Osteoderm

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Osteoderm of the sauropod dinosaur Ampelosaurus atacis . The titanosaurs are so far the only sauropod group in which such skin ossification has been reliably detected.
Osteoderms of the extinct crocodile Deinosuchus
A massive dermal armor, here at Sauropelta , offered ankylosaurs protection from predators .

As osteoderms (from ancient Greek ὀστέον ostéon 'bone' and δέρμα dérma 'skin'), osteodermata or cutaneous bone plates are usually flat, dome-shaped or spiky pointed bones located and formed within the dermis in terrestrial vertebrates . Osteoderms are part of the so-called dermal skeleton , which also includes all other bones that the skin plays a part in, including the roof of the skull and the collarbones . Adjacent osteoderms can, depending on their function, be connected by joints and ligaments (as in crocodiles and their early relatives ) or interlocked or fused along their edges (as in armadillos ). In this way, osteoderms can form coherent bone armor .

Osteoderms are the only group of mammals found in today's reptiles , amphibians and armadillos . They have been found in many extinct groups that are not closely related to one another and were already present in some early land vertebrates of the Carboniferous (around 360 to 300 million years ago). Contrary to the similarities in the structure of their bone tissue , the scattered occurrence of osteoderms indicates that in the course of the tribal history they often developed independently of one another through convergent evolution and that at most a certain predisposition to the formation of osteoderms can be traced back to a common ancestor.

The position and extent of body cover with osteoderms are related to their function: In addition to armoring , osteoderms can be part of an active defense mechanism, for example a tail club, serve for display, provide a calcium supply for times of increased calcium requirement or for regulating the heat balance matter. In animals that live in water, osteoderms can help increase body density. In many cases, rows of dorsal osteoderms prevent the trunk from moving excessively and can thus keep the spine away from harmful stresses that occur when moving on land .

The scientific investigation of osteoderms, including their appearance (within the framework of morphology ), their tissue structure (within the framework of skeletal histology ) and their development processes (within the framework of developmental biology ), in order to draw conclusions about the function, lifestyle and phylogenetic history of extinct animals , is currently an active field of research in paleobiology .

Differentiation from other skeletal components of vertebrates

Osteoderms are bony components of the skin of terrestrial vertebrates which, together with the bones of the skull and shoulder girdle, which the skin is involved in, form what is known as the dermal skeleton . They differ from the skeletal elements in the skin of some fish by the absence of dentin (dentin), which differs from bone tissue in its composition, microstructure and type of education.

Ossified skin scales , which occur both in land vertebrate- like fish (the meat-finisher group ) and in many primeval terrestrial vertebrates of the Temnospondyli group, are often considered to be another type of skin ossification to be distinguished from osteoderms. The delimitation is based on the fact that ossified skin flakes, in contrast to osteoderms, show recurring overlapping patterns that are also typical for non-ossified skin flakes, have no ornamentation on the outside and are formed further outside in the dermis near the border with the epidermis . So there are some of the primeval terrestrial vertebrates in which both ossified scales and osteoderms appear - sometimes lying on top of one another.

Some authors, on the other hand, refer to the diversity of osteoderms in terms of their location, shape, surface structure, compactness, tissue structure and ontogenesis and understand ossified skin flakes as part of the spectrum of land vertebrate skin ossifications for which they collectively use the term "osteoderm". However, these authors also define ossification of the skin in fish - including those without any dentin component - and the scales of the caecilica with dentin components as not belonging to the osteoderms. In the following, the term osteoderm is used in the narrower sense, i.e. without including ossified terrestrial vertebrate skin flakes.

Skeletogenesis and bone microstructure

The formation of the skeleton in the course of the development of an individual from the embryo to the adult animal is also known as skeletogenesis . Osteoderms and other dermal bones differ in their skeletogenesis from many bones of the vertebrate inner skeleton in that they do not emerge as so-called replacement bones from a bone precursor consisting of cartilage . The only exception were osteoderms of the extinct marine reptile group Placodontia , which had parts of cartilage. Most osteoderms are either the product of (intra-) membranous ossification or the conversion of skin tissue into bone, also known as metaplastic ossification , or a combination of both.

Intramembranous and metaplastic ossification

Skeletogenesis of the osteoderms: A – C metaplastic ossification followed by intramembranous ossification, D – F intramembranous ossification. Blood vessels and bone cells have been omitted from the schematic representations.

In intramembranous ossification, a compression of connective tissue cells forms within the dermis , which secrete osteoid . The osteoid tissue is of bone-forming cells ( osteoblasts ) and bone skin (periosteum) rearranges. These convert the osteoid into real bone tissue and, similar to the growth of the inner skeleton, continuously store new bone tissue from the inside out. The bone tissue of osteoderms formed in this way usually consists of parallel-fiber bone or lamellar bone. In these types of bone tissue, the collagen fibers, on which the mineral hydroxyapatite is deposited in the form of fine crystals ( crystallites ), are aligned parallel to the bone surface. Differences in growth cause differences in the density, cell structure and fiber orientation of successive layers, which are visible as zoning under the light microscope .

Osteoderm tissue, which has emerged from the transformation ( metaplasia ) of dense skin tissue, lacks this layer. The metaplastic ossification leads to apatite mineralization of areas of the dermis without a periosteum. The connective tissue fibers of the skin, which are often arranged in regular interweaving, remain after the transformation as mineralized structural fibers in the metaplastic bone. This tissue can be surrounded on the outside by bone, which was formed by a periosteum.

Remodeling and Sharpey Fibers

Often, in the course of the development of the osteoderm carrier, an internal remodeling of the osteoderm takes place comparable to the remodeling of other bones. In the process, bones that are formed (primary bones ) are first dissolved by osteoclasts and a spongy middle layer is created that resembles the cancellous bone of other bones and is also known as Diploë . Cavities can in turn be lined with newly formed bone (secondary bone). The more compact cortical tissue of the osteoderm, which surrounds the middle layer, often contains fibers of the adjacent skin tissue growing in perpendicular or oblique to the surface as well as muscles or ligaments , which are referred to as Sharpey fibers .

Timing of osteoderm formation in the course of individual development

The onset of osteoderm skeletogenesis in the course of individual development is delayed in many osteoderm carriers compared to the formation of the rest of the skeleton: In some osteoderm-bearing species of frogs , the absence of osteoderms shortly after metamorphosis indicates the late formation of these skin ossifications.

In the Mississippi alligator , osteoderm formation begins much later than that of the rest of the skeleton (including the dermal bones of the skull, which ossify particularly early) after the hatchlings. The ossification of the shell starts with the osteoderms in the area of ​​the cervical vertebrae and continues from there gradually to the side and towards the tail.

A late formation of the osteoderms in the course of the individual development can also be observed in the scale lizards: The pearl-shaped osteoderms of the crusty lizards belong to the skeletal elements of this group, whose ossification occurs the last. In the case of blindworms , even adolescent (subadult) individuals still show no signs of skin ossification (as is the case in adult animals).

In the nine-banded armadillo , the skeletogenesis of the osteoderms begins later than that of the rest of the skeleton, but before birth in the course of embryogenesis. The formation of the osteoderms takes place asynchronously, i.e. staggered in time from armor segment to armor segment and within each armor segment. Usually the ossification of the individual segments begins at the middle front end and continues (in a similar way to the alligator back shield) to the side and towards the tail.

For extinct groups of osteoderm-bearing animals, individuals of different ages and stages of development are only sometimes available, so that the question of the timing of osteoderm development is rarely answered in the same way as for stegosaur osteoderms, whose formation and growth is significantly delayed, as in today's osteoderm carriers leaves.

Occurrence, origin and evolution

Osteoderms are found in representatives of today's frogs , turtles , lizards , crocodiles and armadillos and thus in three of the four conventional classes of terrestrial vertebrates. In addition, there are numerous records in prehistorically extinct groups that are only documented by fossils . The following branch diagram shows the relationships between the terrestrial vertebrates in a simplified manner and, with the exception of the little-known reptile group Hupehsuchia , lists all groups for which osteoderms have been identified (simplified diagram based on Ruta et al. 2003 and Hill 2005):

 Terrestrial vertebrates  

 Primordial terrestrial vertebrates: possibly in Colosteiden


   


 Modern amphibians : in different groups of the frogs ( toad frogs , tree frogs , saddle toads and leptodactylids )


   

 temnospondyle amphibians : in the groups Edopoidea , Dissorophoidea , Dvinosauria , Zatracheidae and Stereospondylomorpha



   

 Main group of the Amniota: in Chroniosuchiern and Microsauriern


  Amniota  

 Synapsida : in varanopids and mammals belonging to the group of articular animals


  Reptiles  

 Parareptiles : among the procolophonids and pareiasaurs


   



 Scale reptiles : among crawlers , geckos and skinks


   

 ? Turtles : bone shell ; single osteoderms can occur in softshell turtles


   

 Sauropterygia : in placodont animals , especially in the cyamodontoid group



   

 Archosaurs stem group: Euparkeria and Proterochampsiden


  Archosaurs  

 Crurotarsi : in almost all groups, including crocodiles and aetosaurs


   

 Ornithodira : in dinosaurs , especially in the Thyreophora and Titanosauria groups










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Since the individual osteoderm carrier groups listed here often belong to higher-level groups whose last common ancestor did not show any ossification of the skin, so in the case of the secondary articulated animals , which, like all other mammalian groups today, are descended from a mammal without osteoderms, osteoderms have often been independent of one another in the course of the tribal history of the terrestrial vertebrates have arisen through massive convergent evolution . For example, within the archosaurs, the osteoderms of the crocodiles , the thyrophore dinosaurs and the sauropod dinosaurs are not homologous structures , that is, they cannot be traced back to a common precursor osteoderm type. According to current knowledge, osteoderms have also emerged several times in parallel within the frog.

Osteoderms as "depth homology"?

Embryological studies show that the formation of functionally identical organs in the course of embryonic development of distantly related groups - for example the formation of the legs of arthropods and terrestrial vertebrates - is based on the action of the same shape-forming (morphogenic) substances and the activity of the same shape-forming genes . Therefore, have evolutionary biologists and developmental biologists the theory of low homology established (deep homology), which states that the independent occurrence of similar features in appearance ( phenotype ) of different groups of organisms in a homology are the mechanisms that control the development of these qualities in the course of development returned, can.

The hypothesis that osteoderms are "depth homologous" states that the ability to form osteoderms first appeared within the common ancestors of all osteoderm carriers and was then passed on to the offspring. After that, the osteoderm-forming genetic control mechanisms were initiated in some descendants and not in many other descendants, although they have the necessary genetic predispositions. In comparative studies on osteoderm bone tissues, “depth homology” is often cited as the reason for similarities in the structure and development of the osteoderms.

The oldest evidence and the question of the origin of the first osteoderms

Row of dorsal osteoderms in a Permian temnospondylum amphib of the group Dissorophidae .

Already the extinct groups of the meat- floss (Sarcopterygii) of the Devonian and Carboniferous , which are closer to the terrestrial vertebrates (their descendants) than any current fish group and which are also known as the tribal group of the terrestrial vertebrates or "tribal terrestrial vertebrates", have ossified scales on which are attached bring back the ossified skin flakes of the early terrestrial vertebrates.

Osteoderms in the narrower sense are documented for the first time for primeval land vertebrates of the Carboniferous and Permian : some representatives of the Temnospondyli , from which today's amphibians ( Lissamphibia ) probably arose , and Chroniosuchier , which probably belong to the parent group of amniotic animals ( Amniota ), have comparatively thick bone plates which, like the bones of the skull, often show a sculptured surface with an ornamental relief. For osteoderms of both groups, the ability to form skin bone by transforming skin tissue (metaplastic ossification) has been demonstrated, which has not been proven for fossil fish.

Insofar as the osteoderms of the Temnospondyles and today's amphibians are homologous with those of the Chroniosuchier, reptiles and mammals with regard to the osteoderm development processes and their control mechanisms, the last common ancestor of all these groups, who probably lived in the early Carboniferous approx. 350 million years ago, must already have been possessed the potential to develop osteoderms. Thus the origin of osteoderms would possibly be the earliest evolution of land vertebrates after landfall been associated bone fish in Devon. A later representative of the near-native group Colosteidae shows skin ossification, which is sometimes interpreted as osteoderms.

Evolutionary trends within individual groups of osteoderm carriers

Row of dorsal osteoderms in a reptile-like terrestrial vertebrate of the Chroniosuchia group

In several groups, osteoderms appear in primitive representatives initially as a series of narrow shields along the midline of the back, while later representatives have a more extensive osteoderm covering or a bone shell: For pareiasaurs, such a trend is from large primitive forms such as Bradysaurus with a central row of osteoderms to smaller forms assumed as anthodon , whose back was completely covered with osteoderms. Within the placodon animals , a group of marine reptiles, a middle row of osteoderms is found in Placodus and other placodontoidea , while the later derived cyamodontoidea have turtle-like carapaces. Chroniosuchier show rows of dorsal osteoderms of different widths, although very broad osteoderm systems, as in Chroniosaurus , are among the oldest and possibly most primeval, so that a trend towards increased armor is not clear.

In close relatives of the archosaurs such as Euparkeria and Proterochampsa as well as in many primeval Triassic archosaurs of the crocodile line ( Crurotarsi ) such as phytosaurs , some Rauisuchi , ornithosuchids and sphenosuchi , simple osteoderm systems also initially appear, which consist of one or two rows along the midline of the back . The more extensive shields and armor of the aetosaurs and crocodiles emerge from such forms . The aetosaurs, which only appeared in the Triassic, carry a centrally located double row of strongly broadened osteoderms, which are supplemented by rows of lateral osteoderms and a belly armor. In the later crocodiles, especially the representatives of the Eusuchia group , to which all modern crocodiles also belong, there are usually four or more rows of osteoderms that cover a large part of the back of the body. Often a belly armor is added.

Within the crocodiles, a flexibilization of the back-osteoderm system can be observed: While in primitive crocodiles the shield segments connected to the spine by ligaments show a clear overlap along the longitudinal axis of the body and allow only little movement against each other due to cone-shaped extensions and laterally angled sections, with today's crocodiles Crocodiles hardly overlap. This change was probably related to the reconstruction of the spine: ancient crocodiles had biconcave (amphicoele) vertebrae and therefore had vertebral joints that were susceptible to buckling loads, but which were blocked by the interlocking osteoderms. Representatives of the Eusuchia, on the other hand, have convex- concave (procoele) vertebrae and consequently less stress-prone vertebral joints that do not require blocking by the osteoderm system.

In the shield-bearing dinosaurs (group Thyreophora ), the osteoderms show a trend towards extreme growth in thickness: Compared to the relatively flat back plates of the similar forms Scelidosaurus and Scutellosaurus , the derived ankylosaurs and stegosaurs have spurs, long spines, upright back plates and tail lobes. These changes are apparently based on a function change or an expansion of functions.

Origin of the turtle shell

The bony back armor of the turtle (right), the segmentation of which differs from the horn armor lying above it (left), could have its origin in the merging of osteoderms.

Two opposing hypotheses are discussed about the origin of the turtle shell:

  1. Since embryological studies show that the plate-like bones that build up the shell emerge in the course of embryonic development from outgrowths of the ribs and vertebrae, it can be assumed that the turtle shell is a new formation within the trunk line of the turtle that is not on skin bones of a forerunner form can lead back.
  2. The turtle shell emerged from the growing together of many individual osteoderms and finally from the growing together of the osteoderm shield with the underlying inner skeleton.

In agreement with the second hypothesis that skin bones were also involved in the formation of the shell, structural fibers of ossified skin tissue (signs of metaplastic ossification) were found in the shell segments of early turtles. If, in the course of tribal history, there has been a fusion of the embryonic structures for the bones of the shell and for the ribs or vertebrae, then that would be one reason why there are no additional ossification centers in the skin in the embryonic development of today's turtles, although the shell was originally actually originated from the osteoderms and not from the internal skeleton.

The recently published discovery of Chinlechelys tenerstesta from the Late Triassic Chinle Formation of New Mexico has particularly contributed to the debate on the origin of the turtle shell. Chinlechelys shows ribs in the area of ​​the thorax, which are hardly connected to the overlying costal bones of the back armor, which contradicts an interpretation of these armor segments as rib protrusions according to the first-mentioned theory. The presence of osteoderms along the neck and tail of chinlechelys suggests, according to the study's authors, that the shell is merely an agglomeration of part of the osteoderms that covered the bodies of original turtles. Since Chinlechelys and other Triassic turtles such as Proganochelys quenstedti and Proterochersis robusta are likely to live on land based on the characteristics of the limbs and the bone tissue of the shell, the turtle shell, like the osteoderm systems of crocodiles and their relatives , could have originated on land.

The authors of the first description of Odontochelys semitestacea from the late Triassic Falang Formation of southwest China , published in 2008, come to almost contradicting conclusions : Since the skeleton lacks osteoderms and only the abdominal shell (plastron) is present as a coherent bony shield, while the back armor (carapace ) similar to how in an embryonic stage of development of today's turtles only the middle row of the neural bones is present and the ribs are spread in the trunk area, Odontochelys is proof of the evolutionary development of the back armor after that of the belly armor and for an emergence of the turtle armor without osteoderm precursors from the Ribs and vertebrae of the internal skeleton. Since Odontochelys comes from marine deposits and the (sole) formation of a belly armor could most likely have benefited a resident of the sea coast, the first turtles were probably aquatic. This interpretation was questioned, among other things, in an article published parallel to the first description: Similar to later aquatic turtles such as the softshell turtles , Odontochelys had back armor , but was not fully ossified. However, this does not indicate the general condition of the shell of the first turtles, but only the regression of the back shell in an already specialized form adapted to the water.

function

The osteoderm shield of the aetosaur is divided into a neck, trunk and tail section (cervical, dorsal and caudal armor)
The carapace of the Ice Age mammal Glyptodon consisted of interlocking osteoderms

Armor

The protection against predators by an osteoderm armor is seen in many groups as an important factor that triggered the evolution of osteoderm systems or influenced them to a significant extent. As with many thyrophore dinosaurs and some lizards, areas of the skin surface can develop, which are covered by many separate skin bone plates, usually located at a small distance, or the extensive formation of osteoderms that touch each other and by ligaments , overlapping areas or even complicated joints are more or less movably connected to one another. The latter applies, among other things, to the back armor of many crocodiles, aetosaurs , some Chroniosuchids and some pareiasaurs . Back armor can be supplemented by a belly armor - for example with some crocodiles and aetosaurs.

Rigid back and / or abdominal armor made of interlocking or welded osteoderms occur in placodont animals of the group Cyamodontoidea , armored secondary animals (Cingulata) and probably in turtles (see discussion of the osteoderm origin of turtle shells above), with armadillos as survivors Representatives of the Cingulate have several mutually movable shield segments, each of which consists of immovably interlocking (but rarely welded) osteoderms.

Active defense

Spiky osteoderms on the tail end of Stegosaurus
Tail lobe of Ankylosaurus

Club-shaped thickened or prickly osteoderms near the tail end occurred convergently in stegosaurs , ankylosaurids , primeval sauropods such as Shunosaurus and Spinophorosaurus as well as armored articulated animals of the extinct group Glyptodontidae . Such skin bones did not function as part of the armor, but instead formed a tail weapon, which is supported by the structure of their bone tissue and biomechanical modeling of possible strikes.

Thermoregulation

The fact that osteoderms contributed to thermoregulation by promoting heat exchange with the environment was suggested , among other things, for the upright back plates of the stegosaurs . These are lightly built and tightly crisscrossed by vessels, which seemed to suggest that instead of a protective function, they had a solar collector- like function. In a more recent study, however, it was shown that the vessels of the osteoderm inner and outer sides were not connected in the sense of a circulatory system, but the tight vascular system was more of a "structural artifact ", which is not due to a thermoregulatory function in the adult animal , but rather on the intermittently high growth rates of the plates, which required good blood circulation. A purely external vascular system for heat exchange could have existed, but there is no evidence for this. In addition, the outer osteoderm sheath made of horn also required a supply from blood vessels. Therefore, a thermoregulatory function, if at all, only occurred in a few stegosaurs and at most played a subordinate role.

The osteoderm system of today's Mississippi alligator has also been considered to have helped regulate body temperature. Thermoregulation was also suggested as a possible function in a recent histological study for the lightly built plates and spines of some ankylosaurs of the subgroups Ankylosauridae and Polacanthidae , which have networks of pipe-like vessels similar to those of the alligator osteoderms.

display

For the towering plates and laterally projecting spines some stegosaur , Ankylosaurier and Aetosaurier is drawn taking into account their histological features into account that they do not or not only served as a defensive facilities or organs for thermoregulation, but (also) the display under the Imponierverhaltens against Sexual partners or intra-species competitors.

The hypothesis that the plate-shaped stegosaur dorsal osteoderms were mainly used for display follows the observation that their vascular system hardly supported fluid circulation as in an organ for thermoregulation and that these osteoderms developed very late in comparison to bones of the inner skeleton and therefore possibly only during sexually mature animals had a function. In addition, there is the argument that the stegosaur back plates in particular enlarged the body silhouette significantly when viewed from the side and that for this reason animals with relatively large plates might have an advantage in sexual selection .

The stegosaur back plates possibly served more to impress than to thermoregulate.

The spiky and plate-shaped osteoderms of some ankylosaurs could only have had a secondary protective function. Especially the spines of members of the Polacanthidae family and the armor plates of some ankylosaurids have bone tissue of poor compactness and, like the stegosaur dorsal plates, may have been used primarily for intra-species display. The same may be true of some ankylosaurid tail lobes, which from a biomechanical point of view were not all equally suitable as weapons of defense.

For the closely related aetosaurs Longosuchus meadei and Lucasuchus hunti , which are similar in their spatial and temporal distribution, the hypothesis was made that they were animals of different sexes of the same species, whose differences in the shape of the osteoderm back armor were misinterpreted as species-specific characteristics have been. Such gender differences would, in turn, indicate the influence of sexual selection in the evolution of the shell and osteoderms as a means of display. However, based on the current state of knowledge, the evidence for this hypothesis is insufficient.

For the horn and spiked skulls of the ceratopsid dinosaurs , a function for display or, alternatively, for intra-species rival fights is also being considered, and skull injuries can be demonstrated for Triceratops that can be traced back to fights with rivals or predators. However, it has not been conclusively clarified to what extent the horns, spines and plate-shaped attachments of the ceratopsid skull are skin formations that can be compared with osteoderms, or skeletal elements that can be distinguished from them.

Part of the torso carrying system

In crocodiles like this fossil alligator, back osteoderm systems are part of the supporting structure of the trunk.

Movement on land requires a different physique than swimming in water, because on land there is no static buoyancy of the water as a counterforce to the weight force and the points of application of the supporting forces and thrust forces of the limbs acting instead differ from those of the weight force (see position of the body's center of gravity ) . In four-legged runners, the spine and back muscles in particular contribute to the supporting structure of the torso, which arches over the area between the front and rear limbs.

A special feature of the crocodile's trunk support system is the criss-cross tension of the back muscles between the centrally located back osteoderms, which are each firmly connected to a vertebra by cartilage and ligaments (belt ligaments ), and the transverse processes of another distant vertebra. At an angle of less than 10 ° to the longitudinal axis of the body, around five segments of the spine are spanned. In the course of the tribal history of the crocodiles, this type of construction enabled even large representatives to walk ashore without the belly touching the ground and plays an important role , especially for rural dwellers and representatives of the group who live amphibiously . In addition to anchoring the muscles, the osteoderm system protects the spine by overlapping and wedging neighboring osteoderms from damaging torso bends and the resulting shear and torsional loads , which was particularly important for the more vulnerable vertebral joints of primitive crocodiles.

Also for the back osteoderm series of the Chroniosuchier and the Dissorophiden , a group of temnospondylic amphibians , functions in connection with locomotion on land similar to those of the crocodile osteoderms are discussed.

Increasing the density of the body in aquatic life

Osteoderm pachyosteosclerosis.png

Just as the bones of the inner skeleton differ in their compactness, the compactness of the osteoderms, that is, the proportion of bone volume in their total volume, can vary depending on the lifestyle of the osteoderm carrier. Compared to related land dwellers, land vertebrates, which are more adapted to aquatic life, show a reduction in the cavities inside the bones (increased compactness due to osteosclerosis ) or a thickening of the cortical tissue due to increased external deposition of bone tissue ( pachyostosis ) or a combination of both (pachy osteosclerosis). The background to the greater compactness and increased bone substance in aquatic animals is the function of the skeleton as an aid to increasing the density of the entire body, i.e. it helps to keep the animal under water.

A high compactness of the osteoderms, which is related to the aquatic way of life, can be found among other things in some temnospondyl amphibians , placodont animals and extinct sea crocodiles.

Protection against fluid loss

The armor of some placodon animals consisted of osteoderms of high compactness, the weight of which counteracts the buoyancy forces in the water.

Since osteoderms can reduce the transport of fluid through the skin, it was considered that the ossification of the frog's corrugation might have served to protect against dehydration. However, these effects play only a minor role, since in osteoderm-bearing frogs, contrary to the protection against loss of fluid, the tissue between the osteoderms and epidermis is often well supplied with blood and other protective mechanisms such as the secretion of skin wax exist.

Calcium reservoir

The formation of osteoderms from hydroxyapatite offers the possibility of compensating for a temporary calcium deficiency by breaking down the osteoderm bone substance without having to resort to the substance of other skeletal parts. For the aquatic Temnospondylum gerrothorax , trough-like depressions on the surface of the osteoderm at different times of osteoderm growth are documented, which indicate recurring phases of resorption of bone substance. This histological finding can be related to the habitat of gerrothorax : The Temnospondyle inhabited brackish coastal areas that were subject to regular fluctuations in the salt content , which had an effect on the animals' calcium requirements.

Studies on the bone histology of the limb bones and osteoderms of today's Mississippi alligator show that bone remodeling is particularly pronounced in the osteoderms of females, which is related to the increased calcium requirement for the formation of the eggshell in the time before the egg is laid. In addition, the nitrogen and carbon isotopic composition of the osteoderm bone substance differs significantly from that of the limb bones and the carbon and nitrogen sources in the alligators' habitat, which also indicates the frequent alternation of bone substance storage and dissolution.

Even for adult titanosaurs whose osteoderms have not formed effective protective armor, the function of the osteoderms as calcium reservoirs is being considered.

Osteoderms as a research subject

History of exploration

The Russian paleontologist Alexei Petrovich Bystrow took on a pioneering role in the histological examination of fossil skeletons in the middle of the 20th century and, among other things, dealt with the skin bones of early terrestrial vertebrates. While previously osteoderms were mostly only taken into account when describing individual species and smaller groups, several research projects followed in the 1960s to 1980s which were devoted to the osteoderms of various large groups living today.

Studies of the microstructure of fossil bones, including the examination of thin sections of bones with the polarizing microscope , were revived in the 1980s and 1990s, especially by the research work of Armand de Ricqlès' group and subsequently belonged to the standard methods of paleobiology , which in many cases have also been applied to osteoderms. Many studies of the 2000s were devoted to the osteoderms of individual fossil and living groups today and tried to elucidate their origins and changes in the course of individual development , their phylogenetic change and their function (s). In the last ten years in particular, they have led to a more diverse overall picture of the biology of osteoderms.

Systematics and phylogenetics

In some groups of extinct osteoderm carriers such as the Chroniosuchiern , aetosaurs and shield-bearing dinosaurs , osteoderm series and armor belong to the skeletal elements that change particularly quickly in the course of evolution and in which subgroups and species differ most clearly from one another. Therefore, new fossil finds from these groups can be systematically classified on the basis of features of external shape, bone tissue structure, and the arrangement of the osteoderms. In addition, osteoderm characteristics of these groups are used for research on the history of the tribes ( phylogenetics ), which aims to clarify the relationships and the course of tribal history. In particular, the phylogenetic analyzes of the aetosaurs are mostly based on features of the particularly diverse osteodermic shell.

In other groups, osteoderm features are only of secondary importance for the identification of kinship relationships, but are of genealogical importance because their change in shape is accompanied by a change in function - as in crocodiles, whose osteoderm system is closely related to the functioning of the musculoskeletal system - and therefore, factors that control the evolution of these groups can be identified.

Developmental biology

More recent studies on osteoderm histology and on the skin of osteoderm carriers who are alive today mostly attempt to make statements about the structure, development and changes in osteoderms in the course of individual development. This is necessary because, on the one hand, an evolutionary change in the appearance and function of the osteoderms often manifests itself in the form of changes in the formation and growth processes and, on the other hand, a common origin of different types of skin ossification can be proven or refuted based on similarities and differences in the developmental processes . For example, based on developmental findings, the osteoderms of certain extinct reptile groups could be excluded as precursor structures of the turtle shell. With regard to the need to clarify evolutionary processes with the aid of developmental methods and data, osteoderms are a typical research subject in evolutionary developmental biology .

Bibliography

  • H. Francillon-Vieillot, V. de Buffrénil, J. Castanet, J. Géraudie, FJ Meunier, J.-Y. Sire, L. Zylberberg, A. de Ricqlès: Microstructure and mineralization of vertebrate skeletal tissues. In: JG Carter (Ed.): Skeletal Biomineralization: Patterns, Process and Evolutionary Trends. Van Nostrand Reinhold, New York 1990, pp. 471-548.
  • WG Joyce, SG Lucas, TM Scheyer, AB Heckert, AP Hunt: A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell. In: Proceedings of the Royal Society. Series B 276, 2009, pp. 507-513
  • N. Klein, TM Scheyer, T. Tütken: Skeletochronology and isotopic analysis of a captive individual of Alligator mississippiensis Daudin, 1802. In: Fossil Record 12, 2009, pp. 121-131.
  • RP Main, A. de Ricqlès, JR Horner, K. Padian: The evolution and function of thyreophoran dinosaur scutes: implications for plate function in stegosaurs. In: Paleobiology. 31, 2005, pp. 291-314.
  • R. Ruibal, V. Shoemaker: Osteoderms in anurans. In: Journal of Herpetology. 18, 1984, pp. 313-328.
  • TM Scheyer, PM Sander: Histology of ankylosaur osteoderms: implications for systematics and function. In: Journal of Vertebrate Paleontology. 24, 2004, pp. 874-893.
  • MK Vickaryous, J.-Y. Sire: The integumentary skeleton of tetrapods: origin, evolution, and development. In: Journal of Anatomy . 214, 2009, pp. 441-464.
  • F. Witzmann, R. Soler-Gijón: The bone histology of osteoderms in temnospondyl amphibians and in the chroniosuchian Bystrowiella . In: Acta Zoologica . 91, 2010, pp. 96-114.

Individual evidence

  1. ^ Wilhelm Gemoll : Greek-German school and hand dictionary . G. Freytag Verlag / Hölder-Pichler-Tempsky, Munich / Vienna 1965.
  2. ↑ a b c d e f g H. Francillon-Vieillot, V. de Buffrénil, J. Castanet, J. Géraudie, FJ Meunier, J.-Y. Sire, L. Zylberberg, A. de Ricqlès: Microstructure and mineralization of vertebrate skeletal tissues. In: JG Carter (Ed.): Skeletal Biomineralization: Patterns, Process and Evolutionary Trends. Van Nostrand Reinhold, New York 1990, pp. 471-548.
  3. ↑ a b c d e f g h i j k M. K. Vickaryous, J.-Y. Sire: The integumentary skeleton of tetrapods: origin, evolution, and development. In: Journal of Anatomy. 214, 2009, pp. 441-464.
  4. ^ A b F. Witzmann: The evolution of scalation patterns in temnospondyl amphibians. In: Zoological Journal of the Linnean Society. 150, 2007. pp. 815-134.
  5. ^ A b c d e J. Castanet, H. Francillon-Vieillot, A. de Ricqles, L. Zylberberg: The skeletal histology of the Amphibia. In: H. Heatwole, M. Davies: Amphibian Biology. Vol. 5: Osteology. Surrey Beatty & Sons, Chipping Norton, 2003, pp. 1598-1683.
  6. ↑ a b c d e f g h i F. Witzmann, R. Soler-Gijón: The bone histology of osteoderms in temnospondyl amphibians and in the chroniosuchian Bystrowiella . In: Acta Zoologica. 91, 2010, pp. 96-114.
  7. ↑ a b c d e f g R. Ruibal, V. Shoemaker: Osteoderms in anurans. In: Journal of Herpetology. 18, 1984, pp. 313-328.
  8. ↑ a b K. V. Kardong: Vertebrates - comparative anatomy, function, evolution. 2nd edition, WCB McGraw-Hill, Boston, 1998, 747 S; see chapter 5.
  9. ↑ a b c d e f T. M. Scheyer: Skeletal histology of the dermal armor of Placodontia: the occurrence of 'postcranial fibro-cartilaginous bone' and its developmental implications. In: Journal of Anatomy. 211, 2007, pp. 737-753.
  10. ↑ RW Haines, A. Mohuiddin: Metaplastic bone. In: Journal of Anatomy. 103, 1968, pp. 527-538.
  11. ↑ a b c d e f g h i j k R. P. Main, A. de Ricqlès, JR Horner, K. Padian: The evolution and function of thyreophoran dinosaur scutes: implications for plate function in stegosaurs. In: Paleobiology. 31, 2005, pp. 291-314.
  12. ↑ a b M. K. Vickaryous, BK Hall: Development of the Dermal Skeleton in Alligator mississippiensis (Archosauria, Crocodylia) With Comments on the Homology of Osteoderms. In: Journal of Morphology 269, 2008, pp. 398-422.
  13. ↑ L. Zylberberg, J. Castanet: New data on the structure and the growth of the osteoderms in the reptile Anguis fragilis L. (Anguidae, Squamata). In: Journal of Morphology 186, 1985, pp. 327-342.
  14. ↑ MK Vickaryous, BK Hall: Osteoderm Morphology and Development in the Nine-Banded Armadillo, Dasypus novemcinctus (Mammalia, Xenarthra, Cingulata). In: Journal of Morphology 267, 2006, pp. 1273-1283.
  15. ↑ a b c d e f g S. Hayashi, K. Carpenter, D. Suzuki: Different growth patterns between the skeleton and osteoderms of Stegosaurus stenops : implications for plate and spike growth. In: Journal of Vertebrate Paleontology. 29, 2009, pp. 123-131.
  16. ^ RL Carroll, Dong Z.-M .: Hupehsuchus , an enigmatic aquatic reptile from the Triassic of China, and the problem of establishing relationships. In: Philosophical Transactions of the Royal Society of London Series B 331, 1991, pp. 131-153.
  17. ↑ a b M. Ruta, MI Coates, DLJ Quicke: Early tetrapod relationships revisited. In: Biological Reviews. 78, 2003, pp. 251-345.
  18. ↑ a b R. V. Hill: Integration of morphological data sets for phylogenetic analysis of Amniota: the importance of integumentary characters and increased taxonomic sampling. In: Systematic Biology. 54, 2005, pp. 1-18.
  19. ^ A b c d e I. V. Novikov, MA Shishkin, VK Golubev: Permian and Triassic anthracosaurs from Eastern Europe. In: MA Shishkin, MJ Benton, DM Unwin, EN Kurochkin (eds.): The Age of Dinosaurs in Russia and Mongolia. Cambridge University Press, Cambridge 2000, pp. 60-70.
  20. ^ RL Carroll, P. Gaskill: The Order Microsauria. The American Philosophical Society, Philadelphia 1978.
  21. ↑ J. Botha-Brink, SP Modesto: A mixed-age classed 'pelycosaur' aggregation from South Africa: earliest evidence of parental care in amniotes? In: Proceeding of the Royal Society London. Series B, 274, 2007, pp. 2829-2834.
  22. ↑ a b R. V. Hill: Comparative anatomy and histology of xenarthran osteoderms. In: Journal of Morphology 267, 2006, pp. 1441-1460.
  23. ↑ a b c d e T. M. Scheyer, PM Sander: Bone microstructures and mode of skeletogenesis in osteoderms of three pareiasaur taxa from the Permian of South Africa. In: Journal of Evolutionary Biology. 22, 2009, pp. 1153-1162.
  24. ↑ a b c J. M. Parrish: Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. In: Journal of Vertebrate Paleontology. 13, 1993, pp. 287-308.
  25. ↑ a b c T. M. Scheyer, PM Sander: Histology of ankylosaur osteoderms: implications for systematics and function. In: Journal of Vertebrate Paleontology. 24, 2004, pp. 874-893.
  26. ↑ MD d'Emic, JA Wilson, S. Chatterjee: The titanosaur (Dinosauria: Sauropoda) osteoderm record: review and first definitive specimen from India. In: Journal of Vertebrate Paleontology. 29, 2009, pp. 165-177.
  27. ↑ N. Shubin, C. Tabin, S. Carroll: Fossils, genes and the evolution of animal limbs. In: Nature. 388, 1997, pp. 639-648.
  28. ↑ N. Shubin, C. Tabin, S. Carroll: Deep homology and the origins of evolutionary novelty. In: Nature. 457, 2009, pp. 818-823.
  29. ↑ J.-Y. Sire, A. Huysseune: Formation of dermal skeletal and dental tissues in fish: a comparative and evolutionary approach. In: Biological Reviews of the Cambridge Philosophical Society. 78, 2003, pp. 219-249.
  30. ^ A b V. K. Golubev: Revision of the Late Permian chroniosuchians (Amphibia, Anthracosauromorpha) from Eastern Europe. In: Paleontological Journal. 32, 1998, pp. 390-401.
  31. ^ A b c R. A. Long, PA Murry: Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. In: New Mexico Museum of Natural History and Science Bulletin. 4, 1995, pp. 1-254.
  32. ↑ a b c W. G. Parker: Reassessment of the aetosaur 'Desmatosuchus' chamaensis with a reanalysis of the phylogeny of the Aetosauria (Archosauria: Pseudosuchia). In: Journal of Systematic Palaeontology. 5, 2007, pp. 41-68.
  33. ↑ a b c d e E. Frey: The crocodile's support system - a biomechanical and phylogenetic analysis. Stuttgart Contributions to Natural History Series A 426, 1988, pp. 1-60.
  34. ↑ a b c d e f S. W. Salisbury, E. Frey: A biomechanical transformation model for the evolution of semi-spheroidal articulations between adjoining vertebral bodies in crocodilians. In: GC Grigg, F. Seebacher, CE Franklin (eds.): Crocodilian biology and evolution. Surrey Beatty and Sons, Chipping Norton 2000, pp. 85-134.
  35. ↑ a b c d e f g h S. Hayashi, K. Carpenter, TM Scheyer, M. Watabe, D. Suzuki: Function and evolution of ankylosaur dermal armor. In: Acta Palaeontologica Polonica. 55, 2010, pp. 213-228.
  36. ^ SF Gilbert, GA Loredo, A. Brukman, AC Burke: Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution. In: Evolution & Development. 3, 2001, pp. 47-58.
  37. ↑ a b c d W. G. Joyce, SG Lucas, TM Scheyer, AB Heckert, AP Hunt: A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell. In: Proceedings of the Royal Society. Series B 276, 2009, pp. 507-513.
  38. ↑ a b c T. M Scheyer, PM Sander: Terrestrial paleoecology for basal turtles indicated by shell bone histology. In: Proceedings of the Royal Society. Series B 274, 2008, pp. 1885-1893.
  39. ↑ C. Li, X.-C. Wu, O. Rieppel, L.-T. Wang, L.-J. Zhao: An ancestral turtle from the Late Triassic of southwestern China. In: Nature. 456, 2008. pp. 497-501.
  40. ^ RR Reisz, JJ Head: Turtle origins out to sea. In: Nature. 456, 2008, pp. 450-451.
  41. ↑ a b J. A. Clack, J. Klembara: An articulated specimen of Chroniosaurus dongusensis , and the morphology and relationships of the chroniosuchids. In: Special Papers in Palaeontology. 81, 2009, pp. 15-42.
  42. ↑ R. Steel: The fossil crocodiles. Die Neue Brehmbücherei Volume 488, A. Ziemsen Verlag, Wittenberg, 1975, 76 pp.
  43. ↑ MK Vickaryous, BK Hall BK: Osteoderm morphology and development in the Nine-Banded Armadillo, Dasyceps novemcinctus (Mammalia, Xenarthra, Cingulata). In: Journal of Morphology. 267, 2006, pp. 1273-1283.
  44. ^ Y. Zhang: The Middle Jurassic dinosaur fauna from Dashanpu, Zigong, Sichuan. Volume I: Sauropod dinosaur (I). Shunosaurus . Sichuan Publishing House of Science and Technology, Chengdu 1988.
  45. ↑ K. Remes, F. Ortega, I. Fierro, U. Joger, R. Kosma, JMM Ferrer, OA Ide, A. Maga: A new basal sauropod dinosaur from the Middle Jurassic of Niger and the early evolution of Sauropoda. In: PLoS ONE. 4 (9), 2009, e6924. PMC 2737122 (free full text)
  46. ^ A b R. E. Blanco, WW Jones, A. Rinderknecht: The sweet spot of a biological hammer: the center of percussion of glyptodont (Mammalia: Xenarthra) tail clubs. In: Proceedings of the Royal Society. series B 276, 2009, pp. 3971-3978.
  47. ^ A b V. M. Arbor: Estimating impact forces of tail club strikes by ankylosaurid dinosaurs. In: PLoS ONE. 4 (8), 2009, e6738. PMC 2726940 (free full text)
  48. ↑ V. de. Buffrénil, JO Farlow, A. de Ricqlès: Growth and function of Stegosaurus plates: evidence from bone histology. In: Paleobiology. 12, 1986, pp. 459-473.
  49. ↑ MR Seidel: The osteoderms of the American alligator and their functional significance. In: Herpetologica. 35, 1979, pp. 375-380.
  50. ↑ WG Parker, JW Martz: Using positional homology in aetosaur (Archosauria: Pseudosuchia) osteoderms to evaluate the taxonomic status of Lucasuchus hunti . In: Journal of Vertebrate Paleontology. 30, 2010, pp. 1100-1108.
  51. ↑ a b J. H. Horner, MB Goodwin: Ontogeny of cranial epi-ossifications in Triceratops . In: Journal of Vertebrate Paleontology. 28, 2008, pp. 134-144.
  52. ↑ a b T. L. Hieronymus, LM Witmer, DH Tanke, PJ Currie: The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. In: The anatomical record. 292, 2009, pp. 1370-1396.
  53. ↑ a b A. A. Farke, EDS Wolff, DH Tanke: Evidence of Combat in Triceratops . In: PLoS ONE. 4 (1), 2009, e4252.
  54. ↑ D. Dilkes: Comparison and biomechanical interpretations of the vertebrae and osteoderms of Cacops aspidephorus and Dissorophus multicinctus (Temnospondyli, Dissorophidae). In: Journal of Vertebrate Paleontology. 29, 2009, pp. 1013-1021.
  55. ↑ A. de Ricqlès, V. de Buffrénil: Bone histology, heterochronies, and the return of tetrapods to life in water: Where are we? In: J.-M. Mazin, V. de Buffrénil: Secondary adaptations of tetrapods to life in water. Verlag Pfeil, Munich 2001, pp. 289-310.
  56. ^ HB Lillywhite: Water relations of tetrapod integument. In: Journal of Experimental Biology. 209, 2006, pp. 202-226.
  57. ↑ N. Klein, TM Scheyer, T. Tütken: Skeletochronology and isotopic analysis of a captive individual of Alligator mississippiensis Daudin, 1802. In: Fossil Record 12, 2009, pp. 121-131.
  58. ↑ T. da Silva Marinho: Functional aspects of titanosaur osteoderms. Nature Precedings, 2007.
  59. ↑ see list of works in the article Alexei Petrovich Bystrow
  60. ↑ ML Moss: Comparative histology of dermal sclerifications in reptiles. In: Acta Anatomica 73, 1969, pp 510-533.
  61. ↑ A. de Ricqlès, FJ Meunier, J. Castanet, H. Francillon-Vieillot: Comparative microstructure of bone. In: BK Hall (Ed.): Bone. vol. 3. Bone matrix and bone specific products. CRC Press, Boca Raton, 1991, pp. 1-78.
  62. ↑ a b A. B. Heckert, SG Lucas: A new aetosaur from the Upper Triassic of Texas and phylogeny of aetosaurs. In: Journal of Vertebrate Paleontology 19, 1999, pp. 50-68.
  63. ↑ K. Carpenter: Phylogenetic analysis of the Ankylosauria. In: K. Carpenter (Ed.): The Armored Dinosaurs. Indiana University Press, Bloomington & Indianapolis, 2001, pp. 455-483.
  64. ^ ME Burns: Taxonomic utility of ankylosaur (Dinosauria, Ornithischia) osteoderms: Glyptodontopelta mimus FORD, 2000: a test case. In: Journal of Vertebrate Paleontology 28, 2008, pp. 1102-1109.
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