• Arcidae;
  • Bivalvia;
  • functional morphology;
  • ontogeny;
  • Plio-Pleistocene

The unusual Plio-Pleistocene arcid Ambrogia mytiloides (Brocchi, 1814) has a large, elongate, smooth and streamlined shell. On the basis of these characters and the occurrence of moderate shell torsion, the mode of life of this species was formerly thought to be semi-infaunal endobyssate, obliquely oriented like the twisted arcid Trisidos. The discovery of shells in life position suggests that this species lived in a subvertical position. Rather than a recliner, this arcid was then a sticker, whose stability was provided by the byssus, which also was used to aid the bivalve in burrowing, and by its large size. The morphology of juvenile valves, smaller than 4 mm, suggests an epibyssate mode of life in its early growth stages. Ambrogia represents a remarkable pathway in the secondary return of arcids to soft bottoms: with this genus, they reached their deepest burrowing level. However, this strategy was not very successful, probably because of evolutionary constraints on the Arcoida.