Müller Johannes, Hipsley Christy A, Maisano Jessica A
Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany.
Museum Victoria, Melbourne, VIC, Australia.
J Anat. 2016 Nov;229(5):615-630. doi: 10.1111/joa.12513. Epub 2016 Jun 22.
The fossorial amphisbaenians, or worm lizards, are characterized by a suite of specialized characters in the skull and postcranium, however fossil evidence suggests that at least some of these shared derived traits evolved convergently. Unfortunately the lack of detailed knowledge of many fossil taxa has rendered a more precise interpretation difficult. Here we describe the cranial anatomy of the oldest-known well-preserved amphisbaenian, Spathorhynchus fossorium, from the Eocene Green River Formation, Wyoming, USA, using high-resolution X-ray computed tomography (HRXCT). This taxon possesses one of the most strongly reinforced crania known among amphisbaenians, with many dermal bones overlapping each other internally. In contrast to modern taxa, S. fossorium has a paired orbitosphenoid, lacks a true compound bone in the mandible, and possesses a fully enclosed orbital rim. The last feature represents a highly derived structure in that the jugal establishes contact with the frontal internally, reinforcing the posterior orbital margin. S. fossorium also possesses a strongly modified Vidian canal with a previously unknown connection to the ventral surface of the parabasisphenoid. Comparison with the closely related fossil taxon Dyticonastis rensbergeri reveals that these derived traits are also shared by the latter species and potentially represent synapopmorphies of an extinct Paleogene clade of amphisbaenians. The presence of a reinforced orbital rim suggests selection against the loss of a functional eye and indicates an ecology potentially different from modern taxa. Given the currently accepted phylogenetic position of Spathorhynchus and Dyticonastis, we predict that supposedly 'unique' cranial traits traditionally linked to fossoriality such as a fused orbitosphenoid and the reduction of the eye show a more complex character history than previously assumed, including both parallel evolution and reversals to superficially primitive conditions.
穴居的蚓蜥类，即蚓蜥，其特征是在头骨和颅后骨骼中有一系列特化特征，然而化石证据表明，这些共同的衍生特征中至少有一些是趋同进化而来的。不幸的是，由于对许多化石分类单元缺乏详细了解，使得更精确的解释变得困难。在这里，我们使用高分辨率X射线计算机断层扫描（HRXCT）描述了来自美国怀俄明州始新世绿河组的已知最古老且保存完好的蚓蜥——斯帕托鼻蚓蜥（Spathorhynchus fossorium）的颅骨解剖结构。这个分类单元拥有蚓蜥类中已知强化程度最高的颅骨之一，许多真皮骨在内部相互重叠。与现代分类单元不同，斯帕托鼻蚓蜥有一对眶蝶骨，下颌缺乏真正的复合骨，并且拥有完全封闭的眶缘。最后一个特征代表了一种高度衍生的结构，因为颧骨在内部与额骨接触，加强了眶后缘。斯帕托鼻蚓蜥还拥有一个经过强烈改造的维杜斯管，与副蝶骨腹面有一个以前未知的连接。与密切相关的化石分类单元伦氏掘蚓蜥（Dyticonastis rensbergeri）比较表明，这些衍生特征后者也有，可能代表了一个已灭绝的古近纪蚓蜥类分支的共有衍征。强化眶缘的存在表明对失去功能性眼睛的选择，这表明其生态可能与现代分类单元不同。鉴于目前所接受的斯帕托鼻蚓蜥和掘蚓蜥的系统发育位置，我们预测，传统上与穴居性相关的所谓“独特”颅骨特征，如融合的眶蝶骨和眼睛的缩小，其特征历史比以前假设的更为复杂，包括平行进化和向表面原始状态的逆转。
