Non-hexapod crustaceans
The fundamental assumption of evolution is that all living things on Earth are, at some level, related by descent from a common ancestor. In some cases, this common ancestor lived recently, as is the case for humans and chimpanzees. In the vast majority of cases the common ancestor is far more ancient; our DNA tells us that we and the bacteria that causes Salmonella are also related, but that shared ancestor lived far more than a billion years ago. Note that “sharing a common ancestor” is not the same as “descended from,” especially when talking about living forms. We are not descended from chimpanzees; we are both, however, descended from the same ancestor; whether this ancestor would be considered a chimpanzee (probably not) is a separate question.
Working out the patterns of common ancestry and shared descent, the phylogeny of groups of organisms (taxa), has a been a chief work of biologists and paleontologists since the 1859 publication of The Origin of Species. Over this time, it has undergone multiple revolutions in both data and method and is still a changing and often contentious field. Until the 1960’s, nearly all of the basic data was the morphology (structure of anatomical features) of a taxon (a species, genus, family, etc.) and the methodological approach was the somewhat subjective appraisal by experts of which similar features among taxa were most crucial in revealing relationships. This meant recognizing that some kinds of “similarity,” such as the bones of the limbs in bats and dolphins, were more important than others; e.g. streamlined shape of sharks and dolphins. The first methodological development was to quantify morphologic similarity using detailed measurements and the use of computer based statistical analysis. The hope of this approach, known as phenetics, was that such quantitative analyses would be far more objective in revealing relationships. Phenetics, although still useful for many purposes, has become secondary relative to another approach introduced at about the same time. Known as cladistics, it formalized the logic used by previous investigators. Features (characters) shared among taxa, such as humans and other primates, were divided into three general groups: ones shared by those organisms and all of their more distant relatives (five fingers on all terrestrial vertebrate limbs); those uniquely shared among them (an opposable thumb without claws in humans and nearly all primates) and those independently acquired from different ancestors (hairlessness in humans and some breeds of cats). Only the uniquely shared characters, novel features acquired by the common ancestor and passed on to it descendants, count for determining relationships. Taxa joined by these unique shared characters and thus sharing a single common ancestor are known as clades. Cladistics and methods derived from it have become the principal method for reconstructing phylogenies (for a brief introduction to the method, see: https://timescavengers.blog/evolution/reading-the-tree-of-life/).
The earliest cladistic studies of phylogeny were qualitative. It quickly became obvious, however, that this was not practical with large data sets made of many taxa and large numbers of morphological characters. With the advent of rapid and relatively cheap DNA sequencing, independent and even larger data sets became available. The result is that a host of new methods have been developed that use sophisticated computer programs to analyze the data. Among other thing, these methods differ in underlying concepts (e.g., parsimony versus maximum likelihood) and in what data they utilize. Some use only molecular or morphologic data, some attempt to combine both, and they differ in how fossils should be treated. The end results have been major advances in our understanding of the relationships among taxa; the topology of the tree of life. Questions that have long bedeviled us, such as the phylogenetic relationships among the many kinds of arthropods or the origins of modern bird groups, are far closer to a consensus answer. Birds are a group of theropod dinosaurs; the insects and other hexapods share a common ancestor with a group of crustaceans. Others, such as the relationship of sponges to the other animals, still roil the waters.
Parallel to the unraveling of the historical and hereditary relationships between groups of organisms has been the assigning of names to the clades. Many of these clade names are familiar and of long standing: Mammalia, Primates, Gastropoda. Others are of recent vintage and are not commonly known: Eumaniraptora; Cetartiodactyla; Pancrustacea. The first are the modern birds, ancestral bird-like forms such as Archaeopteryx, and their closest relatives among the theropods, such as the familiar Velociraptor. The second is the group comprised of both Cetacea (whales) and Artiodactyla (even-toed ungulates, such as cows, camels, and deer); it recognizes that the ancestors of whales were artiodactyls. The last is the group that includes crustaceans (e.g., lobsters, shrimp) and insects and other hexapods. There has been a proliferation of such clade names in recent years. Some of this comes from revisions of relationships, but much comes from the seeming insistence that nearly every branch point in the tree of life (nodes), and the corresponding clade must have a name. Most of these names, I suspect, will only be known to specialists in these groups. I must admit some discomfort with this. If every node must be named, then the number of names will soon be a very large subset of the total number of taxa. And given that nodes are still not robust against further change, many clade names may end up abandoned or their meaning changed. And they are a burden to teach. I don’t think I will mention Avebrevicauda in my next introduction to paleontology class. (It should also be noted that the huge number of hierarchical levels in the trees are leading to the gradual abandonment of the classic Linnaean hierarchy of species-genus-family-order…).
There is also the long-standing issue of the difference between informal and formal names for taxa. A formally named clade should include all of the descendants of the common ancestor; if it does not, it is said to be paraphyletic. Dinosaurs without birds is paraphyletic, as would be artiodactyls without whales or crustacea without insects. Therefore, we have Dinosauria, which includes the Aves; Cetartiodactyl; and Pancrustacea. I have no issue with these formal names. What does bother me is the insistence that informal names should also avoid paraphyletic groups — a case in point is the use of the awkward phrase “non-avian dinosaurs” to refer to the classic dinosaurs minus the birds, with the birds being “avian dinosaurs.” As shown by the Google Ngram https://books.google.com/ngrams at the top, the phrase non-avian dinosaurs has become increasingly standard since its introduction by (I believe) Gauthier and Padian. If this is the case, why don’t we say “non-cetacean artiodactyls” or “non-hexapod crustaceans” etc.? I think the bird-dinosaur relationship is well enough understood and accepted that if I say “dinosaur” it is understood that I am talking about the groups that went extinct at the end of the Cretaceous and if I say “bird”, it is understood that I know they are dinosaur descendants. If I am coerced to use “non-avian dinosaurs,” then expect me to force my colleagues to use ‘non-hexapod crustaceans” or “non-arachnid chelicerates.” So there.
