Data from: Post-embryonic development of Dalmanitina, and the evolution of facial suture fusion in Phacopina

A large sample of post-embryonic specimens of Dalmanitina proaeva elfrida
and D. socialis from the Upper Ordovician (Sandbian to Katian) Prague
Basin allows for the first reasonably complete ontogenetic sequence of
Dalmanitoidea (Phacopina). The material provides an abundance of
morphological information, including well-preserved marginal spines in
protaspides and meraspides, and hypostome external surfaces throughout.
The development of D. proaeva elfrida is unusual due to variability in
timing of the first trunk articulation. While broadening our developmental
understanding of Phacopina, a diverse group of phacopid trilobites, we are
also able to study the evolution of their specializations in exoskeletal
molting behavior. Adult phacopines, unlike most other trilobites, had
fused facial sutures. This means that rather than molting through the
Sutural Gape mode, characterized by opening of the facial sutures and
separation of the librigenae, they disarticulated the entire cephalon in
Salter’s mode of molting. For other phacopine clades (Phacopoidea) the
transition to Salter’s mode occurs during the meraspid period or at the
onset of holaspis, and its developmental timing is intraspecifically
fixed. However, owing to the large sample size we can see that facial
suture fusion likely occurred later in Dalmanitina, usually during the
holaspid period, and was intraspecifically variable with holaspides of
varying sizes showing unfused sutures. Further, D. proaeva elfrida
specimens showed an initial librigenal-rostral plate fusion event, where
the librigenae began as separate entities but appear fused with the
rostral plate as one structure (the ‘Lower Cephalic Unit’) from M1, and
are discarded as such during molting. Dalmanitoidea is considered to
represent the first phacopine divergence, occurring earliest in the fossil
record. This therefore provides insight into how linked morphologies and
behaviors evolved, potentially suggesting the timing of facial suture
fusion in Phacopina moved earlier during development and became more
intraspecifically fixed over geological time.

采自奥陶纪晚期（桑比阶至凯迪阶）布拉格盆地的大量德姆三叶虫（Dalmanitina）proaeva elfrida与D. socialis的胚后发育标本，首次构建了德姆三叶虫超科（Dalmanitoidea）、镜眼三叶虫亚目（Phacopina）相对完整的个体发育序列。该标本群蕴含丰富的形态学信息，包括保存完好的原幼体期（protaspides）与分节幼体期（meraspides）边缘棘，以及各发育阶段口板（hypostome）的外表面结构。D. proaeva elfrida的发育过程存在特殊之处：其首次躯干关节形成的时间存在种内变异。本研究在加深我们对镜眼三叶虫亚目——一类多样的镜眼三叶虫类三叶虫——发育认知的同时，还得以探讨其外骨骼蜕皮行为特化的演化历程。与多数其他三叶虫不同，成年镜眼三叶虫拥有愈合的面线（facial sutures）：这意味着它们并非通过以面线开裂、活动颊（librigenae）分离为特征的面线开裂模式（Sutural Gape mode）蜕皮，而是采用索特蜕皮模式（Salter’s mode）脱卸整个头甲。对于其他镜眼三叶虫超科（Phacopoidea）类群，向索特蜕皮模式的转变发生在分节幼体阶段或成体期起始阶段，且其发育时间在种内是固定的。然而得益于本次研究的大样本量，我们发现德姆三叶虫的面线愈合时间通常更晚，多发生在成体阶段，且种内存在变异：不同体型的成体个体仍可能存在未愈合的面线。进一步观察显示，D. proaeva elfrida的标本存在初始的活动颊-喙板（rostral plate）融合事件：活动颊最初为独立结构，但从M1阶段起便与喙板融合为单一结构（即“下头甲单元”），并在蜕皮过程中整体脱卸。德姆三叶虫超科被认为是镜眼三叶虫亚目最早的分化类群，在化石记录中出现时间最早。因此本研究可为关联形态与行为的演化提供重要启示，这也暗示镜眼三叶虫亚目的面线愈合时间在地质演化历程中逐渐提前，且种内固定程度不断提升。