ABSTRACT

The porolepiform Holoptychius Agassiz, 1839 from the Upper Devonian is one of the most widely found vertebrates in the Palaeozoic fossil record. Holoptychius is considered to display a more ubiquitous mode of life and a greater potential of dispersion than any other sarcopterygian taxon, consistent with its cosmopolitan distribution and profuse fossil record, mainly represented by scales. Previous studies on the squamation of Holoptychius have discussed the variable ornamentation of the superficial layer; however, the basal plate has been less explored. Here we furnish new histological data on the scales of Holoptychius, focusing on the mineralisation rate and organisation of the lamellar bone. Ground sections show that the basal plate is made of a stacking of thick collagenous layers with a plywood-like structure. Collagenous fibres are parallel within layers, with two successive layers orthogonally arranged and with every second layer rotated by an angle of 36º. This condition, known as a double-twisted plywood-like organisation, is similar to that of other sarcopterygians like Eusthenopteron and extant coelacanths and dipnoans. The new palaeohistological data provide insights into the morphofunctional, palaeoecological and phylogenetical implications of the microstructural characteristics of the scales, adding to our knowledge of the histological diversity of osteichthyans.

摘要

1839 年来自上泥盆纪的 porolepiform Holoptychius Agassiz 是古生代化石记录中发现最广泛的脊椎动物之一。Holoptychius被认为显示出比任何其他肉翅目分类群更普遍的生活方式和更大的分散潜力，这与其全球分布和丰富的化石记录一致，主要以鳞片为代表。以前对海参鳞片的研究讨论了表层的可变纹饰；然而，对基底板的探索较少。在这里，我们提供了关于Holoptychius尺度的新组织学数据，重点关注板层骨的矿化率和组织。地面剖面显示，基底板由一层厚厚的胶原层堆叠而成，具有类似胶合板的结构。胶原纤维在层内平行，两个连续的层正交排列，每隔一层旋转 36°。这种情况被称为双扭曲胶合板状组织，类似于其他肉翅目动物，如Eusthenopteron和现存的腔棘鱼和 Dipnoans。新的古组织学数据提供了对鳞片微观结构特征的形态功能、古生态学和系统发育影响的见解，增加了我们对骨鱼的组织学多样性的了解。