Brasilodon

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.

Abstract: We report here on a new prozostrodontian cynodont, Botucaraitherium belarminoi gen. et sp. nov., from the Late Triassic Riograndia Assemblage Zone (AZ) of the Candelaria Sequence (Santa Maria Supersequence), collected in the Botucarai Hill Site, Candelaria Municipality, state of Rio Grande do Sul, Brazil. The new taxon is based on a single specimen (holotype MMACR-PV-003-T) which includes the left lower jaw, without postdentary bones, bearing the root of the last incisor, canine and four postcanines plus one partial crown inside the dentary, not erupted, and two maxillary fragments, one with a broken canine and another with one postcanine. The features of the lower jaw and lower/upper postcanines resemble those of the prozostrodontians Prozostrodon brasiliensis from the older Hyperodapedon AZ and Brasilodon quadrangularis and Brasilitherium riograndensis from the same Riograndia AZ. The inclusion of Botucaraitherium within a broad phylogenetic analysis, positioned it as a more derived taxon than tritylodontids, being the sister-taxon of Brasilodon, Brasilitherium plus Mammaliaformes. Although the new taxon is based on few cranial elements, it represents a additional faunal component of the Triassic Riograndia AZ of southern Brazil, in which small-sized derived non-mammaliaform cynodonts, closely related to the origin of mammaliaforms, were ecologically well succeed and taxonomically diverse.

Abstract: n New discoveries of articulated skulls and lower jaws of Brasilodon and Brasilitherium from the Late Triassic of Brazil provide additional information on these small cynodonts, allowing us to establish the family Brasilodontidae nov. The basicranium, lateral wall of the braincase, primary and secondary palates and the quadrate of Brasilitherium are described, as well as the basicranium, primary palate and lower jaw of Brasilodon. Brasilitherium exhibits more derived characters than Brasilodon in the periotic, the promontorium, and the articular process of the dentary. Brasilodon is more derived in the lower dentition, the mode of tooth replacement, and the vascular features in the istapedial recessi. Both genera have interpterygoid vacuities that are considered here as possibly derived or reversed characters of the mammalian primary palate. The new phylogenetic analysis presented here corroborates the position of both genera as sister-group of mammals as previously proposed.