Stratigraphy and Depositional Environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the Type Area, Western Kansas

Strata of the Smoky Hill Chalk Member (Coniacian-Campanian) are exposed extensively in the type area, where 12 key sections are the basis for a composite section that is 181.8 m (596.3 ft) thick. The member consists mainly of olive gray, well-laminated to nonlaminated, flaky-weathering, fecal-pellet-speckled, impure chalk consisting mostly of foraminiferal pelmicrite with wackestone or, less commonly, packstone texture, and characterized by well-stratified grain fabric. Ubiquitous constituents include micritic matrix, coccolith-rich fecal pellets, planktonic foraminiferal tests, wisps and angular silt-sized grains of black organic matter, skeletal remains of fish, and minute framboids of pyrite or its oxidized equivalent. Sparse grains of angular silt-sized quartz are the only detrital grains detected commonly in thin sections. Scattered through the member are very thin to thick beds of lighter-colored, bioturbated chalk and granular (probably microbioturbated) chalk, which form conspicuous bands on little-weathered exposures. Bioturbated and granular chalks contain the same basic components as stratified chalks, but fecal pellets are less obvious, black organic matter is less abundant, and pyrite framboids are less common in the matrix. The Smoky Hill composite section contains more than 100 seams of bentonite, which range in thickness to as much as 11.3 cm (0.37 ft). Gypsum, jarosite, and limonite, usually in some combination, are common along weathered seams. Principal clay minerals in the bentonites are kaolinite (dominant) and smectite, and the most common accessory minerals are quartz, gypsum, and calcite. Diagenetic phenomena of Smoky Hill laminated to nonlaminated chalks include compactional deformation of fecal pellets, foraminifers, sediment-filled burrows, macroinvertebrate remains, and grain fabric around large allochems; incipient microstylolites; dissolution of aragonitic skeletal remains; sparry calcite cement in foraminiferal chambers; interstitial micritic calcite cement; and secondary calcite overgrowths on skeletal remains, especially in matrix portions of the rock. Bioturbated and granular chalks are less well compacted than the well-stratified chalks, and have greater amounts of secondary calcite as interstitial cement and overgrowths on coccoliths. In such chalks, lithification was initiated earlier than in the well-stratified chalks. Bioturbation of these and other Kansas chalk deposits produced textures similar to those ascribed by others to deep-burial diagenesis and solution transfer. Most extensively altered by diagenesis are lenses of Uintacrinus limestone, in which skeletal elements have been altered to microsparite and the rock has been microstylolitized. Several stratigraphic intervals, none more than about 1.5 m (4.9 ft) in thickness, are exceptionally rich in macroinvertebrate skeletal remains, which litter eroded slopes with shelly debris. Lenses of hard, brittle limestone, consisting mainly of Uintacrinus skeletal remains, occur sparingly in the zone of Clioscaphites choteauensis. Sparse biomicrudite lenses, consisting mainly of inoceramid bivalve debris, occur locally in the lower half of the member. Bioturbated chalk beds, granular chalk beds, bentonite seams, shelly zones, and organic-rich chalk beds are useful stratigraphic markers. Twenty-three marker units are described in detail, and are indispensable tools for precise determination of stratigraphic position. The Smoky Hill Member contains vast numbers of macroinvertebrate body fossils, but at most horizons diversity is very low. Ammonites, which are the basis for the standard Western Interior zonation of Smoky Hill and equivalent strata elsewhere, are sparse in all but a few intervals, and many of the standard zonal indices have not been recorded in Kansas. The lower half of the composite section contains four easily recognizable zones, namely those of Inoceramus (Volviceramus) grandis, I. (Cladoceramus) undulatoplicatus, Clioscaphites vermiformis, and C. choteauensis. The upper half of the member contains the single, broadly defined zone of Inoceramus (Endocostea) balticus. On the basis of macroinvertebrate fossils and physical correlation (i.e., marker beds) the composite section has been determined to extend from the Upper Coniacian into the Lower Campanian. Coccolith-rich Smoky Hill muds were deposited on the eastern shelf of the Western Interior Sea. Chalky strata of Kansas grade westward into progressively less calcareous beds, which in the western portion of the ancient seaway are dominated by terrigenous detritus. Stratigraphically upward variations in detrital components reflect varying rates of terrigenous detrital influx from the west. However, the principal component of most Smoky Hill strata is low-magnesium calcite, which consists largely of coccoliths, coccolith debris, coccolith-rich fecal pellets, tests of planktonic foraminifers, interstitial cement, calcite overgrowths on coccoliths, forams and other skeletal remains, and remains of inoceramid and ostreid bivalves. On the basis of biotic and physical considerations, depth of Smoky Hill deposition is estimated to have ranged from a minimum of 150 or 200 m (492-656 ft) to a maximum that probably exceeded 300 m (984 ft). What is now Kansas lay in a warm- or mild-temperate climatic zone. During deposition, Smoky Hill muds were mainly soft, perhaps even soupy oozes. Evidence of bottom currents is minimal, and bottom waters were at most times poorly circulated. Although interstitial conditions were largely anoxic, near absence of infaunal suspension feeders was apparently owing mainly to substrate fluidity. Wide lateral persistence of marker beds, coupled with absence of organically constructed banks, scour channels, or wedge-shaped stratigraphic units, is evidence for almost perfectly flat depositional topography. Based on data deriving from remains of pelagic organisms (e.g., low diversity of coccoliths and planktonic foraminifers and paucity of crinoids and ammonoids) the salinity is judged to have been somewhat lower than normal. Inoceramids, rudists, and conchs of dead cephalopods were the principal substrates for other benthic organisms, such as ostreid bivalves, acrothoracican cirripeds, lepadomorph cirripeds, and clionid sponges. Inoceramids were nearly ubiquitous inhabitants of the sea floor, and manifest a variety of growth forms that reflect adaptation to soft substrates. The numerically predominant ostreid bivalves, represented by as many as four generations on a single host, commonly encrusted all available substrate area, and apparently grew even on the undersides of host inoceramids and rudists. Deposit-feeding organisms, lacking or very sparse in well-stratified chalks, produced bioturbated intervals whenever suitable substrates (i.e., purer chalk beds) were developed. Short intervals of increased circulation fostered proliferation of the macro invertebrate epibenthos, and produced the present shelly zones in which biotic diversity is greater than normal. Nonfragmented epibenthic macroinvertebrates are preserved in life position, bivalve articulation ratios are high, and assemblages have remarkably uniform composition. These features suggest that Smoky Hill assemblages are true fossil communities. Except for infaunal deposit-feeding worms(?), Smoky Hill benthic macro invertebrates were exclusively suspension feeders. Exceedingly soft substrates, possibly combined with marginally oxygenated bottom waters, seem the best explanation for absence of infaunal deposit feeders other than worms(?). Slow depositional rates (0.036 mm per year) help to account for the gigantic size of many Smoky Hill inoceramids and heavy encrustation of substrates by epizoans. The Smoky Hill Member contains significant reserves of biogenic gas, which are being exploited currently in northwesternmost Kansas and adjacent parts of the bordering states. Knowledge of Smoky Hill stratigraphy, petrology, diagenesis, and correlation should enhance exploration for this valuable energy resource

Stratigraphy of the Carlile Shale (Upper Cretaceous) in Kansas

The Carlile Shale of Kansas was examined at numerous places, fossils were collected at 56 localities, and 47 sections were measured. Stratigraphic relations are depicted in graphic columns. The known ranges of macrofossils in the Kansas Carlile are tabulated graphically and characteristic species are figured. From the standpoints of general lithology, chemistry, environment of deposition, mappability, and fauna, the Fairport Chalk Member should be accorded formational status, as should the combined Blue Hill Shale and Codell Sandstone Members. Fairport marker beds include 13 layers of chalky limestone, marly chalk, and bentonite, and Blue Hill marker beds include 7 layers of clay-ironstone concretions, septarian concretions, and bentonite that provide a framework for more detailed studies. The Greenhorn Limestone and Fairport Member of the Carlile are entirely conformable, the Fairport-Blue Hill contact is conformable, and the Blue Hill-Codell contact is gradational both vertically and laterally, but absence of the zones of Scaphites nigricollensis and S. corvensis, which characterize highest Carlile strata in the Black Hills region, in combination with stratigraphic and lithologic evidence, is proof that the Niobrara Chalk rests on the Carlile Shale with regional diastem. Carbonate sediments of the Fairport were generated wholly within the depositional basin and consist of micro- and macrofossils, calcareous ooze, recrystallized calcite, sparry calcite, and fecal pellets of unknown organisms that fed upon coccolithophores. Terrigenous detritus, consisting mostly of clay and very fine silt. is nearly lacking in lower Fairport beds but is more abundant toward the top of the member. Sparseness of detritus, lamination of chalky shales, and paucity of structural features associated with turbulence are evidence that Fairport sediment was deposited under generally quiet conditions, far from shore, in relatively clear to very clear water, and below the depth of normal wave activity. Beds of chalky limestone and marly chalk represent sediment that was reworked by gentle turbulence; calcarenite lenses are the product of short intervals of severe stirring of bottom sediments by waves. Blue Hill and Codell strata are composed chiefly of fine-grained terrigenous detritus deposited rapidly from turbid waters of a nearer-shore environment than that represented by the Fairport, but mostly deeper than that in which wave and current activity is normal. The upward coarsening of detritus in the Blue Hill-Codell section and local development of cross-laminated and ripple-marked beds near the top of the Carlile indicates the approach to the Kansas area of a regressive shoreline. Abundant septarian concretions and pyrite nodules of the Blue Hill Member are products of early diagenesis. Slow sedimentation and general lack of turbidity during most of Fairport deposition favored development of an extensive benthonic fauna in well-oxygenated water. Inoceramus cuvieri, which commonly reached a breadth of 3 feet, provided the substratum for an extensive epizoal growth of Ostrea congesta, bryozoans, serpulids, and barnacles. Normal salinity is indicated by the fauna. Increased turbidity during Blue Hill deposition reduced the sessile benthos to a small number of O. congesta; salinity remained normal. The benthonic fauna is evidence of well-oxygenated water; the near absence of fossils at many exposures is the result of nonpreservation rather than inhospitable bottom conditions. The Fairport Member lies wholly within the range of Collignoniceras woollgari. The Blue Hill Member coincides with the range of Collignoniceras hyatti. Codell strata of Kansas contain few macroinvertebrates, but Prionocyclus wyomingensis is abundant at one locality. The zone of Scaphites warreni, which normally lies between the zones of C. hyatti and P. wyomingensis, cannot be distinguished in Kansas, but strata in that part of the section are entirely conformable. Evidence is summarized for correlation of the Kansas Carlile with strata of the Western Interior and Gulf Coastal Plain and with the zone of Terebratulina lata and at least the lower part of the zone of Holaster planus of the English Middle Chalk on the basis of stratigraphic distribution of C. woollgari, Inoceramus labiatus (broad form), I. latus, and Prionocyclus. Strata of the Carlile Shale of Kansas represent the regressive half of the first major Late Cretaceous cycle of sedimentation in the Western Interior region. Chief elements of the cycle are (1) siltstone or sandstone, (2) dark-gray silty and sandy clay shale, (3) dark-gray locally concretionary silty clay shale, (4) chalky shale and limestone, (5) dark-gray concretionary silty clay shale, (6) dark-gray silty and sandy shale, and (7) siltstone or sandstone. The first phase of deposition is represented by the Dakota Formation, the second and third by the lower and upper parts of the Graneros Shale, respectively. The fourth phase (maximum transgression) is represented by the Greenhorn and Fairport; regression began during Fairport sedimentation. The fifth and sixth phases are represented by the lower and upper parts of the Blue Hill, respectively, and the last phase by the Codell

Stratigraphy and Depositional Environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the Type Area, Western Kansas

Strata of the Smoky Hill Chalk Member (Coniacian-Campanian) are exposed extensively in the type area, where 12 key sections are the basis for a composite section that is 181.8 m (596.3 ft) thick. The member consists mainly of olive gray, well-laminated to nonlaminated, flaky-weathering, fecal-pellet-speckled, impure chalk consisting mostly of foraminiferal pelmicrite with wackestone or, less commonly, packstone texture, and characterized by well-stratified grain fabric. Ubiquitous constituents include micritic matrix, coccolith-rich fecal pellets, planktonic foraminiferal tests, wisps and angular silt-sized grains of black organic matter, skeletal remains of fish, and minute framboids of pyrite or its oxidized equivalent. Sparse grains of angular silt-sized quartz are the only detrital grains detected commonly in thin sections. Scattered through the member are very thin to thick beds of lighter-colored, bioturbated chalk and granular (probably microbioturbated) chalk, which form conspicuous bands on little-weathered exposures. Bioturbated and granular chalks contain the same basic components as stratified chalks, but fecal pellets are less obvious, black organic matter is less abundant, and pyrite framboids are less common in the matrix. The Smoky Hill composite section contains more than 100 seams of bentonite, which range in thickness to as much as 11.3 cm (0.37 ft). Gypsum, jarosite, and limonite, usually in some combination, are common along weathered seams. Principal clay minerals in the bentonites are kaolinite (dominant) and smectite, and the most common accessory minerals are quartz, gypsum, and calcite. Diagenetic phenomena of Smoky Hill laminated to nonlaminated chalks include compactional deformation of fecal pellets, foraminifers, sediment-filled burrows, macroinvertebrate remains, and grain fabric around large allochems; incipient microstylolites; dissolution of aragonitic skeletal remains; sparry calcite cement in foraminiferal chambers; interstitial micritic calcite cement; and secondary calcite overgrowths on skeletal remains, especially in matrix portions of the rock. Bioturbated and granular chalks are less well compacted than the well-stratified chalks, and have greater amounts of secondary calcite as interstitial cement and overgrowths on coccoliths. In such chalks, lithification was initiated earlier than in the well-stratified chalks. Bioturbation of these and other Kansas chalk deposits produced textures similar to those ascribed by others to deep-burial diagenesis and solution transfer. Most extensively altered by diagenesis are lenses of Uintacrinus limestone, in which skeletal elements have been altered to microsparite and the rock has been microstylolitized. Several stratigraphic intervals, none more than about 1.5 m (4.9 ft) in thickness, are exceptionally rich in macroinvertebrate skeletal remains, which litter eroded slopes with shelly debris. Lenses of hard, brittle limestone, consisting mainly of Uintacrinus skeletal remains, occur sparingly in the zone of Clioscaphites choteauensis. Sparse biomicrudite lenses, consisting mainly of inoceramid bivalve debris, occur locally in the lower half of the member. Bioturbated chalk beds, granular chalk beds, bentonite seams, shelly zones, and organic-rich chalk beds are useful stratigraphic markers. Twenty-three marker units are described in detail, and are indispensable tools for precise determination of stratigraphic position. The Smoky Hill Member contains vast numbers of macroinvertebrate body fossils, but at most horizons diversity is very low. Ammonites, which are the basis for the standard Western Interior zonation of Smoky Hill and equivalent strata elsewhere, are sparse in all but a few intervals, and many of the standard zonal indices have not been recorded in Kansas. The lower half of the composite section contains four easily recognizable zones, namely those of Inoceramus (Volviceramus) grandis, I. (Cladoceramus) undulatoplicatus, Clioscaphites vermiformis, and C. choteauensis. The upper half of the member contains the single, broadly defined zone of Inoceramus (Endocostea) balticus. On the basis of macroinvertebrate fossils and physical correlation (i.e., marker beds) the composite section has been determined to extend from the Upper Coniacian into the Lower Campanian. Coccolith-rich Smoky Hill muds were deposited on the eastern shelf of the Western Interior Sea. Chalky strata of Kansas grade westward into progressively less calcareous beds, which in the western portion of the ancient seaway are dominated by terrigenous detritus. Stratigraphically upward variations in detrital components reflect varying rates of terrigenous detrital influx from the west. However, the principal component of most Smoky Hill strata is low-magnesium calcite, which consists largely of coccoliths, coccolith debris, coccolith-rich fecal pellets, tests of planktonic foraminifers, interstitial cement, calcite overgrowths on coccoliths, forams and other skeletal remains, and remains of inoceramid and ostreid bivalves. On the basis of biotic and physical considerations, depth of Smoky Hill deposition is estimated to have ranged from a minimum of 150 or 200 m (492-656 ft) to a maximum that probably exceeded 300 m (984 ft). What is now Kansas lay in a warm- or mild-temperate climatic zone. During deposition, Smoky Hill muds were mainly soft, perhaps even soupy oozes. Evidence of bottom currents is minimal, and bottom waters were at most times poorly circulated. Although interstitial conditions were largely anoxic, near absence of infaunal suspension feeders was apparently owing mainly to substrate fluidity. Wide lateral persistence of marker beds, coupled with absence of organically constructed banks, scour channels, or wedge-shaped stratigraphic units, is evidence for almost perfectly flat depositional topography. Based on data deriving from remains of pelagic organisms (e.g., low diversity of coccoliths and planktonic foraminifers and paucity of crinoids and ammonoids) the salinity is judged to have been somewhat lower than normal. Inoceramids, rudists, and conchs of dead cephalopods were the principal substrates for other benthic organisms, such as ostreid bivalves, acrothoracican cirripeds, lepadomorph cirripeds, and clionid sponges. Inoceramids were nearly ubiquitous inhabitants of the sea floor, and manifest a variety of growth forms that reflect adaptation to soft substrates. The numerically predominant ostreid bivalves, represented by as many as four generations on a single host, commonly encrusted all available substrate area, and apparently grew even on the undersides of host inoceramids and rudists. Deposit-feeding organisms, lacking or very sparse in well-stratified chalks, produced bioturbated intervals whenever suitable substrates (i.e., purer chalk beds) were developed. Short intervals of increased circulation fostered proliferation of the macro invertebrate epibenthos, and produced the present shelly zones in which biotic diversity is greater than normal. Nonfragmented epibenthic macroinvertebrates are preserved in life position, bivalve articulation ratios are high, and assemblages have remarkably uniform composition. These features suggest that Smoky Hill assemblages are true fossil communities. Except for infaunal deposit-feeding worms(?), Smoky Hill benthic macro invertebrates were exclusively suspension feeders. Exceedingly soft substrates, possibly combined with marginally oxygenated bottom waters, seem the best explanation for absence of infaunal deposit feeders other than worms(?). Slow depositional rates (0.036 mm per year) help to account for the gigantic size of many Smoky Hill inoceramids and heavy encrustation of substrates by epizoans. The Smoky Hill Member contains significant reserves of biogenic gas, which are being exploited currently in northwesternmost Kansas and adjacent parts of the bordering states. Knowledge of Smoky Hill stratigraphy, petrology, diagenesis, and correlation should enhance exploration for this valuable energy resource

Stratigraphy of the Carlile Shale (Upper Cretaceous) in Kansas

The Carlile Shale of Kansas was examined at numerous places, fossils were collected at 56 localities, and 47 sections were measured. Stratigraphic relations are depicted in graphic columns. The known ranges of macrofossils in the Kansas Carlile are tabulated graphically and characteristic species are figured. From the standpoints of general lithology, chemistry, environment of deposition, mappability, and fauna, the Fairport Chalk Member should be accorded formational status, as should the combined Blue Hill Shale and Codell Sandstone Members. Fairport marker beds include 13 layers of chalky limestone, marly chalk, and bentonite, and Blue Hill marker beds include 7 layers of clay-ironstone concretions, septarian concretions, and bentonite that provide a framework for more detailed studies. The Greenhorn Limestone and Fairport Member of the Carlile are entirely conformable, the Fairport-Blue Hill contact is conformable, and the Blue Hill-Codell contact is gradational both vertically and laterally, but absence of the zones of Scaphites nigricollensis and S. corvensis, which characterize highest Carlile strata in the Black Hills region, in combination with stratigraphic and lithologic evidence, is proof that the Niobrara Chalk rests on the Carlile Shale with regional diastem. Carbonate sediments of the Fairport were generated wholly within the depositional basin and consist of micro- and macrofossils, calcareous ooze, recrystallized calcite, sparry calcite, and fecal pellets of unknown organisms that fed upon coccolithophores. Terrigenous detritus, consisting mostly of clay and very fine silt. is nearly lacking in lower Fairport beds but is more abundant toward the top of the member. Sparseness of detritus, lamination of chalky shales, and paucity of structural features associated with turbulence are evidence that Fairport sediment was deposited under generally quiet conditions, far from shore, in relatively clear to very clear water, and below the depth of normal wave activity. Beds of chalky limestone and marly chalk represent sediment that was reworked by gentle turbulence; calcarenite lenses are the product of short intervals of severe stirring of bottom sediments by waves. Blue Hill and Codell strata are composed chiefly of fine-grained terrigenous detritus deposited rapidly from turbid waters of a nearer-shore environment than that represented by the Fairport, but mostly deeper than that in which wave and current activity is normal. The upward coarsening of detritus in the Blue Hill-Codell section and local development of cross-laminated and ripple-marked beds near the top of the Carlile indicates the approach to the Kansas area of a regressive shoreline. Abundant septarian concretions and pyrite nodules of the Blue Hill Member are products of early diagenesis. Slow sedimentation and general lack of turbidity during most of Fairport deposition favored development of an extensive benthonic fauna in well-oxygenated water. Inoceramus cuvieri, which commonly reached a breadth of 3 feet, provided the substratum for an extensive epizoal growth of Ostrea congesta, bryozoans, serpulids, and barnacles. Normal salinity is indicated by the fauna. Increased turbidity during Blue Hill deposition reduced the sessile benthos to a small number of O. congesta; salinity remained normal. The benthonic fauna is evidence of well-oxygenated water; the near absence of fossils at many exposures is the result of nonpreservation rather than inhospitable bottom conditions. The Fairport Member lies wholly within the range of Collignoniceras woollgari. The Blue Hill Member coincides with the range of Collignoniceras hyatti. Codell strata of Kansas contain few macroinvertebrates, but Prionocyclus wyomingensis is abundant at one locality. The zone of Scaphites warreni, which normally lies between the zones of C. hyatti and P. wyomingensis, cannot be distinguished in Kansas, but strata in that part of the section are entirely conformable. Evidence is summarized for correlation of the Kansas Carlile with strata of the Western Interior and Gulf Coastal Plain and with the zone of Terebratulina lata and at least the lower part of the zone of Holaster planus of the English Middle Chalk on the basis of stratigraphic distribution of C. woollgari, Inoceramus labiatus (broad form), I. latus, and Prionocyclus. Strata of the Carlile Shale of Kansas represent the regressive half of the first major Late Cretaceous cycle of sedimentation in the Western Interior region. Chief elements of the cycle are (1) siltstone or sandstone, (2) dark-gray silty and sandy clay shale, (3) dark-gray locally concretionary silty clay shale, (4) chalky shale and limestone, (5) dark-gray concretionary silty clay shale, (6) dark-gray silty and sandy shale, and (7) siltstone or sandstone. The first phase of deposition is represented by the Dakota Formation, the second and third by the lower and upper parts of the Graneros Shale, respectively. The fourth phase (maximum transgression) is represented by the Greenhorn and Fairport; regression began during Fairport sedimentation. The fifth and sixth phases are represented by the lower and upper parts of the Blue Hill, respectively, and the last phase by the Codell

Redox‐controlled preservation of organic matter during “OAE 3” within the Western Interior Seaway

During the Cretaceous, widespread black shale deposition occurred during a series of Oceanic Anoxic Events (OAEs). Multiple processes are known to control the deposition of marine black shales, including changes in primary productivity, organic matter preservation, and dilution. OAEs offer an opportunity to evaluate the relative roles of these forcing factors. The youngest of these events—the Coniacian to Santonian OAE 3—resulted in a prolonged organic carbon burial event in shallow and restricted marine environments including the Western Interior Seaway. New high‐resolution isotope, organic, and trace metal records from the latest Turonian to early Santonian Niobrara Formation are used to characterize the amount and composition of organic matter preserved, as well as the geochemical conditions under which it accumulated. Redox sensitive metals (Mo, Mn, and Re) indicate a gradual drawdown of oxygen leading into the abrupt onset of organic carbon‐rich (up to 8%) deposition. High Hydrogen Indices (HI) and organic carbon to total nitrogen ratios (C:N) demonstrate that the elemental composition of preserved marine organic matter is distinct under different redox conditions. Local changes in δ13C indicate that redox‐controlled early diagenesis can also significantly alter δ13Corg records. These results demonstrate that the development of anoxia is of primary importance in triggering the prolonged carbon burial in the Niobrara Formation. Sea level reconstructions, δ18O results, and Mo/total organic carbon ratios suggest that stratification and enhanced bottom water restriction caused the drawdown of bottom water oxygen. Increased nutrients from benthic regeneration and/or continental runoff may have sustained primary productivity.Key PointsBottom water redox changes triggered carbon burial within the WIS during OAE 3Anoxia developed due to O2 drawdown in a stratified water columnRedox‐controlled changes in OM preservation altered primary δ13Corg signalsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112294/1/palo20210.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/112294/2/palo20210-sup-0001-SupportingInfo.pd

Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur

Mosasaurs (family Mosasauridae) are a diverse group of secondarily aquatic lizards that radiated into marine environments during the Late Cretaceous (98–65 million years ago). For the most part, they have been considered to be simple anguilliform swimmers – i.e., their propulsive force was generated by means of lateral undulations incorporating the greater part of the body – with unremarkable, dorsoventrally narrow tails and long, lizard-like bodies. Convergence with the specialized fusiform body shape and inferred carangiform locomotory style (in which only a portion of the posterior body participates in the thrust-producing flexure) of ichthyosaurs and metriorhynchid crocodyliform reptiles, along with cetaceans, has so far only been recognized in Plotosaurus, the most highly derived member of the Mosasauridae. Here we report on an exceptionally complete specimen (LACM 128319) of the moderately derived genus Platecarpus that preserves soft tissues and anatomical details (e.g., large portions of integument, a partial body outline, putative skin color markings, a downturned tail, branching bronchial tubes, and probable visceral traces) to an extent that has never been seen previously in any mosasaur. Our study demonstrates that a streamlined body plan and crescent-shaped caudal fin were already well established in Platecarpus, a taxon that preceded Plotosaurus by 20 million years. These new data expand our understanding of convergent evolution among marine reptiles, and provide insights into their evolution's tempo and mode