Dilophus, Krantzii L. Heyden, 1870

Skartveit, John & Wedmann, Sonja, 2021, A Revision of fossil Bibionidae (Insecta: Diptera) from the Oligocene of Germany, Zootaxa 4909 (1), pp. 1-77 : 55-75

Dilophus sp. 2

Localities: Kleinkems

Material examined: (2 impressions) NMB F 2110, F 2246

Two relatively poorly preserved impressions from Kleinkems. The two impressions are part and counterpart of the same male specimen. These specimens were not further discussed by Skartveit & Nel (2017).

Description:

Total length 5.5 mm

Wings not preserved. Thorax, front and hind legs and abdomen reasonably well preserved. The specimen appears yellowish-brown.

Head: Outline vaguely preserved, length 1.06 mm. Relatively large, ocellar triangle small and not prominent.

Thorax: Length 2.0 mm, brownish-black. Thoracic spine rows not well preserved, apparently of very short spines. Haltere yellowish-brown.

Legs: Yellowish-brown. Protibia robust, mesal spines apparently small and not prominent, apical spine circlet with strong, short and blunt spines, not possible to count. Hind leg relatively short and slender, length of hind tibia 1.2 mm. All tarsi slender.

Wing: not preserved.

Abdomen: Length 3.0 mm, yellowish-brown, conical.

Terminalia: not well preserved. Gonostylus apparently square-tipped.

For all the following figures, scale bar = 1 mm.

General discussion

The present work is an attempt to revise the species mainly from Rott and Enspel, two abundant fossil faunas from Germany, fairly close to each other in time and space. Though much of the material is in an excellent state of preservation, it is a problem that we have not been able to locate a number of the type specimens, several of which may have been lost or destroyed during World War II. Most of the specimens available are females, and most of the previously named species were based on female specimens - for the species described by Statz (1943), consistently so. In modern Diptera systematics, the male terminalia are very important for the classification. In our view, this set of characters should be utilised as much as possible even in fossil species, and, accordingly, new taxa should preferrably be based on male specimens, where available. Associating male and female specimens in sexually dimorphic species is sometimes challenging even in modern insect faunas, and much more so with fossils, though specimens are occasionally fossilised in copula. We have chosen to place male and female specimens in the same species where we consider that the likelihood that they represent the same species is high. These associations are, and are likely to remain, tentative. This approach will lead to conservative estimates of the number of species in a fossil fauna. Most 19th Century (e.g, Heer 1849, 1856, Heyden 1859, Heyden & Heyden 1865) workers named any distinctive morphotype, male or female, as a separate species, this obviously has led to highly inflated species numbers ( Skartveit & Nel 2017). Compared to studies of recent species, paleontological studies have to make do with a limited amount of information – many important morphological characters are not possible to see, and molecular characters are generally not available. In this situation we believe the burden of the evidence should be on decisions to split taxa, that is, that two specimens should be assigned to the same taxon unless there is good evidence for regarding them as different. If no such evidence is found for two previously named taxa, they should be synonymised. Though this approach will probably lead to an under-estimate of the number of biological species in a fossil fauna (this is a lesser concern since presumably only a fraction of those species which once existed have been preserved as fossils anyway), it has the crucial advantage that the recognised taxa will be identifiable by available, objective traits.

Sensu Statz vs. Original descriptions – how large are the discrepancies?

It is a general problem with European fossil bibionids, that most authors have not studied the type material of previous publications before proceeding to describe new species. This has led to a lot of duplication and synonymy, severely so with the material from France ( Skartveit & Nel 2017). In the work of Statz (1943), the most important treatise of the fossil fauna of Rott, species described by previous authors (mainly Heyden & Heyden) were included, but their inclusion was to a large extent based on the descriptions of the species and not on the type material. Much of the relevant type material is in the Natural History Museum, London, and obviously was not available to Statz during World War II. This has lead to numerous discrepancies, in that the identity of a species sensu Statz is not the same as in the original description. This is particularly unfortunate since Statz’ descriptions are generally quite good and well illustrated, and could be a good foundation for future work if the naming was correct. On the other hand, the descriptions of Meunier (1915, 1917, 1920) are lacking in most respects, they are inadequate and sometimes outright wrong (see Handlirsch 1906 for a general critique of Meunier’s work), and the type material we have found from these publications is not unambiguously labeled, making their inclusion difficult. Fortunately, most of Meunier’s descriptions of German Oligocene bibionids are backed by good photographs of the type specimens.

Comparison of species diversity of Rott and Enspel and implications for palaeoclimate

This revision documents 39 species from Rott, while only four species are recorded from Enspel ( Tab. 1). This clearly shows that the diversity of bibionids in Rott was much higher than in Enspel, or in any other Oligocene site from Europe. We can only speculate on the reasons for this. Most of the richest localities including Rott and Enspel are Lagerstätten, into which the specimens are believed to have been transported. Perhaps the striking differences in diversity is linked to Rott having had a wider range of environments in the surroundings than Enspel? Another possible reason could be a change in climate just before or during the deposition of the Rott specimens. Although the deposits were laid down during a limited stretch of time, bibionid species appear to be able to respond fast to climate change, since several relatively thermophilic species are currently spreading rapidly in Norway due to a warming climate (JS, personal observations).

It is notable that there are apparently no shared species between the localities of Enspel and Rott except the uncommon Plecia dubia , although the sites are quite close to each other and believed to be fairly similar in age. However, two of the species described by Germar (1837) from the Rott formation appear to occur at Enspel. In connection with this it is notable that the rock surrounding the Germar (1837) specimens (quite dark) looks quite different from the very pale rocks surrounding most of the specimens in the Heyden & Heyden and Statz Rott materials. This may suggest that the material of Germar (1837) may have come from a different outcrop, probably one that was deposited under similar conditions to Enspel. Collomb et al. (2008) suggested that the ratio between specimens of the mainly tropical genus Plecia and the temperate genera Penthetria and Bibio may be a useful indicator of the local climate at the site. For Rott, 19 species of Plecia are recorded, while there are 16 species of Penthetria and Bibio documented. For Enspel, there is 1 species of Plecia documented, and 3 species of Penthetria and Bibio together. By this measure, the climate of Rott (c. 24 mya) should be considerably warmer than that of Enspel (c. 25 mya), suggesting a substantial warming in the intervening period. A possible alternative explanation of higher temperatures in Rott could be due to the fact that Enspel was situated topographically higher than Rott ( Wedmann 2000: p 131) and therefore had a colder climate.

Species shared with other localities

Not surprisingly, the fauna of Kleinkems apparently shares numerous species with French Oligocene localities. Skartveit & Nel (2017) found six such shared species. The fossils from Rott are considered to be almost exactly the same age as the French locality of Aix-en-Provence, and the two species Plecia pinguis (treated as Plecia lapidaria by Skartveit & Nel 2017) and Plecia rhenana are apparently shared between the two sites ( Skartveit & Nel, 2017). Though the two localities are currently separated by a distance of c. 700 km and have substantially different climates, it is not implausible that species would occur in both localities – in the present fauna, several bibionid species (e.g., Bibio marci , Bibio hortulanus , Dilophus febrilis ) are likely to live in both areas.

The distinctive species Bibio giganteus is apparently shared between Rott and the Miocene locality Radoboj in Croatia, and the equally distinctive Plecia pinguis between the localities of Rott and Kleinkems. The dating of Radoboj is somewhat in doubt ( Skartveit & Krizmanić 2020), but according to the most commonly held view it is> 10 MYr younger than Rott, though it could also be considerably older than this date, whereas Kleinkems is believed to be 4–9 MYr older than Rott. The longevity of fossil bibionid morphospecies appears to be considerable: several species are apparently shared between the French Middle Oligocene site of Céreste and the late Oligocene site of Aix-en-Provence ( Skartveit & Nel 2017) and a mid-Miocene specimen from Bosnia-Hercegovina is indistinguishable from a recent species ( Wedmann & Skartveit 2020). On the other hand, no species are shared between Rott and the German Miocene locality of Öhningen ( Skartveit & Pika 2014), nor have any of the species from the Oligocene material been found in the Eocene Baltic amber ( Skartveit 2009) or from the Eocene site of Grube Messel ( Skartveit & Wedmann 2016).