Why does the genus “Varanus” contain all known monitor lizards?

14 08 2009

Good tidings and well-wishes!

When I wrote an article about Varanus priscus (aka: “Megalania”) back in April, Zach made the following comment:

“Seems like every monitor lizard and its mother are within the Varanus genus. In fact, I can’t think of one that’s not. Have there been any large-scale comparisons between Varanids? Are they all really that similar?”

Having never considered that point, I replied that I’d enlist the services of the gang at “Ask A Biologist” to provide an answer. Unfortunately, after a few weeks, I forgot about it (sorry, Zach…). To redeem myself, I’ve posted their comments below.

First, Corwin Sullivan wrote:

“Yes, as far as I know, all living monitors are currently assigned to the genus Varanus. This is indeed an unusual situation, since a few dozen species exist and they vary widely in size and to some extent in body structure. Some taxonomists do recognise “subgenera” within Varanus, such as one called Odatria for the Australian “pygmy monitors”, and it is certainly possible that Varanus will some day be broken up into multiple genera.

The real issue here is that genera, like other ranks in the Linnean system, are entirely arbitrary. Every taxonomist who uses these ranks has some intuitive sense of how broadly a genus, class or family should be defined, but the intuitive senses of any two taxonomists might not agree. This tends to be especially true if they work on different groups of organisms, so there’s no guarantee that a genus of lizards is in any way comparable to a genus of flies or mushrooms. It may look like the genus Varanus includes an impressive range of variation, but all this really means is that the genus has been rather broadly defined. There is no central taxonomic authority to, er, monitor genus definitions and keep them meaningful and consistent.”

Shortly thereafter, Mike Taylor added:

“… and the genus Quercus (the oak tree) contains at lease FOUR HUNDRED species, some of which look so different from what we usually think of as an oak that, from my vertebrate-o-centric perspective, it beggars belief that they all belong to the same genus.  It just goes to show that one man’s genus is another man’s family (or species).

That said, at least within groups, systematists really ought to try to keep the disparity encompassed at each rank similar as far as possible — e.g. one lizard genus should encompass about as much disparity (morphological difference) as another lizard genus, even if we accept at the outset that oak taxonomy is never going to follow the same standard!  And on that basis you do have to wonder whether Varanus is overlumped.  As it happens, I have before me right now the partially prepared (and rather smelly) skull of a mature savannah monitor lizard (Varanus exanthematicus), and it looks DRAMATICALLY different from the skull of the komodo dragon (Varanus komodoensis) shown on the Skulls Unlimited site at http://www.skullsunlimited.com/komodo-dragon-skull.html

BTW., you can look over a nice 3d scan of the V. exanthematicus skull at http://musom.marshall.edu/anatomy/holli … skull.html “

It can therefore hardly be argued that monitors, as a group, are far more diverse and taxonomically interesting than their current classification suggests.

May the fossil record (and the rest of the natural world) continue to enchant us all!

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12 responses

14 08 2009
Zach Miller

Ah, wonderful! I’d forgotten all about this, thanks for bringing it back up! I imagine that, if monitor lizards were extinct and we dug up their skeletons and treated them like most other extinct animals, most of them would be placed in unique genera. 😛

14 08 2009
220mya

I tend to disagree with Mike’s assessment. If you are a phylogenetic systematist (like me), you want your supra-specific taxonomy to reflect phylogeny rather than disparity. So I would say that disparity is not a valid criterion for splitting up Varanus. There are plenty of examples of taxa that are very closely related, diverged very recently (evolutionarily speaking), but are very morphologically disparate. I would argue these should still be placed in the same “genus-level” clade. However, you might find it useful to give sub-clades of Varanus a name so you can talk about them and demonstrate that some species are more closely related to each other than others within Varanus. If we could ever do away with the Linnean binomen, it would be really nice, because you could use Varanus as the clade including all living monitors, and then have separate names for all the subclades, but you would have to pin down exactly which name is a genus – afterall, they’re all nest clades anyways, and genera are abitrary!

14 08 2009
220mya

Sorry – meant to say in the last sentence “you wouldn’t have to pin down exactly which name is a genus.”

16 08 2009
Zach Miller

So Varanus would become the family representing the crown group, and then different species would get new genera? You’d just be raising Varanus one level, keeping species where they are, and coming up with new genera. Seems like you could just do that right now.

And like I said, if modern varanids were fossils, I’d betcha money most of them would get separate genera.

19 08 2009
Neil

Have been wrapping my head around this same question for quite a while–actually both questions A) “is Varanus overlumped?” B) “What, if anything, is a ‘genus’?”

As to the first, it’s worth noting that Lanthanotus is often considered a “monitor” in the most inclusive sense although Lanthanotus and Varanus probably diverged in the Paleogene at latest. So, we might say there are two genera of monitor – one monotypic and the other relatively speciose. That in and of itself is an interesting situation though it sort of begs the question.

As to the second, effed if I know.

I have heard rumors now and then that Varanus might be split…we’ll see I suppose.

19 08 2009
Neil

Also note that there are far more speciose genera of lizard – Anolis, Sceloporus for instance. One wonders what role size plays in the “genus concept” of particular groups….

19 08 2009
Neil

…missing italics tab there obviously.

19 08 2009
Zach Miller

TRIPLE POST FOR THE FAIL. 😀

I didn’t know anoles were overlumped.

23 08 2009
220mya

Zach – first off, I’m not advocating raising Varanus to family level, because I don’t use Linnean ranks – they are artificial etc. etc., so most modern systematists do not use them above the genus. Because I want my taxonomy to be philosophically consistent, I say we should get rid of genus and species too, but that another discussion altogether. Right now, you can’t do what I advocate for Varanus because there is only one available Linnean rank between genus and species: the subgenus. However, we’ve known for a while that there is alot more phylogenetic structure below the “genus-level” within Varanus. There are certainly cases where we would want to name two nodes nested within each other and within Varanus, but the current ranked system doesn’t allow that, because you only have subgenus to work with.

The problem with ranks is people automatically try to make them “equivalent”, which is a frivolous exercise. Exisiting subgenera for Varanus are in no way equivalent (in either diversity or disparity), and nor are they “equivalent” with other lizard subgenera. Lets just dispense with ranks, the linnean binomen, and name nodes where needed. Then we can really appreciate the nested hierarchy of taxonomy and not worry about whether a certain clade is “equivalent” or under-split.

23 08 2009
220mya

Forgot to mention that you couldn’t raise Varanus to the family level even in the Linnean system, because we already have Varanidae, which includes Varanus, Lanthanotus, and a host of extinct species. Thus, you’d basically have to raise up one level every rank in the taxonomic hierarchy of Varanus. This would obviously be ridiculous, and why systematists started inventing things like “sub-class”, “infra-order”, “super-family” etc. Now you can begin to see why Linnean ranks are such a pain and should just be tossed.

3 09 2009
Bill Bartmann

Cool site, love the info.

21 09 2009
Nick Gardner

Wow, I never found this site until today. Just wanted to let you know, the Holliday lab has a new website, and the model of V. exanthematicus (and other lizards) is now here instead:

http://web.missouri.edu/~hollidayca/3DAnatomy/lizard_symphyses.htm

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