Moritz Leif, Wesener Thomas, Koch Markus
Zoologisches Forschungsmuseum Alexander Koenig, Leibniz Institute for Animal Biodiversity, Section Myriapoda, Adenauerallee 160, 53113 Bonn, Germany.
Institute of Evolutionary Biology and Ecology, University of Bonn, An der Immenburg 1, 53121 Bonn, Germany.
Zookeys. 2018 Mar 7(741):77-91. doi: 10.3897/zookeys.741.21909. eCollection 2018.
The presence of a swinging tentorium is a key apomorphy of Myriapoda, but this character has been studied in detail in only few species. Here the tentorium, i.e., the peristomatic skeleton of the preoral chamber, is comparatively studied in three species of the millipede order Sphaerotheriida Brandt, 1833. Since dissections of the fragile tentorial components proved to be difficult, despite the large head size, they were analysed mainly in situ via micro-computed tomography. Our results confirm previous observations of large differences in the tentorial construction in the giant pill-millipedes compared to chilognathan diplopods. The tentorium of Sphaerotheriida consists of a curved, plate-like epipharyngeal bar with distal projections, an elongate and thin hypopharyngeal bar, and a plate-like triangular posterior process; a transverse bar is absent. Only seven muscles attach at the tentorium in giant pill-millipedes, including two antennal muscles and two muscles of the gnathochilarium. Within the order Sphaerotheriida, the composition of the tentorium and its muscular equipment seems to be conserved, except for some variability in the shape of the epipharyngeal bar. As the transverse bar has been considered essential for the mobility of the tentorium in myriapods, its absence in Sphaerotheriida may indicate that their tentorium is not capable of performing a swing. Loss of tentorial mobility may also pertain to the order Glomerida Brandt, 1833, inferred here from the absence of a posterior process. An apparently immobile tentorium in Glomerida and Sphaerotheriida can straightforwardly be correlated with transformations of the head related to their ability of volvation. The different transformations of the tentorium, here hypothesised to cause immobility, may support current assumptions that the ability of volvation evolved convergently in Glomerida and Sphaerotheriida. This conclusion, however, still requires more detailed studies of the head anatomy in Glomerida and Glomeridesmida Cook, 1895.
摆动幕骨的存在是多足纲的一个关键衍征，但仅在少数物种中对这一特征进行了详细研究。在此，对1833年布兰特所定球马陆目三种马陆的幕骨（即口前腔的围口骨骼）进行了比较研究。尽管头部尺寸较大，但由于脆弱的幕骨组成部分难以解剖，因此主要通过显微计算机断层扫描对其进行原位分析。我们的结果证实了先前的观察结果，即与唇颚类倍足纲动物相比，巨型丸状马陆的幕骨结构存在很大差异。球马陆目的幕骨由一根弯曲的、板状的咽上杆和远端突起、一根细长且薄的咽下杆以及一个板状三角形后突组成；没有横杆。巨型丸状马陆只有七块肌肉附着在幕骨上，包括两块触角肌和两块颚唇肌。在球马陆目内，幕骨的组成及其肌肉装置似乎是保守的，除了咽上杆的形状存在一些变化。由于横杆被认为是多足纲动物幕骨活动所必需的，球马陆目中没有横杆可能表明它们的幕骨无法进行摆动。幕骨活动能力的丧失也可能与1833年布兰特所定的球马陆目有关，在此是根据后突的缺失推断出来的。球马陆目和球马陆目中明显不能活动的幕骨可以直接与与其卷曲能力相关的头部变化联系起来。在此假设导致不能活动的幕骨的不同变化可能支持当前的假设，即卷曲能力在球马陆目和球马陆目中是趋同进化的。然而，这一结论仍需要对球马陆目和1895年库克所定的球马陆亚目动物的头部解剖结构进行更详细的研究。
