Sink or swim?
Bone density as a mechanism for buoyancy control in early cetaceans Noel-Marie Gray cs 2007
Anat Rec 290:638-653 doi 10.1002/ar.20533.
Previous analyses have shown:
secondarily aquatic tetrapods (incl. whales) exhibit osteological adaptations to life in water, as part of their complex buoyancy control systems.
These structural specializations of bone span hyperostosis through osteoporosis.
The past 15 yrs of paleontological effort has provided an unprecedented opportunity to examine the osteological transfm of whales:
the transition to an obligate aquatic lifestyle over a 10-My period.
Do whales manifest their osteological specialization in the same manner as extant semi- & fully aquatic mammals?
We present a micro-structural analysis of bone in early & late archaic Cetacea, in a comparative sample of modern terrestrial, semi-aquatic & aquatic mammals.
Bone histology was examined from the ribs of 10 fossilized individuals of 5 early cetacean families: Pakicetidae, Ambulocetidae, Protocetidae, Remintonocetidae, Basilosauridae.
Comparisons were made with rib histology from 9 genera of extant mammals: Odocoileus (deer), Bos (cow), Equus (horse), Canis (dog), Lutra (river otter), Enhydra (sea otter), Choeropsis (pygmy hippo), Trichechus (sea cow), Delphinus (dolphin).
Results:
the transition from terrestrial, to semi- to bligate aquatic locomotion in Archaeocetes involved a radical shift in bone function, achieved by means of profound changes at the micro-structural level.
Surprisingly, micro-structural change predates gross anatomical shift in archaeocetes ass.x swimming.
Histological analysis shows:
- high bone density is an aquatic specialization that provides static buoyancy control (ballast) for animals living in shallow water,
- low bone density is ass.x dynamic buoyancy control for animals living in deep water:
there was a shift from the typical terrestrial form, to osteopetrosis & pachyosteosclerosis, and then to osteoporosis in the 1st quarter of cetacean evol.history.
_____
IOW, only ridiculous kudu runners deny:
H.erectus' osteopetrosis & pachyosteosclerosis (POS) indicates shallow-diving.
Sorry for the title (becoming too old? too fast? too angry?):
should be: "incredible" instead of "incridibly", of course...
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