https://crimsonpublishers.com/oara/fulltext/OARA.000537.php
Open Access Research in Anatomy
Correlation of Bone Density in Aquatic and Semiaquatic Animals to Ecological and Dietary Specializations
Sulman J Rahmat, Madelyn G Crowell & Irina A Koretsky 2020
doi 10.31031/OARA.2020.02.000537

One of the most obvious adaptations of animals re-introduced to an aquatic environment is the difference in bone density.
Numerous marine mammals & marine reptiles exhibit changes in bone density that correlate to their habitat (ecological niche) & dietary specializations, not phylogenetic relationships.
Increased bone density (either pachyostosis, osteosclerosis, or pachyosteosclerosis) was observed early in the transition of terrestrial taxa to the aquatic environment.
Animals such as early Cetacea & Sirenia clearly exhibit these adaptive features, and even retain many terrestrial characters such as hind-limbs & behaviors such as paddle swimming.
The increase in bone density is a more energetically efficient hydrostatic mechanism for buoyancy for marine mammals with large lung volumes.
As the taxa became more specialized for the aquatic environment morphologically (evolving fins, flippers & flukes) & behaviorally (evolving an oscillating swimming mechanism), variation in bone density correlates with their ecological niche.
Modern Sirenia retain increased bone density, allowing these large-sized mammals to remain submerged in shallow waters, to feed on sessile littoral foods (sea grasses),
but the bone density in modern Cetacea became more osteoporotic, allowing them to swim faster, and hunt faster moving prey.
Pinnipeds live in a wide range of habitats (from cold to warm waters) and demonstrate varying feeding mechanisms: filter-feeding on krill, bottom-feeding on mollusks, and/or catching fast moving prey.
Bone density is one vital character that can be used to predict the specific ecological niche & feeding preference for pinnipeds.
Some early hominids have been shown to have an increase in bone density.
These heavier, thicker bones would make it easier for early Homo to hunt in waters for littoral food sources, and would compensate for the lack of stability from bipedalism.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)