ABSTRACT

Several shark species produce egg cases as protective casings in which their embryos develop. These casings are composed of multiple layers of collagen and are extremely durable, making them prone to fossilisation. Here we document Palaeoxyris (Spirangium) ‒ fossil shark egg cases from Lower Jurassic successions of southern Sweden. We present high-resolution 3D images of Palaeoxyris based on microfocus X-ray computed tomography (μCT) of seven specimens, including fossils that were embedded within a sandstone matrix. Our examination of the internal structure of the egg cases revealed the possible remnants of a yolk and foetus in one specimen. The cases were most likely produced by hybodont sharks, as outlined in previous studies, and the occurrence of hybodont teeth from Lower Jurassic successions of Sweden support this. Palynological analysis of the matrix from one of the specimens hosting Palaeoxyris, indicates an early Hettangian age. The high percentage of spores (c. 60%) reveals that the egg cases were laid during the Transitional Spore Spike Interval following the end-Triassic mass extinction. The egg cases are found in conjunction with fossil horsetails; with the broader palynological and sedimentological evidence, this suggests an estuarine depositional setting, and potentially indicates that newborn sharks were living in habitats comparable to modern mangroves, as is often the case today.

摘要

一些鲨鱼物种产生卵壳作为胚胎发育的保护壳。这些肠衣由多层胶原蛋白组成，非常耐用，容易化石化。在这里，我们记录了Palaeoxyris ( Spirangium ) - 来自瑞典南部下侏罗纪系列的化石鲨鱼卵案例。我们展示了Palaeoxyris的高分辨率 3D 图像基于七个标本的微焦点 X 射线计算机断层扫描 (μCT)，包括嵌入砂岩基质中的化石。我们对卵壳内部结构的检查揭示了一个样本中可能存在蛋黄和胎儿的残留物。这些病例很可能是由之前的研究中概述的弓齿鲨产生的，瑞典下侏罗纪系列的弓齿鲨的出现支持了这一点。对寄居 Palaeoxyris 的其中一个标本的基质进行孢粉学分析, 表示早期的赫唐阶时代。孢子的高百分比（约 60%）表明，卵壳是在三叠纪末大灭绝后的过渡孢子尖刺间隔期间产下的。蛋壳与化石马尾一起被发现；有了更广泛的孢粉学和沉积学证据，这表明存在河口沉积环境，并可能表明新生鲨鱼生活在与现代红树林相当的栖息地，就像今天的情况一样。