Osteohistology of Late Triassic prozostrodontian cynodonts from Brazil

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.

The postcranial anatomy of Brasilodon quadrangularis and the acquisition of mammaliaform traits among non-mammaliaform cynodonts

“…Here, Brasilitherium riograndensis is considered as the junior synonym of Brasilodon quadrangularis , following Liu and Olsen, [7]. This hypothesis is also supported by the histological study (see [31]).…”

“…The neural arches are poorly preserved, almost all vertebral processes are broken. The neural arch is fused with the centrum with no evidence of neurocentral suture, indicating the adult nature of the specimen (one of the largest known Brasilodon specimen; see also [31]). The neural canal is relatively large, with thin lateral walls.…”

“…Most postcranial elements are well-preserved, including the epiphysis in long bones. Both UFRGS-PV-0765T and UFRGS-PV-1043T were used in the histological study by Botha-Brink et al [31]. Here, Brasilitherium riograndensis is considered as the junior synonym of Brasilodon quadrangularis , following Liu and Olsen, [7].…”

Enamel formation and growth in non-mammalian cynodonts

“…Preliminary studies on bone histology in Irajatherium show a rapid early growth with annual interruptions later in ontogeny [ 118 ], which may be consistent with an intermediate pattern of skeletal growth. Tooth replacement data in Irajatherium are still limited, but it has been shown to have the alternate postcanine replacement with loss of the anterior postcanines typical of many more stemward non-mammalian cynodonts [ 130 , 131 ].…”

“…Bone microstructure studies show that growth is not ‘indeterminate’ in these more stemward cynodont species (because growth does reduce in rate and eventually ceases in adulthood), and that there is variability in growth strategies of non-mammalian cynodonts and therapsids [ 57 , 58 , 117 ]. However, overall, there is a trend towards more rapid, mammal-like bone deposition in more crownward groups [ 57 , 58 , 118 ].…”