School of Environmental and Rural Science, University of New England, Armidale, NSW, Australia.
Unidad Ejecutora Lillo, CONICET-Fundación Miguel Lillo, Miguel Lillo, San Miguel de Tucumán, Tucumán, Argentina.
BMC Biol. 2022 Jun 7;20(1):132. doi: 10.1186/s12915-022-01329-9.
In egg-laying amniotes, the developing embryo is tethered to a number of the extraembryonic membranes including the yolk sac and allantois that deliver oxygen and nutrients and remove metabolic waste products throughout embryonic development. Prior to, or soon after hatching, these membranes detach from the animal leaving a temporary or permanent umbilical scar (umbilicus) equivalent to the navel or 'belly button' in some placental mammals, including humans. Although ubiquitous in modern mammals and reptiles (including birds), at least early in their ontogeny, the umbilicus has not been identified in any pre-Cenozoic amniote.
We report the oldest preserved umbilicus in a fossil amniote from a ~130-million-year-old early-branching ceratopsian dinosaur, Psittacosaurus. Under laser-stimulated fluorescence (LSF), the umbilicus is revealed as an elongate midline structure delimited by a row of paired scales on the abdomen. The relatively late ontogenetic stage (close to sexual maturity) estimated for the individual indicates that the umbilicus was probably retained throughout life.
Unlike most extant reptiles and birds that lose this scar within days to weeks after hatching, the umbilicus of Psittacosaurus persisted at least until sexual maturity, similar to some lizards and crocodylians with which it shares the closest morphological resemblance. This discovery is the oldest record of an amniote umbilicus and the first in a non-avian dinosaur. However, given the variability of this structure in extant reptilian analogues, a persistent umbilical scar may not have been present in all non-avian dinosaurs.
在卵生羊膜动物中，发育中的胚胎与许多胚外膜相连，包括卵黄囊和尿囊，这些膜在胚胎发育过程中输送氧气和营养物质，并清除代谢废物。在孵化之前或之后不久，这些膜与动物分离，留下一个临时或永久性的脐痕（脐），相当于一些胎盘哺乳动物（包括人类）的肚脐或“肚脐眼”。尽管在现代哺乳动物和爬行动物（包括鸟类）中普遍存在，但至少在它们的个体发育早期，在任何前新生代羊膜动物中都没有发现脐。
我们报告了一种来自约 1.3 亿年前早期分支角龙类恐龙 Psittacosaurus 的化石羊膜动物中保存最古老的脐。在激光激发荧光（LSF）下，脐呈现为沿腹部排列的一对鳞片界定的长中线结构。对个体的相对较晚的个体发育阶段（接近性成熟）的估计表明，脐可能终生保留。
与大多数现生爬行动物和鸟类在孵化后数天至数周内失去这个疤痕不同，Psittacosaurus 的脐至少在性成熟时保留下来，类似于一些与它共享最密切形态相似性的蜥蜴和鳄鱼类。这一发现是羊膜动物脐的最古老记录，也是首例非鸟类恐龙的记录。然而，鉴于现存爬行动物类似物中这种结构的可变性，并非所有非鸟类恐龙都可能存在持续的脐痕。
