Palaeoecology of Placodus gigas (Reptilia) and other placodontids — Middle Triassic macroalgae feeders in the Germanic Basin of central Europe — and evidence for convergent evolution with Sirenia

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Abstract

The classical descriptions of Middle Triassic marine Placodus gigas Agassiz, 1833 (Reptilia) from the Germanic Basin of central Europe as being shell-crushing durophagous placodontids are revised in this paper through analyses in convergent anatomy. In particular, the jaw morphologies of three placodontid genera, Paraplacodus, Placodus and Cyamodus, are compared to those of dugongs (Mammalia) such as the central European Tertiary Halitherium schinzii and the modern Dugong dugon of the Arabian Gulf. The anatomies of Paraplacodus, Placodus and Cyamodus exhibit convergences to Halitherium and Dugong. Whereas mammalian dugongs developed pachyostotic thoracic ribs to enhance their body weight, the placodontid reptiles achieved a similar result in different ways: Paraplacodus developed enlarged thoracic ribs; Placodus had pachyostotic gastral ribs, and Cyamodus had a thoracic osteoderm shield. The teeth of the placodontids are also convergent with those of Halitherium and Dugong in their general function and jaw morphology. Whereas Halitherium and the modern Dugong possess a horny oral pad and counterpart, and a specialized rasp-like tongue with which to grind the seagrass and its roots, placodontids had large teeth that covered the whole of their upper and lower jaws forming a similar crushing or grinding pad. Both of the extinct groups must have fed on sea-plants, as does the modern Dugong, although Halitherium possibly fed on both seagrass and macroalgae. A study of the wear stages of many Placodus teeth, skulls and jaws has revealed a large proportion of highly worn anterior teeth, indicating a usage similar to that of the procumbent front teeth of modern Dugong which are used to scrape plant roots from the sea-floor. In contrast, highly worn (wear stage 3) teeth are rare (0.5%) amongst all other upper palatal, maxillary, or lower-jaw dentary teeth, suggesting a relatively soft diet. Placodus must have used their broadly spaced anterior teeth, to dig macroalgae from carbonate sands in shallow marine, sand bar environments; indirect evidence for the existence of such environments is provided by the benthic communities of the Germanic Basin and the northern Tethys. Sea-plants would have been only crushed and swallowed by the placodontids and not chewed with jaw rotation, in a similar feeding strategy to that used by modern Dugong feeding on seagrass.

Introduction

The three most common Middle Triassic placodontid genera in the European fossil record, Placodus, Paraplacodus, and Cyamodus (e.g. Agassiz, 1833–1843, Drevermann, 1933, Westphal, 1988) are, in this paper, compared palaeoecologically to Neogene and modern Sirenia.

Placodus is known from two valid species in the Germanic Basin: Placodus gigas and Placodus antiquior. Placodus gigas Agassiz, 1833 was first identified two hundred years ago in the Germanic Triassic “Muschelkalk,” as a “large Triassic fish” (Muenster, 1830; Fig. 1) which was named by Agassiz (1833). It was subsequently referred to as a reptile by Owen (1858). Individual teeth, bones and skulls of Placodus (Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8) from the Upper Muschelkalk (Middle Triassic) have been illustrated, mainly from Bindlach near Bayreuth in southern Germany, by Meyer (1847–1855, cf. Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8), and later from Crailsheim to the southwest of Bindlach, by Westphal, 1967, Westphal, 1988. The only known skeletal remains were discovered in the Upper Muschelkalk of Steinsfurt, also in southern Germany (Drevermann, 1933), from which casts were made for the Senckenberg Museum in Frankfurt, the Staatliche Museum für Naturkunde in Stuttgart, and the University of Tuebingen (Drevermann, 1933, von Huene, 1946, Westphal, 1967). The bones have been well described and illustrated by Drevermann (1933), who carried out the preparation and reconstruction work (Fig. 13A). The phylogeny has been discussed by several authors (Meyer, 1863, Peyer, 1931a, Peyer, 1931b, Kuhn, 1969, Suess, 1987, Rieppel, 2000, Rieppel, 2001a, Rieppel, 2001b) and the palaeoecological interpretations made by Westphal, 1967, Westphal, 1988, who believed that Placodus was a “shell-crushing durophag” reptile which he concluded solely on the basis of its “shell-crushing tooth morphology,” have still been used in more recent studies (cf. Rieppel, 1995).

Paraplacodus was thought to have been restricted stratigraphically to the Middle Triassic Anisian/Ladinian boundary (Peyer, 1931b, Rieppel, 2000) and in its distribution to the coastlines of the Tethys ocean, with the only known valid species for this genus being Paraplacodus broilii Peyer, 1931a, Peyer, 1931b found in the black shales of Monte San Giorgio, Switzerland (Peyer, 1931b). A single tooth has, however, been more recently illustrated from the basal Lower Muschelkalk (Bithynian) at Winterswijk in the Netherlands, which is several million years older (Oosterink et al., 2003). The presence of Paraplacodus can now also be proven for the first time in the Germanic Basin, at Freyburg a. d. U. in Central Germany, with the identification of another anterior tooth (Fig. 2.4). This discovery from the muS (Illyrian) member falls within the time frame between the two previously mentioned discoveries. A much wider temporal and geographic distribution has now therefore been proven for this animal than was previously thought within the Germanic Basin, where it appears to have arrived much earlier than in the northern Tethys and to have possibly also been more common. New reports of Placodus inexpectatus in the eastern Tethys of China indicate an even wider distribution of the placodontids around the Pangean globe (cf. Yiang et al., 2008), although these might represent nothing more than the species formerly described from the Aegean to Pelsonian of Romania (cf. Jurcsák, 1977). This precise problem of systematics without palaeoenvironmental and palaeobiogeographical analyses has possibly resulted in many species being mis-identified as “endemic,” especially in central Europe.

A few species of Cyamodus have been described from crania and from single teeth (cf. Meyer, 1847–1855, Schmidt, 1928, Rieppel, 2000); Cyamodus rostratus Münster, 1839 was used as a valid species in this study (Fig. 10), as well as the skull of Cyamodus kuhnschnyderi Nosotti and Pinna, 1993 from the mo3 Member (Erfurt Fm, Upper Muschelkalk) of Tiefenbach (Fig. 11, Fig. 12) and, finally, the skeletal remains of Cyamodus hildegardis Peyer, 1931a, Peyer, 1931b from the northern Tethys at Monte San Giorgio (Peyer, 1931a) — also recorded herein from the Germanic Basin (Fig. 11.2). New discoveries of Cyamodus can now be added from several sites in the Germanic Basin, in the famous bone beds of Freyburg a. d. U. in central Germany, together with some tooth remains from the muS member of the Lower Muschelkalk (Illyrian, Middle Triassic) and Upper Muschelkalk (Anisian/Ladinain boundary, Middle Triassic) of Bissendorf in north-west Germany (Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6), which combine to extend both the palaeobiogeographic and temporal ranges of this reptile. There are also many postcranial bones in various collections, that have only recently been recognised as being attributable to placodontids. The number of placodontids is therefore shown to have been much higher than was previously believed and described, as a result of studies in “skull taxonomy.” Most of the known and accessible placodontid skulls from the Germanic Basin are herein presented in a new palaeoecological context.

Many new interpretations of Triassic reptile palaeoecology have resulted from several new and detailed palaeogeographic maps generated within the “Middle European Middle Triassic Megatracksite Project” (Diedrich, 2005; Diedrich and Trostheide, 2007), which explained reptile distributions and adaptations in the Germanic Basin over a time frame of about six million years (Diedrich, 2008b). In particular, the differences in sedimentary facies — i.e. habitat zones — and the restriction of Placodus teeth and bones largely to carbonate-sand sediments, combined with other information from both new excavations (Diedrich, 2008b, Diedrich, 2009b) and studies on older material, have provided new insights into the lives of these large, fully aquatic, extinct reptiles. A new palaeoecological interpretation of the feeding habits of placodontids, in which Placodus is believed to be a macroalgae feeder, has been suggested by Diedrich and Trostheide (2007). These ideas are further explained and discussed here in detail for various placodontid reptiles, with actuopalaeontological comparisons to Neogene and modern dugongid mammals and their habitats, in particular to Dugong dugon in the Arabian Gulf, which can be considered a modern analogue of the Germanic Basin in its genesis (Knaust, 1997), its association with carbonate sediments and its life as a large vertebrate animal (Diedrich, 2005, Diedrich, 2008b).

Section snippets

Materials and methods

The bone material examined in this study is housed in the Geological Department of the Martin-Luther-University, Halle/Saale (abbreviation: MLU.IfG), the Humboldt Museum of Berlin University (abbreviation: MB), the Staatliche Museum fuer Naturkunde, Stuttgart (abbreviation: SMNS), the Museum für Natur und Umwelt, Osnabrück (abbreviation: NMO), the Urwelt-Museum Oberfranken, Bayreuth (abbreviation: UM-O), the Muschelkalkmuseum, Ingelfingen (abbreviation: MHI), and the Naturkundemuseum, Erfurt

Discussion

The taphonomical fossil record of placodontid remains is a first important source of information for understanding their primary habitats. Placodus remains, together with many isolated bones and teeth, have been found in central Germany, especially in the Jena “Saurierkalk” and Freyburg a. d. U. “Schaumkalk” carbonate sand bar deposits (muS member, Lower Muschelkalk, basal Illyrian). The typical black (diagenetically coloured) teeth from these carbonates have been illustrated in numerous

Acknowledgements

I am grateful to Dr. E. Heinzmann and Dr. R. Schoch for permission to reproduce the skeletons and bone material in the Staatliche Museum für Naturkunde Stuttgart. Placodus material from the Lower Muschelkalk of Freyburg a. d. U. in eastern Germany was studied in the collections of Halle/Saale University with the support of Dr. N. Hauschke. Dr. H. Hagdorn kindly permitted the work on vertebrates in the Muschelkalkmuseum Ingelfingen. Dr. R. Riedel provided access to the collection in the

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