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. 2011 Dec 22;278(1725):3679-86.
doi: 10.1098/rspb.2011.0729. Epub 2011 May 4.

Intercontinental dispersal of giant thermophilic ants across the Arctic during early Eocene hyperthermals

S Bruce Archibald  1 , Kirk R Johnson, Rolf W Mathewes, David R Greenwood
Affiliations

Intercontinental dispersal of giant thermophilic ants across the Arctic during early Eocene hyperthermals

S Bruce Archibald et al. Proc Biol Sci. .

Abstract

Early Eocene land bridges allowed numerous plant and animal species to cross between Europe and North America via the Arctic. While many species suited to prevailing cool Arctic climates would have been able to cross throughout much of this period, others would have found dispersal opportunities only during limited intervals when their requirements for higher temperatures were met. Here, we present Titanomyrma lubei gen. et sp. nov. from Wyoming, USA, a new giant (greater than 5 cm long) formiciine ant from the early Eocene (approx. 49.5 Ma) Green River Formation. We show that the extinct ant subfamily Formiciinae is only known from localities with an estimated mean annual temperature of about 20°C or greater, consistent with the tropical ranges of almost all of the largest living ant species. This is, to our knowledge, the first known formiciine of gigantic size in the Western Hemisphere and the first reported cross-Arctic dispersal by a thermophilic insect group. This implies intercontinental migration during one or more brief high-temperature episodes (hyperthermals) sometime between the latest Palaeocene establishment of intercontinental land connections and the presence of giant formiciines in Europe and North America by the early middle Eocene.

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Figures

Figure 1.
Figure 1.
Reconstructed early Eocene northern continental positions and shorelines in polar view with Formiciinae fossil localities (G, Germany; B, Britain; W, Wyoming; T, Tennessee), and dispersal routes across the Arctic indicated by red arrows.
Figure 2.
Figure 2.
Holotype of T. lubei gen. et sp. nov.: (a) photograph and (b) drawing.
Figure 3.
Figure 3.
(a–c) Comparative bodies (wings not drawn) of (a) T. lubei, (b) T. simillimum, (c) T. giganteum and (d) T. lubei holotype with rufous hummingbird (Selasphorus rufus) for size comparison; (b,c) redrawn from Lutz [20]. P, petiole; s, spiracle; A3–7, abdominal segments three through to seven (=gaster segments I–V).
Figure 4.
Figure 4.
Climates and distributions of formiciine ants and modern ants with any caste over 3 cm in length. (a) Mean annual temperature (MAT) values for the largest living ants (box plots: minimum, 25%, median, 75% and maximum values), the Eocene localities where formiciine ants have been recovered and Green River Formation localities comparable with the Farson Fish Beds where T. lubei was found (references for fossil site MAT values as in table 1); (b) ranges of modern ant species with any caste over 3 cm in length (locality references in the electronic supplementary material), equator, tropics of Capricorn and Cancer indicated: South America, Dinoponera spp.; Africa, D. wilverthi; Southeast Asia, C. gigas; eastern Australia, M. brevinoda (in part, see text).
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