Original paper

Stunting in some invertebrates from the Cassian Formation (Late Triassic, Carnian) of the Dolomites (Italy)

Urlichs, Max

Abstract

Stunting occurs in several fossil assemblages of the Late Triassic Cassian Formation (Early Carnian, Dolomites). It can be shown for the bivalves Palaeonucula strigilata and Prosoleptus lineatus, the scaphopod Plagioglypta undulata, the gastropod Rhaphistomella radians, and for the brachiopods Koninckina leonhardi and Camerothyris subangusta. Previously, stunting had been investigated with statistical methods only in the ammonoid Lobites. Other small ammonoids represent normal-sized early juveniles. Stunting is also absent in sponges, corals, nautiloids, ostracods, and echinoderms. Normal-sized, intermediate, and stunted faunas alternate several times within the Cassian Formation. Hence, the small size is not genetically fixed and is explained as stunting depending on environmental factors. The high percentage of framboidal and finely dispersed pyrite in beds with stunting indicates restricted life conditions at the seafloor. Probably during poorly oxygenated intervals the toxic sulfoxide, which also retards growth and development in Recent marine invertebrates, caused the stunting in the Cassian Formation. The occurrence of stunting correlates with the ends of regressive cycles, when dysoxic conditions occurred more frequently at the termination of the progradation of the adjacent carbonate platforms into the St. Cassian Basin.

Keywords

stuntingbivalvesscaphopodsgastropodsbrachiopodscassian formationearly carnianlate triassicdolomitesitaly