Abstract

The Prozostrodontia includes a group of Late Triassic-Early Cretaceous eucynodonts plus the clade Mammaliaformes, in which Mammalia is nested. Analysing their growth patterns is thus important for understanding the evolution of mammalian life histories. Obtaining material for osteohistological analysis is difficult due to the rare and delicate nature of most of the prozostrodontian taxa, much of which comprises mostly of crania or sometimes even only teeth. Here we present a rare opportunity to observe the osteohistology of several postcranial elements of the basal prozostrodontid Prozostrodon brasiliensis, the tritheledontid Irajatherium hernandezi, and the brasilodontids Brasilodon quadrangularis and Brasilitherium riograndensis from the Late Triassic of Brazil (Santa Maria Supersequence). Prozostrodon and Irajatherium reveal similar growth patterns of rapid early growth with annual interruptions later in ontogeny. These interruptions are associated with wide zones of slow growing bone tissue. Brasilodon and Brasilitherium exhibit a mixture of woven-fibered bone tissue and slower growing parallel-fibered and lamellar bone. The slower growing bone tissues are present even during early ontogeny. The relatively slower growth in Brasilodon and Brasilitherium may be related to their small body size compared to Prozostrodon and Irajatherium. These brasilodontids also exhibit osteohistological similarities with the Late Triassic/Early Jurassic mammaliaform Morganucodon and the Late Cretaceous multituberculate mammals Kryptobaatar and Nemegtbaatar. This may be due to similar small body sizes, but may also reflect their close phylogenetic affinities as Brasilodon and Brasilitherium are the closest relatives to Mammaliaformes. However, when compared with similar-sized extant placental mammals, they may have grown more slowly to adult size as their osteohistology shows it took more than one year for growth to attenuate. Thus, although they exhibit rapid juvenile growth, the small derived, brasilodontid prozostrodontians still exhibit an extended growth period compared to similar-sized extant mammals.

Highlights

  • The non-mammaliaform cynodonts are the most derived and mammal-like clade of non-mammalian synapsids (Therapsida)

  • The Probainognathia appeared during the early Late Triassic and gave rise to the Prozostrodontia, a clade that became progressively smaller-bodied during its evolution before evolving into crown group mammals during the Early Jurassic (Liu & Olsen, 2010; Ruta et al, 2013; Martinelli & Soares, 2016)

  • We describe here the limb bone osteohistology of four nonmammaliaform prozostrodontian probainognathian taxa from the Carnian and Norian of Brazil

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Summary

Introduction

The non-mammaliaform cynodonts are the most derived and mammal-like clade of non-mammalian synapsids (Therapsida) They first appeared during the late Permian. Within the Cynognathia, a group of largebodied herbivorous/omnivorous taxa known as the Gomphodontia arose during the Middle Triassic This clade includes the families Diademodontidae, Trirachodontidae and Traversodontidae (Crompton, 1955; Hopson & Kitching, 2001; Abdala, 2007). The latter group includes highly diverse, globally distributed non-mammaliaform cynodonts that became especially abundant during the Late Triassic, with an apomorphic masticatory system for cutting and grinding food (Abdala & Ribeiro, 2010). The Tritheledontidae are a group of small (BSL ∼30–70 mm), faunivorous/frugivorous forms that existed from the Late Triassic to Early Jurassic (Kemp, 2005; Gow, 1980; Martinelli et al, 2005; Martinelli & Rougier, 2007; Soares, Schultz & Horn, 2011)

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