Limb Bones Microstructure in Rauisuchia (Archosauria: Pseudosuchia): New Insights in the Growth Strategies

Abstract

Recently discovered specimens of "Rauisuchia" have boosted interest in this enigmatic group of pseudosuchians, because they provide crucial information on major patterns and processes in continental Triassic archosaur evolution. Here we analyze limb bone histology of Decuriasuchus quartacolonia MCNPV10.005b-X (humerus, ulna, radius, tibia) and Prestosuchus chiniquensis BSPG AS XXV 10, 11b, 35 (humerus, femur, fibula) from the Middle Triassic (Santa Maria Supersequence), southern Brazil, to establish their ontogenetic stage and infer growth dynamics. Both taxa exhibit cyclically deposited highly vascularized cortices. Whereas a maximum of 4 lines of arrested growth (LAGs) were recorded in Prestosuchus, only 1 is preserved in Decuriasuchus. While the cortical bone is formed by fibrolamellar tissue in Decuriasuchus, parallel fibered bone predominates in Prestosuchus. Evidence of secondary remodeling is only observed in Prestosuchus. An outer circumferential layer (indicative of somatic maturity) is absent in all samples. Furthermore, since no clear reduction of the distance between growth marks is observed, sexual maturity also seems to not be achieved. Comparing with the large bodied "rauisuchian" Batrachotomus which exhibits a higher proportion of fibrolamellar bone, the relative growth rate of Prestosuchus appears to be lower. Comparison with Decuriasuchus is difficult, due to the younger condition of the sampled individual. Other pseudosuchians such as Effigia, Terrestrisuchus, and Postosuchus grew fast in early development, as shown by fibrolamellar bone matrix in the inner cortex, changing to lamellar-zonal after reaching sexual maturity. This condition is also evident in the taxa here sampled, likely reflecting the primitive condition for archosaurs. Bone histology thus argues for rauisuchians reaching massive body size during early development instead of protracted longevity.