An ant, 30 million years too early

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Chronomyrmex1
Chronomyrmex, a new dolichoderine ant from the Cretaceous. Image modified from McKellar et al 2013, Figure 2.

Fossils tell a mostly consistent story about the evolution of life on our planet. But every now and again a puzzling new animal turns up that doesn’t quite fit. Such is the aptly-named Chronomyrmex, a 78 million year old dolichoderine from Alberta.

To my eye, Chronomyrmex doesn’t look much like any modern dolichoderine. This shouldn’t be surprising, since its age dates the fossil 10 million years before previous estimates for the radiation of extant members of the subfamily. Yet, this new ant sports jaws that are nearly identical to present-day Linepithema, a young genus from the neotropics. Both genera bear the same medially-indented clypeus and the same pattern of alternating large teeth with small denticles on the mandibles. Look:

Chronomyrmex2
Chronomyrmex (left) and Linepithema (right), showing similar teeth.

The paper’s authors interpret the morphology as placing this ant within the young dolichoderine tribe Leptomyrmicini, the group that contains Linepithema and a number of other genera. This is a radical finding. If Chronomyrmex is truly a leptomyrmicine, Dolichoderinae should be much older than surmised, as should ants as a whole. It’s hard to maintain ants as being 120 million years old when specimens a mere 40 million years on are far along in a modern lineage.

But I’m not so sure this isn’t a case of convergence.

Ant mandibles are a highly functional part of the insect and are especially prone to convergence under similar ecological conditions. Trap-jaw ants, for example, emerge separately from many lineages in the ant tree. It’s possible this ancient insect was similar enough in biology to modern Linepithema that both groups arrived at solution so similar as to be indistinguishable in amber.

All the same, this one is a head-scratcher.


source: McKellar RC, Glasier JRN, Engel MS. 2013. New ants (Hymenoptera: Formicidae: Dolichoderinae) from Canadian Late Cretaceous amber. Bulletin of Geosciences 88: 583-594.

 

12 thoughts on “An ant, 30 million years too early”

  1. Interesting post Alex. On a side note, what are the ins and outs of using images from scientific articles? Is it OK to use them as long as you give credit? Or did you have to personally contact the authors of the paper?

    1. Hi Tom. Good question. In my interpretation of copyright law, my reproduction of these images is editorial fair use. The point of posting these excerpts is to comment on the paper itself. Had I posted these images to illustrate a piece not related to the paper, I’d clearly be in the wrong. I can use them, but only in this very narrow context.

      More on fair use: http://www.law.cornell.edu/uscode/text/17/107

  2. Why shouldn’t the evolution of ants be any different from any other taxa with respect to ‘uneveness’ of morphological change ?

    There are tons of animal groups with histories of supposed ‘very fast’ morphological change followed by long periods of little or no visible change.

    Millions of years are incomprehensible to humans and insect species can make large changes in 25 years or less. The written history of humans is less than 3 -5,000 years in comparison and there are plenty of head-scratchers from only that long ago.

  3. The similarities are striking. Now I am no expert on ants, but I do see lots of small differences between the mandibles of the two species. I suppose those are e a demonstration that convergent evolution will not make a perfect match.

  4. Rather than focusing exclusively on the mandibles, what about the other anatomical features, such as the antennae? Surely ant genera aren’t defined solely by their dentition?

  5. I agree with Alex (which always makes me scratch my head). There is far too much convergence in evolution and that is especially true when filtered through our interpretations of what we are seeing. Alternating runs of big and small teeth probably does have a functional aspect to it – I know of similar patterns in the chelicerae of predatory mites. Speaking of mites, that Grassy Lake amber is fantastic: a recent cover of the International Journal of Acarology has one of Ryan’s mites on it that pushes back the first known occurrence of the family 30 million years.

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