NAPC 2001

Abstracts, Be - Bo
(5/17/01)

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POSSIBLE PREY ACQUISITION BEHAVIOR FOR THE CRETACEOUS FISH XIPHACTINUS AUDAX

BEAMON, Joseph C., Museum of the Rockies, Montana State University, Bozeman, MT, USA

Prey acquisition behavior of the Cretaceous marine fish Xiphactinus audax can be inferred from a combination of its morphology and preserved gut contents. Many specimens have been found with relatively large fishes in the gut region (6 ft. Gillicus arcuatus inside the 15 foot X. audax specimen at the Sternberg Museum of Natural History), seemingly swallowed head first. The hypothesis presented here is that Xiphactinus audax was equivalent to a modern piscivorous "lie-in-wait" predator (i.e., tuna, barracuda) with the ability to take relatively large fishes.

Adaptations for acquiring large prey include a fully ossified and multifaceted vomeroethmoid unit and a basipterygoid process anchoring the palate to the neurocranium, restricting antero-posterior palatal movement and bracing the hyopalatine arch against strong postero-laterally directed forces of large, struggling prey. Epaxial muscles attached to the supraoccipital crest could be used to lift the entire nuerocranium, allowing the mouth to open wider for large prey.

Xiphactinus audax had the body outline and fin placement of a modern "lie-in-wait" predator. The body is fusiform in lateral view and laterally compressed, with the anal, dorsal and pelvic fins placed posteriorly along the body and the pectoral and pelvic fins set ventrally. The caudal fin is lunate in shape, with a narrow caudal peduncle. Modern fishes with the above features approach their prey slowly, waiting until they are within range for a swift lunge. The eyes and mouth are both oriented dorsally, indicating that prey was approached from below. In addition, the teeth are large and conical, considered adaptations for piscivory in modern fishes, and the premaxillary teeth are enlarged, possibly for dealing with large prey.

THE EVOLUTION OF THE SHOULDER JOINT IN NON-MAMMALIAN SYNAPSIDS

BECK, Allison L., Committee on Evolutionary Biology, University of Chicago, Chicago, IL, USA

The synapsid fossil record preserves evidence for a major reorganization of the appendicular skeleton from primitive amniotes to therian mammals, including the transition from sprawling locomotion to more upright stance and gait. Recent work on the locomotor evolution of terrestrial vertebrates has provided a framework for understanding the origins of parasagittal locomotion in modern mammals. Despite significant morphological changes in the shoulder girdle from pelycosaurs to early therapsids, it has been suggested that the synapsid forelimb retained its sprawling posture until the cynodont grade of evolution. Osteological changes are presumably accompanied by changes in muscle morphology and function. Very little work has been done on the functional evolution of the shoulder girdle, and what has been done is largely restricted to cynodonts and Mesozoic mammals, neglecting the changes that occurred in early therapsid evolution. Integrating morphological, morphometric, kinematic, and cineradiographic data on the shoulder joint of extant mammals and reptiles provides a phylogenetic bracket within which the evolution of the mammalian shoulder can be reconstructed. Inclusion of anatomical and morphometric data from fossil taxa and biomechanical studies imparts support to predictions based on modern taxa alone.

Data were collected from the skeletons of extant taxa of lizards, crocodylians, therian mammals, and monotremes as well as fossil synapsids. Morphological and myological data were synthesized from the literature and from dissections of extant taxa and examination of fossil specimens. In addition, measurements were taken from the humeri and scapulae of study specimens. Kinematic data obtained from literature were combined with multivariate statistical and biomechanical analyses and phylogenetic techniques to create an outline of the functional evolution of the synapsid shoulder joint. Such methods inform the study of the evolution of function by accounting for features that are not preserved in the fossil record but are known to influence form and function.

COMPARISON OF THE GROWTH OF ADAPTIVE STRUCTURE IN ARTIFICIAL LIFE MODELS AND IN THE FOSSIL RECORD

BEDAU, Mark A., Dept. of Zoology, University of Oklahoma, Norman, OK, USA

The origin and growth of complex adaptive structure is one of the most striking aspects of the history of life, but it is not known how to explain this phenomenon. One problem is the difficulty of quantifying an evolving system's adaptive structure. Another is the difficulty of creating any simple model that produces the phenomenon. This talk proposes a feasible method for quantifying a system's adaptive structure and applies this method to data from artificial life models and from the fossil record. The results of this analysis suggest two conclusions: (i) the quantification method is a promising approach for measuring adaptive structure, and (ii) the fossil record shows a striking kind of growth in adaptive structure that is not exhibited by any existing artificial life model. These conclusions suggest that a fundamentally new kind of model is needed to explain the growth of adaptive structure in the history of life.

TEST-DRIVING THE ETE DATABASE IN THE LATE CENOZOIC OF AFRICA

BEHRENSMEYER, Anna K., and Gerry E. McBrinn, Evolution of Terrestrial Ecosystems Program, Dept. of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA

The Evolution of Terrestrial Ecosystems (ETE) Program was established in 1988 to explore the relationship between ecology and evolution through study of ecosystems and species associations through geological time. The ETE database provides a standardized basis for analyzing continental faunas, floras, and ecosystem structures throughout the Phanerozoic. The database consists of 248 fields in 28 multirelational tables; mandatory fields are locality name, latitude-longitude, and geological age. ETE's customized format is locality-based but includes species tables with ecomorphic characters that allow comparisons across different taxonomic groups. Locality information includes sedimentology and taphonomy fields used to test the effects of paleoenvironmental context and taphonomic biases on proposed biotic patterns. A separate time table can be modified independently so that searches on particular time intervals will use current chronologies. Update tables provide an automatic record of all changes to locality or species data. The Neogene Old World (NOW) database is a clone of the ETE structure and has over 1055 Neogene Eurasian mammal localities and 9263 records. A web-accessible, ArcExplorer 3.0 version of the ETE database (http://etedata.si.edu) provides map-based locality information including species lists for the African dataset. The ETE database consists of 1418 localities, 4516 species and 5,388 records, primarily from published data on the African late Cenozoic but including localities for plants, invertebrates and vertebrates on various continents. Analysis of the African Plio-Pleistocene mammal data has revealed a complex relationship between global and regional environmental trends, patterns of species turnover, and ecomorphic change. This analysis also demonstrates that sedimentary context and taphonomic biases can have significant effects on long-term faunal trends.

FABRIC PATTERNS OF THE PERMIAN (GUADALUPIAN) MIDDLE AND UPPER CAPITAN REEF, TEXAS AND NEW MEXICO, USA: A PROGRESS REPORT

BELL, Gordon L., Jr., National Park Service, Guadalupe Mountains National Park, Salt Flat, TX, USA; and Christopher J. Crow, Dept. of Geological Sciences, University of Alabama, Tuscaloosa, AL, USA

The origin of the Capitan Limestone has been subject to ongoing debate for nearly a century. Issues making interpretation difficult include (1) fabric recognition problems, (2) the scale of the Capitan depositional system, (3) outcrops in rugged terrain with difficult access, (4) the underlying environmental heterogeneity of reef systems, and (5) wide-ranging extrapolations from studies of limited aerial extent. Our studies of selected Middle and Upper Capitan reef outcrops support previous interpretations that significant lithological and paleontological differences exist throughout the Capitan reef. Surveys conducted along the Middle Capitan reef outcrops exposed on the McKittrick Canyon Permian Reef Geology Trail (Guadalupe Mountains National Park, Texas, USA) detected three lithological end-member fabrics (Micrite Rich, Crypt, and Occluded Framework Cavity) as well as a number of recurrent fabrics that are transitional between these end members. Microbial micrite and early marine cements are important constituents of these rocks and surely made significant contributions to the rigidity of the framework. Patch (or facies) size in the Middle Capitan reef can be measured (temporally and along strike) on the scale of a few meters to a few tens of meters. The Middle Capitan reef was deposited in a complex patchwork of facies, resulting in abrupt and, as yet, unpredictable fabric changes. Significant differences are seen in outcrops of the Upper Capitan reef in Eddy County, New Mexico, USA, where the dominant fabrics (Phylloid-algal "Bafflestone," Archaeolithoporella Boundstone, and Shamovella (Tubiphytes) Boundstone) are present in a predictable sequence for many kilometers along strike, reflecting less depositional heterogeneity and/or greater community variability than in the Middle Capitan. Microbial micrite is scarce, as are crypts and mixed fabrics. Early marine cements are more abundant, representing up to 70% of rock volume in the Archaeolithoporella boundstone facies.

THE TROPHIC HISTORY OF FISHES ON CORAL REEFS

BELLWOOD, David R., Dept. of Marine Biology, James Cook University, Townsville, QLD, Australia

Coral reefs and their associated reef fish faunas present one of the most complex and interesting systems for examining biotic interactions, in the past and present. Reefs and fishes both have a long tenure in the fossil record, but the history of their interactions remains obscure. The consequences of changes in the taxonomic composition of marine fish assemblages during the Mesozoic and Cenozoic were therefore evaluated based on analysis of the functional morphology of the jaws. Measurements of the oral jaws of fossil and Recent fishes were used to construct plots of functional morphospace: an expression of the range of potential feeding modes. Measurements were restricted to those values which have known biomechanical significance in extant fishes and which have strong links to their trophic status.

Analyses of the fish assemblages in the vicinity of coral reefs or coral-bearing hardground in the Triassic, Jurassic, Eocene and Recent, reveal marked changes in fish functional morphospace occupation, with the appearance of novel forms in the Cenozoic. The functional capabilities of these taxa point to a shift in the nature of the interaction between fishes and coral reefs. The Eocene marks not only the first record of extant reef fish families but also the first occupation of an area of functional morphospace that characterizes herbivory in Recent marine fishes. It appears that the Cenozoic witnessed a marked shift in the nature of reef-fish interactions, with the origins of piscine herbivory.

SIGNIFICANCE OF OXYGEN MINIMUM ZONES UNDER UPWELLING SYSTEMS IN LATE ORDOVICIAN-EARLY SILURIAN GRAPTOLITE EXTINCTIONS AND RADIATIONS

Berry, William B. N., Dept. of Earth and Planetary Science, University of California, Berkeley, CA, USA

Many associations of graptolite taxa have been analyzed in the context of both the tectonic setting and depositional environments of strata in which they occur. Certain of the most detailed of these analyses have been conducted in Late Ordovician-Early Silurian stratal successions in a number of parts of the world. These analyses indicate that the majority of graptolite species lived in hypoxic waters near the margins of oxygen minimum zones. Most of the oxygen minimum zones developed under upwelling conditions close to the shelf-slope break, as is seen in modern oceans. Study of the pattern of Late Ordovician graptolite occurrences indicates that the majority of those taxa that lived in hypoxic waters became extinct when upwelling conditions and their related oxygen minimum zones vanished when glacio-eustatic sea-level fall led to changes in ocean surface water circulation. The graptolite taxa that lived in relatively oxic waters not only survived this extinction event but also radiated modestly during the time interval during which the graptolites living in hypoxic waters became extinct. Most of these survivor taxa are normalograptids. Deglaciation and sea level rise led to changes in ocean surface circulation. Gradually, sites of upwelling expanded as did oxygen minimum zones related to them. Many of these sites were relatively local in the earliest Silurian. As a consequence, earliest Silurian graptolites exhbit significant provincialism. As oxygen minimum zones expanded during the Early Silurian, those graptolite radiations that characterize that part of the Silurian took place. Late Ordovician graptolite extinctions and Early Silurian radiations are linked closely with changes in oceanic oxygen minimum zones that are, in turn, linked with climate changes.

THE EVOLUTIONARY BIOGEOGRAPHY OF PINNIPEDIMORPHS

BERTA, Annalisa, Dept. of Biology, San Diego State University, San Diego, CA, USA; Thomas A. Deméré, Dept. of Paleontology, San Diego Museum of Natural History, San Diego, CA, USA; and Peter J. Adam, Dept. of Biology, San Diego State University, San Diego, CA, and Dept. of Organismal Biology, Ecology, and Evolution, University of California, Los Angeles, CA, USA

A testable hypothesis is presented to explain the evolutionary biogeography of pinnipedimorphs (fur seals, sea lions, walruses, seals, and their fossil relatives) based on both dispersal and vicariant events and within the context of a species-level phylogenetic framework. This integrated hypothesis considers many lines of evidence including physical and ecologic factors controlling the distribution of modern taxa, new information regarding the nature and timing of paleoceanographic events, recent fossil discoveries, and improved resolution of phylogenetic relationships. Paleobiogeographic hypotheses for each of the major lineages are presented using cladograms and paleogeographic maps.

Our biogeographic hypothesis supports an eastern North Pacific origin for pinnipedimorphs in the late Oligocene. Otariids (fur seals and sea lions) are first known from the late Miocene in the North Pacific where they remained until the late Pliocene. A transequatorial dispersal into the western South Pacific preceded the rapid diversification of this group during the Pleistocene. Odobenids (walruses) evolved in the North Pacific during the late early Miocene and underwent dramatic diversification with later members of the odobenine lineage dispersing into the North Atlantic most likely via an Arctic route. Extinct archaic phocoids, the Miocene desmatophocids, were confined to the North Pacific. Phocids, although postulated here to have had a North Pacific origin, are first known from the middle Miocene in the North Atlantic region, as well as in the Paratethys. Both monachine and phocine seals are distinct lineages beginning in the middle Miocene in the North Atlantic. The early biogeographic history of phocine seals is centered in the Arctic and North Atlantic. Subsequent dispersal of phocines into the Paratethys and the Pacific occurred during the Pleistocene. In contrast, monachine seals have a Southern Hemisphere center of diversity. The pagophilic nature of extant phocine and lobodontine seals is largely a function of Pleistocene glacioeustatic events.

SHELLS IN TROPICAL SEDIMENTS: SKELETONS, SUBSTRATES, AND THE SIGNIFICANCE OF EARLY MARINE
DIAGENESIS

BEST, M.M.R., Dept. of Geology, University of Toronto, Toronto, ON, Canada; T.C.W. Ku, Dept. of Geological Sciences, University of Michigan, Ann Arbor, MI, USA; Susan M. Kidwell, Dept. of Geophysical Sciences, University of Chicago, Chicago, Il, USA; and L.M. Walter, Dept. of Geological Sciences, University of Michigan, Ann Arbor, MI, USA

Coordinated taphonomic and geochemical studies of the bivalve death assemblages and their sedimentary environments of San Blas, Panama, permits us to test whether shell condition can indicate environmental history. Ten sites were studied in detail, along environmental axes including water nutrients, grain size, and sediment chemistry (carbonate, organic carbon, iron). Taphonomic data (e.g., encrustation, fine scale surface alteration) were derived from total bivalve death assemblages and experimentally deployed Mercenaria mercenariaand Mytilus edulis. Pore water and sediment analyses (Ca/Cl, SO₄/Cl, alkalinity, sulfate d¹⁸O and ³⁴S, d¹³C-DIC, Fe extractions, SEM, XRD) were used to determine the degree and rates of diagenetic evolution. Death assemblages showed taphonomic separation into 3 groups using NMDS: reefal and non-reefal carbonate, and siliciclastics. Shell epibionts showed trends correlating with bottom water quality. Experimental shells grouped according to degree of exposure above the sediment/water interface, buried shells further divided along lines of sediment chemistry and shell organic content. Buried shell interiors from carbonate sites showed signs of dissolution, those from siliciclastic sites showed secondary precipitates. In fact at siliciclastic sites, microbial reduction of Fe-aluminosilicates is more important than previously realized, which promotes carbonate supersaturation by increasing the pore water alkalinity, limiting sulfate reduction, and precipitating hydrogen sulfide as FeS. Thus the most important controls on post-mortem condition are (i) exposure above the sediment/water interface, (ii) sediment chemistry, particularly iron reactivity, and (iii) shell microstructure, particularly organic content. Time-averaged skeletal death assemblages can provide a baseline of change over decades or more where the noise of short-term spatial and temporal heterogeneity is dampened. More volatile changes can be captured in the water column using shell epibionts and in the sediments by dissolution or precipitation textures.

STRATIGRAPHY AND PALEOENVIRONMENTS OF THE MID-MIOCENE MASCALL FORMATION (JOHN DAY BASIN, OREGON, USA)

BESTLAND, Erick A., Earth Sciences (SoCPES), Flinders University, Adelaide, SA, Australia

The Mascall Formation consists of a minimum of 348 m of middle Miocene age alluvial deposits that are dominated by moderately to well-developed paleosols. The formation is devisable into three stratigraphic packages based on paleosol type and the occurrence of tuff beds and conglomerates. They are in stratigraphic order: (1) Lower Tuffaceous Unit (53 m) which includes the Mascall Tuff Bed, (2) Red-Brown Claystone Unit (72 m), and (3) Upper Mascall Formation (223 m). The Lower Tuffaceous Unit contains numerous Alfisol and Inceptisol-like paleosols with siliceous features, thin vitric tuff beds, and diatomaceous and lignitic lacustrine deposits that are locally present at the base of the unit. The Red-Brown Claystone Unit contains the most well-developed paleosols as defined by macroscopic features (clay skins, peds, clay content, and color) and bulk rock geochemistry. Pebbly conglomerates are common in the middle unit and consist of eroded clasts of indurated soil fragments (pedolithic).

Paleoenvironments of the Mascall Formation are assessed by combining paleobotanical evidence from leaf fossils and paleosol types. Clayey Alfisol-like paleosols are common throughout the formation and indicate general humid climatic conditions. Leaf fossil assemblages from near the base of the formation are interpreted as the remains of hardwood forest resembling the oak-madrona woodland of northern California or of hardwood forests similar to ones in the lower Mississippi basin (Cheney, 1925, 1956). The lack of calcareous soil features throughout the formation indicates that precipitation levels did not drop below humid to sub-humid levels. Paleosols with siliceous soil features are interpreted as indicators of a humid Mediterranean climate. The Red-Brown Claystone Unit with its sequences of well-developed Alfisol-like paleosols and estimated age of between 16.0­15.0 Ma probably represents wetter and warmer climatic conditions of the mid-Miocene climatic optimum (16­15 Ma).

ENVIRONMENTAL VARIABILITY, EXPANDING GRASSLANDS, AND THE RISE OF THE GENUS HOMO IN AFRICA

BOBE, Rene, Anna K. Behrensmeyer, and Ralph E. Chapman, Dept. of Paleobiology, Evolution of Terrestrial Ecosystems Program, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA

The emergence and diversification of the genus Homo in Africa has been linked to environmental change using evidence from fossil faunas and floras and other paleoenvironmental indicators. According to a widely cited hypothesis, the hominid genera Homo and Paranthropus diverged from a more primitive hominid species of Australopithecus between 2.5 and 3.0 Ma as a consequence of global climatic cooling that caused increased aridity and grassland expansion in Africa. Interpretations of environmental change in the African Plio-Pleistocene remain controversial, and testable hypotheses linking early hominids to particular environmental processes are limited by taphonomic and collection biases that affect most hominid localities as well as poor stratigraphic and chronological resolution. Fine-scale evidence of relationships between faunal and environmental change are recorded in the well-dated and abundant fossil record of the lower Omo Valley, Ethiopia, where it is possible to control for sampling biases and compare successive faunas from time intervals <10⁵ yrs. Faunal evidence based on shifting species abundances as well as species turnover indicates that major environmental changes occurred in the Late Pliocene of East Africa as closed habitats gave way to more open, grass-dominated habitats. Taphonomic features of the fossil assemblages also changed through time, but these changes are not correlated with the faunal shifts. The tempo of faunal change itself varied significantly during this interval, with a period of change between 3.2 and 2.85 Ma, an interval of stability from 2.85­2.55 Ma, and then a distinct pattern of variable, accelerated change beginning at 2.55 Ma that continued through 2.0 Ma. The appearance of Homo and the first occurrence of stone artifacts in the Omo correlate in time with this shift in faunal variability, suggesting that ecosystem instability provided opportunities and selection pressures important to the dispersal, and possibly also the origination, of the genus Homo.

THE BEGINNING OF THE MESOZOIC: PANGEAN BREAKUP WITH 60 MILLION YEARS OF ENVIRONMENTAL STRESS AND BIOTIC CRISIS

BOTTJER, David J., Dept. of Earth Sciences, University of Southern California, Los Angeles, CA, USA

The Big 5 mass extinctions, identified through global marine taxonomic diversity analyses, occurred at varying time intervals with 130 million years as the largest separation between events. Compared to the temporal spacing of other Big 5 mass extinctions, the end-Permian and end-Triassic mass extinctions are relatively close, on the order of 40 million years apart. Along with examining trends in taxonomic diversity, evaluations of paleoecological changes through life's long history can also be made. A method to evaluate such changes, developed by Droser et al. (1997), includes four paleoecological levels of change, with level 1 representing the largest change, and level 4 the smallest. Second level changes are the largest to occur in Phanerozoic marine environments, and it is apparent that ecological losses at the second level occurred with both Big 5 mass extinctions that affected life at the beginning of the Mesozoic (Bottjer et al., 2001), confirming that unusual ecological disruption also marks this time interval.

Given that the end-Permian and end-Triassic mass extinctions are also associated with prolonged recoveries, and that several minor mass extinctions occurred during this time, it is likely that as much as half of the first 60 million years of the Mesozoic occurred under heightened environmental stress. Although events causing these two mass extinctions may not be related, their temporal proximity, as well as other associated features, indicate that they may have been due to the same underlying internal causes. The root mechanism is very likely that this was the time of the early breakup of Pangea, with stressful tectonic, oceanographic and climatic conditions associated with the breakup leading to the biotic crises that characterize the beginning of the Mesozoic.

ANALYZING TAPHONOMIC VARIATION IN LEAF COMPRESSIONS FROM CRETACEOUS FLOODPLAIN SUBENVIRONMENTS

BOUCHER, Lisa D., Dept. of Biology, University of Nebraska-Omaha, Omaha, NE, USA; and K. Sian Davies-Vollum, Dept. of Geology, Pomona College, Claremont, CA, USA

Floras deposited in fluvial environments are subjected to various taphonomic influences, and subenvironments within such a system may have characteristic taphonomic signatures. In this study, we propose a method of data collection and quantitative analysis using compression floras to detect and compare taphonomic biases among floodplain subenvironments. Six different sites were sampled from within the Fruitland-Kirtland Formations in the San Juan Basin, NM. Sites were classified into depositional subenvironments using field and analytical data and included ash deposits in swamp, swamp flooded with an influx of sediment, silting up of swamp, overbank flood onto soil and overbank splay/levee. At each site, 54­119 specimens were identified to morphotype, and then scored for type of damage, completeness, and position relative to the bedding plane. Specimens consisted of fern, conifer, monocot and dicot leaves. Also noted were biological characteristics inherent to particular morphotypes, such as leaf thickness and petiole diameter, which may impact taphonomic signatures. The data were compared by using percent frequencies of character states at each site and analyzed using uni- and multivariate statistics. Most sites have similar leaf position and completeness profiles and this may be partially due to preferential selection of sites with identifiable remains. Damage to leaves by angular breaks was present in higher frequency at sites with greater silt content than clay. Sites with a greater frequency of specimens with "robust" features (thick leaves, large petioles) were not strongly correlated with sites having more complete or less damaged leaves. These results imply that subenvironments within floodplain settings have subtle differences in their taphonomic signatures as measured by these leaf parameters. In particular, the type and extent of damage to the leaves are the most notable and have implications for interpreting quantitative data collected in abundance and diversity studies.