# Static Osteogenesis versus Dynamic Osteogenesis:
## 3 SO Versus DO during Ontogeny and the Origin of Bone Tissue Diversity
In the context of the commonly accepted tetrapod phylogeny, using the parsimony method the authors [32] concluded that (1) the ontogenetic sequence observed in chickens and in rabbits, according to which SO precedes DO, is convergent, and (2) the ontogenetic pattern observed in _Pleurodes_ larvae and _Pogoun_ embryos, according to which SO does not precede DO, corresponds to the elisomorphic condition for both Tetrapoda and Amniota. This evidence is reflected in the different bone architectures between growing small amphibians/squamates and growing mammals/birds. [PERSON] and coworkers [34] tested the hypothesis suggested by [PERSON] et al. [10], according to which \"without preexisting osteocytes only woven bone can form (through SO), because an orderly recruitment of osteocytes can only take place by signals issued by a preexisting osteocyte syncytium\". At the onset of bone formation, the presence of a cohort of stationary osteoblasts is necessary, independently of whether static or dynamic osteogenesis is taking place; without such stationary osteoblasts, the first layers of bone tissue would not contain osteocytes. The authors [34] also affirmed that, in Pleurodes and Pogona, DO occurs even without a rigid mineralized framework, whereas in chick embryos and newborn rabbits it seems to take place over a mineralized support in bone (as observed in perichondral ossification by [PERSON] et al. [7] and by [PERSON] et al. [10]) or calcified cartilage (as observed in endochondral ossification by [PERSON] et al. [11]). [PERSON] and coworkers [34] also explained that in the bone collar (i.e., the thick vascularized bone tissue layer surrounding the cartilaginous shaft) further maturation of pre-osteoblasts is controlled by either SO or DO processes. As SO starts to take place, randomly oriented mesenchymal cells condensate into 1-3-cell-thick layers halfway between the adjacent capillaries of the bone collar and start to differentiate into stationary osteoblasts. At the same time, in other regions of the perigoteum DO-controlled osteoblasts are organized into osteogenic laminae. While static osteoblasts produce the first thin trabeculae of porous irregularly fibered woven bone between the capillaries, the DO-derived osteogenic laminae start to deposit highly organized bone matrix on already existing surfaces like the forming SO-derived trabeculae or the limb precursor. Since the SO-derived woven framework incorporates large vascular spaces into the bony substance within a short period of time, rapid diametrical growth can be realized by means of rapid volume expansion of the forming limb bone shaft. SO osteoblasts start to mineralize their secreted matrix very early, filling it up with apatite crystallites. After the sauropod embryo develops the diametrical expansion in parallel to compaction of the developing limb bone (performed by SO and DO processes, respectively), the successive active growth phases of postembryonic ontogeny continue in time in the same way. As DO proceeds and compacts the primary vascular spaces, at some point the osteogenic lamina may start to form lamellae around the progressively narrowing vascular space.