The first ontogenetic model for non‐crocodylomorph loricatans, based on osteohistology of the ontogenetic series of Prestosuchus chiniquensis from the Middle Triassic of Brazil

Abstract

Prestosuchus chiniquensis is an iconic non-crocodylomorph loricatan fromthe Brazilian Triassic beds and the best-known taxon, represented by severalspecimens. The completeness and preservation of its skeleton make it a valuabletaxon for paleobiological studies. We explore the microstructure of bonetissues of appendicular elements and ribs of three specimens of Prestosuchusto access a variety of aspects of its paleobiology, such as histovariability,ontogeny, and growth patterns. Integrating our data and other osteohistologicallystudied P. chiniquensis specimens, we proposed for the first time anontogenetic model for non-crocodylomorph loricatans. The model encompassessix distinct age classes (I–VI) that allow us to infer the growth patternsof P. chiniquensis and possibly expand to other phylogenetically closetaxa. During early ontogeny (age classes I–II), sustained fast growth wasmaintained by a fibrolamellar complex. In mid ontogeny (age classes III–IV), highly vascularized parallel-fibered bone predominates, suggestingintermediary growth rates. A change for a poorly vascularized parallelfibered/lamellar bone would mark the attainment of sexual (age classes IV–V) and skeletal maturity, comprising the age class VI. An externalfundamental system (EFS) present in the outermost cortex is the main histologicalfeature that characterize the age class VI. Major histovariability featuresare present between appendicular bones and ribs of skeletallyimmature individuals. The most prominent of them is the presence of fibrolamellarcomplex and highly vascularized parallel-fibered bone in appendicularbones and poorly vascularized parallel-fibered bone in ribs. In advancedontogenetic stages, the histovariability between appendicular bones and ribstends to be minor. Our data also support previous hypothesis of the presenceof one new taxon among the specimens assigned to P. chiniquensis, increasingthe diversity of non-crocodylomorph loricatans. The new taxon, representedby the specimen UFRGS-PV-0152-T, awaits a formal anatomicaldescription. Our study advances the preliminary understand of the ontogenyand growth patterns of non-crocodylomorphs loricatans and Pseudosuchiaas a whole.