Lystrosaurus was one of the few tetrapods to survive the end-Permian mass extinction (EPME), the most catastrophic biotic crisis in Phanerozoic history. The significant increased abundance of this genus during the post-extinction Early Triassic recovery period has made Lystrosaurus an iconic survivor taxon globally and ideal for studying changes in growth dynamics during a mass extinction. There is potential evidence of a Lilliput effect in Lystrosaurus in South Africa as the two Triassic species that became highly abundant after the EPME are relatively smaller than the two Permian species. In order to test this hypothesis a detailed examination of the body size and life history of Permo-Triassic Lystrosaurus is required. In this study, the basal skull length and growth patterns of the four South African Lystrosaurus species from the Karoo Basin, L. maccaigi, L. curvatus, L. murrayi and L. declivis, were examined using cranial measurements and bone histology. The basal skull length measurements show that the Triassic species are smaller than the Permian species and supports previous studies. The osteohistology examination of all four species reveal rapidly forming fibrolamellar bone tissues during early to mid-ontogeny. Growth marks are common in L. maccaigi and L. curvatus, but rare and inconsistent in the purely Triassic L. murrayi and L. declivis. The inconsistency of the growth marks in these latter two taxa suggests the presence of developmental plasticity. This feature may have been advantageous in allowing these species to alter their growth patterns in response to environmental cues in the post-extinction Early Triassic climate. An overall transition to slower forming parallel-fibered bone is observed in the largest individuals of L. maccaigi, but absent from the limb bones of the other species. The absence of such bone tissue or outer circumferential lamellae in L. curvatus, L. murrayi and L. declivis indicates that even the largest collected specimens do not represent fully grown individuals. Although L. murrayi and L. declivis are smaller in size, the lack of a growth asymptote in the largest specimens indicates that adult individuals would have been notably larger and may have been similar in size to large L. maccaigi and L. curvatus when fully grown. Thus, the previously described Lilliput effect, recognized by some authors in the Karoo fossil record (such as the therocephalian Moschorhinus kitchingi), may be a product of high juvenile excess mortality in the Triassic rather than a strict “dwarfing” of Lystrosaurus species. The lifestyle of Lystrosaurus was also re-examined. Although previous studies have proposed an aquatic lifestyle for the genus, the similar morphology and bone microanatomy to several other large terrestrial Permo-Triassic dicynodonts supports a fully terrestrial mode of life.

中文翻译：

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南非水龙的古生物学和古生态学

水龙是少数在二叠纪末大灭绝（EPME）中幸存下来的四足动物之一，这是显生宙历史上最具灾难性的生物危机。在灭绝后的早三叠世恢复期间，该属的丰度显着增加，使水龙龙成为全球标志性的幸存者分类群，是研究大灭绝期间生长动态变化的理想选择。有潜在证据表明南非的水龙龙中存在小人国效应，因为在 EPME 之后变得高度丰富的两个三叠纪物种比两个二叠纪物种要小。为了验证这个假设，需要详细检查二叠纪-三叠纪水龙的体型和生活史。在这项研究中，来自卡鲁盆地的四种南非水龙属物种的基底颅骨长度和生长模式，L. maccaigi、L. curvatus、L. murrayi 和 L. declivis，使用颅骨测量和骨组织学进行检查。基底颅骨长度测量显示三叠纪物种比二叠纪物种小，支持先前的研究。所有四个物种的骨组织学检查显示在个体发育的早期到中期快速形成纤维板层骨组织。生长痕迹在 L. maccaigi 和 L. curvatus 中很常见，但在纯三叠纪的 L. murrayi 和 L. declivis 中很少见且不一致。后两个分类群中生长标记的不一致表明存在发育可塑性。这一特征可能有利于允许这些物种改变其生长模式以响应灭绝后早三叠世气候中的环境线索。在最大的 L. maccaigi 个体中观察到向较慢形成平行纤维骨的整体过渡，但在其他物种的四肢骨中不存在。L. curvatus、L. murrayi 和 L. declivis 中没有这种骨组织或外周薄片表明即使是最大的收集标本也不能代表完全成长的个体。尽管 L. murrayi 和 L. declivis 的体型较小，但最大标本中缺乏生长渐近线表明成年个体会明显更大，并且在完全发育时可能与大型 L. maccaigi 和 L. curvatus 的体型相似长大的。因此，先前描述的小人国效应，被卡鲁化石记录中的一些作者（例如 therocephalian Moschorhinus kitchingi）认可，可能是三叠纪幼鱼高死亡率的产物，而不是水龙属物种的严格“矮化”。水龙的生活方式也被重新审视。尽管之前的研究提出了该属的水生生活方式，但与其他几种大型陆生二叠纪-三叠纪二齿兽类相似的形态和骨骼显微解剖结构支持完全陆生的生活方式。