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Abstract Cranial synchondroses are cartilaginous joints between basicranial bones or between basicranial bones and septal cartilage, and have been implicated as having a potential active role in determining craniofacial form. However, few studies have examined them histologically. Using histological and immunohistochemical methods, we examined all basicranial joints in serial sagittal sections of newborn heads from nine genera of primates (five anthropoids, four strepsirrhines). Each synchondrosis was examined for characteristics of active growth centers, including a zonal distribution of proliferating and hypertrophic chondrocytes, as well as corresponding changes in matrix characteristics (i.e., density and organization of Type II collagen). Results reveal three midline and three bilateral synchondroses possess attributes of active growth centers in all species (sphenooccipital, intrasphenoidal, presphenoseptal). One midline synchondrosis (ethmoseptal) and one bilateral synchondrosis (alibasisphenoidal synchondrosis [ABS]) are active growth centers in some but not all newborn primates. ABS is oriented more anteriorly in monkeys compared to lemurs and bushbabies. The sphenoethmoidal synchondrosis (SES) varies at birth: in monkeys, it is a suture‐like joint (i.e., fibrous tissue between the two bones); however, in strepsirrhines, the jugum sphenoidale is ossified while the mesethmoid remains cartilaginous. No species possesses an SES that has the organization of a growth plate. Overall, our findings demonstrate that only four midline synchondroses have the potential to actively affect basicranial angularity and facial orientation during the perinatal timeframe, while the SES of anthropoids essentially transitions toward a “suture‐like” function, permitting passive growth postnatally. Loss of cartilaginous continuity at SES and reorientation of ABS distinguish monkeys from strepsirrhines.
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Histology of the preparietal: a neomorphic cranial element in dicynodont therapsids
The preparietal, a neomorphic midline ossification on the skull roof, is thought to have evolved three times in therapsids, but its development and homology remain poorly understood. Here, we provide preliminary data on the histology of this element in specimens referred to Diictodon feliceps and an indeterminate species of Lystrosaurus. The preparietal has previously been noted to vary substantially in its shape on the dorsal surface of the skull in several dicynodonts, and we found similar variation in thin section. In Diictodon, the preparietal forms a prong that embeds itself entirely within the frontals and shows evidence of a midline suture anteriorly. The sectioned specimen of Lystrosaurus shows histological evidence of immaturity and features a well-defined midline suture at the posterior end of the preparietal, although an anterior prong was not present. In both taxa, the anteroventral portion of the preparietal forms a strongly interdigitating suture with the underlying frontals and parietals. More posteriorly, the preparietal is composed of fibrolamellar bone suggestive of rapid posteroventral growth. In large dicynodont species, the dorsal expression of the preparietal appears to show negative allometry compared with other cranial roofing elements during ontogeny, but the significance of this geometry is unclear. In addition, histological work is needed on the preparietal in gorgonopsians and biarmosuchians to determine whether the features characterizing dicynodonts are also seen in the other two groups of therapsids that evolved a preparietal. The therapsid preparietal provides a rare opportunity to study the development and evolution of a neomorphic cranial element in the vertebrate fossil record.
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- Award ID(s):
- 1713787
- PAR ID:
- 10178863
- Date Published:
- Journal Name:
- Journal of Vertebrate Paleontology
- ISSN:
- 0272-4634
- Page Range / eLocation ID:
- e1770775
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1080/02724634.2020.1770775
