Prolucanus beipiaoensis gen. et sp. nov.: The First Fossil Species of Lucaninae (Coleoptera: Lucanidae) from the Early Cretaceous of Northeastern China

Simple Summary The family Lucanidae Latreille, 1804, has long been considered to be one of the most archaic groups in the superfamily Scarabaeoidea and contains more than 110 genera and about 1300 extant species. Most adult lucanids display obvious sexual dimorphism as males are often larger and possess well-developed and variable mandibles used to compete with other males. For a long period of time, researchers of stag beetles have had problems and disputes, because many views are only speculation and cannot be supported by the evidence of fossil specimens. In this paper, we describe and illustrate a new fossil species from the Early Cretaceous of northeastern China: Prolucanus beipiaoensis gen. et sp. nov. Based on this well-preserved specimen, a new genus is established, representing the first record of the fossil Lucaninae in China and the oldest reliable record of the subfamily. Abstract A new stag beetle fossil is described from the Yixian Formation in northeastern China. A new genus is established based on this well-preserved specimen. Prolucanus beipiaoensis gen. et sp. nov. is firmly placed in the extant lucanid subfamily Lucaninae based on its broad prosternal process and geniculate antennae. Our discovery represents the first fossil record of Lucaninae from the Late Mesozoic of China and the oldest reliable record belonging to the subfamily. We also summarize the fossil species of stag beetles found in China and the described fossil species of the subfamily Lucaninae found in the world. A key to all fossil genera of Lucanidae from China is provided.


Introduction
The family Lucanidae Latreille, 1804, is long considered to be one of the earliest diverging lineages of the superfamily Scarabaeoidea Latreille, 1802, and contains more than 110 genera and about 1300 extant species distributed throughout all main zoogeographical regions except Antarctica [1][2][3][4][5][6]. Most adult lucanids display obvious sexual dimorphism. Males are often larger and possess well-developed mandibles used to compete with other males, whereas females often possess smaller mandibles, which are more conducive for activities such as oviposition. Crown-group representatives of Lucanidae are divided into four extant subfamilies: Lucaninae, Lampriminae, Syndesinae, and Aesalinae [2]. Four extinct subfamilies (Protolucaninae, Paralucaninae, Ceruchitinae, and Litholampriminae) have been established [7,8], although the validity of some of them has been questioned.
Fewer than 30 species of fossil stag beetles have been described, including two recently discovered Cretaceous species preserved in amber from northern Myanmar [9,10]. To date, six lucanid fossils have been reported in China [8,11,12] Up until now, only nine fossil species of Lucaninae have been described in the world [4,13]. There is an extinct genus from the Mesozoic, including three species [14]: Cretolucanus longus Nikolajev 2007, C. ordinarius Nikolajev 2007, and C. sibericus Nikolajev 2007. They were previously considered to be the oldest fossil species of Lucaninae, originating from the Early Cretaceous in Pad Semen, Russia. However, no convincing characters can be found on three very vague fossil specimens to support their classification as Lucaninae (see Discussion). There are five genera (two extinct and three extant) and six species from the Cenozoic: Succiniplatycerus berendti (Zang, 1905) originates from the Eocene in Baltic Amber. Miocenidorcus andancensis Riou, 1999 originates from the Miocene in Andance, France. Platycerus sepultus Germar, 1837 originates from the Oligocene, "incarbone fossili territorii Rheni prope Bonnam", Germany. Platycerus zherichini Nikolajev, 1990 originates from the Oligocene, Pozhar region in Russia. Dorcus primigenius Deichmüller, 1881 originates from the Eocene, Kučlín near Bílina in the Czech Republic. Lucanus fossilis Wickham, 1913 originates from the Oligocene, Florissant in the USA. In conclusion, the previous reliable record of the oldest fossil Lucaninae was determined to be from the Eocene.
In this paper, we describe and illustrate a new fossil species from the Lower Cretaceous Yixian Formation near the Huangbanjigou Village in Beipiao City, Liaoning Province. Based on this well-preserved specimen, a new genus is established, representing the first record of fossil Lucaninae in China and the oldest reliable record of the subfamily.

Material and Methods
The present specimen originates from the fossiliferous yellowish tuffs of the Lower Cretaceous Yixian Formation from the locality near the Huangbanjigou Village of Beipiao City in the Liaoning Province of northeastern China. This stratum represents the famous Jehol Biota known for its feathered dinosaurs, mammals, birds, angiosperm plants, and numerous insects [15]. Its geological age has been indicated with respect to the Early Cretaceous, circa 125 MYA [16].
Images were taken by using a Canon 5D mark IV digital camera with a 100 mm f/ 2.8 macro lens. A Canon MT-26EX twin flash was used as the light source. Morphological details were photographed by using a Keyence VHX-5000 digital microscope with the Keyence VH-Z20R zoom lens (20-200×). The images were processed and combined into figures by using Adobe Photoshop CC 2019. The type specimen is housed at the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences, in Nanjing, China.
Measurement criteria in millimeters (mm) are used as follows: Body length: length between the apex of mandible to the elytral apex along the midline; Elytral length: length between the basal border and the apex of elytra along suture; Elytral width: widest part of both elytra combined; Head length: length between the anterior apex of clypeus and the posterior margin of occiput along the midline; Head width: widest part of head; Mandible length: length from the apex of mandible to its base at anterior margin of the head; Pronotal length: length of the pronotum along the midline; Pronotal width: widest part of pronotum.

Systematic Palaeontology.
Order Coleoptera Linnaeus, 1758 Superfamily Scarabaeoidea Latreille, 1802 Family Lucanidae Latreille, 1804 Subfamily Lucaninae Latreille, 1804 Genus Prolucanus gen. nov. Type species: Prolucanus beipiaoensis sp. nov. Diagnosis. The new genus is firmly placed in the subfamily Lucaninae based on broad prosternal process and geniculate antennae, and it can be distinguished from almost all fossil or extant genera by a combination of the following features: (1) body broadly-oval; (2) eyes large, nearly entire, slightly divided by ocular canthus; (3) antenna geniculate, antennal club with four antennomeres; (4) pronotum transverse, nearly trapezoidal, over twice as broad as long; and (5) mesotibia only with one long and sharp spur at apex (maybe only one preserved).
Description. Body broadly-oval. Head large, wider than long; preocular angle rounded; eyes large, slightly divided by ocular canthus; antenna geniculate with 10 antennomeres, antennomere I elongate and strong, antennal club with four antennomeres, antennomere VII slender and sharply pointed apically, antennomeres VIII-X lamellate; mandible robust, widest and strongly inward curved near the base, mandibular apex bidentate, incisor edge of mandible smooth, without tooth (female). Pronotum transverse, over twice as broad as long, front angle rounded. Elytra elongate, densely punctate. Legs strong, protibia with 3-4 teeth along outer margin, progressively larger from base to apex, apex bifurcate, short, nearly right angle; mesotibia with one large tooth on outer margin and at least one long and sharp spur at apex. Prosternal and mesosternal process broad. Abdomen five-segmented, apex slightly rounded.
Etymology. The Latin term "pro-", meaning ancient or primitive, and Lucanus after the type genus of Lucaninae.
Prolucanus beipiaoensis sp. nov. (Figures 1-3) 1  Material examined. Holotype: NIGP176633, probably female, a well-preserved and almost complete body collected from the Yixian Formation near the Huangbanjigou Village of Beipiao City, Liaoning Province, and is now housed in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China.
Pronotum (Figures 2A,B and 3A) transverse, form 2.4 times as wide as long, widest near base. Anterior margin is a relatively straight arc, front angle ( Figure 3A) protruding forward and rounded. Lateral margin nearly smooth. Posterior margin longer than anterior margin and protrudes obviously to the rear at the midline.
Etymology. The specific name is derived from the type locality.

Discussion
Extant stag beetles can be classified relatively accurately into subfamilies by a combination of external and genital morphology and molecular means. The genital morphology of lucanids is an important character [17,18], but it is usually not visible in fossil species and can only be classified by external morphological characters. Holloway [19] considered the canthus as absent in only a small number of Lucanidae, and the eyes are stated to be entire. Lampriminae, Aesalinae, and Syndesinae all have entire eyes, whereas Lucaninae all have varied forms of canthus. However, Howden and Lawrence [20] expressed reservations and proposed Aesalinae to also have eyes partly divided by a canthus, and the tribe Platycerini (Lucaninae) may have the eyes entire; thusm this character should be used with caution. According to Howden and Lawrence [20], the subfamily Lucaninae is characterized by the following combination of characters: (1) prosternal process not or only slightly narrowed between coxae so that the latter are distinctly separated; (2) antenna geniculate; and (3) body elongate and depressed. Although the above characters selected by the authors only cover North American species, these morphological characters are applicable to most Lucaninae (except Chiasognathus, Ryssonotus, Pholidotus, etc., in which the body is usually oval, convex, and the prosternal process is narrow). In 2007, Holloway [2] regarded the following two characters as the unique characters of Lucaninae in New Zealand: (1) prementum situated near the middle of the mentum and not visible when the head is viewed from below; (2) basal first segment of labial palp concealed by the mentum. However, these two characters also apply to all species of Lucaninae, worldwide. Unfortunately, these parts are difficult to preserve intactly in most fossil specimens and in our fossil specimen. Prolucanus gen. nov. can be firmly placed in the extant subfamily Lucaninae based on the combination of morphological characters mentioned above. Since most lucanid adults display obvious sexual dimorphism, with males being larger and possessing prominent mandibles, the fossil specimen with a broadly oval body, short and robust mandible, and possessing only one inner tooth is likely to be female.
Based on three fossil specimens discovered from the Early Cretaceous in Pad Semen, Russia, Nikolajev [14] established the extinct genus Cretolucanus Nikolajev, 2007 and described three species (C. longus, C. ordinarius, and C. sibericus), classified them into Lucaninae. However, the description, diagnosis, and illustrations provided by the author are confusing. The author distinguished Cretolucanus from members of Penichrolucaninae, Aesalinae, and Nicaginae (merged with Aesalinae) only by its elongated body, small head, and the eyes partly divided by ocular canthus; moreover, unfortunately, the author did not even provide any evidence supporting its subfamilial placement in Lucaninae. In fact, it can be seen from the illustrations that the three fossil specimens are very vague, and it is almost impossible to extract effective characters for comparison. Only C. longus preserved part of the antennal structure, which can be seen as not geniculate, which is obviously in contradiction with Lucaninae. In addition, there are also insufficient characters to indicate that it belongs to the family Lucanidae. Therefore, we recommend that Cretolucanus should be removed from Lucaninae and tentatively designated as Lucanidae subfamilia incerta or Scarabaeoidea familia incerta until more complete specimens are found.

Conclusions
Lucaninae is the most species-rich and morphologically diverse subfamily of stag beetles. Since there are many genera and species of Lucaninae and convergent evolution is common, it is difficult to clarify the relationship between various genera and tribes by using traditional morphological classification. In 2015, Kim and Farrell [4] reconstructed the first molecular phylogeny of world stag beetles, and a time-calibrated phylogeny based on five fossil data was estimated. They demonstrated that the subfamilies Lucaninae and Lampriminae appeared monophyletic under both methods of phylogenetic inferences; however, Aesalinae and Syndesinae were found to be polyphyletic. Within Lucaninae, the most primitive Platycerini lineage branched off first during the mid-Early Cretaceous around 125 MYA, and the other lineage further diverged into the two main lineages at the end of Lower Cretaceous circa 108 MYA, which then diversified rapidly in each hemisphere. Prolucanus beipiaoensis gen. et sp. nov. is basically consistent with the research results of Kim and Farrell [4]. Based on morphological comparison, we believe that the new genus does not belong to the branch of Platycerini lineage because of its broadly oval body, transverse and large head, trapezoidal and transverse pronotum, outer margin of protibia with a few isolated teeth, and abdomen with five visible ventrites. However, Platycerini usually with elongated body, longitudinal and smaller head, nearly oval pronotum, outer margin of protibia serrate continuously, and abdomen with six visible ventrites. In addition, the character of having large eyes that are nearly entire and slightly divided by an ocular canthus as found in Prolucanus gen. nov. and Platycerini may be a plesiomorphic feature, which is different from most Lucaninae. Therefore, we suggest that Prolucanus gen. nov. is one of the earliest diverging lineages in Lucaninae.

Institutional Review Board Statement:
The study did not involve humans or animals.

Informed Consent Statement: Not applicable.
Data Availability Statement: The study did not report any data.