This post is about taxonomy and “The Genus Question.” If you do not want to read, I suggest not going below the fold. If you wish to continue, you will be rewarded with a pretty picture.
The Paleobiology Database, is a taxonomic database started by and largely run by the Australian Research Council, and contains values such as changes in taxonomy, citing authors, locality information, geologic settings, and so forth, a broad range of data that is useful to the paleobiologist. Anne Lagomarcino of the University of Cincinnati’s Dept. of Geology joined with Arnold Miller combed this database for Cenomanian through Maastrichtian marine taxa.
They localized these taxa for informativeness and for region, choosing to focus on three: the Gulf Coast, the North American Cretaceous Seaway, and the European Epicontinental Sea, and used maps of these regions for each of four segments of time (Cenomanian, a combined Turonian-Coniancian-Santonian bin, Campanian, and Maastrichtian); while the Gulf Coast represented open-ocean, the other two represented epicontinental seas, and thus are influenced by pelagic versus near-shore or shallow-water taxa. Using a variety of metrics including the power law function (S = cAz), the authors computed their taxa against area and regressed concentrations for each area with each other.
Some outliers exist: the Turo-Coni-Santonian taxon sampling for area is particularly low on the Gulf Coast, while for the Maastrictian this diversity is low in the NA Cretaceous Seaway (likely due to the near absence of the seaway post-Campanian); the Tur-Coni-Santonian samples for all regions display relatively high slopes, indicative of high diversity in small and broad areas, but not moderate areas, while these slopes suggest rather that during other periods slopes are generally consistent.
Some of this is over my head, so some should pardon me for not covering all the details. The paper is free (as from PLoS One, it is open-access), and I encourage the interested to check it out for themselves. My concern is something that readers familiar with my blog will anticipate. In their abstract, Lagomarcino and Miller write:
Recent studies indicate that spatial distributions of taxa and the kinetics of taxon origination and extinction may have differed in these two settings. Against this backdrop, we analyze regional Genus-Area Relationships (GARs) of Late Cretaceous marine invertebrates in epicontinental sea and open-ocean settings using data from the Paleobiology Database.
This is enough to generally pique my interest, as I have been looking forward to finding whether there are useful metrics for evaluating the quality, if any, of a genus. The authors continue in their introduction:
While most present-day studies of the relationship between area and diversity have been conducted at the species level, one could just as easily look at these relationships at higher taxonomic levels, and, because of concerns about data quality, all analyses in this paper were conducted at the genus level, which has been the level of choice for a broad suite of investigations of ancient marine diversity conducted over the past two decades.
So the challenge is on: Is there a difference between genus-level versus species-level diversity over time in these three regions? What might this question resolve?
First, it should be noted that even given narrow qualifications of the term “genus,” “genera” or “genera-equivalent” nomenclature generally encapsulate more than one species in extant organisms, it is considered a sort of metataxon, qualifying a form of diversity by which scientists can measure “richness,” “likelihood of speciation,” etc. Yet “genera” do not exist, in at least the same way that species do. What then is meant when scientists use “genera” in the stead of species? For the most part, they assume that “genera” represent a form of actual genetic diversity, which can be quanitified (despite no one having done so, in at least a phenetic manner; and the same, generally, cannot be said of species, due to the plethora of species definitions, for none are forthcoming for “genera” that themselves do not depend on species). Hence, “genus” more resembles a metataxon, and not an actual taxon, and the same is true for all other supra-specific Linnaean ranks.
When Lagomarcino & Miller note that their option of taxon choice was the “genus,” it is because it is easier to compare this to previous studies; however, this ignores the fact that, unlike species, “genera” can shift by containing fewer or more species, and thus species remain constant while moving in and out of particular containing clades. The question similarly becomes moot if, say, we were to simply collapse all “genera” to their type species, and thus expand all fossil species to have a “genus” and species (we’d just call this a binomen: a species name preceded by a praenomen). This solves some tricky issues, but of course it brings up new ones, especially given that current ecological and conservation studies depend on the broader categories, rather than the specific, in order to define what is largely an irrelevant distinction: It is easier to conserve a taxon if it seems rarer, by further refining the taxonomy to restrict the possible population, or alternatively to subsume diversity into large categories and define “genera” with lone species to encapsulate how rare the breadth of lineage being considered really is.
Let me just say, that while I applaud conservation and preservation of modern diversity as far as man’s interference goes, playing political gamesmanship by alternately expanding or narrowing the nomenclature of these animals does nothing for scientific analysis by itself and should not be considered when assessing the genetic diversity of living or extinct organisms. Taxic diversity doesn’t necessarily equate to genetic diversity, nor the inverse.
The authors do not consider the species-level when analyzing their datasets, nor then do they consider relative naming conventions have on genetic diversity being approximated. Instead, they (and most other analyses of fossil organisms before them, as cited in the paper) use “genus” as a proxy for this concept, and assume it is effective in conveying the broader concept of diversity. I, respectfully, disagree. The question, of relative species and “genus” diversity, never comes up, and thus the question asked of the authors is instead mapping relative open-ocean and epicontinental sea diversity by generic defintions of their taxonomic proxies. I cannot help but think that this paper, well written and relevant in many ways to assessing diversity, fudges the quality and relevance of the work by not using the database at hand and use species instead.
Lagomarcino, A. J. & Miller, A. I. 2012. The relationship between genus richness and geographic area in Late Cretaceous marine biotas: Epicontinental sea versus open-ocean-facing settings. PLoS One 7(8):e40472.
As promised, a pretty picture: