Norman MacLeod

The Cretaceous-Tertiary biotic transition

Mass extinctions are recognized through the study of fossil groups across event horizons, and from analyses of long-term trends in taxonomic richness and diversity. Both approaches have inherent flaws, and data that once seemed reliable can be readily superseded by the discovery of new fossils and/or the application of new analytical techniques. Herein the current state of the Cretaceous-Tertiary (K-T) biostratigraphical record is reviewed for most major fossil clades, including: calcareous nannoplankton, dinoflagellates, diatoms, radiolaria, foraminifera, ostracodes, scleractinian corals, bryozoans, brachio-pods, molluscs, echinoderms, fish, amphibians, reptiles and terrestrial plants (macrofossils and palynomorphs). These reviews take account of possible biasing factors in the fossil record in order to extract the most comprehensive picture of the K-T biotic crisis available. Results suggest that many faunal and floral groups (ostracodes, bryozoa, ammonite cephalopods, bivalves, archosaurs) were in decline throughout the latest Maastrichtian while others (diatoms, radiolaria, benthic foraminifera, brachiopods, gastropods, fish, amphibians, lepidosaurs, terrestrial plants) passed through the K-T event horizon with only minor taxonomic richness and/or diversity changes. A few microfossil groups (calcareous nannoplankton, dinoflagellates, planktonic foraminifera) did experience a turnover of varying magnitudes in the latest Maastrichtian-earliest Danian. However, many of these turnovers, along with changes in ecological dominance patterns among benthic foraminifera, began in the latest Maastrichtian. Improved taxonomic estimates of the overall pattern and magnitude of the K-T extinction event must await the development of more reliable systematic and phylogenetic data for all Upper Cretaceous clades.

The Cretaceous/Tertiary boundary stratotype section at El Kef, Tunisia: how catastrophic was the mass extinction?

Abstract The Cretaceous/Tertiary (K/T) boundary stratotype section at El Kef, Tunisia, represents the most complete and expanded sedimentary record across this important mass extinction horizon presently known. High resolution analysis of planktic foraminifera in two outcrops (El Kef I—stratotype and El Kef II) along with comparisons between planktic and benthic foraminifera, calcareous nannofossils, ostracods, pollen and spores, and dinoflagellates indicate that major changes across the K/T boundary are registered only in benthic and planktic foraminifera and calcareous nannofossils. Biotic changes in benthic foraminifera are unique to El Kef and similarly shallow continental shelf sections and appear to be the result of a sea-level regression in the latest Maastrichtian followed by a sea-level rise across the K/T boundary that was accompanied by expansion of the local oxygen minimum zone (OMZ). Biotic changes in planktic foraminifera appear partly related to these conditions also, but in general reflect more global oceanographic changes. For instance, species extinctions are gradual and selective as observed in K/T sections worldwide, rather than random and abrupt. Although there is a 69% decline in species richness between 25 cm below and 10 cm above the K/T boundary, only rare species disappeared. Their combined relative abundance constitute less than 20% of the total population. About 52% of these extinct taxa (8% of the population) are large, ornate, morphologically complex tropical-subtropical forms that lived at or below the thermocline. No planktic foraminifera from this depth range survived the K/T boundary event. All survivor taxa were surface dwellers living within the photic zone. Their relative abundance (∼80%) dominates both Cretaceous and early Tertiary populations. These data indicate that the K/T biotic record in the shallow continental shelf section at El Kef was significantly influenced by local conditions which, combined with the latest Maastrichtian sea-level regression and subsequent sea-level rise, resulted in shallowing of the local OMZ relative to the sea-surface. Shallowing of the local OMZ lead to the selective disappearance of benthic faunas and may have adversely affected the surviving photic zone dwellers. The selective nature of species extinctions, however, appear to be related partly to long-term global oceanographic changes which were accelerated at the K/T boundary possibly by a bolide impact.

Generalizing and extending the eigenshape method of shape space visualization and analysis

Outline-based morphometric methods have been more or less restricted to the consideration of closed curves and plagued by problems related to the maintenance of close biological correspondence across all forms within a sample. Methods developed herein generalize and extend the eigenshape method of outline analysis along the following lines: (1) consideration of open curves, (2) improvement of interobject correspondence via incorporation of information provided by landmarks, and (3) extension to the analysis of three-dimensional (open and closed) curves. In addition, techniques for using eigenshape results to create models of shape variation and for more consistently assessing the digital resolution necessary to represent an object are discussed and illustrated. These improvements are then placed in context via discussions of previous attempts to extend morphometric outline analysis methods, the relation between landmark and outline-based morphometric methods, the use of morphometric analyses to test biological hypotheses, and the nature of morphometric shape spaces (with special reference to studies of morphological disparity).

Morphology, shape and phylogeny

Introduction: Morphology, Shape and Phylogenetics, N. Macleod and P. Forey Homology, Characters and Continuous Variables, C.J. Humphries Quantitative Characters, Phylogenies, and Morphometrics, J. Felsenstein Scaling, Polymorphism and Cladistic Analysis, T.C. Rae Overlapping Variables in Botanical Systems, G. Reid and K. Sidwell Comparability, Morphometrics and Phylogenetic Systematics, D.L. Swiderski, M.L. Zelditch, and W.L. Fink Phylogenetic Signals in Morphometric Data, N. Macleod Creases as Morphometric Characters, F.L. Bookstein Geometric Morphometrics and Phylogeny, F.J. Rohlf A Parametric Bootstrap Approach to the Detection of Phylogenetic Signals in Landmark Data, T.M. Cole III, S. Lele, and J.T. Richtsmeier Phylogenetic Tests for Differences in Shape and the Importance of Divergence Times: Eldredges Enigma Explored, P.D. Polly Ancestral States and Evolutionary Rates of Continuous Characters, A.J. Webster and A. Purvis Modelling the Evolution of Continuously Varying Characters on Phylogenetic Trees: The Case of Hominid Cranial Capacity Summary

Time to automate identification

Taxonomists should work with specialists in pattern recognition, machine learning and artificial intelligence, say Norman MacLeod, Mark Benfield and Phil Culverhouse — more accuracy and less drudgery will result.

How complete are Cretaceous /Tertiary boundary sections? A chronostratigraphic estimate based on graphic correlation

Cogent interpretations of data bearing on the Cretaceous/Tertiary (K/T) extinction controversy depend on the existence of accurate chronostratigraphic models for the various K/T boundary sections. We have employed the graphic correlation technique to summarize biostratigraphic and lithostratigraphic data from 15 intensively sampled K/T boundary sections within a common chronostratigraphic model. Our results indicate that almost all of these sections, along with 13 additional boundary sections not used to construct the model, contain prolonged and in many cases multiple hiatuses. Of these 28 boundary sections, only six were found to contain a continuous record of sediment accumulation across the K/T boundary itself. These six K/T-complete sections are El Kef (Tunisia), Agost (Spain), Caravaca (Spain), and three sections along the Brazos River (Texas). A comparative analysis of hiatus distributions among these 28 K/T boundary sections also reveals the presence of systematic differences between continental-shelf and deep-sea depositional environments. The lower Danian interval immediately following the K/T boundary, and extending into biochronozones P0 and P1a, is typically missing from the deep sea, whereas boundary sections deposited in shallower middle-neritic to upper-slope environments are in most cases complete across the K/T boundary. These shallow, neritic boundary sections, however, are in many instances disrupted by hiatuses at the P0/P1a boundary and again in the upper part of Zone P1a. These differential patterns of hiatus distribution between deep-sea and continental-shelf depositional settings appear to be linked to sea-level fluctuations. Our data suggest that the apparently sudden mass extinction of planktonic Foraminifera and anomalies in the occurrence of geochemical tracers that are characteristic of the K/T boundary in deep-sea sections may be artifacts of a temporally incomplete deep-sea stratigraphic record.

Hiatus distributions and mass extinctions at the Cretaceous/Tertiary boundary

Much disagreement over the interpretation of data bearing on various Cretaceous/Tertiary (K/T) extinction scenarios results from a failure to view these data within their appropriate stratigraphic context. Combined biostratigraphic and chronostratigraphic analyses of K/T boundary sequences have revealed systematic differences in patterns of sediment accumulation within continental-shelf and deep-sea depositional settings. Although virtually all deep-sea boundary sequences are marked by intervals of nondeposition or hiatus formation during the latest Cretaceous and earliest Tertiary, many continental shelf-slope sequences appear to be temporally complete over this same interval. This differential pattern of sediment accumulation can be related to the latest Maastrichtian-earliest Danian sea-level rise, during which deep-sea sediment- accumulation rates would be expected to drop as the locus of sediment deposition migrated across the continental shelf. Our data suggest that the abrupt shifts in carbon-isotope abundances, single-peak Ir anomalies, and apparently instantaneous mass extinctions of marine plankton—which are routinely reported from deep-sea K/T boundary sequences and used to support a causal relation between Late Cretaceous bolide impacts and K/T mass extinctions—may be artifacts of a temporally incomplete (or extremely condensed) deep-sea stratigraphic record.

Cretaceous Extinctions: Multiple Causes

![Figure][1] Deccan plateau basalts. Lava from Deccan volcanism formed distinct layering. CREDIT: GSFC/NASA In the Review “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene boundary” (P. Schulte et al. , 5 March, p. [1214][2]), the terminal Cretaceous

Arabidopsis thaliana Leaf Form Evolved via Loss of KNOX Expression in Leaves in Association with a Selective Sweep

Morphological diversity is often caused by altered gene expression of key developmental regulators. However, the precise developmental trajectories through which morphologies evolved remain poorly understood. It is also unclear to what degree genetic changes contributing to morphological divergence were fixed by natural selection. Here we investigate these problems in the context of evolutionary developmental transitions that produced the simple unlobed leaf of the model species Arabidopsis thaliana. We demonstrate that A. thaliana leaf shape likely derived from a more complex lobed ancestral state that persists in extant Arabidopsis species. We also show that evolution of the unlobed leaf form in A. thaliana involved loss of expression of the knotted1-like homeobox gene SHOOTMERISTEMLESS (STM) in leaves and that cis-regulatory divergence contributed to this process. Further, we provide evidence for a selective sweep at the A. thaliana STM locus, indicating that loss of STM expression in A. thaliana leaves may have been fixed by positive selection. In summary, our data provide key information as to when and how the characteristic leaf form of A. thaliana evolved.

Punctuated anagenesis and the importance of stratigraphy to paleobiology

-The depositional history of Upper Miocene through Recent sediments from DSDP Site 214 (Ninetyeast Ridge, Indian Ocean) is reexamined. Samples of the Globorotalia tumida planktic foraminiferal lineage, originally obtained from these sediments by Malmgren et al. (1983), serve as the empirical basis for the recognition of punctuated anagenesis as a distinct mode of phenotypic evolution and have been the subject of numerous additional investigations. However, conclusions reached by previous authors depend strictly on the validity of the original chronostratigraphic interpretation of these sediments. Graphic correlation analysis of firstand last-appearance datum levels for a total of 41 planktic foraminiferal, radiolarian, and calcareous nannoplankton taxa provides evidence for a more complex depositional history at this deep-sea site than originally believed. Based on a conservative model of variation in the pattern of sediment accumulation rates, the lowermost portion of the studied section (6.5-4.3 Ma) represents an interval of temporally condensed sediment accumulation (1.88 cm/1,000 yr) followed by an interval (4.3-2.8 Ma) of temporally expanded sediment accumulation (3.97 cm/1,000 yr). This interval, in turn, is followed by a depositional hiatus or an extremely condensed interval, at least 800,000 yr in duration, which is followed by another relatively condensed (1.36 cm/1,000 yr) interval from 2.0 Ma-Recent. Although this chronostratigraphic reinterpretation deviates substantially from the original, which recognized Site 214 as being both temporally continuous and exhibiting a constant sediment accumulation rate from the Upper Miocene through the Upper Pliocene, it is more consistent with expectations based on Neogene eustatic sea-level fluctuations and global surveys of Neogene hiatus distributions. Age assignments for samples of the Gr. tumida lineage based on the revised chronostratigraphic model reverse some findings of previous investigators with respect to the distinctiveness of phenotypic evolutionary rates characterizing the transition from Gr. plesiotumida to Gr. tumida. Finally, a brief survey of similar marine invertebrate lineage studies shows that changes in the rate of phenotypic evolution often appear to coincide with major physical changes in the paleoceanographic environment. Such correspondences may be due, at least in part, to the effect of these environmental changes on sediment accumulation rates. Paleobiologists who seek to understand patterns of phenotypic change over time must remove the effects of variations in sediment accumulation rates from their data before evolutionary hypothesis testing and remain aware of the limitations imposed on their interpretations by the uncertain nature of chronostratigraphic inference. Norman MacLeod. Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Accepted: December 15, 1990