NAPC 2001

Abstracts, Hu - Iv
(5/17/01)

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TAGMATA, SEGMENT SPECIFICATION, AND THE DYNAMICS OF POST-CEPHALIC SEGMENT ACCRETION IN TRILOBITES

HUGHES, Nigel C., Dept. of Earth Sciences, University of California, Riverside, CA, USA.; Giuseppe Fusco and Alessandro Minelli, Dept. of Biology, University of Padova, Padova, Italy; and Mark Webster, University of California, Riverside, CA, USA

While variation in the number of trilobite cephalic segments remains debatable, it is clear that the postcephalic region exhibited wide variation in segment numbers among different clades. Intraspecific variability in the number of thoracic segments is known for at least 25 species, mainly Cambrian, so the boundary between thorax and pygidium was placed at a different segment in different adults of the same species. The thorax to transitory pygidium boundary also shifted during ontogeny. Among Recent arthropods, intraspecific variability in segment number is known in several clades, but in all cases tagmata have definitive boundaries as soon as they become expressed as distinct morphological entities. Equating trilobite body regions with expression domains of Hox genes is not straightforward because although the dorsal exoskeleton is divided into two posterior units, the thorax and the pygidium, the ventral appendages associated with these units share a similar basic structure. A similar mismatch between dorsal and ventral structures is also known among Recent arthropods, but trilobites are apparently unique in that the regional identity of individual postcephalic segments changed during their ontogeny, as segments formed in the pygidium were later released into the thorax. The pattern of growth in the postcephalic region of trilobites is hemianamorphic, i.e., only the early molts were accompnied by an addition of new thoracic segments (meraspid period = anamorphic phase), then the animal continued to molt without further addition of thoracic segments (holaspid period = epimorphic phase). As in some Recent arthropods, the shape of the adult appears more tightly constrained through ontogeny than the number of segments: in adult Aulacopleura konincki, specimens with 18 to 22 thoracic segments shared almost identical sagittal proportions. These results suggest that there was some developmental flexibility in trilobite body architecture, thus raising the question of whether trilobite postcephalic tagmata are strictly comparable to those of the remaining arthropods.

EVENT BED DEPOSITION IN THE CINCINNATIAN SERIES: IMPLICATIONS FOR TAPHONOMIC PROCESSES AND ASSESSING MICROEVOLUTIONARY CHANGES WITHIN FLEXICALYMENE

HUNDA, Brenda R., and Nigel C. Hughes, Dept. of Earth Sciences, University of California, Riverside, CA, USA

Despite intense scientific interest there is currently little high resolution data available to understand how organisms respond to multiple episodes of environmental change at time-scales ranging from 1,000 to 1,000,000 years. The prerequisites for such a study include exceptional preservation of multiple fossiliferous horizons and a high-resolution stratigraphic framework. Recent stratigraphic analyses of the Cincinnatian Series now provide an exceptionally precise spatial and temporal framework for assessing micro-evolutionary changes within individual species and species lineages.

Initial studies in the area have shown that storm-related episodes of rapid mud deposition smothered populations of live Flexicalymene, preserving unique and substantial samples of contemporary individuals alive some 450 million years ago. Thin section analysis has revealed a similar depositional regime at Mt. Orab, within the Arnheim Formation, with at least seven gradational beds within a 0.46 m mudstone unit. Differences in size distribution, density, and attitude of the trilobites suggests that Mt. Orab beds differ from the specific behavioral aggregations seen within the "granulosa" bed of the Kope Formation, although both are thought to represent contemporaneous populations. Similar "event bed" preservation occurs throughout the Cincinnatian Series, offering the possibility of repeated sampling of contemporary populations sequentially throughout the Series. Morphometric analysis of these contemporary populations of Flexicalymene will constrain patterns of morphological variability at the population level, providing a firm basis for understanding the morphological variation evident within Cincinnatian Flexicalymene, and its relationship to multiple episodes of environmental change within a sequence stratigraphic framework.

TIME-AVERAGING AND MORPHOMETRIC DATA: DO FOSSIL SAMPLES ACCURATELY REFLECT POPULATION-LEVEL VARIABILITY?

HUNT, Gene, Committee on Evolutionary Biology, University of Chicago, Chicago, IL, USA

While the importance of time-averaging is widely appreciated in paleoecological studies, its effects on morphological data have been less explored. Morphological change during the interval in which a sample accumulates will inflate the phenotypic variance of that sample. In this study, I consider two related questions: (1) By how much are time-averaged samples expected to overestimate population variance under different models of evolution? (2) Does analysis of published data suggest that the variance inflation in fossil samples is intolerable large? If fossil samples tend to grossly overestimate variability, paleontologists must be cautious when attempting to infer biological variation in fossil taxa.

To address the first question, I derive the expected degree to which fossil samples should overestimate population variances under three models of evolution: (i) constant (linear) evolution, (ii) unbiased random walk and (iii) biased random walk. For these models, the expected variance inflation can be expressed as simple functions of time and rate parameters, but more empirical work is necessary to establish realistic ranges for these rates. In order to address the second question, I first compiled morphologic data on fossil and recent samples of the same species from the literature, and compared their variability. As a complementary approach, I also used published data from examples of within-lineage morphological evolution to examine the effects on variability of artificially time-averaging adjacent samples.

Results indicate that (1) fossil and modern samples of the same species usually show comparable levels of variability, and (2) often, one can lump samples spanning thousands to even millions of years without doing great violence to estimates of within-sample variance. There are, of course, instances where variances estimated from fossil samples will be gross overestimates. However, it appears that for most paleontological purposes, fossil variability is an adequate proxy for population-level variability.

REVISING PALEOGENE WEST ANTARCTIC CLIMATE AND VEGETATION HISTORY: NEW GEOCHRONOLOGY AND PALAEOENVIRONMENTAL DATA

HUNT, Richard J., School of Earth Sciences, University of Leeds, Leeds, UK

Paleocene-Oligocene volcanics with plant-bearing sediment intercalations crop out extensively on King George Island, Antarctica. These plants are the most complete Paleogene terrestrial foliar record in Antarctica and variations in the composition of the flora are the basis of climate change models for this period (e.g., Birkenmajer, 1997).

The flora is preserved in lacustrine deposits and grew in the fore-arc region of the Antarctic Peninsula, where it was prone to inundation by arc volcanics. It comprises impressions and carbonised compressions of leaves, fertile material and silicified wood. Gymnosperms, angiosperms and ferns are represented.

The Wawel flora from Point Hennequin is currently regarded as an impoverished Oligocene post-glacial flora. However, ⁴⁰Ar/³⁹Ar dating of the encapsulating lavas indicates that the sequence is early Eocene in age (44­49 Ma) and recent field collections have yielded a diverse range of plant fossils from the locality. Thirty-two angiosperm leaf morphotypes have been described and are referred to families including the Nothofagaceae, Proteaceae, Sterculiaceae, and Lauraceae. This increases the recognised diversity of Eocene vegetation in West Antarctica and is important because the composition of the flora is demonstrably different from other Eocene localities on King George Island. Leaf margin analysis of the Wawel flora, based on simple linear regression, yields a mean annual temperature of ~9° C.

In overall composition, the closest modern analogue for the King George Island flora is the Valdivian rainforests of Chile. These forests are controlled disturbance, largely of volcanic origin. Local variations in vegetational composition previously attributed to climatic change during the Paleogene could be explained in terms of disturbance and related succession dynamics.

Birkenmajer, K. 1997. Bulletin of the Polish Academy of Sciences, Earth Sciences 44(3):157­181.

GEOLOGY AND PALEONTOLOGY OF THE UPPER JOHN DAY BEDS, JOHN DAY RIVER VALLEY, OREGON: LITHOSTRATIGRAPHIC AND BIOCHRONOLOGIC REVISION IN THE HAYSTACK VALLEY AND KIMBERLY AREAS

HUNT, Robert M., and Ellen Stepleton, Div. of Vertebrate Paleontology, University of Nebraska, Lincoln, NE, USA

Fine-grained pyroclastic sediments of the John Day Formation in central Oregon preserve superposed mammalian faunas of Oligocene through early Miocene age. Youngest faunas from the upper part of the formation date the final phase of continental deposition prior to outpouring of the Columbia River Group basalts. Upper John Day rocks, first described by J.C.Merriam (1900­1901) in the Kimberly and Haystack Valley areas, were subsequently divided by Fisher and Rensberger (1972) into two members. We focused our study on this geographic area and these units. Upper John Day sedimentation in the Kimberly-Haystack Valley area was punctuated by at least two major erosional episodes prior to basalt flooding. South of Kimberly, the Kimberly Member is disconformably incised and overlain by tuffaceous late Arikareean siltstones (~22.6 Ma) with fluvial coarse gravels. This sequence was later tilted, faulted, and eroded, and on the eroded surface coarse gravels and tuffs, containing a Hemingfordian fauna was deposited in angular unconformity. After a final erosive interval, the entire sequence was capped by basalts. In Haystack Valley, Balm Creek dissects the southern limb of a syncline, which preserves the most complete local section of upper John Day rocks. Four sedimentary units are identified, the lowest two bearing Arikareean faunas, and the latter two without fossil mammals. The uppermost of these units, comprising coarse gravels and tuffaceous sandstones, cuts deeply into the underlying rocks, and is believed to correlate to the fossiliferous Hemingfordian unit south of Kimberly. Because the Hemingfordian unit is of regional extent, and lies with angular unconformity on older John Day rocks, it probably deserves separate formation rank. The complexity of upper John Day rocks suggests greater variety in early Miocene depositional settings relative to more uniform lower John Day Oligocene environments.

SPATIAL PATTERNING AMONG LATE CRETACEOUS (LANCIAN) MAMMALS IN NORTH AMERICA

HUNTER, John P., Dept. of Anatomy, New York College of Osteopathic Medicine, Old Westbury, NY, USA

I tested the previously proposed hypothesis that Late Cretaceous (Maastrichtian) mammalian faunas in North America differ in composition and dominance along a north-south gradient. For this purpose, I compiled a dataset of occurrences and abundances of Lancian mammalian faunas from the northern North American Western Interior. Initial ordination using correspondence analysis revealed variation among these faunas in both composition and dominance associated with latitude. To further test whether these patterns were due to simple spatial autocorrelation (i.e., resemblance solely due to geographic proximity) rather than a large-scale north-south gradient, I examined the partial matrix correspondence (Mantel) structure among faunal dissimilarity, spatial separation (surface distance), and latitude. There was no association between composition or dominance and latitude when surface distance was held analytically constant. Significant association between composition and dominance remained with surface distance, however, when latitude was held constant. Spatial patterning among faunas is more likely explained by spatial separation than by a north-south gradient. Further examination of the correspondence between individual species distributions and surface distance, however, revealed interpretable patterns, such as instances of congeneric species, presumed to be ecologically similar to one another, replacing one another along the landscape. Thus, lower-level ecological processes, such as competition, may underlie the observed larger-scale spatial pattern.

COMPARATIVE PALEOBIOLOGY OF TWO DIFFERENT MIDDLE TRIASSIC MARINE ENVIRONMENTS FROM WESTERN PANGEA (USA)

HUYNH, Tran T., and David J. Bottjer, Dept. of Earth Sciences, University of Southern California, Los Angeles, CA, USA

Following the end-Permian mass extinction, it wasn't until Middle Triassic times that normal benthic conditions returned and various groups were re-established worldwide. This period marks the initiation of the Mesozoic marine radiation, as measured by global diversity trends. Recent work on the Early Triassic recovery interval has allowed for better assessment of the paleobiological changes that occurred in the benthic realm at the transition from the Early to Middle Triassic.

Preliminary examination of different paleoenvironments represented by Middle Triassic Star Peak Group strata (west ern US) suggests that the ecological and evolutionary character of the initial Mesozoic marine radiation varied between depositional settings. Compared to Early Triassic assemblages of nearshore environments, less cosmopolitan members and increased levels of tiering complexity characterize the Middle Triassic fauna. Lazarus taxa continued to return to level-bottom nearshore settings at this time, filling in more Bambachian megaguilds. In contrast, deep-water assemblages of the Middle Triassic are significantly different from those found in the Early Triassic from similar depositional settings. The fossil associations of deeper-water environments are comprised of cephalopod and bivalve taxa that are more typical of later Mesozoic assemblages. Thus, while nearshore environments exhibited community-level patterns of re-assembly by Lazarus taxa, evolutionarily distinct offshore assemblages emerged at the initiation of the Mesozoic radiation in the western US.

PLUME WINTER SCENARIO FOR THE PERMO-TRIASSIC BOUNDARY MASS EXTINCTION

ISOZAKI, Yukio, Dept. of Earth Sciences and Astronomy, University of Tokyo, Komaba, Tokyo, Japan

The end-Permian mass extinction, the greatest in the Phanerozoic, occurred through two distinct steps, i.e., the first big decline in biodiversity across the Guadalupian/Wuchapingian boundary and the well-known main one across the Chanhsingian/Griesbachian boundary. The deep-sea cherts in Japan also recorded changes across the two horizons in fauna, lithofacies and redox. The intercalation of tuff at the two horizons, in stratotypes in South China strongly suggests an intimate cause-effect relation between volcanism and extinction. In particular, the ryholitic nature of tuff beds indicates explosive volcanic eruption derived from highly viscous acidic magma rather than contribution from the nearly coeval Siberian flood basalt. The "Plume Winter" scenario is proposed to explain a possible link among the following three unusual geologic phenomena occurred across the Permo-Triassic boundary (PTB); the existence of Pangea, long-term oxygen depletion in deep-sea (superanoxia), and mass dying. These three are all global in context and unusual in the Phanerozoic but unique to PTB. The coincidence in timing of the initial breakup of Pangea, superanoxia, and biosphere crisis probably indicates a sequence of events triggered by a superplume activity in Earth's mantle. When a plume head penetrated into the bottom of preexisting continent to break up, partial melting of crust may have occurred to generate rhyolitic magma. Unusually violent volcanism associated with continental breakup may have caused (1) formation of dust/aerosol screen, (2) stop of photosynthesis, (3) collapse of food web, (4) mass extinction, and (5) superanoxia. The noble gas evidence for extraterrestrial impact (Becker et al., 2001) from an erroneously assigned PTB horizon in Japan is not accepted straightforward.

CLIMATIC AND TECTONIC SIGNIFICANCE OF UPPER
MIOCENE AND LOWER PLIOCENE DIATOMITES IN CENTRAL MEXICO

ISRADE, Isabel, and Victor Hugo Garduño, Instituto de Investigaciones Metalurgicas, Depto. de Geología, Edif. U. Universidad Michoacana de San Nicolás de Hidalgo, México; and John A. Barron, USGS, Menlo Park, CA, USA

An initial Miocene (11­7 Ma) phase of lacustrine sedimentation in west central Mexico occurred during a period of widespread tectonic extension. Lacustrine diatomaceous rocks of this period contain the genus Mesodictyon,which also characterizes upper Miocene deposits in the western US, Peru, France and North Africa. Although climatic conditions in west central Mexico during this interval were relatively warm and humid, the lakes were shallow and limited in extent.

Following a latest Miocene (8­5.2 Ma) stage of reduced precipitation and very limited lacustrine sediments, a period of extensive, relatively deep, perennial lakes extending for more than 250 km in an east-west direction marked the early Pliocene period between 5.2 and 4.0 Ma. Pliocene diatomites contain Stephanodiscus carconensis and Cyclotella "Tertiarius" spp., taxa found in rocks of this age in the western US. Pliocene vertebrate faunas from Rancho El Ocote and Coecillo (Guanajuato) reflect a savanna setting with moist and warm conditions. This interval of the early Pliocene is marked by relatively high global sea level and increased high latitude temperatures. Restriction of the Central American Seaway during the latest Miocene may have also acted to increase precipitation in central Mexico during the early Pliocene.

Shallow lake and fluvial deposits dominate after 4.0 Ma, and vertebrate remains of La Goleta in Michoacan and Rancho Viejo in Guanajuato document a period of dispersal from North and South America. A combination of reduced volcanic tectonic processes and reduced precipitation are presumed to have been the cause for this middle Pliocene reduction in lake size.

XENOTURBELLA (PHYLUM UNCERTAIN): THE APPEARANCE AND LOSS OF EVERYTHING DURING ITS DEVELOPMENT

ISRAELSSON, Olle, Dept. of Integrative Biology and Museum of Paleontology, University of California, Berkeley, CA, USA

Xenoturbella Westblad, 1949 is a strange marine animal (up to 4 cm long) with epithelial epidermis and gastrodermis, a basiepidermal nerve plexus and a ventral mouth, but without an anus, concentrated nervous system, or any other distinct organs except for a "statocyst" containing flagellated "statoconia." It is marine and not parasitic. Despite its peculiarities and possible relevance to the early phases of the metazoan evolution, Xenoturbella has been neglected by most textbooks. Since its discovery, it has been the subject of most diverse interpretations: as one of the most primitive metazoans due to its simplicity in organization, as a primitive relative of acoelomorph flatworm, or as a paedomorphic holothurian or enteropneust.

However, the simple body plan of Xenoturbella is neither due to paedomorphy nor to plesiomorphy but to metamorphosis from a complex larva. No living embryos have been observed before. The larvae have alimentary tract, mantle cavity, gills and concentrated nervous system while the adults lack such structures. The embryology as well as molecular data indicate that Xenoturbella actually is a lophotrochozoan, probably within or close to molluscs. Solving its systematic position does not make it less puzzling. Why would an animal that is neither parasitic, microscopic nor short-lived lose all its organs and change its concentrated nervous system with ganglia to a loose network of neurons, especially when its larva has all the features the adults lacks?

Xenoturbella has a general importance for the understanding of evolution. It shows that it is possible for an organism with a specialized body plan (larvae with complex morphology) to develop into a totally different type of organism (adults without organs). Xenoturbella is not an ancient group, a Cambrian relict. Instead, the embryonic data suggest that it is fairly recent.

PALEONTOLOGICAL EVIDENCES OF THE EARLY EVOLUTION OF THE CARIBBEAN SEAWAY

ITURRALDE-VINENT, Manuel A., Museo Nacional de Historia Natural, La Habana, Cuba; and Zulma Gasparini, Museo de Historia Natural, Depto. Paleontología Vertebrados, La Plata, Argentina

Similarities among several groups of Jurassic marine animals that occur both in the western Tethys and the southwestern Pacific suggest that since the Sinemurian (nearly 195­200 m.y. ago) there was an aquatic connection between both regions. Although such a connection, theoretically, may have been throughout some gateway located south of Pangaea, there are several biogeographic reasons to suggest that the preferred passway was placed within central Pangaea. According to recent paleogeographic reconstructions, this passway was established before the break-up of Pangaea, probably across a dominantly shallow siliciclastic epicontinental basin in the interior of western Pangaea. This sea should not be thought of as the Caribbean basin, but as a precursor. The opening of an oceanic Caribbean Seaway, located between North and South America, is corroborated by the occurrence of an important assemblage of Oxfordian marine animals in the Guaniguanico terrain of western Cuba, an assemblage that is clearly related with both the western Tethys and the southwestern Pacific faunas. The opening of the Caribbean Seaway represent an important paleoceanographic event, as only from that moment may had had started the Mesozoic deep circumequatorial oceanic circulation that linked the Tethys, the Central Atlantic and the Pacific oceans.

LOCOMOTION TRAILS OF THE VENDIAN INVERTEBRATES PRESERVED WITH THE PRODUCER'S BODY FOSSILS, WHITE SEA, RUSSIA

IVANTSOV, Andrei Yu., and Mikhail A. Fedonkin, Paleontological Institute, Russian Academy of Sciences, Moscow, Russia

Body fossils of two invertebrate species are found in direct association with their locomotion trails in the Vendian siliciclastic deposits about 555 Ma old (Zimnii Bereg, White Sea). Bilateral and segmented Yorgia (up to 25 cm long) is preserved in negative relief by the chains or clusters of its ventral surface imprints preserved in the positive relief on the lower bedding plane. Positive and smooth relief, absence of the sharp border line between the trail and the surrounding microrelief of the bedding plane and uniform morphology make the fundamental difference of the ichnofossil from the associating body imprint that normally shows a wide spectrum of preservation forms. While crawling over the mud Yorgia produced an abundant mucus like the recent flat worms or gastropods. The mucus impregnated, glued and sealed the sediment, thus protecting the trail from erosion during the sedimentation event. Short impulses of gliding locomotion alternated with longer resting phases during which more mucus was produced and exact imprint of the ventral side was fixed in the sediment. In the light of this study we have to reconsider the nature of other segmented fossils preserved in a positive relief—they may well be locomotion or feeding trails. Short trails are found behind a few small (1 cm long) specimens of Kimberella. These trails preserved in negative relief on the sole of the sandstone bed were produced by the dorsal shell of an animal while it was trying to escape the sand after the sedimentation event. Another kind of ichnofossil produced by Kimberella is a fan-shape scratch mark that goes from one end of the body fossil. These feeding tracks may reflect the work of the proboscis that bears the hook-like organs on its end.

USING STABLE ISOTOPIC DATA TO RESOLVE VARIATION IN RATE AND DURATION OF SEASONAL GROWTH THROUGHOUT ONTOGENY

IVANY, Linda C., Dept. of Earth Sciences, Syracuse University, Syracuse, NY, USA; Douglas S. Jones, Florida Museum of Natural History, University of Florida, Gainesville, FL, USA; and Bruce H. Wilkinson, Dept. of Geological Sciences, University of Michigan, Ann Arbor, MI, USA

Understanding variation in the rate and timing of growth with ontogeny is a fundamental component of many paleobiological and paleoecological studies. Examining variation through ontogeny in the stable isotopic composition of accretionary biogenic hard parts is becoming more commonplace as a technique for providing estimates of age and for gaining insights into change in growth rate through ontogeny, as well as for inferring the environmental conditions under which organisms lived. The noise associated with such records, however, can make interpretation and inter-sample comparison difficult. We have developed a computational approach that quantitatively resolves secular (seasonal) change in environmental parameters such as temperature and salinity with spatial (hard part) variation in isotopic composition. Output consists of determinations of mean annual isotopic composition, seasonal range in isotopic composition and variations in hard part growth rate for any interval of data in an accretionary transect. The advantage of this method for paleontologists, beyond the obvious paleoenvironmental applications, is that rate and duration of growth can be approximated at seasonal to subseasonal levels of resolution for any living or fossil organism. We have applied this technique to d¹⁸O data from the surf clam, Spisula solidissima, collected from a range of latitudes and water depths along the Atlantic coast. Quantitative estimates of absolute growth rates, as well as the proportion of each season represented by hard part accretion, are derived for each shell with error estimates for every season of the clam's life. Such information from a suite of individuals can reveal how growth within a species is affected by changes in latitude, depth, productivity, and other environmental gradients. In addition, changes in rate and timing of growth within a lineage are the mainstay of heterochrony studies, and quantitative methods for assessing such variables through time in fossil specimens will allow more rigorous testing of hypotheses.