
It was established that the synapsids were very early divergents from the main ‘reptile’ bush, and that so-called ‘mammal-like reptiles’ were far more closely related to mammals than to ‘other’ reptiles. At about the same time it was established that the so-called euryapsids were (a) not monophyletic – ichthyosaurs did not share common ancestry with the others – and (b) probably the result of a secondary closure of the temporal fenestra in two groups of early diapsids, not a distinct group at all. It’s quite possible that the anapsids resulted from secondary closure of both fenestrae as well – the earliest amniotes are diapsids.
With the careful taxonomy and paleontology that established that birds diverged from a specific stock in the dinosaurs, things shifted again. Effectively there are two main lineages of amniotes: the one leading through the pelycosaurs and synapsids to the mammals, and the one that includes all the diapsids in the new sense, including dinosaurs in the old sense, birds, pterosaurs, crocodiles, lizards, snakes, and virtually all the extinct ‘reptiles’ other than pelycosaurs and synapsids. At the same time, cladistics substituted for the old Linnaean hyerarchy (KPCOFGS) a more flexible system that means that if you have five early families splitting off from ‘the main line’ of a family or onder, you don’t end up with a confusing hierarchy of supercohorts, infraorders, and hyperfamilies, but a sequence of Chinese-doll clades, each containing the ‘lower’ clades and their closest relative group.
At the beginning of the 6th paragraph was:
“The mammalian line of descent diverged from an amniote line at the end of the Carboniferous period. One line of amniotes would lead to reptiles, while the other would lead to synapsids.”
I understand Wikipedia does not necessarily give “truth”; what I would like to know is if this terminology is accepted by a significant fraction of vertebrate researches (researchers into vertebrates, that is) - or if it’s just a fringe usage. If this usage is “true” then mammals branched from the rest of the amniote line before turtles did. Not hard to swallow, but still, calling non-mammal clade “reptiles” seems strange.
The “in” thing these days is Phylogenetic Systematics. What this means is that our taxonomies ought to represent, as best they can, actual evolutionary lineages. The Linnean system has a significant problem with this, in that it obscures the evolutionary origins of birds by placing them in an equivalent “rank” with “reptiles” and mammals. However, birds descended directly from classical reptiles - namely, certain theropod dinosaurs. As such, cladistics and phylogenetic systematics are often favored amongst those who study those relationships. Trying to shoehorn an extant bird into Linnean Reptilia is a nightmare of supra-thises, and infra-thats, and infra-sub-whoseits. Better to simply do away with the whole “rank” concept entirely, since it doesn’t tell us anything anyway. Unfortunately, many folks still do use those ranks.
Cladstics divides the amniotes into Synapsida and Sauropsida. Synapsida is the branch that eventually would lead to Mammlia, and includes the previously-considered “mammal-like reptiles”, which, in truth, are neither reptiles nor mammals. Sauropsida contains what would become modern reptiles; Sauropsida is further dividied into Anapsida, which contains turtles, and Eureptilia, which contains the other “traditional” reptiles (snakes, lizards, crocodiles, etc.), as well as dinosaurs and birds.
I had known that the traditional “reptiles” was not a good clade and I assumed that taxonomists would stay away from the term as too baggage-laden. So now that it’s been cladified, birds are reptiles and pelycosaurs are not.
Essentially, they still exist side-by-side. In practice, the Linnean hierarchical system still exists, but people have been trying to make it more compatible with cladistic reslationships. It’s a uncomfortable fit, since cladistic relationships can’t conveniently be expressed by a hierarchical system like the Linnean one.
The old system is better for pigeonholing things. If you have to catalog a museum collection of bird specimens, for example, it’s much easier to organize them by order, family, and genus. So I expect the Linnean system to still be used for this kind of purpose for the foreseeable future.
Right. However, a distinction should be made between the non-technical and technical use of a word. I think that there’s no problem with continuing to use “reptiles” informally to include lizards, snakes, crocodilians, turtles, and the the tuatara and exclude birds. And even scientists are not going to change the names of their journals to “The Journal of Avian Reptiles”.