Browsing by Author "O'Leary, Maureen A."
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Item A composite section of fossiliferous Late Cretaceous--Early Paleogene localities in Senegal and preliminary description of a new late Maastrichtian vertebrate fossil assemblage (American Museum novitates, no. 4013)(American Museum of Natural History., 2024-05-08) Sarr, Raphaël.; Hill, Robert V.; Jenkins, Xavier A.; Tapanila, Leif.; O'Leary, Maureen A.We describe new macro- and microfossils recovered on a field expedition to the North Quarry of Poponguine, a locality in western Senegal that spans the Late Cretaceous through the Early Paleogene, albeit with a likely unconformity at the base of the Danian. Newly discovered macrovertebrates from the Maastrichtian Cap de Naze Formation include pycnodonts, dyrosaurids, and chelonians, the latter two the oldest and first Cretaceous representatives of these clades from Senegal. Screenwashing of this deposit revealed that the matrix also consists of abundant microscopic biological clasts comprising osteichthyan vertebrae, spines, and cranial fragments, shark dermal denticles, invertebrates, and numerous ovoid coprolites among other fragments. These microfossils document a much less conspicuous portion of the paleobiodiversity. We describe the stratigraphy of this new locality and incorporate it into a new correlated section that ties together three other Late Cretaceous--Early Paleogene fossiliferous localities of the Senegalese-Mauritanian Basin. The presence of fossiliferous Danian rocks, which are notably rare in West Africa, is elucidated in our section, which synthesizes prior geological and paleontological work around Poponguine and in the nearby coastal region of Ndayane. Primarily dated using the biostratigraphy of ostracods and foraminiferans, the correlated rock units include the Paki and the Cap de Naze Formations of the Late Cretaceous Diass Group and the Ndayane and the Poponguine Formations of the Paleocene Cap-Vert Group, deposits that capture ancient near shore marine environments. The localities described, although separated by only a few kilometers, exhibit dramatic differences in thickness across faulted blocks of the Diass Horst. Dyrosaurids, which are common fossils in nearshore marine outcrops of West Africa, have been hypothesized to be of African origin and to have dispersed to South America in the Late Cretaceous. This dyrosaurid specimen, although fragmentary, documents a western extreme in the geographic range of African dyrosaurids where the clade would have been well situated for broader trans-Atlantic dispersal.Item Acleistochelys, a new side-necked turtle (Pelomedusoides, Bothremydidae) from the Paleocene of Mali ; American Museum novitates, no. 3549(New York, NY : American Museum of Natural History, 2007) Gaffney, Eugene S.; Roberts, Eric (Eric M.); Sissoko, Famory.; Bouaré, Mohamed L.; Tapanila, Leif Michael.; O'Leary, Maureen A.The Paleocene Teberemt Formation south of the Adrar des Iforas Mountains, between Saguirilidad and In Fargas, Mali, yielded a nearly complete skull of a new genus and species of side-necked turtle, Acleistochelys maliensis. Acleistochelys is a member of the family Bothremydidae Baur, 1891, because: (1) the fossa precolumellaris is absent, (2) the foramen stapedio-temporale faces anteriorly, (3) the eustachian tube is separated from the stapes by bone, and (4) an exoccipital-quadrate contact is present. Within the Bothremydidae, Acleistochelys belongs to the tribe Taphrosphyini because: (1) the maxilla-quadratojugal contact is absent, (2) the palate is dorsally arched, (3) there is only a small contribution of the palatine to the triturating surfaces, and (4) the septum orbitotemporale is at least partially open. Acleistochelys is most closely related to Azabbaremys because both share a narrow vomer lacking a posterior attachment to the palatines. The specimen was found in a marine limestone associated with crocodiles, echinoids, and mollusks.Item An anatomical and phylogenetic study of the osteology of the petrosal of extant and extinct artiodactylans (Mammalia) and relatives. (Bulletin of the American Museum of Natural History, no. 335)(American Museum of Natural History., 2010) O'Leary, Maureen A.I describe and figure petrosal bones for a sample of 35 (12 extinct, 23 extant) artiodactylans, perissodactylans, †mesonychids, and archaic ungulates. Detailed herein are the cladistic characters of the petrosal used in the recent combined analysis of molecular and morphological data by Spaulding et al. (2009). That analysis, the largest in taxa and characters for artiodactylans (including cetaceans) to date, showed that hippopotamids are the closest living relatives of cetaceans. It also showed that in the shortest trees †Indohyus is on the stem lineage to Cetacea and that †mesonychians are positioned outside Artiodactyla; however, these positions for fossils are highly unstable, as †mesonychians are more closely related to cetaceans than is †Indohyus in trees only two steps longer. I show that in many ways the osteology of the hippopotamid ear resembles that of certain stem cetaceamorphans more than it resembles the ear regions of suines (pigs and peccaries). Previous studies have suggested that many artiodactylans lacked an inflated tegmen tympani of the petrosal; however, that generalization is not supported by data presented herein. Petrosal characters, such as the presence of the prefacial commissure fossa, presence of a convex and hyperinflated tegmen tympani, and the absence of a subarcuate fossa, are shown to be synapomorphies of hippopotamids and cetaceans. Some of these features were previously argued to represent a special similarity between †mesonychids and cetaceans, but these are here interpreted as homoplasies. Other features previously argued to be extremely similar between †mesonychians and cetaceans to the exclusion of other ungulates, such as the presence of the anterior process of the tegmen tympani, are shown to be more widely distributed among ungulates than previously recognized. A number of artiodactylans, including ruminants, are also shown to have transpromontorial sulci on the petrosal despite reports that the internal carotid artery is absent in the neck of ruminants. The petrosals of †anthracothere and †entelodont species exhibit varied morphology, with the †anthracothere †Bothriogenys having the greatest gross similarity to the hippopotamid and cetaceamorphan condition; however, shortest trees indicate that these similarities are convergent.Item Comparative basicranial anatomy of extant terrestrial and semiaquatic Artiodactyla. (Bulletin of the American Museum of Natural History, no. 409)(American Museum of Natural History., 2016-10-07) O'Leary, Maureen A.Comparative data from the ear region has played an important role in recent combined-data phylogenetic analyses of the relationships of living and extinct Artiodactyla and the position of that clade among Euungulata. These studies have also been important for establishing the phylogenetic position of Cetacea and for understanding the relationships of a diversity of euungulate species to their fossil relatives. Detailed and standardized descriptive reference works of the basicranium for a range of living artiodactylans are not, however, readily available. Here I describe exemplar species from the four major extant terrestrial and semiaquatic artiodactylan clades (Hippopotamidae, Ruminantia, Suina, and Camelidae) and illustrate the anatomy of the ear region with the auditory bulla both in place and removed. Terrestrial artiodactyls exhibit varying degrees of expansion of the bony external acoustic meatus laterally relative to the mediolateral dimensions of the rounded, medial aspect of the auditory bulla, a characteristic that is least developed in Tragulidae. A relatively elongate external acoustic meatus has previously been described as entirely absent in living and fossil cetaceans and in some fossil species such as Diacodexis pakistanensis. Variation also exists in the proximity of the petrosal-bullar complex to midline basicranial bones. Isolation of these bones from other basicranial structures has been previously interpreted as functionally important for underwater hearing in Cetacea. Many artiodactylans have contact between the auditory bulla and the basioccipital but no contact between the deeper pars cochlearis of the petrosal bone and the basioccipital/basisphenoid. Exceptions are species of Hippopotamidae in which both the bulla and the petrosal are separated from midline bones. The functional interpretation of this separation has previously been linked to aquatic hearing, but this association may be more complex than originally thought. Other features observed in the basicrania of terrestrial artiodactylans described here are a general coalescence of basicranial foramina (i.e., the basicapsular fissure, carotid foramen, piriform fenestra, and sometimes the foramen ovale), the development of large and ornate styliform processes in species of Ruminantia, and widespread contact between the auditory bulla and the paracondylar process of the exoccipital.Item Dyrosaurid (Crocodyliformes, Mesoeucrocodylia) fossils from the Upper Cretaceous and Paleogene of Mali : implications for phylogeny and survivorship across the K-T boundary. American Museum novitates, no. 3631.(New York, NY : American Museum of Natural History., 2008) Hill, Robert V.; McCartney, Jacob A.; Roberts, Eric (Eric M.); Bouaré, Mohamed L.; Sissoko, Famory.; O'Leary, Maureen A.We describe new dyrosaurid fossils from three localities in Mali, representing strata of Maastrichtian, Paleocene, and Eocene ages. The fossils significantly extend the temporal and geographic ranges of several known dyrosaurid taxa. Rhabdognathus keiniensis and Chenanisuchus lateroculi are identified for the first time from Maastrichtian sediments. Additional material is referred to Phosphatosaurus gavialoides and, tentatively, the genus Sokotosuchus. These discoveries represent the first occurrence of Chenanisuchus and possibly of Sokotosuchus from Mali. Previously unknown morphological character states are incorporated into existing data matrices, reducing the amount of missing data. Phylogenetic analyses largely corroborate prior hypotheses of dyrosaurid relationships, but indicate a need for new characters to resolve the relationships of certain genera and species. The occurrence of both basal (e.g., Chenanisuchus lateroculi) and highly nested (e.g., Rhabdognathus keiniensis) members of Dyrosauridae on both sides of the K/T boundary indicates that dyrosaurid diversification was well underway by the latest Cretaceous, and that most, if not all dyrosaurid species survived the extinction event. The geology of the Mali’s Tilemsi Valley is clarified; some rocks previously assigned to the Iullemmeden Basin actually represent extensions of other basins: the Taoudeni Basin and Gao Trench.Item Morphology of the humerus of Hapalodectes (Mammalia, Mesonychia). American Museum novitates ; no. 3242(New York, NY : American Museum of Natural History, 1998) O'Leary, Maureen A."A distal humerus of the primitive mesonychian Hapalodectes cf. leptognathus from the early Eocene of Colorado reveals that Hapalodectes was less cursorially derived than were other members of the Mesonychidae such as Dissacus and Mesonyx. The humerus has a moderately long deltopectoral crest, a relatively wide distal articular surface with a cylindrical capitulum, a relatively broad entepicondyle with an entepicondylar foramen, and a shallow olecranon fossa that lacks a supratrochlear foramen. These skeletal features are associated with terrestrial locomotion but not with cursoriality. This specimen represents some of the first data on the postcranial skeleton of this taxon, data that are relevant both to determination of the sister taxon of Cetacea and for resolving whether or not Mesonychia is paraphyletic"--P. [1].Item Stratigraphy and paleobiology of the Upper Cretaceous-Lower Paleogene sediments from the Trans-Saharan Seaway in Mali. (Bulletin of the American Museum of Natural History, no. 436)(American Museum of Natural History., 2019-06-28) O'Leary, Maureen A.; Bouaré, Mohamed L.; Claeson, Kerin M.; Heilbronn, Kelly.; Hill, Robert V.; McCartney, Jacob A.; Sessa, Jocelyn A.; Sissoko, Famory.; Tapanila, Leif.; Wheeler, E. A.; Roberts, Eric (Eric M.)An epicontinental sea bisected West Africa periodically from the Late Cretaceous to the early Eocene, in dramatic contrast to the current Sahara Desert that dominates the same region today. Known as the Trans-Saharan Seaway, this warm and shallow ocean was a manifestation of globally elevated sea level associated with the rapid break-up of the supercontinent Gondwana in the late Mesozoic. Although it varied in size through time, the Trans-Saharan Seaway is estimated to have covered as much as 3000 km2 of the African continent and was approximately 50 m deep. The edges of the sea were defined in part by the high topography of the Precambrian cratons and mobile belts of West Africa. Over its approximately 50 million year episodic existence, through six major periods of transgression and regression, the Trans-Saharan Seaway left behind extensive nearshore marine sedimentary strata with abundant fossils. The waters that yielded these deposits supported and preserved the remains of numerous vertebrate, invertebrate, plant, and microbial species that are now extinct. These species document a regional picture of ancient tropical life that spanned two major Earth events: the Cretaceous-Paleogene (K-Pg) boundary and the Paleocene-Eocene Thermal Maximum (PETM). Whereas extensive epeiric seas flooded the interior portions of most continents during these intervals, the emerging multicontinental narrative has often overlooked the Trans-Saharan Seaway, in part because fundamental research, including the naming of geological formations and the primary description and analysis of fossil species, remained to be done. We provide such synthesis here based on two decades of fieldwork and analyses of sedimentary deposits in the Republic of Mali. Northern parts of the Republic of Mali today include some of the farthest inland reaches of the ancient sea. We bring together and expand on our prior geological and paleontological publications and provide new information on ancient sedimentary rocks and fossils that document paleoequatorial life of the past. Ours is the first formal description of and nomenclature for the Upper Cretaceous and Lower Paleogene geological formations of this region and we tie these names to regional correlations over multiple modern territorial boundaries. The ancient seaway left intriguing and previously unclassified phosphate deposits that, quite possibly, represent the most extensive vertebrate macrofossil bone beds known from anywhere on Earth. These bone beds, and the paper shales and carbonates associated with them, have preserved a diverse assemblage of fossils, including a variety of new species of invertebrates and vertebrates, rare mammals, and trace fossils. The shallow marine waters included a wide range of paleoenvironments from delta systems, to hypersaline embayments, protected lagoons, and carbonate shoals. Our overarching goal has been to collect vertebrate fossils tied to a K-Pg stratigraphic section in Africa. We provide such a section and, consistent with prior ideas, indicate that there is a gap in sedimentation in Malian rocks in the earliest Paleocene, an unconformity also proposed elsewhere in West Africa. Our phylogenetic analyses of several vertebrate clades across the K-Pg boundary have clarified clade-by-clade species-level survivorship and range extensions for multiple taxa. Few macrofossil species from the Trans-Saharan Seaway show conspicuous change at either the K-Pg boundary or the PETM based on current evidence, although results are very preliminary. Building on our earlier report of the first record of rock-boring bivalves from the Paleocene of West Africa, we further describe here a Cretaceous and Paleogene mollusk fauna dominated by taxa characteristic of the modern tropics. Among the newly discovered fossil osteichthyans, large body size characterizes both the pycnodonts and a new freshwater Eocene catfish species, one of the largest fossil catfishes found in Africa. Our new paleoecological and faunal reconstructions show an evergreen, broadleaf forest that included some of the oldest mangroves known. The ancient Malian ecosystem had numerous apex predators including Crocodyliformes, Serpentes, and Amiidae, some of which were among the largest species in their clades. The Trans-Saharan Seaway exhibited intermittent isolation from major seas. This environmental variable may have created aquatic centers of endemism, stimulating selection for gigantism as previously observed for species on terrestrial islands.