stratigraphic distribution and habitat segregation of ... - BioOne

Journal of Vertebrate Paleontology 22(1):91–103, March 2002 q 2002 by the Society of Vertebrate Paleontology

STRATIGRAPHIC DISTRIBUTION AND HABITAT SEGREGATION OF MOSASAURS IN THE UPPER CRETACEOUS OF WESTERN AND CENTRAL ALABAMA, WITH AN HISTORICAL REVIEW OF ALABAMA MOSASAUR DISCOVERIES CAITLIN R. KIERNAN #304 Liberty House, 2301 1st Avenue North, Birmingham, Alabama 35203-4320, [email protected]

ABSTRACT—A survey of mosasaur material from the Eutaw Formation (Santonian) and Selma Group (Late Santonian–Late Maastrichtian) of western and central Alabama has revealed significant stratigraphic segregation among taxa. Three distinct biozones are recognized based on this survey: the Tylosaurus Acme-zone (from the base of the Tombigbee Sand Member of the Eutaw Formation to approximately 12 m above the base of the Mooreville Chalk), the Clidastes Acme-zone (from approximately 12 m above the base of the Mooreville Chalk to the base of the Demopolis Chalk), and the Mosasaurus Acme-zone (from the base of the Demopolis Chalk to the K-T boundary at the top of the Prairie Bluff Chalk). In each case, the biozone is named for the genus comprising the majority of the specimens examined from that horizon. The previously-recognized Globidens alabamaensis Acme-zone, including the Arcola Limestone Member of the Mooreville Chalk, has been abandoned. The transition from the Tylosaurus biozone to the Clidastes biozone has been interpreted primarily as an artifact of paleoecology, reflecting habitat preference; additional support for this interpretation has come from the mosasaur fauna of the Blufftown Formation (?Late Santonian–Early Campanian) of eastern Alabama and western Georgia. The transition from the Clidastes biozone to the Mosasaurus biozone appears to represent the local expression of a reorganization within North American mosasaur communities. The uppermost portion of the Selma Group, comprised of the Ripley Formation and Prairie Bluff Chalk, has been tentatively included in the Mosasaurus Acme-zone on the basis of poorly-known faunas.

sented before the Southeastern Section of the Geological Society of America by G. L. Bell, Jr. (1985). Although refined versions were later read before the 63rd annual meeting of the Alabama Academy of Science and the Fourth North American Paleontological Convention (Wright 1986a, 1986b, respectively), this is the first time the database justifying the establishment of mosasaur biozones in western and central Alabama, as well as an attempt at explaining these phenomena, has been presented in detail.

INTRODUCTION Between October 1982 and October 2000, the author examined more than 600 mosasaur specimens from the Tombigbee Sand Member (Eutaw Formation) and Selma Group of west and central Alabama. This allowed the construction of a large database organized by stratigraphic units. Only specimens identifiable to genus and accompanied by reliable locality and stratigraphic records were included. Degree of completeness was not a criterion for selection of specimens for the sample. Material included ranges from isolated teeth and bones (or fragments of bones) to virtually complete individuals. The total sample meeting these criteria was 448 specimens. Considering the generally large size of mosasaurs, collection bias in the interpretation of the relative abundance of taxa is not believed to be a problem for this study. Material from the following collections was examined during the course of this study: Philadelphia Academy of Science (ANSP); Auburn University (AUMP); Field Museum of Natural History (P, PR); Geological Survey of Alabama (GSA-V, GSATC); Red Mountain Museum (RMM); Alabama Museum of Natural History (ALMNH PV); United States National Museum (USNM); Uppsala Palaeontological Institute (UPI); and University of New Orleans (UNO). The specimens comprising the database were collected from a ten-county area in west and central Alabama (Fig. 1) including (in order from west to east) Pickens, Sumter, Greene, Marengo, Hale, Perry, Wilcox, Dallas, Lowndes, and Montgomery counties. The concept of successive North American mosasaur faunas was first expressed by D. A. Russell (1967) and first applied to the Upper Cretaceous of Alabama by S. W. Shannon (1975, 1977). The author began a refinement of the concept late in 1984, which resulted largely from the opportunity to examine significant collections of Alabama mosasaurs recovered outside the Mooreville Chalk. The resulting model of mosasaur biostratigraphy within the Eutaw Formation and Selma Group, including the erection of four successive biozones, was first pre-

GEOLOGY Within the study area, a virtually unbroken series of Upper Cretaceous sediments is exposed at the surface, from the basal beds of the Tuscaloosa Group (Cenomanian; Raymond et al., 1988) to the K–T boundary (Figs. 1, 2). In ascending order, these stratigraphic units are the Tuscaloosa Group; the Eutaw Formation (lower unnamed member and Tombigbee Sand Member); and the Selma Group (Mooreville Chalk Formation [lower unnamed member and Arcola Limestone Member], Demopolis Chalk Formation [lower unnamed member and Bluffport Marl Member], Ripley Formation, and Prairie Bluff Chalk Formation). No mosasaurs have been recovered from pre-Tombigbee Sand beds, owing largely to their nonmarine origins, poor exposure, and generally nonfossiliferous nature. Cretaceous marine lithostratigraphic units in Alabama exhibit lateral facies changes from clastic sediments in the eastern part of the state to predominantly carbonate sediments in the west, shifting back to clastics in northeastern Mississippi. These shifts reflect increasing and decreasing proximity to sources of clastic sediment input. Most of the chalky facies within the study area have southeastern and northwestern clastic equivalents. The Tombigbee Sand Member is the earliest post-Paleozoic unit exposed in Alabama containing an abundance of marine macrofossils. In the study area, it varies in thickness from about 1.5 to 6.0 m and consists of a clayey, highly glauconitic sand with localized beds of calcareous sandstone and sandy chalk.

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FIGURE 1.

Geological map of the ten-county study area in western Alabama.

The Tombigbee Sand contains abundant mollusks, including the ostreids Exogyra upatoiensis and E. ponderosa. Puckett (1996) reviewed efforts to date the Tombigbee Sand on the basis of calcareous nannoplankton, planktonic foraminifera, and ammonites and assigned it a Santonian age. The contact between the Tombigbee Sand and the overlying Mooreville Chalk Formation is conformable but appears to be diachronous, ranging from Middle Santonian in Dallas County, Alabama to Latest Santonian–Earliest Campanian at the type locality near Columbus, Mississippi. Soens (1984) concluded that the Tombigbee Sand was deposited during a transgression, in inner shelf (in the lower beds) to middle shelf (in the upper beds) conditions. In eastern Alabama, the Eutaw Formation is undifferentiated and the Tombigbee Sand is replaced by the basal beds of the clastic Blufftown Formation. The lower unnamed member of the Mooreville Chalk consists primarily of fossiliferous clayey compact chalk and chalky marl, with beds of clayey, glauconitic marl and calcareous sand near the base. Within the study area, the unit ranges in thickness from 82 m in western Alabama to about 183 m in Montgomery County (Raymond et al., 1988). Invertebrate diversity is lower than in the Tombigbee Sand, perhaps due to the soft substrate and dysoxic bottom conditions. Several species of pteriomorph pelecypod are common, including small, thin oysters such as Ostrea plumosa and Agerostrea falcata and much larger Exogyra spp. and inoceramids. Puckett (1996) reviewed efforts to date the lower Mooreville based on planktonic forams, calcareous nannofossils, and ostracodes and assigned the unit to the Late Santonian–Early Campanian. Wylie and King (1986) concluded that the primary depositional setting was a poorly-oxygenated, middle-shelf environment, below storm wave base. In

eastern Alabama, the Mooreville Chalk grades into its lateral clastic equivalent, the Blufftown Formation. The Arcola Limestone Member of the Mooreville Chalk consists of two to four beds of impure, fossiliferous limestone intercalated with marl beds and is about 3 m thick in the study area (Raymond et al., 1988). Based on the presence of the ostracode Ascetolebris plummeri, Puckett (1996) placed the Arcola Limestone within the A. plummeri Interval Zone of Hazel and Brouwers (1982) of Late Early–Early Middle Campanian age. The origin of the Arcola Limestone remains controversial, but Lins et al. (1977) interpreted the small (40 to 60 mm) calcispheres that comprise the bulk of the limestone beds as reproductive spores of benthonic dasycladacean algae. The Arcola may have resulted from the uplift of the Monroe-Sharkey Platform (western Mississippi, northeastern Louisiana, and southeastern Arkansas) which caused a period of cessation of upwelling nutrients on which nannoplankton fed, allowing the benthonic algae to grow. Florian (1984) concluded that, since dasycladacean algae require very shallow, clear water, the Arcola was likely deposited at depths of not more than 6 m. Lithology of the lower unnamed Member of the Demopolis Chalk ranges from marls indistinguishable from the uppermost Mooreville (below the Arcola) low in the section to purer chalks composed chiefly of calcareous nannoplankton higher in the section (Puckett, 1996). In the study area, thickness varies from about 151 m in Sumter County to 128 m in Montgomery County. As in the Mooreville Chalk, pteriomorph pelecypods (Ostrea spp., Inoceramus sp.) are the most common invertebrate macrofossils. Smith and Mancini (1983), Taylor (1985), and Puckett (1996) have assigned a Late Early Campanian– Early Maastrichtian age to the lower Demopolis Chalk on the

KIERNAN—ALABAMA MOSASAURS basis of calcareous nannoplankton, planktonic foraminifera, and ostracodes. Puckett (1996) places the Campanian–Maastrichtian boundary 91.4 m above the base of the Demopolis Chalk based on the highest occurrence surface of the planktonic foram Globotruncanita calcarata. The lower Demopolis Chalk marks a return to offshore, open-shelf carbonate deposition following the Arcola uplift. The Bluffport Marl Member of the Demopolis Chalk consists of chalky marl, very clayey chalk, and calcareous clay and varies from 15 to 20 m in thickness within the study area (Raymond et al., 1988). Large and small pteriomorph pelecypods are abundant throughout the unit, including Exogyra cancellata, E. costata, Pycnodonte mutabilis, Inoceramus sp., and Agerostrea falcata. Smith and Mancini (1983) dated the Bluffport Marl as Early Maastrichtian. Cagle (1985) examined the benthic foraminifera of the Bluffport Marl and determined that deposition occurred in an outer shelf setting at depths between 91.4 and 183.0 m. In eastern Alabama, the lower Demopolis is replaced by the clastic Cusseta Sand Member of the Ripley Formation and the Bluffport Marl is replaced by the lower unnamed member of the Ripley Formation. Within the study area, the Ripley Formation is an undifferentiated and heterogeneous unit consisting chiefly of massive glauconitic sand, sandy calcareous clay, sandy chalk, and thin beds of indurated fossiliferous sandstone. The unit varies in thickness from about 45 to 76 m but thins to only 10.6 m in Sumter County (Raymond et al., 1988). A diverse molluscan assemblage is present, and on the basis of calcareous nannofossils, Smith and Mancini (1983) assigned the basal Ripley to the Early Maastrichtian and the upper beds to the Middle Maastrichtian. Skotnicki and King (1989) discussed the wide range of depositional environments and concluded that the majority of the formation was deposited under lower shoreface and inner-shelf conditions. In western and central Alabama, the uppermost unit of the Selma Group is the Prairie Bluff Chalk, which rests disconformably on the Ripley Formation. Though thickness varies significantly, the formation reaches a maximum of 33.5 m in Lowndes County (Raymond et al., 1988). The dominant lithology is a sandy fossiliferous chalk which preserves a diverse invertebrate macrofauna. Smith and Mancini (1983) assigned a Middle–Late Middle Maastrichtian age to the Prairie Bluff on the basis of calcareous nannofossils. Moshkovitz and Habib (1993) have dated the uppermost Prairie Bluff Chalk at Braggs, in Lowndes County, as Latest Maastrichtian, based on the presence of the calcareous nannofossil Micula prinsii. The Prairie Bluff Chalk was deposited offshore and is overlain disconformably by the Paleocene (Danian) Clayton Formation. In eastern Alabama, the Prairie Bluff Chalk grades laterally into the Providence Sand and is replaced in northeastern Mississippi by the clastic Owl Creek Formation. HISTORICAL BACKGROUND Mosasaurs have been known from the Upper Cretaceous of Alabama since Gibbes (1850) described Mosasaurus minor from three vertebrae and two teeth from unspecified localities and horizons in Georgia and Alabama. The three vertebrae are probably referable to Platecarpus and both teeth are indeterminate. Furthermore, the holotype cannot be located, and on these grounds, Russell (1967:183–184) considered the species a ‘‘nomen vanum.’’ Gibbes (1851) described two additional taxa based in part on Alabama material, Amphorosteus brumbyi, based on two weathered and indeterminate vertebrae from an unspecified horizon in Alabama, and Holcodus acutidens, based on three teeth (only one of which is actually mosasauroid, probably Platecarpus) from unspecified sites in Alabama, New Jersey, and South Carolina. Again, owing to the inadequacy of the

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type material, Russell (1967:178–179) considered both taxa ‘‘nomina vana.’’ Michael Tuomey, the first state geologist of Alabama, presented the remains of a ‘‘fossil lacertian reptile, belonging to the genus Leiodon from the cretaceous of Alabama’’ (Tuomey, 1850) before the American Academy for the Advancement of Science. Discovered by Samuel Sherman of Howard College (now Samford University), the specimen was praised by Alexander Agassiz as ‘‘one of the most splendid additions to the paleontology of the United States ever made.’’ Unfortunately, the present whereabouts of the specimen is unknown and it may have been destroyed during the Civil War. E. D. Cope (1869) described Clidastes propython (ANSP 10193) based on a specimen collected from the lower unnamed member of the Mooreville Chalk in Lowndes County. A juvenile, it remains one of the best preserved and most complete mosasaurs collected from the state and has recently supplanted the indeterminate holotype of C. iguanavus (Cope, 1868) as the generic holotype of Clidastes (Kiernan, 1992). Cope (1869) described Platecarpus tympaniticus from a fragmentary specimen (ANSP 8484, 8487, 8488, 8491, 8558–9, 8562; a single specimen, including a surangular, quadrate, opisthotic, ?prootic, pterygoid, and three cervical vertebrae) from the Tombigbee Sand near Columbus, Mississippi. This report is important, not only because Platecarpus material recovered from the Tombigbee Sand and Mooreville Chalk of Alabama is identical, but because the name P. tympaniticus may prove to be a senior synonym for both P. ictericus (Cope, 1871) and P. coryphaeus (Cope, 1872). E. L. Nicholls (1988) and the author, while working with Pierre Shale and Niobrara Chalk specimens, independently concluded that Cope’s species were synonymous, the characters which Russell (1967:153–155) used to distinguish between the two being artifacts of individual variation and ontogeny, not phylogeny. All attempts to find characters separating the Alabama material from the Western Interior specimens have proven fruitless. If the three are indeed conspecific, Cope’s P. tympaniticus is the earliest applicable name for the taxon. Furthermore, the author has been unable to find characters distinguishing P. somenensis (The´venin, 1896) from P. tympaniticus and suspects that it too is a junior synonym of the type species. A thorough analysis of the problem has yet to be carried out, however, and although the author has chosen to refer all Platecarpus material from Alabama to P. tympaniticus, these conclusions remain provisional. Cope (1869–1870) reported two additional mosasaurs based on Alabama material; Liodon congrops and L. perlatus were based on a single cervical vertebra of Clidastes and another isolated vertebra probably belonging to Tylosaurus, respectively. Russell (1967:178, 184) considered both ‘‘nomina vana.’’ Joseph Leidy (1870) described Clidastes intermedius from the Mooreville Chalk of Alabama. Russell (1967:156) tentatively placed C. intermedius in the genus Platecarpus. However, the holotype (ANSP 9023–4, 9029, 9092–4; a single specimen, including three vertebrae and two dentary fragments with short, inflated teeth) compares favorably with material referred to Globidens alabamaensis. The vertebrae bear well-developed zygosphenes and zygantra and circular articular surfaces, traits characteristic of globidensines, generally absent or incipient in plioplatecarpines. In all likelihood, Leidy’s specimen is a subadult Globidens alabamaensis, and the author recommends that C. intermedius (5‘‘P.’’ intermedius of Russell, 1967) heretofore be regarded as a nomen dubium. Globidens alabamaensis was based on a very fragmentary skull (USNM 6527; right maxilla, splenial, basisphenoid, postorbitofrontal, and fragmentary frontal) and a single trunk vertebra from the ‘‘ . . . Bogue Chitto Prairies west of Hamberg . . . Perry and Dallas Co; Ala . . . ’’ (Gilmore, 1912:479). The description of the type locality makes it difficult to determine

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the holotype’s stratigraphic position, which could have been either the lower unnamed member or the Arcola Limestone Member of the Mooreville Chalk. Future analysis of sediment from USNM 6527 for calcareous nannofossils should resolve this problem. Renger (1935) described the excavation of a Tylosaurus skeleton from an undisclosed part of the Selma Group (probably the Mooreville Chalk), but this material cannot be located at present. Perhaps the finest skeleton of Platecarpus yet recovered from Alabama was described very briefly in a semi-technical account by H. G. Dowling (1941). The only published photograph of the specimen shows a complete, disarticulated skull, much of the axial skeleton, and forelimbs. The specimen was displayed for years in the Alabama Museum of Natural History and then removed by Dowling when he left the University of Alabama in the 1960s. Since 1975, numerous inquiries have been made to Dr. Dowling concerning the specimen’s whereabouts; all have gone unanswered, and its fate remains a mystery. The skeleton was reported to have come from exposures near Eutaw, in Greene County, Alabama, and therefore probably came from low in the Mooreville Chalk. The first attempt to systematically collect fossil vertebrates from the Selma Group was initiated in the summer of 1945 by the noted amateur collector C. M. Barber, who discovered a number of fragmentary fish and turtles from Mooreville Chalk exposures in Greene County, following an unsuccessful search for prospectable outcrops in the Late Cretaceous of Georgia. That same year, R. Zangerl returned with Barber to the Greene County sites and discovered more extensive outcrops farther east in the Marion Junction—Harrell Station area, about 10 mi west of Selma, Dallas County, Alabama. A second party from the Field Museum, including Barber, Zangerl, and W. D. Turnbull, returned to the Dallas County exposures in May 1946. The two seasons produced some 220 specimens of Mooreville Chalk vertebrates, including selachians, pycnodonts, teleosts, four families of marine turtles, three families of dinosaurs, and many mosasaurs (Zangerl, 1948a). The mosasaurs were described in a report by D. A. Russell (1970), who noted at least eight distinct species from the Mooreville Chalk, based on the material gathered by the Field Museum expeditions and earlier reports, including Halisaurus sternbergi, Clidastes propython, Globidens alabamaensis (although PR 196, identified as G. alabamaensis, is indeterminate), Platecarpus sp. (probably P. tympaniticus), ‘‘Platecarpus’’ intermedius (5Globidens alabamaensis, see above), Prognathodon sp., Tylosaurus sp., and Tylosaurus zangerli (the discovery of two well-preserved juvenile tylosaurs [RMM 3189, 5610] in beds roughly equivalent to those from which the holotype [P 27443] of this species came leaves little doubt that Russell’s tiny tylosaur humerus and femur belong to a subadult T. proriger and that T. zangerli is a nomen dubium). Throughout the late 1960s and 70s, specimens were collected sporadically, often as salvage operations after farmers or road crews reported fortuitous vertebrate finds to local universities. Most notably, some careful excavation and preparation was done by Drs. D. E. Jones (University of Alabama) and J. L. Dobie (Auburn University), each responsible for the recovery of a number of important mosasaur specimens from the Mooreville Chalk and Demopolis Chalk during this period. Shannon (1975, 1977) provided the first description of Alabama mosasaur material from units other than the lower unnamed member of the Mooreville Chalk. This includes the first reports of mosasaurs from the Tombigbee Sand Member of the Eutaw Formation in Alabama and the first reports of mosasaurs from the post-Mooreville Chalk units of the Selma Group, based on a small number of well-preserved specimens in the collections of the Geological Survey of Alabama, the Alabama

Museum of Natural History, and Auburn University. Tylosaurus proriger, Platecarpus cf. P. somenensis (5P. tympaniticus), and Mosasaurus cf. M. missouriensis were reported for the first time from Alabama (although the tylosaur reported by Renger [1935] was probably T. proriger). A new species of Clidastes, recently named C. ‘‘moorevillensis’’ (Bell, 1997), was recognized from the Mooreville Chalk, but the taxon is currently a nomen nudum. Shannon also noted the presence of a new genus of plioplatecarpine, Selmasaurus russelli (Wright and Shannon, 1988), but could not determine the stratigraphic position of the holotype and only known specimen due to poor locality data. In 1998, the author was permitted to extract a small amount of matrix from the basilar canal of the basioccipital, which was examined for calcareous nannoplankton. The sample yielded an abundance of nannofossils, including taxa indicating the specimen was collected from basal Campanian beds within the lower unnamed Mooreville Chalk (C. C. Smith, pers. comm.). The paleontological investigations of the Red Mountain Museum from the late 1970s through the late 1980s appreciably advanced knowledge of the Late Cretaceous vertebrate faunas of western Alabama. This collection is the largest made from this important and poorly-worked area, consisting of some three thousand cataloged Late Cretaceous specimens, with at least as many more awaiting curation. While most of this attention was focused on the well-exposed, highly-fossiliferous, and easilycollected lower unnamed member of the Mooreville Chalk, Red Mountain parties also collected the scarcer, less-productive, and less-accessible sites in the Eutaw Formation and upper Selma Group. For the first time, relatively large samples of the mosasaur assemblages of the Tombigbee Sand and Demopolis Chalk were recovered and studied. It was this expansion of the sample beyond the confines of the Mooreville Chalk that has made the old view of a ‘‘Selma Chalk mosasaur fauna’’ obsolete, permitting the development of a more comprehensive and dynamic picture of mosasaur evolution and paleoecology in the eastern part of the Mississippi Embayment. The Red Mountain collection also includes over one hundred subadult mosasaurs (see Bell and Sheldon, 1986; Sheldon, 1987), perhaps the largest such assemblage known, including subadult individuals of Clidastes, Platecarpus, Halisaurus, Tylosaurus, and Plioplatecarpus. In the case of Clidastes, this material permits the reconstruction of the ontogenetic development of a mosasaur genus from fetus to old age. The excellent preservation of these specimens facilitated innovative studies on mosasaur reproduction not previously possible (Sheldon, 1993, 1995). Wright (1988) referred a specimen (RMM 1578) from the lowermost beds of the Mooreville Chalk to Clidastes liodontus, the first record of this species outside the Smoky Hill Member of the Niobrara Chalk of Logan County, Kansas. K. Derstler (1988) made the first comprehensive report on the vertebrate fauna of the Bluffport Marl Member and uppermost beds of the lower unnamed member of the Demopolis Chalk based on specimens collected from western Alabama and northeastern Mississippi. The assemblage is dominated by Mosasaurus conodon and Plioplatecarpus sp. (probably P. primaevus) and also includes material referred to Prognathodon cf. P. solvayi. D. Burnham (1991) reported a possible new species of Plioplatecarpus (UNO 8611–2) from the lower member of the Demopolis Chalk in Sumter County, Alabama. In the early 1990s, funding for the vertebrate paleontology program at the Red Mountain Museum was suspended and this important collection remained in storage, unavailable to investigators, for almost a decade. Recently, the collection has been transferred to the McWane Center (Birmingham) and is once again accessible. Currently, the Alabama Museum of Natural History has the most active vertebrate paleontology collection and conservation program in the state.

KIERNAN—ALABAMA MOSASAURS MOSASAUR BIOSTRATIGRAPHY AND PALEOECOLOGY The author recognizes the following mosasaur biozones (Fig. 2) within the study area, superseding all previous descriptions of Alabama mosasaur biozones (Bell, 1985; Wright, 1986a, b; Russell, 1988): Tylosaurus Acme-zone Bell (1985) set the lower boundary of the Tylosaurus Acmezone at the base of the Tombigbee Sand Member of the Eutaw Formation and the upper boundary at a point 12 m above the base of the Mooreville Chalk. Within this biozone, Tylosaurus accounts for 57.7% of the total mosasaur sample; Platecarpus tympaniticus accounts for another 19.2%, Clidastes spp. 12.8%, and Halisaurus sternbergi the remaining 10.3% (Fig. 3). A typographical error in Wright (1986a) omitted the Tylosaurus Acme-zone, erroneously placing the lower boundary of the Clidastes Acme-zone at the base of the Tombigbee Sand. In the Cretaceous of the Alabama Coastal Plain, mosasaurs first appear in the Tombigbee Sand. It is assumed that their sudden appearance at this stratigraphic level is a result of the general scarcity of vertebrate fossils in older Cretaceous beds in western and central Alabama. The predominantly fluviatile origin of the Tuscaloosa Group (Cenomanian–?Coniacian; Raymond et al., 1988) probably precludes mosasaur finds from that unit. To date, the nearshore sands and clays of the lower unnamed member of the Eutaw Formation (Santonian; Puckett, 1996) have not yielded significant vertebrate remains. Only modest effort has been expended collecting from these older and poorly-exposed beds. Certainly mosasaurs must have inhabited the region since at least Late Coniacian times, considering the diverse assemblages of Late Coniacian/Early Santonian mosasaurs preserved in the Smoky Hill Chalk of the Western Interior and the Austin Chalk of Texas (see Russell, 1967, 1988). No known paleogeographical barrier precludes their presence in equivalent strata of the eastern Gulf Coastal Plain. The vertebrate fauna of the Tombigbee Sand remains a ‘‘scrap’’ fauna, although a very large one. It consists chiefly of isolated bones and teeth, broken and sometimes abraded by wave action in a high-energy depositional environment. In this respect, it is typical of scrap faunas recovered from Late Cretaceous coastal clastics along the Atlantic and Gulf Coastal plains. Only rarely are semi-articulated skeletons recovered. Exposures are scarce, and most consist of cuts in stream and river banks. The most successful collecting strategy for macrovertebrates has been on-site wet screening of Holocene alluvium derived from the Tombigbee Sand. Microvertebrates have been successfully recovered from lag deposits within the Tombigbee Sand at both the Vinton Bluff locality in Clay County, Mississippi (Emry et al., 1981) and from a sand pit within the Montgomery city limits, Montgomery County, Alabama (RMM locality AMg-2; Whetstone and Collins, 1982). Only one site within the study area, a series of exposures along Catoma Creek (RMM localities AMg-3 and AMg-4 [Lamb et al., 1991]), also in Montgomery County, has produced a significant in situ macrovertebrate fauna. Within the study area, the Tombigbee Sand vertebrate assemblage includes a very diverse elasmobranch fauna (Meyer, 1974; Thurmond and Jones, 1981; Schultze et al., 1982), a pycnodontid (Anomeodus sp. [Thurmond and Jones, 1981]), an aspidorhynchid (Belonostomus sp. [Schultze et al., 1982]), a poorly-known teleost fauna (Enchodus sp., Xiphactinus sp., Pachyrhizodus sp., Albula dunklei, Banannogmiidae incertae sedis, Saurodontidae incertae sedis), elasmosaurids (Shannon, 1974; Shannon and Thurmond, 1981) and polycotylids, crocodilians, dinosaurs (Theropoda incertae sedis, Nodosauridae incertae sedis, Hadrosauridae incertae sedis), marine turtles (Pro-

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tostega sp., Toxochelyidae incertae sedis, Bothremys sp.), an undetermined trionychid, and mosasaurs. The mosasaurs are dominated by Tylosaurus aff. T. nepaeolicus and include Platecarpus tympaniticus, an undetermined species of Clidastes, and Halisaurus sternbergi (Fig. 2). The lowermost Mooreville Chalk is extremely fossiliferous, with well-preserved isolated bones and semi-articulated skeletons in varying stages of completeness, indicating quieter bottom conditions than prevailed during the deposition of the Tombigbee Sand. Plesiosaurs appear to be absent altogether from these beds, as do hybodontoids, myliobatoids, dinosaurs, and crocodilians, probably reflecting the increase in depth and distance from shore. The elasmobranch fauna is less diverse than in the Tombigbee Sand, limited to lamnoids (Cretolamna appendiculata, Cretoxyrhina mantelli, Scapanorhynchus texanus, Squalicorax kaupi, and Pseudocorax laevis). The osteichthyan assemblage (see Applegate, 1970; Russell, 1988) is typical of that found higher in the Mooreville Chalk. The pelomedusid Bothremys barberi, the protostegid Protostega gigas (5P. ‘‘dixie’’; Hooks, 1998), and the toxochelyid Toxochelys moorevillensis are the dominant chelonians. Tylosaurus aff. T. nepaeolicus is replaced in the lowermost Mooreville by the gigantic Tylosaurus proriger, some Alabama specimens estimated to have reached lengths in excess of 10 m. Platecarpus and Halisaurus persist, and Clidastes liodontus is also present. Clidastes Acme-zone Bell (1985) erected the Clidastes Acme-zone, placing its lower boundary at the top of the Tylosaurus Acme-zone (12 m above the base of the Mooreville Chalk) and its upper boundary at the contact between the lower unnamed member and the Arcola Limestone member of the Mooreville Chalk. The upper boundary of the Clidastes Acme-zone is herein extended upwards to the Arcola Limestone Member—Demopolis Chalk contact. The source of Alabama’s most diverse Cretaceous vertebrate fauna, the Mooreville Chalk sediments, extensively exposed as erosional gullies, have yielded a large elasmobranch and osteichthyan assemblage (see Applegate, 1970; Meyer, 1974; Russell, 1988), nearly twenty species of pelomedusid, protostegid, dermochelyid, and toxochelyid sea turtles (see Zangerl, 1948b, 1953a, b, 1960; Hooks, 1998), pterosaurs (cf. Pteranodon sp.), a polycotylid (cf. Trinacromerum sp.), a crocodilian (Shannon and Thurmond, 1981), dinosaurs (Lophorothon atopus, Nodosauridae incertae sedis, Theropoda incertae sedis [Langston, 1960]), ichthyornithiform (Olson, 1975, 1985) and enantiornithine birds, and eight genera of mosasaurs. Bone preservation is often pristine, free from the crushing characteristic of the Niobrara Chalk and the gypsum encrustation common in the Pierre Shale. Careful study of Mooreville Chalk specimens should add considerably to knowledge of the morphology of Early Campanian marine vertebrates. The Clidastes Acme-zone mosasaur assemblage is the most diverse known from North America east of the Mississippi River, with a total of at least nine species. Russell (1970) found that Clidastes comprised the bulk of this fauna, a conclusion corroborated by Wright (1988) and the Mooreville Chalk sample for this study (Fig. 3). Two species (C. propython and C. ‘‘moorevillensis’’; see Shannon, 1975, 1977; Bell and Sheldon, 1986; Wright, 1987, 1988) of this genus account for 78.2% of the generically-identifiable mosasaur material between the 12 m level in the Mooreville Chalk and the base of the overlying Demopolis Chalk. Tylosaurus (8.2%) and Halisaurus sternbergi (6.5%) are fairly common, while the remaining taxa (Platecarpus tympaniticus [3.8%], Globidens alabamaensis [1.5%], Prognathodon sp. [1.2%], an undescribed species of cf. Ectenosaurus [0.3%], and Selmasaurus russelli [0.3%]) account for

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FIGURE 2. Generalized geologic section of the Eutaw Formation and Selma Group, including stratigraphic distribution of mosasaur taxa, NAMVAs (after Russell, 1988) and mosasaur Acme-zones.

such a small portion of the total assemblage that these occurrences may represent ‘‘strays’’ from adjacent biogeographic provinces (Fig. 3). Zangerl (1953b:261–262) arrived at a similar conclusion regarding rare taxa in his study of the Mooreville Chalk toxochelyids.

Elsewhere (Wright, 1986a, b), the author has argued that the displacement of Tylosaurus as the predominant mosasaur taxon by Clidastes and the relative decrease in numbers of Platecarpus should be interpreted as an example of habitat segregation and not as a genuine reduction in the local population density

KIERNAN—ALABAMA MOSASAURS

FIGURE 3.

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Relative abundance of mosasaur taxa in the Tylosaurus, Clidastes, and Mosasaurus Acme-zones.

of tylosaurs or of Platecarpus. This interpretation rests on the continued presence of both Tylosaurus and Platecarpus in the higher Mooreville Chalk, the scarcity of Clidastes in the lowermost Mooreville Chalk, and the gradual northeastward migration of the nearshore/inner shelf setting that produced the sediments of the Tylosaurus Acme-zone. As the Mississippi Embayment expanded during Early Campanian times, the shoreline migrated northeast, taking with it the nearshore biota. Marine sands and sandy clays (Eutaw Formation) covered flooded deltaics (Tuscaloosa Formation) and were in turn buried by the marls and chalks of an advancing outer shelf (Mooreville Chalk). Two contiguous ecologic zones, or biotopes, existed: a nearshore/inner-shelf zone, dominated by Tylosaurus, and a deeper, outer-shelf zone dominated by Clidastes (Fig. 4). The stratigraphic transition from sediments deposited in the Tylosaurus biozone into the overlying beds of the Clidastes biozone creates an illusion of faunal succession. The transition can be viewed as both a vertical (biostratigraphic) and horizontal (paleoecologic) phenomenon. The fact that the shift in dominance coincides precisely with the appearance of open-shelf carbonates seems to support such an interpretation. Additional support for this model comes from the ‘‘scrap’’ fauna of the Blufftown Formation, a clastic equivalent of the Mooreville Chalk in southeastern Alabama and southwestern Georgia (Schwimmer, 1986; Russell, 1988). The Blufftown consists of sands, marls, and clays (Raymond et al., 1988) originating in nearshore, back-barrier and lagoonal settings. The bulk of this material is currently housed in the collections of Columbus State College, Columbus, Georgia. The author first examined material collected from the Blufftown in December

1988, and again in 1999, and on both occasions found the assemblage dominated by Platecarpus tympaniticus, Tylosaurus sp., and Halisaurus sp.; Clidastes is conspicuously absent. Clearly, the mosasaur fauna that thrived in western Alabama during the deposition of the Tombigbee Sand (Santonian) continued to dominate nearshore environments in eastern Alabama during Mooreville times (Late Santonian–Early Campanian). This model of habitat preference for Tylosaurus, Platecarpus, and Clidastes conflicts with an earlier interpretation by Russell (1967:191), who concluded that ‘‘In the Niobrara Chalk Platecarpus and Tylosaurus may have frequented deeper water farther from the coast than did Clidastes.’’ Russell (1967:187) demonstrated that the collection of mosasaurs made from the uppermost Niobrara Chalk (upper Smoky Hill Member, presumed to have originated in a shallower sea, nearer shore, than the underlying beds) by O. C. Marsh’s 1871 Yale expedition, shows a higher percentage of Clidastes than do collections made lower in the section. Of the 36 generically identifiable skulls collected, 30% are Clidastes; of the total number of skulls taken from the Niobrara Chalk by Marsh and his collectors (398), only 13% are Clidastes. This seems to indicate an increase in the relative abundance of the genus higher in the Niobrara Chalk and therefore a greater population density in the shallower environment. However, Everhart (2001) has discussed the generally poor locality data available for the Yale mosasaurs, stressing the need for better data for mosasaur biostratigraphic studies, and Stewart (1988) has noted exceptions to the pattern of mosasaur distribution described by Russell for the Smoky Hill Chalk. Russell (1970:378–379) also noted the scarcity of Platecar-

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FIGURE 4.

Hypothetical paleoecologic/depositional relationship between Tylosaurus and Clidastes Acme-zones.

pus in the Mooreville Chalk as compared to the Niobrara Chalk. This genus accounted for only 13% of the Field Museum’s Mooreville Chalk collection, while it made up 60% of Marsh’s Niobrara Chalk collections. In this study’s larger Mooreville Chalk sample, Platecarpus comprises only 3.8% of the generically-identifiable mosasaur material collected above the 12 m level, increasing to 19.2% in the lowermost Mooreville Chalk and Tombigbee Sand. Russell (1970) speculated that a diet of belemnites, a group of cephalopods absent in the Mooreville Chalk, may have restricted Platecarpus to the cooler, more northerly waters within the boreal belemnite zone. This proposition rests on (1) a single instance of belemnite endoskeletons associated with the Maastrichtian Belgian species Plioplatecarpus houzeaui and (2) the gross similarity between the odontology of the extant delphinid cetacean Globicephala, which feeds on squid and cuttlefish, and that of P. houzeaui (Russell, 1967). A number of difficulties exist with the belemnite diet hypothesis, the greatest being the appearance of Plioplatecarpus primaevus as a major element (30%) of the Demopolis Chalk fauna, which completely lacks belemnites. Also, belemnites are unknown in the Blufftown Formation, where Platecarpus may be the most common mosasaur. Nicholls (1986, 1988) suggested that mosasaur diversity decreases in higher latitudes, with Platecarpus being the dominant northern genus. Indeed, Platecarpus makes up 85% of the mosasaur assemblage of the Pembina Member of the Pierre Shale from southern Manitoba (Nicholls, 1987). The author’s examination of the University of Colorado (Boulder) collection of mosasaurs made from the Sharon Springs Member of the Pierre Shale at Red Bird, Niobrara County, Wyoming found Platecarpus tympaniticus dominant, almost to the exclusion of any other mosasaur taxon; of 30 specimens, 27 were Platecarpus, the remaining three being Clidastes ‘‘moorevillensis.’’ It is safe to conclude that in North America Platecarpus was restricted largely to the northern latitudes, where it dominated mosasaur assemblages. Its presence in the Gulf may mark the southernmost extent of the taxon’s range in the western hemisphere, and its near absence throughout the higher Mooreville Chalk probably indicates a regional restriction to a nearshore niche. Likewise, it appears that Clidastes favored southern latitudes. The Niobrara Chalk, where, according to Russell (1967), Clidastes makes up only 13% of the fauna, probably represents the northernmost limit of the genus as an important element in North American mosasaur assemblages. It is interesting to note the faunal succession within the Smoky Hill indicated by Marsh’s Niobrara mosasaur collection.

Russell (1967:187) postulated ‘‘ . . . that the Smoky Hill Chalk may be divisible into an upper Clidastes propython–Platecarpus ictericus–Tylosaurus proriger zone and a lower zone in which C. liodontus–P. coryphaeus–T. nepaeolicus occur, perhaps to the exclusion of other forms.’’ Several workers have recently revised this succession (Schumacher, 1993; Sheldon, 1996; Everhart, 2001), extending the range of C. liodontus to the top of the Smoky Hill and adding Platecarpus planifrons and an undescribed species of Tylosaurus to Russell’s lower zone. A very similar succession is seen in the transition from the Tombigbee Sand to the Mooreville Chalk, in the replacement of the T. nepaeolicus-like tylosaur by T. proriger, and in the transition from the Tylosaurus Acme-zone to the Clidastes Acme-zone, in the replacement of C. liodontus by C. propython and C. ‘‘moorevillensis,’’ and in the presence of P. tympaniticus in both biozones. Halisaurus sternbergi appears to have been affected little in the transition from the nearshore/inner-shelf environment of the Tylosaurus Acme-zone to the more open sea of the Clidastes Acme-zone. The minor disparity in Halisaurus numbers between the two biozones, a drop from 10.3% to 6.4%, is probably not significant. The determining factor behind the distribution of mosasaur taxa within either latitudinal or, on a smaller scale, locally-differentiated environmental boundaries cannot at present be resolved. The prime controlling factor was probably the biogeography and paleoecology of mosasaur prey. This could account for both the latitudinal range limits and the sort of variation with regard to habitat preference within those ranges witnessed for Platecarpus and Clidastes. Moreover, a better understanding of the functional significance of the diversity of body forms among mosasaurs (see, for example, Lingham-Solair, 1992; Nicholls and Godfrey, 1994), combined with biostratigraphic data, should prove useful in resolving paleoecological problems. Apparently the biotope preferred by a given mosasaur genus or species can vary at different points along its geographic range. Thus, we can find a taxon, such as Clidastes, favoring an open shelf, deep water environment in the Mississippi Embayment and coastal waters within the Western Interior Seaway. Bell (1985) erected the Globidens alabamaensis Acme-zone, setting its boundaries at the lowermost and uppermost beds of the Arcola Limestone Member of the Mooreville Chalk. However, reevaluation of the available data has failed to support this zonation as (1) the mosasaur sample from the Arcola Limestone is too small (only about a half dozen specimens) to be statistically significant and (2) some of the Globidens material orig-

KIERNAN—ALABAMA MOSASAURS inally believed to have come from the Arcola Limestone Member may have come instead from the uppermost beds of the lower unnamed member of the Mooreville Chalk. On these grounds, the author places the Arcola Limestone Member in the uppermost part of the Clidastes Acme-zone and abandons the Globidens alabamaensis Acme-zone. Mosasaurus Acme-zone Vertebrate remains are less common and considerably less well preserved in the Demopolis Chalk than in the underlying Mooreville Chalk. Little is known of the elasmobranchs and bony fishes. Lamnoid sharks (Squalicorax pristodontus [Thurmond and Jones, 1981], Scapanorhynchus sp., Cretolamna appendiculata) are fairly common, but diversity is low. A sclerorhynchid (Ischyrhiza mira [Thurmond and Jones, 1981]) is also present. By Mooreville Chalk standards, the known osteichthyian assemblage (Protosphyraena gladius [Stewart, 1988], Enchodus petrosus) is sparse, but this is almost certainly an artifact of a collecting bias towards larger vertebrates. Elasmosaurs are present, but very rare (Shannon and Thurmond, 1981). Although Demopolis Chalk outcrops are extensive throughout the western part of the study area, the unit’s lower yield of vertebrate fossils, coupled with the difficulties of excavation and preparation posed by a harder, more compact matrix, has led survey crews to favor the Mooreville Chalk. Poor preservation has also played a role in discouraging work in the Demopolis Chalk. A slower rate of deposition, as compared to the relatively rapid Mooreville deposition, apparently left skeletons at the mercy of decay, scavengers, and current action for greater lengths of time before complete burial. Specimens are often badly disarticulated and scattered. The situation is worsened by a tendency for pyrite crystals to form around almost any available nucleus. This often obliterates surface detail and may even encase entire elements (RMM 2487 [Plioplatecarpus primaevus] and RMM 3037 [Mosasaurus cf. M. conodon] illustrate this problem). Bell (1985) erected the Mosasaurus/Plioplatecarpus Acmezone (5Mosasaurus missouriensis/Plioplatecarpus primaevus Acme-zone of Wright 1986a, b). When this zone was originally conceived, Plioplatecarpus was believed to comprise almost as large a percentage of the Demopolis Chalk fauna as Mosasaurus; it is now known to account for less than a third of the assemblage (Fig. 3). Therefore, for simplicity and consistency’s sake, the name of this biozone is herein shortened to the Mosasaurus Acme-zone. The upper limit of this biozone is here provisionally extended to the top of the Selma Group in western and central Alabama. Preliminary evidence suggests that Mosasaurus remains the dominant taxon all the way to the K–T boundary. The author is aware that the genus Mosasaurus is problematic. Russell (1967) suggests it is polyphyletic, uniting at least two distinct lineages derived independently from Clidastes. The taxon Mosasaurus, as recognized for the purposes of this study, is based on Russell’s (1967) diagnosis for the genus. In the interval between the deposition of the Mooreville Chalk and the beginning of the Demopolis Chalk deposition, the composition of the marine reptile fauna of the eastern Mississippi Embayment was radically altered. The Clidastes/Tylosaurus dominated assemblage of the Mooreville Chalk was completely replaced by a new regime of mosasaurs dominated by Mosasaurus. This transition marks a dramatic decrease in local marine reptile diversity. Mosasaur diversity drops from eight genera to three. Mosasaurus spp. and Plioplatecarpus primaevus account for 93.3% of the entire Demopolis Chalk mosasaur fauna, with the remaining 6.7% made up of a single

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occurrence each of Prognathodon rapax and Prognathodon cf. P. solvayi. This event was not limited to the eastern Mississippi Embayment. Russell (1967) noted a fundamental change in the composition of North American mosasaur faunas by the latter half of the Campanian. North American mosasaur faunas can readily be divided into those predating the Middle Campanian (e.g., Lower Pierre, Niobrara, Eagle Ford, Austin, Taylor, Eutaw, Mooreville, and Merchantville formations/groups) and postdating it (e.g., Moreno, Bearpaw, Upper Pierre, Fox Hills, Navarro, Marlbrook, Demopolis, Ripley, Peedee, Severn, Navesink, and lower Hornerstown formations/groups). The former are characterized by the predominance of Platecarpus, Tylosaurus, and/or Clidastes, in association with Halisaurus, Globidens, Hainosaurus, Ectenosaurus, and/or Prognathodon. The latter faunas are dominated by Mosasaurus (having made its first North American appearance with M. ivoensis in the Niobrara Formation), Plioplatecarpus, Prognathodon, Halisaurus (surviving only along the Atlantic Coast), and on the Pacific Coast, Plotosaurus and Plesiotylosaurus. Massare (1987) has documented periodic reorganizations in the composition and feeding guild structure of Mesozoic marine reptile faunas. One such event towards the close of the Lower Cretaceous included the extinction of the ichthyosaurs and marine mesosuchians (teleosaurs and metriorhynchids) and possibly a drop in diversity among plesiosaurs. It was most likely this crisis that triggered the sudden appearance and rapid radiation of the ancestral mosasauroids as they exploited niches vacated or left inefficiently occupied. It would seem that the abrupt drop in mosasaur diversity within the Mississippi Embayment at the end of the Mooreville deposition is the local expression of another such event, one in which mosasaur genera that had dominated North American faunas since the Coniacian were replaced by survivors of the crisis. The mechanism responsible for this event remains unknown. Russell (1988) divided the Upper Cretaceous North American marine vertebrate faunas into a Niobraran ‘‘age’’ (Late Cenomanian–Early Campanian) and Navesinkan ‘‘age’’ (Middle Campanian–Late Maastrichtian). Russell’s NAMVAs (North American Marine Vertebrate ‘‘Ages’’) take their conceptual basis from the North American Land Mammal ‘‘Ages’’ as defined by Lillegraven and McKenna (1986). The close of the Niobraran ‘‘age’’ is marked by the disappearance of Platecarpus and the tylosaurines from North American mosasaur assemblages (although the latter survives in Europe well into the Maastrichtian). The beginning of the Navesinkan ‘‘age’’ is marked by the appearance of Mosasaurus conodon and M. missouriensis as well as M. hoffmani (5M. maximus; see Mulder, 1999) and Plioplatecarpus. This seems to describe perfectly the mosasaur turnover in the Selma Group. Therefore, this study provides further evidence of a major marine vertebrate turnover during Middle Campanian times and strengthens the case for Russell’s Niobraran—Navesinkan division. It is not yet clear to what degree other vertebrate groups in the Mississippi Embayment, such as the fishes and marine turtles, may have been affected by this event. Derstler (1988) reported a turtle assemblage from the uppermost Demopolis Chalk which included Prionochelys nauta, Toxochelys barberi, T. moorevillensis, Protostega sp., ?Bothremys sp., ?Peritresius sp., and an undetermined chelosphargine. Only the possible presence of Peritresius distinguishes this assemblage from the turtle fauna of the underlying Mooreville Chalk. The Ripley and Prairie Bluff Chalk formations have received little attention from vertebrate paleontologists, again partially owing to the small number of accessible exposures. Sporadic collection of Ripley Formation outcrops near Starkville, Oktibbeha County, Mississippi by the Red Mountain Museum and Mississippi State University has produced a few mosasaur frag-

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ments (Plioplatecarpinae incertae sedis), an eosuchian crocodile (Thoracosaurus neocesariensis [Carpenter, 1983]), and elasmobranch and teleost scraps. In the late 1970s, BirminghamSouthern College deposited the fragmentary remains of a small nodosaur (RMM 1224), apparently collected from the Ripley Formation of Lowndes County, Alabama, in the Red Mountain Museum collection. In the mid-1980s, collection of Ripley Formation outcrops in Wilcox County, Alabama, by the Red Mountain Museum produced evidence of a diverse vertebrate fauna, including an advanced toxochelyid turtle and mosasaur scraps (Mosasaurus sp., ?Plioplatecarpus sp.). Recently, the author has located an extensive Ripley outcrop near Sandy Ridge in Lowndes County, where the unit is being exposed in a mining operation, but has not yet gained access to the site. In 1990, a very large and relatively complete mosasaurine (ALMNH PV 990.0003), probably Mosasaurus hoffmani, was recovered from the Ripley near Sandy Ridge. Vertebrate remains thus far recovered from the Prairie Bluff Chalk are mostly limited to a sparse shark and teleost microvertebrate fauna. A tooth collected from the uppermost Prairie Bluff Chalk at Braggs, Lowndes County, Alabama (see Bryan and Jones, 1986) resembles teeth from the late Maastrichtian of New Jersey that have been referred to Mosasaurus dekayi. The large size of a single caudal vertebra in the Red Mountain collection (unnumbered specimen) may also indicate the presence of Mosasaurus hoffmani in the Prairie Bluff Chalk of Alabama. In early 1988, a very large Mosasaurus was discovered by the Alabama Museum of Natural History, from the lowermost Prairie Bluff Chalk, a few cm above its contact with the Ripley Formation, near Braggs. This specimen (ALMNH PV 988.0018) may be the largest mosasaur yet discovered in Alabama (estimated body length about 17 m), and by far the most complete mosasaur (or other fossil vertebrate) to come from the Prairie Bluff Chalk, represented by much of the skull and axial skeleton. Though the specimen remains mostly unprepared, it too appears to be referable to M. hoffmani. Early in 2000, the author recovered a single large tooth of Mosasaurus sp., less than a meter below the K-T boundary, at Moscow Landing in Sumter County, where the Prairie Bluff Chalk—Clayton Formation contact is exposed. The small number of mosasaurs known from the Ripley and Prairie Bluff Chalk were not included in the database (Appendix 1), but have been noted in a stratigraphically organized list of taxa (Fig. 2). SUMMARY 1. Three biostratigraphic zones have been erected based on the peak occurrence of mosasaur taxa in the Eutaw Formation and Selma Group of western and central Alabama (Figs. 2–4): a. The Tylosaurus Acme-zone (Bell, 1985; Wright, 1986b), extending from the base of the Tombigbee Sand Member of the Eutaw Formation to a point approximately 12 m above the base of the Mooreville Chalk. b. The Clidastes Acme-zone (Bell, 1985; Wright, 1986a, b), extending from a point approximately 12 m above the base of the Mooreville Chalk to the base of the Demopolis Chalk. c. The Mosasaurus Acme-zone, extending from the base of the Demopolis Chalk to the top of the Selma Group (local K–T contact). This biozone supersedes the Mosasaurus/Plioplatecarpus Acme-zone of Bell (1985) and the Mosasaurus missouriensis/Plioplatecarpus primaevus Acme-zone of Wright (1986a, b). 2. The stratigraphic transition from the Tylosaurus Acmezone to the Clidastes Acme-zone is here interpreted as an artifact of local habitat segregation among mosasaurs. Tylosaurus and Platecarpus are believed to have preferred the shallow

nearshore and inner shelf zone, while Clidastes frequented deeper, open shelf waters farther from shore. Halisaurus probably fared equally well in both biotopes. 3. The previously recognized Globidens alabamaensis Acmezone (Bell, 1985; Wright, 1986a, b) is here abandoned on the grounds that the small number of mosasaur specimens known from this unit does not constitute a statistically-significant sample relative to the larger samples on which the Tylosaurus, Clidastes, and Mosasaurus biozones are based. 4. The transition from the Clidastes Acme-zone to the Mosasaurus Acme-zone is the local expression of a reorganization among North American marine reptiles during the Middle Campanian, replacing Clidastes—Platecarpus—Tylosaurus dominated faunas with faunas usually dominated by Mosasaurus. In the Selma Group, mosasaur diversity falls from nine species in the Mooreville Chalk to five species in the overlying Demopolis Chalk. ACKNOWLEDGMENTS The author is indebted to Gorden L. Bell, Jr. and the late Richard D. Estes, both of whom lent invaluable support to this project. Additional thanks go to the following for their varied assistance in the course of this study: Robert T. Bakker, Scott Brande, Jonathan R. Bryan, Charles W. Copeland, Jr., Kraig Derstler, James L. Dobie, Michael J. Everhart, Craig Guyer, John C. Hall, Ed Hooks, James P. Lamb, Judy A. Massare, T. Lingham-Solair, James I. Kirkland, Dale A. Russell, Samuel W. Shannon, Charles C. Smith, Richard Thurn, Daniel Varner, and G. Dent Williams. Special thanks go to Andrew K. Rindsberg and David R. Schwimmer for encouraging the completion of this project after a long hiatus. Kenneth Carpenter, Elizabeth L. Nicholls, and Kenneth D. Rose read an earlier draft of the manuscript and made numerous valuable comments. Robert B. Holmes and Michael W. Caldwell reviewed the final draft. Mosasaur silhouettes in Figure 4 were adapted from restorations by Russell Hawley. Jada Walker and Jennifer M. Caudle provided secretarial and editorial assistance. Partial support for this project came from the Red Mountain Museum Society and National Science Foundation grant BSR-8709233 to the late Richard Estes. LITERATURE CITED Applegate, S. P. 1970. The vertebrate fauna of the Selma Formation of Alabama. Part VIII: the fishes. Fieldiana: Geology Memoirs 3:381– 433. Bell, G. L., Jr. 1985. Vertebrate faunal zones in the Upper Cretaceous of west-central Alabama. Geological Society of America, Southeastern Section, Abstracts with Programs 17:80. ——— 1997. A phylogenetic revision of North American and Adriatic Mosasauroidea; pp. 293–332 in J. M. Callaway and E. L. Nicholls (eds.), Ancient Marine Reptiles. Academic Press, San Diego. ———, and M. A. Sheldon. 1986. Description of a very young mosasaur from Greene County, Alabama. Journal of the Alabama Academy of Science 57:76–82. Bryan, J. R., and D. S. Jones. 1986. Macrofaunal changes across the Cretaceous-Tertiary (K–T) boundary, Braggs, Alabama. Abstracts, North American Paleontological Convention IV:A7. Burnham, D. A. 1991. A new mosasaur from the Upper Demopolis Formation of Sumter County, Alabama. M.S. thesis, University of New Orleans, New Orleans, 63 pp. Cagle, D. A. 1985. Foraminifera and paleobathymetry of the Bluffport Marl Member of the Demopolis Chalk (Upper Cretaceous) in eastern Mississippi and west-central Alabama. M.S. thesis, University of New Orleans, New Orleans, 219 pp. Carpenter, K. 1983. Thoracosaurus neocesariensis (DeKay, 1842) (Crocodilia, Crocodylidae) from the Late Cretaceous Ripley Formation of Mississippi. Mississippi Geology 4:1–10. Cope, E. D. 1868. (Remarks on Clidastes iguanavus, Nectoportheus

KIERNAN—ALABAMA MOSASAURS validus, and Elasmosaurus.) Academy of Natural Sciences of Philadelphia, Proceedings 20:181. ——— 1869. On the reptilian orders Pythonomorpha and Streptosauria. Proceedings, Boston Society of Natural History 12:250–256. ——— 1869–1870. Synopsis of the extinct Batrachia, Reptilia, and Aves of North America. Transactions, American Philosophical Society, issued in parts: 1 (1869):1–105, 2(1870):106–235, 3(1870): i–vii, 236–252. ——— 1871. (Verbal communication on pythonomorphs) Proceedings, American Philosophical Society 11:571–572. ——— 1872. Catalogue of the Pythonomorpha found in the Cretaceous strata of Kansas. Proceedings, American Philosophical Society 12: 264–287. Derstler, K. 1988. A rich vertebrate fossil assemblage from the upper Demopolis Formation of Alabama and Mississippi. Journal of the Alabama Academy of Science 59:144. Dowling, H. G., Jr. 1941. A new mosasaur skeleton from the Cretaceous in Alabama. Journal of the Alabama Academy Science 13:46–48. Emry, R. J., J. D. Archibald, and C. C. Smith. 1981. A mammalian molar from the Late Cretaceous of northern Mississippi. Journal of Paleontology 55:953–956. Everhart, M. J. 2001. Revisions to the biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk (Upper Cretaceous) of Kansas. Kansas Academy of Science, Transactions 104:56–75. Florian, M. D. 1984. Factors controlling deposition of the Arcola Member of the Mooreville Formation (Upper Cretaceous) in east-central Mississippi and west-central Alabama. M.S. thesis, Michigan State University, East Lansing, 105 pp. Gibbes, R. W. 1850. On Mosasaurus and other allied genera in the United States. Proceedings, American Academy for the Advancement of Science, second meeting, Cambridge (1849):77. ——— 1851. A memoir on Mosasaurus and three allied genera, Holcodus, Conosaurus, and Amphorosteus. Smithsonian Institution Contributions to Knowledge 2:1–13. Gilmore, C. W. 1912. A new mosasauroid reptile from the Cretaceous of Alabama. Proceedings, United States National Museum 41:479– 484. Hazel, J. E., and E. M. Brouwers. 1982. Biostratigraphic and chronostratigraphic distribution of ostracodes in the Coniacian–Maastrichtian (Austinian–Navarron) in the Atlantic and Gulf Coastal Province; pp. 166–198 in R. F. Maddocks (ed.), Texas Ostracoda: Guidebook for the Eighth International Symposium on Ostracoda. University of Houston Press, Houston. Hooks, G. E. III. 1998. Systematic revision of the Protostegidae, with a redescription of Calcarichelys gemma Zangerl, 1953. Journal of Vertebrate Paleontology 18:85–98. Kiernan, C. R. 1992. Clidastes Cope, 1868 (Reptilia, Sauria): proposed designation of Clidastes propython Cope, 1869 as the type species. Bulletin of Zoological Nomenclature 49:137–139. Lamb, J. P., G. L. Bell, Jr., and A. K. Rindsberg. 1991. The Catoma Creek scrap fauna (Late Cretaceous) from Montgomery County, Alabama. Journal of Vertebrate Paleontology 11(3, suppl.):41A. Langston, W., Jr. 1960. The vertebrate fauna of the Selma Formation of Alabama. Part VI: the dinosaurs. Fieldiana, Geology Memoirs 3:313–361. Leidy, J. 1870. (Remarks on Poicilopleuron valens, Clidastes intermedius, Leiodon proriger, Baptemys wyomingensis, and Emys stevensonianus.) Proceedings, Academy of Natural Science, Philadelphia 22:3–5. Lillegraven, J. A., and M. C. McKenna. 1986. Fossil mammals from the ‘‘Mesaverde’’ Formation (Late Cretaceous, Judithian) of the Bighorn and Wind River Basins, Wyoming, with definitions of Late Cretaceous land-mammal ‘‘ages.’’ American Museum of Natural History, Novitates 2840:68 pp. Lingham-Solair, T. 1992. A new mode of locomotion in mosasaurs: subaqueous flying in Plioplatecarpus marshi. Journal of Vertebrate Paleontology 12:405–421. Lins, T. W., F. E. Johnson, D. M. Keady, and E. E. Russell. 1977. The Arcola Limestone: a Cretaceous calcisphere wackestone and grainstone. Geological Society of America, Abstracts with Programs 9(2):159. Massare, J. A. 1987. Tooth morphology and prey preference of Mesozoic marine reptiles. Journal of Vertebrate Paleontology 7:121–137. Meyer, R. L. 1974. Late Cretaceous elasmobranchs from the Mississippi

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Skotnicki, M. C., and D. T. King, Jr. 1989. Depositional facies and eustatic effects in the Upper Cretaceous (Maastrichtian) Ripley Formation, central and eastern Alabama. Gulf Coast Association of Geological Societies Transactions 39:275–284. Smith, C. C., and E. A. Mancini. 1983. Calcareous nannofossil and planktonic foraminiferal biostratigraphy; pp. 16–28 in E. E. Russell, D. M. Keady, E. A. Mancini, and C. C. Smith (eds.), Upper Cretaceous Lithostratigraphy and Biostratigraphy in Northeast Mississippi, Southwest Tennessee and Northwest Alabama, Shelf chalks and Coastal Clastics. Gulf Coast Section, Society of Economic Paleontologists and Mineralologists, Guidebook for Spring Field Trip. Soens, D. D. 1984. Stratigraphy and sedimentology of the Tombigbee Sand Member, Eutaw Formation (Cretaceous–Campanian Stage) of northeastern Mississippi. M.S. thesis, University of Alabama, Tuscaloosa, 184 pp. Stewart, J. D. 1988. The stratigraphic distribution of Late Cretaceous Protosphyraena in Kansas and Alabama; pp. 80–94 in M. E. Green (ed.), Geology, Paleontology and Biostratigraphy of Western Kansas: Articles in Honor of Myrl V. Walker, Fort Hays Studies 3rd Series (10). Taylor, R. H. 1985. Planktonic foraminiferal biostratigraphy of the Demopolis Formation (Campanian/Maastrichtian) in Lowndes and Oktibbeha counties, Mississippi. M.S. thesis, Mississippi State University, Starkville, 145 pp. ´ clusier The´venin, A. 1896. Mosasauriens de la Craie Grise de Vaux-E pre`s Pe´ronne (Somme). Socie´te´ Ge´ologique de la France Bulletin (3rd series) 24:900–916. Thurmond, J. T., and D. E. Jones. 1981. Fossil Vertebrates of Alabama. University of Alabama Press, Tuscaloosa, 244 pp. Tuomey, M. 1850. [Exhibition of a fossil reptile belonging to the genus Leiodon.] Proceedings of the American Association for the Advancement of Science 1:74. Whetstone, K. N., and J. S. H. Collins. 1982. Fossil crabs (Crustacea: Decapoda) from the Upper Cretaceous Eutaw Formation of Alabama. Journal of Paleontology 56:1218–1222. Wright, K. R. 1986a. A preliminary report on the biostratigraphic zonation of Alabama mosasaurs. Journal of the Alabama Academy of Science 57:146. ——— 1986b. On the stratigraphic distribution of mosasaurs in western and central Alabama. Abstracts, North American Paleontological Convention IV:A51. ——— 1987. The mosasaur Clidastes: new specimens and new problems. Journal of the Alabama Academy of Science 58:99. ——— 1988. The first record of Clidastes liodontus (Squamata, Mosasauridae) from the eastern United States. Journal of Vertebrate Paleontology 8:343–345. ———, and S. W. Shannon. 1988. Selmasaurus russelli, a new plioplatecarpine mosasaur from Alabama. Journal of Vertebrate Paleontology 8:102–107. Wylie, J. A., and D. T. King, Jr. 1986. Mooreville Chalk (Upper Cretaceous), sedimentary facies and sea-level. Journal of the Alabama Academy of Science 57:145. Zangerl, R. 1948a. The vertebrate fauna of the Selma Formation of Alabama. Part I: introduction. Fieldiana, Geology Memoirs 3:1–18. ——— 1948b. The vertebrate fauna of the Selma Formation of Alabama. Part II: the pleurodiran turtles. Fieldiana, Geology Memoirs 3:19–58. ——— 1953a. The vertebrate fauna of the Selma Formation of Alabama. Part III: the turtles of the family Protostegidae. Fieldiana, Geology Memoirs 3:59–136. ——— 1953b. The vertebrate fauna of the Selma Formation of Alabama. Part IV: the turtles of the family Toxochelyidae. Fieldiana, Geology Memoirs 3:137–280. ——— 1960. The vertebrate fauna of the Selma Formation of Alabama. Part V, an advanced cheloniid sea turtle. Fieldiana, Geology Memoirs 3:281–312. Received 20 October 2000; accepted 18 May 2001.

APPENDIX 1 A key to specimens (arranged taxonomically and by biozone) comprising the database used in computing the relative frequencies of taxa as shown in Figure 3.

Tylosaurus Acme-zone: Halisaurus sternbergi—AUMP 2087 (four unassociated specimens); RMM 3546, 5633, 5806, 5828. Clidastes liodontus—RMM 1578, 5879. Clidastes sp.—RMM 1701, 1871, 1872, 2639, 3202, 3210, 5771, 5796. Platecarpus tympaniticus—RMM 1144, 1903, 3188, 3577, 5636, 5647, 5646; ALMNH PV 985.0021, 988.0020.0217, –.0254, 993.0002.0024, –.0132, 994.0002.0063a, –.0063b, –.0072. Tylosaurus aff. T. nepaeolicus—AUMP 982, 2087 (12 unassociated individuals), 2703, 2709; GSA-V one unnumbered specimen; RMM 2475, 5811; ALMNH PV 994.0001.0021. Tylosaurus proriger—GSA-V 1051 and two unnumbered specimens; P 27443 (T. ‘‘zangerli’’); RMM 1574, 1613, 1915, 3031, 3189, 3253, 5610. Tylosaurus sp.—GSA-V one unnumbered specimen; RMM 1864, 2522, 2606, 3556, 3565, 5648, 5652, 5834; ALMNH PV 985.0063, 988.0020.0481, 993.0002.0181, 994.0002.0020, –.0039, –.0046.001. Clidastes Acme-zone: Halisaurus sternbergi—AUMP 235, 1253; GSA-V one unnumbered specimen; PR 186, 195; RMM 1525, 1873a, 2497, 2567, 2602, 2826, 3152, 3221, 3284, 5730; ALMNH PV 992.0006.0010, 993.0002.0101.001, -.0237, 998.0020.0358. Halisaurus novum sp. (after Bell, 1997)—AUMP 408; RMM 3284, 6890. Clidastes propython—ANSP 10193; AUMP 403, 1287, 2032, 2057; GSATC 219, 1016, GSA-V 1029, 1045, 1046, 1102; P 27234, 27248, 27439, 27449, 27478, 27484, 27545, 27546, PR 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 53, 138, 142, 148, 150, 156, 162, 164, 192, 200, 211, 238; RMM 1788, 2473, 2479, 2496, 2825, 3212; ALMNH PV 985.0012., 985.0048.0001. Clidastes ‘‘moorevillensis’’—AUMP 404b, 414c, 2702; GSATC 218, GSA-V one unnumbered specimen; RMM 070, 1287, 1912, 1994, 2986, 3408, 5844; ALMNH PV 985.0023. Clidastes sp.—AUMP 234, 268, 405, 407, 409, 410, 413, 438, 672, 676, 687, 1306, 1309, 1476, 2027, 2039, 2053; GSA-V 1019, 1024, 1034, 1063, 1100, 1104, 6201; RMM 1246, 1279, 1296, 1532, 1534, 1543, 1827, 1828, 1848, 1873b, 1883, 1905, 1906, 1961, 1963, 1982, 1992, 2004, 2018, 2028, 2079, 2197, 2441, 2498, 2536, 2538, 2565, 2647, 2648, 2657, 2694, 2745, 2750, 2758, 2762, 2779, 2783, 2784, 2801, 2827, 2828, 2908, 2909, 3146, 3156, 3165, 3173, 3186, 3190, 3222, 3231, 3239, 3281, 3287, 3294, 3320, 3354, 3367, 3430, 3530, 3571, 3578, 3619, 5624, 5625, 5627, 5629, 5630, 5650, 5782, 5799, 5800, 5802, 5804, 5807, 5814, 5833, 5836, 5840, 5852, 5890, 5896; ALMNH PV 985.0002, 985.0031, 988.0020.0042.001, –.0052, –.0058a, –.0083, –.0086, –.0094, –.0107.002, –.0118, –.0119, –.0140, –.0144, –.0146, –.0147.001, –.0150, –.0162, –.0172, –.0174.002, –.0192, –.0203, –.0212, –.0214, –.0228, –.0236.001, –.0237.002, –.0238.001a, –.0238.001b, –.0263.006, –.0278, –.0290, –.0383.001, 990.0023.0047, 991.0004.0005, –0012, –0021 991.0014.0006, –.0008, 991.0015.0001, 991.0021.0006, 992.0001.0003, 992.0005.0005, –.0011, 992.0035.0003, 992.0037.0011, –.0008, 992.0047, 993.0002.0002.002, –.0009.003, –.0009.004, –.0011.001, –.0015.001, –.0019.001, –.0026.001, –.0028.001, –.0053.001, –.0067.001, –.0069.001, –.0071.001, –.0073.001, –.0083.007, –.0086, –.0092, –.0100, –.0100.001, –.0109.001, –.0110.001, –.0119.001, –.0119.002, –.0128, –.0166.001, –.0171.001, –.0188, –.0195.003, –.0197, –.0206, –.0209.002, –.0211, –.0212, –.0218, 993.0004.0003 994.0002.0018, –.0022, –.0025, –.0027, –.0033, –.0044, –.0048.002, –.0052.002, –.0057, –.0074, –.0077, –.0080. Globidens alabamaensis—ANSP 9023-4/9029/9092-4 (single individual); AUMP 2747; GSA-V 1014, 1054 (part of ALMNH PV 985.0017); USNM 6527; ALMNH PV 985.0017. Prognathodon sp.—PR 143, 146, 165, 193. Platecarpus tympaniticus—AUMP 2701; P 27399; GSATC 220; RMM 2604, 2766, and one unnumbered specimen; ALMNH PV 985.0005.0001, 988.0020.0058b, –.0232.002, –.0237.001a, –.0237.001b, –.0244.001, 993.0002.0010.001. cf. Ectenosaurus novum sp.—GSATC 1048. Selmasaurus russelli—GSATC 221. Tylosaurus proriger—AUMP 104, 401, 176a; RMM 5628, 5733; ALMNH PV 985.0011, 985.0019, 985.0022, 993.0001.0001. Tylosaurus sp.—AUMP 404, 1264, 1310, 1682, 1684; GSA-V one unnumbered specimen; P 204, PR 27474; RMM 1905, 5732, 5846; ALMNH PV 985.0072.0016, –.0017/–.0019 (single individual),

KIERNAN—ALABAMA MOSASAURS 985.0064, 988.0020.0221.003, 992.0040.0002, 993.0002.0153.003, –.0225, 994.0002.0002.0086. Mosasaurus Acme-zone: Mosasaurus missouriensis—GSA-V one unnumbered specimen; RMM 2204, 5853, 5898, and one unnumbered specimen. Mosasaurus cf. M. conodon—RMM 3037, 3038.

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Mosasaurus sp.—AUMP 296 and one unnumbered specimen; GSAV 1017, 1021, 1023, 1036, 1037, 1041, and one unnumbered specimen; RMM 2906, 5897; ALMNH PV 985.0020. Prognathodon rapax—RMM 2194. Prognathodon cf. P. solvayi—UNO one unnumbered specimen. Plioplatecarpus primaevus—AUMP 420 and one unnumbered specimen; RMM 2048, 2477, 2910, 3038, 5852; ALMNH PV 988.0020.0293, –.0320.001.