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interests / sci.anthropology.paleo / PO & OS in H.erectus & other salt-water-diving tetrapods

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o PO & OS in H.erectus & other salt-water-diving tetrapodslittor...@gmail.com

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PO & OS in H.erectus & other salt-water-diving tetrapods

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Subject: PO & OS in H.erectus & other salt-water-diving tetrapods
From: littoral...@gmail.com (littor...@gmail.com)
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Bone histology in extant and fossil penguins (Aves: Sphenisciformes)
Daniel T Ksepka cs 2015
Anat.227: 611–630 doi 10.1111/joa.12367

Substantial changes in bone histology accompany the secondary adaptation to life in the water.
This transition is well-documented in several lineages of mammals & non-avian reptiles, but it has received rel.little attention in birds.
We present new observations on the long -bone micro-structure of penguins, based on histological sections from
- 2 extant taxa: Spheniscus & Aptenodytes,
- 8 fossil spms, belonging to stem-lineages †Palaeospheniscus & several indeterminate Eocene taxa.
High bone density in penguins results from compaction of the internal cortical tissues: penguin bones are best considered osteo-sclerotic (OS), rather than pachy-ostotic (PO).
The oldest spms sampled in this study represent stages of penguin evolution at least 25 My after the loss of flight,
but major differences in humeral structure were observed between these Eocene stem-taxa & extant taxa:
the modification of flipper-bone micro-structure continued long after the initial loss of flight in penguins.
It is proposed:
2 key transitions occurred during the shift from the typical hollow avian humerus to the dense osteo-sclerotic humerus in penguin:
1) a reduction of the medullary cavity occurred, due to a decrease in the amount of peri-medullary osteo-clastic activity,
2) a more solid cortex was achieved by compaction.
In extant penguins & †Palaeospheniscus, most of the inner cortex is formed by rapid osteo-genesis, resulting an initial lattice-work of woven-fibered bone: open spaces are filled by slower, centri-petal deposition of parallel-fibered bone.
Eocene stem-penguins formed the initial lattice-work, but the subsequent round of compaction was less complete: open spaces remained in the adult bone.
In contrast to the humerus, hind-limb bones from Eocene stem-penguins had smaller medullary cavities & thus higher compactness values than extant taxa..
Cortical lines of arrested growth have been observed in extant penguins, but none was observed in any of the current sampled spms:
likely, even these ‘giant’ penguin taxa completed their growth cycle without a major pause in bone deposition:
they did not undergo a prolonged fasting interval before reaching adult size.

________

Evolution of Sirenian Pachyosteosclerosis,
a Model-case for the Study of Bone Structure in Aquatic Tetrapods
Vivian de Buffrénil cs 2010
J Mamm Evol 17:101-120
doi 10.1007/s10914-010-9130-1

OS (inner bone compaction) & PO (outer hyperplasy of bone cortices, swollen bones) are typical features of tetrapods secondarily adapted to life in water.
These peculiarities are spectacularly exemplified by the ribs of extant & extinct Sirenia:
sea-cows are the best model for studying this kind of bone structural specializations.
How did these features differentiate during sirenian evolution?
The ribs of 15 spp (incl. most basal form Pezosiren portelli & extant taxa) were studied & compared to other mammalian spp, morphometrically & histologically.
-- PO was the first of these 2 specializations to occur (mid-Eocene), it is a basal feature of the Sirenia,
it subsequently regressed in some taxa that do not exhibit hyperplastic rib cortices.
-- OS was only incipient in P.portelli: its full development occurred later, end-Eocene.
These 2 structural specializations of bone are variably pronounced in extinct & extant Sirenia, rel.independent from each other, although frequently associated.
They are possibly due to similar hetero-chronic mechanisms bearing on the timing of osteoblast activity.
These results are discussed with respect to the functional constraints of locomotion in water.

______
Research in Anatomy open access Preliminary Report:
Pachyosteosclerotic Bones in Seals
Irina Koretsky & Sulman J Rahmat 2017
doi10.31031/OARA.2017.01.000501

Despite extensive knowledge about the distribution of POS (increased bone volume & density) among some modern groups of marine mammals, this aquatic adaptation is not well known in Phocidae (true seals).
POS bones reduce buoyancy, and permit easier submergence for some marine mammals.
PO & PS are 2 vastly different bone adaptations, which have co-occurred independently (POS):
- PO describes the thickening of bone in cross-sectional area,
- OS is the replacement of cancellous bone with compact bone.
OS, PO & bone lightening consecutively occurred to various degrees as adaptations of marine mammals to different environmental niches & life-styles.
Differing extents of POS has been demonstrated in Recent phocids, otariids & odobenids, Cetacea, Sirenia, sea-birds, fish, reptiles & other aquatic tetrapods.
There have been descriptions of POS among different modern (semi)aquatic animals, but this osteological condition has never been studied in fossil pinnipeds (e.g. Phocidae true seals), and has not been compared with recent representatives.
Geological evidence suggests:
marine mammals have evolutionarily undergone 3 distinct stages of bone modifications.
1) OS first occurred in tetrapods secondarily adapted to life in water, seen in Sirenia & Cetacea early- to mid-Eocene ~45-50 Ma.
2) Subsequent to OS (replacement of cancellous bone with compact bone) was PO (bone thickening), which appeared from the mid-Eocene.
All lineages of aquatic tetrapods went through OS & PO during the initial stages of aquatic adaptation.
3) The 3rd stage of bone modification was an osteoporotic-like skeletal lightening, occurring only in advanced evolutionary stages as an adaptation for *deep* diving & *fast* swimming: Cetacea exhibits such light bones.
During preliminary examination, the first 2 stages of bone modification (OS & PO) are observed among both extinct & extant seals, extinct walruses & Sirenia.
Most likely, POS served as a hydrostatic adaptation (ballast), maintaining static equilibrium in water, during the transition from terrestrial to aquatic life.
Demonstration of variability in bone density is observed in 2 modern spp of seals:
- Pusa hispida ringed seal,
- Pagophilus groenlandicus harp seal.
These 2 taxa are morphologically distinguishable:
- Pusa hispida has dense (increased compact bone), POS bones,
- Pag.groenlandicus has lighter (more cancellous bone), OS bones.
This difference may in part be explained by the diets of these 2 spp & their diving habits:
- Pusa hispida feeds on cod, herring, smelt, whitefish, sculpin, perch & other organisms primarily found in shallow Arctic waters,
- Pag.groenlandicus routinely dives up to 100 m to feed on capelin, cod, halibut, herring, redfish & some crustaceans.
Thus,
- P.hispida is able to reduce buoyancy, and remain submerged underwater by having high bone density,
- Pag.groenlandicus can dive deeper, and swim faster.
Despite their sympatric populations in marine Arctic & N-most Atlantic oceans, differing diving depths of these modern seals suggest dietary disparities, due to availability of prey.

POS was observed in the bones of the 1st fossil record of the subfamily Cystophorinae:
some extinct true seals did have POS bones.
Morphological examination of these fossil postcrania (mid-Miocene, mid-Sarmatian 11.2-12.3 Ma, S-Ukraine) led to the description of a new genus:
Pachyphoca, with 2 new spp: Pachyphoca ukrainica & chapskii + a mosaic of primitive characters.
Anatomical traits were studied with corresponding morphological functionality, e.g.
the well-developed lesser trochanter of the femur in the smaller species (P.ukrainica) suggests that it was more adapted to terrestrial locomotion than its larger relative P.chapskii,
both new spp are more primitive & better adapted for terrestrial locomotion than any living representatives of the subfamily Cystophorinae.
-- The larger species P.chapskii has innominate bones with a deep, conical acetabulum,
the margins of the acetabular fossa are raised high above the plane surface of the bone.
-- In contrast, the smaller P.ukrainica has:
- a pubis with a big, well-developed ridge for attachment of the obturator muscles (which cause outward rotation of the hip joint),
- a thick, wide & robust ischial spine for attachment of the biceps femoris muscle (hip extensor),
- a deep fossa on the medial aspect of the ilium for attachment of the gluteus medius muscle (also a hip extensor).
Discussion:
Koretsky [17] briefly detailed that some fossil postcrania of seals demonstrate thick & swollen (POS) bones that can be mistaken for those of Sirenia such as Manatus maeoticus.
If hyper-saline closed basins developed when the ancient sea in C-Europe dried out, then POS seals & Sirenia would have evolved in parallel, but separately, during the same time periods.
Increased skeletal mass would allow taxa to remain submerged for longer periods of time,
it is likely a dietary adaptation for feeding in shallow waters.
POS bones would mean that these taxa swim at slow speeds, and dive only shallow depths, suggesting that they ate slow-moving prey near the ocean floor.
POS among fossil seals is a rel.new discovery: it is hardly remarked at all in literature:
future studies are needed to determine the cause & frequency of POS in marine mammals, esp. true seals.
Upcoming morphological examinations will demonstrate:
is POS an adaptation of true seals that may have helped them successfully adjust from terrestrial to fully aquatic life & to different salinity levels?
or is there an interspecific difference in bone mass, resulting in varying dietary preferences, diving depths and/or ecological niches?
PO in fossil seals (~3-24 Ma) from the Para-Tethys (Europe) & N.America will be examined & compared to representatives of Recent seals, who present this ostological condition.
Future studies will examine diving depths, dietary specializations & ecological niches of taxa with & without POS, to demonstrate the specific cause of this condition.


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