bluuebunny/crossref_metadata_2025_split huggingface dataset:
Analysis of the preservation of community structure in assemblages of fossil mammals
Authors: Damuth John 
Published: July 2016

The degree to which the relative abundances of the species in a fossil assemblage represent those in the living community from which it came is an indication of the fidelity of the assemblage in preserving most of the aspects of community structure that we may hope to find in the fossil record. I have developed a model for the formation of attritional fossil mammal assemblages that shows that the logarithm of the number of individuals of each species in an ideal unbiased assemblage, when regressed on the logarithm of body mass, will exhibit a slope of approximately −1.05 ± 0.25 (approximate 95% confidence interval). This value is based upon ubiquitous scaling relationships of population density and turnover rate with body mass among extant mammals. These relationships are the results of widespread energetic and metabolic regularities in community structure and physiology and can be expected to have been essentially the same in the remote past. The slope of −1.05 provides for the first time a reference standard for community structure of sufficient generality for use in the analysis of this aspect of fossil assemblages. We can use this value as a sort of “null hypothesis” of no bias against which we can compare actual fossil assemblages.Analysis of some promising assemblages reveals that they exhibit badly biased relative abundances. However, it is likely that in these cases we can explain this bias as almost entirely the result of pre-burial differential destruction of the carcasses of different-sized species.The combination of this new, biological approach with the largely physical approaches of taphonomy and sedimentology will allow us to ask more specific questions about bias in fossil assemblages, avoiding circular arguments. We can now distinguish, in a large number of situations, assemblages that preserve reliable community-structure information from those that do not.
A functional analysis of flying and walking in pterosaurs
Authors: Padian Kevin 
Published: July 2016

An analysis of the structure and kinematics of the forelimbs and hindlimbs of pterosaurs, and functional analogy with recent and fossil vertebrates, supports a reappraisal of the locomotory abilities of pterosaurs. A hypothesis of structural, aerodynamic, and evolutionary differences distinguishing vertebrate gliders from fliers is proposed; pterosaurs fit all the criteria of fliers but none pertaining to gliders. The kinematics of the reconstructed pterosaur flight stroke reveal a down-and-forward component found also in birds and bats; structural features of the shoulder girdle and sternum unique to pterosaurs may be explained in light of this motion. The recovery stroke of flight was accomplished, in birdlike fashion, by a functional reversal of the action of theM. supracoracoideusby the pronounced enlargement of the acrocoracoid process, which acted as a pulley. The wing membrane was supported and controlled through a system of stiffened, intercalated fibers, which were oriented like the main structural elements in the wings of birds and bats.The hindlimbs of pterosaurs were independent of the wing membrane, and articulated like those of other advanced archosaurs and birds, not like those of bats. The gait was parasagittal and the stance digitigrade. Because of limitations on the motion of the forelimb at the shoulder, pterosaurs could not have walked quadrupedally. However, bipedal locomotion appears to have been normal and quite sufficient in all pterosaurs. There is nothing batlike about pterosaur anatomy; on the other hand, pterosaurs bear close structural resemblances to birds and dinosaurs, to which they are most closely related phylogenetically.
Early ontogeny ofEutrephocerascompared to RecentNautilusand Mesozoic ammonites: evidence from shell morphology and light stable isotopes
Authors: Landman Neil H. , Rye Danny M. , Shelton Kevin L. 
Published: July 2016

Observations on the morphology of the early whorls ofEutrephoceras dekayi(Morton), a widespread Cretaceous nautilid, are supplemented with oxygen and carbon isotopic analyses (δ18O and δ13C) of the early septa of five well-preserved specimens to help identify the point of hatching on the shell. Septa 4 and 5 are more closely spaced than preceding septa and probably correspond in time of formation with a constriction or first broken aperture on the outer shell one-third whorl forward of the fourth septum. In modernNautilus, morphologic, isotopic, and observational data suggest that similar features mark hatching. Between the fourth and fifth septa inE. dekayi, δ18O values show a shift of variable magnitude from heavy to lighter values followed by a return to heavier values over the next one to three septa. This isotopic shift is compatible with a hatching interpretation and may be explained as the result of kinetic and equilibrium effects on emergence from an egg capsule.Eutrephoceras dekayihatched at about 9 mm in diameter, one-third the hatching size of modernNautilus.LikeNautilus, E. dekayiprobably produced few young, all of which were active swimmers at hatching. In contrast, Mesozoic ammonoids produced numerous offspring ranging from 0.5 to 1.5 mm in diameter which may have spent some time in the plankton. These differences in life history may correlate with differences in ecologic specialization, environmental tolerance, and habitat between ammonoids and nautilids and may have contributed to their disparate rates of evolution during the Mesozoic.
Mechanical and photosynthetic constraints on the evolution of plant shape
Authors: Niklas Karl J. , Kerchner Vincent 
Published: July 2016

A computer model is presented which is capable of calculating both the photosynthetic efficiency (I) of any specified plant shape and the stress related to the total moment arm (M) imposed on vertical branching patterns. Computer simulations indicate that a flattened plant thallus and an erect branching growth habit are two plant shapes capable of optimizing photosynthetic efficiency during indeterminate growth. These two morphologies have geometric analogues in the dorsiventral thalli of some bryophytes and in the vertical axes of mosses and tracheophytes, respectively.Extension of the model to complex, three-dimensional branching patterns indicates thatIandI/Mare maximized when branching is overtopped (treelike, with lateral branches on a main axis) and when lateral branching systems are planated (frondlike). Geometric alterations of branching patterns that result in optimization ofIandI/Mcan be simulated by computer and are shown to be similar to morphologic alterations attending the early evolution of vascular land plants. It is suggested that a number of major evolutionary trends seen in Upper Silurian to Upper Devonian times can be expressed in terms of optimizing the display of photosynthetic tissues (I) or the balance between photosynthetic efficiency and incurred moment arms (I/M).
Radiations and extinctions in relation to environmental change in the marine Lower Jurassic of northwest Europe
Authors: Hallam A. 
Published: July 2016

A diversity and turnover analysis has been undertaken for a number of invertebrate groups in the Liassic of northwest Europe. There is a more or less steady rise in diversity from the early Hettangian through to the Pliensbachian, followed by a marked decline into the early Toarcian, after which it tends once more to increase. Ammonites stand out from the other invertebrates as having had an exceptionally high rate of turnover, with very short species durations.Increase of neritic habitat area due to rise of sea level, and recolonization following the end-Triassic mass extinction event appear to be the promoters of diversity increase or radiation. Severe reductions of neritic habitat area with associated environmental deterioration, related either to episodic marine regressions or spreads of anoxic bottom waters, and bound up respectively with sea-level fall and rise, are seen as the prime factors responsible for increase of extinction rate. While the environmentally sensitive ammonites were affected by even minor regressions, the other, more eurytopic groups were evidently more resistant to these. The only event that warrants the term mass extinction, affecting nearly all the benthos and nekton but not the plankton, correlates precisely with the early Toarcian anoxic event. Several episodes can be recognized of migrations of organisms into Europe following extinctions.
Larval ecology, life history strategies, and patterns of extinction and survivorship among Ordovician trilobites
Authors: Chatterton Brian D. E. , Speyer Stephen E. 
Published: July 2016

Differences in the larval ecology of Ordovician trilobites directly influenced the outcome of the Ashgill extinction (latest Ordovician) and indirectly governed the pattern of evolution in post-Ordovician trilobites. Larval ecology also affected survivorship patterns within the Ordovician, particularly between the Llandeilo and the Caradoc stages. All taxa with pelagic adults became extinct by the end of the Ordovician. Similarly, trilobites with entirely planktonic larvae had all but disappeared by the end of the Ordovician. Although suffering a significant loss in diversity, taxa with benthic larvae provided the ancestral stock for the majority of post-Ordovician lineages. Trilobites with a two-stage protaspid period (single planktonic followed by benthic larvae) suffered least during the Ashgill extinction, giving rise to approximately 23% of the new genera appearing in the early Silurian.Patterns of extinction/survivorship among trilobite taxa with different developmental strategies indicate that the so-called Ashgill extinction was most likely the result of a composite phenomenon, including environmental perturbations, ecosystem breakdown and biogeographic restriction, and was not the consequence of a single catastrophic event (e.g., bolide collision). Indeed, our results are consistent with extinction models which invoke periodic global cooling and sea level regression associated with glaciation. Taxa with planktonic larvae first suffered a marked decline in diversity between the Llandeilo and the Caradoc (end of Middle Ordovician), coincident with the onset of the Ordovician-Silurian glaciation. During this time interval, the paleogeographic ranges of most surviving genera with solely planktonic larvae were severely constricted to lower latitude paleobiogeographic provinces. During the Ashgill (Rawtheyan to Hirnantian), climatic and sea level fluctuations were most extreme because of continued and more extensive glaciation. At this time, when extinctions were particularly severe, there was also a significant effect upon the survivorship of taxa with benthic larvae.Our results indicate that the proportion of the life cycle spent in the water column, dependent upon a planktonic food source, was a critical factor in the survivorship of trilobite taxa during Middle to Late Ordovician time. Consequently, differences in life history patterns predisposed individual taxa to survival or extinction.
Life-history and the evolution of ontogeny in the ostracode genus Cyprideis
Authors: Schweitzer Peter N. , Lohmann G. P. 
Published: July 2016

A large body of paleontological literature concerns the importance of ontogeny as a source of morphological variation for evolution; morphologies that appear during one stage of an organism's development are made available for use in another simply by modifying the developmental program. Paleontologists need to know why this occurs, so they can study the process of evolution in extinct animals and so they can discuss the fossil record in terms that are applicable to modern forms. If most cases of heterochrony can be attributed to life-history evolution then the fossil record provides evidence of the nature of selection (in particular the age-specific mortality) that extinct animals experienced. The hypothesis of interest here is that species in which maturity is accelerated will also show generalized morphology and small size, while those with delayed maturity will have more specialized morphology and large size.Four species of the ostracode genus Cyprideis were studied to determine whether differences in age at maturity are correlated with heterochrony in the expected manner. For each species the changes in size and shape through geological time were evaluated in the statistical context of modern geographic and seasonal variation. Living populations were sampled regularly to detect differences in seasonality and to estimate the duration of development.Evolution of ontogeny is apparent at the level of species in this group, but it is not simply related to differences in life-history. In comparisons among species, we find evidence of heterochrony where there is no difference in the age at maturity, and a difference in age at maturity where there is no heterochrony. Similarly, three of the four species show the expected positive correlation between size and age at maturity, yet the fourth species is relatively large and matures rapidly. Cyprideis does not support the generalization that life-history evolution causes heterochrony, and casts doubt on the inference of life-history evolution from heterochrony where the data are drawn exclusively from extinct forms.
Sutural pattern and shell stress inBaculiteswith implications for other cephalopod shell morphologies
Authors: Jacobs David K. 
Published: July 2016

InBaculites, a straight shelled ammonite, the constructional limits on shell shape resulting from the limited strength of nacre in tension are circumvented by a system of vaults in the phragmocone. Vaults bridge between regions of the phragmocone supported by the complex ammonite septal suture, and maintain the shell wall in compression when hydrostatic load induces bending moments. To determine how these vaults interact in the phragmocone to resist hydrostatic loading, measurements were made on a suite ofBaculitesspecimens. InBaculitesthere is a statistically significant inverse relationship between circumferential curvature (radius of curvature) of the vaulted shell surface and the size of vaults spanning between sutural elements supporting the phragmocone. The inverse relationship between radius of curvature and the sizes of spans in this system of vaults results in the generation of comparable reactive forces at the ends of the vault spans where adjacent vaults interact. The equivalence of these reactive forces prevents the lateral displacement of the vault ends. Consequently, compressive stresses from adjacent vaults are superimposed on, and reduce, the tensional stress component of bending. Limiting tensile stress is of utmost importance in a lightweight shell composed of a brittle material such as nacre, which is strong in compression but weak in tension.Baculitesis particularly appropriate for this study because its straight shell is curved only in the circumferential direction, thus simplifying the problem. However, sutural patterns in coiled ammonites appear to be similarly constrained to produce vaults in the phragmocone which vary inversely in curvature and span size.
Incumbent replacement: evidence for long-term evolutionary progress
Authors: Rosenzweig Michael L. , McCord Robert D. 
Published: July 2016

Evolutionary progress is a trend that relaxes trade-off rules. It begins with the evolution of a key adaptation. It continues with the spread of the key adaptation as the clade that contains it replaces some older clade that lacks it. Key adaptations are those that allow for improvement in at least one organismal function at a reduced fitness cost in other functions.Replacement almost certainly involves more than pure chance. It may not often involve competitive extinction. Instead, species from the new clade produce new species to replace already extinct species from the old clade. The key adaptation gives them a higher competitive speciation rate than old-clade sources of replacement. The process, termedincumbent replacement,proceeds at a rate limited by extinction rate. Thus, replacement often seems linked to mass extinction events.The incumbent-replacement hypothesis explains what we know about the replacement of straight-neck turtles (Amphichelydia) by those that can flex their necks and protect their heads in their shells. This replacement occurred four or five times in different biotic provinces. It happened as long ago as the Cretaceous in Eurasia, and as recently as the Pleistocene in mainland Australia. It was accomplished in Gondwanaland by turtles flexing their necks sideways (Pleurodira), and in the north by those flexing their necks into an S-curve (Cryptodira). As is typical of replacements, amphichelydian replacement took millions of years to accomplish wherever it occurred, and much of it in North America took place in a burst associated with and immediately subsequent to a mass extinction.
The biogeography of origin and radiation: dendrasterid sand dollars in the northeastern Pacific
Authors: Beadle Steven C. 
Published: July 2016

Under favorable circumstances, biogeographic and biostratigraphic data can be combined to identify accurately the time and place of origin of a given taxon, and to reconstruct the pattern of its subsequent radiation. This study considers the dendrasterid sand dollars, which are abundant today along the Pacific Coast of North America. The Neogene sand dollar record in this region is particularly good; in fact, sand dollars have traditionally been used as provincial index fossils.The dendrasterids originated in central California at the end of the Miocene; the oldest forms are dated at about 6.0–6.5 Ma. They spread south to Baja California during the Pliocene, and then north to Alaska during the Quaternary. This historical pattern is not an artifact of the record; it is consistent with independent paleogeographic evidence. The dendrasterids supplanted an older Mio-Pliocene sand dollar fauna; they are now completely dominant in the temperate coastal waters of the northeastern Pacific. They have reached this position in less than 7 m.y. since their first local appearance. The rapid rise of dendrasterids could be related to their aberrant morphology and behavior; these adaptations allow dendrasterids to suspension-feed, in a manner unique among living echinoids.Dendrasterids are characterized by “eccentric” test morphologies. Even the oldest species are highly eccentric; transitional forms are unknown. The first dendrasterids appear suddenly in the provincial “Jacalitos Stage,” above an unconformity which represents no more than about 1 m.y. They do not occur in the underlying units, although other fossil sand dollars are abundant. The dendrasterids may have arisen rapidly, through a heterochronic change in the development of older, noneccentric forms. Recent ontogenetic studies have documented the feasibility of this process.
Biased extinction and evolutionary trends
Authors: Norris Richard D. 
Published: July 2016

The directionality of long-term trends can be strongly biased by forces intrinsic to a clade. Trends in body size and skeletal shape may be dictated more by variations in survivorship that reflect differences in ecology than by long-term directional changes in the environment. Hence, mass extinctions can help drive evolutionary trends by selectively eliminating some morphologies and permitting the survivors to found the next radiation.Examples include repeated trends toward larger maximum body size and the evolution of keeled species from those with globose tests in planktonic foraminifera. Both the trends in size and shape develop because small species with globose tests are significantly more resistant to extinction than species that are large or have peripheral keels. Hence, the survivors of both the Cretaceous-Tertiary and Eocene-Oligocene extinction episodes are small, unkeeled taxa. Large species and species with keels evolved convergently after both mass extinctions as the founders radiated anew.Comparison of three radiations of planktonic foraminifera suggest that the convergent evolution of similar test shapes and sizes is not due to synchronous changes in oceanography that track evolutionary trends. Instead, the reestablishment of habitat heterogeneity is needed to permit the ensuing radiation to unfold rather than to closely guide its progress. Similar evolutionary trends will develop in each radiation as long as the founders have similar morphology and the evolution of variants present in the previous radiation is not precluded by the environment.
Systematics and paleobiology
Authors: Eldredge Niles , Novacek Michael J. 
Published: July 2016

Systematics, ostensibly the “old” paleontology, actually plays a central and crucial role in modern paleobiology. We argue that a revised ontology has recently clarified the nature of species and has expressly added monophyletic groups to the roster of spatiotemporally bounded entities—“individuals”—that are now seen as participants in the evolutionary process. Systematics is the study of species and monophyletic taxa, and fossils alone provide the data on the temporal boundedness of such taxa.Cladistics (phylogenetic systematics) is explicitly geared to the recognition of monophyletic taxa. We review aspects of the core problem of character analysis in systematics, particularly addressing the still contended issue of the seemingly competitive claims of three methodologies: out-group comparison, comparative ontogeny, and the “paleontological method.” We find that these methods overlap in their basic assumptions to a significant extent, yet each retains a characteristic and distinctive flavor. They are not all “the same,” nor are they always “complementary”—and no one method is superior to the others in all circumstances.Far from being the Victorian symbol of a moribund science, systematics lies at the very heart of modern paleobiological research, providing the central data for paleobiology's truly unique contribution, both real and potential, to evolutionary biology in general.