Craniogenetic studies in Sus scrofa: With emphasis on the ‘orbitosphenoid’ problem

Abstract

The orbitosphenoid is a skeletal element of the endocranium of extant mammals. However, it has also been described in many of their fossil ancestors. Craniogenetic studies show that it is composed of two types of bone: first, the cartilaginous ala orbitalis and parts of the trabecular plate are transformed by endochondral ossification; second, so‐called ‘appositional bone’ (‘Zuwachsknochen’) arises directly from the perichondrium of the two optic pilae and spreads in all directions and overlays the remaining cartilage and the endochondral ossifications. For some time, both bone types can be distinguished microscopically, but later in craniogenesis they fuse completely to become the presphenoid sensu lato of the osteocranium. We interpret the ‘appositional bone’ as neomorphic mode to reinforce the endocranial bone structures, which are the ossification of the delicate cartilaginous template of the chondrocranium. We studied the ossifications of the presphenoidal skull region in a series of ontogenetic stages of the pig Sus scrofa. We applied conventional histology as well as stained and unstained μCT scans. We can show the above‐mentioned modes of ossification, and we can demonstrate the contribution of ‘appositional bone’ well into neonatal and infantile stages. The ossifications of the presphenoid (including the orbitosphenoid) are very slender in therapsids and early mammaliaforms as previously described by other authors. In mammaliaforms, they tend to become thicker and closely connected with the frontal bone, which may be due to the contribution of neomorphic appositional bone. We assume that thereby the presphenoid sensu lato becomes an enforcement of the orbital pillars.

The skull base in Cingulata (Xenarthra, Mammalia): early ossification, homologies, and consequences (presphenoid and mesethmoid, not a child's play)

“…Its development varies among mammals, and it might arise from 1) a single ossi cation center, as in Monodelphis [9]; 2) two ossi cation centers, as in Canis domesticus [10]; or even 3) not ossi ed and replaced by a medial fusion of the orbitosphenoids, in which case the ossi cation center of the presphenoid bone itself is absent, as described for Dasypus novemcinctus [11,12] and Euphractus sexcintus [8]. Recently, Maier et al [13] proposed an alternative mode for the development of the "presphenoid region" of Sus scrofa. According to these authors, the presphenoid, which they called the "sphenethmoid", develops from a rapid and early fusion of a central endochondral ossi cation and two lateral appositional bone tissues that correspond to part of the orbitosphenoids.…”