Bombers and Bivalves: First North American occurrence of the rudist Durania sp. (Bivalvia: Radiolitidae) in the Upper Cretaceous (Cenomanian) Greenhorn Limestone of southeastern ColoradoBruce A. Schumacher

USDA Forest Service, 1420 E. 3rd St., La Junta, CO 81050 baschumacher@fs.fed.us

A colonial monospecific cluster of rudist bivalves from the lowermost Bridge Creek Limestone Member, Greenhorn Limestone (Upper Cenomanian) are attributable to Durania cf. D. cornupastoris. This discovery marks only the eighth recorded pre-Coniacian occurrence of rudist bivalves in the Cretaceous Western Interior and the only Cenomanian record of rudist Durania in North America. Discovered in 2011, the specimen was unearthed by aerial bombing at a training facility utilized during World War II. The appearance of rudist bivalves at mid-latitudes coincident with marked change in marine sediments likely represents the onset of mid-Cretaceous global warming.

Keywords: Cenomanian, climate, Durania, Greenhorn, rudist

TransacTions of The Kansas academy of science

Vol. 115, no. 3-4 p. 117-124 (2012)

Introduction Some seventy years ago southeastern Colorado was utilized during World War II (1943 – 1945) as a training area for precision bombing practice and air-to-ground gunnery. The La Junta Municipal Airport was created in April 1940 as La Junta Army Air Field (Thole 1999) and was used by the United States Army Air Forces for an advanced flying school and training base for B-25 Mitchell bomber crews (Fig. 1). In addition to training American pilots, Chinese pilots also trained at La Junta Army Air Field (Foley 2007). Some pilots training at this location used a rare variant of the Mitchell bomber with a 75mm cannon mounted in the nose, as well as the more common version with up to four .50 caliber nose-mounted machine guns. The former Pueblo Precision Bombing Range #2 is located roughly twenty-five miles (40 km) southwest of La Junta, Colorado, and is now part of the Comanche National Grassland. Locations of nine bombing targets plus submarine and ship skip-bombing targets have been located (Foley 2008) using LiDAR imagery (Figure 2). Historic use of these areas is still evident in the form of metallic bomb debris and extensively cratered pasture lands from high explosive ordinance.

The Greenhorn Limestone in southeastern Colorado (Fig. 3) is divided into the three subunits (Cobban and Scott 1972; Hattin 1975; Kauffman 1986). Roughly the lower two-thirds of the unit is comprised of the basal Lincoln Limestone Member (5 m) and the Hartland Shale Member (19 m). The dominant lithology of the lower members is calcareous shale with minor amounts of thin calcarenite beds. The Bridge Creek Limestone Member (Figure 4) forms the upper one-third of the unit (13 m) and consists of resistant chalky limestone

Figure 1. Restored B-25D-30 Mitchell medium bomber known as “Miss Mitchell”. Various pro-duction versions of such planes were delivered to the USAAF’s advanced flying school at the 402nd Army Air Force Base Unit at La Junta, Colorado in October, 1943 (photo used with permission of Abbots International Airshow Society, John T. Sessions Chairman).

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layers (5 – 28 cm) interbedded with soft and thinly stratified intervals of calcareous shale (25 – 150 cm). The base of the Bridge Creek Limestone is marked by the thickest of its limestone beds (28 cm), a characteristic light gray micritic limestone (BC-1) which exhibits conchoidal fracture and contains various and abundant ammonite impressions of the Sciponoceras gracile biozone. The BC-1 bed has been miscorrelated as the Fencepost limestone bed of central Kansas due to its thickness and resistant nature. However bed BC-1 is much older than the Fencepost Limestone bed and correlates with the lower Hartland Shale of central Kansas (marker bed HL-1, Hattin 1975). Additionally, the Bridge Creek bed BC-1 is durable but brittle. Fresh exposures of BC-1 will initially yield regular jointed blocks, but upon weathering develops abundant conchoidal fractures. Bed BC-1 is locally used as road material though it is hard on tires, and has only been employed in a few local instances to construct small buildings.

The summer of 2011 saw intense drought in southeastern Colorado and reduced vegetation made many of the historic bomb craters at the Pueblo Precision Bombing range visible. In some places the high explosive ordnance welled up underlying limestone layers of the Greenhorn Limestone as ejecta berms surrounding the blast craters, in particular in those places where the basal Bridge Creek limestone bed (BC-1) lay in the shallow subsurface. Craters in such locations are surprisingly evident given seventy years of weathering due to the resistant nature of the BC-1 limestone bed. In May 2011 a small colony of rudist bivalves (FHSM IP-1506) was discovered by the author amid the broken up limestone around one such crater (Fig. 5).

Rudists are a group of tube or ring shaped marine heterodont bivalves that arose in the Jurassic and became prolific reef builders in tropical waters during the Cretaceous. Although better known in younger Cretaceous

Figure 2. LiDAR imagery of bombing target 4 of the former Pueblo Precision Bombing Range #2 (taken from Foley 2008). The outlines of a large cross-hair and four ship targets are clearly visible, as are clusters of high explosive impact craters near each target.

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rocks such as the Niobrara Formation in Kansas (Hattin 1988) and adjacent states, occurrences of rudist bivalves in rocks of Greenhorn age (Upper Cenomanian – Lower Turonian) are rare at mid-latitude locations such as in Colorado. Although warmer temperatures prevailed during the latter half of the Cretaceous Period (Caldeira 1991), rudist bivalves in the North American mid-continent are at or near the northern periphery for the occurrence of tropical species such as this and their appearance likely reflects climate change (El-Shazly 2011).

Institutional Abbreviations: FHSM – Sternberg Museum of Natural History, Fort Hays State University, Hays, Kansas; USNM – United States National Museum (National Museum of Natural History), Washington, D.C.

SYSTEMATIC PALEONTOLOGY

Class Bivalvia Family RADIOLITIDAE Gray, 1847 Genus Durania Douvillé, 1908 Durania cf. D. cornupastoris Des Moulins, 1826

Material: FHSM IP-1506, fragments of a laterally crushed cluster of conjoined lower valves (Fig. 6).

Occurrence: From the calcareous shale interval just above basal limestone bed (BC-1) of Bridge Creek Limestone (Upper Cenomanian), Greenhorn Limestone, south-central Otero County, Colorado. The basal limestone bed contains a relative abundance of ammonite molds including Calycoceras cf. C. naviculare, Kanabiceras septemseriatum, Metoicoceras sp., and Sciponoceras gracile (Fig. 7).

Description: Light yellow, fine grained calcareous matrix clings to the specimen, unlike the light gray, sparry limestone matrix of the basal bed (BC-1) of the Bridge Creek Limestone. Beds below the basal Bridge Creek in this area are typically darker and

less calcareous, thus it is likely that the rudists occurred above BC-1. There is no evidence of epifauna such as oysters or other invertebrates attached to the valves which is atypical for younger and more common rudist occurrences in the Western Interior Seaway (Hattin 1988). A slight curvature to the valves suggests the individuals trended toward a partially recumbent position during growth.

Figure 3. Composite measured section of Bridge Creek Limestone Member, Greenhorn Limestone compiled from several locations in the Otero County portion of the Comanche Na-tional Grassland. Location of rudist colony and select ammonite occurrences noted.

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The specimen includes fragmented portions of minimally five individual lower valves. The conjoined nature of the valves leaves little doubt that the individuals form a single clustered colony as is typical of rudists (Kauffman and Sohl 1974). Individual lower valves are cone-shaped, tapering to a small area at the base, with uncrushed rim diameters of from 20 to 25 mm and heights from 80 to more than 100 mm. Closely spaced, uniform vertical ridges or grooves 2 mm in width surround the exterior surface of valves and form a distinct zigzag suture on the dorsal rim where two valves remain joined. More widely spaced horizontal growth rugae cut across the vertical ribbing, as many as eight times observed on one individual.

Where well preserved, the dorsal rim (growth surface) of lower valves is formed by small (0.5

– 1 mm) and thin pentagonal and hexagonal plates, referred to as the “sauvagesiine polygonal cell network” typical of radiolitid rudists (Cobban et al. 1991). There exists no vestige of an arête cardinal or other in-folding of the lower valve as noted in other rudist families. The inner surface of valves is smooth, but in the form of dark and light tones there are extensive bifurcating networks of radiating ‘vascular’ impressions frequently noted on the inner valve rims of Durania (Skelton and Wright, 1987).

Remarks: The closely spaced, regular ribbing and clear polygonal cell structure on the dorsal rim of lower valves identify them as sauvagesiine radiolitids, and the absence of inward folds of the valve walls place them in the genus Durania (Douvillé, 1910; Cobban

Figure 4. An arroyo within the historical Pueblo Precision Bombing Range #2 (now the Comanche National Grassland) is visited by sixth grade students of La Junta, CO who record alternating layers of limestone and calcareous shale of the Bridge Creek Limestone Member, Greenhorn Limestone.

Transactions of the Kansas Academy of Science 115(3-4), 2012 121

et al. 1991). Given similar geographic and stratigraphic positions to USNM 442109 (Cobban et al. 1991) the species that invites closest comparison is D. cornupastoris Des Moulins. However, the species-level taxonomy of the specimens will first require systematic revision of the group as a whole.

Discussion

Durania is a widespread rudist known from the Tethys region, Mexico and the North American mid-continent. Most reported occurrences are of Turonian and younger age and the oldest reported occurrence of the genus is that of Durania arnaudi from the Late Cenomanian of North Africa (Eulàlia et al. 2003).

Figure 5. Historic high explosive crater at former Pueblo Precision Bombing Range #2, feature is approximately 1.5 m in depth and 4.5 m in diameter. Note heaped up fragments of the basal bed of Bridge Creek Limestone which forms raised berm around crater. Fragmented colonial rudist speci-men FHSM IP-1506 was discovered at location of GPS unit (circle) at left.

Figure 6. a) Four fragments of rudist colony lower valves (FHSM IP-1506), pieces at right and left in various lateral/medial views and pieces in middle and bottom in dorsal view; b) two conjoined lower valves in dorsal view, note lateral crushing.

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Pre-Coniacian records of rudists in the Western Interior are rare and only seven previous occurrences have been noted (Cobban et al. 1991; Schumacher 2011). Regarding Cenomanian occurrences, only scant records of radiolitid rudist fragments are mentioned from the Graneros Shale and Greenhorn Limestone of south-central Colorado and northern New Mexico (Cobban et al. 1991). Previous records lack taxonomic and/or stratigraphic detail, and FHSM IP-1506 represents the only well documented rudist specimen reported from Cenomanian strata of the Western Interior Seaway (WIS). Thus the appearance of radiolitid rudist bivalves coupled with a sudden change in depositional character (see Fig. 3) of the Greenhorn Limestone is suggested to reflect the onset of mid-Cretaceous global warming (Caldeira 1991; Bralower et al. 2002) although further testing of this supposition is needed. The appearance of radiolitid rudists at this particular stratigraphic interval is especially curious, just prior to onset of global oceanic anoxic event II (Kauffman 1995; Sageman et al. 2006).

Rudists populations are concentrated in tropical waters of equatorial latitudes, generally around what are now Meso-America, the Mediterranean, the Middle East, and Southeast Asia (Arthur et al. 1996). Rudists in the WIS occur as isolated colonies or single individuals, and are generally considered to be fringe populations that expand northward in relatively small numbers during favorable environmental conditions (Hattin 1982). In the interest of filling in geographic and stratigraphic gaps, I note here to have located several rudist fragments attributable to Ichthyosarcolites from the middle portion of the Pierre Shale in the northern Black Hills, South Dakota. Also of note are the occurrences of specimens attributable to Durania maxima Logan from the Fort Hays Limestone Member (FHSM IP-1507) and lower Smoky Hill Chalk Member, Niobrara Formation (FHSM IP-1472, FHSM IP-1473) of southeastern Colorado.

Figure 7. Limestone sample from FHSM IP-1506 locality, exhibiting molds of Sciponoceras gracile (top) and Calycoceras cf. C. naviculare (bottom).

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Conclusion

A colonial monospecific cluster of rudist bivalves from the lowermost Bridge Creek Limestone Member (Upper Cenomanian), Greenhorn Limestone of southeastern Colorado are attributable to Durania cf. D. cornupastoris. The specimens were located in bedrock debris rimming the edge of a blast crater created by aerial bombing practice during World War II. This discovery marks only the eighth recorded pre-Coniacian occurrence of rudist bivalves in the Western Interior (Cobban et al. 1991; Schumacher 2011) and the only Cenomanian record of Durania in North America. The appearance of rudist bivalves in the Bridge Creek Limestone may correlate to the onset of warmer temperatures in the latter half of the Cretaceous Period.

Acknowledgments

Thanks to Kevin Lindahl for prompting my interest in the historic bombing range, which led to the inadvertent discovery of the rudist colony.

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