A full description is given of the first scarabaeid beetle documented from the Mesozoic (Late Cretaceous) of Africa (Orapa, Botswana). Brief and partial descriptions of this beetle have appeared in two previous review papers of fossils from the Orapa Diamond Mine. This work represents an updated analysis and interpretation of the fossil based on character comparison and phylogenetic analysis. The new genus and species, Ceafornotensis archratiras, are described in Melolonthinaeincertae sedis. Phylogenetic analysis suggests a sister group relationship with the tribe Phaenognathini (Melolonthinaeincertae sedis). Although phytophagy in the adult beetle cannot be confirmed, it is proposed that angiosperms could have contributed to a proportion of its diet (larval or adult). This beetle may belong to a stem lineage of pleurostict scarabaeids which later experienced major radiation in response to widespread environmental changes brought about by continuing radiation of the angiosperms.

Scarabaeoidea , scarab, palaeoenvironment, Turonian, Late Cretaceous, Mesozoic, Botswana, Orapa, diamond mine, crater lake

Compression fossils of insects from the Orapa Diamond Mine (Central District, Botswana, 21°18.465'S; 25°22.177'E) are known for their excellent preservation (Rayner 1993). The relative scarcity of Cretaceous deposits in the Southern Hemisphere, lends Orapa particular importance amongst the known lagerstätte (Rasnitsyn and Brothers 2007). The deposit’s age, estimated at 91 mya (Turonian age - 89.0 ± 0.5 to 93.5 ± 0.2 mya), (Brothers and Rasnitsyn 2003; Davis 1977; Haggerty et al. 1983), is also significant with respect to the angiosperms and their contemporaneous radiation (Rayner et al. 1997).

The insects’ burial and preservation is thought to have come about through the infilling of a crater lake by mass flows of epiclastic material from the crater’s sides (Rayner et al. 1991). That a large assemblage of aquatic insects is not found in the deposits has led to the suggestion that volcanic leachates produced inimical conditions for life in the lake’s waters (McKay and Rayner 1986). Insects that were buried in lake sediments are therefore thought to be those which either fell into the lake or alighted on the surface before becoming trapped. Toxic leachates could also account for the absence of trace fossils (McKay and Rayner 1986) and reduced bioactivity may have limited decomposition thereby further enhancing the preservation of these fossils.

The presence of deciduous leaves, together with a southerly shift in the present-day distributions of several taxa: Empidae (Waters 1989), Bibionidae (Rayner 1987), Promecognathinae (McKay 1990), suggest that the palaeoclimate of Orapa was both seasonal, and cooler and wetter than found today. Plant remains are predominately from angiosperms and the diversity of leaf types (entire and serrated margins) suggest a mixed successional vegetation within the crater being periodically disturbed by mass flows from the unstable slopes. Mesic forest may have predominated over the wider area contrasting with sparser and earlier successional vegetation on the crater sides (Bamford 1990). Sedimentology, dating methodology, palaeoenvironment and selected references to the fossil flora and fauna are dealt with by McKay and Rayner (1986), Rayner et al. (1991), Rayner (1993) and Rayner et al. (1997).

Coleoptera, which account for approximately one third of the Orapa specimens, have so far been addressed in two papers, one describing a ground beetle Palaeoaxinidium orapensis McKay, 1991 (Promecognathinae, Carabidae) (McKay 1991), and another, a weevil Orapaeus cretaceous Kuschel & Oberprieler, 1994 (Brentidae, Curculionoidea) (Kuschel et al. 1994), together with a doctoral thesis on the Carabidae (McKay 1990).

The Orapa collection holds two fossil specimens belonging to the superfamily Scarabaeoidea (= Lamellicornia). The fossil described in this paper has been documented in a review of the Orapa fossils in McKay and Rayner (1986) and Rayner et al. (1997). A brief description and photograph was provided in the former publication with reference to its likely placement within the family Scarabaeidae. In the latter publication, it was suggested that the fossil may be that of a dung beetle (Scarabaeinae). In the following interpretation it is argued that this specimen is more likely of the ‘pleurostict’ lineage (McKenna et al. 2015) and could belong to the Melolonthinae, a large and diverse family which includes ‘chafers’ and ‘maybugs’.

The described specimen (BP/2/18654) is housed in the collection at the Evolutionary Studies Institute, University of the Witwatersrand.

Characters which pose some uncertainty in their interpretation are the mouthparts and the distal portion of the protibia. The mandibles and labrum of the Orapa fossil clearly extend beyond the clypeus which is considered the plesiomorphic condition (Scholtz et al. 1988). Krell (2006) however states that this extension could result from gaseous pressure or disarticulation during decay and therefore the degree to which the mandibles are exposed in the Orapa specimen could be exaggerated. There is evidence to suggest that the labrum may be broad and similar in width to the clypeus. The lateral margins of the labrum are however not distinct enough to be certain and the labrum could be the width of the intermandibular space.

The apex of the protibia (Figs 2, 4a) displays a well-defined ‘lobe’ which appears to abruptly truncate the apex whilst expanding it laterally on the inner margin. Damage or orientation could result in this unusual form although another possibility is that the apex is partially obscured by a superimposed structure. It is suggested that this could be the proximal segments of the protarsi, however, although the proximal segments of the meso- and metatarsi appear fairly broad and robust (segment 2 of the mesotarsus appears to have a ‘scalloped’ apical margin which may have accommodated stout spines), this condition would be particularly evident in the protarsi if this were the case. Adaptions of the protarsi are seen in some male melolonthines (Evans 2000) and superimposed impressions of what could be articulation of the first tarsal joint and apical teeth of the protibia give some support to this.

Figure 2. 

Ceafornotensis archratiras gen. and sp. n.: holotype BP/2/18654, habitus. Abbreviations: ae – aedeagus, a.l. – antennal lamellae, b.l. – basal lobe of mandible, cl – clypeus, g.l. – genal lobe, la? – labrum?, ma – mandible, ms – metasternum, msc – mesocoxa, msp – mesopisternum, mtc – metacoxa, mtp – metepisternum, pa – parameres, prc – procoxa, pt. I.? – protarsus I?, te? – tentorium?. Scale bar: 1 mm.

Figure 3. 

Ceafornotensis archratiras gen. and sp. n.: holotype BP/2/18654, general appearance. Micrographs taken under: A polarised light B low angle unpolarised light. Scale bar: 1 mm.

Figure 4. 

Ceafornotensis archratiras gen. and sp. n.: (A, B, C) holotype BP/2/18654, characters enlarged. Micrographs of: A right protibia B left mandible C aedeagus. Scale bar: 0.5 mm.

Alternatively, the form of protibae taken ‘as is’ could be a ‘spade-like’ fossorial adaption, possibly for digging in soft earth or sand. Some sand-inhabiting Pachysoma Macleay, 1821 species (wingless dung beetles) possess arcuate protibae which are widened on the inner margin; although their form is far more elongate and dentate. Arguably though, the dentate protibae of most scarbaeids are already fossorially adapted, for example in the Ochodaeidae (sand-loving scarabs), which possess no unique specialisation to their environment other than variation in the typical dentate condition. The more likely scenario then is that the distal ‘profile’ of the protibae in the Orapa fossil is not of its living form. If we accept that the specimen possesses no distinct adaptation, then the small, single tooth of the outer margin of the protibia, the lack of pronounced apical widening of the tibae, and the clypeus and head also not distinctly widened, would indicate that the beetle was not a strong digger (compared to dung beetles for example). This species may have lived much of its adult life above ground or in the upper soil and litter layer where bioturbation would produce a loose, non-compacted matrix. An alternative explanation could be that the form of the tibae was sexual dimorphic with females possessing a more fossorially adapted tibae to aid nest building, as is observed in the Lichnia Erichson, 1835, Lichninii (Hawkins 2006).

The form of the left mandible (Fig. 4b) (the right mandible partially obscured); a sclerotised apiculus (distal part of the mandible), terminating apically in a single tooth and teeth occurring proximally, is typical of several basal and derived taxa (Nel and Murray De Villiers 1988). The form does not appear indicative of specialised diets such as carrion (Hybosoridae) or dung feeding (Scarabaeinae, Aphodiinae) and therefore a phytophagous or saprophagous /fungivorous diet would seem most likely. If dietary preference was shared with extant adult melolonthines, then a strictly phytophagous diet should be expected which could include leaves, flowers or young fruits (Ritcher 1958). According to Nel and Scholtz (1990), the forms of mouthparts of extant melolonthines can be divided into five groups of which the mandible of the Orapa specimen most closely resembles group 2. Functionally, this form is adapted for cutting and grinding plant matter, however, as the mandible also corresponds to a plesiomorphic form shared by other taxa, its dietary preference is unclear. A fungivorous (Scholtz and Chown 1996) or saprophagous (Cambefort 1991) diet, the suggested ancestral diets of adult scarabaeoidea, could be possible. However, estimates of the origin of the pleurostict crown group suggest that phytophagy emerged between 109 and 128 Ma (Ahrens et al. 2014) which gives some support to the proposal that the adult beetle may have fed on living plants. That the paleoenvironment of the inner crater was likely dominated by angiosperms (Bamford 1990) does at least indicate that a proportion of its sustenance, either through adult feeding or larval feeding on plant roots, humus or rotting wood, was likely derived from angiosperms, particularly if the specimen was an inhabitant of the crater environment.

According to estimated divergence times given by Ahrens et al. (2014), the Orapa specimen existed early in the radiation of the pleurostict scarabaeids and before the divergence of many of the extant melolonthine tribes and the subfamilies of the ‘Higher Scarabaeidae’ (Rutelinae, Dynastinae and Cetoniinae). The age of the Orapa specimen and its apparent basal positon in what is a very diverse and still poorly understood phylogeny (Jameson and Ratcliffe 2000), limits what can be inferred regarding its relationship to extent taxa. For instance, whether it represents a stem lineage either to the extant melolonthine tribes or other more derived groups is unknown. The Orapa specimen’s relationship to angiosperm radiation is equally unclear although its existence prior to the apparent large radiation of pleurostict scarabs in the Tertiary may suggest that even the dominance of angiosperms, approximately 60% of species during the Turonian (Lidgard and Crane 1990), was not yet sufficient to drive a major radiation in the pleurosticts. Ahrens et al. (2014) suggest that this later radiation was the result of a large propagation in available niches brought about by increased angiosperm productivity and synergistic and self-propagating changes in litter and microbial conditions over wide biogeographical regions. This in turn corresponded with enrichment in pleurostict endosymbiotic gut microflora.

In summary, evidence is presented in support of the Orapa specimen as a basal member of the sub-family Melolonthinae. The phylogeny of the Melolonthinae is at present unresolved and probably paraphyletic. Assignment of fossil specimens to this group will therefore be problematic until further work is undertaken to address inconsistencies between morphological and molecular derived phylogenies. Further work on resolving the phylogenetic relationships of the basal taxa Phaenognathini and Lichniini is welcomed together with a greater understanding of the Melolonthinae ‘grade’ and how this might relate to other scarabaeid subfamilies.

The work was funded by the DST-NRF Centre of Excellence in Palaeosciences funding initiative. I am grateful to Prof Marion Bamford, Dr Riaan Stals and the Wits Research Office for respectively providing equipment, specimens for comparative study, and additional research-related funding in support of this work.