How catastrophic were massextinction events

Determining whether a given extinction event was catastrophic or merely gradual is not a straightforward matter,

Table 3.1 The five biggest mass extinction events of the Phanerozoic

(1) End-Ordovician event (26)

The graptolites underwent the worst crisis in their history, and the conodonts were also severely affected. The dominant benthic (bottom-living) trilobites and brachiopods suffered major extinctions.

(2) Late Devonian event (Frasnian-Famennian boundary) (22)

Reef ecosystems, comprising rugose and tabulate corals and stromatoporoid sclerosponges, underwent a major crisis. Among the rest, both marine invertebrates (ammon-oids, brachiopods, trilobites) and vertebrates (conodonts, agnathan and armoured placoderm fish) suffered severe extinctions. The land record is much less clear, but there could have been an important extinction among plants.

Reefs were again seriously affected, with the final disappearance of rugose and tabulate corals. Major extinctions took place among the echinoderms, with the loss of important Palaeozoic crinoid groups and blastoids, and the bryozoans. There were also significant losses among the brachiopods, marking the end of their numerical domination of benthic communities, and foraminifera, with the final disappearance of a major, large-sized group, the fusulinids. On land, there was a significant loss of because of the limitations imposed by the stratigraphic record. Consider the case represented in Fig. 3.3, in which a gradual extinction can appear to be catastrophic if a hiatus exists (caused either by failure of sedimentary deposition or by erosion in the region studied), leading to the absence of one vertebrates, with over half the families disappearing, including all large herbivores. Insects suffered the greatest turnover in their long history, and there were regionally important extinctions among the plants.

(4) End-Triassic event (22)

A further crisis was undergone by reef ecosystems, with a major group of calcareous sponges almost completely disappearing. The rich and diverse Triassic ammonite faunas were almost totally wiped out, and the conodonts ended their long history. Significant extinctions took place among the now-dominant benthic bivalves, together with brachio-pods. An important regional extinction event took place among terrestrial plants, but claims of a major land vertebrate extinction are disputed.

(5) End-Cretaceous event (16)

In the marine record the most striking event is the mass extinction of two planktonic groups with calcite skeletons, the foraminifera and coccolithophorids. Many benthic invertebrates were also strongly affected, and certain groups such as the ammonite and belemnite cephalopods and inoceramid and rudist bivalves finally disappeared from the record. On land the most obvious victims were the dinosaurs; other groups were much less strongly affected.

Figures in parentheses are the percentages of marine families that became extinct.

or more biozones. This emphasizes the vital importance of having the best possible biostratigraphic control on the stratal successions that we study.

We have, however, to take into account the converse effect. What are known as 'range charts' record the series of occur-






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Species ranges

Fig. 3.3 Diagram to illustrate how a stratigraphic hiatus can create a false impression of a mass extinction from a continuous set of species ranges, represented by vertical bars. The left-hand portion of the diagram (a) exhibits continuous sedimentation, but in the right-hand portion (b) the band marked by oblique lines has been removed by erosion or non-sedimentation.

rences of fossil species against a stratigraphic column. Such charts give a feel for the relative frequency of occurrence of species; more common species will have shorter gaps between their occurrences than rarer ones. An important point to be remembered when interpreting extinctions in range charts is that the highest (i.e. latest) occurrence of a fossil species is unlikely to represent the very last individual of that species. Thus all species ranges are artificially truncated to some extent. This will tend to make mass extinctions appear more gradual than they actually were (Fig. 3.4). This effect is known as 'backsmearing' or the 'Signor-Lipps effect', after the two American palaeontologists who first pointed it out.

The Signor-Lipps effect is least important for the commonest species, which tend to have short gap lengths. The differ-

Butler Tourism Model

Fig. 3.4 The Signor-Lipps effect: the effect of random range truncations on an abrupt mass extinction. Artificial range truncations produce an apparently gradual extinction.

Fig. 3.4 The Signor-Lipps effect: the effect of random range truncations on an abrupt mass extinction. Artificial range truncations produce an apparently gradual extinction.

ence in gap length between common and rare species is the fundamental attribute that is used in several attempts to assign error bars to the last (and first) occurrences of a species. In recent years increasingly sophisticated statistical techniques have been applied to the problem, most notably with data from the Cretaceous-Tertiary (K-T) boundary mass extinction, but the subject matter presents some natural limitations. Thus it is easier to collect reliable data for tiny foraminifera, which occur in prolific quantities in small volumes of rock, than for the frequently huge but much rarer dinosaurs.

A further complication has been pointed out and analysed by another American palaeontologist, Steve Holland. He showed that a gradual mass extinction can be made to appear abrupt by rapid changes in sea-water depth, as determined by the analysis of sedimentary facies. One of the most significant of such changes occurs with the water deepening that occurs during marine transgressions, which are characterized by marked reductions in rates of sedimentation, leading to what is called 'condensation' by stratigraphers. This will clearly exacerbate the abruptness of organic change through time as it appears in the stratigraphic record. Thus, for those mass extinctions that are associated with rapid transgression of the sea over the land the fossil record must be searched in the expanded, near-shore stratal successions because the condensed, offshore successions may give an abrupt pattern of last occurrences that is merely the result of low sedimentation rates. Some fossil groups disappear from the stratigraphic record only to reappear much later. Such groups have been called Lazarus taxa by the University of Chicago palaeontologist David Jablonski, after the biblical character who returned from the dead. (Taxon (plural taxa) is a useful term for a group of organisms of any rank, such as a species, a genus, or a family.) The phenomenon is commonly due to the fact that the appropriate facies, which represents the environment favoured by the organisms, has temporarily disappeared. If this is widespread, representing a deteriorating environment across an extensive part of the world, the organisms will have retreated to a refuge of limited geographic extent, which is relatively unlikely to be sampled in the stratigraphic record.

A potential problem of quite another sort arises from the type of classification used for the fossils. Traditional taxonomy (the formal classification of organisms) is based primarily on overall degrees of morphological resemblance, leading to the distinction of grades. Thus, reptiles are a category clearly distinct from mammals and birds. In recent years, the more rigorous and coherent methods of cladistic taxonomy (from clados, the Greek for branch) have swept the board. Under this method, reptiles are paraphyletic; that is, they exclude their evolutionary descendants, the mammals and birds. But in cladistics degrees of affinity are based on the inferred recency of common ancestry, and paraphyletic groups are not permitted; only monophyletic groups or clades, embracing all the descendants, are allowed (Fig. 3.5).

In the context of extinction one can foresee a potential prob-

Salmon Lungflsh Dog

Salmon Lungflsh Dog

Salmon Lungflsh Dog

Salmon Lungflsh Dog

Fig. 3.5 The difference between traditional and cladistic methods of classification, illustrated by reference to three familiar vertebrates. In the traditional method (a) closeness of evolutionary affinity is determined by degree of morphological resemblance, whereas in the cladistic method (b) it is determined by recency of common ancestry.

Fig. 3.5 The difference between traditional and cladistic methods of classification, illustrated by reference to three familiar vertebrates. In the traditional method (a) closeness of evolutionary affinity is determined by degree of morphological resemblance, whereas in the cladistic method (b) it is determined by recency of common ancestry.

lem here. It is now generally accepted that dinosaurs gave rise to birds, which would make the dinosaurs paraphyletic and thus an artificial grouping. Cladists get round this by calling the dinosaurs non-avian dinosaurs, which some might consider, a little unfairly perhaps, as intellectual sleight of hand. There is a school of thought that considers that if particular fossil groups, whether monophyletic or paraphyletic, represent distinct biological entities their final disappearance from the stratigraphic record then represents significant information. While there is some truth in this, we have been taught by the cladists to be aware of the problems of pseudoextinction. A particular taxon may be reported as becoming extinct at a stratigraphic horizon, whereas all it has done is evolve into its descendant. The overall consequence for the study of mass extinctions is that traditional taxonomic methods have probably overestimated the extent of extinction. This means that whereas the 'big five' mass extinctions still stand out significantly from background extinctions, some of the lesser events that have been claimed may be open to question.

Those events that can, after rigorous analysis, be genuinely classed as catastrophic in a geological sense are unlikely ever to be pinned down in time more precisely than to a few tens of thousands of years, although exceptionally this limit may be reduced to a few thousand years. This conclusion is based on the study of particular examples in which ingenious juggling of inferred sedimentation rates in a given region is combined with the most refined radiometric dating that is possible. Extinction scenarios that involve events as geologically instantaneous as a few months to a few years are consequently not amenable to rigorous testing from the stratigraphic record. We have to bear this in mind in what follows.

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