“Pachyostosis” in aquatic amniotes: a review
Alexandra HOUSSAYE 2009 houssaye@mnhn.fr
doi org/10.1111/j.1749-4877.2009.00146.x

During the course of amniote evolution, numerous taxa secondarily adapted to an aquatic life,
many of these taxa primitively display “pachyostosis”: an osseous specialization characterized by an increase in bone compactness and/or volume.
The term is used in morphological & histological descriptions, to describe what in fact corresponds to different patterns.
I present the current state of knowledge re. this adaptation among aquatic amniotes.
All the taxa that have returned to an aquatic environment are listed,
their degree of adaptation to the marine environment, their life environment & the nature of their “pachyostotic” pattern (when present) are described.
This inventory enables the evaluation of the current quality of the data re. this specialization,
it provides an indication of the work that remains to be done.
The functional consequences of “pachyostosis” & notably its importance for buoyancy control (hydrostatic regulation of the body trim) are discussed, and opposed to the requirement of improved swimming abilities in the case of a hydrodynamic mode of
trim regulation.
Questions are posed about the signification of the polymorphism displayed by this specialization between different taxa, different spms of the same taxon & different bones of the same spm, and the problem of quantification of pachyostosis is discussed.

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H.erectus was pachyostotic: there's no doubt they frequently dived: shellfish engravings (google "Joordens Munro"), fossilizations amid shellfish, stone use, larger brain (DHA), coastal dispersal etc.

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