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

摘要

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 modem 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 modem 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 modem Dugong feeding on seagrass.