Enspel Formation Fossil

The following is from Jesson (2020):

The volcanogenic Lake Enspel is an oil shale deposit with abundant and diverse fossil insects.

Lake Enspel was surrounded by a relatively high crater rim (Pirrung 1998). During the duration of sedimentation, the rim was episodically eroded, deposited in part temporarily on alluvial fans, and then episodically resedimented on the lake floor. Gullies formed on the subaerial part of the steep inner crater rim, and mudflows occurred in the local gullies (Schindler and Wuttke 2015). The walls and the ground of the gullies were lightly vegetated, displayed by subaerially dessicated owl pellets that were washed episodically into the lake by flowing water (Smith and Wuttke 2015).

The late Oligocene was characterised by pronounced climatic changes that were accompanied by vast vegetational changes. The mean annual temperature of this terrestrial environment was estimated at about 15–17 °C, the warmest month about 25 °C, the coldest one about 5–7 °C and the mean annual precipitation was between 900 and 1355 mm/year (Uhl and Herrmann 2010). Compared to the “World Map of the Köppen-Geiger Climate Classification”, temperature in Enspel was similar to the contemporary Mediterranean, but humidity was significantly higher.

The vegetation was dominated by zonal assemblages of a mesophytic forest with a strong East Asian influence. Based on the fossil macroflora found in Enspel, four communities of plants have been described by Köhler and Uhl (2014): water plants, azonal riparian vegetation (of the lake shore), azonal riparian forest, zonal mesophytic forest/mixed mesophytic forest. The forest consisted of four storeys and reached the lake in non-disturbed parts of the crater rim that was characterised by mostly steep margins. Azonal elements (aquatic and semi-aquatic plants, riparian forest) are only represented based on approximately 15 % of the taphoflora (Köhler and Uhl 2014). Banks of the supposed gullies or streamlets feeding into the lake or shallow marginal areas can also be assumed as the potential growth areas of these plants (Schindler and Wuttke 2015).

Lake Enspel Fossil Ants
According to Wedmann (2000), the fossil insects are predominately 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. 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).

Between 1995 and 2013, 354 ant fossils were found in Enspel [during yearly excavation campaigns between 1995 and 2013 conducted by the Directorate General for Cultural Heritage Rhineland-Palatinate, Directorate Archaeology, Department of Earth History]. For 287 of these, the subfamily could be identified (for 42 specimens the subfamily was indeterminable, and in addition there are 25 isolated wings). At present, fossil ants from the formicoid clade (Formicinae, Dolichoderinae, Myrmicinae) comprise 96.5% of the total. Myrmicinae make up 36.6% (n = 105) of the specimens, mainly belonging to the morphogenus Paraphaenogaster (personal communication G. Dlussky, cited in Wedmann et al. 2010). Formicinae and Dolichoderinae combined constitute 59.9% (n = 172). The distinction of the latter two subfamilies is difficult; 80 fossils could be identified with quite high probability as Formicinae, only 4 as Dolichoderinae, and the remainder were not clearly assignable to either of the two subfamilies. Specimens belonging to the poneroid clade are represented by 3.5% (n = 10). Ponerinae make about 2.1% (n = 6), Agroecomyrmecinae 1.4% (n = 4). By far the majority of the specimens are males and alate females, and the number of workers is low: n = 19. This is equivalent to 6.1%, when 25 isolated ant wings are included (n = 312 in total). Fourteen of the 25 isolated wings show the typical “Paraphaenogaster wing venation pattern”.

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).