Notes on the Morphology and Systematic Position of Archaeolus Lin, 1986, from the Jurassic of South China (Coleoptera: Elateroidea)

Simple Summary Elateroidea is one of the large superfamilies in the beetle suborder Polyphaga. Many adpression-type elateroid fossils were insufficiently described, which hinders the interpretation of their systematic position. Here, we figure and re-describe an elateroid fossil, Archaeolus funestus, from the Jurassic of South China. Our observations support that Archaeolus might be a member of the Throscidae family. Abstract The morphology of the Jurassic fossil Archaeolus funestus Lin, 1986, which was previously placed in the extinct click-beetle subfamily Protagrypninae (Coleoptera: Elateridae), is revised based on a re-examination of the type specimen. The validity of Protagrypninae is discussed and further questioned, partly based on the newly observed characters in A. funestus, including the surface sculpture of the mesoventrite. A possible Throscidae affinity of monotypic Archaeolus Lin, 1986, as suggested in a recent study, is further critically reviewed.


Introduction
Elateroidea is one of the large superfamilies in the beetle suborder Polyphaga and contains both hard-and soft-bodied forms. Some hard-bodied elateroid families (i.e., Eucnemidae, Throscidae, Cerophytidae, and Elateridae) share a somewhat uniform appearance, especially due to the presence of a pro-mesothoracic clicking mechanism. Though recent molecular studies have revealed that Eucnemidae, Throscidae, and Cerophytidae are distantly related to the much more diverse and common family Elateridae [1][2][3][4], historically these four families were thought to form a monophyletic clade based on their morphology [5]. In adpression fossils of hard-bodied elateroids, due to limited available characters and often taphonomic artefacts, it is even more difficult to determine their precise systematic position [6,7]. In the present paper, we re-examine the elateroid fossil, Archaeolus funestus Lin, 1986, from the Jurassic of South China, and discuss its systematic position.

Materials and Methods
The holotype of A. funestus was collected from the Shiti Formation (Middle Jurassic according to Zhang [8] and Yin et al. [9]) at the Xiwan Coal Mine, Pinggui District, Hezhou City, Guangxi, China [10]. The specimen is deposited in the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences, Nanjing, China.
Photographs under incident light were taken with a Zeiss Discovery V20 stereo microscope and stacked in Helicon Focus 7.0.2. Scanning electron microscopic (SEM) images were obtained with a Hitachi SU 3500 scanning electron microscope, operating with an accelerating voltage of 15 kV and a pressure of 70 Pa. Images were further processed in Adobe Photoshop CC to enhance contrast.
Remarks. Since there is a relatively wide space between the prosternum and the pronotal hypomeron (at least well shown on one side), we suppose there should be an antennal groove along the pronotosternal suture. The specimen NIGP70071b is generally an impression of the ventral side of the beetle; thus, a groove on the beetle body should appear as a ridge on the impression. However, the pronotosternal suture does not appear as a distinct ridge on that specimen, which we suppose is caused by the damage during fossilisation and (or) fossil preparation.
In previously known throscids, the prosternal carinae, if present, are always continuous with the lateral edges of prosternal process (e.g., [11]). In Archaeolus, however, the distance between the prosternal carinae is much wider than the prosternal process, and the carinae are not continuous with the lateral edges of prosternal process, which might be an apomorphy of the genus.

Discussion
Archaeolus was originally included in Elateridae without a specified subfamilial placement [10]. After re-studying the type material, Dong et al. [12] and Ponomarenko et al. [13] placed Archaeolus in tribe Protagrypnini within the subfamily Protagrypninae, which are both extinct taxa attributed to Elateridae [14]. However, the validity of Protagrypninae itself is in question. As noticed by previous researchers (e.g., [6,7,15,16]), many species previously classified in Protagrypninae may belong to other extant elaterid subfamilies or even other hard-bodied elateroid families (i.e., Cerophytidae, Eucnemidae, and Throscidae). The extinct tribe Protagrypnini, as defined by Dolin [14,17], is characterised by the presence of a groove along the pronotosternal sutures and the presence of a transverse suture on the mesoventrite, which has been uncritically adopted by some following researchers (e.g., [18][19][20][21][22]). The first character, pronotosternal grooves, can be found in many other extant Elateridae, as well as Throscidae and Eucnemidae. The second diagnostic character, based on our observations, is not a suture at all, and instead, it represents the line between the mesoventrite body and procoxal rests, which is in agreement with the general observations by Kundrata et al. [7]. In the mesoventrite of Archaeolus, the portion below the transverse line (mesoventrite body) is developed with small punctures, a type of common surface decoration in elateroids, while the portion above the line is relatively smooth, suggesting it is not usually exposed as the outer surface, supporting an interpretation as procoxal rests ( Figure 2B). An additional character indicative of Protagrypninae (including species in Protagrypnini) is the median plate-like structure on the prosternum. A similar structure can also be found in Throscidae, Eucnemidae, and some members of extant Elateridae, and therefore it may have a limited taxonomic value [6,7].
Muona et al. [6], based on the images and the descriptions in Dong et al. [12], transferred Archaeolus to the extant family Throscidae. However, our re-examination of the holotype revealed some problems in the illustration and description by Dong et al. [12], which also partly questions the interpretation by Muona et al. [6]. The key character for a placement of Archaeolus in Throscidae, as claimed by Muona et al. [6], is the "antennal groove running close to the [pro]notosternal suture and then turning towards the hind corners of the prothorax above the protibial groove". However, we found no clear evidence supporting the fact that the antennal groove extends beyond the posterior end of the pronotosternal suture and turns towards the hind corners of the prothorax (Figure 2A). Thus, its validity cannot be guaranteed. Dong et al. [12] claimed that Archaeolus has distinct antennal clubs, which if true, as noticed by Muona et al. [6], is another typical throscid feature. However, based on our observations, the antennae of Archaeolus are essentially not preserved at all (Figure 2A). Metacoxal plates medially extending posteriorly could serve as an additional feature to characterise throscids [23]. Unfortunately, the metacoxae are not well-preserved in the holotype of A. funestus ( Figure 2D). Therefore, we argue that the systematic position of Archaeolus cannot be confidently solved based on the available characters. Nevertheless, considering its general appearance and prosternal structure, we agree with Muona et al. [6] that, at this moment, it is better to place Archaeolus to Throscidae rather than retain it in the "wastebin taxon" Protagrypninae.