Cephalopods from reef limestone of the Vasalemma Formation, northern Estonia (latest Sandbian, Upper Ordovician) and the establishment of a local warm-water fauna

Abstract

The cephalopods of the reef limestones of the Vasalemma Formation, northern Estonia, are highly diverse and comprise a number of 22 species, of 10 families and seven orders in a sample of +300 specimens. Most of the specimens were collected from shell concentrations in synsedimentary cavities and are interpreted parautochthonous as washed in from nearby palaeo-habitats. Nearly all of the shells are fragmented and nearly 15% are partially encrusted by epibionts. The assemblage is dominated by small (mostly less than 30 mm wide) straight shelled actinocerids and orthocerids, additionally coiled tarphycerid are common. The higher taxonomic composition of the Vasalemma cephalopod assemblage with a dominance of actinocerids and an absence of endocerids is in agreement with an interpretation of the depositional environment under a warm water (tropical or subtropical) shallow subtidal regime. At species level the assemblage is highly endemic, but the generic composition allows for a statistical comparison with other faunas. A cluster analysis of contemporary assemblages reveals a high degree of similarity with late Sandbian cephalopod faunas of epicontinental Laurentia. The palaeogeographical distribution pattern is similar than in brachiopods, which supports earlier interpretations of these clusters as mainly controlled by water temperature and depositional depth. Several of the Vasalemma genera became conspicuous elements of epicontinental Laurentia during the Katian, which emphasizes that immigration toward Laurentia was an important factor in Late Ordovician diversity dynamics. Of the described taxa, the following are new:

Conch structures, soft-tissue imprints and taphonomy of the Middle Ordovician cephalopod Tragoceras falcatum from Estonia

“…This observation can be explained by Calcite Sea conditions, where aragonitic shells dissolved rapidly (Palmer et al 1988, Palmer andWilson 2004). Cephalopods with encrusting bryozoans are less common in Estonia, but locally abundant in reef environments such as the Late Ordovician Vasalemma Formation (Kröger and Aubrechtová 2018). Recently, Vinn et al (2018) described encrusting bryozoans on cryptic surfaces (e.g.…”

The cephalopods of the Kullsberg Limestone Formation, Upper Ordovician, central Sweden and the effects of reef diversification on cephalopod diversity

“…In a recent review, Kröger et al (2016) argued that the shallow water reefs of the Norwegian Mjøsa and Steinvika Limestone formations and of the Estonian Vasalemma Formation are nearly time equivalent to the Kullsberg Limestone Formation. The reefal part of the Vasalemma Limestone (except for the stratigraphically younger Saku Member) was interpreted as exclusively latest Sandbian (Kröger & Aubrechtová 2017). The termination of the Kullsberg mound growth probably occurred slightly later, but the exact timing is difficult to determine because currently, the correlation with the inter-mound facies is not fully resolved (Calner et al 2010).…”

“…Remarks. The species was described in detail by Kröger & Aubrechtová (2017). The additional material from the Kullsberg Limestone Formation provides further information on the variability of the angle of expansion (Fig.…”

“…This faunal analysis is the last of a series of three papers, all of them outline fossil cephalopod assemblages within Late Ordovician reef settings of Baltoscandia, and none of the assemblages was described before in its entirety. The first of these analyses monographically covered the cephalopod assemblage of the Boda Limestone Formation, late Katian-Hirnantian stages, of central Sweden (Kröger 2013) and the second paper is a description of the cephalopods of the Vasalemma Formation, late Sandbian Stage, of northern Estonia (Kröger & Aubrechtová 2017). The cephalopods of the Kullsberg Limestone Formation, described herein, connect the two former assemblages stratigraphically and geographically, and this triangle allows for the distinction of global evolutionary trends and local habitat pattern.…”

“…Did the establishing of a widespread reef and mound habitat affect the in-between habitat diversity (β diversity) of cephalopods and did during the Late Ordovician a reef-related cephalopod fauna appear? These are the questions which we addressed here based on the existing framework of taxonomic studies (Kröger 2013;Kröger & Aubrechtová 2017).…”