Oryctoantiquus borealis Ratcliffe and Smith

Ratcliffe, Brett C., Smith, Dena M. & Erwin, Diane, 2005, Oryctoantiquus borealis, New Genus And Species From The Eocene Of Oregon, U. S. A., The World’ S Oldest Fossil Dynastine And Largest Fossil Scarabaeid (Coleoptera: Scarabaeidae: Dynastinae), The Coleopterists Bulletin 59 (1), pp. 127-135 : 130-132

Oryctoantiquus borealis Ratcliffe and Smith , new species ( Figs. 1–2 )

Type Material. Holotype labeled UCMP (University of California Museum of Paleontology ) PA503//157049 . Holotype deposited at the Museum of Paleontology, University of California, Berkeley , CA .

Occurrence. Middle Eocene, Clarno Formation, West Branch Creek, Wheeler County, Oregon, U. S. A. UCMP locality 3904, USGS locality 8637, UF locality 230.

Diagnosis. Metasternum proportionately large, transverse (19 mm wide, 6.9 mm long along central, longitudinal sulcus); surface punctate on lateral half of each side; punctures dense, moderate in size ( Figs. 1–2 ). Metepimeron similarly punctate. Mesocoxal cavities (coxae not present) and metacoxae large, transverse (ca. 2.5 3 9.0 mm), contiguous at midline. Posterior femora enlarged (ca. 6.1 3 12.4 mm) (division between trochanter and femur not evident). Right posterior tibia apically widened, with apparent small, lateral tooth near middle and another small, lateral tooth at apex; longest length 11.5 mm.

Etymology. The species name is derived from the Latin borealis , in reference to the northerly locality of the species; most oryctine species today are southerly in their distribution because their habitats are mostly subtropical or tropical.

Classification. Ratcliffe compared the fossil with extant species of Scarabaeoidea . Aside from the fact that O. borealis is much larger than any of those compared, it still did not resemble any species of Passalidae , Lucanidae , Geotrupidae , Pleocomidae , Trogidae , or Scarabaeidae ( Scarabaeinae , Melolonthinae , Rutelinae , or Cetoniinae ). Oryctoantiquus borealis is unique among fossil scarabs, and the few leg and sternum characters it possesses allow us to confidently place it with the tribe Oryctini in the subfamily Dynastinae . Characters that place it in the tribe Oryctini are (1) the enlarged femora, (2) apically expanded posterior tibiae that are toothed at their apices, (3) large, transverse, coxal cavities, (4) large metasternum with distinctive punctation on the anterolateral surface, and (5) large size and robust body form. The Oryctini today contains mostly larger (20–70 mm) scarabs in 26 genera with 230 species. Species in the tribe occur worldwide except for Australia and the northernmost parts of Europe, Asia, and North America.

The metasternum and legs of O. borealis ( Fig. 2 ) bear a strong resemblance to those of the extant Strategus aloeus (L) (Fig. 3), a species that is found from the southern United States to South America. And yet they are different. We postulate that O. borealis is not congeneric with Strategus Hope since Strategus is South American in origin and spread northwards into Central and North America ( Ratcliffe 1976) only after the establishment of the isthmus of Panama in the Miocene about 7.1 Ma ( Coates 2004). Oryctoantiquus borealis was already established in northwestern North America 46–44 Ma, a full 39–37 million years before ancestral Strategus species entered North America. Furthermore, the metasternum in all extant species of Strategus is either entirely punctate or, more typically, with the anterior half punctate (Fig. 3). The metasternum of O. borealis has a different pattern of sculpturing where the punctures are broadly separated along the midline ( Fig. 2 ).

Using the assumption that northwestern North America may have been more broadly joined with Asia 45 Ma, we also considered the possibility of affinities with Asian genera of Oryctini whose ancestors may have dispersed to North America. Ratcliffe examined extant genera of Asian oryctines to determine if O. borealis could be considered congeneric with any of them. Species of Oryctes Illiger , Trichogomphus Burmeister , Scapanes Burmeister , Clyster Arrow , and Dichodontus Arrow were examined, and none resembled O. borealis in size, proportions, or punctation pattern.

Of the four fossil taxa currently placed in the subfamily Dynastinae , three (two

Fig. 3. Strategus aloeus , line drawing of extant species in same position as that of fossil Oryctoantiquus borealis .

Tomarus species and the single Pentodon species ) are in the tribe Pentodontini , and one ( Strategus cessatus Wickham ) is placed in the Oryctini . Strategus cessatus is known from a single wing cover from the late Eocene (34 Ma) Florissant shales in Colorado. The Florissant climate at this time was temperate to warm temperate ( Gregory 1994; Gregory and Chase 1992; Meyer 1992, 2003; Wolfe 1992 a, 1994, 1998) although a few subtropical to tropical taxa are found in the assemblage (Moe and Smith, in review), which is in keeping with the climate requirements of extant Strategus species. Ratcliffe examined the specimen ( Ratcliffe 1976) and concluded that it did most closely resemble the extant Strategus cessus LeConte from Arizona. Whether the Florissant fossil represents the genus Strategus is now doubtful, especially considering that it predates the establishment of the Panamanian isthmus, which was the dispersal route of ancestral Strategus from South America to North America. As Krell (2000) noted, ‘‘... most Tertiary fossil Scarabaeoidea were classified in an extant genus because the latter resembles the shape of the fossil and was known to the classifying author. In most of these cases, the fossil does not show any diagnostic character of the extant genus, much less any generic autapomorphy.’’ While the specimen of Oryctoantiquus is 15 million years older than S. cessatus and is a larger and more robust beetle, we have no way of knowing if they are congeneric because there is not enough of each specimen preserved to provide sufficient evidence. But we do believe that Oryctoantiquus is a distinct genus based upon its size, puncture pattern on the metasternum, and location in time and space.

Size. The largest fossil in the superfamily Scarabaeoidea is a stag beetle (family Lucanidae ), Protognathinus spielbergei Chalumeau and Brochier , from the Eocene (49 Ma) oil shale deposits of Messel, Germany ( Hoffmann 2000; Chalumeau and Brochier 2001). It is 55.0 mm in length. The largest known fossil in the family Scarabaeidae up until now was Cheirotonus otai Ueda (1989) (subfamily Euchirinae ) from the Middle Miocene of Japan. This specimen is 45.0 mm long from the apex of the clypeus to the apex of the abdomen. We believe that Oryctoantiquus borealis is now the largest fossil specimen in the family Scarabaeidae . The proportions of posterior tibial length do not differ significantly among Dynastinae and especially in members of the Oryctini , which are robust and compact beetles adapted for digging. The posterior tibial length of O. borealis is 11.5 mm whereas the posterior tibial length of C. otai is 9.2 mm. Extrapolating from many specimens of the abundant Strategus aloeus (L.) (a modern oryctine analog occurring from the southern U.S. to South America; Fig. 3) with the same posterior tibial length as that of O. borealis , we arrive at a body length (apex of clypeus to apex of abdomen) for O. borealis of 50.0 mm, or 5.0 mm longer than C. otai .

Geological Setting. The West Branch Creek fossil locality is west of the town of Mitchell, Oregon. The laminated shales have limited exposure and are primarily found along a tributary that drains into the West Branch Creek. This locality is a lacustrine deposit from the middle Eocene Clarno Formation ( Manchester 1990; Meyer and Manchester 1997). Although the Clarno Formation consists primarily of volcanigenic sediments that range in age from 39–54 million years, the localities that yield insect and plant fossils are usually composed of mixed-debris flow deposits, tuff beds and lacustrine units (Bestland et al. 1999). Major collections of West Branch Creek plant material have been made in the past and include leaves, fruits, seeds, and flowers. The flora includes many subtropical plant taxa, including a fossil banana ( Musaceae ) (Manchester and Kress 1993). Although a few insects, primarily beetles, have been collected from the West Branch Creek locality, none of them have been described before.

Fossils from the Clarno Formation record an important interval in the Tertiary of North America. During the Eocene and Oligocene, there was dramatic change in climate from generally warm and tropical to much cooler and temperate. This transition is well-documented in both the marine and terrestrial fossil record (Berggren and Prothero 1992) and is especially pronounced in the Pacific Northwest where it appears that mean annual temperatures may have declined by approximately 88C ( Wolfe 1992 b), and the mean annual range in temperature may have increased by as much as 158C ( Wolfe 1978). The habitat of Oryctoantiquus borealis would have been forests that are most similar to modern subtropical and tropical forests ( Wing 1998; Manchester and Meyer 1987). After the Eocene-Oligocene cooling event, forests of the Pacific Northwest became much more temperate, hosting a combination of conifers, broad-leaved evergreens, and deciduous plant taxa ( Wing 1998). Oryctoantiquus and its lineage began to attenuate and ultimately become extinct because the new, temperate climate and habitat could not support a tropical lineage.

Figures

Fig. 1. Oryctoantiquus borealis. Scale bar equals 10 mm.

Fig. 2. Oryctoantiquus borealis, line drawing of fossil.