Histology of the Preparietal: A Neomorphic Cranial Element in Dicynodont Therapsids
Histology of the Preparietal: A Neomorphic Cranial Element in Dicynodont Therapsids
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DOI：
10.1080/02724634.2020.1770775
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发表时间：
2020-03
影响因子：
1.4
通讯作者：
L. Marilao;Z. Kulik;C. Sidor
中科院分区：
文献类型：
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作者：
L. Marilao;Z. Kulik;C. Sidor
摘要颅顶骨化是一种新形态的中线骨化，在兽孔目动物中经历了三次进化，但其发展和同源性仍然知之甚少。在这里，我们提供了初步的组织学数据，这个元素的标本中提到的Diictodon feliceps和一个不确定的物种的水龙。先前已经注意到，在几种双齿兽的头骨背表面上，骨突的形状有很大的变化，我们在薄片上也发现了类似的变化。在双齿龙中，齿突形成一个叉，完全嵌入额骨内，并显示出前方中线缝合的证据。水龙兽的切片标本显示出不成熟的组织学证据，其特征是在背骨后端有一条清晰的中线缝，尽管没有前叉。在这两个类群中，前腹侧的部分，形成一个强烈的交错缝与下面的额和顶骨。更靠后，骨板由纤维板层骨组成，提示快速的后腹生长。在大型的双齿齿类物种中，背侧的表达与个体发育过程中的其他颅顶元素相比，似乎表现出负的异速生长，但这种几何学的意义尚不清楚。此外，还需要对柳珊瑚目和双头鳄目中的齿骨进行组织学研究，以确定双齿兽的特征是否也出现在其他两组进化出齿骨的兽孔目中。该therapsid napietal提供了一个难得的机会，研究的发展和进化的一个新的颅元素在脊椎动物化石记录。
ABSTRACT The preparietal, a neomorphic midline ossification on the skull roof, is thought to have evolved three times in therapsids, but its development and homology remain poorly understood. Here, we provide preliminary data on the histology of this element in specimens referred to Diictodon feliceps and an indeterminate species of Lystrosaurus. The preparietal has previously been noted to vary substantially in its shape on the dorsal surface of the skull in several dicynodonts, and we found similar variation in thin section. In Diictodon, the preparietal forms a prong that embeds itself entirely within the frontals and shows evidence of a midline suture anteriorly. The sectioned specimen of Lystrosaurus shows histological evidence of immaturity and features a well-defined midline suture at the posterior end of the preparietal, although an anterior prong was not present. In both taxa, the anteroventral portion of the preparietal forms a strongly interdigitating suture with the underlying frontals and parietals. More posteriorly, the preparietal is composed of fibrolamellar bone suggestive of rapid posteroventral growth. In large dicynodont species, the dorsal expression of the preparietal appears to show negative allometry compared with other cranial roofing elements during ontogeny, but the significance of this geometry is unclear. In addition, histological work is needed on the preparietal in gorgonopsians and biarmosuchians to determine whether the features characterizing dicynodonts are also seen in the other two groups of therapsids that evolved a preparietal. The therapsid preparietal provides a rare opportunity to study the development and evolution of a neomorphic cranial element in the vertebrate fossil record.