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      Osteohistology of Late Triassic prozostrodontian cynodonts from Brazil

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          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.

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          Sexual maturity in growing dinosaurs does not fit reptilian growth models.

          Recent histological studies suggest relatively rapid growth in dinosaurs. However, the timing of reproductive maturity (RM) in dinosaurs is poorly known because unambiguous indicators of RM are rare. One exception is medullary bone (MB), which is an ephemeral bony tissue that forms before ovulation in the marrow cavities of birds as a calcium source for eggshelling. Recently, MB also was described in a single specimen of the saurischian dinosaur Tyrannosaurus rex. Here, we report two other occurrences of MB: in another saurischian dinosaur, Allosaurus, and in the ornithischian dinosaur Tenontosaurus. We show by counting lines of arrested growth and performing growth curve reconstructions that Tenontosaurus, Allosaurus, and Tyrannosaurus were reproductively mature by 8, 10, and 18 years, respectively. RM in these dinosaurs coincided with a transition from growth acceleration to deceleration. It also far precedes predictions based on the growth rates of living reptiles scaled to similar size. Despite relatively rapid growth, dinosaurs were similar to reptiles in that RM developed before reaching asymptotic size. However, this reproductive strategy also occurs in medium- to large-sized mammals and correlates with a strategy of prolonged multiyear growth. RM in actively growing individuals suggests that these dinosaurs were born relatively precocial and experienced high adult mortality. The origin of the modern avian reproductive strategy in ornithuran birds likely coincided with their extreme elevations in growth rate and truncations to growth duration.
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            Sequence stratigraphy of continental Triassic strata of Southernmost Brazil: a contribution to Southwestern Gondwana palaeogeography and palaeoclimate

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              U-Pb age constraints on dinosaur rise from south Brazil

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                Author and article information

                Contributors
                Journal
                PeerJ
                PeerJ
                peerj
                peerj
                PeerJ
                PeerJ Inc. (San Francisco, USA )
                2167-8359
                27 June 2018
                2018
                : 6
                : e5029
                Affiliations
                [1 ]Department of Karoo Palaeontology, National Museum , Bloemfontein, South Africa
                [2 ]Department of Zoology and Entomology, University of the Free State , Bloemfontein, South Africa
                [3 ]Departamento de Paleontologia e Estratigrafia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul , Porto Alegre, Brazil
                Article
                5029
                10.7717/peerj.5029
                6026457
                29967724
                18a7c302-a832-408d-a30c-b60a404c1153
                ©2018 Botha-Brink et al.

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ) and either DOI or URL of the article must be cited.

                History
                : 26 February 2018
                : 31 May 2018
                Funding
                Funded by: National Research Foundation
                Award ID: UID 98819, 104688
                Funded by: Palaeontological Scientific Trust (PAST), Johannesburg, South Africa
                Funded by: DST-NRF Centre of Excellence in Palaeosciences (CoE-Pal)
                This work was supported by the National Research Foundation (UID 98819, 104688), the Palaeontological Scientific Trust (PAST), Johannesburg, South Africa, and DST-NRF Centre of Excellence in Palaeosciences (CoE-Pal). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Evolutionary Studies
                Paleontology
                Histology

                non-mammaliaform cynodonts,prozostrodontia,osteohistology,growth patterns,paleobiology

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