George R. McGhee

The ‘multiple impacts hypothesis’ for mass extinction: a comparison of the Late Devonian and the late Eocene

Abstract Application of the lag-time multiple impacts hypothesis [Poag, 1997b. Palaios 12, 582–590; Poag et al., 2001. Columbia Univ. Press] to the Late Devonian leads to the prediction that the Frasnian–Famennian pulsed extinctions were triggered by a rapid drop in global temperature that followed an impact-produced anomalous warm interval, which interrupted the global cooling trend from the Middle Devonian greenhouse to the Early Carboniferous icehouse. In actualistic comparison with the late Eocene, the lag-time multiple impacts hypothesis would predict that a Frasnian interval of multiple impacts should have occurred between 367.7 and 366.7 Ma. The fact that three impacts (the Alamo, Siljan and Flynn Creek) do occur either within this predicted interval, or close to it in time, is corroborative evidence that the lag-time multiple impacts hypothesis may indeed provide the causal mechanism for the Frasnian–Famennian mass extinction. Based on the application of the lag-time multiple impacts hypothesis to the Frasnian–Famennian mass extinction, it is here suggested that future searches for evidence of impact events in the Late Devonian be concentrated in strata that occur in the Frasnian transitans to Early hassi zonal interval, and not in strata immediately below or above the Frasnian–Famennian boundary.

Evolutionary replacement of ecological equivalents in late devonian benthic marine communities

Abstract Temporal changes in taxonomic diversity, dominance structure, trophic structure, niche structure, and community composition are examined in equivalent nearshore and offshore Late Devonian benthic marine communities of New York and the central Applachians. The study interval spans an approximately 5-m.y. record of Frasnian tropical marine environments. Community structures remain relatively constant during the five million year interval, though offshore communities appear to be more unstable than nearshore communities. Considerable change in community composition occurs, however, with the total replacement of dominant spiriferacean brachiopods in the early Frasnian by their ecological equivalents in the late Frasnian. Ecological replacement of dominant species affected both offshore and nearshore communities, and does not appear to be a function of spatial environmental gradients.

Fenestrate theoretical morphology: geometric constraints on lophophore shape and arrangement in extinct Bryozoa

Abstract A geometric analysis of fenestrate bryozoan lophophore shape and arrangement is conducted by creating a theoretical morphospace of apertural positioning within the colonial meshwork. Working from the assumption that fenestrate bryozoans needed to form a continuous filtering surface with contact between adjacent lophophores, we show that within the morphospace three regions exist for optimum close-packing of lophophores with circular projections; all other close-packing configurations in the morphospace require the existence of noncircular lophophores. Examination of the actual distribution of 251 fenestrate colonies within the morphospace reveals that the morphospace regions occupied by fenestellids and polyporids are displaced and have little overlap, but that they are very similar in size and shape, and that the colonies scale similarly. With increasing size, fenestrate meshworks expand laterally faster than the branches widen and the proximodistal spacing of the apertures increases, apparently because the larger zooids require disproportionately more room for their lophophores. Two of the optimum close-packing regions of the morphospace are occupied by fenestrates. The positioning of the fenestellid region within the morphospace suggests that these biserial bryozoans followed a proximodistal-row placement of the lophophores, and that the lophophores were generally equitentacular, with circular projections. The positioning of the polyporid region within the morphospace suggests that these polyserial bryozoans followed a diagonal-row placement of the lophophores, and that the lophophores were heteromorphic, with medial lophophores on the branch being more equitentacular whereas the laterally placed lophophores were obliquely truncate. The third optimum close-packing region in the morphospace, corresponding to a hypothetical lateral-row placement of the lophophores within the colony, is unoccupied. We suggest that hypothetical fenestrate morphologies in the vacant region of morphospace have branches that would be too narrow to support normally shaped zooids, and that the lateral-row placement of the lophophores would have required the branches of the colony to have been perfectly aligned throughout growth.

Chapter 3Modelling late devonian extinction hypotheses

Abstract Modelling the conditions of extinction hypotheses can lead to the discernment of the critical observations that must be made in order to definitively test those hypotheses. Models considered here are based upon the beginning ning assumption that the end-Frasnian biodiversity crisis was triggered in large part by pulses of global cooling. Both classes of models are catastrophic: large-igneous-province volcanism and asteroid or comet impact. A single impact model can be ruled out, as its predicted effects do not match those of the extinction pulses seen in the Late Fransian. An episodic global-winter model would, but those effects can be produced by both the catastrophic volcanism and multiple impacts models. Alternative models suggest that both catastrophic volcanism and multiple impacts may trigger a global greenhouse-induced temperature increase, a predicted effect that does not match the empirical data. However, if both of those mechanisms are shifted back in time to produce lag-time models of short-term global temperature increase and then rapid temperature drops as the anomalous greenhouse interval collapsed, then both produce effects that do match the empirical data. To definitively choose among the several model predictions two critical data are needed: (1) an accurate radiometric date on the Frasnian/Famennian (F/F) boundary, and (2) an accurate Late Devonian global temperature curve. Recent research results suggesting a 376 Ma age for the F/F boundary would indicate that several known impact craters post-date the F/F extinctions, and make the impact-induced extinction models less likely. High-research priority should thus be given to obtaining a biostratigraphic date or an accurate radiometric date on the Viluy Traps, as a test to corroborate the volcanic-induced extinction models.

FENESTRATE GRAPTOLITE THEORETICAL MORPHOLOGY: GEOMETRIC CONSTRAINTS ON LOPHOPHORE SHAPE AND ARRANGEMENT IN EXTINCT HEMICHORDATES

Abstract A geometric analysis of lophophore shape and arrangement in the fenestrate dendroid graptolite genus Dictyonema reveals that the shape of the zooid domain in the majority of Dictyonema species colonies is highly elliptical, with the long axis of the ellipse perpendicular to the proximodistal axis of the stipe. A complex lophophore, bilaterally symmetrical and consisting of two tentaculated arms that are laterally directed and perpendicular to the stipe axis, provides the best geometric solution to completely filling the elliptical zooid domain seen in the majority of Dictyonema species. Working under the assumption that fenestrate graptolites, like fenestrate bryozoans, needed to form a continuous filtering surface with contact between adjacent lophophores, two optimum close-parking models exist for lophophores with elliptical zooid domains: either a proximodistal-row arrangement or a lateral-row arrangement of the lophophores. Of the two possible geometries, the most probable close-packing arrangement of hypothetical biplumed lophophores within the fenestrate graptolite meshwork is in proximodistal rows.

Can evolution be directional without being teleological

Convergent evolution reveals to us that the number of possibilities available for contingent events is limited, that historically contingent evolution is constrained to occur within a finite number of limited pathways, and that contingent evolution is thus probabilistic and predictable. That is, the phenomenon of convergence proves that truly contingent evolutionary processes can repeatedly produce the same, or very similar, organic designs in nature and that evolution is directional in these cases. For this reason it is argued in this paper that evolution can be directional without being teleological, and that the dichotomy that evolution must either be directionless and unpredictable or directional and predetermined (teleological) is false.

GEOMETRIC MODELS OF LOPHOPHORE SHAPE AND ARRANGEMENT IN EXTINCT MODULAR ORGANISMS: AN ADDENDUM

Geometric models for the reconstuction of soft-tissue lophophore shape and arrangement have been designed for extinct fenestrate bryozoans (Starcher and McGhee, 2000) and for extinct fenestrate graptolites (Starcher and McGhee, 2003). We wish to add the material given in the following three tables as an addendum to our paper in the March 2003 issue of the Journal of Paleontology. These tables will enable other researchers to characterize quickly and quantitatively the geometry of lophophore shapes and arrangements in the colonial meshworks of extinct organisms, and will add quantitative measures of soft-tissue characteristics of extinct modular organisms to the already existing morphometric characterizations of hard-tissue skeletal aspects of colonial meshworks (Hageman, 1991; Holdener, 1994; Hageman et al., 1998 …