I just read this very extensive & beautifully illustrated article on osteosclerosis (OS) & pachy-ostosis (PO): a shortened version, without illustrations & refs:

Adaptive Patterns in Aquatic Amniote Bone Microanatomy -
More Complex than Previously Thought
Alexandra Houssaye, Martin Sander & Nicole Klein 2021
Integrative & Comparative Biology 56:1349-1369

Numerous amniote groups adapted to an aquatic life.
This change of habitat naturally led to numerous convergences.
The various adaptive traits vary depending on the degree of adaptation to an aquatic life, notably between
- shallow-water taxa (still able to occasionally locomote on land) vs
- open-marine forms (totally independent from the terrestrial environment),
- surface swimmers vs
- deep divers:
despite convergences, there is a high diversity within aquatic amniotes in e.g.
- shape,
- size,
- physiology,
- swimming mode.

Bone micro-anatomy (considered to be strongly ass.x bone bio-mechanics) is a powerful tool to
- understand bone adaptation to functional constraints,
- make functional inferences on extinct taxa.

2 opposing major micro-anatomical specializations have been described in aquatic amniotes:
- bone mass increase,
- a spongious organization.

They are assumed to be essentially linked with
- the hydro-static or -dynamic control of buoyancy & body trim,
- with swimming abilities & velocity.
Between extremes in these specializations, a wide range of intermediary patterns occurs.

The present study provides a state-of-the-art review of these inner bone adaptations in (semi)aquatic amniotes.

The analysis of the various micro-anatomical patterns observed in long bones, vertebrae & ribs of a large sample of (semi)aquatic extant & extinct amniotes reveals
- the wide diversity in micro-anatomical patterns &
- the variation in combination of these different patterns within a single skeleton.

This enables us to discuss the link between micro-anatomical features & habitat, swimming abilities:
functional requirements in the context of amniote adaptation to an aquatic lifestyle.

....

Discussion

Terminology issue

In numerous previous studies, the term “pachy-ostosis” (PO) has been widely employed to describe significant bone mass increase, either corresponding to
- true PO,
- osteo-sclerosis OS,
- pachy-osteo-sclerosis POS (PO + OS).

Recent studies insisted on the necessity to distinguish OS from PO: these specializations
- rely on different processes,
- have probably different causes,
- can also have different functional consequences.
....
where to put a threshold to determine whether there is OS or PO?
when is the increase in inner bone compactness (1), or change in outer shape caused by additional cortical bone deposits (2), significant enough to objectively state that there is OS (1) or PO (2)?

De Buffrénil cs proposed a threshold to determine the occurrence of these specializations, based on ribs:
- compactness index >81.7 % for OS,
- cortical development index >17.7 % for PO (mean rib circumference/length)..

But these values were based on an estimated 95 % confidence interval of the plesiomorphic (unspecialized terrestrial) condition, calculated on a specific sample, thus influenced by the composition of this sample.

This first attempt is promising, nevertheless, and underscores the importance of the threshold question:
as shown in our study, numerous taxa display some (limited) increase in bone compactness vs terrestrial taxa,
but it is extremely difficult to objectively determine if the observed pattern corresponds to OS or not.

Should OS be defined based on the compactness index only?
or general bone tissue distribution also?
Should the definition of OS be based on the pattern of increase in bone mass?
or on specific processes leading to this pattern?

Here we focus on the patterns -
a detailed discussion on the processes, already preliminary discussed in some papers, is to follow.

The same problems surround the term “spongious organization”.
Spongious organization was previously used to describe a pattern resembling the pathology osteo-porosis:
“osteoporotic-like pattern” was used since a decrease in bone mass was assumed.
But despite a spongious structure generally looking like it contains less bone tissue than a tubular (terrestrial-like) organization, quantification of compactness indices shows:
there is not always a decrease in bone mass going along with a spongious organization (e.g. mosasaurs):
the definition & diagnosis of these specializations should not be based on compactness alone.

A general decrease of skeletal mass (vs tetrapods of similar size & mass) was documented for some of the taxa showing a spongious organization,
but the skeletal mass decrease is the result of general skeletal reduction (total bone volume), not of a decrease in the compactness of the bones themselves.

The spongious organization involves a strong change in inner organization (vs a tubular organization), but without any significant change in bone compactness.

When to call an inner organization “spongious”?
“Spongiosa” is generally used for all trabecular organization, although spongiosa is commonly defined as the part of a bone displaying a lower volume of bone tissue than of pore space:
a compactness index of <50 %.

Ichthyosaurs display a very compact trabecular organization, that does not correspond to a true spongiosa:
the volume of bone tissue is higher than that of pore space:
where to put a threshold value for spongious inner organization?
the distinction of e.g. a trabecular network with a compactness index of 46 % & one of 54 % is not obvious.

It is usu.possible to circumvent these definition issues when the difference of organization between the spmn analyzed & the comparative material is obvious,
but the issue becomes more complex when differences are more subtle.

Various intra-skeletal combinations

A major scope of this review (beyond providing large-scale review of the state of the art) is to make it possible to combine data from 4 bone types from the axial & appendicular skeleton, to better understand the functional impact of bone mass increase & spongious organization.

The review clearly highlights the strong variability in the patterns observed within the skeleton,
this enables to discuss the main functional constraints acting on bone micro-anatomical features.

Buoyancy is one of the main constraints for diving aquatics:
they are generally positively buoyant because of their lungs, with possible additional contribution of their fur.

Shallow divers generally stay in the zone of maximal buoyancy in the water column:
they benefit most from strongly decreasing their buoyancy by bone mass increase:
otherwise they would have to spend muscular energy to change their position in the water column, e.g.
- slow swimmers hovering at a given shallow depth, that should ideally be close to the "neutral depth" (where animal's buoyancy = surrounding water),
- bottom-walkers (such ecologies are inferred for taxa displaying a strong increase in bone mass, helping them to hydrostatically control buoyancy & body trim).

An extreme OS in the vertebrae
- does not occur in mammals & birds,
- is only found in some long-bodied reptiles considered to still display rather "terrestrial-like" limbs (except hind-limbed snakes), e.g.
stem-ophidiomorphs, pachypleurosaurs & mesosaurs show
- strong OS in their ribs as well,
- generally, PO in their axial skeleton, e.g.
--- extreme in hind-limbed snakes, some pachypleurosaurs & mesosaurs,
--- limited in ophidiomorph Pontosaurus & mosasauroid Haasiasaurus.

Limb data for these taxa were generally not available, except for many pachypleurosaurs (strong OS – to a lesser extent in Anarosaurus).
In these long-tailed animals (anguilliform sub-surface swimmers?), PO probably limited movements between adjacent vertebrae or ribs,
- increasing the rigidity of the body,
- decreasing the locomotion cost of animals whose propulsion could be essentially ensured by their long tail (that shows no bone mass increase), but also limiting Carrier's constraint (disturbance in breathing, engendered by flexion of the trunk during lateral movements of the body).
The additional bone mass increase also played a role in hydrostatic buoyancy & body trim.

In Sirenia, bone mass increase
- is extreme in the ribs,
- limited in the vertebrae.
There is also OS in the humerus.
They have a large spindle-shaped body.
The high O2 store in the lungs increases buoyancy – compensated for by OS or PO ribs.
Beyond decreasing buoyancy & increasing body mass anteriorly (to facilitate a horizontal body trim), OS in the sirenian humerus might also play a role in improving underwater stability (by lowering the centre of mass).

Some Desmostylia (Behemotops, Ashoroa) display a pattern very similar to Sirenia:
this suggests similar ecologies, despite clearly different swimming modes in these 4-limbed animals.

Aquatic sloths Thalassocnus display
- ribs & long bones closer to the taxa cited above,
- vertebrae more similar to Desmostylus (requires further investigations functionally & ecologically).

Crocodiles & hippos swim essentially along the bottom of shallow waters, touching the ground with their legs (crocs additionally wriggle the tail).
This is also the case for the marine iguana, although it swims slightly more in the water column than the taxa cited above.
All these taxa still have efficient terrestrial locomotion:
they still require micro-anatomical features compatible with locomotion in a high-gravity environment:
crocs & the marine iguana still retain a tubular organization in their limb bones (perfectly adapted to an environment where gravity is the main constraint).

Op zaterdag 14 augustus 2021 om 14:12:41 UTC+2 schreef littor...@gmail.com:

Sorry, the paper is from 2016, no 2021.

> I just read this very extensive & beautifully illustrated article on osteosclerosis (OS) & pachy-ostosis (PO): a shortened version, without illustrations & refs:
>
>
>
> Adaptive Patterns in Aquatic Amniote Bone Microanatomy -
> More Complex than Previously Thought
> Alexandra Houssaye, Martin Sander & Nicole Klein 2021
> Integrative & Comparative Biology 56:1349-1369
>
>
> Numerous amniote groups adapted to an aquatic life.
> This change of habitat naturally led to numerous convergences.
> The various adaptive traits vary depending on the degree of adaptation to an aquatic life, notably between
> - shallow-water taxa (still able to occasionally locomote on land) vs
> - open-marine forms (totally independent from the terrestrial environment),
> - surface swimmers vs
> - deep divers:
> despite convergences, there is a high diversity within aquatic amniotes in e.g.
> - shape,
> - size,
> - physiology,
> - swimming mode.
>
> Bone micro-anatomy (considered to be strongly ass.x bone bio-mechanics) is a powerful tool to
> - understand bone adaptation to functional constraints,
> - make functional inferences on extinct taxa.
>
> 2 opposing major micro-anatomical specializations have been described in aquatic amniotes:
> - bone mass increase,
> - a spongious organization.
>
> They are assumed to be essentially linked with
> - the hydro-static or -dynamic control of buoyancy & body trim,
> - with swimming abilities & velocity.
> Between extremes in these specializations, a wide range of intermediary patterns occurs.
>
> The present study provides a state-of-the-art review of these inner bone adaptations in (semi)aquatic amniotes.
>
> The analysis of the various micro-anatomical patterns observed in long bones, vertebrae & ribs of a large sample of (semi)aquatic extant & extinct amniotes reveals
> - the wide diversity in micro-anatomical patterns &
> - the variation in combination of these different patterns within a single skeleton.
>
> This enables us to discuss the link between micro-anatomical features & habitat, swimming abilities:
> functional requirements in the context of amniote adaptation to an aquatic lifestyle.
>
>
> ...
>
>
> Discussion
>
> Terminology issue
>
> In numerous previous studies, the term “pachy-ostosis” (PO) has been widely employed to describe significant bone mass increase, either corresponding to
> - true PO,
> - osteo-sclerosis OS,
> - pachy-osteo-sclerosis POS (PO + OS).
>
> Recent studies insisted on the necessity to distinguish OS from PO: these specializations
> - rely on different processes,
> - have probably different causes,
> - can also have different functional consequences.
> ...
> where to put a threshold to determine whether there is OS or PO?
> when is the increase in inner bone compactness (1), or change in outer shape caused by additional cortical bone deposits (2), significant enough to objectively state that there is OS (1) or PO (2)?
>
> De Buffrénil cs proposed a threshold to determine the occurrence of these specializations, based on ribs:
> - compactness index >81.7 % for OS,
> - cortical development index >17.7 % for PO (mean rib circumference/length).
>
> But these values were based on an estimated 95 % confidence interval of the plesiomorphic (unspecialized terrestrial) condition, calculated on a specific sample, thus influenced by the composition of this sample.
>
> This first attempt is promising, nevertheless, and underscores the importance of the threshold question:
> as shown in our study, numerous taxa display some (limited) increase in bone compactness vs terrestrial taxa,
> but it is extremely difficult to objectively determine if the observed pattern corresponds to OS or not.
>
> Should OS be defined based on the compactness index only?
> or general bone tissue distribution also?
> Should the definition of OS be based on the pattern of increase in bone mass?
> or on specific processes leading to this pattern?
>
> Here we focus on the patterns -
> a detailed discussion on the processes, already preliminary discussed in some papers, is to follow.
>
> The same problems surround the term “spongious organization”.
> Spongious organization was previously used to describe a pattern resembling the pathology osteo-porosis:
> “osteoporotic-like pattern” was used since a decrease in bone mass was assumed.
> But despite a spongious structure generally looking like it contains less bone tissue than a tubular (terrestrial-like) organization, quantification of compactness indices shows:
> there is not always a decrease in bone mass going along with a spongious organization (e.g. mosasaurs):
> the definition & diagnosis of these specializations should not be based on compactness alone.
>
> A general decrease of skeletal mass (vs tetrapods of similar size & mass) was documented for some of the taxa showing a spongious organization,
> but the skeletal mass decrease is the result of general skeletal reduction (total bone volume), not of a decrease in the compactness of the bones themselves.
>
> The spongious organization involves a strong change in inner organization (vs a tubular organization), but without any significant change in bone compactness.
>
> When to call an inner organization “spongious”?
> “Spongiosa” is generally used for all trabecular organization, although spongiosa is commonly defined as the part of a bone displaying a lower volume of bone tissue than of pore space:
> a compactness index of <50 %.
>
> Ichthyosaurs display a very compact trabecular organization, that does not correspond to a true spongiosa:
> the volume of bone tissue is higher than that of pore space:
> where to put a threshold value for spongious inner organization?
> the distinction of e.g. a trabecular network with a compactness index of 46 % & one of 54 % is not obvious.
>
> It is usu.possible to circumvent these definition issues when the difference of organization between the spmn analyzed & the comparative material is obvious,
> but the issue becomes more complex when differences are more subtle.
>
> Various intra-skeletal combinations
>
> A major scope of this review (beyond providing large-scale review of the state of the art) is to make it possible to combine data from 4 bone types from the axial & appendicular skeleton, to better understand the functional impact of bone mass increase & spongious organization.
>
> The review clearly highlights the strong variability in the patterns observed within the skeleton,
> this enables to discuss the main functional constraints acting on bone micro-anatomical features.
>
> Buoyancy is one of the main constraints for diving aquatics:
> they are generally positively buoyant because of their lungs, with possible additional contribution of their fur.
>
> Shallow divers generally stay in the zone of maximal buoyancy in the water column:
> they benefit most from strongly decreasing their buoyancy by bone mass increase:
> otherwise they would have to spend muscular energy to change their position in the water column, e.g.
> - slow swimmers hovering at a given shallow depth, that should ideally be close to the "neutral depth" (where animal's buoyancy = surrounding water),
> - bottom-walkers (such ecologies are inferred for taxa displaying a strong increase in bone mass, helping them to hydrostatically control buoyancy & body trim).
>
> An extreme OS in the vertebrae
> - does not occur in mammals & birds,
> - is only found in some long-bodied reptiles considered to still display rather "terrestrial-like" limbs (except hind-limbed snakes), e.g.
> stem-ophidiomorphs, pachypleurosaurs & mesosaurs show
> - strong OS in their ribs as well,
> - generally, PO in their axial skeleton, e.g.
> --- extreme in hind-limbed snakes, some pachypleurosaurs & mesosaurs,
> --- limited in ophidiomorph Pontosaurus & mosasauroid Haasiasaurus.
>
> Limb data for these taxa were generally not available, except for many pachypleurosaurs (strong OS – to a lesser extent in Anarosaurus).
> In these long-tailed animals (anguilliform sub-surface swimmers?), PO probably limited movements between adjacent vertebrae or ribs,
> - increasing the rigidity of the body,
> - decreasing the locomotion cost of animals whose propulsion could be essentially ensured by their long tail (that shows no bone mass increase), but also limiting Carrier's constraint (disturbance in breathing, engendered by flexion of the trunk during lateral movements of the body).
> The additional bone mass increase also played a role in hydrostatic buoyancy & body trim.
>
> In Sirenia, bone mass increase
> - is extreme in the ribs,
> - limited in the vertebrae.
> There is also OS in the humerus.
> They have a large spindle-shaped body.
> The high O2 store in the lungs increases buoyancy – compensated for by OS or PO ribs.
> Beyond decreasing buoyancy & increasing body mass anteriorly (to facilitate a horizontal body trim), OS in the sirenian humerus might also play a role in improving underwater stability (by lowering the centre of mass).
>
> Some Desmostylia (Behemotops, Ashoroa) display a pattern very similar to Sirenia:
> this suggests similar ecologies, despite clearly different swimming modes in these 4-limbed animals.
>
> Aquatic sloths Thalassocnus display
> - ribs & long bones closer to the taxa cited above,
> - vertebrae more similar to Desmostylus (requires further investigations functionally & ecologically).
>
>
>
> Crocodiles & hippos swim essentially along the bottom of shallow waters, touching the ground with their legs (crocs additionally wriggle the tail).
> This is also the case for the marine iguana, although it swims slightly more in the water column than the taxa cited above.
> All these taxa still have efficient terrestrial locomotion:
> they still require micro-anatomical features compatible with locomotion in a high-gravity environment:
> crocs & the marine iguana still retain a tubular organization in their limb bones (perfectly adapted to an environment where gravity is the main constraint).
>
>
> In non-graviportal terrestrial taxa, forces in long bones are
> - maximal near the surface,
> - almost negligible at their core:
> an empty medullary cavity reduces weight, without altering the bone’s effective strength.
>
> The varanoid lizard Pachyvaranus & placodonts (considered exclusively aquatic) were probably also bottom-dwellers in shallow waters.
> OS is functionally advantageous in all of these bottom-dwellers for buoyancy & body trim control.
> Strongly OS bones are considered brittle, incompatible with efficient terrestrial locomotion:
> - OS bones are found essentially in exclusively aquatic forms,
> - semi-aquatic forms display only a limited increase in bone mass in their long bones (except Aptenodytes):
> in these semi-aquatic animals, OS is stronger in the ribs (ribs are less actively involved in locomotion, and less subject to fracture engendered by locomotion).
>
> Hippos are graviportal: no open medullary cavity replaced by a spongiosa.
>
> Polar bears no bone mass increase:
> they swim at the surface or at shallow depth in deep water
> (vs hippos with the limbs temporarily touch the ground).
>
> Coypu & beaver are also primarily surface-swimmers,
> platypus & otters primarily dive,
> but all of these still have efficient terrestrial locomotion:
> they have a non-wettable fur, that provides positive buoyancy.
> A slight increase in bone compactness is observed in the coypu humerus,
> but it is higher in the platypus & otters, although there is no strong OS..
>
> In the sea-otter, longitudinal sections indicated:
> - the increase in bone mass is localized around the growth center,
> - the bone away from this region is spongious.
> The sea otter is almost exclusively aquatic
> (some other otters spend no more than 20 % of their time in water, e.g. Lutra lutra).
> Sea otters are also strongly buoyant, because of their lungs;
> strong buoyancy helps their unique feeding mode, but that needs to be counter-acted for diving.
>
> Variations linked to their various swimming modes should occur within otters.
>
>
>
> Flightless diving-birds appear to display strong OS in their long bones. Previous studies suggested:
> OS was higher in the humerus than in the femur (cf BP terrestrial locomotion?),
> but Canoville cs recently highlighted an equally strong OS in the femora (further investigation required).
>
> Aptenodytes penguins dive to great depth, but (vs Pinnipedia, Cetacea) dive on inspiration.
> Highly buoyant like all birds,
> - at shallow depth, it performs vigorous flipper-beating to work against positive buoyancy,
> - further descent occurs through passive gliding.
> The OS in its long bones could thus reflect a hydrostatic help in buoyancy reduction,
> but buoyancy control remains essentially hydrodynamic.
> But why is there no significant bone mass increase (though ribs are naturally much more compact than in flying birds) in the axial skeleton, at least in the ribs?
>
> The long-bodied plesio-pedal & plesio-pelvic mosa-sauroids (previously "aigialosaurs") display bone mass increase in their axial skeleton (long bone data are not available),
> they are assumed to have lived in coastal environments.
>
> Nothosaurs (also long-bodied) display a wide range of micro-anatomical specializations, that render their interpretation difficult, and probably reflect different swimming ecologies, from surface to bottom swimmers & coastal, to more open marine forms.
> Most nothosaur material has been studied from isolated bones, belonging to various morphotypes, that may reflect both taxonomic diversity & sex.dimorphism,
> but these morphotypes cannot be assigned to specific nothosaur taxa for each bone:
> comparisons between different bones cannot be performed.
> The "thin-walled organization" in the humeri of some large forms (a true decrease in bone mass) is unique among amniotes (flying birds excepted, but their bones are also pneumatized),
> it remains unexplained.
>
>
>
> Like some placodonts, pistosaurs display strongly OS humeri & less compact femora,
> but, as these taxa are the sister-group of plesiosaurs, longitudinal sections are required:
> does this increase in compactness affect the whole bone, or is it only local?
>
> Plesiosaurs seem to display
> - a spongious organization,
> - but a strong increase in long bone compactness near the growth centre.
>
>
>
> Pinnipeds seem to all display a spongiosa in the medullary area in the bones sampled, despite differences in the tightness of this spongiosa & in the thickness of the peripheral layer of compact cortical.
> They retain only limited terrestrial locomotor capabilities, esp.phocids (seals s.s.):
> phocids (vs otariids sea-lions) have limbs unable to carry their weight,
> they swim using pelvic oscillation (otariids use pectoral oscillation).
> A thicker cortex is observed in otariids than in phocids.
> How can the limbs of sea-lions carry their high body weight?
> Longitudinal sections would be required to more clearly observe the diversity of micro-anatomical specializations in pinnipeds.
>
> The spongious organization is nevertheless compatible with a homogeneous distribution of stresses in a milieu not dominated by uni-directional gravity, but by multi-directional drag for animals requiring high acceleration & maneuverability abilities & diving:
> bone mass increase
> - offers buoyancy & body trim cô at low energy expenditure in poorly active swimmers,
> - but increases mass, and is also often ass.x an increase in lung volume & ribcage volume:
> - increases inertia & drag, reducing acceleration & maneuverability, esp. in faster swimmers.
>
>
> Pinnipedia & Cetacea (highly diving aquatic mammals) overcome positive buoyancy by lung collapse & thoracic compression:
> they would not benefit from bone mass increase.
>
>
> Hydropedal mosasauroids, most Cetacea & the leatherback turtle also display a spongious organization in all bones analyzed, with only a thin layer of compact cortex.
> This is also the case in ichthyosaurs, with the difference that the long bones show a high compactness near the growth center, except Mixosaurus that displays an organization more similar to some pinnipeds. This local strong increase in compactness near the growth center of stylopod bones is observed in various taxa with different body shapes & swimming ecologies:
> ichthyosaurs, plesiosaurs, Enhydra lutris, protocetid archaeocetes.
> Its functional significance remains currently unexplained.
>
>
> The growth center is the region of the long bone where
> - bone of periosteal origin (cortical bone) is the thickest:
> - a local increase in bone mass could easily occur (through an inhibition of bone remodeling turning compact into cancellous bone):
> this pattern might be the evolutionarily easiest way to increase compactness only slightly,
> but in ichthyosaurs, remodeling does occur:
> increased compactness occurs through active remodeling characterized by excessive secondary bone deposits, which clearly does not appear as the cheapest strategy.
>
>
> Strongly reduced femora were analysed only in Basilosaurus (Cetacea) & the hind-limbed snake Eupodophis.
> - Basilosaurus shows a strong increase in bone mass (Houssaye et al. 2015),
> - Eupodophis displays a terrestrial-like inner structure, with no increase in bone mass:
> the occurrence of OS in limbs under evolutionary reduction cannot be attributed to general bone reduction only.
>
>
> Some taxa display comparable micro-anatomical features in their humeri & femora (e.g. plesiosaurs, crocs),
> but this is sometimes clearly not the case, e.g. in some placodonts, pistosaurs, the plesiopedal but hydropelvic mosasauroid Dallasaurus:
> this implies distinct constraints acting on these bones, and thus distinct uses.
>
> There are also taxa where usage & morphology differ between fore- & hind-limb,
> but micro-anatomy does not (e.g. the pachypleurosaurs, the nothosaur Ceresiosaurus, the leatherback turtle), requiring other’s explanations than bio-mechanics of use.
>
>
> As previously observed, PO is a specialization primarily of the axial skeleton.
> Based on the spms for which micro-anatomical data are available, PO appears always ass.x a strong increase in inner bone compactness, but also with “incomplete OS”:
> is this also the case in some of the taxa for which micro-anatomical data are not available yet, e.g.
> some choristoderans, juvenile plesiosaurs & the rhynchocephalian Ankylosphenodon?
> Could PO be
> (1) advantageous in limiting movements between adjacent vertebrae, and thus in straightening the body?
> (2) the consequence of the need to increase body mass beyond the strongest increase in inner bone mass (through OS) functionally possible for the organism?
> (3) both?
>
> A functional role beyond an increase in bone mass appears nevertheless poorly possible for ribs.
> The occurrence of bone mass increase in the rib-cage of some taxa (Basilosaurus, some plesiosaurs e.g. Pachycostasaurus) remains unexplained because these were highly aquatic.
>
>
> Some really peculiar combinations of bone specializations occur, e.g.
> - a strongly OS humerus + a tubular (terrestrial-like) femur & spongious vertebrae in the mosasauroid Dallasaurus,
> - the extremely tight spongiosa of the vertebra + strongly OS long bones of Champsosaurus.
> To understand these peculiar extinct taxa, much work on extant taxa for which ecological data are available is required.
>
>
>
> Signals in bone microanatomy
>
> Various studies have shown that there is a phylogenetic signal in bone micro-anatomical adaptation.
> As for the specializations ass.x an aquatic lifestyle, important convergences clearly occur.
> This shows the strong impact of functional constraints, but there is naturally a phylogenetic heritage.
> However, the phylogenetic signal can also be artificially strengthened, depending on the sample analyzed:
> only a limited nr of evol.events corresponding to a shift from a terrestrial to an aquatic lifestyle occur.
> Structural constraints ass.to physical & architectural requirements also act on bone micro-anatomy, as previously described for the variations in the tightness of the trabecular network with size in amniote vertebrae.
>
> But by comparing mosasaurs, ichthyosaurs & plesiosaurs of similar size, it also clearly appears:
> a functional impact, ass.x constraints essentially linked to swimming-mode, is superimposed to this structural effect.
>
> All these effects should thus be taken into consideration when analyzing bone micro-anatomical adaptation.
>
>
>
> Conclusion
>
> This review of the state of the art clearly illustrates the wide diversity in microanatomical patterns observed in bones of (semi)aquatic amniotes.
>
> Unfortunately, because of the limits of the current terminology, these have been generally dichotomized under the general terms
> - bone mass increase &
> - spongious organization.
>
> Our review also highlights the variation in combination of these different patterns among different skeletal elements.
>
> To better understand the link between functional requirements & bone inner structure, taking phylogenetic & structural constraints into consideration, it appears now necessary to analyze differences within specific taxonomical groups in more details, as well as among taxa from different groups, but with closely similar ecologies.
>
> 3D analyses should make it possible to obtain data that differentiate much more precisely between taxa.
>
> More detailed analyses on extant taxa will make much more precise inferences on the paleo-ecology of extinct taxa possible, notably of the forms displaying currently enigmatic combinations of micro-anatomical features in their skeleton.
>
> _____
>
> This beautiful comparative work suggests H.erectus (PO+OS, e.g. occiput, femora, parts of pelvis ...) spent most of their time bottom-dwelling (& back-floating for between dives - sea-otter-like?), probably mostly in salt waters.
> It also helps explain H.erectus fur loss.
> Apparently our ancestors were still partly aquatic until not long ago (late-Pleistocene?).
>
> Only complete idiots still believe erectus ran antelopes to exhaustion.

On Saturday, August 14, 2021 at 8:12:41 AM UTC-4, littor...@gmail.com wrote:
> I just read this very extensive & beautifully illustrated article on osteosclerosis (OS) & pachy-ostosis (PO): a shortened version, without illustrations & refs:
>
>
>
> Adaptive Patterns in Aquatic Amniote Bone Microanatomy -
> More Complex than Previously Thought
> Alexandra Houssaye, Martin Sander & Nicole Klein 2021
> Integrative & Comparative Biology 56:1349-1369
>
>
> Numerous amniote groups adapted to an aquatic life.
> This change of habitat naturally led to numerous convergences.
> The various adaptive traits vary depending on the degree of adaptation to an aquatic life, notably between
> - shallow-water taxa (still able to occasionally locomote on land) vs
> - open-marine forms (totally independent from the terrestrial environment),
> - surface swimmers vs
> - deep divers:
> despite convergences, there is a high diversity within aquatic amniotes in e.g.
> - shape,
> - size,
> - physiology,
> - swimming mode.
>
> Bone micro-anatomy (considered to be strongly ass.x bone bio-mechanics) is a powerful tool to
> - understand bone adaptation to functional constraints,
> - make functional inferences on extinct taxa.
>
> 2 opposing major micro-anatomical specializations have been described in aquatic amniotes:
> - bone mass increase,
> - a spongious organization.
>
> They are assumed to be essentially linked with
> - the hydro-static or -dynamic control of buoyancy & body trim,
> - with swimming abilities & velocity.
> Between extremes in these specializations, a wide range of intermediary patterns occurs.
>
> The present study provides a state-of-the-art review of these inner bone adaptations in (semi)aquatic amniotes.
>
> The analysis of the various micro-anatomical patterns observed in long bones, vertebrae & ribs of a large sample of (semi)aquatic extant & extinct amniotes reveals
> - the wide diversity in micro-anatomical patterns &
> - the variation in combination of these different patterns within a single skeleton.
>
> This enables us to discuss the link between micro-anatomical features & habitat, swimming abilities:
> functional requirements in the context of amniote adaptation to an aquatic lifestyle.
>
>
> ...
>
>
> Discussion
>
> Terminology issue
>
> In numerous previous studies, the term “pachy-ostosis” (PO) has been widely employed to describe significant bone mass increase, either corresponding to
> - true PO,
> - osteo-sclerosis OS,
> - pachy-osteo-sclerosis POS (PO + OS).
>
> Recent studies insisted on the necessity to distinguish OS from PO: these specializations
> - rely on different processes,
> - have probably different causes,
> - can also have different functional consequences.
> ...
> where to put a threshold to determine whether there is OS or PO?
> when is the increase in inner bone compactness (1), or change in outer shape caused by additional cortical bone deposits (2), significant enough to objectively state that there is OS (1) or PO (2)?
>
> De Buffrénil cs proposed a threshold to determine the occurrence of these specializations, based on ribs:
> - compactness index >81.7 % for OS,
> - cortical development index >17.7 % for PO (mean rib circumference/length).
>
> But these values were based on an estimated 95 % confidence interval of the plesiomorphic (unspecialized terrestrial) condition, calculated on a specific sample, thus influenced by the composition of this sample.
>
> This first attempt is promising, nevertheless, and underscores the importance of the threshold question:
> as shown in our study, numerous taxa display some (limited) increase in bone compactness vs terrestrial taxa,
> but it is extremely difficult to objectively determine if the observed pattern corresponds to OS or not.
>
> Should OS be defined based on the compactness index only?
> or general bone tissue distribution also?
> Should the definition of OS be based on the pattern of increase in bone mass?
> or on specific processes leading to this pattern?
>
> Here we focus on the patterns -
> a detailed discussion on the processes, already preliminary discussed in some papers, is to follow.
>
> The same problems surround the term “spongious organization”.
> Spongious organization was previously used to describe a pattern resembling the pathology osteo-porosis:
> “osteoporotic-like pattern” was used since a decrease in bone mass was assumed.
> But despite a spongious structure generally looking like it contains less bone tissue than a tubular (terrestrial-like) organization, quantification of compactness indices shows:
> there is not always a decrease in bone mass going along with a spongious organization (e.g. mosasaurs):
> the definition & diagnosis of these specializations should not be based on compactness alone.
>
> A general decrease of skeletal mass (vs tetrapods of similar size & mass) was documented for some of the taxa showing a spongious organization,
> but the skeletal mass decrease is the result of general skeletal reduction (total bone volume), not of a decrease in the compactness of the bones themselves.
>
> The spongious organization involves a strong change in inner organization (vs a tubular organization), but without any significant change in bone compactness.
>
> When to call an inner organization “spongious”?
> “Spongiosa” is generally used for all trabecular organization, although spongiosa is commonly defined as the part of a bone displaying a lower volume of bone tissue than of pore space:
> a compactness index of <50 %.
>
> Ichthyosaurs display a very compact trabecular organization, that does not correspond to a true spongiosa:
> the volume of bone tissue is higher than that of pore space:
> where to put a threshold value for spongious inner organization?
> the distinction of e.g. a trabecular network with a compactness index of 46 % & one of 54 % is not obvious.
>
> It is usu.possible to circumvent these definition issues when the difference of organization between the spmn analyzed & the comparative material is obvious,
> but the issue becomes more complex when differences are more subtle.
>
> Various intra-skeletal combinations
>
> A major scope of this review (beyond providing large-scale review of the state of the art) is to make it possible to combine data from 4 bone types from the axial & appendicular skeleton, to better understand the functional impact of bone mass increase & spongious organization.
>
> The review clearly highlights the strong variability in the patterns observed within the skeleton,
> this enables to discuss the main functional constraints acting on bone micro-anatomical features.
>
> Buoyancy is one of the main constraints for diving aquatics:
> they are generally positively buoyant because of their lungs, with possible additional contribution of their fur.
>
> Shallow divers generally stay in the zone of maximal buoyancy in the water column:
> they benefit most from strongly decreasing their buoyancy by bone mass increase:
> otherwise they would have to spend muscular energy to change their position in the water column, e.g.
> - slow swimmers hovering at a given shallow depth, that should ideally be close to the "neutral depth" (where animal's buoyancy = surrounding water),
> - bottom-walkers (such ecologies are inferred for taxa displaying a strong increase in bone mass, helping them to hydrostatically control buoyancy & body trim).
>
> An extreme OS in the vertebrae
> - does not occur in mammals & birds,
> - is only found in some long-bodied reptiles considered to still display rather "terrestrial-like" limbs (except hind-limbed snakes), e.g.
> stem-ophidiomorphs, pachypleurosaurs & mesosaurs show
> - strong OS in their ribs as well,
> - generally, PO in their axial skeleton, e.g.
> --- extreme in hind-limbed snakes, some pachypleurosaurs & mesosaurs,
> --- limited in ophidiomorph Pontosaurus & mosasauroid Haasiasaurus.
>
> Limb data for these taxa were generally not available, except for many pachypleurosaurs (strong OS – to a lesser extent in Anarosaurus).
> In these long-tailed animals (anguilliform sub-surface swimmers?), PO probably limited movements between adjacent vertebrae or ribs,
> - increasing the rigidity of the body,
> - decreasing the locomotion cost of animals whose propulsion could be essentially ensured by their long tail (that shows no bone mass increase), but also limiting Carrier's constraint (disturbance in breathing, engendered by flexion of the trunk during lateral movements of the body).
> The additional bone mass increase also played a role in hydrostatic buoyancy & body trim.
>
> In Sirenia, bone mass increase
> - is extreme in the ribs,
> - limited in the vertebrae.
> There is also OS in the humerus.
> They have a large spindle-shaped body.
> The high O2 store in the lungs increases buoyancy – compensated for by OS or PO ribs.
> Beyond decreasing buoyancy & increasing body mass anteriorly (to facilitate a horizontal body trim), OS in the sirenian humerus might also play a role in improving underwater stability (by lowering the centre of mass).
>
> Some Desmostylia (Behemotops, Ashoroa) display a pattern very similar to Sirenia:
> this suggests similar ecologies, despite clearly different swimming modes in these 4-limbed animals.
>
> Aquatic sloths Thalassocnus display
> - ribs & long bones closer to the taxa cited above,
> - vertebrae more similar to Desmostylus (requires further investigations functionally & ecologically).
>
>
>
> Crocodiles & hippos swim essentially along the bottom of shallow waters, touching the ground with their legs (crocs additionally wriggle the tail).
> This is also the case for the marine iguana, although it swims slightly more in the water column than the taxa cited above.
> All these taxa still have efficient terrestrial locomotion:
> they still require micro-anatomical features compatible with locomotion in a high-gravity environment:
> crocs & the marine iguana still retain a tubular organization in their limb bones (perfectly adapted to an environment where gravity is the main constraint).
>
>
> In non-graviportal terrestrial taxa, forces in long bones are
> - maximal near the surface,
> - almost negligible at their core:
> an empty medullary cavity reduces weight, without altering the bone’s effective strength.
>
> The varanoid lizard Pachyvaranus & placodonts (considered exclusively aquatic) were probably also bottom-dwellers in shallow waters.
> OS is functionally advantageous in all of these bottom-dwellers for buoyancy & body trim control.
> Strongly OS bones are considered brittle, incompatible with efficient terrestrial locomotion:
> - OS bones are found essentially in exclusively aquatic forms,
> - semi-aquatic forms display only a limited increase in bone mass in their long bones (except Aptenodytes):
> in these semi-aquatic animals, OS is stronger in the ribs (ribs are less actively involved in locomotion, and less subject to fracture engendered by locomotion).
>
> Hippos are graviportal: no open medullary cavity replaced by a spongiosa.
>
> Polar bears no bone mass increase:
> they swim at the surface or at shallow depth in deep water
> (vs hippos with the limbs temporarily touch the ground).
>
> Coypu & beaver are also primarily surface-swimmers,
> platypus & otters primarily dive,
> but all of these still have efficient terrestrial locomotion:
> they have a non-wettable fur, that provides positive buoyancy.
> A slight increase in bone compactness is observed in the coypu humerus,
> but it is higher in the platypus & otters, although there is no strong OS..
>
> In the sea-otter, longitudinal sections indicated:
> - the increase in bone mass is localized around the growth center,
> - the bone away from this region is spongious.
> The sea otter is almost exclusively aquatic
> (some other otters spend no more than 20 % of their time in water, e.g. Lutra lutra).
> Sea otters are also strongly buoyant, because of their lungs;
> strong buoyancy helps their unique feeding mode, but that needs to be counter-acted for diving.
>
> Variations linked to their various swimming modes should occur within otters.
>
>
>
> Flightless diving-birds appear to display strong OS in their long bones. Previous studies suggested:
> OS was higher in the humerus than in the femur (cf BP terrestrial locomotion?),
> but Canoville cs recently highlighted an equally strong OS in the femora (further investigation required).
>
> Aptenodytes penguins dive to great depth, but (vs Pinnipedia, Cetacea) dive on inspiration.
> Highly buoyant like all birds,
> - at shallow depth, it performs vigorous flipper-beating to work against positive buoyancy,
> - further descent occurs through passive gliding.
> The OS in its long bones could thus reflect a hydrostatic help in buoyancy reduction,
> but buoyancy control remains essentially hydrodynamic.
> But why is there no significant bone mass increase (though ribs are naturally much more compact than in flying birds) in the axial skeleton, at least in the ribs?
>
> The long-bodied plesio-pedal & plesio-pelvic mosa-sauroids (previously "aigialosaurs") display bone mass increase in their axial skeleton (long bone data are not available),
> they are assumed to have lived in coastal environments.
>
> Nothosaurs (also long-bodied) display a wide range of micro-anatomical specializations, that render their interpretation difficult, and probably reflect different swimming ecologies, from surface to bottom swimmers & coastal, to more open marine forms.
> Most nothosaur material has been studied from isolated bones, belonging to various morphotypes, that may reflect both taxonomic diversity & sex.dimorphism,
> but these morphotypes cannot be assigned to specific nothosaur taxa for each bone:
> comparisons between different bones cannot be performed.
> The "thin-walled organization" in the humeri of some large forms (a true decrease in bone mass) is unique among amniotes (flying birds excepted, but their bones are also pneumatized),
> it remains unexplained.
>
>
>
> Like some placodonts, pistosaurs display strongly OS humeri & less compact femora,
> but, as these taxa are the sister-group of plesiosaurs, longitudinal sections are required:
> does this increase in compactness affect the whole bone, or is it only local?
>
> Plesiosaurs seem to display
> - a spongious organization,
> - but a strong increase in long bone compactness near the growth centre.
>
>
>
> Pinnipeds seem to all display a spongiosa in the medullary area in the bones sampled, despite differences in the tightness of this spongiosa & in the thickness of the peripheral layer of compact cortical.
> They retain only limited terrestrial locomotor capabilities, esp.phocids (seals s.s.):
> phocids (vs otariids sea-lions) have limbs unable to carry their weight,
> they swim using pelvic oscillation (otariids use pectoral oscillation).
> A thicker cortex is observed in otariids than in phocids.
> How can the limbs of sea-lions carry their high body weight?
> Longitudinal sections would be required to more clearly observe the diversity of micro-anatomical specializations in pinnipeds.
>
> The spongious organization is nevertheless compatible with a homogeneous distribution of stresses in a milieu not dominated by uni-directional gravity, but by multi-directional drag for animals requiring high acceleration & maneuverability abilities & diving:
> bone mass increase
> - offers buoyancy & body trim cô at low energy expenditure in poorly active swimmers,
> - but increases mass, and is also often ass.x an increase in lung volume & ribcage volume:
> - increases inertia & drag, reducing acceleration & maneuverability, esp. in faster swimmers.
>
>
> Pinnipedia & Cetacea (highly diving aquatic mammals) overcome positive buoyancy by lung collapse & thoracic compression:
> they would not benefit from bone mass increase.
>
>
> Hydropedal mosasauroids, most Cetacea & the leatherback turtle also display a spongious organization in all bones analyzed, with only a thin layer of compact cortex.
> This is also the case in ichthyosaurs, with the difference that the long bones show a high compactness near the growth center, except Mixosaurus that displays an organization more similar to some pinnipeds. This local strong increase in compactness near the growth center of stylopod bones is observed in various taxa with different body shapes & swimming ecologies:
> ichthyosaurs, plesiosaurs, Enhydra lutris, protocetid archaeocetes.
> Its functional significance remains currently unexplained.
>
>
> The growth center is the region of the long bone where
> - bone of periosteal origin (cortical bone) is the thickest:
> - a local increase in bone mass could easily occur (through an inhibition of bone remodeling turning compact into cancellous bone):
> this pattern might be the evolutionarily easiest way to increase compactness only slightly,
> but in ichthyosaurs, remodeling does occur:
> increased compactness occurs through active remodeling characterized by excessive secondary bone deposits, which clearly does not appear as the cheapest strategy.
>
>
> Strongly reduced femora were analysed only in Basilosaurus (Cetacea) & the hind-limbed snake Eupodophis.
> - Basilosaurus shows a strong increase in bone mass (Houssaye et al. 2015),
> - Eupodophis displays a terrestrial-like inner structure, with no increase in bone mass:
> the occurrence of OS in limbs under evolutionary reduction cannot be attributed to general bone reduction only.
>
>
> Some taxa display comparable micro-anatomical features in their humeri & femora (e.g. plesiosaurs, crocs),
> but this is sometimes clearly not the case, e.g. in some placodonts, pistosaurs, the plesiopedal but hydropelvic mosasauroid Dallasaurus:
> this implies distinct constraints acting on these bones, and thus distinct uses.
>
> There are also taxa where usage & morphology differ between fore- & hind-limb,
> but micro-anatomy does not (e.g. the pachypleurosaurs, the nothosaur Ceresiosaurus, the leatherback turtle), requiring other’s explanations than bio-mechanics of use.
>
>
> As previously observed, PO is a specialization primarily of the axial skeleton.
> Based on the spms for which micro-anatomical data are available, PO appears always ass.x a strong increase in inner bone compactness, but also with “incomplete OS”:
> is this also the case in some of the taxa for which micro-anatomical data are not available yet, e.g.
> some choristoderans, juvenile plesiosaurs & the rhynchocephalian Ankylosphenodon?
> Could PO be
> (1) advantageous in limiting movements between adjacent vertebrae, and thus in straightening the body?
> (2) the consequence of the need to increase body mass beyond the strongest increase in inner bone mass (through OS) functionally possible for the organism?
> (3) both?
>
> A functional role beyond an increase in bone mass appears nevertheless poorly possible for ribs.
> The occurrence of bone mass increase in the rib-cage of some taxa (Basilosaurus, some plesiosaurs e.g. Pachycostasaurus) remains unexplained because these were highly aquatic.
>
>
> Some really peculiar combinations of bone specializations occur, e.g.
> - a strongly OS humerus + a tubular (terrestrial-like) femur & spongious vertebrae in the mosasauroid Dallasaurus,
> - the extremely tight spongiosa of the vertebra + strongly OS long bones of Champsosaurus.
> To understand these peculiar extinct taxa, much work on extant taxa for which ecological data are available is required.
>
>
>
> Signals in bone microanatomy
>
> Various studies have shown that there is a phylogenetic signal in bone micro-anatomical adaptation.
> As for the specializations ass.x an aquatic lifestyle, important convergences clearly occur.
> This shows the strong impact of functional constraints, but there is naturally a phylogenetic heritage.
> However, the phylogenetic signal can also be artificially strengthened, depending on the sample analyzed:
> only a limited nr of evol.events corresponding to a shift from a terrestrial to an aquatic lifestyle occur.
> Structural constraints ass.to physical & architectural requirements also act on bone micro-anatomy, as previously described for the variations in the tightness of the trabecular network with size in amniote vertebrae.
>
> But by comparing mosasaurs, ichthyosaurs & plesiosaurs of similar size, it also clearly appears:
> a functional impact, ass.x constraints essentially linked to swimming-mode, is superimposed to this structural effect.
>
> All these effects should thus be taken into consideration when analyzing bone micro-anatomical adaptation.
>
>
>
> Conclusion
>
> This review of the state of the art clearly illustrates the wide diversity in microanatomical patterns observed in bones of (semi)aquatic amniotes.
>
> Unfortunately, because of the limits of the current terminology, these have been generally dichotomized under the general terms
> - bone mass increase &
> - spongious organization.
>
> Our review also highlights the variation in combination of these different patterns among different skeletal elements.
>
> To better understand the link between functional requirements & bone inner structure, taking phylogenetic & structural constraints into consideration, it appears now necessary to analyze differences within specific taxonomical groups in more details, as well as among taxa from different groups, but with closely similar ecologies.
>
> 3D analyses should make it possible to obtain data that differentiate much more precisely between taxa.
>
> More detailed analyses on extant taxa will make much more precise inferences on the paleo-ecology of extinct taxa possible, notably of the forms displaying currently enigmatic combinations of micro-anatomical features in their skeleton.
>
> _____
>
> This beautiful comparative work suggests H.erectus (PO+OS, e.g. occiput, femora, parts of pelvis ...) spent most of their time bottom-dwelling (& back-floating for between dives - sea-otter-like?), probably mostly in salt waters.
> It also helps explain H.erectus fur loss.
> Apparently our ancestors were still partly aquatic until not long ago (late-Pleistocene?).
>
> Only complete idiots still believe erectus ran antelopes to exhaustion.
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