Papers by George Phillips
Revista Mexicana de Ciencias Geológicas
The gryphaeid oyster Exogyra Say, 1820, is ubiquitous in Upper Cretaceous sediments in the southe... more The gryphaeid oyster Exogyra Say, 1820, is ubiquitous in Upper Cretaceous sediments in the southeastern United States. Like many oysters (Order Ostreida), Exogyra attached its shell to hard surfaces on the seafloor by means of cementation. Throughout its lifetime, Exogyra may preserve through bioimmuration the characteristics and even skeletal remains of substrate organisms, including mollusk shells, echinoids, and bryozoans. Exogyra costata of all sizes were collected from three different localities within upper Maastrichtian deposits in northeast Mississippi and their bioimmurations analyzed. Substrates were identified and classified to compare the three localities’ substrate taxa in order to probe differences in benthic population structure. The results of this pilot study provide an overview of available surfaces on the Late Cretaceous seafloor on the eastern side of the Mississippi Embayment. The data suggest that taxonomic diversity among utilized substrates may increase from ...
Journal of Crustacean Biology, 2019
Palaeobiodiversity and Palaeoenvironments, 2014
Different cheilostome bryozoan species construct their skeletons from calcite, aragonite, or a co... more Different cheilostome bryozoan species construct their skeletons from calcite, aragonite, or a combination of these two minerals. Calcite is the primitive biomineral in cheilostomes, but an increasing number of clades since the Late Cretaceous have evolved the ability to biomineralize aragonite. This change in bryozoan mineralogy paralleled the switchover from calcite to aragonite seas. Here, we provide a first test of whether changing bryozoan biomineralogy can be correlated with a particular calcite to aragonite sea transition by comparing the mineralogical composition of bryozoan assemblages from the Eocene with those from the Early Oligocene, a time of major climatic and oceanographical changes that many believe to be a time of transition. Bryozoan assemblages from deposits in Mississippi and western Alabama preserving skeletal aragonite (as indicated by the occurrence of aragonitic molluscs) were sampled, cleaned, and sorted by species for XRD analysis to determine their mineralogy. The hypothesis that the proportion of species employing aragonite increases from the Eocene into the Oligocene through the transition to aragonite seas is not supported by our data. Among 23 Eocene species, 14 (61 %) were found to be calcitic, 7 (30 %) aragonitic and 2 (9 %) bimineralic, whereas among 28 Oligocene species 19 (68 %) were calcitic, 5 (18 %) aragonitic and 4 (14 %) bimineralic. Possible reasons why the expected mineralogical change is lacking include incorrect dating of the calcite to aragonite sea switchover at the Eocene-Oligocene boundary, the compensating effects of cooling at this time favouring calcite over aragonite biomineralization, or high levels of biological control over their mineralogy by the bryozoans.
Bulk sampling and extensive, systematic surface collecting of the Coon Creek Member of the Ripley... more Bulk sampling and extensive, systematic surface collecting of the Coon Creek Member of the Ripley Formation (early Maastrichtian) at the Blue Springs locality and primarily bulk sampling of the Owl Creek Formation (late Maastrichtian) at the Owl Creek type locality, both in northeast Mississippi, USA, have produced the largest and most highly diversified actinopterygian otolith (ear stone) assemblage described from the Mesozoic of North America. The 3,802 otoliths represent 30 taxa of bony fishes representing at least 22 families. In addition, there were two different morphological types of lapilli, which were not identifiable to species level. The large number of otolith specimens as well as the preservation contributed to the recognition of 4 new genera and 13 new species. The otoliths supplied information regarding the presence of bony fishes not available solely on the basis of osteological remains, and the Late Cretaceous bony fish assemblage at the sites would be underestimate...
PeerJ, 2017
Ceratopsids (“horned dinosaurs”) are known from western North America and Asia, a distribution re... more Ceratopsids (“horned dinosaurs”) are known from western North America and Asia, a distribution reflecting an inferred subaerial link between the two landmasses during the Late Cretaceous. However, this clade was previously unknown from eastern North America, presumably due to limited outcrop of the appropriate age and depositional environment as well as the separation of eastern and western North America by the Western Interior Seaway during much of the Late Cretaceous. A dentary tooth from the Owl Creek Formation (late Maastrichtian) of Union County, Mississippi, represents the first reported occurrence of Ceratopsidae from eastern North America. This tooth shows a combination of features typical of Ceratopsidae, including a double root and a prominent, blade-like carina. Based on the age of the fossil, we hypothesize that it is consistent with a dispersal of ceratopsids into eastern North America during the very latest Cretaceous, presumably after the two halves of North America w...
Evidence of the end-Cretaceous impact has been widely observed in the eastern Mississippi Embayme... more Evidence of the end-Cretaceous impact has been widely observed in the eastern Mississippi Embayment and northeastern Mexico. Surprisingly, given its proximity to the impact site, indicators of the Chicxulub impact are missing in the western Mississippi Embayment between the sites at Brazos River, TX and Crowley's Ridge, MO. This study documents impact spherules and a possible tsunami deposit related to the Chicxulub impact for the first time in Arkansas. A nine meter section was exposed along the Ouachita River near Malvern, AR that includes the Arkadelphia Formation and the base of the Midway Group. The Arkadelphia Formation is composed of thick beds of marl interbedded with thin beds of limestone. The uppermost limestone contains large pieces of broken shell material and phosphatic nodules, consistent with the vertebrate bed described by Becker et al. (2006). The occurrence of Discoscaphites iris indicates an uppermost Maastrichtian age, which correlates with the CC26b nannofo...
Paläontologische Zeitschrift, 2013
Icriocarcininae Š tevčić, 2005, an extinct North American subfamily of portunoid decapods, is ele... more Icriocarcininae Š tevčić, 2005, an extinct North American subfamily of portunoid decapods, is elevated to family level to contain two Late Cretaceous genera-Icriocarcinus Bishop, 1988, from the Pacific Coast, and Branchiocarcinus Feldmann and Vega, 1995, from the Gulf of Mexico and Atlantic Coast Plain. The family is centered on Icriocarcinus xestos Bishop, 1988, described from the Late Campanian of Baja California. Closely allied with this species are: ''Eryma'' flecta Rathbun, 1926, originally described from the latest Campanian of Tennessee and now known to occur throughout the Maastrichtian of the adjoining Mississippi; Branchiocarcinus cornatus Feldmann and Vega, 1995, from the Maastrichtian of San Luis Potosí, Mexico; and undescribed material from the latest Maastrichtian of New Jersey. The Gulf and Atlantic populations compose a single new species-Branchiocarcinus flectus (Rathbun). Provisionally regarded as a lobster, on the basis of a single, eroded chela, B. flectus is now known from complete bodies found at several locations in the eastern US. The additional material also clarifies the identity of B. cornatus, which is based on a distorted external mold of a lone dorsal carapace. Members of the family lack the typical portunoid flattened fifth pereiopod but share other characters that enable placement within the Portunoidea. Keywords Icriocarcinidae Á Crustacea Á Upper Cretaceous Á Pacific and Gulf Coast Kurzfassung Icriocarcininae Š tevčić, 2005, eine ausgestorbene nordamerikanische Unterfamilie von portunoiden Dekapoden, wird auf Familienniveau gehoben und beinhaltet die zwei oberkretazischen Gattungen Icriocarcinus Bishop, 1988, von der pazifischen Küste und Branchiocarcinus Feldmann und Vega, 1995, vom Golf von Mexiko und der atlantischen Küstenebene. Die Familie zentriert sich um Icriocarcinus xestos Bishop, 1988, beschrieben aus dem Obercampanium von Baja California. Nahe verwandte Arten sind: ''Eryma'' flecta Rathbun, 1926, ursprünglich beschrieben aus dem obersten Campanium von Tenessee, heute bekannt aus dem gesamten Maastrichtium des angrenzenden Mississipi; Branchiocarcinus cornatus Feldmann und Vega, 1995, aus dem Maastrichtium von San Luis Potosi, Mexiko; sowie unbeschriebenes Material aus dem obersten Maastrichtium von New Jersey. Die Populationen vom Golf und Atlantik stellen eine einzige neue Art dar: Branchiocarcinus flectus (Rathbun). Aufgrund von ungenügenden Merkmalen ursprünglich als Hummer angesehen, ist B. flectus heute in Form von vollständigen Körpern aus verschiedenen Lokalitäten der östlichen Vereinigten Staaten bekannt. Das zusätzliche Material klärt auch die Identität von B. cornatus, welcher auf einem deformierten Häutungshemd eines dorsalen Karapax basiert. Mitgliedern dieser Familie fehlt das charakteristische portunoide flache fünfte Pereiopod, aber sie besitzen
Journal of South American Earth Sciences, 2013
The portunoid crab Ophthalmoplax Rathbun, 1935, is known from late Cretaceous deposits of Africa ... more The portunoid crab Ophthalmoplax Rathbun, 1935, is known from late Cretaceous deposits of Africa and the Americas. A review of 76 specimens from many localities in North and South America reveals that the genus is represented by only two species d one in Africa (recently described) and the other in the Americas. Ophthalmoplax brasiliana (Maury, 1930) was distributed along the Atlantic and Gulf coasts of the Americas throughout the Maastrichtian d from Brazil to North Carolina. In early Maastrichtian deposits of North America (w69.0 Ma), the species is represented by local populations of medium-sized individuals, and by the late Maastrichtian (w67.0 Ma), populations of larger individuals became abundant. This size increase may be related to a decrease in ocean water temperatures. Populations of medium-sized individuals are found again in the latest Maastrichtian (w66.2 Ma), below strata with ejecta deposits in Coahuila, Mexico, and in the uppermost Owl Creek Formation, Mississippi. This size decrease is possibly linked to an increase in seawater temperature occurring just below the K/P boundary, when Ophthalmoplax became extinct.
Sedimentary Geology, 2021
Abstract Upper Cretaceous marine sequences in the Gulf Coastal Plain (USA) span the Cretaceous-Pa... more Abstract Upper Cretaceous marine sequences in the Gulf Coastal Plain (USA) span the Cretaceous-Paleogene (K-Pg) transition, allowing for detailed studies of one of the most severe extinction events of the Phanerozoic. To improve the temporal resolution of the stratigraphic record that represents environmental change leading up to the K-Pg boundary, we constructed a high-resolution chemostratigraphy and cyclostratigraphy of upper Maastrichtian shallow marine deposits located in the state of Mississippi (USA). Upper Maastrichtian strata in this area consist of alternating decimeter scale chalk and marl rhythmites deposited in a hemipelagic setting. New geochemical proxy records were used to test whether rhythmic sedimentation was driven by Milankovitch cycles. Stable isotopes (δ13Ccarb, δ18Ocarb), carbonate content (wt% CaCO3), and elemental concentrations (Ti, K, Fe) integrated with microfossil and ammonite biostratigraphy reveal astronomical forcing in the studied record. Spectral estimation suggests that rhythmic bedding was associated with climate change driven by precession (~20 kyr). Obliquity signals are also apparent in our analysis, and short eccentricity (~100 kyr) is inferred from amplitude modulation of precession. Studied sections were correlated at the precession scale with the recently tuned K-Pg succession near Morello, Italy which is stratigraphically equivalent to the well-characterized K-Pg sites in Gubbio, Italy (Bottaccione, Contessa Highway). Additionally, carbon isotope records from the study area exhibit large scale trends throughout the latest Maastrichtian, similar to those observed in the Morello and Bottaccione sections. Thus, we show that Milankovitch-scale climatic signals signals and low-amplitude carbon isotope shifts (
Cretaceous Research, 2016
Abstract The Cretaceous outcrop belt of the Mississippi Embayment in the Gulf Coastal Plain (GCP)... more Abstract The Cretaceous outcrop belt of the Mississippi Embayment in the Gulf Coastal Plain (GCP) spans the Cretaceous/Paleogene (K/Pg) boundary. A detailed reconstruction of this time interval is critical for understanding the nature of biotic and environmental changes preceding the end-Cretaceous Mass Extinction event and for deciphering the likely extinction mechanism (i.e., bolide impact versus volcanism). Eight sections encompassing the K/Pg succession across the Mississippi Embayment were analyzed using biostratigraphic sampling of ammonites, dinoflagellates, and nannofossils. An upper Maastrichtian ammonite zonation is proposed as follows, from oldest to youngest: Discoscaphites conradi Zone, D. minardi Zone, and D. iris Zone. Our study documents that the ammonite zonation established in the Atlantic Coastal Plain (ACP) extends to the GCP. This zonation is integrated with nannofossil and dinoflagellate biostratigraphy to provide a framework to more accurately determine the age relationships in this region. We demonstrate that ammonites and dinoflagellates are more reliable stratigraphic indicators in this area than nannofossils because age-diagnostic nannofossils are not consistently present within the upper Maastrichtian in the GCP. This biostratigraphic framework has the potential to become a useful tool for correlation of strata both within the GCP and between the GCP, Western Interior, and ACP. The presence of the uppermost Maastrichtian ammonite D. iris , calcareous nannofossil Micula prinsii , and dinoflagellates Palynodinium grallator and Disphaerogena carposphaeropsis suggests that the K/Pg succession in the GCP is nearly complete. Consequently, the GCP is an excellent setting for investigating fine scale temporal changes across the K/Pg boundary and ultimately elucidating the mechanisms causing extinction.
The portunoid crab Ophthalmoplax Rathbun, 1935, is known from late Cretaceous deposits of Africa ... more The portunoid crab Ophthalmoplax Rathbun, 1935, is known from late Cretaceous deposits of Africa and the Americas. A review of 76 specimens from many localities in North and South America reveals that the genus is represented by only two species d one in Africa (recently described) and the other in the Americas. Ophthalmoplax brasiliana (Maury, 1930) was distributed along the Atlantic and Gulf coasts of the Americas throughout the Maastrichtian d from Brazil to North Carolina. In early Maastrichtian deposits of North America (w69.0 Ma), the species is represented by local populations of medium-sized individuals, and by the late Maastrichtian (w67.0 Ma), populations of larger individuals became abundant. This size increase may be related to a decrease in ocean water temperatures. Populations of medium-sized individuals are found again in the latest Maastrichtian (w66.2 Ma), below strata with ejecta deposits in Coahuila, Mexico, and in the uppermost Owl Creek Formation, Mississippi. This size decrease is possibly linked to an increase in seawater temperature occurring just below the K/P boundary, when Ophthalmoplax became extinct.
Icriocarcininae Š tevčić, 2005, an extinct North
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Papers by George Phillips