Diverse bone microanatomy in cetaceans from the Eocene of Ukraine further documents early adaptations to fully aquatic lifestyle

Abstract

Basilosauridae, fully aquatic archaeocetes from the Eocene, had osteosclerotic or pachyosteosclerotic structure of ribs and, sometimes, other bones. Such a structure is far different from osteoporotic-like bones of modern cetaceans. A microanatomical and histological study was conducted on axial and limb skeleton of several basilosaurid specimens assigned to the genus Basilotritus, from Bartonian (late middle Eocene) deposits of Ukraine, remarkable for its pachyostotic bones. The postcranial skeleton of these specimens is a complex mosaic of diverse types of bone structure, which include pachyosteosclerotic, osteosclerotic and cancellous elements. The vertebrae have a pachyostotic layered cortex reaching its greatest thickness in the lumbar region. This cortex was strongly vascularized, and its layered structure is due to concentric circles mostly made by longitudinal vascular canals, in addition to cyclical growth lines. Heavy bones are concentrated in the dorsal and ventral areas. Swollen distal ends of thoracic ribs are interpreted as serving as ballast in the ventral area, as also previously proposed for Basilosaurus cetoides. Cortical bone tissue in vertebrae and ribs showed signs of intensive resorption and remodeling. This indicates the use of the axial skeleton not only for buoyancy control but also possibly for calcium and phosphorus recycling.

A vertebra of a small species of Pachycetus from the North Sea and its inner structure and vascularity compared with other basilosaurid vertebrae from the same site

“…Toward the epiphyseal sides of the vertebrae, less and less periosteal bone layers are deposited, because here bone deposition occurs later in the individual’s ontogenetic age. Remodelling and restructuring of bone with the appearance of secondary osteons, was seen in thin sections of the inner cortical layers of vertebra, NMNH-P Ngr-12, a large species of Basilotritus ( Pachycetus ) sp., from Ukraine ( Davydenko, Tretiakov & Gol’din, 2023 ). However, in the here investigated vertebrae, the persisting layering of the inner parts of the cortex indicates that remodelling of periosteal bone was not complete, but reduced, similar to what is observed in ribs of Basilosaurus cetoides or Zygorhiza kochii ( De Buffrénil et al, 1990 ).…”

“…The endocortical VC were described and figured by Davydenko, Tretiakov & Gol’din ( 2023 : fig. 6C) in microscopic sections of the cortex of lumbar vertebra NMNH-P Ngr-12, Basilotritus ( Pachycetus ) sp.…”

“…The accessory VC were described and figured by Davydenko, Tretiakov & Gol’din ( 2023 : figs. 1B and 2A) in thoracic vertebra NMNH-P OF-2096, Basilotritus ( Pachycetus ) uheni and lumbar vertebra NMNH-P Ngr-12, Basilotritus ( Pachycetus ) sp.…”

“…The cortex in vertebrae of Morphotype 1b is divided into two parts: a very compact one (1a) and a less compact one (1b).The transition between the two parts is more or less fluent, whereas the transition to the amorphous inner part is rather sharply delineated. Davydenko, Tretiakov & Gol’din ( 2023 : p. 5) discerned three parts in the cortex of vertebrae of Basilotritus ( Pachycetus ) sp. : a compact outer part, a transition zone and a less compact inner part.…”