Limb bone histology and growth in Placerias hesternus (Therapsida: Anomodontia) from the Upper Triassic of North America

Abstract

Patterns of bone deposition are reported and deduced from mid-shaft sections of 21 limb bones of the dicynodont Placerias hesternus from the Placerias Quarry (Upper Triassic), Arizona, USA. All sampled elements of P. hesternus have a large medullary cavity completely filled with bony trabeculae surrounded by dense cortical bone. Dense Haversian bone extends from the perimedullary region to at least the mid-cortex in all sampled bones. Primary bone in the outer cortex of limb elements of P. hesternus is generally zonal fibrolamellar with a peripheral layer of parallelfibred bone. These data suggest periodic rapid osteogenesis followed by slower growth. Among dicynodonts, this strategy is most similar to growth previously reported in other Triassic (Lystrosaurus, Wadiasaurus) and some Permian taxa (Oudenodon, Tropidostoma). An external fundamental system (EFS), suggesting complete or near complete cessation of appositional growth, is present in the largest tibia. This is the first report of EFS in dicynodonts and may represent the attainment of maximum size in P. hesternus. Slow-growing peripheral bone was observed in elements of varying size in our sample and may support a differential growth pattern between P. hesternus individuals from this locality. A complete growth series of P. hesternus, analysis of Placerias specimens from other localities, and further sampling of other Upper Triassic dicynodonts are needed to better understand a more complete picture of the growth and remodelling patterns that we have initially investigated.

The paleobiology and paleoecology of South African Lystrosaurus

“…Outer circumferential lamellae or OCL (also known as an External Fundamental System) in the form of avascular lamellar bone and or many closely spaced LAGs at the bone periphery, which would indicate that maximum growth had been reached, are absent. An OCL has been observed in the Triassic kannemeyeriiform dicynodonts Kannemeyeria and Placerias (Botha-Brink & Angielczyk, 2010;Green et al, 2010). Secondary osteons in this individual are more abundant and extend to the sub-periosteal surface in places and have probably resorbed some of the inner growth marks.…”

Osteogenesis, homology, and function of the intercostal plates in ornithischian dinosaurs (Tetrapoda, Sauropsida)

“…Ground sections were taken from each sample along a set of orthogonal planes to investigate whether the direction of sampling aVected our observations. Histological samples were prepared following methods described in Green et al (2010).…”

New dicynodonts (Therapsida, Anomodontia) from near the Permo-Triassic boundary of Laos: implications for dicynodont survivorship across the Permo-Triassic mass extinction and the paleobiogeography of Southeast Asian blocks

“…and Kannemeyeria (Ray et al, 2005(Ray et al, , 2012Wall 1983). However, whilst a semiaquatic lifestyle based on microanatomy is proposed for Placerias (Green et al, 2010), Kannemeyeria, and Wadiasaurus were supposed to be terrestrial (Ray et al, 2010(Ray et al, , 2012. Evidence for dicynodont lifestyle remains equivocal, but even if some taxa did have a semi-aquatic but freshwater lifestyle, this would be unlikely to allow dispersal across a wide oceanic domain.…”