Ontogenetic variation in the cranium of Mixosaurus cornalianus, with implications for the evolution of ichthyosaurian cranial development.

IF 3 2区 地球科学 Q1 PALEONTOLOGY Swiss Journal of Palaeontology Pub Date : 2023-01-01 Epub Date: 2023-10-05 DOI:10.1186/s13358-023-00289-z
Feiko Miedema, Gabriele Bindellini, Cristiano Dal Sasso, Torsten M Scheyer, Erin E Maxwell
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Abstract

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 Mixosaurus cornalianus, 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 Mixosaurus 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 Mixosaurus is therefore likely still very akin to the ancestral condition, even though the adult cranium diverges from the standard diapsid morphology.

Supplementary information: The online version contains supplementary material available at 10.1186/s13358-023-00289-z.

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角混合龙颅骨的个体遗传学变异,对鱼龙类颅骨发育的进化具有启示意义。
相对完整的个体发生系列在脊椎动物化石记录中相对罕见。这可能会在我们对形态和进化的理解中产生偏见,因为不成熟可能是骨骼解剖和生态学中未被识别的种内变异的来源。在已灭绝的海生爬行动物Ichthypterygia分支中,个体发生系列仅在侏罗纪的一些属中被广泛研究,而该分支中最古老和最基础的成员的个体发生却知之甚少。在这里,我们研究了瑞士和意大利阿尔卑斯山脉中三叠纪贝萨诺组的角腹混合龙的头骨个体发育。这个小型分类单元以来自多个大小类别的丰富物质为代表,包括胎儿物质。这使我们能够评估颅骨形态的个体发生变化,并确定个体发生轨迹中与更多衍生鱼龙发生分化的阶段。Mixosaurus的早期个体发生阶段显示出的发育模式让人想起了推测的祖先(早期分化的蜥脚类)状况。这在胎儿晚期的基枕部和眶后部都明显可见,基枕部的形态类似于基底结节,眶后部有三放射状头部。因此,Mixosaurus至少一些头骨元素的个体发育轨迹可能仍然与祖先的状况非常相似,尽管成年头骨与标准的底辟形态不同。补充信息:在线版本包含补充材料,可访问10.1186/s13358-023-00289-z。
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来源期刊
Swiss Journal of Palaeontology
Swiss Journal of Palaeontology Earth and Planetary Sciences-Paleontology
CiteScore
4.30
自引率
16.70%
发文量
17
审稿时长
4 weeks
期刊介绍: The Swiss Journal of Palaeontology publishes original research and review articles of interest to the international community in the fields of palaeontology, taxonomy and systematics, while recognising at the same time the importance of documenting high-quality palaeontological data in a regional context. Palaeobiology in combination with alpha taxonomy is a core topic of the journal. Submitted papers should have an appeal as wide as possible, directed towards an international readership. Contributions should not have been simultaneously submitted elsewhere, and the overlap of content between related articles should be minimal. Duplications of text and the use of previously published illustrations without adequate citation are unacceptable. If a manuscript has two or more authors, both or all have to sign to confirm they all were involved in the work and have agreed to its submission. The preferred manuscript language is UK English, but consistently used US English is also acceptable. We encourage the publication of proceedings of international meetings as well as special thematic issues. Short contributions and book reviews are also accepted. An international editorial team as well as guest editors guarantee that the thematic issues as well as all articles in regular issues are peer-reviewed and meet the highest standards.
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