Morphological fluctuations of ammonoid assemblages from the Muschelkalk (Middle Triassic) of the Germanic Basin—indicators of their ecology, extinctions, and immigrations

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Abstract

For a stratophenetic analysis of Middle Triassic ammonoids from the German Muschelkalk (Anisian, Ladinian), whorl expansion rates, whorl width indexes, umbilical width indexes, maximal conch sizes, body chamber lengths, the orientations of the aperture, and a number of sculpture parameters of approximately 500 specimens were identified. 274 of these data sets, sorted according to their stratigraphic age, were evaluated in scatter plots as well as canonical discriminant function analyses. Several of the diagrams that were produced in these analyses reflect more or less steady changes in conch morphology through geological time, except for some intervals with abrupt and rather drastic transformations. These morphological discontinuities are synchronous with immigrations into the Germanic Basin of crinoid and brachiopod taxa. This discovery indicates disturbances in the endemic evolution of the ammonoids caused by such immigrations. At a small scale, this case study demonstrates that a rising sea level may have boosted the faunal exchange between an open marine and a restricted epicontinental basin, causing a minor regional increase in biodiversity. It also demonstrates that the evolution of ‘Germanic’ ceratites happened mainly within the Germanic Basin but partially probably also within the Tethyan Sea. The ecology of the ceratites from this Basin is discussed. Accordingly, they were stenohaline, good backward swimmers with a good manoeuvrability, and able to achieve neutral buoyancy.

Introduction

Cephalopod remains are rather abundant in the sedimentary rocks of the Muschelkalk (Anisian and Ladinian, Middle Triassic) of central Europe. In contrast to many other epicontinental marine basins of Mesozoic age, specimens of only a few cephalopod genera have been recorded. Among these, specimens of the ammonoid genera Ceratites (the heraldic animal of this journal) and Paraceratites as well as of the nautiloid genus Germanonautilus by far outnumber representatives of other genera with respect to diversity, morphological disparity, and abundance. During the late Anisian and Ladinian, the species of Ceratites and Paraceratites underwent continuous morphological transformation. This transformation is reflected in a fairly large number of species and subspecies as well as in successive biozones (e.g. Philippi, 1898, Philippi, 1901, Riedel, 1916, Penndorf, 1951, Wenger, 1957, Urlichs and Mundlos, 1987, Urlichs and Vath, 1990, Urlichs, 1993).

Because of the putatively simple phylogenetic patterns within Ceratites and Paraceratites (e.g. Müller, 1954, Wenger, 1957, Rein, 1988), the large number of specimens, and the well known sedimentological (Aigner, 1985, Aigner and Bachmann, 1992, Aigner et al., 1999) as well as stratigraphic framework, specimens of these genera are excellent subjects for study. Their morphological fluctuations and dependence on palaeoenvironment, their mode of life (also discussed in Wang and Westermann, 1993, Rein, 1996, Rein, 1999a, Rein, 2000a, Klug et al., 2004), and immigrations into the German Muschelkalk Basin (Urlichs and Mundlos, 1985, Urlichs, 1999) are the focus of the following account. The only limiting factor of this study was the often moderate to poor preservation of the specimens, which made it difficult to find a statistically relevant number of sufficiently well preserved specimens.

Immigration of ammonoids into the German Muschelkalk Basin has been documented for various genera, which are also known from the wider Tethyan realm (Urlichs and Mundlos, 1985, Urlichs, 1999): Acrochordiceras, Arcestes, Balatonites, Beneckeia, Bulogites, Discoptychites, Flexoptychites, Judicarites, Noetlingites, Paraceratites, Parapinacoceras, Protrachyceras (Nevadites according to Urlichs, 1999), and Trachyceras (Assmann, 1937; Balatonites according to Urlichs, 1999). A few nautiloid genera also immigrated into this epicontinental basin: Michelinoceras, Germanonautilus, Paranautilus, and Pleuronautilus. Some of the listed genera, such as Germanonautilus, have intruded the German Muschelkalk Basin repeatedly. This genus is well known from both the Lower as well as the Upper Muschelkalk but there are no records from the Middle Muschelkalk; this can be explained by the hypersaline conditions during the timespan of deposition of the evaporites of the Middle Muschelkalk.

In this study, we present the results of a stratophenetic analysis and the morphological fluctuations of the ammonoids of the Upper Muschelkalk (1), we discuss the relationships between changes in their morphology and changes in their mode of life (2), we review potential extinctions of ammonoid taxa of the Germanic Basin (3), and we report subsequent renewed immigrations of ammonoids (4).

Section snippets

Material

Because of deformation or insufficient completeness of many specimens, approximately 500 specimens were selected out of several thousand. Only 274 of these specimens yielded satisfying data sets for the stratophenetic analysis. The measurements used in this study were taken from specimens that are deposited in the following institutions: The majority is stored in the Staatliches Museum für Naturkunde in Stuttgart (SMNS). A large number of specimens was made accessible at the Muschelkalkmuseum

Methods

From 257 specimens, conch parameters and dimensions were measured (see Fig. 2). The measurements are listed in Appendix A. In order to obtain an idea of the morphological trends and pathways, univariate and multivariate statistical methods were applied. The simplest way to document trends is to plot morphological parameters trough time. Therefore, we computed the dispersal of different morphological parameters (diameter, whorl height/diameter ratio, umbilical width/diameter ratio, whorl

Stratigraphy

The main focus of this study concerned the cephalopods from the Upper Muschelkalk. Consequently, the stratigraphy of the Lower Muschelkalk will not be discussed in detail here (Fig. 1). The sediments of the Muschelkalk are of Anisian to Early Ladinian age (Aegean/Bithynian to Longobardian substage, 244 to 231 Ma according to Menning and the German Stratigraphic Commission, 2002). The Lower Muschelkalk belongs to the late Aegean/early Bithynian to early Illyrian substages, whereas the carbonates

Conch parameters

It is certainly the merit of authors such as Sandberger and Sandberger (1850–56) and later Trueman (1941), Raup, 1966, Raup, 1967, as well as Saunders and Shapiro (1986) that they recognised the importance of the geometry of ammonoid coiling for systematics and ecology. Chamberlain, 1976, Chamberlain, 1981, Jacobs (1992), and Jacobs and Chamberlain (1996) contributed important details on energy consumption, drag, the connection between conch size and flow resistance.

General

The canonical discriminant function analysis revealed a strong discrimination along the first function (Appendix C). The function 1 represents 68.7% of overall variance. Structure features (sa, sbc and sph) especially correlate with this function. The parameters correlating with function 2 (18.2%) are associated to the parameters WER and ww/dm; these are parameters that are in a context to the orientation of the aperture during life (see above). The interpretations of the morphological

Physical conditions (sea-level)

Changes in the sea-level within the Germanic Basin are documented in the Upper and Lower Muschelkalk in repeated changes of carbonate content, grain size, faunal composition, and other factors (see Aigner, 1985; Fig. 1). During Triassic times, the sedimentary environment changed from largely terrestrial or fluvial from the earliest Indusian to the earliest Anisian to shallow marine during most of the Anisian and Ladinian (with a phase of evaporite deposition in the late Anisian) to

Environment

In previous decades, various workers dealt with the marine palaeoenvironment during Middle Triassic times in the Germanic Basin from both palaeontological and sedimentological points of view (e.g. Aigner, 1985, Hagdorn, 1985, Hagdorn and Simon, 1993, Ockert, 1993, Aigner et al., 1999, Beutler and Szulc, 1999; Fig. 1). In contrast to the evaporites of the Middle Muschelkalk, the carbonates of the Lower and the Upper Muschelkalk were largely deposited under euhaline conditions. Fluctuations in

Migrations and palaeobiogeography

During the early Anisian, marine conditions expanded from the easternmost Germanic Basin towards the west. Various authors have documented the subsequent immigration of organisms into this peri-Tethyal epicontinental basin (Assmann, 1944, Kozur, 1974, Senkowiczowa, 1975, Hagdorn, 1985, Hagdorn and Gluchowski, 1993, Kedzierski and Szulc, 1996, Beutler and Szulc, 1999, Urlichs, 1999; Fig. 8). A number of ammonoid taxa arrived particularly in the eastern part of the Germanic Basin during Bithynian

Buoyancy

Like most fossil ectocochleate cephalopods and like all Recent ectocochleate cephalopods, most ammonoids were most likely capable to achieve neutral buoyancy (see review in Jacobs and Chamberlain, 1996). This was doubted by some authors (Ebel, 1990, Rein, 1996, Rein, 1999a, Rein, 2000a). As discussed by Jacobs and Chamberlain (1996) and in the methods chapter, these doubts appear to be not very reasonable for the following reasons: (1) Buoyancy calculations come either close to neutral buoyancy

Conclusions

Our stratophenetic case-study on the fluctuations in morphometric parameters of cephalopods through time shows how regional biodiversity in a marine epicontinental basin can be affected by sea-level changes (compare Monnet et al., 2003). Discontinuities in the morphological change rather reflect small-scale extinctions and iterative immigrations than a purely endemic evolutionary process which was restricted to this basin. This hypothesis is supported by the repeated immigrations of other

Acknowledgements

Hans Hagdorn (Ingelfingen) put his collection of Muschelkalk ammonoids at our disposal and discussed several aspects of ceratite palaeobiology with us. Hans Hagdorn, Susan Turner (Kenmore, Queensland) and Nicolas Goudemand (Zürich) thoughtfully proof-read the manuscript and offered substantial help in improving language as well as structure and content of the manuscript. Hans Rieber and Hugo Bucher (both Zürich) contributed some important details and discussed some of our results with us. Ana

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