Botha-Brink Jennifer, Bento Soares Marina, Martinelli Agustín G
Department of Karoo Palaeontology, National Museum, Bloemfontein, South Africa.
Department of Zoology and Entomology, University of the Free State, Bloemfontein, South Africa.
PeerJ. 2018 Jun 27;6:e5029. doi: 10.7717/peerj.5029. eCollection 2018.
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 , the tritheledontid , and the brasilodontids and from the Late Triassic of Brazil (Santa Maria Supersequence). and 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. and 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 and may be related to their small body size compared to and These brasilodontids also exhibit osteohistological similarities with the Late Triassic/Early Jurassic mammaliaform and the Late Cretaceous multituberculate mammals and This may be due to similar small body sizes, but may also reflect their close phylogenetic affinities as and 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.
原兽齿类包括一群晚三叠世-早白垩世的真犬齿兽类以及哺乳形类分支，哺乳动物嵌套在哺乳形类之中。因此，分析它们的生长模式对于理解哺乳动物生活史的演化至关重要。由于大多数原兽齿类分类群稀少且脆弱，获取用于骨组织学分析的材料很困难，其中许多主要由颅骨组成，有时甚至只有牙齿。在这里，我们有一个难得的机会来观察来自巴西晚三叠世（圣玛丽亚超级层序）的基础原兽齿类、三列齿兽类和巴西齿兽类几个颅后骨骼元素的骨组织学。它们揭示了早期快速生长、个体发育后期有年度生长中断的相似生长模式。这些中断与生长缓慢的骨组织宽区域有关。它们呈现出编织纤维骨组织与生长较慢的平行纤维和板层骨的混合。即使在个体发育早期也存在生长较慢的骨组织。与其他两者相比，巴西齿兽类相对较慢的生长可能与其较小的体型有关。这些巴西齿兽类在骨组织学上也与晚三叠世/早侏罗世的哺乳形类以及晚白垩世的多瘤齿兽类有相似之处。这可能是由于相似的小体型，但也可能反映了它们密切的系统发育亲缘关系，因为巴西齿兽类是哺乳形类最亲近的亲属。然而，与体型相似的现存胎盘哺乳动物相比，它们成年时的生长可能更慢，因为它们的骨组织学显示生长衰减需要一年以上时间。因此，尽管它们幼年生长迅速，但与体型相似的现存哺乳动物相比，小型的衍生巴西齿兽类原兽齿类仍表现出延长的生长周期。
