Coelurosauria Maniraptora Avialae Aves

Where does something like Archaeopteryx go in this classification? It might be tempting simply to say that Archaeopteryx is a bird and should naturally go under the formal category Aves. But the problem is trickier because of some recent developments in the theory of classification. To dispatch Archaeopteryx properly, I must first digress a bit and mention those recent arguments. Traditionally, ranks of the Linnaean hierarchy call for a general name—a species belongs to a genus, a genus to a family, a family to an order, an order to a class, a class to a phylum, and a phylum to a kingdom. In some cases, the names are easily applied—we recognize a five-kingdom system for life, one of which includes the Animalia. But naming ranks also leads to problems. As we become more knowledgeable about the relationships of groups, we can be more specific about various levels in the classification, and we can proliferate greatly the number of ranks. Likewise, proliferation of general rank names then becomes cumbersome; we start designating things as superfamilies or super-superfamilies, as more levels are recognized.

There is another problem with rank names. A rank like family suggests more knowledge than we actually have. It suggests somehow that the family Protoceratopsidae and the family Hominidae (the group including us—Homo sapiens) are equivalent in some biological way. But this equivalence in ranks means nothing really. Protoceratopsidae and Hominidae simply happened to "fall out" as families as we moved up along different branches on the tree of life. Certain families, like Protoceratopsidae or Oviraptoridae, may have only a few species within them. Others, like many of the prolific families of insects, may have huge numbers of species. Why, then, should the two groups be given equivalent rank names? These annoyances can be avoided if we simply rid ourselves of these rank names. After all, the indented list above doesn't need all these rank names; it tells clearly where the various groups belong. It also allows the insertion of other groups in the hierarchy at some later time. For this reason, I have throughout this book avoided referring to the groups as families, orders, classes, or superclasses.

This provision is important because we would like names of groups to have an explicit meaning. The Maniraptora should mean "a group including the maniraptoran ancestor and all its descendants." It should not simply mean "a group that, because it is somehow different, isn't another kind of dinosaur." Unfortunately, the traditional scientific literature is plagued with names whose original meanings are very foggy. Invertebrata, for instance, the great suite of creatures that includes flatworms, sponges, worms, insects, snails, and starfish, are simply those animals that for some reason are not vertebrates. There is nothing that makes invertebrates a real, i.e., monophyletic, group. Some so-called invertebrates are on the same evolutionary spine leading to vertebrates, while other so-called invertebrates are not. But all "invertebrates" cannot claim to share an ancestor unique to themselves, in other words one that excludes vertebrates. In this sense, the common textbook and encyclopedia category Invertebrata is no better than Linnaeus' primitively obsolete term Beastiae.

Despite the long history of taxonomy, this attempt to use group names consistently to indicate ancestry and descent is rather new. The approach was clarified in the works of the German zoologist Willi Hennig in the late 1950s and 1960s, some two hundred and twenty years after the publication of Linnaeus' Systema Naturae. Hennig was also responsible for the development of the methods later called cladistics. The rules of classification have been further refined by two brilliant colleagues, Jacques Gauthier, a dinosaur and reptile specialist in his own right and one responsible for many of the clarifications of dinosaur groupings noted above, and Kevin de Queiroz, a curator at the Smithsonian Natural History Museum. Jacques and Kevin have outlined a set of rules they call phylogenetic taxonomy. First, the names of ranks are abandoned; no more families, super-families, or super-superfamilies. Secondly, names of groups are defined by their genealogical connections, not by their characters or contents per se. Thus Dinosauria is defined as the clade stemming from the last common ancestor shared by the two dinosaur subgroups, the Ornithischia and the Saurischia. It so happens that dinosaurs also have certain features that enable us to recognize them—such as the hole in the hip socket mentioned in Chapter 2. This trait is then a handy way of diagnosing the group; it is the shared-derived feature that demonstrates the monophyly of Di-nosauria. But Dinosauria is defined by its basic connection—ornithischians and saurischians through their last common ancestor. This way a group can change in content (by the inclusion of more taxa) or its diagnostic traits may be modified, but the group's name can be retained. Such a method offers more stability in naming biological groups.

Jacques and Kevin have also sought to retain some of the familiar names. They use them for clades (called crown clades) issuing from two or more extant taxa. Mammalia is the clade issuing from the last common ancestor of the three major branches of living mammals: monotremes (duckbilled platypus, echidna), marsupials (pouched kangaroos and kin), and placentals (our own group as well as that for many other mammals). Their ideas are constructive, especially if some of the original group names—the Mammalia, Aves, Crocodylia, and so forth—can be retained at levels that make sense in the hierarchy of life. But phylogenetic taxonomy is relatively new, being first described in papers dating back to the late 1980s, and it is controversial. It has not been widely or enthusiastically embraced by all taxonomists.

It is easy to see how this new system impacts on our designations of birds and dinosaurs. As noted, the formal equivalent of birds is Aves. As Jacques and Kevin would have it, Aves is defined as the group that includes the last common ancestor of all the living groups of birds. Under this scheme Archaeopteryx is not a member of Aves. There are other fossil birds, including Gobipteryx, that are more intimately nested within the clade including all living birds. In other words, there are some fossil taxa that are closer to the ancestry of all living birds than Archaeopteryx. A phylogenetic classification for these players might then read:

0 0

Responses

  • tuuli
    What did the Maniraptora Subgroup "Avialae" include ?
    8 years ago

Post a comment