Karoo Palaeontology Department, National Museum, Bloemfontein 9300, South Africa; Department of Zoology and Entomology, University of the Free State, Bloemfontein 9300, South Africa.
Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg 2050, South Africa.
Curr Biol. 2022 Oct 24;32(20):4501-4507.e2. doi: 10.1016/j.cub.2022.08.031. Epub 2022 Sep 8.
Sauropod dinosaurs include the largest land animals to have walked the earth, mostly weighing 10-70 tons (e.g., Sander et al. and Carballido et al.). Osteohistology suggests that derived physiological traits evolved near the origin of sauropod gigantism, including both rapid and uninterrupted growth from juvenile to adult with little developmental plasticity. This differs from the slower, seasonally interrupted growth of their direct ancestors, as evident in most non-sauropodan sauropodomorphs, which also show developmental plasticity in some groups. Accelerated but seasonally interrupted growth is present in Lessemsauridae, the sister clade to Sauropoda, which also attained giant adult body sizes (>10 tons). These observations suggest a correlation between giant size and accelerated growth. However, testing this evolutionary connection has been limited by the incomplete understanding of the growth patterns in some of the closest non-giant relatives of sauropods. We present the osteohistology of two such taxa, Aardonyx celestae and Sefapanosaurus zatronensis. Both exhibit highly vascularized woven-parallel complexes, with fibrolamellar complexes during early to mid-ontogeny, containing regular growth marks. These observations provide strong evidence for rapid but seasonally interrupted growth with limited developmental plasticity (indicated by the regular spacing of growth marks). Combined with our review of early branching sauropodomorph osteohistology, these results show that highly accelerated growth rates originated among smaller, non-sauropodan sauropodomorphs weighing 1 to 2 tons but preceded the origins of giant size (>10 tons). Therefore, the capacity for rapid bone tissue formation, a derived aspect of rapid growth seen in sauropods, did not evolve specifically to enable giant body sizes but may have been a prerequisite for them.
蜥脚类恐龙包括有史以来最大的陆生动物，大多数体重在 10-70 吨之间（例如，Sander 等人和 Carballido 等人）。骨骼组织学表明，衍生的生理特征在蜥脚类巨型动物起源附近进化，包括从幼年到成年的快速和不间断生长，发育可塑性很小。这与它们直接祖先的较慢、季节性中断的生长方式不同，这在大多数非蜥脚类蜥脚形亚目动物中显而易见，其中一些群体也表现出发育可塑性。加速但季节性中断的生长存在于 Lessemsauridae 中，这是蜥脚类的姐妹群，也达到了成年巨大体型（>10 吨）。这些观察结果表明，巨型体型和加速生长之间存在相关性。然而，由于对一些最接近的蜥脚类非巨型亲属的生长模式的理解不完整，对这种进化联系的测试受到限制。我们展示了两个这样的分类群的骨骼组织学，Aardonyx celestae 和 Sefapanosaurus zatronensis。两者都表现出高度血管化的编织-平行复合体，在早期到中期的个体发育过程中具有纤维层状复合体，包含有规律的生长标记。这些观察结果为快速但季节性中断的生长提供了强有力的证据，具有有限的发育可塑性（由生长标记的规则间隔表示）。结合我们对早期分支的蜥脚形亚目骨骼组织学的综述，这些结果表明，高加速生长率起源于体重 1 至 2 吨的较小的、非蜥脚类蜥脚形亚目动物中，但先于巨型体型（>10 吨）的起源。因此，快速骨组织形成的能力，即蜥脚类快速生长的一个衍生方面，并不是专门为了实现巨大体型而进化的，但可能是实现巨大体型的前提。
