Early post-embryonic development in Ellipsostrenua (Trilobita, Cambrian, Sweden) and the developmental patterns in Ellipsocephaloidea

This study documents the early post-embryonic developmental stages (protaspides and early meraspides) of the Cambrian trilobite Ellipsostrenua granulosa (Ahlberg, 1984) from the Gärdsjön Formation of Jämtland, Sweden. The early protaspid stage is characterized by a circular outline of the exoskeleton, two pairs of fixigenal spines, a short preglabellar field, a genal swelling, and prominent bacullae. The late protaspid stage differs only in having the trunk portion discernible. Early meraspid cranidia are sub-rectangular with prominent palpebral lobes, a wide anterior margin, a proportionally long anterior branch of the facial suture and intergenal spines. Meraspid pygidia tentatively assigned to this species possess comparatively long macrospines. Small hypostomes associated with E. granulosa bear at least four pairs of marginal spines. A comparison of the early developmental stages of E. granulosa with some other species of Ellipsocephalidae and with species of the closely related Est...本研究记述了采自瑞典耶姆特兰省加德松（Gärdsjön）组的寒武纪三叶虫颗粒椭圆肋盾虫（Ellipsostrenua granulosa, Ahlberg, 1984）的胚胎后早期发育阶段（原盾期（protaspides）与早期后盾期（meraspides））。早期原盾期以其外骨骼呈圆形轮廓、两对固定颊刺（fixigenal spines）、短小的颊前区（preglabellar field）、颊部隆起（genal swelling）以及显著的轴脊（bacullae）为特征。晚期原盾期仅在可辨认出躯干部分这一点上与早期原盾期存在差异。早期后盾期的头盖（cranidia）呈近矩形，具显著的眼叶（palpebral lobes）、宽阔的前缘、比例修长的面线（facial suture）前支以及间颊刺（intergenal spines）。暂归入该种的后盾期尾甲（pygidia）具相对修长的大刺（macrospines）。与该种伴生的小型口下板（hypostomes）至少具四对边缘刺。本研究将颗粒椭圆肋盾虫的早期发育阶段与椭头虫科（Ellipsocephalidae）的其他若干物种，以及近缘的Est...类群物种进行了对比。