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:
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.