Mooreville Chalk Research Articles

Sequential stable (δ18O and δ13C) isotopic analysis of ammonite aptychi from Upper Cretaceous eastern Gulf Coastal Plain

This study reports sequential δ18O and δ13C values measured in ammonite aptychi (n=8) and assesses their preservation via scanning electron microscopy and electron backscatter diffraction techniques. The samples were collected from the Late Cretaceous Mooreville Chalk in Greene and Dallas Counties, Alabama, USA. These ontogenetic data profiles may provide insight into possible migration patterns, potentially including both short-term and long-term movements between cooler and warmer water. The δ18O variations appear primarily caused by temperature variation because it is unlikely that fresh water influenced these samples so far offshore. Estimated age of all species indicate these animals analyzed here did not live >3.5–13 years. Calculated paleotemperature values based on δ18O data vary between 15 °C and 28 °C, similar to other estimates from the Upper Cretaceous eastern Gulf Coastal Plain. While some systematic patterns were observed in the data, no definitive seasonal cyclicity was observed.

The first occurrence of the stem turtle Naomichelys from the Late Cretaceous of eastern North America

ABSTRACT Here we describe the first occurrence of the stem turtle Naomichelys from the Late Cretaceous of eastern North America. The specimen (MSC 41038) was collected as float from locality AMg-1 in Montgomery County, Alabama, USA. Although this locality consists of exposures of both the upper Santonian Tombigbee Sand Member of the Eutaw Formation and Mooreville Chalk, it is likely that the specimen was derived from the latter unit, which is known to possess both nearshore and terrestrial components. The specimen consists of a single incomplete peripheral and is referred to Naomichelys based on its Santonian age and the presence of numerous cylindrical tubercles covering the surface of the element that never coalesce and are easily dislodged. The presence of Naomichelys in the Gulf Coastal Plain greatly expands the geographical range of helochelydrids in North America. This occurrence indicates that helochelydrid turtles were likely a well-established component of the Appalachian terrestrial fauna and reinforces the emerging pattern of faunal similarities between Laramidia and Appalachia seen in Late Cretaceous non-avian dinosaurs. The recovery of helochelydrid remains from the Santonian of Alabama therefore represents an important contribution to our understanding of the terrestrial palaeoecology of Appalachia.

Putative Remains of an Enigmatic Cretaceous Bony Fish, Palaeonotopterus greenwoodi (Teleostei: Osteoglossomorpha), from Alabama, U.S.A.

Palaeonotopterus greenwoodi Forey is an enigmatic Cretaceous bony fish (Teleostei: Osteoglossomorpha) currently known only from Albian–Cenomanian deposits of Morocco. Here, we describe nine fossil tooth plate specimens from the Eutaw Formation and Mooreville Chalk Formation from Alabama, U.S.A., tentatively assigned to P. greenwoodi. If they indeed belong to the taxon, these specimens not only represent the geologically youngest material ranging from late Santonian to early Campanian in age, but also constitute the first record of the species outside of Morocco.

Prionochelys matutina Zangerl, 1953 (Testudines: Pan-Cheloniidae) from the Late Cretaceous of the United States and the evolution of epithecal ossifications in marine turtles.

BackgroundMany neritic to nearshore species of marine adapted turtle from the Late Cretaceous of North America are thought to represent the stem lineage of Cheloniidae but due to fragmentary holotypes, low total specimen counts, and resultantly incomplete morphological character suites, are routinely placed either within or outside of crown group Chelonioidea leaving their precise cladistic affinities uncertain. Despite this systematic ambiguity, the referral of these species to either the stem of Cheloniidae or Chelonioidea belies the critical importance of these taxa in any investigation into the origins of extant marine turtles. The adequate incorporation of these species into phylogenetic studies requires the formal description of relatively complete specimens, particularly those possessing associated cranial and post-cranial material.MethodsRemarkably complete fossil specimens of several adult and juvenile marine turtles from the Mooreville Chalk and Eutaw Formations (Alabama, USA) are formally described and assigned to Prionochelys matutina. This material provides new information into the anatomy, ontogeny, and cladistic affinities of the species. A phylogenetic hypothesis for Late Cretaceous marine turtles is then generated through the consilience of stratigraphic, morphological, and molecular data.ResultsPhylogenetic analysis places Prionochelys matutina on the stem of Cheloniidae as a member of a monophyletic clade with other putative pan-cheloniids, including Ctenochelys stenoporus, Ctenochelys acris, Peritresius martini, and Peritresius ornatus. The members of this clade possess incipient secondary palates, pronounced carapacial and plastral fontanelles at all stages of development, and are characterized by the presence of superficial ossifications at the apices of the neural keel elevations along the dorsal midline of the carapace.DiscussionThe epithecal osteoderms dorsal to the neural series (epineurals) found in Ctenochelyidae are unique among turtles. The presence of epineurals in ctenochelyid turtles shows that epithecal ossifications arose independently in both leatherback (Dermochelyidae) and hard-shelled (Cheloniidae) marine turtles. Whether or not the epineurals of Ctenochelyidae are homologous with the dermal ossicles comprising the carapace of Dermochelys coriacea remains untested however, histological thin sectioning of dermochelyid and ctenochelyd epithecal elements may reveal meaningful information in future studies.

FEEDING TRACES ON A PTERANODON (REPTILIA: PTEROSAURIA) BONE FROM THE LATE CRETACEOUS (CAMPANIAN) MOOREVILLE CHALK IN ALABAMA, USA

Pterosaur remains are exceptionally rare in the Late Cretaceous marine chalks of Alabama and the few specimens found are typically very fragmentary. We report the occurrence of a metacarpal of Pteranodon cf. longiceps from the Mooreville Chalk (Campanian, 83 million years old) of Dallas County, Alabama. The Pteranodon specimen exhibits serrated teeth marks on the surface of the bone and a second set of larger, unserrated teeth marks unlike those of any contemporary shark species. These feeding traces compare favorably with the tooth spacing and morphology of Squalicorax kaupi, and a small to moderate-sized saurodontid fish, such as Saurodon or Saurocephalus, respectively. In both instances, feeding traces appear to be scavenging events due to the lack of any healing or bone remodeling. During the Campanian, Dallas County, Alabama, was a shallow-marine environment comprising part of the Mississippi Embayment. It is hypothesized that the specimen represents a pterosaur that either fell into marine waters or was washed out from nearshore areas and then scavenged by both a chondrichthyan and osteichthyan. This type of scavenging behavior has been recorded on other taxonomic groups from Alabama during the Late Cretaceous. However, the fragile, hollow bones of pterosaurs make their preservation rare.

The first report of <em>Toxochelys latiremis</em> Cope, 1873 (Testudines: Panchelonioidea) from the early Campanian of Alabama, USA

Toxochelys latiremis Cope, 1873 is currently thought to be one of the oldest members of the clade originating from the last common ancestor of all extant species of marine-adapted turtles (Chelonioidea). Fossil material of this species has been reported from numerous lower Campanian marine formations across North America; however, reported occurrences have been conspicuously absent from the upper Santonian-to-lower Campanian Mooreville Chalk of Alabama and Mississippi, USA, the type stratum for the only other valid species within the genus, Toxochelys moorevillensis Zangerl, 1953. The apparent absence of T. latiremis from the Mooreville Chalk, and from the southern expanse of the Mississippi Embayment, has made T. latiremis one of the few outliers in previously proposed paleobiogeographic models for marine turtles in the Late Cretaceous Western Interior Seaway. This absence also confounded attempts at reconciling the distribution and phylogeny of these taxa. Here we report the first material of T. latiremis identified from the Mooreville Chalk of Alabama, which represents the southern-most occurrence of this taxon. The discovery of this species in the Mooreville Chalk of Alabama helps to reconcile the previously hypothesized paleobiogeography of North American Late Cretaceous chelonioids with their fossil occurrence and provides the first evidence for overlapping ranges of the only two currently recognized species of Toxochelys.

A new species of Cretalamna sensu stricto (Lamniformes, Otodontidae) from the Late Cretaceous (Santonian-Campanian) of Alabama, USA.

Decades of collecting from exposures of the Upper Cretaceous Tombigbee Sand Member of the Eutaw Formation and Mooreville Chalk in Alabama, USA has produced large numbers of isolated Cretalamna (sensu stricto) teeth. Many of these teeth had formerly been assigned to the extinct Late Cretaceous shark Cretalamna appendiculata (Agassiz, 1843), a taxon that is now considered largely restricted to the Turonian of Europe. Recent studies have shed light on the diversity of Late Cretaceous Cretalamna (s.s.) taxa, and here we recognize a new species from Alabama, Cretalamna bryanti. The teeth of C. bryanti sp. nov. appear aligned with the members of the Cretalamna borealis species group, but can be distinguished from these other species by a combination of the following: anterior teeth with a more pronounced and triangular lingual root protuberance, broader triangular cusp, and a taller root relative to the height of the crown; anteriorly situated lateroposterior teeth have a distally inclined or hooked main cusp and more than one pair of lateral cusplets; and lateroposterior teeth have a strong distally hooked main cusp and a root that is largely symmetrical in basal view. At present, C. bryanti sp. nov. is stratigraphically confined to the Santonian/Campanian Dicarinella asymetrica Sigal, 1952 and Globotruncanita elevata Brotzen, 1934 Planktonic Foraminiferal Zones within the Tombigbee Sand Member of the Eutaw Formation and Mooreville Chalk, and teeth have been collected from only four counties in central and western Alabama. The recognition of C. bryanti sp. nov. in Alabama adds to our knowledge on the diversity and distribution of Late Cretaceous otodontids in the region.

New material of the Late Cretaceous marine turtle Ctenochelys acris Zangerl, 1953 and a phylogenetic reassessment of the ‘toxochelyid’-grade taxa

Late Cretaceous marine turtles formerly referred to the family Toxochelyidae represent perhaps the earliest members of the clade including extant marine species of cryptodire (Chelonioidea). Though more pelagically specialized marine taxa such as Protostegidae (i.e. Archelon, Desmatochelys) predate the first occurrence of any known ‘toxochelyid’ species, phylogenetic analyses of modern and fossil Testudines have shown that the protostegids may be a separate radiation of stem cryptodires only distantly related to modern marine turtles. This hypothesis has since been supported by ancillary evidence from the stratigraphic record and has been argued from a perspective of rampant homoplasy. This information brings into question our understanding of chelonioid evolution with regard to their previously assumed monophyly, and supports the placement of ‘toxochelyids’ as perhaps the earliest definitive members of total group Chelonioidea (Pan-Chelonioidea). However, many ‘toxochelyids’ are known from partial holotypes, preventing accurate species diagnosis and precluding these species from character-based phylogenetic analyses. A poorly known member of this clade, Ctenochelys acris, is herein redescribed based on several nearly complete specimens from the early Campanian Mooreville Chalk of Alabama. Ctenochelys acris is characterized by large, dorsolaterally oriented orbits, significant contributions of the palatines to the secondary palate, a broad maxillary triturating surface, and almost equilaterally pentagonal posterior peripherals. This redescription not only identifies previously unknown apomorphies of Ctenochelys spp. but also provides an opportunity to examine the ‘toxochelyids’ within a global phylogenetic context. A constraint tree reflecting the recognized molecular topology for extant Testudines is used as a backbone and the resulting phylogeny is then placed within a chronostratigraphic framework in an effort to present a scenario that reconciles anatomical, temporal and phylogenomic patterns for marine turtles. This analysis supports the placement of protostegids outside of Chelonioidea as marine eucryptodirans and establishes Ctenochelys spp. as perhaps some of the earliest representatives of Pan-Cheloniidae.

Anatomy and osteohistology of the basal hadrosaurid dinosaur Eotrachodon from the uppermost Santonian (Cretaceous) of southern Appalachia.

The cranial and postcranial anatomy of the basal hadrosaurid dinosaur Eotrachodon orientalis, from the uppermost Santonian of southern Appalachia (southeastern U.S.A.), is described in detail. This animal is the only known pre-Campanian non-lambeosaurine hadrosaurid, and the most complete hadrosauroid known from Appalachia. E. orientalis possesses a mosaic of plesiomorphic and derived characters in the context of Hadrosauroidea. Characters shared with basal hadrosauroids include a short and sloping maxillary ectopterygoid shelf, caudally prominent maxillary jugal process, one functional tooth per alveolus on the maxillary occlusal plane, a jugal rostral process with a shallow caudodorsal margin and medioventrally facing articular facet, a vertical dentary coronoid process with a poorly expanded apex, and tooth crowns with accessory ridges. Derived characters shared with other hadrosaurids include a circumnarial depression compartmented into three fossae (as in brachylophosaurins and Edmontosaurus), a thin everted premaxillary oral margin (as in Gryposaurus, Prosaurolophus, and Saurolophus), and a maxilla with a deep and rostrocaudally extensive rostrodorsal region with a steeply sloping premaxillary margin (as in Gryposaurus). Eotrachodon orientalis differs primarily from the other hadrosauroid from the Mooreville Chalk of Alabama, Lophorhothon atopus, in having a slender and crestless nasal whose caudodorsal margin is not invaded by the circumnarial depression. Hadrosaurus foulkii, the only other known hadrosaurid from Appalachia, is distinct from E. orientalis in having dentary teeth lacking accessory ridges and a dorsally curved shaft of the ischium. A histological section of the tibia of the E. orientalis holotype (MSC 7949) suggests that this individual was actively growing at the time of death and, thus, had the potential to become a larger animal later in development.

Endothermic mosasaurs? Possible thermoregulation of Late Cretaceous mosasaurs (Reptilia, Squamata) indicated by stable oxygen isotopes in fossil bioapatite in comparison with coeval marine fish and pelagic seabirds

AbstractThe thermoregulatory style of Late Cretaceous mosasaurs has become a highly controversial subject in vertebrate palaeontology. These extinct marine reptiles have previously been described as poikilothermic, endothermic or gigantothermic. Here we analyse three genera of mosasaurs from the Mooreville Chalk in Alabama (USA) of differing body mass, and compare their δ18OPO4 derived body temperatures (Tb) with those of coeval poikilothermic fish (Enchodus) and endothermic pelagic seabirds (Ichthyornis). Results show that all mosasaurs, Clidastes (Tb = 33.1°C), Platecarpus (Tb = 36.3°C), and Tylosaurus (Tb = 34.3°C), had elevated average body temperatures in relation to those of the fish (Tb = 28.3°C) and were closer to those of Ichthyornis (Tb = 38.6°C). The temperatures calculated for Enchodus compare well with previously reported temperature estimates for the Mooreville Chalk and the Tb of Ichthyornis compares well with temperatures that have been reported for modern seabirds, suggesting that this method provides accurate results. Finally, although there are small differences of body temperature among mosasaur genera, these are independent of size, and thus inferred body mass, suggesting that mosasaurs were not gigantotherms, but rather endotherms.

Late Cretaceous chimaeroids (Chondrichthyes: Holocephali) from Alabama, USA

Tooth plates of three extinct species of callorhynchid holocephalans, Edaphodon mirificus, E. barberi, and Ischyodus bifurcatus have been collected from Upper Cretaceous strata of Alabama. Of the two species of Edaphodon, E. mirificus is represented by isolated tooth plates as well as associated dentitions. Edaphodon barberi was based on a small left mandibular tooth plate, but additional mandibular tooth plates in museum collections show that the diagnostic features seen on the E. barberi holotype are consistently present and therefore useful for species differentiation. Ischyodus bifurcatus is reported for the first time in Alabama and is known from a partial associated dentition and several isolated tooth plates. Most of the fossils are from the upper Santonian to lower Campanian Mooreville Chalk, but two specimens of Edaphodon from a lower Campanian component of the Tombigbee Sand Member of the Eutaw Formation and one from the lower Maastrichtian Ripley Formation represent the first holocephalan records from these lithostratigraphic units in Alabama.

A new approach for the determination of ammonite and nautilid habitats.

Externally shelled cephalopods were important elements in open marine habitats throughout Earth history. Paleotemperatures calculated on the basis of the oxygen isotope composition of their shells can provide insights into ancient marine systems as well as the ecology of this important group of organisms. In some sedimentary deposits, however, the aragonitic shell of the ammonite or nautilid is poorly or not preserved at all, while the calcitic structures belonging to the jaws are present. This study tests for the first time if the calcitic jaw structures in fossil cephalopods can be used as a proxy for paleotemperature. We first analyzed the calcitic structures on the jaws of Recent Nautilus and compared the calculated temperatures of precipitation with those from the aragonitic shell in the same individuals. Our results indicate that the jaws of Recent Nautilus are secreted in isotopic equilibrium, and the calculated temperatures approximately match those of the shell. We then extended our study to ammonites from the Upper Cretaceous (Campanian) Pierre Shale of the U.S. Western Interior and the age-equivalent Mooreville Chalk of the Gulf Coastal Plain. In the Pierre Shale, jaws occur in situ inside the body chambers of well-preserved Baculites while in the Mooreville Chalk, the jaw elements appear as isolated occurrences in the sediment and the aragonitic shell material is not preserved. For the Pierre Shale specimens, the calculated temperatures of well-preserved jaw material match those of well-preserved shell material in the same individual. Analyses of the jaw elements in the Mooreville Chalk permit a comparison of the paleotemperatures between the two sites, and show that the Western Interior is warmer than the Gulf Coast at that time. In summary, our data indicate that the calcitic jaw elements of cephalopods can provide a reliable geochemical archive of the habitat of fossil forms.

The First Associated Teeth of the Late Cretaceous Anacoracid Shark,Pseudocorax laevis(Leriche), from the Mooreville Chalk of Alabama

Pseudocorax laevis (Leriche) is a Late Cretaceous anacoracid shark. Here, I describe the first associated teeth of P. laevis (along with unidentifiable fragments of calcified cartilage) from the Mooreville Chalk of Alabama. Although the specimen consists only of four teeth, morphological variation observed confirms the presence of heterodonty in the dentition of this taxon.

New Material of North American Side-Necked Turtles (Pleurodira: Bothremydidae)

New cranial and postcranial material of the pleurodire family Bothremydidae, subtribe Bothremydina, from the Gulf Coastal Plain of North America, clarifies the distribution of the genera Bothremys and Chedighaii. New skull material from the Campanian Tar Heel Formation of North Carolina shows the presence of both Bothremys and Chedighaii, based on a maxilla and lower jaw. Additionally, a series of otic chambers and basicrania, while providing important information on morphology, are identified as subtribe Bothremydina, genus and species indeterminate, as the basicranium alone is insufficient to distinguish Bothremys and Chedighaii.An associated skull-shell specimen belonging to the pleurodire subtribe Bothremydina from the Campanian Mooreville Chalk of Alabama, FMNH PR 247, identified in Gaffney et al. (2006) as Chedighaii barberi, is reinterpreted as belonging to the genus Bothremys. A maxilla and jugal fragment found among the material belonging to FMNH PR 247 shows that this specimen has a skull with deep pits on its triturating surface, diagnostic of the genus Bothremys, and in contrast to the flat triturating surface of Chedighaii. Because FMNH PR 247 has an associated partial skull and shell, it was the basis for placing the species “Podocnemis” barberi Schmidt, 1940, the type of which is a shell, in Chedighaii Gaffney et al., 2006. A result of this identification is that the species “Podocnemis” barberi Schmidt, 1940, cannot be assigned to a genus, as the shell morphology of Chedighaii Gaffney et al., 2006, and Bothremys Leidy, 1865, cannot be distinguished at present. A review of the shell material in the subtribe Bothremydina concludes that shells alone are inadequate to reliably distinguish alpha-level taxa in this group at present.It is likely that ALAB PV 2001.2, NCSM 23681, and YPM PU 12951, here referred to Chedighaii sp., belong to a new species of Chedighaii, distinct from Chedighaii hutchisoni, but the material is too incomplete at present to adequately diagnosis it.

Oxygen and carbon isotope analysis of the Mooreville Chalk and late Santonian-early Campanian sea level and sea surface temperature changes, northeastern Gulf of Mexico, U.S.A.

Oxygen and carbon isotope profiles for strata of the Mooreville Chalk (upper Santonian-lower Campanian) of the eastern Gulf Coastal Plain, U.S.A., show correlations with published curves for these isotopes. The δ 18O curve exhibits a strong similarity to the Exmouth Plateau δ 18O curve from ODP drilling sites offshore northwestern Australia, and the δ 13C curve can be correlated with the δ 13C curve from the English Chalk Trunch section. A thermal maximum probably occurred in the northeastern Gulf of Mexico during the latest Santonian (∼83.8 Ma), as evidenced by the minimum δ 18O values in the lower Mooreville beds. A δ 13C positive excursion occurs at the same stratigraphic level, which has been recognized as the “Santonian/Campanian boundary event” worldwide. After this event, ocean surface water temperature decreased throughout the early Campanian. This carbon isotope excursion is followed by a plateau in δ 13C values with a peak value occurring in a condensed section (∼80 Ma), which has been correlated to a downlap or maximum flooding surface on seismic data from offshore Alabama. The section characterized by increasing δ 13C values corresponds to a marine transgression. The interval characterized by decreasing δ 13C values corresponds to regression and progradation. The maximum flooding event occurred ∼0.8 Ma later than the thermal maximum event. The Mooreville chalk/marl cycles are most likely a product of fluctuations in siliciclastic sediment influx into the northeastern Gulf of Mexico modulated by the precession band of the Earth's orbital cycles. Higher carbon isotope values occur in the marl beds indicating that these beds were formed in a more anoxic/dysoxic environment characterized by higher clay, silt input and higher organic carbon accumulation.

Sequence stratigraphy and orbital cyclostratigraphy of the Mooreville Chalk (Santonian–Campanian), northeastern Gulf of Mexico area, USA

Cycles from the depositional sequence level to decimeter-scale rhythmic alteration of chalk and marl beds in the Mooreville Chalk (Santonian–Campanian) in the northeastern Gulf of Mexico area are studied employing an integrated biostratigraphic, cyclostratigraphic, and sequence stratigraphic approach. Two dominant frequencies, identified from the natural gamma-ray log from the Alabama Power Selma Site Test well, site 3 hole 3, are attributed to two of the Milankovitch cycles, the long eccentricity (413 ka) and eccentricity (100 ka) cycles. Parasequences, the building blocks of systems tracts, appear to be modulated by long eccentricity cycles. Individual eccentricity cycles were counted to establish a high-resolution orbital chronology, which agrees with established biostratigraphic chronology. A high-resolution record of temporal changes in sedimentation rate is established on the basis of cyclostratigraphic chronology. Two condensed sections are identified. The lower condensed section is found to correspond to a downlap surface recognized in seismic data in the offshore Alabama and Mississippi area. This condensed section and downlap surface are interpreted to represent the maximum flooding event of a third-order depositional sequence. The maximum flooding event is also represented by a marine flooding surface in eastern Mississippi and by the Mooreville tongue in eastern Alabama. The upper condensed section is found to correspond to a faunal abundance peak recognized in data from outcrop studies, and is interpreted to represent the maximum bathometric surface that was achieved as a result of a condition of rising sea level in association with low sediment input during the deposition of the early highstand systems tract.

Santonian-Maastrichtian planktonic foraminiferal and ostracode biostratigraphy of the northern Gulf Coastal Plain, USA

The Santonian-Maastrichtian deposits of the northern Gulf Coastal Plain of the United States (Mississippi-Alabama) are richly fossiliferous and structurally uncomplicated, offering an excellent setting in which to study calcareous microfossil biostratigraphy. Eight planktonic foraminiferal zones and seven ostracode zones are recognized in these strata, based on the stratigraphic occurrence of 45 species of planktonic foraminifera and 112 taxa of ostracodes. The planktonic foraminiferal zonation is based on standard global biozones. These include the Dicarinella asymetrica Taxon Range Zone, the Globotruncanita elevata Interval Zone, the Globotruncana ventricosa Interval Zone, the Globotruncanita calcarata Taxon Range Zone, the Globotruncanella havanensis Interval Zone, the Globotruncana aegyptiaca Interval Zone, the Gansserina gansseri Interval Zone, and the Contusotruncana contusa-Racemiguembelina fructicosa Interval Zone. Three new ostracode zones are defined, and the upper boundary of two previously defined zones are amended. These zones are the Veenia quadrialira Interval Zone (amended), the Acuminobrachycythere acuminata Interval Zone (new), the Brachycythere pyriforma Interval Zone (new), the Ascetoleberis plummeri Taxon Range Zone (amended), the Curfsina communis Interval Zone (new), the Escharacytheridea pinochii Interval Zone, and the Platycosta lixula Interval Zone. The planktonic foraminiferal biostratigraphy indicates that the age of these deposits range from early Santonian to early Maastrichtian. The Santonian-Campanian boundary is recognized on the basis of the highest occurrence surface of Dicarinella asymetrica, and demonstrates that the contact between the Tombigbee Sand Member of the Eutaw Formation and the Mooreville Chalk is diachronous. The Campanian-Maastrichtian boundary cannot be defined by calcareous microfossils due to the lack of a proxy for the newly established definition. The high-resolution biostratigraphy of the ostracodes, which are indigenous to the North American Coastal Plain, will enable subsequent studies of Late Cretaceous paleobiogeography and Gulf of Mexico-Caribbean tectonic development to be conducted.

On the cranial anatomy of the polycotylid plesiosaurs, including new material of Polycotylus latipinnis, Cope, from Alabama

ABSTRACT The cranial anatomy of plesiosaurs in the family Polycotylidae (Reptilia: Sauropterygia) has received renewed attention recently because various skull characters are thought to indicate plesiosauroid, rather than pliosauroid, affinities for this family. New data on the cranial anatomy of polycotylid plesiosaurs is presented, and is shown to compare closely to the structure of cryptocleidoid plesiosaurs. The morphology of known polycotylid taxa is reported and discussed, and a preliminary phylogenetic analysis is used to establish ingroup relationships of the Cryptocleidoidea. This study also presents new material referable to Polycotylus latipinnis from the Mooreville Chalk Formation of Alabama. This skeleton is largely complete, and although the skull is fragmentary it does preserve several regions not previously represented. The preserved portions demonstrate that the palate of Polycotylus is similar to that of Trina-cromerum in the possession of robust epipterygoids, and similar to that of all other polycotylids in the possession of a large anterior interpterygoid vacuity. Polycotylus therefore displays the derived palate morphology common to all polycotylids, but also displays some plesiomorphic features. This impression is confirmed by the results of the cladistic analysis.

Stratigraphic distribution and habitat segregation of mosasaurs in the Upper Cretaceous of western and central Alabama, with an historical review of alabama mosasaur discoveries

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 Glohidens 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.

Climatically Controlled Redox Cycles of the Upper Cretaceous Mooreville Formation in Mississippi

ABSTRACT The Upper Cretaceous Mooreville Formation in Noxubee County, Mississippi, conformably overlies the sandstones of the Eutaw Formation and is separated from the overlying Demopolis Chalk by the Arcola Limestone. The Mooreville Formation is 268 feet (82 m) thick in this area and consists of 53 redox cycles. The cycles range in thickness from 1 to 11 feet (0.3 to 3.6 m) and display three distinct intervals: a basal laminated, a middle partly bioturbated, and an upper burrowed interval. The basal laminated section indicates sedimentation under anoxic bottom water conditions, hostile to burrowing organisms. The laminated layer grades upward into a gray, partly burrowed interval, suggesting that bottom waters became slightly oxygenated and organisms tolerant of low oxygen conditions were sparsely present. The cycles are capped by thoroughly bioturbated layers indicative of deposition under fully oxygenated conditions. Cycles were initiated during wet climatic periods with the introduction of siliciclastics and terrestrial organic matter into the basin. Oxidation of organic matter at the sediment-water interface or in the uppermost portion of the sediment column resulted in oxygen depletion and bottom water anoxia, leading to the deposition of laminated strata. Riverine input decreased as climatic conditions became arid, limiting the input of terrestrial organic matter. As a result, the amount of oxygen in the sediments increased to the point that deposition of the dysoxic section of the cycle began. Eventually, organic matter input decreased, leading to full oxygenation of bottom water when heavily bioturbated sedimentation resumed. The lower and upper contacts of the Mooreville Formation are approximately marked by the base and top of the Globotruncanita elevata biozone, suggesting 3.5 My of sedimentation at a rate of 2.34 cm/ky. At this sedimentation rate, cycle durations range from 13 ky to 143 ky (average = 64 ky), with major peak frequencies at 30-40 ky, 50-60 ky, and 60-70 ky, none of which correspond to Milankovitch orbital forcing mechanisms of climate change.