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H.erectus = incipient slow shallow water predator of hard-shelled prey?

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  • Marc Verhaegen
    The Correlation between High Limb-Bone Density and Aquatic Habits in Recent Mammals WP Wall 1983 Journal of Paleontology 57:197-207 Measurements of volume and
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      The Correlation between High Limb-Bone Density and Aquatic Habits in Recent
      Mammals
      WP Wall 1983 Journal of Paleontology 57:197-207
      Measurements of volume and weight for limb bones from a series of Recent
      mammals resulted in the recognition of 2 groups corresponding to different
      habitats, aquatic and terrestrial. Most aquatic mammals studied exhibited
      significantly higher limb-bone density than did the terrestrial mammals. It
      is proposed that the higher bone density of aquatic mammals is an adaptation
      to reduce problems of buoyancy. Cetaceans and some "pinnipeds" have
      secondarily reduced bone density with the acquisition of lung collapse
      during deep dives. The greater bone density of aquatic mammals is at least
      partially achieved through increased deposition of compact bone along the
      shafts of limb bones. As a result it is possible to differentiate visually
      aquatic from terrestrial limb elements by sectioning the bone and examining
      the percentage of compact bone present. Fossil bones can be interpreted in
      the same manner, thus providing a quantifiable test for the determination of
      life habits in extinct mammals.

      Stomach Stones for Feeding or Buoyancy?
      The Occurrence and Function of Gastroliths in Marine Tetrapods
      MA Taylor 1993 Philos.Transactions: Biol.Sciences 341:163-175
      Gastroliths or 'stomach stones' occur frequently in some, but not all,
      groups of fossil and living marine tetrapods. Comparative analysis of
      gastrolith distribution suggests a role in buoyancy control rather than food
      processing. Once accidental ingestion by bottom-feeding animals is excluded,
      gastroliths occur in most tetrapods which 'fly' underwater with hydrofoil
      limbs, including plesiosaurs, penguins, and otariid pinnipeds, but not the
      marine chelonians. They do not usually occur in cetaceans, ichthyosaurs,
      mosasaurs, and odobenid and phocid pinnipeds, which swim with a caudal fin
      or the equivalent. Occurrence in amphibious forms is variable; crocodilians
      often have gastroliths, but nothosaurs and placodonts do not. The
      correlation of gastroliths and underwater flight is corroborated by a
      comparative analysis which takes phylogenetic factors into account. There is
      no correlation with diet. Consideration of function and occurrence in
      terrestrial forms suggests that the use of gastroliths in digestion would
      not be useful, and might even be harmful, to a carnivorous marine tetrapod.
      Gastroliths are more efficient than skeletal bone (as in pachyostosis) in
      terms of sinking force per unit of added mass or volume. As well as
      driftwood and ice, marine tetrapods should be considered as a potential
      source of erratic stones in freshwater and marine sediments. Gastroliths may
      have evolved by the accidental ingestion of stones, the retention into
      adulthood of stones used by juveniles to process insect or plant food, or as
      a compensatory replacement for dense bones habitually filling the stomach.
      Their presence or absence should be more carefully recorded and further
      studies should be carried out on their function.

      Sink or swim?
      Bone density as a mechanism for buoyancy control in early cetaceans
      Noel-Marie Gray, Kimberly Kainec, Sandra Madar, Lucas Tomko & Scott Wolfe
      Anat Rec 290:638-653, 2007
      Previous analyses have shown that secondarily aquatic tetrapods, including
      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 years of paleontological
      effort has provided an unprecedented opportunity to examine the osteological
      transformation of whales as they make their transition to an obligate
      aquatic lifestyle over a 10-million-year period. It is hypothesized that
      whales manifest their osteological specialization in the same manner as
      extant semiaquatic and fully aquatic mammals. This study presents an
      analysis of the microstructural features of bone in early and late archaic
      cetaceans, and in a comparative sample of modern terrestrial, semiaquatic,
      and aquatic mammals. Bone histology was examined from the ribs of 10
      fossilized individuals representing 5 early cetacean families, including
      Pakicetidae, Ambulocetidae, Protocetidae, Remintonocetidae, and
      Basilosauridae. Comparisons were then made with rib histology from 9 genera
      of extant mammals including: Odocoileus (deer), Bos (cow), Equus (horse),
      Canis (dog), Lutra (river otter), Enhydra (sea otter), Choeropsis (pygmy
      hippo), Trichechus (sea cow), and Delphinus (dolphin). Results show that the
      transition from terrestrial, to semiaquatic, to obligate aquatic locomotion
      in archaeocetes involved a radical shift in bone function achieved by means
      of profound changes at the microstructural level. A surprising finding was
      that microstructural change predates gross anatomical shift in archaeocetes
      associated with swimming. Histological analysis shows that high bone density
      is an aquatic specialization that provides static buoyancy control (ballast)
      for animals living in shallow water, while low bone density is associated
      with dynamic buoyancy control for animals living in deep water. Thus, there
      was a shift from the typical terrestrial form, to osteopetrosis and
      pachyosteosclerosis, and then to osteoporosis in the first quarter of
      cetacean evolutionary history.

      Marine Mammals - Evolutionary Biology 2d Edition
      Annalisa Berta, James Sumich & Kit Kovacs 2005 Elsevier
      Ch.5: Sirenian and Other Marine Mammals: Evolution and Systematics
      The dense and swollen ribs of prorastomids point to a partially aquatic
      lifestyle,as does their occurrence in lagoonal deposits.
      A stable isotope study of tooth enamel from Desmostylussuggests that this
      taxon spent time in estuarine or freshwater environments rather than
      exclusively marine ecosystems and likely foraged on sea grasses as well as a
      wide range ofaquatic vegetation (Clementz et al.2003).
      Kolponomos had a massive skull with a markedly downturned snout and broad,
      crushing teeth. ... Kolponomos was probably coastal in distribution, because
      all specimens have been discovered in near-shore marine rocks. The crushing
      teeth would have been suited to a diet of hard-shelled marine invertebrates.
      Kolponomos probably fed on marine invertebrates living on rocky substrates,
      prying them off with the incisors and canines, crushing their shells, and
      consuming the soft parts as sea otters often do. Kolponomos represents a
      unique adaptation for marine carnivores; its mode of living and ecological
      niche are approached only by the sea otter (Tedford et al.1994).

      MA Taylor 2000 Historical Biology 14: 15-31
      Functional significance of bone ballast in in the evolution of buoyancy
      control strategies by aquatic tetrapods
      The primary function of pachyostosis, pachyosteosclerosis, and
      osteosclerosis may be to act as ballast, not so much (as previously
      suggested) to neutralise the buoyancy of existing lungs, but to allow
      enlargement of the lungs. Enlarged lungs cause an animal to lose buoyancy
      more rapidly with depth. They also provide a larger oxygen store. These
      features are useful for slow swimmers and shallow divers, such as feeders on
      benthic plants and invertebrates. Examples are sirenians, primitive
      sauropterygians ('nothosaurs'), placodonts, and the sea otter Enhydra. These
      last 2 show convergent evolution of adaptations to feeding on hard-shelled
      invertebrate prey in shallow water. Mesosaurids are problematical. Bone
      ballast uses body mass and volume less efficiently than other buoyancy
      control strategies. Theoretical analysis predicts that bone ballast should
      not occur in semiaquatic forms, fast swimmers or deep divers. It does not
      usually occur in such organisms. Marine iguanas of the Galapagos,
      desmostylians, and the aquatic sloth Thalassocnus are all littoral feeders
      and all lack bone ballast as predicted. Plesiosaurs adopted varied
      strategies: some used bone ballast, and others used gastroliths.
      Biomechanical considerations lead to the prediction that a new marine
      tetrapod clade will typically evolve bone ballast as part of its adaptation
      to life in water. Slow swimmers and grazers on sessile food, like sirenians
      and placodonts, develop it more strongly, but active predators like
      ichthyosaurs and cetaceans secondarily lose this character.
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