Registered users receive a variety of benefits including the ability to customize email alerts, create favorite journals list, and save searches.
Please note that a BioOne web account does not automatically grant access to full-text content. An institutional or society member subscription is required to view non-Open Access content.
Contact email@example.com with any questions.
Reefs containing abundant calcified metazoans occur at several stratigraphic levels within carbonate platforms of the terminal Proterozoic Nama Group, central and southern Namibia. The reef-bearing strata span an interval ranging from approximately 550 Ma to 543 Ma. The reefs are composed of thrombolites (clotted internal texture) and stromatolites (laminated internal texture) that form laterally continuous biostromes, isolated patch reefs, and isolated pinnacle reefs ranging in scale from a meter to several kilometers in width. Stromatolite-dominated reefs occur in depositionally updip positions within carbonate ramps, whereas thrombolite-dominated reefs occur broadly across the ramp profile and are well developed as pinnacle reefs in downdip positions.
The three-dimensional morphology of reef-associated fossils was reconstructed by computer, based on digitized images of sections taken at 25-micron intervals through 15 fossil specimens and additionally supported by observations of over 90 sets of serial sections. Most variation observed in outcrop can be accounted for by a single species of cm-scale, lightly calcified goblet-shaped fossils herein described as Namacalathus hermanastes gen. et sp. nov. These fossils are characterized by a hollow stem open at both ends attached to a broadly spheroidal cup marked by a circular opening with a downturned lip and six (or seven) side holes interpreted as diagenetic features of underlying biological structure. The goblets lived atop the rough topography created by ecologically complex microbial-algal carpets; they appear to have been sessile benthos attached either to the biohermal substrate or to soft-bodied macrobenthos such as seaweeds that grew on the reef surface. The phylogenetic affinities of Namacalathus are uncertain, although preserved morphology is consistent with a cnidarian-like bodyplan. In general aspect, these fossils resemble some of the unmineralized, radially symmetric taxa found in contemporaneous sandstones and shales, but do not appear to be closely related to the well-skeletonized bilaterian animals that radiated in younger oceans. Nama reefs demonstrate that biohermal associations of invertebrates and thrombolite-forming microorganisms antedate the Cambrian Period.
Vase-shaped microfossils (VSMs) occur globally in Neoproterozoic rocks, but until now their biological relationships have remained problematic. Exceptionally preserved new populations from the uppermost Chuar Group, Grand Canyon, Arizona, display details of morphology and taphonomy that collectively point to affinities with the testate amoebae. The fossils are tear-shaped tests, ∼20–300 μm long and ∼10–200 μm wide, that are circular in transverse section, expand aborally toward a rounded or slightly pointed pole, and taper orally toward a “neck” that ends in a single aperture. Apertures may be circular, hexagonal, triangular, or crenulate, and may be rimmed by a distinct collar. Approximately 25% of the Chuar VSMs are curved, such that the oral and aboral poles do not lie opposite each other. Tests are preserved as mineralized casts and molds, commonly coated with organic debris or iron minerals, but they were originally composed of nonresistant organic matter. Approximately 1% have a “honeycomb-patterned” wall attributable to the original presence of mineralized scales whose bases were arranged regularly in the test wall. Scale-bearing testate amoebae, such as members of the Euglyphidae, are essentially identical to the honeycomb VSMs, and a close relationship between other Grand Canyon VSMs and additional testate amoebae, both lobose and filose, is likely. The VSM population therefore most likely represents a multispecies assemblage whose spatial association reflects a common habitat and/or taphonomic circumstances that favor test preservation. The assignment of these fossils to the testate amoebae strengthens the case for a major diversification of eukaryotic organisms by mid-Neoproterozoic times and, more significantly, provides the earliest morphological evidence for heterotrophic eukaryotes in marine ecosystems.
Multicellular filaments from the ca. 1200-Ma Hunting Formation (Somerset Island, arctic Canada) are identified as bangiacean red algae on the basis of diagnostic cell-division patterns. As the oldest taxonomically resolved eukaryote on record Bangiomorpha pubescens n. gen. n. sp. provides a key datum point for constraining protistan phylogeny. Combined with an increasingly resolved record of other Proterozoic eukaryotes, these fossils mark the onset of a major protistan radiation near the Mesoproterozoic/Neoproterozoic boundary.
Differential spore/gamete formation shows Bangiomorpha pubescens to have been sexually reproducing, the oldest reported occurrence in the fossil record. Sex was critical for the subsequent success of eukaryotes, not so much for the advantages of genetic recombination, but because it allowed for complex multicellularity. The selective advantages of complex multicellularity are considered sufficient for it to have arisen immediately following the appearance of sexual reproduction. As such, the most reliable proxy for the first appearance of sex will be the first stratigraphic occurrence of complex multicellularity.
Bangiomorpha pubescens is the first occurrence of complex multicellularity in the fossil record. A differentiated basal holdfast structure allowed for positive substrate attachment and thus the selective advantages of vertical orientation; i.e., an early example of ecological tiering. More generally, eukaryotic multicellularity is the innovation that established organismal morphology as a significant factor in the evolutionary process. As complex eukaryotes modified, and created entirely novel, environments, their inherent capacity for reciprocal morphological adaptation, gave rise to the “biological environment” of directional evolution and “progress.” The evolution of sex, as a proximal cause of complex multicellularity, may thus account for the Mesoproterozoic/Neoproterozoic radiation of eukaryotes.
New morphometric data gathered from cross-sections of two Lower Devonian land plants (Rhynia gwynne-vaughanii and Asteroxylon mackiei) are interpreted in terms of the evolution of the function of vascular bundles in early land plants. The following conclusions can be drawn from these new data: (1) The ratio of the cross-sectional area of the xylem (representing the conducting volume supplying the axis with water) to the xylem perimeter (representing the “contact area” between xylem and parenchyma through which water leaves the xylem and enters the parenchyma) is not constant for Rhynia axes, almost constant for Asteroxylon axes, and different between Rhynia and Asteroxylon. Thus, Bower´s hypothesis that the ratio of cross-sectional area of the xylem to xylem perimeter is constant during ontogenetic development is true for Asteroxylon. That this ratio is constant during phylogeny, however, is not supported by our data. (2) The ratio between cross-sectional area of xylem to parenchyma is higher in Asteroxylon than in Rhynia. (3) As predicted by previous computer simulations, the ratio of the xylem perimeter to the axis perimeter plays a major role in determining water transport performance of the transpiring axis. This ratio is constant within ontogeny but is different in Asteroxylon and Rhynia. In Asteroxylon axes, this ratio is about twice as large as in Rhynia axes. (4) Contrary to the expectations, the distance between the outermost layer of the xylem and the transpiring surface, which represents the low-conductivity pathway through the parenchyma, appears not to be a limiting factor for the water transport in axes of Rhynia and Asteroxylon. (5) From the analysis of the geometric parameters, it is evident that Rhynia and Asteroxylon with their distinct stelar geometries represent two different constructional types for which no transitional stages are known.
Phylogenetic analysis of the metazoan evolutionary tree as a whole, and of trees of component major clades, indicates that marine herbivores, defined here as macrophagous consumers of living multicellular attached marine plants, always occupy terminal positions at several scales of analysis. Nearly all living benthic marine herbivores are derived from microphages, detritivores, or predators, and most have post-Paleozoic origins. The derived nature of herbivory in the sea parallels the evolutionary situation among land animals. Pre-Mesozoic marine benthic ecosystems, characterized by relatively low rates of flow of energy and nutrients, may have relied even more heavily on decomposers for the transfer of carbon from primary producers to animals than do living marine ecosystems in the photic zone.
Paleobiologists frequently hypothesize that a taxon's duration (i.e., the true span from origination to extinction) exceeds its stratigraphic range (i.e., the span from first appearance to last appearance in the fossil record). One can test hypothesized duration by assessing the plausibility of the implicitly hypothesized gaps between origination and first appearance and/or between last appearance and extinction. Several tests assess the probability of not finding a taxon over some stratigraphic gap. Because the likelihood of a hypothesis given data reflects the probability of the data given that hypothesis, these probabilities also give the likelihood of a hypothesized duration. However, many probability/likelihood tests require simplifying assumptions about unknown sampling parameters such as the consistency of sampling over time, sampling intensities for unknown ancestors, and actual sampling intensities themselves.
This paper examines the effects of sampling parameters on probability/likelihood tests and presents methods for testing hypotheses about these unknowns while testing hypotheses about true durations. Two data sets are used here as examples. One analysis tests the origination times among Paleozoic gastropods implied by phylogenetic inferences. The other analysis tests the extinction times among Maastrichtian ammonites implied by different numbers of extinction events. In both cases, hypotheses positing many gaps in the fossil record become more likely after accommodating uncertainty about sampling. However, the increased likelihoods are insufficient to prevent these hypotheses from being rejected in favor of hypotheses positing fewer gaps. In both cases, the conclusions are identical to those derived by simple methods using simple models for unknown sampling parameters. Although numerous factors can exaggerate the implausibility of gaps, making these factors parts of testable hypotheses is possible. Thus, excessive assumptions about sampling parameters need not hinder empirical testing of hypothesized durations.
Ceratopsid dinosaurs traditionally have been restored with sprawling forelimbs and were considered unable to run at high speeds. An alternative view restores the ceratopsids as rhinoceros-like with parasagittal forelimb kinematics and the ability to run faster than extant elephants. Several anatomical difficulties concerning the mounting of ceratopsid skeletons with nearly parasagittal forelimbs stem not from the forelimb itself, but from errors in rib and vertebral articulation. Matching a skeletal restoration to a probable ceratopsid trackway shows that the hands were placed directly beneath the glenoids, and that manual impressions were directed laterally, not medially as in sprawling reptiles. Pedal impressions in trackways are medial to the manual impressions, owing to the slightly averted elbow and to the asymmetrical distal femoral condyles, which directed the crus slightly medially. The limbs of ceratopsians of all sizes display substantial joint flexure, strongly indicating that the elephantine forelimb posture that has sometimes been suggested as the alternative to a sprawling posture is erroneous. The articular surfaces of uncrushed ceratopsian scapulocoracoids and forelimb joints confirm that the forelimb operated in a near-parasagittal plane with the elbows only slightly averted. The maximal running speed of even the largest ceratopsids is inferred to have significantly exceeded that of elephants and was probably broadly similar to that of rhinos.
A new sampling technique for fossil bone (coring with a 5/8” bit) was used to sample longbones of all four sauropod genera from the Upper Jurassic Tendaguru beds of Tanzania for paleohistological study. Brachiosaurus and Barosaurus are represented by growth series of humeri and femora, while Dicraeosaurus could be sampled in fewer specimens and only one bone of Janenschia was available. Although all samples are dominated by fibrolamellar bone tissue, taxa can be distinguished by the degree and nature of bone remodeling and the presence and spacing of a peculiar kind of growth line (here termed “polish lines”). In addition, Barosaurus bone revealed two types of histology, tentatively interpreted as sexual morphs. The Tendaguru sauropods show a common growth pattern in which growth is determinate but sexual maturity is achieved well before maximum size is reached. For Brachiosaurus and Barosaurus, size at sexual maturity can be estimated and was reached at about 40% and 70% maximum size, respectively. Quantification of growth is possible in Janenschia using polish lines: the specimen studied reached sexual maturity at ≥11 years, attained maximum size at ≥26 years, and died at ≥38 years.
In this paper, I survey hindlimb and pelvic anatomy across non-avian dinosaurs and analyze these within a cladistic framework to quantify patterns of change within the locomotor apparatus. Specifically, I attempt to identify where homoplasy constitutes parallelism and may thereby be used to infer similar selective pressures on hindlimb function. Traditional methods of discrete character optimization are used along with two methods for evaluating changes in continuous characters in a phylogenetic context (squared-change parsimony and clade rank correlation). Resultant patterns are evaluated in light of the biomechanics of locomotion and the relationship between form and function in extant terrestrial vertebrates.
Although non-avian dinosaurian locomotor morphology is strikingly uniform, these analyses reveal the repeated derivations of several morphological features that have potential relevance for hindlimb locomotor function. Anterior and posterior iliac expansion, a medially oriented femoral head, and an elevated femoral lesser trochanter each evolved independently multiple times within Dinosauria. These changes probably reflect enlargement of several hindlimb muscles as well as a general switch in their predominant function from abduction-adduction (characteristic of “sprawling” limb postures) to protraction-retraction (characteristic of parasagittal, or “erect,” limb postures). Several “avian” characteristics are shared with more basal theropods, and many were acquired convergently in other dinosaurian lineages. The evolution of the avian hindlimb therefore represents a cumulative acquisition of characters, many of which were quite far removed in time and function from the origin of flight.
Morphologically complex metazoans appear abruptly during the Cambrian explosion. Suggested measures of metazoan complexity include number of cell morphotypes and aspects of the genome such as the amount of DNA, the number of genes, and the information content of the genome or egg. Estimates of gene numbers are now available for metazoan species belonging to five different phyla or subphyla. There is little correlation between gene number and morphological complexity in the invertebrates: relatively complex forms can have fewer genes than relatively simple forms. Presumably, the more complex forms use more gene-expression events during development, implying that, on average, cis-regulatory elements of more complex invertebrates are richer in binding sites than are those of simpler forms. Vertebrates have many more genes than invertebrates and therefore have more total gene-expression events during development, although they may have, on average, fewer expression events per gene than the invertebrates. There are thus two genomic pathways in the evolution of metazoan complexity: one involves increasing the number of genes, the other involves increasing the number of cis-regulatory binding sites. Both modes were associated with the origin of bodyplans that first appear as fossils during the Cambrian explosion.