C. Diedrich

Academic paper

Historical Biology

23

4

391-409

The oldest record of Cyamodus is a skull of Cyamodus tarnowitzensis (Gu¨rich 1884, Zt dt Geol Ges. 36:125�144) from Pelsonian shallow marine deposits. During the middle Illyrian, placodonts disappeared from the Germanic Basin. With renewed upper Illyrian transgression, Cyamodus rostratus (Mu¨nster 1839,U¨ ber einige ausgezeichnete fossile Fischza¨hne aus dem Muschelkalk bei Bayreuth. Birner, Bayreuth, p. 14) appeared which was found in terebratulid shell-rich shallow marine deposits. Abundant remains of Cyamodus muensteri (Agassiz 1839), traditionally referred to as Cyamodus hildegardis and here synonymised with C. muensteri, have been reported from the upper Illyrian/middle Fassanian. Skeletal remains of this species are from the Grenzbitumenzone of the Monte San Giorgio lagoons. The youngest species, Cyamodus kuhnschnyderi (Nosotti and Pinna 1993, Compt Rend Acad Sci Paris. 317:847�850), has been found in the upper Fassanian/lower Longobardian of the southern Germanic Basin or Burgundian Gate when marine facies in the Germanic Basin had nearly disappeared. These successive species provide evidence of monophylogenetic development with a trend towards anterior upper and lower jaw teeth reduction, along with a shortening of the rostrum, over an interval of five million years (243� 238 Ma). This evolutionary trend most probably reflects adaptation to specialised feeding on seaplants. The Cyamodus osteoderm carapace was not fused to the vertebral column, and appears to have been primarily a body enhancement that produced neutral or negative buoyancy to facilitate long-period diving.

Cyamodus; Placodontia; marine reptile; Middle Triassic; evolution; biostratigraphy

English