Bone histology of two pareiasaurs from Russia (Deltavjatia rossica and Scutosaurus karpinskii) with implications for pareiasaurian palaeobiology

Abstract

Pareiasaurs were one of the main clades of large herbivorous tetrapods in Middle–Late Permian continental ecosystems. Despite abundant pareiasaur material, many aspects of their biology remain poorly known. This paper provides a description of ontogenetic changes in long-bone and rib microanatomy/histology of two Upper Permian pareiasaurs from Russia, Deltavjatia rossica and Scutosaurus karpinskii. Analysis of a growth series of bones of Deltavjatia and Scutosaurus revealed rapid and cyclical growth early in ontogeny (as indicated by fast-growing fibrolamellar bone with lines of arrested growth). This was followed by a change in the growth pattern (as indicated by an outer avascular layer of lamellar bone in the cortex) and a decrease in the growth rate after 50% of maximum body size was reached in Deltavjatia and 75% in Scutosaurus (larger body sizes in Scutosaurus were attained through an extended initial period of fast skeletal growth). The study confirms that the bones of all pareiasaurs are histologically and microanatomically uniform [spongious (porous) microstructure and a very thin compact cortex] and indicate a similar growth strategy (a short initial period of rapid and cyclical growth followed by a long period of slow growth). The microanatomical characteristics of pareiasaurs do not provide a clear indication of their lifestyle.

Passive-Dynamic Walkers of the Late Paleozoic

“…A possible important evidence for the validity of the pareiasaurian walking technique proposed here is supplied by the available data on the limb bone histology of pareiasaurs (Canoville & Chinsamy, 2017;Boitsova et al, 2019). One of the major purposes of the investigations cited was to solve the old-standing terrestrial-vs.-semiaquatic dilemma of the pareiasaurian lifestyle.…”

“…Therefore, a reciprocal reinforcement of a number of hypotheses is immediately obtained, namely, that Sukhonopus was produced by a pareiasaur, that pareiasaurs were terrestrial, that their gait was the rocking pace in spite of the sprawling limb posture, and that their unusually spongy limb bones served for terrestrial locomotion as shock absorbers. Some other large continental Permian tetrapods, namely caseids (Synapsida: Caseasauria), shared with pareiasaurs the specific spongy bone tissue in the limb segments (Canoville, Chinsamy, 2017;Boitsova et al, 2019). Probably, this peculiar and yet unexplained feature is indicative of a similar locomotion technique, that is the rocking pace, as the inward-turned manual imprints can manifest this locomotion technique on the trackways.…”

“…The classic tubular bones could be expected for pareiasaurs, based on the recent isotopic analysis that supports their terrestriality (Canoville et al, 2014). However, the bone histology shows a much unexpected structure (Canoville & Chinsamy, 2017;Boitsova et al, 2019). Both the peripheral compacta and the marrow cavity were replaced by spongiosa in the long bones of the pareiasaurian limbs (and to a less extent in the other bones).…”

“…Therefore, it can be concluded that the spongy structure could have been better for pareiasaurians because it saved their limb bones from compression fractures at high-energy impacts. Furthermore, the intense internal remodeling of their bones (Canoville & Chinsamy, 2017;Boitsova et al, 2019) may indicate that the microscopic fractures did frequently occur but were persistently repaired. It is herein hypothesized that the lifetime microscopic compression fractures will be found in future studies of pareiasaurian bone histology.…”

“…Plausibility of the reconstruction have been tested by a reverse-engineering of the natural-sized walking model with pareiasaurian body proportions. The required mechanical properties of the limb skeleton have been tested against pareiasaurian bone histology which was recently discovered (Canoville & Chinsamy, 2017;Boitsova et al, 2019).…”