Miedema Feiko, Bindellini Gabriele, Dal Sasso Cristiano, Scheyer Torsten M, Maxwell Erin E
Staatliches Museum für Naturkunde Stuttgart, Rosenstein 1, 70191 Stuttgart, Germany.
Hohenheim University, Schloss Hohenheim 1A, 70599 Stuttgart, Germany.
Swiss J Palaeontol. 2023;142(1):27. doi: 10.1186/s13358-023-00289-z. Epub 2023 Oct 5.
Relatively complete ontogenetic series are comparatively rare in the vertebrate fossil record. This can create biases in our understanding of morphology and evolution, since immaturity can represent a source of unrecognized intraspecific variation in both skeletal anatomy and ecology. In the extinct marine reptile clade Ichthyopterygia, ontogenetic series were widely studied only in some Jurassic genera, while the ontogeny of the oldest and most basal members of the clade is very poorly understood. Here, we investigate cranial ontogeny in , from the Middle Triassic Besano Formation of the Swiss and Italian Alps. This small-bodied taxon is represented by a wealth of material from multiple size classes, including fetal material. This allows us to assess ontogenetic changes in cranial morphology, and identify stages in the ontogenetic trajectory where divergence with more derived ichthyosaurs has occurred. Early ontogenetic stages of show developmental patterns that are reminiscent of the presumed ancestral (early diverging sauropsid) condition. This is prominently visible in the late fetal stage in both the basioccipital, which shows morphology akin to basal tubera, and in the postorbital, which has a triradiate head. The ontogenetic trajectory of at least some of the cranial elements of is therefore likely still very akin to the ancestral condition, even though the adult cranium diverges from the standard diapsid morphology.
The online version contains supplementary material available at 10.1186/s13358-023-00289-z.
相对完整的个体发育系列在脊椎动物化石记录中较为罕见。这可能会在我们对形态学和进化的理解上产生偏差，因为不成熟可能代表着骨骼解剖学和生态学中未被认识的种内变异来源。在已灭绝的海洋爬行动物类群鱼龙目（Ichthyopterygia）中，个体发育系列仅在一些侏罗纪属中得到广泛研究，而该类群最古老和最基部成员的个体发育情况却知之甚少。在这里，我们研究了来自瑞士和意大利阿尔卑斯山三叠纪中期贝萨诺组的 的颅骨个体发育。这个小型分类单元有来自多个大小类别的丰富材料，包括胎儿材料。这使我们能够评估颅骨形态的个体发育变化，并确定个体发育轨迹中与更进化的鱼龙出现分歧的阶段。 的早期个体发育阶段显示出的发育模式让人联想到假定的祖先（早期分化的蜥形纲）状态。这在胎儿后期的枕基部尤为明显，其形态类似于基部瘤，在眶后骨中也很明显，眶后骨有一个三叉头。因此，即使成年颅骨与标准双孔类形态不同， 的至少一些颅骨元素的个体发育轨迹可能仍然与祖先状态非常相似。
在线版本包含可在10.1186/s13358 - 023 - 00289 - z获取的补充材料。
