The histological characteristics of the ribs of several sirenians from the middle Eocene of the Pyrenees (the dugongid Prototherium montserratense from Catalonia and two undetermined dugongid species from Navarre) reveal that the histogenetical mechanisms causing bone pachyosteosclerosis in the extant Sirenia, i.e. hyperplasy of periosteal and endosteal deposits, and inhibition of the chondroclastic and osteoclastic activities involved in bone remodelling, were already present by the middle of the Eocene, and have not greatly changed since then. However, in one of the two undetermined specimens examined, bone remodelling in the cortical and medullar regions remained fairly active, a characteristic that is reminiscent of the condition commonly encountered in mammals. These observations suggest that the mechanisms involved in the peculiar form of pachyosteosclerosis displayed by the sirenians did not act with the same intensity, and with identical results, in all Eocene forms.
Mammalia, Sirenia, Prototherium, palaeohistology, pachyosteosclerosis, middle Eocene, Pyrenees