McKenna and Bell on ranked categories

I learned mammalian systematics and cladistics around the same time that Malcolm McKenna and Susan Bell published their 1997 book, Classification of Mammals: Above the Species Level. The movie-title placement of the colon in their book title suggests the epic nature of the task they took on.

McKenna began during the 1960s to undertake the task of updating Simpson’s 1945 mammal classification to accord with the rules of cladistics, and published an interim part of the work in 1975. When I learned mammal paleontology, it was with class notes drawn from mimeograph copies of old notes. McKenna’s 1975 classification had a prominent place in this—sometimes as the only available classification for some groups, sometimes as one among other conflicting alternatives. From it I learned the placement of many extinct branches of early mammals, and saw systematics as an important part of understanding the fossil record.

How can systematists generate a classification that makes sense given the phylogenetic arrangement of mammals? Much of the evolutionary diversity of mammals has historically been recognized at the level of Linnaean orders – Primates, Carnivora, Perissodactyla, and so on. It happens that many of these orders have a very similar time depth, because they originated at or shortly after the Cretaceous-Paleogene impact event 66 million years ago. But relationships below the level of these orders are diverse – some lineages diversified enormously with sudden adaptive radiations at different times, others were more conservative. And when extinct mammals come into the picture, the diversity and time depth expected of “orders” and other higher level groups become less clear.

Today, genomic evidence indicates that the order Primates is a sister to the order Dermoptera (colugos). The group including both is known as Primatomorpha. Rodentia and Lagomorpha (rabbits) are likewise sisters, grouped as Glires, and this group appears to be a sister to Scandentia (tree shrews), although possibly Scandentia is closer to Primatomorpha. All these together form a group with the name Euarchontoglires. There remain several levels above Euarchontoglires but below the class Mammalia. Primates themselves have an extinct group of relatives known as Plesiadapiformes—sometimes included as a stem group within Primates, but sometimes included within Primatomorpha as a sister to Primates. Each of these higher-level branches of the mammal tree belongs to a distinct level of the hierarchy.

To deal with this complexity, systematists must multiply levels. But how many levels? Linnaeus could stack as many orders into a class as he liked, because he was not working with a bifurcating tree. A modern cladistic classification involves many, many bifurcations, each successive bifurcation in the tree representing a distinct level in the hierarchy. To get from one class to 40 orders requires at least six bifurcations: five hierarchical levels of classification between the class and order. Five is not enough for mammals, because of all the extinct stem branches represented by the known fossils. Each new fossil discovery of early mammals potentially introduces another level.

Simpson (1945) had included fifteen levels from class to species; McKenna and Bell (1997) increased this to 25 levels—recognizing categories such as “magnorder” and “supercohort” above the order, and “parvorder” and “subtribe” for lower levels.

They addressed the interesting difference between a classification and a tree, by discussing how prefixes relate to the hierarchy. As I re-read this passage from page 18, I thought it worth sharing:

Certain taxonomic categories came to bear prefixes suggestive of special hierarchical linkage (e.g., in the family-group, subfamilies are always subsumed in families). Others did not (e.g., tribes might alternatively have been dubbed "microfamilies" or some such term connoting subordination). use of a prefixed category implies that the category to which the prefix applies is also used. In the Linnaean system we do not cconstruct superfamilies directly from subfamilies without also employing families. Logically, however, above the species level there is nothing special about sub- or supercategories. They all could have received unprefixed cardinal names or simply be referred to as taxa. Such names are, after all, just labels (recognition symbols) (Mayr 1953:391). That prefixed categories did not receive unprefixed cardinal names, free of reference to another rank, seems to us to be partly a matter of practicality and memorability, and partly a function of their authors' essentialistic belief in the objective reality (beyond a construct of human language) and commensurability of various examples of such taxonomic levels as classes, orders, families, and genera (see Slaughter 1982). We employ prefixed names for the sake of stability, because they have been long in use, but we do not hesitate to allocate to an incertae sedis position some taxa whose names happen to be prefixed. For reasons of stability we might not wish to change their rank or to list the lower-ranked contents but not the valid but prefixed monophyletic taxon containing them.

It is a thoughtful observation. A tree is a logical structure that does not care whether humans can recognize and remember its parts. One advantage of a system of classification is that it is built with human memory in mind. The use of categories that bear a hierarchical relationship not only in definition but also in the form of the category names themselves has utility. Once a student learns that a parvorder is below the level of an infraorder, and a mirorder is above the order but below the grandorder, they’re not likely to confuse them.

Yet.

Any set of taxonomic levels faces a problem as soon as any new stem branch emerges between two adjacent levels of the hierarchy. Taxonomists who recognize lots and lots of levels have a buffer against taxonomic changes, because there will be empty levels. But with new discoveries of stem groups, the empty levels may eventually be filled.

We are in that situation with hominins. Hominini is a “tribe”. The group used to be called “Hominidae”, at the family level, but the discovery of the branching order of the apes argued for recognizing the family at a higher level of the tree, so that Hominidae includes great apes and humans, the subfamily Homininae includes African apes and humans, and the tribe Hominini includes only humans and fossil species closer to humans than to chimpanzees and bonobos. But clearly that still does not leave enough levels. The McKenna and Bell classification only provides family, subfamily, and tribe. The branch including chimpanzees, bonobos, and humans lacks a level in this hierarchy. Some scientists advocate recognizing this branch as the tribe Hominini, which would make humans and their fossil relatives a subtribe, Hominina. A different approach would be to introduce more levels: Historically, below the family level, taxonomists have used categories like “infrafamily”,”hypersubfamily” and “supersubfamily”.

None of this would matter very much to everyday use of these groups, if the names of them were not connected to the level. McKenna and Bell discuss this as well. What I didn’t realize is that the use of level-specific suffixes was itself a post-Linnaean innovation with the laudable aim of making levels more consistent:

With the proliferation of ranked categories that had increased steadily from Linnaeus's original six, came also a perceived need to encode the names of taxa themselves as signifying that the taxa for which they stood were members of some particular rank. In each particular discipline, the names of family-group taxa came to have various standardized inflected suffixes linked to the perceived rank. Thus, in zoology, a name ending in "-idae", signifies a taxon at family rank. Latreille (1796), who introduced the family category to zoology, did not use the suffix "-idae". That modification was provided later by Kirby (1815), and has not only stuck but is now legislated by the ICZN. We think of these inflective conventions as part of the "Linnaean System" but, in fact, they are arbitrary post-Linnaean additions to it, originally added for the mnemonic usefulness but now the occasion of much pedantic drudgery whenever taxonomic rank is changed or organisms are transferred from one kingdom to another.

There has been plenty of pedantic drudgery associated with changing hominin taxonomy, and that’s not counting the many holdouts.

Anyway, what if our taxonomies routinely use more and more levels? Doesn’t that get hard to keep track of? It’s fascinating to me that McKenna and Bell defend their 1997 classification of 25 levels by noting that “it’s no harder to learn than the alphabet”. But this passage really made my jaw drop:

In the present classification of more than 5000 mammalian taxa that are assigned generic or subgeneric rank, additional categories have proven useful in depicting in words a somewhat richer hierarchical arrangement of mammals than that found in Simpson's (1945) classification. There are now many more mammalian taxa to classify than was the case in 1945, both in real terms and because of the efforts of "splitters" and paleontological "apparent lineage choppers". Increasingly, most of these names organisms are made known from fossil materials only, sometimes very poorly represented. Moreover, the cladistic revolution in systematics has resulted in far more attention to phylogeny than was the case in the 1940s. The 25 taxonomic levels used in our classification actually fall closer to the theoretical minimum, 13 (see below for formula) than to the thousands that would be required if the classification reflected a completely pectinate (and very unlikely) sequence of taxa. The hierarchical level sequence is no more difficult (for humans) to learn than the alphabet, or probably less so in that some of the levels are very easy to remember because of meaningful prefixes and suffixes. We see no particular reason why, if useful, additional categories (or simply unranked taxa) should not be proposed (or revived). Computers can remember them for us. Indeed, in the program Unitaxon (TM) used to process the data resulting from this classification, facilities exist to expand and keep track of the names, number and sequence of taxonomic levels indefinitely, if deemed appropriate.

Ha! We don’t need to remember taxonomic categories because the computers can remember them for us!

If you’re interested in outsourcing your taxonomic knowledge to a computer, you can still see the Web 1.0 page for Unitaxon, listed as a “software product from yesteryear”. Here’s an excerpt:

Unitaxon Browser 2.0 is available directly from its developer, Mathemaesthetics, Inc. The application is distributed on CD for both Macintosh (System 8 and 9) and Windows (95 or later) operating systems. The Browser will work in Classic compatibity mode under Mac OS X. For maximum performance reasons, the Browser reads the entire classification into memory when you open the file. Depending on the level of taxon commenting in the database, the overhead is currently about 1MB RAM per 1200 taxa on average.
For instance, the most recent classification of the Mammals has been placed on the net in Unitaxon Browser format. It is our expectation and hope that other large taxonomic databases will follow suit.
The price per copy for the Browser is US $128, plus shipping/handling.

Well, that’s one solution.

The changes in 20 years have enormous. Even the link in the Unitaxon website to “vertebrate paleontologists” at the AMNH no longer connects to vertebrate paleontologists — the AMNH site now redirects the link to its “Center for Biodiversity and Conservation”. Malcolm McKenna passed away in 2008.

Anyway, the McKenna and Bell introduction has a lot of really interesting and useful thoughts about taxonomy and classification. The volume was published at the height of cladistic morphological classification, just as DNA evidence was starting to become a potent source of information about the deep relationships of mammal groups. As such, the McKenna-Bell classification has become outmoded in many details, even if some of the guiding concepts behind their taxonomy remain valuable.