Extreme growth plasticity in the early branching sauropodomorphMassospondylus carinatus

Abstract

There is growing evidence of developmental plasticity in early branching dinosaurs and their outgroups. This is reflected in disparate patterns of morphological and histological change during ontogeny. In fossils, only the osteohistological assessment of annual lines of arrested growth (LAGs) can reveal the pace of skeletal growth. Some later branching non-bird dinosaur species appear to have followed an asymptotic growth pattern, with declining growth rates at increasing ontogenetic ages. By contrast, the early branching sauropodomorph Plateosaurus trossingensis appears to have had plastic growth, suggesting that this was the plesiomorphic condition for dinosaurs. The South African sauropodomorph Massospondylus carinatus is an ideal taxon in which to test this because it is known from a comprehensive ontogenetic series, it has recently been stratigraphically and taxonomically revised, and it lived at a time of ecosystem upheaval following the end-Triassic extinction. Here, we report on the results of a femoral osteohistological study of M. carinatus comprising 20 individuals ranging from embryo to skeletally mature. We find major variability in the spacing of the LAGs and infer disparate body masses for M. carinatus individuals at given ontogenetic ages, contradicting previous studies. These findings are consistent with a high degree of growth plasticity in M. carinatus .

Palaeobiology of the early sauropodomorph Mussaurus patagonicus inferred from its long bone histology

“…Developmental plasticity can result in widely differing growth rates and final sizes in individuals of the same species and has been invoked to explain the variation evident in growth rates and adult size in two early sauropodomorphs: Plateosaurus engelhardti (Sander & Klein 2005) and Massosspondylus carinatus (Chapelle et al . 2021). As recorded here for Mussaurus , the adult body sizes of Plateosaurus and Massosspondylus are variable and there are records of fully grown individuals with less than 50% LKI in the case of Plateosaurus .…”

“…2016), including early sauropodomorphs such as Plateosaurus and Massospondylus (Sander & Klein 2005; Chapelle et al . 2021). Previous studies have proposed that there is a close correlation between body size and ontogenetic stage in sauropod dinosaurs, which have lost the developmental plasticity (Starck & Chinsamy 2002) present in early sauropodomorphs such as Plateosaurus .…”

“…2019; Chapelle et al . 2021). In both cases, palaeobiological inferences are robustly based on the analysis of numerous individuals representing an ontogenetic series.…”

40 new specimens ofIchthyornisprovide unprecedented insight into the postcranial morphology of crownward stem group birds

“…The extent to which this mismatch between size and apparent levels of ontogenetic maturity reflects intraspecific or interspecific variation is challenging to assess at present, but recalls similar patterns observed in other groups of fossil and extant archosaurs (Bailleul et al, 2016;Griffin et al, 2021), and patchy stratigraphic sampling precludes our ability to evaluate any potential anagenetic patterns. Notable variation in adult size has been previously inferred to be ancestral to archosaurs, and documented in many stem-birds (Sander & Klein, 2005;Hone, 2016;Griffin & Nesbitt, 2016;Carr, 2020;Chapelle et al, 2021). Whether Ichthyornis exhibited comparable variability in adult size is difficult to establish, but this interpretation provides a possible explanation for the considerable variation in size and morphology we observe, despite the near-crown position of Ichthyornis and its similar growth patterns to extant birds (Chinsamy et al, 1998).…”

Osteohistology and taphonomy support social aggregation in the early ornithischian dinosaur Lesothosaurus diagnosticus

“…However, given previous findings of developmental plasticity in non‐sauropod sauropodomorphs (Sander & Klein 2005; Chapelle et al . 2021) and the ornithischian Gasparinisaura (Cerda & Chinsamy 2012), it is possible that these two largest femora in the sample represent individual variation. However, a large sample of elements is required to confirm developmental plasticity, such as those used in the studies on Plateosaurus (a broad sample of limb bone and girdle elements: Sander & Klein 2005) and Massospondylus (a large sample of femora: Chapelle et al .…”