A new subtropical-temperate brooding echinoid with no marsupium: the first Mediterranean and the last European Temnopleuridae from the Early Pleistocene of Italy

Abstract

The regular echinoid Placentinechinus davolii gen. et sp. nov. is described from eight Early Pleistocene (GelasianÀCalabrian) sites in north and south Italy. It is the most recent record known for the family Temnopleuridae in the European domain and the first in the Mediterranean area. A review of temnopleurid palaeobiogeography, and morphological comparisons, suggest that P. davolii was derived from a marsupiate brooder that lived in the north-east Atlantic during MessinianÀmiddle Gelasian time. This indicates a brooding reproductive strategy for P. davolii, although it does not bear any evidence of a marsupium. The stratigraphy and climatic setting inferred for the study sites indicate that Placentinechinus represents a significant southward shift of the European Temnopleuridae, triggered by a progressive cooling that changed the Mediterranean climate from tropical to subtropical-temperate. A further climatic deterioration, perhaps that at about 0.8 Ma with the onset of the 100 kyr-controlled Ice Ages, caused its extinction. The palaeoecology of Placentinechinus, as deduced from palaeoenvironmental reconstructions of the study sites, indicates that it lived in shoreface to shallow offshore, moderately agitated waters, often together with the scallop-polychaete Aequipecten-Ditrupa association. It tolerated more or less marked conditions of turbidity, but flourished in trophically well-structured palaeocommunities. The main morphological characters distinguishing Placentinechinus are the very depressed test and the extremely large apical opening, up to 82% of the test diameter, which is so far the largest known for an adult echinacean echinoid. The statistically exhaustive morphometric data collected from more than 100 Placentinechinus tests indicate that inferring sexual dimorphism for Temnopleuridea echinoids based only on the apical disc width could be misleading. For P. davolii, both climate and environmental stability, in terms of sedimentation rate and nutrient supply, may have been concurrent drivers of its evolutionary history.http://zoobank.org/urn:lsid:zoobank.