Diversity and palaeoecology of a highly diverse Late Triassic marine biota from the Cassian Formation of north Italy

Abstract

A sample of marine invertebrates from the Late Triassic Cassian Formation (north Italy) yielded one of the most diverse Early Mesozoic fossil assemblages ever reported (c. 170 species). The assemblage was found in basin clays, but was transported from nearby carbonate platforms as indicated by fragmentation, microbial encrustation and the presence of coated grains and ooids. Most of the specimens are small (< 1 cm) reflecting both, small adult sizes and size sorting during transport. Rarefaction analysis suggests that diversity of surface collection and bulk sampling is the same. However, rank abundance, species richness and taxonomic composition differ strongly according to sampling method. Low‐grade lithification of the sediments is the main reason that high diversity can be recognized, because it facilitates disaggregation and finding of small molluscs. Sample standardization shows that the studied assemblage is much more diverse than known Early Triassic assemblages. However, its diversity is similar to that of Anisian assemblages. This suggests that recovery from the end‐Permian mass‐extinction was quite advanced in the Middle Triassic and alpha‐diversity remained high until the Late Triassic. According to current models, Early Triassic and Anisian faunas match the niche overlap phase of recovery during which diversity is built up by increasing alpha‐diversity, whereas beta‐diversity rises slowly. Subsequently, habitat width of species contracts because of increasing competition, making beta‐diversity the principal drive of overall diversity increase. The diversity pattern of various Late Triassic Cassian associations meets the predictions for the transition from the niche overlap to the habitat contraction phase.: Triassic, Cassian Formation, palaeoecology, diversity, mollusc dominance.

Drill hole convergence and a quantitative analysis of drill holes in mollusks and brachiopods from the Triassic of Italy and Poland

“…This species appears too rare to produce that many drill holes. It was, however, reported from the Stuores Wiesen (where the Stuores 2010-3 sample was also collected), which yielded a shallow-water assemblage that was transported to the basin by mass flows (Hausmann and Nützel, 2015). The shell shape of P. alata is reminiscent of species that can attach to substrates such as platyceratids and capulids.…”

“…To our knowledge, this rare species has never been found attached to a possible victim. Nevertheless, Hausmann and Nützel (2015) referred to this species as parasitic, citing Bandel (1992b). Could it be possible that the drill holes from the Cassian Formation were caused by P. alata?…”

“…The large and diverse allochthonous assemblage from the Stuores Meadows (Hausmann and Nützel, 2015), contained no drilled specimens at all. However, specimens from this collection are commonly encrusted so that drilling may be obscured.…”

“…A C C E P T E D M A N U S C R I P T Hausmann and Nützel, 2015). The low grade of diagenetic and tectonic alteration resulted in a good preservation of fossils.…”

“…The low grade of diagenetic and tectonic alteration resulted in a good preservation of fossils. Moreover, the low grade of lithification facilitates continuous weathering out of even small fossils and also allows for bulk sampling by wet sieving (Hausmann and Nützel, 2015).…”