Correlation of Bone Density in Semi-Aquatic and Aquatic Animals with Ecological and Dietary Specializations
Madelyn Galimore Crowell, Sulman Rahmat, Irina Koretsky 2020 doi org/10.1096/fasebj.2020.34.s1.01860

A successful transition from a terrestrial to an aquatic environment requires the acquisition of unique adaptations that fit the distinct habitat.
POS pachyosteosclerosis is a combination of pachy-ostosis PO (thickening of cortical bone) & osteo-sclerosis OS (thickening of medullary bone).
This accumulation of bone increases the density & Wt of the animals that exhibit these characters.

POS is commonly found in (semi)aquatic animals: Sirenia sea-cows, Pinnipedia odobenids, sea-lions & true seals, Cetacea whales.
Increased bone density occurred early in the transition from a terrestrial to a marine environment, before many of the other morphological characters appear: flukes, flippers & fins, & behavioral characters: oscillatory swimming ass.x aquatic life.
POS helps to counteract buoyancy, allowing taxa to remain submerged longer, to feed while expending little energy.
- This is clearly demonstrated in Sirenia, which feed on sea-grasses in shallow waters near the coast-line.
- In most other aquatic mammals incl. modern Cetacea, POS is followed by an osteoporosis-like bone lightening for increased swimming speeds, that allows for the active hunting of fast-moving prey.
- Pinnipedia live in a wide range of habitats (from cold to warm, from marine to fresh water), they demonstrate varying feeding-mechanisms, from filter-feeding to bottom-feeding to catching fast prey.
Increased bone density is seen in multiple spp of seals in different geological ages from the Miocene (23- Ma) to present.
The morphological & ecological significance for seals with thicker bones is that they swim slower, and hunt in shallower water,
seals with lighter bones swim faster, and hunt fast-moving prey.
Bone micro-structural disparity allows different spp of seals to successfully live in sympatric habitats, due to their ecological & dietary specializations.
Bone density is one vital character that can be used to predict the specific ecological niche & feeding preference for Pinnipedia.

Some early hominids (Homo erectus) have been shown to have increased density in the bones also.
These heavier, thicker bones would make it easier for early Homo to hunt in coastal waters for littoral food sources, and would compensate for the lack of stability from bipedalism.

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