Palaeobiology of rauisuchian archosaurs based on limb bone histology.

JULIA B. DESOJO; BRODSKY D. M. FARIAS; TORSTEN M. SCHEYER; IGNACIO A. CERDA

Simposio; 5th International Symposium on Palaeohistology?, 2019, Julio de 2019, Cape Town, South Africa; 2019

Recent discoveries of new specimens of "Rauisuchia" have boosted the interest in this enigmatic group of pseudosuchian archosaurs, especially because they provide crucial information for understanding the major patterns and processes in the continental Triassic archosaur evolution. Here we analyze the histology of limb bones 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 in southern Brazil, in order to establish their ontogenetic stages and infer the growth dynamics. Both individuals exhibit highly vascularized cortices, which are interrupted by lines of arrested growth (LAGs). Whereas a maximum of four LAGs were recorded in Prestosuchus, only one 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, however. An outer circumferential layer indicative of somatic maturity achievement is absent in all the bone samples. Furthermore, since no clear reduction of the space between successive growthmarks is observed, sexual maturity also seems to not be achieved yet. Comparing with another large bodied rauisuchian, the relative growth rate of Prestosuchus appears to be lower than that reported for Batrachotomus kupferzellensis from the Late Triassic of Germany, which exhibits a higher proportion of fibrolamellar bone tissue. Comparison with Decuriasuchus is more difficult to perform, because of the younger condition of the sampled individual. The bonehistology of other pseudosuchians such as Effigia, Terrestrisuchus, and Postosuchus indicates that these taxa grew fast in early development, as shown by a fibrolamellar bone matrix in the inner cortex, and later changing to lamellar-zonal after reaching sexual maturity. Such condition, which is also evident in the taxa sampled here, correspond with the primitive condition for archosaurs and possibly was important for the achievement of large body sizesin these top predators of the Triassic ecosystems. The large body size found in all rauisuchian specimens sampled so far and the histological evidence of ongoing growth indicate that these animals reached their massive body size during early development instead of through protracted longevity.