Cranial bone histology ofMetoposaurus krasiejowensis(Amphibia, Temnospondyli) from the Late Triassic of Poland

Abstract

In this study, 21 skull bones of Metoposaurus krasiejowensis from the Late Triassic of Poland were investigated histologically. Dermal bones show a diploë structure, with an ornamented external surface. The ridges consist of mostly well vascularized parallel-fibered bone; the valleys are built of an avascular layer of lamellar bone. The thick middle region consists of cancellous bone, with varying porosity. The thin and less vascularized internal cortex consists of parallel-fibered bone. The numerous Sharpey’s fibers and ISF are present in all bones. The cyclicity of growth is manifested as an alternation of thick, avascular annuli and high vascularized zones as well as a sequence of resting lines. The detailed histological framework of dermal bones varies even within a single bone; this seems to be related to the local biomechanical loading of the particular part of the skull. The dynamic processes observed during the ornamentation creation indicate that the positions of the ridges and grooves change during growth and could be a specific adaptation to changing biomechanical conditions and stress distribution during bone development. In the supratemporal, the cementing lines show that the remodeling process could be involved in the creations of sculpture. The common occurrence of ISF suggests that metaplastic ossification plays an important role during cranial development. Endochondral bones preserved the numerous remains of calcified cartilage. This indicates that ossification follows a pattern known for stereospondyl intercentra, with relatively slow ossification of the trabecular part and late development of the periosteal cortex. The large accumulation of Sharpey’s fibers in the occipital condyles indicates the presence of strong muscles and ligaments connecting the skull to the vertebral column.

Morphology and preliminary biomechanical interpretation of mandibular sutures in Metoposaurus krasiejowensis (Temnospondyli, Stereospondyli) from the Upper Triassic of Poland

“…Thus, the presence of ISF in this case represent the connection between two ossification systems. However, IFS also represent a strong connection of bone with the soft tissues, increasing the biomechanical strength of the bony tissue and, similar to the skull dermal bones of Metoposaurus krasiejowensis (Gruntmejer et al 2016), the occurrence of ISF in mandible dermal bones also indicates their metaplastic origin.…”

“…This feature could be important in our understanding of the mechanical system for flat and relatively large cranial bones, but also in mandibles. Together with the highly fibred matrix of dermal bones (Gruntmejer 2015;Gruntmejer et al 2016), the entire skull and lower jaw were strongly, yet elastically, connected to the external cover of the body, but also the bones to each other. In contrast to the skull, mandibular sutures in Metoposaurus reveal a complex variability as far as cross sectional morphology is concerned and suggest the important role of the mandible in feeding behaviours.…”

“…Interestingly, in the entire skull examined, sutures associated with tension (i.e., abutting or tongueand-groove) do not occur (Gruntmejer 2012). In contrast to the skull, where the external traces of sutures between skull bones are fused and invisible (Gruntmejer 2012;Gruntmejer et al 2016), the external course of all sutures in the mandible is visible and can be followed. In addition, well-mineralised Sharpey's fibres occur in dense bundles along the periphery of the sutural edges.…”

“…These structures indicate the point of attachment of skin, ligaments and skeletal muscles into the bone (Francillon-Vieillot et al 1990). Sharpey's fibres are very numerous in the skull bones of Metoposaurus krasiejowensis, but are short and thin (Gruntmejer et al 2016;Konietzko-Meier et al 2018), whereas they are much thicker and longer in the mandible (Gruntmejer 2015).…”

“…Skeletal remains of these temnospondyls are known from North America (Koskinonodon and Apachesaurus; Hunt 1993), North Africa (Dutuitosaurus ouazzoui and Arganasaurus lyazidi ;Dutuit 1976;Hunt 1993), Madagascar (Dutuit 1978), India (Phantasaurus maleriensis; Chowdhury 1965, Sengupta 1992Chakravorti and Sengupta 2016) and from Europe, including the following species: Metoposaurus diagnosticus (von Meyer 1842), Metoposaurus krasiejowensis-previously recognised as a subspecies, Metoposaurus diagnosticus krasiejowensis (see Sulej 2002), and Metoposaurus algarvensis (Brusatte et al 2015). Material of the Polish taxon Metoposaurus krasiejowensis, recovered at Krasiejów, has been examined largely osteologically and histologically (Dzik et al 2000;Sulej 2002Sulej , 2007Dzik and Sulej 2007;Konietzko-Meier and Wawro 2007;Gruntmejer 2012;Konietzko-Meier and Klein 2013;Brusatte et al 2015;Gruntmejer et al 2016;Konietzko-Meier et al 2018;Teschner et al 2018). However, quite of lot of unexplained issues surrounding the ecology and feeding of these animals remain.…”