A chiton without a foot

Abstract: The palaeoloricate ‘polyplacophorans’ are an extinct paraphyletic group of basal chiton‐like organisms known primarily from their fossilized valves. Their phylogenetic placement remains contentious, but they are likely to include both stem‐group Polyplacophora and stem‐group Aplacophora. Candidates for the latter position include ‘Helminthochiton’thraivensis from the Ordovician of Scotland, which we redescribe here through a combined optical and micro‐CT (XMT) restudy of the type material. The 11 specimens in the type series are all articulated, presenting partial or complete valve series as well as mouldic preservation of the girdle armature; they demonstrate a vermiform body plan. The valves are typically palaeoloricate in aspect, but differ in detail from all existing palaeoloricate genera; we hence erect Phthipodochiton gen. nov. to contain the species. The most notable feature of the fossils is the spicular girdle; this is impersistently preserved, but demonstrably wraps entirely around the ventral surface of the animal, implying that a ‘true’ (i.e. polyplacophoran like) foot was absent, although we do not exclude the possibility of a narrow solenogastre‐like median pedal groove having been present. Phthipodochiton thraivensis presents an apparent mosaic of aplacophoran and polyplacophoran features and as such will inform our understanding of the relationship between these groups of extant molluscs. An inference may also be drawn that at least some other palaeoloricates possessed an ‘armoured aplacophoran’ body plan, in contrast to the ‘limpet‐like’ body plan of extant Polyplacophora.     

Individual inter‐annual nest‐site relocation behaviour drives dynamics of a recently established Barnacle Goose Branta leucopsis colony in sub‐arctic Russia

Avian breeding colonies are generally in a continuous state of flux, some parts growing whilst others shrink as individuals move within the colony on the search for better nest‐sites. We examined the role of experience in breeding patch choice by individually marked Barnacle Geese Branta leucopsis in a recently established colony in sub‐arctic Russia. Individuals failing to reproduce successfully tended to shift nest location further the following season than did successful pairs, and they did so towards the most dense nest aggregations within the colony, where reproductive success was higher. We suggest that individual decisions on nest‐site choice shape the spatial dynamics of this colony.