Daniel C. Fisher

Oxygen isotope variation in the tusks of extinct proboscideans: A measure of season of death and seasonality

Centimetre-scale laminae in tusk and molar dentine of late Pleistocene mastodonts and mammoths have been interpreted as annual growth bands produced, in part, by seasonal variation in growth rate. To test this interpretation, we measured the oxygen isotope composition (δ 18 O) of the CO 3 fraction of dentinal hydroxyapatite from samples covering consecutive inferred years of growth in tusks. In mammals, changes in the δ 18 O value of dental tissues within individuals predominantly reflect variation in the δ 18 O value of body fluids, which is controlled mainly by the isotopic composition of ingested water. In Northern Hemisphere continental regions, winter precipitation has substantially lower δ 18 O values than does precipitation in other seasons. If ingested water tracks focal precipitation, then seasonal variations in dentinal isotope composition should result, the lowest δ 18 O values representing winter growth. We demonstrate that there are substantial variations in the oxygen isotope composition of proboscidean dentinal apatite, and that isotopic identifications of winter (i.e., low δ 18 O values) coincide with those based on growth rate (i.e., slow-growth zones). Finally, the potential of oxygen isotope analyses of terrestrial mammals for assessing the seasonality of paleoclimates is considered.

Regulation of Dental Enamel Shape and Hardness

Epithelial-mesenchymal interactions guide tooth development through its early stages and establish the morphology of the dentin surface upon which enamel will be deposited. Starting with the onset of amelogenesis beneath the future cusp tips, the shape of the enamel layer covering the crown is determined by five growth parameters: the (1) appositional growth rate, (2) duration of appositional growth (at the cusp tip), (3) ameloblast extension rate, (4) duration of ameloblast extension, and (5) spreading rate of appositional termination. Appositional growth occurs at a mineralization front along the ameloblast distal membrane in which amorphous calcium phosphate (ACP) ribbons form and lengthen. The ACP ribbons convert into hydroxyapatite crystallites as the ribbons elongate. Appositional growth involves a secretory cycle that is reflected in a series of incremental lines. A potentially important function of enamel proteins is to ensure alignment of successive mineral increments on the tips of enamel ribbons deposited in the previous cycle, causing the crystallites to lengthen with each cycle. Enamel hardens in a maturation process that involves mineral deposition onto the sides of existing crystallites until they interlock with adjacent crystallites. Neutralization of acidity generated by hydroxyapatite formation is a key part of the mechanism. Here we review the growth parameters that determine the shape of the enamel crown as well as the mechanisms of enamel appositional growth and maturation.

Lightning Strike Fusion: Extreme Reduction and Metal-Silicate Liquid Immiscibility

A glassy fulgurite, formed recently on a morainal ridge in southeastern Michigan, contains micrometer- to centimeter-sized metallic globules rich in native silicon, which unmixed from a silica-rich liquid. The unusual character of these globules and their potential for elucidating conditions of fulgurite formation prompted further study. Thermodynamic calculations indicate that temperatures in excess of 2000 K and reducing conditions approaching those of the SiO2-Si buffer were needed to form the coexisting metallic and silicate liquids. The phases produced are among the most highly reduced naturally occurring materials known. Some occurrences of other highly reduced minerals may also be due to lightning strike reduction. Extreme reduction and volatilization may also occur during high-temperature events such as lightning strikes in presolar nebulae and impacts of extraterrestrial bodies. As a result of scavenging of platinum-group elements by highly reduced metallic liquids, geochemical anomalies associated with the Cretaceous-Tertiary boundary may have a significant terrestrial component even if produced through bolide impact.

Taphonomic analysis of late Pleistocene mastodon occurrences; evidence of butchery by North American paleo-Indians

Taphonomic analysis of several late Pleistocene mastodon ( Mammut americanum ) skeletons excavated in southern Michigan provides compelling evidence of mastodon butchery by Paleo-Indians. The occurrence of butchery and details of butchering technique are inferred primarily from patterns of bone modification. An important aspect of butchering practice was production and use of tools fashioned from bones of the animal being butchered. Evidence for butchery and bone tool use includes matching marks on the conarticular surfaces of disarticulated pairs of bones; cutmarks on bones; green bone fracturing; use wear, secondary flaking, and impact features on bone fragments; and burned bone. Interpretation of these features is facilitated by information on patterns of bone distribution and disarticulation preserved in a primary depositional context. Preliminary comparisons among nine sites indicate that putative butchering sites differ consistently and in a variety of ways from sites that appear to record no human involvement. Although based on a small sample of sites, the apparent frequency of butchered individuals relative to those that were not butchered is unexpectedly high. These findings provide new evidence of a well-developed “bone technology” employed by the late Pleistocene human inhabitants of eastern North America. In addition, these data offer circumstantial support for the hypothesis that human hunting was an important factor in the late Pleistocene extinction of mastodons.

Comparing the fit of stratigraphic and morphologic data in phylogenetic analysis

Stratigraphic data are compared to morphologic data in terms of their fit to phylogenetic hypotheses for 29 data sets taken from the literature. Stratigraphic fit is measured using MacClades stratigraphic character, which tracks the number of independent discrepancies between observed order and the order of occurrence that would be expected on the basis of a given phylogenetic hypothesis. Acceptance of a phylogenetic hypothesis despite such discrepancies requires ad hoc hypotheses concerning differential probabilities of preservation and recovery. These stratigraphic ad hoc hypotheses are treated as logically equivalent to morphologic ad hoc hypotheses of homoplasy. The retention index is used to compare the number of stratigraphic and morphologic ad hoc hypotheses required by given phylogenetic hypotheses. Each data set is subjected to five analyses, varying in the constraints imposed on the structure of the phylogenetic tree against which fit is measured. Analyses 1–4 compare the stratigraphic and morphologic retention indices using phylogenetic trees consistent with the morphologically most-parsimonious cladogram reported in the original study. Analysis 5 compares retention indices using the overall (stratigraphically and morphologically) most-parsimonious phylogenetic tree, which may be, but is not necessarily, consistent with the reported cladogram. Proceeding from Analysis 1 to Analysis 5, stratigraphic data are allowed greater influence in determining the structure of phylogenetic trees, with the trees in Analysis 1 derived without reference to the stratigraphic character and the trees in Analysis 5 derived from full interaction of stratigraphic and morphologic characters. Morphologic and stratigraphic retention indices for these 29 studies cannot be statistically distinguished in comparisons 3–5, suggesting very similar degrees of fit. The values of these retention indices are high, indicating a generally high level of congruence under these phylogenetic hypotheses. Significant gains (49%) in stratigraphic fit can be realized without significant loss (4%) in morphologic fit as the stratigraphic and morphologic evidence are both allowed to participate in constraining the structure of phylogenetic hypotheses. These results suggest that arguments based on alleged “noisiness” of stratigraphic data offer inadequate grounds for ignoring stratigraphic order in phylogenetic analysis. In terms of congruence, stratigraphic and morphologic data perform about equally well.

Mastodont Procurement by Paleoindians of the Great Lakes Region: Hunting or Scavenging?

Investigations of the evolution of human hunting behavior are complex in many respects, but one of the most challenging issues they must address is the distinction between hunting and scavenging. The difficulty of deciding whether instances of carcass utilization represent hunting or scavenging exists in part because the means of carcass procurement frequently leaves no record on preserved materials. Even when it does, such a record may be obscured by, or confused with, the modification sustained during butchery or other postmortem events (papers by Klein and Behrensmeyer, this volume). The context and nature of an assemblage (as in many Great Plains bison kills; Frison 1978; Todd, this volume) or the preservation of structures showing a unique functional association with hunting (Frison, this volume) sometimes make it possible to reject scavenging as an explanation for a given site. However, judged over the whole history of human utilization of animal carcasses, such a clear choice is more the exception than the rule.

Evolutionary morphology; beyond the analogous, the anecdotal, and the ad hoc

Many questions have emerged recently regarding the importance and methodology of analysis of adaptation. Divergent views reflect both problems of definition and more substantive issues of interpretation. Defining the state of adaptation in terms of its contribution to current fitness, rather than origin by natural selection, is essential if natural selection is to be considered an explanation of adaptation. The context dependency and relativity of fitness apply also to adaptation. Design criteria are essential components of adaptation, but only to the extent that they are subsumed as elements of the causal interactions determining relative reproductive potential. The local, relational, contingent character of adaptation supports only limited reference to improvement. Most long-term patterns of change can be better described as diffusion within a structured design-space than as progressive improvement of design. The analysis of adaptation is part of a broader inquiry into the processes and constraints that control form and the history of changing form. It offers one perspective on how organisms operate on ecological time scales and how their configurations might be maintained or transformed over evolutionary time. Hypotheses concerning adaptation are sometimes tested by reference to predictions concerning the central tendency or trend of some aspect of an anatomical-behavioral system. These can be interpreted with minimal reference to assumptions of optimality if the analysis is viewed in terms of Bayesian inference. However, an alternative and frequently preferable approach to testing relies on limit-oriented predictions. Analysis of adaptation can be visualized as inferring the pattern and nature of interactions comprising the causal plexus that determines fitness. A comprehensive understanding of form and form-change requires that this be integrated with the perspective offered by studies of development, genetics, phylogenetic history, and external perturbations acting on the system.

STRATOCLADISTIC ANALYSIS OF PALEOCENE CARPOLESTIDAE (MAMMALIA, PLESIADAPIFORMES) WITH DESCRIPTION OF A NEW LATE TIFFANIAN GENUS

Abstract “Carpodaptes” jepseni is a morphologic intermediate between Carpodaptes and Carpolestes, with the number and position of cusps on p4 more consistent with placement in Carpodaptes but relative size of p4 more like Carpolestes. The type and only previously known specimen of “C.” jepseni, a partial dentary with p4–m2, is from Divide Quarry (Tiffanian Land-Mammal Age) in the Fort Union Formation of the Bighorn Basin, Wyoming. New specimens of “C.” jepseni from Divide Quarry include a nearly complete dentary with p4–m3 and alveoli for all anterior teeth, and the first known upper dentitions with P1–M2 and an alveolus for C1. Specimens of Carpodaptes cygneus are also described from Divide Quarry, demonstrating the occurrence of two distinct carpolestid species at the same locality. Stratocladistic analysis of the thirteen known carpolestid species, using thirty-two morphologic characters and stratigraphic order, produced eight most-parsimonious phylogenetic trees associated with a single cladogram. The topology of the cladogram generated using stratocladistics is identical to that of the single most-parsimonious cladogram from cladistic analysis of the same morphologic data, but stratocladistics allows greater resolution than cladistics at the level of phylogenetic trees. New specimens demonstrate extreme shortening of the anterior jaw of C. jepseni, a derived state not present in other carpolestids. This suggests that C. jepseni occupies a side-branch close to, but not at, the ancestry of the Carpolestes clade (an ancestor-descendant lineage composed of the sequence C. dubius, C. nigridens, and C. simpsoni, in that order). “C.” jepseni is here placed in a new genus, Carpomegodon.

The Xiphosurida: Archetypes of Bradytely?

The Xiphosurida, or horseshoe crabs, are often cited as a classic example of arrested evolution. They have been so consistently associated with this concept, in both professional and popular literature, that their reputation for extreme conservatism in form and behavior is probably more widely known than any other single aspect of their biology or history. Ironically, however, there have been no published measurements of evolutionary rates (either morphologic or taxonomic) for horseshoe crabs, let alone any rigorous comparisons of evolutionary rates between horseshoe crabs and groups that supposedly evolve more rapidly. Rather, their status as an archetypal bradytelic group has been founded primarily on the judgment and authority of specialists who have perceived only minor morphologic differences between the living forms and certain fossil relatives (Fig. 1A-D).

Complete Columbian mammoth mitogenome suggests interbreeding with woolly mammoths

BackgroundLate Pleistocene North America hosted at least two divergent and ecologically distinct species of mammoth: the periglacial woolly mammoth (Mammuthus primigenius) and the subglacial Columbian mammoth (Mammuthus columbi). To date, mammoth genetic research has been entirely restricted to woolly mammoths, rendering their genetic evolution difficult to contextualize within broader Pleistocene paleoecology and biogeography. Here, we take an interspecific approach to clarifying mammoth phylogeny by targeting Columbian mammoth remains for mitogenomic sequencing.ResultsWe sequenced the first complete mitochondrial genome of a classic Columbian mammoth, as well as the first complete mitochondrial genome of a North American woolly mammoth. Somewhat contrary to conventional paleontological models, which posit that the two species were highly divergent, the M. columbi mitogenome we obtained falls securely within a subclade of endemic North American M. primigenius.ConclusionsThough limited, our data suggest that the two species interbred at some point in their evolutionary histories. One potential explanation is that woolly mammoth haplotypes entered Columbian mammoth populations via introgression at subglacial ecotones, a scenario with compelling parallels in extant elephants and consistent with certain regional paleontological observations. This highlights the need for multi-genomic data to sufficiently characterize mammoth evolutionary history. Our results demonstrate that the use of next-generation sequencing technologies holds promise in obtaining such data, even from non-cave, non-permafrost Pleistocene depositional contexts.