Enspel Formation Fossil

The following is from Jesson (2020):

The volcanogenic Lake Enspel is an oil shale deposit with abundant and diverse fossil insects. According to Wedmann (2000), there is a predominance of Coleoptera (53%), followed by Diptera (24%) and Hymenoptera (12%). Six percent of the fossils belong to Trichoptera, mainly represented as larvae (n = 4222). Among the Hymenoptera, ants make about 50%. Wedmann et al. (2010) give a preliminary study on ant fossils from Enspel. In this study, subfamilies were identified but not quantified. A high proportion of Myrmicinae was found, mainly belonging to the morphogenus Paraphaenogaster (personal communication G. Dlussky, cited in Wedmann et al. 2010).

In Enspel, the quality of preservation of ants is often exceptional, as shown by chitin preserved in its primary organic molecular structure (Stankievics et al. 1997; Colleary et al. 2015) and by original colours of insects (McNamara 2013). While color can be preserved it may be inconsistent within taxa, i.e., is not taxonomically relevant. Deviations in the preservation of structural colours from sample to sample within the same fossil location were documented by McNamara et al. (2012).

The sculpture of the sclerites often is well preserved, as are sometimes fragile parts, like bristles and tibial spurs. Normally in papers dealing with fossil ants, the terms “imprint fossil” or “impression fossil” are used. For ants from Enspel, these terms are not appropriate. It has been shown by Stankiewicz et al. (1997) that the original chitin molecular structure has been preserved in insect body fossils from Enspel. In addition, for fossil weevils from Enspel Gupta et al. (2007) showed that the aliphatic polymer in the insect fossils did not derive from migration from the organic-rich host sediment. Therefore, the terms insect body fossil or compression fossil are more appropriate.

Posteriorly to the pars stridens of the stridulation organ, there are integumentary foldings on the presclerite of the first gastral tergite. These foldings are preserved in some specimens described here. They are most likely functionally linked to the stridulation organ. When the gaster moves back and forth, the male produces a sound. Studies on extant species show that the sound differs between the species (Castro et al. 2015). Àlvarez (2009) named these structures “pillars”. The term “pillars” will also be used here, although “furrows and ridges” would often describe their morphology better. Most likely they evolved from the girdling constriction respectively “cinctus3” (see Serna and Mackay 2010).