TRANSACTIONS OF THE KANSAS Vol. 115, no. 3-4 ACADEMY OF SCIENCE p. 117-124 (2012) Bombers and Bivalves: First North American occurrence of the rudist Durania sp. (Bivalvia: Radiolitidae) in the Upper Cretaceous (Cenomanian) Greenhorn Limestone of southeastern Colorado Bruce A. Schumacher USDA Forest Service, 1420 E. 3rd St., La Junta, CO 81050 [email protected] 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 Introduction The Greenhorn Limestone in southeastern Colorado (Fig. 3) is divided into the three Some seventy years ago southeastern Colorado subunits (Cobban and Scott 1972; Hattin 1975; was utilized during World War II (1943 – 1945) Kauffman 1986). Roughly the lower two-thirds as a training area for precision bombing practice of the unit is comprised of the basal Lincoln and air-to-ground gunnery. The La Junta Limestone Member (5 m) and the Hartland Municipal Airport was created in April 1940 as Shale Member (19 m). The dominant lithology La Junta Army Air Field (Thole 1999) and was of the lower members is calcareous shale with used by the United States Army Air Forces for minor amounts of thin calcarenite beds. The an advanced flying school and training base for Bridge Creek Limestone Member (Figure B-25 Mitchell bomber crews (Fig. 1). In addition 4) forms the upper one-third of the unit (13 to training American pilots, Chinese pilots m) and consists of resistant chalky limestone 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 Figure 1. Restored B-25D-30 Mitchell medium of nine bombing targets plus submarine and bomber known as “Miss Mitchell”. Various pro- ship skip-bombing targets have been located duction versions of such planes were delivered (Foley 2008) using LiDAR imagery (Figure 2). to the USAAF’s advanced flying school at the Historic use of these areas is still evident in the 402nd Army Air Force Base Unit at La Junta, form of metallic bomb debris and extensively Colorado in October, 1943 (photo used with cratered pasture lands from high explosive permission of Abbots International Airshow ordinance. Society, John T. Sessions Chairman). 118 Schumacher 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. layers (5 – 28 cm) interbedded with soft and The summer of 2011 saw intense drought in thinly stratified intervals of calcareous shale southeastern Colorado and reduced vegetation (25 – 150 cm). The base of the Bridge Creek made many of the historic bomb craters at Limestone is marked by the thickest of its the Pueblo Precision Bombing range visible. limestone beds (28 cm), a characteristic light In some places the high explosive ordnance gray micritic limestone (BC-1) which exhibits welled up underlying limestone layers of conchoidal fracture and contains various the Greenhorn Limestone as ejecta berms and abundant ammonite impressions of the surrounding the blast craters, in particular Sciponoceras gracile biozone. The BC-1 bed has in those places where the basal Bridge Creek been miscorrelated as the Fencepost limestone limestone bed (BC-1) lay in the shallow bed of central Kansas due to its thickness and subsurface. Craters in such locations are resistant nature. However bed BC-1 is much surprisingly evident given seventy years of older than the Fencepost Limestone bed and weathering due to the resistant nature of the correlates with the lower Hartland Shale of BC-1 limestone bed. In May 2011 a small central Kansas (marker bed HL-1, Hattin colony of rudist bivalves (FHSM IP-1506) was 1975). Additionally, the Bridge Creek bed discovered by the author amid the broken up BC-1 is durable but brittle. Fresh exposures of limestone around one such crater (Fig. 5). BC-1 will initially yield regular jointed blocks, but upon weathering develops abundant Rudists are a group of tube or ring shaped conchoidal fractures. Bed BC-1 is locally used marine heterodont bivalves that arose in the as road material though it is hard on tires, and Jurassic and became prolific reef builders has only been employed in a few local instances in tropical waters during the Cretaceous. to construct small buildings. Although better known in younger Cretaceous Transactions of the Kansas Academy of Science 115(3-4), 2012 119 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 Figure 3. Composite measured section of just above basal limestone bed (BC-1) of Bridge Creek Limestone Member, Greenhorn Bridge Creek Limestone (Upper Cenomanian), Limestone compiled from several locations in Greenhorn Limestone, south-central Otero the Otero County portion of the Comanche Na- County, Colorado. The basal limestone bed tional Grassland. Location of rudist colony and contains a relative abundance of ammonite select ammonite occurrences noted. molds including Calycoceras cf. C. naviculare, less calcareous, thus it is likely that the rudists Kanabiceras septemseriatum, Metoicoceras sp., occurred above BC-1. There is no evidence of and Sciponoceras gracile (Fig. 7). epifauna such as oysters or other invertebrates attached to the valves which is atypical for Description: Light yellow, fine grained younger and more common rudist occurrences calcareous matrix clings to the specimen, in the Western Interior Seaway (Hattin 1988). unlike the light gray, sparry limestone matrix A slight curvature to the valves suggests of the basal bed (BC-1) of the Bridge Creek the individuals trended toward a partially Limestone. Beds below the basal Bridge recumbent position during growth. Creek in this area are typically darker and 120 Schumacher 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. The specimen includes fragmented portions – 1 mm) and thin pentagonal and hexagonal of minimally five individual lower valves. The plates, referred to as the “sauvagesiine conjoined nature of the valves leaves little doubt polygonal cell network” typical of radiolitid that the individuals form a single clustered rudists (Cobban et al. 1991). There exists no colony as is typical of rudists (Kauffman and vestige of an arête cardinal or other in-folding Sohl 1974). Individual lower valves are cone- of the lower valve as noted in other rudist shaped, tapering to a small area at the base, families. The inner surface of valves is smooth, with uncrushed rim diameters of from 20 to but in the form of dark and light tones there 25 mm and heights from 80 to more than 100 are extensive bifurcating networks of radiating mm. Closely spaced, uniform vertical ridges or ‘vascular’ impressions frequently noted on grooves 2 mm in width surround the exterior the inner valve rims of Durania (Skelton and surface of valves and form a distinct zigzag Wright, 1987). suture on the dorsal rim where two valves remain joined. More widely spaced horizontal Remarks: The closely spaced, regular ribbing growth rugae cut across the vertical ribbing, as and clear polygonal cell structure on the many as eight times observed on one individual. dorsal rim of lower valves identify them as sauvagesiine radiolitids, and the absence of Where well preserved, the dorsal rim (growth inward folds of the valve walls place them in surface) of lower valves is formed by small (0.5 the genus Durania (Douvillé, 1910; Cobban Transactions of the Kansas Academy of Science 115(3-4), 2012 121 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.