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Pages 172 To FOSSIL MARINE MAMMALS OF THE LOWER MIOCENE POLLACK FARM SITE, DELAWARE1 David J. Bohaska2 ABSTRACT The Pollack Farm Site, a borrow pit (now covered) near Cheswold, Delaware, in the Cheswold sands of the lower Miocene Calvert Formation, produced a fragmentary marine mammal fauna. At least six cetaceans are present: five porpois- es comprising a squalodont, Squalodon calvertensis, the squalodelphid Phocageneus venustus, the long-beaked platanistid Zarhachis flagellator, a rhabdosteid Rhabdosteus sp., and a small kentriodontid; and a sperm whale. Also present is a dugong, and one of the earliest records of a true seal, Leptophoca lenis. Although less diverse than the Calvert fauna as known from Maryland and Virginia, this is most likely a product of sampling. The absence of the common Calvert porpoise Eurhinodelphis from the Pollack Farm Site may be significant; the absence of mysticetes (whalebone or baleen whales) matches their unex- plained absence from beds of similar age in the lower Calvert of Maryland and Virginia. INTRODUCTION Simonson not only for providing many of the specimens used The Pollack Farm Site (Delaware Geological Survey in this and other papers but for his logistical support. locality Id11-a), a borrow pit excavated near Cheswold, Specimens cited in this paper were donated by David Delaware, for road bed material, exposed the Cheswold Duke, Gordon Simonson and Lauck W. Ward, and loaned by sands of the lower Miocene Calvert Formation (Ramsey et John Clark, Thomas R. O’Hagen and John Ricard. al., 1992). The site was the subject of intensive field study Additional cetacean specimens were donated by the above and fossil collecting from November 1991 through May and Kenneth Webb, and loaned by David L. Anker, Michael 1992 by field crews from the Delaware Geological Survey, Miskofsky, Mark L. Richerson, and Greg Turner. Virginia Museum of Natural History, and Smithsonian Lauck W. Ward introduced me to the site, and Ralph E. Institution, and members of the construction crews and ama- Eshelman and Robert W. Purdy collected for the Smithsonian. teur fossil collectors. Frederick Von Hofe Grady’s skill in processing and picking The beds at the site are equivalent in age to Bed 3A of the bags of concentrated matrix added several marine mammal the Fairhaven Member (diatomaceous earth) of the Calvert specimens to the collection. Use of the Calvert Marine Formation in Maryland (Ward, 1993; Jones et al., 1995) and Museum collection was courtesy of curator Michael D. are discussed in more detail in other papers in this volume Gottfried. Norman L. Riker permitted examination of his (Benson, 1998; Ramsey, 1998, Jones et al., 1998). On the Popes Creek Sand Member collection, and he donated basis of stratigraphic position, they are possibly the same age Phocageneus specimens to extend the known range of that as the Popes Creek Sand Member (Gibson, 1983) of the genus. William “Pat” Gotsis found the first known physterid in Calvert Formation (Wetmore and Andrews, 1990; Gibson and the Popes Creek Sand while this paper was in preparation. Andrews, 1994), which does contain a diverse vertebrate Mario A. Cozzuol first recognized the complete peri- fauna (largely unpublished). Specimens from the Popes Creek otic described as physeterid, and Clayton E. Ray the single Sand are cited within this paper as the earliest occurrences of seal bone. I discussed squalodonts with Alton Dooley, sire- the represented taxa within the Calvert Formation in Maryland nians with Daryl Domning and sperm whales with James G. and Virginia and the nearest in age to those from the Pollack Mead. Farm Site. Unlike Bed 3A and the Popes Creek Sand, the Photography was by Peter Kroehler, Laurie Minor- Pollack Farm Site includes beds with a well preserved, abun- Penland, and Victor Krantz. dant, and diverse molluscan fauna (Ward, 1998). The site also Clayton E. Ray and Frank C. Whitmore, Jr., reviewed produced a diverse vertebrate fauna, distinguished from the the manuscript and provided many useful comments. other Chesapeake Group outcrops of Maryland and Virginia in Funding was provided by the National Geographic containing a more diverse terrestrial mammal, terrestrial rep- Society and the Remington and Marguerite Kellogg tile, and amphibian assemblage, adding many new elements to Memorial fund. the Calvert fauna. By contrast, the marine mammal collection, the subject of this paper, is more fragmentary and less diverse COLLECTING METHODS AND than the marine mammal fauna from the Calvert Formation of METHODS OF INVESTIGATION Maryland and Virginia, lacking the more nearly complete Identifications of the scrappy material in this report are skulls and skeletons found there. based primarily on comparisons with the Miocene Calvert fauna of Maryland and Virginia in the collections of the Acknowledgments Department of Paleobiology, National Museum of Natural Field work at the Pollack Farm Site was a pleasant expe- History, Smithsonian Institution (USNM) and of the Calvert rience owing to the cooperation of the crews from the Marine Museum (CMM-V). Since sampling techniques Delaware Department of Transportation, Pierson Engineering, undoubtedly affect conclusions regarding relative abun- and Century Engineering. Particular thanks go to Gordon dances, a description follows. 1 In Benson. R.N., ed., 1998, Geology and paleontology of the lower Miocene Pollack Farm Fossil Site, Delaware: Delaware Geological Survey Special Publication No. 21, p. 179–191. 179 Collecting was done by careful inspection of spoil roots of molariform tooth, original found by and in collec- piles, particularly those rich in material derived from the tion of J. Ricard. shell beds. Spoil pile and in situ shell bed matrix was screen Description.—The presence of at least one of the het- washed and inspected in the field; all fine concentrate and erodont squalodonts or shark-toothed porpoises is most evi- most of the coarse concentrate was fully picked in the labo- dent in the several molariform teeth in the collection ratory. Picking of some of the fine concentrate (particularly (Fordyce [1982] argued that premolars and molars cannot be that collected late in the field work) was facilitated by dis- differentiated in squalodonts, so the term “molariform” is solving most of the carbonate shell fragments with acetic used here). USNM 487256 (Plate 1A) and USNM 488602 acid following a first picking to remove most of the identifi- (Plate 1B) preserve the double roots, and the triangular, lat- able mollusks and otoliths. All vertebrate material collected erally compressed, wrinkled enamel crowns of the molari- by the three institutions was retained. form teeth. The characteristic cusps along the cutting edges Although most material described in this report was can be seen as one cusp on the posterior edge of USNM found loose in spoil piles, or received with no specific data, 487256, parts of two and possibly three cusps on USNM most was probably derived from the lower shell bed (see 487257, and hints of cusps on the broken edges of USNM Benson, 1998, fig. 2), since spoil of this material was most 488602. In both USNM 487256 and USNM 488602, the two intensively inspected. All terrestrial mammals found are con- roots are joined for a distance below the crown, more so in sistent with one age, early Hemingfordian (Emry and USNM 487256 (22 mm vs. 19 mm measured on the lingual Eshelman, 1998), confirmed by the early Miocene age based side). USNM 488602 has swollen-appearing roots, owing to on radiolarians from a sandy silt between the lower and a thickened layer of cementum. upper shell bed (Benson, 1998) and the strontium-isotope The conical anterior incisors (USNM 488600, Plate age estimate of 17.9 Ma on mollusks from the lower shell 1C,D; USNM 487253) are large (compared with typical por- bed (Jones et al., 1998). poise teeth), have prominent anterior and posterior cutting Almost all of the vertebrate fossils found by amateurs edges (particularly visible on USNM 488600, Plate 1D), and were inspected by Smithsonian staff. Because cetacean wrinkled enamel. The enamel-root boundary is sigmoid bones and teeth tend to be simpler, they are inherently less (most visible on USNM 487253) typical of, but not restrict- identifiable from teeth, postcranial, and scrap material than ed to, squalodonts. All are interpreted as anterior incisors terrestrial mammals. More nearly complete cetacean materi- because they are relatively straight; reconstructions of al (skulls, jaws, and skeletons) has been and is found regu- Squalodon show these teeth extending directly anteriorly larly in Maryland and Virginia. For these reasons, cetacean from the end of the rostrum and mandibles. scrap was not solicited for donation or loan as rigorously as USNM 487254 also appears to be a squalodont anteri- terrestrial mammals, which are more poorly known from or incisor. The enamel-root boundary appears to be sigmoid Maryland and Virginia. As a result, the cetaceans represent- (wear prevents this from being certain). The wrinkled nature ed in the institutional Pollack Farm Site collections are lower of the enamel is only debatably visible; the little enamel pre- in number of specimens than would be preserved relative to served is almost entirely worn smooth. Like the molar land mammals in a marine deposit. No actual counts were USNM 488602, the root (single in this case) has a thick layer made, but cetacean scrap was observed more often in the of cementum. field and in amateur collections than land mammals. Part of the crown is missing on USNM 488601, reveal- Potentially identifiable cetacean specimens (teeth, and jaw, ing a 20-mm-deep conical cavity diminishing to a point skull, and limb fragments) were solicited on par with terres- toward the root. Were it not for a 4 mm x 4 mm patch of prob- trial mammal specimens, but vertebrae and ribs generally able enamel preserved on the crown, the cavity could be inter- were not.
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