Crematogaster aurora

Distribution
This taxon was described from the Coal Creek Member, Kishenehn Formation shale, northwestern Montana, USA.

Nomenclature

 * † aurora. †Crematogaster aurora LaPolla & Greenwlalt, 2015: 169, fig. 9 (q.) USA (Eocene).

Taxonomic Notes
Radchenko & Dlussky (2019) - The taxonomic position of C. aurora is unclear. LaPolla and Greenwalt (2015, p. 169) gave as the diagnosis of this species “Presence of a 2-segmented club on antennae, and petiole with distinct subpetiolar process directed anteriorly”. Neither of these features actually “diagnose” Crematogaster. Despite the general structure of the petiole of C. aurora being similar to modern Crematogaster (it is without a distinct node), contrary to modern species it is quite short, not so strongly flattened, and possess well developed, quite wide ventral process. Furthermore, neither the original description of C. aurora nor the high quality photograph of the holotype specimen, indicates the presence of the core diagnostic feature of Crematogaster: a postpetiole that articulates on the dorsal surface of the first gastral segment (opposed to the “normal” myrmicine condition articulating on the anterior surface of the first gastral segment).

This unique structure of the waist in all modern Crematogaster species is believed to be closely connected with defense and hunting behaviors. The sting in Crematogaster species is well developed, but with a blunt, spatulate tip that is unsuitable for pricking (Buren 1959, Kugler 1978), and the venom is applied topically by wiping on a victim instead of injecting it inside the body (Buren 1959, Longino 1993). By holding its gaster raised up and even somewhat forward, over its head, a worker can attack prey or enemies in a 360° radius.

While many of the visible characteristic features of the imprints of the specimens (queens) that were described by the authors are found in genus Crematogaster, they are also common for many other myrmicine genera, e.g. “Head longer than wide; posterior mar gin apparently straight; posterolateral corners broadly rounded; scapes short, not surpassing posterior margin by length of at least 2 funicular segments; … declivity [of propodeum] steep, with two small spines directed slightly entad … petiole without node, longer than postpetiole; petiole with distinct subpetiolar process directed anteriorly; postpetiole short, without node, with smaller subpetiolar process than on petiole; postpetiole narrowly attached to gaster“ (LaPolla and Greenwalt (2015, p. 169).

We can only speculate that big (about 10 mm long) and robust queens of C. aurora might belong to a new fossil genus that could represent the extinct ancestral lineage of the genus Crematogaster or even the whole tribe Crematogastrini (sensu Bolton, 2003).