Environmental stress and atavism in ammonoid evolution Autor(en): Guex, Jean Objekttyp: Article Zeitschrift: Eclogae Geologicae Helvetiae Band (Jahr): 94 (2001) Heft 3 PDF erstellt am: 10.10.2021 Persistenter Link: http://doi.org/10.5169/seals-168897 Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. Einzelne Dateien oder Ausdrucke aus diesem Angebot können zusammen mit diesen Nutzungsbedingungen und den korrekten Herkunftsbezeichnungen weitergegeben werden. Das Veröffentlichen von Bildern in Print- und Online-Publikationen ist nur mit vorheriger Genehmigung der Rechteinhaber erlaubt. 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Basel, 2001 Environmental stress and atavism in ammonoid evolution Jean Guex Keywords: Evolutionary trends, reversals, atavistic forms, environmental stress, ammonites, extinctions ABSTRACT RESUME The most common trends observed in ammonoid evolution during ecologically Les tendances évolutives les plus communément observées chez les ammonites stable periods are characterized by an increase of shell curvature (e.g. evolute au cours des périodes de stabilité environnementale sont caractérisées to involute), by the development of more complex ornamentation (flexu- par une augmentation de la courbure de la coquille (passage de formes osity of ribbing, appearance of nodes and spines) and by a long term increase évolutes vers des formes involutes), par le développement d'une ornementation of the suture line's fractal dimension. Major evolutionary jumps in ammonoids plus complexe (flexuosité des côtes, apparition de nodosités ou d'épines) el occur during severe extinction events, and are characterized by the sudden par une augmentation, dans le king terme, de la dimension fractale de la ligne appearance of simple, primitive-looking forms which are similar to remote de suture (passage de sutures goniatitiques vers des sutures ammonitiques). ancestors of their more complex immediate progenitors. Such forms are Les sauts évolutifs majeurs surviennent au cours des événements d'extinctions interpreted as atavistic. According to this hypothesis, homeomorphic species majeures et sont caractérisés par l'apparition soudaine de formes d'aspect generated during such sublethal stress events can be separated by several millions primitif qui sont semblables aux ancêtres de leurs progéniteurs immédiats, plus of vears. complexes, et que nous interprétons comme ataviques. Les espèces homéo- morphes engendrées au cours de telles périodes de stress sublétal peuvent être séparées par des intervalles de plusieurs millions d'années. Introduction In recent years, biologists have renewed interest in the influence opment. Long periods of environmental stress (see below) of environmental stress on evolution and development. should increase the probability of generating viable modified For example. Rutherford & Lindquist (1998) demonstrated organisms. that impaired or mutant heat shock protein Hsp90 was responsible Other recent work concluded that the effects of external for a great variety of abnormalities in the development of stress on morphological development should increase the rate Drosophila melanogaster. These abnormal forms were shown of mutation and recombination, generating an important to be fertile and their anomalies are inheritable under artificial increase of the intraspecific variability (Williamson 1981) and selection. From their experiments the authors concluded that initiate the appearance of asymmetries (Hoffman & Parsons external stress, such as abnormal temperature or chemical 1991). Phenomena which could be attributed to such effects stress, could uncover morphological variants for selection to have been widely observed in the fossil record (Aimeras & act upon, allowing some of the rapid morphological radiations Elmi 1987; Guex 1992, 1993) and it has been proposed that observed in the fossil record. These conclusions follow the many evolutionary novelties occurred in stressed nearshore fundamental experiments of Waddington (1953, 1956) who environments (Jablonski et al. 1983; Lewin 1983). applied sublethal artificial chemical stress and high temperatures This paper describes the influence of environmental stress to developing Drosophila pupae, thus obtaining inheritable on evolutionary trends in some Mesozoic ammonite lineages. resurgences of atavistic structures, such as the bithorax and Ammonites are characterized by exceedingly rapid evolutionary crossveinless abnormalities which are deleterious under natural rates, encompassing a very wide range of morphologies, selection. Waddington's experiments proved that external which makes them very useful tools for precise chronostratigraphic stress applied to an organism at an early stage of development correlations in marine sediments. In the first part of during a very short time could drastically alter its later devel¬ the paper the major causes of environmental stress which can Dept. of Geology. University of Lausanne. BFSH-2. 1015 Lausanne. Switzerland. E-mail: Jean.Guexts'igp.unil.ch Environmental stress and atavism in ammonoid evolution 321 affect the development of organisms are briefly discussed. The Evolution of ammonites differs from this model because second part describes some of the main patterns of morphological the primitive groups, which are the source of the new changes shown by the evolution of ammonites during evolutionary lineages, are systematically absent from the known periods of extinction generated by high environmental stress. stratigraphie record separating the respective first appearances of these distinct lineages. This absence seems therefore to be real and not due to incomplete fossil documentation. On the Cope's rule and size decrease other hand, when the fossil record is good enough, it is possible Classical scientific literature provides numerous descriptions to follow the more or less gradual transformation of the primitive of evolutionary trends characterized by increases in the size groups into more elaborate forms through time. (Cope's rule) and anatomical and morphological complexity Few studies explore the way some advanced groups of organisms (Newell 1949; Valentine et al. 1994). Paleontological (outcome of evolutionary trends) can yield globally simplified and literature also provides several examples of the repetitive primitive-looking forms. When this question is addressed, and recurrent character of such trends (Foraminifera: heterochronies like progenesis or neoteny are inferred (McKinney Cifelli 1969; Ammonites: Haas 1942; Conodonts: Hirsch 1994). & McNamara 1991). Such forms often appear in an abrupt On the other hand, size reductions generated by environmental manner during episodes of intense environmental stress, constraints such as marine regressions, anoxic episodes and generating polymorphism oriented towards a global simplification thermal or nutritional stress have also been reported (Hallam of the phenotypes belonging to the advanced ancestral group. 1978; Mancini 1978). There is quantitative evidence in several Such abrupt transitions can be schematically represented on foraminiferal lineages that such size reductions in themselves Thorn's fold catastrophe model (Fig. 2) (Thorn 1972; Guex do not necessarily disrupt trends toward size increase or 1981). In such cases the evolutionary jumps are characterized toward increasing geometrical complexity (Hofker 1963; Fig.1). by the appearance of forms which are atavistic and homeo- It is widely considered that small forms with short life morphic with remote ancestors of their own lineage, a spans and rapid proliferation are favoured during stressful phenomenon termed proteromorphosis (i.e. characterized by a episodes to the detriment of larger organisms which are more primitive morphology). vulnerable to extinction (r-selection) (McKinney 1990; Hallam 1998). This idea supports the belief that new evolutionary Stress factors lineages are often developed iteratively from persistant stocks of small opportunistic forms. As a corollary, reversals of the long- For paleontologists, the most obvious and reliable indicators of term trends towards higher complexity have sometimes been major environmental stress consists of extinction periods, explained by an elimination of more complex forms during whatever their causes (Hart ed. 1996; Hallam & Wignall 1997). extinction events (Saunders et al. 1999). The most important stress factors affecting marine faunas and generating major extinctions are: i) large-scale regressions (Grabau 1936), ii) climatic variations (Valentine 1968) and iii) major volcanic