title: Bone inner structure suggests increasing aquatic adaptations in Desmostylia (Mammalia, Afrotheria).
abstract: BACKGROUND
  The paleoecology of desmostylians has been discussed controversially with a general consensus that desmostylians were aquatic or semi-aquatic to some extent. Bone microanatomy can be used as a powerful tool to infer habitat preference of extinct animals. However, bone microanatomical studies of desmostylians are extremely scarce.
  
  
  RESULTS
  We analyzed the histology and microanatomy of several desmostylians using thin-sections and CT scans of ribs, humeri, femora and vertebrae. Comparisons with extant mammals allowed us to better understand the mode of life and evolutionary history of these taxa. Desmostylian ribs and long bones generally lack a medullary cavity. This trait has been interpreted as an aquatic adaptation among amniotes. Behemotops and Paleoparadoxia show osteosclerosis (i.e. increase in bone compactness), and Ashoroa pachyosteosclerosis (i.e. combined increase in bone volume and compactness). Conversely, Desmostylus differs from these desmostylians in displaying an osteoporotic-like pattern.
  
  
  CONCLUSIONS
  In living taxa, bone mass increase provides hydrostatic buoyancy and body trim control suitable for poorly efficient swimmers, while wholly spongy bones are associated with hydrodynamic buoyancy control in active swimmers. Our study suggests that all desmostylians had achieved an essentially, if not exclusively, aquatic lifestyle. Behemotops, Paleoparadoxia and Ashoroa are interpreted as shallow water swimmers, either hovering slowly at a preferred depth, or walking on the bottom, and Desmostylus as a more active swimmer with a peculiar habitat and feeding strategy within Desmostylia. Therefore, desmostylians are, with cetaceans, the second mammal group showing a shift from bone mass increase to a spongy inner organization of bones in their evolutionary history.
authors.affiliation: Steinmann Institut für Geologie, Paläontologie und Mineralogie, Universität Bonn, Bonn, Germany. hayashi@uni-bonn.de
authors.forename: Shoji
authors.initials: S
authors.lastname: Hayashi
authors.forename: Alexandra
authors.initials: A
authors.lastname: Houssaye
authors.forename: Yasuhisa
authors.initials: Y
authors.lastname: Nakajima
authors.forename: Kentaro
authors.initials: K
authors.lastname: Chiba
authors.forename: Tatsuro
authors.initials: T
authors.lastname: Ando
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Sawamura
authors.forename: Norihisa
authors.initials: N
authors.lastname: Inuzuka
authors.forename: Naotomo
authors.initials: N
authors.lastname: Kaneko
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Osaki
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D001842:Bone and Bones / Q000033:anatomy & histology* / Q000166:cytology*; D008322:Mammals / Q000145:classification / Q000502:physiology*; D010163:Paleontology; D010802:Phylogeny; D025341:Principal Component Analysis
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