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Xiphactinus vetus and the distribution of Xiphactinus species in the eastern United States

1997; Taylor & Francis; Volume: 17; Issue: 3 Linguagem: Inglês

10.1080/02724634.1997.10011007

1937-2809

David R. Schwimmer, J. D. Stewart, G.D. Williams,

Paleontology and Evolutionary Biology

Los Angeles County Museum of Natural History, 900 Exposition Boulevard, Los Angeles. California 90007The ichthyodectid genus Xiphactinu.i includes the largest teleost fishknown in North American Late Cretaceous strata. The single speciescurrently recognized in North America (Bardack, 1965) is Xiphactinusaudax, which has been reported to range geographically from Saskatch-ewan to west Texas within the Late Cretaceous interior seaway, andfrom north-central Texas to New Jersey on the Gulf and Atlantic CoastalPlains. The stratigraphic range of X. audax, as the species is presentlyconstrued, is Cenomanian to lower Maastrichtian (Russell, 1988, 1993).Many natural history museums display specimens or casts of com-plete individuals of Xiphactinus audax, including several noteworthylarge (over 4.0 m) specimens with a well-preserved Gillicus arcuatusor other fish in the gut area (Figure 1). Most museum display specimensof X. audax come from the Niobrara Chalk Formation in western Kan-sas, where this species is common and often well preserved.Joseph Leidy and Edward D. Cope independently described the taxonas Xiphactinus audax Leidy 1870 and Portheus molossus Cope, 1871.Although Cope's type specimen was a finely preserved individual,whereas Leidy's type was an isolated pectoral fin spine, the rules ofpriority (Ride, et al. 1985) require that the widely-known name Por-theus molossus be suppressed as a junior synonym. In their early de-scriptions of Xiphactinus and Portheus, Leidy and Cope followed pre-vailing practice and recognized numerous species, all of which, at leastfor North America, were subsequently subsumed into the single speciesX. audax by Bardack (1965). which was the last substantive taxonomicanalysis of the genus.The type description of Xiphactinus audax by Leidy (and that ofPortheus molossus by Cope), and most subsequent studies of X. audax,were based on specimens from the upper Coniacian to lower CampanianSmoky Hill Member of the Niobrara Chalk. Well-preserved Xiphactinusaudax specimens, although incomplete, are also found in the ConiacianEctor Member of the Austin Chalk in north-central Texas. Youngerfossils of Xiphactinus in Western Interior deposits include common, butusually poorly preserved, incomplete skeletons from the lower PierreShale, including the lower Campanian Sharon Springs Member in Kan-sas, Nebraska, South Dakota and Wyoming (Russell, 1988), and thelower Campanian Pembina Member in Manitoba (Bardack. 1968; Nich-olls and Russell, 1990).Xiphactinus is also widely reported from the Gulf and Atlantic Coast-al Plains, either as Xiphactinus sp. or X. audax. Reported occurrencesinclude the upper Campanian Demopolis Formation in northeasternMississippi (Manning and Dockery, 1992), the middle CampanianBlufftown Formation in western Georgia (Case and Schwimmer, 1988),the upper Campanian Black Creek Formation in North Carolina (Robb,1989), and the upper Campanian Marshalltown Formation and the lowerMaastrictian Mount Laurel and Navesink formations in New Jersey(Gallagher, et al., 1986, Gallagher, 1993) and Delaware (Lauginiger,1986). Most of these remains ar e isolated teeth and vertebrae, althoughrare jaw fragments are known (Fig. 2D, U). Attribution of these easternCoastal Plain fossils to Xiphactinus audax is generally based on thepresence of large, relatively slender, hollow-based teeth, with thin,smooth, glossy enamel (see Fig. 2F—Z). Such teeth come presumablyfrom the premaxilla, anterior maxilla or anterior dentary. Also foundassociated in the same strata are proportionately large, teleostean ver-tebral centra (Fig. 3G-L). These centra have paired dorsal and (in some)ventral depressions for neural and hemal arches, which are not fused tothe centra, and two additional pairs of dorso-lateral and lateral depres-sions. This morphology, again, is typical of Xiphactinus audax and otherIchthyodectidae (Patterson and Rosen. 1977).A SECOND NORTH AMERICAN SPECIES OF XIPHACTINUSWe have determined that two. clearly differentiable forms of largeichthyodectid teeth and vertebrae are present in the eastern Coastal PlainUpper Cretaceous outcrop. Both tooth types (Fig. 2) include large in-dividuals (crowns measured to 6.5 cm length and extrapolated to 8.0cm from broken specimens) with glassy, smooth enamel surfaces. Onetooth form (Fig 2D. E) is characteristic of Xiphactinus audax (Fig. 2A—C) in being nearly straight, with a round to oval cross section andanterior and posterior borders without carinae. Deposits including thistooth form also contain axially round ichthyodectid vertebrae (Fig. 3C—F), which are indistinguishable from typical X. audax (Fig. 3A. B).These X. audax morphologies have been observed in the lower Moore-ville Formation in western Alabama (Applegate, 1970), the TombigbeeMember of the Eutaw Formation in eastern Mississippi and central Al-abama (Fig. 3F), and in the lower Blufftown Formation in eastern Al-abama (Fig. 3C-E). These stratigraphic occurrences range from the mid-dle Santonian to the lowermost Campanian stage (chronologically upthrough Nannofossil Zone 19 of Sissingh, 1977).The second large ichthyodectid attributed here to Xiphactinus hasenlarged teeth that are moderately compressed laterally, slightly re-curved (presumably posteriorly and medially), with an anterior or an-tero-lingual carina. Many teeth also have a second, posterior, carina.Some specimens have their external surfaces configured into full-lengthfacetts (e.g.. Fig. 2F-I, Q, R), whereas others are smooth on lingual andlabial surfaces (e.g. Fig. 2M—P, S, T). Six large ichthyodectiform ver-tebrae have been found at one locality in western Georgia, in associationwith more than twenty-five teeth of this second Xiphactinus morphology(Fig. 3G—L). These vertebrae are oval, rather than round, in axial profile,and two (presumably posterior) vertebrae have a flat or slightly concaveventral surface (e.g. Fig. 3G-J). All vertebrae in this assemblage aremarkedly lower dorso-ventrally than are vertebrae of Xiphactinus audax(compare Fig. 3K with Fig. 3B). This second ichthyodectid tooth andvertebral morphology appears in middle Campanian to lower Maastrich-tian deposits in the eastern Gulf of Mexico and Atlantic Coastal Plains(as listed below under Systematics). Most significantly, all occurrencesof the faceted, carinate large teeth and the single known site with flat-tened and oval ichthyodectid vertebrae are in strata younger than alloccurrences of typical Xiphactinus audax. This is true for both the West-ern Interior and the eastern Coastal Plains: X. audax in its entire geo-graphic range occurs in the Turonian through lower Campanian, where-as the ichthyodectid with carinate tooth morphology first occurs in themiddle Campanian.We believe the younger taxon, thus far known only in the easternUnite d States an only from isolate teeth , vertebrae a single jawfragment, i s a separate specie properly named Xiphactinus (Polygon-odon) vetus (Leidy . 1856) Leidy's generic nam Polygonodon precededXiphactinus and , therefore by genera l taxonomic protoco al Xiphac-tinus species shoul d b e replace into Polygonodon. However, sinc Xip-hactinus is in very common use and since Leidy considered Polygon-odon to be a reptile genus, we believe that Articles 23(b) and 79 (thefifty-year rule) of the International Code of Zoological Nomenclature(Ride, et al., 1985) justify retention of Xiphactinus and suppression ofthe generic name Polygonodon. A ruling on this matter will be request-ed from the I.C.Z.N.. and the systematics below assume that permissionis granted to suppress Polygonodon in favor of Xiphactinus.Institutional abbreviations: AMNH. American Museum of NaturalHistory, New York City, New York; ANSP. Academy of Natural Sci-