Temporal range: Lutetian, Middle Eocene Arkansas amber, Malvern, Arkansas, United States
Protrechina carpenteri is known from a single specimen in mid-Eocene Arkansas amber. Unfortunately, the holotype specimen deposited in the MCZC has deteriorated badly and the amber has become so clouded that it is impossible to see the specimen except for a very faint outline of the body shape. Based on Wilson’s (1985) description, the feature used to diagnose this genus from other formicine genera was the lack of a circlet of setae on the acidopore. This lack of setae could have been due to abrasion and needs to be reconfirmed with additional specimens because all other extant and fossil species from the Prenolepis genus-group possess setae around the acidopore. Hung and Brown (1966) did report that many species of the Camponotini have either reduced or no setae around the acidopore, but that clade is distantly related to the Prenolepis genus-group.
Wilson separated Protrechina from Paratrechina (at that time broadly defined to include what are now three distinct genera: Nylanderia, Paratrechina and Paraparatrechina) by the lack of erect mesosomal setae, but based on the short description and illustration provided in the original paper, Protrechina carpenteri could certainly be a Nylanderia species. The overall shape of the species and measured characters (taken from Wilson’s original description: head width [0.23mm], scape length [0.48 mm] and pronotal width [0.23 mm]) all fit with a generalized Nylanderia morphology. The lack of erect mesosomal setae is problematic as all known Nylanderia (extant and fossil species) have erect mesosomal setae. In fact all extant Prenolepis genus-group species have at least a few erect mesosomal setae, although they may be fairly short in some species. However, the setae could have been abraded during capture in the resin or during subsequent fossilization. Wilson was unable to count teeth on the mandible, which would have been very helpful for understanding this genus (the vast majority of Nylanderia species possess 6 teeth) because mandibular tooth count is an important morphological feature in diagnosing Prenolepis genus-group genera (LaPolla et al. 2010).
Because this species cannot be reexamined at the present time it remains difficult to make any decisions regarding the validity of Protrechina as a genus. Our inclination is that, based on what we discussed above regarding the observed morphological features, Protrechina is really a Nylanderia species. In this study we now have a definitive Nylanderia species from Baltic amber, so Nylanderia species could have certainly been present when the resins that would eventually become Arkansas amber were produced. For now we maintain Protrechina as valid genus within the Prenolepis genus-group, but its status remains uncertain until further specimens can be examined. [LaPolla & Dlussky, 2010.]
This taxon was described from Arkansas amber, Malvern, Arkansas, United States (Lutetian, Middle Eocene).
The following information is derived from Barry Bolton's New General Catalogue, a catalogue of the world's ants.
- †carpenteri. †Protrechina carpenteri Wilson, 1985d: 213, fig. 6 (w.) U.S.A. (Eocene). See also: LaPolla & Dlussky, 2010: 269.
- Holotype, worker, Arkansas amber, Malvern, Arkansas (middle Eocene), United States, Museum of Comparative Zoology.
- LaPolla, J.S. & Dlussky, G.M. 2010. Review of fossil Prenolepis genus-group species. Proceedings of the Entomological Society of Washington 112: 258-273.
- Wilson, E. O. 1985f. Ants from the Cretaceous and Eocene amber of North America. Psyche (Camb.) 92: 205-216 (page 213, fig. 6 worker described)