Data from: Letting the “cat” out of the bag: pouch young development of the extinct Tasmanian tiger revealed by X-ray computed tomography

The Tasmanian tiger or thylacine (Thylacinus cynocephalus) was an iconic Australian marsupial predator that was hunted to extinction in the early 1900s. Despite sharing striking similarities with canids, they failed to evolve many of the specialized anatomical features that characterize carnivorous placental mammals. These evolutionary limitations are thought to arise from functional constraints associated with the marsupial mode of reproduction, in which otherwise highly altricial young use their well-developed forelimbs to climb to the pouch and mouth to suckle. Here we present the first 3D digital developmental series of the thylacine throughout its pouch life using X-ray computed tomography on all known ethanol-preserved specimens. Based on detailed skeletal measurements, we refine the species growth curve to improve age estimates for the specimens. Comparison of allometric growth trends in the appendicular skeleton (fore- and hindlimbs) with that of other placental and marsupial mammals revealed that despite their unique adult morphologies, thylacines retained a generalized early marsupial ontogeny. Our approach also revealed mislabelled specimens that possessed large epipubic bones (vestigial in thylacine) and differing vertebral numbers. All of our generated CT models are publicly available, preserving their developmental morphology and providing a novel digital resource for future studies of this unique marsupial.

袋狼（thylacine，学名Thylacinus cynocephalus，俗称塔斯马尼亚虎）是澳大利亚标志性的有袋类（marsupial）捕食者，于20世纪早期因人类猎杀彻底灭绝。尽管其形态与犬科动物高度相似，袋狼却未演化出食肉胎盘类哺乳动物所特有的诸多特化解剖结构。学界认为这类演化局限源于有袋类繁殖模式带来的功能约束：在该繁殖模式下，原本发育极不完全的晚成幼崽依靠自身发育良好的前肢爬向育儿袋，并通过嘴部完成哺乳。本研究针对所有已知的乙醇保存标本开展X射线计算机断层成像（X-ray computed tomography，以下简称CT）扫描，首次构建了袋狼整个育儿袋发育阶段的三维数字化发育序列。基于精细的骨骼测量数据，本研究优化了该物种的生长曲线，以提升标本的年龄估算精度。通过对比附肢骨骼（前肢与后肢）的异速生长（allometric growth）趋势与其他胎盘类、有袋类哺乳动物的对应特征，研究发现：尽管袋狼的成年形态独具特色，但其早期个体发育仍保留了广义有袋类的发育模式。本研究方法还识别出一批错标标本：这些标本拥有大型耻骨前骨（epipubic bones，袋狼的该骨骼已退化），且椎骨数目与袋狼不符。本研究生成的所有CT模型均已公开，完整保留了其发育形态学特征，为这一独特有袋类动物的后续研究提供了全新的数字化科研资源。