George E. Boyajian

Patterns of generic extinction in the fossil record

Analysis of the stratigraphic records of 19,897 fossil genera indicates that most classes and orders show largely congruent rises and falls in extinction intensity throughout the Phanerozoic. Even an ecologically homogeneous sample of reef genera shows the same basic extinction profile. The most likely explanation for the congruence is that extinction is physically rather than biologically driven and that it is dominated by the effects of geographically widespread environmental perturbations influencing most habitats. Significant departures from the congruence are uncommon but important because they indicate physiological or habitat selectivity. The similarity of the extinction records of reef organisms and the marine biota as a whole confirms that reefs and other faunas are responding to the same history of environmental stress.

Evolution of biological complexity and its relation to taxonomic longevity in the Ammonoidea

The increase in biological complexity over evolutionary time has been the subject of extensive study and many hypotheses, but there are few data to document the increase or how organisms may have benefited by becoming more complex. By measuring the fractal dimension of the sutures of >600 ammonoid genera, we document the increase in anatomical complexity of the Ammonoidea and show that there is no differential survival between simple and complex forms. Simple sutures are present throughout the entire history of the Ammonoidea, but the range of variation of suture complexity increased over time as more complex sutures evolved.

Subtle changes in mature predator-prey systems; an example from Neogene Turritella (Gastropoda)

Although predator-prey escalation has been well documented in molluscan ecological systems, relatively little is known about what occurs after these systems have escalated. In order to assess potential changes in already-escalated systems, we examined bulk samples of Turritella shells collected from Neogene strata of the Atlantic and Gulf Coastal Plain, U.S.A. In particular, we examined the intensity, efficiency, and stereotypy of naticid and muricid predation on Turritella as a means of discerning subtle changes in predatory behavior. As in previous studies, we observe little change in the intensity and efficiency of naticid and muricid predation on Turritella during the Neogene. Unlike previous studies, we note significant changes in prey size during this interval. Perhaps more significant is that these changes are associated with concomitant changes in predatory drilling behavior as expressed in drillhole-site selectivity and prey-size selectivity. Together, these findings suggest that escalation was still occurring in this Neogene system, with predators likely making behavioral changes in order to keep pace with morphologic changes occurring within prey populations.

Fractal geometry of ammonoid sutures

Interior chamber walls of ammonites range from smoothly undulating surfaces in some taxa to complex surfaces, corrugated on many scales, in others. The ammonite suture, which is the expression of the intersection of these walls on the exterior of the shell, has been used to assess anatomical complexity. We used the fractal dimension to measure sutural complexity and to inves- tigate complexity over evolutionary time and showed that the range of variation in sutural com- plexity increased through time. In this paper we extend our analyses and consider two new param- eters that measure the range of scales over which fractal geometry is a satisfactory metric of a suture. We use a principal components analysis of these parameters and the fractal dimension to establish a two-dimensional morphospace in which the shapes of sutures can be plotted and in which variations and evolution of suture morphology can be investigated. Our results show that mor- phospace coordinates of ammonitic sutures correspond to visually perceptible differences in suture shape. However, three main classes of sutures (goniatitic, ceratitic, and ammonitic) are not unam- biguously discriminated in this morphospace. Interestingly, ammonitic sutures occupy a smaller morphospace than other suture types (roughly one-half of the morphospace of goniatitic and ceratitic sutures combined), and the space they occupied did not change dimensions from the Jurassic to the late Cretaceous. We also compare two methods commonly used to measure the fractal dimension of linear features: the Box method and the Richardson (or divider) method. Both methods yield comparable results for ammonitic sutures but the Richardson method yields more precise results for less complex sutures.

CHANGING BIOLOGIC SELECTIVITY OF EXTINCTION IN THE FORAMINIFERA OVER THE PAST 150 M.Y.

Extinctions of foraminifera 91 m.y. ago show the greatest number of patterns of selective morphologic and geographic extinction documented in the fossil record. Those taxa at significantly greater risk of extinction include those with agglutinated tests, discoidal shape, tropical ranges, and narrow latitudinal ranges. These results cast doubt on anoxia as the causal mechanism for foraminiferal extinction in the late Cenomanian. Significantly selective extinction of foraminifera with calcareous tests in the upper Eocene and lack of selective extinction at the Cretaceous-Tertiary (K-T) boundary demonstrate that mass extinctions can exhibit numerous patterns of biologic selectivity.

Proportional Change and Patterns of Homoplasy: Sanderson and Donoghue Revisited

The effects of two parameters, (1) the number of taxa and (2) the percentage of characters that change per node (referred to as the amount of proportional change or APC) on the retention index (RI) and the consistency index (CI) were examined. Random data sets were generated with a variety of different combinations of the number of taxa and the APC; five replicates for each unique combination were generated producing a total of 660 data sets. Results suggest that both the number of taxa and the APC are significantly correlated with the CI. In data sets with more than seven taxa, the APC explains more variation in homoplasy than does the number of taxa. The RI was found to be very strongly correlated with the APC and only weakly correlated with the number of taxa.

Taxon age and selectivity of extinction

Taxon-age distributions were compiled for families of marine animals surviving or becoming extinct in each stage of the Phanerozoic. I demonstrate, through the use of a modified bootstrap analysis, that there is no difference between the longevity of families becoming extinct during times of background extinction and times of mass extinction. In both mass and background extinction intervals the mean age of families that become extinct is 2 standard deviations below the geometric mean taxon age of families available for extinction. Young families are more susceptible to extinction, perhaps as the result of lower species richness or of occupying a smaller geographic range. There is no tendency during mass extinctions toward loss of families with different taxon ages other than those that become extinct during background times. Thus, in terms of family survival, mass extinction appears to be an exaggeration of processes of background extinction.

The function of astrorhizae in stromatoporoids: quantitative tests

Using the diameters of the channels at branch points, we quantitatively test three alter- native models of the function of astrorhizae in stromatoporoids. The distribution of diameters at branch points is significantly different from the distribution that would be predicted from models of either a diffusive function or a bulk-flow system in which resistance to flow was constant at all levels of the branching hierarchy. The distribution of channel diameters is virtually identical to that predicted by a model (Murrays law) that simultaneously minimizes resistance to flow and some volume-related cost function. Astrorhizae thus carried a bulk flow of fluid and can be inferred to have been lined with cellular elements; the exchange sites associated with the fluid-transport system were distributed throughout the soft tissues of the stromatoporoid animal. The most parsimonious hypothesis of function, that the fluid-transport system was associated with suspension feeding, implies strong similarities between the structure of the stromatoporoid animal and living sponges.

Selectivity of foraminiferal extinction in the late Eocene

Late Eocene foraminiferal extinction shows diverse patterns of selective morphologic and latitudinal extinction. Taxa with discoidal shape, calcareous tests, and narrow and low-latitudinal ranges are at significantly greater risk of extinction. Elevated extinction intensities in calcareous tests are mainly due to the presence of larger benthic foraminifera that evolved in late Paleocene and diversified through the lower to middle Eocene. Selectivity of late Eocene foraminiferal extinction indicates that this extinction event was not a globally uniform event. Although this result does not verify an extraterrestrial impact or any other proposed cause of extinction, it does constrain the causes of late Eocene extinction. Furthermore, the geography of late Eocene foraminiferal extinction, and previously studied Cenomanian/Turonian extinction, demonstrates that mass extinctions exhibit different patterns of selectivity.

Taxon age, origination, and extinction through geologic time

Changes in the taxon ages of fossil marine families that are alive and those that become extinct in each stage of the Phanerozoic reflect changes in the origination rate, differences in the extinction rate of families with different taxon ages, and mass extinction events. Extinct families are generally much younger than the population from which they were drawn. Periods dominated by higher numbers of younger families are more susceptible to larger size extinctions and greater variation in extinction size. As a result the relative size of extinction peaks must be viewed with regard to the taxon age structure of the population. Mass extinctions cause little change in the taxon age of the fauna. However, adaptive radiations cause a large drop in the average age of the families that are alive at any given time. Families must be treated as dynamic entities in macroevolutionary studies because their probabilities of extinction change over time.