Chemico–structural phylogeny of the discinoid brachiopod shell
Chemico–structural phylogeny of the discinoid brachiopod shell
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盘类腕足动物壳的化学结构系统发育
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
J. Buckman
中科院分区：
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作者：
Alwyn Williams;M. Cusack;J. Buckman
活discinate的层状壳由膜状层状和不同聚集的蛋白质包裹的磷灰石磷灰石颗粒组成，这些颗粒由糖胺聚糖（GAGs）中的蛋白质和几丁质网支撑，形成有节奏的层状集合。这种演替类似于现存的舌鱼，但在膜的结构、杆状体的性质及其有机组成上有很大的不同。特别是，犬牙菌具有较高的氨基酸水平，尽管相对较低的酸性和较高的碱性浓度；它们的整体有机含量较低是由于亲水性成分水平较低，如gag和几丁质。有机成分在石化过程中未完全降解；但目前的数据太少，无法区分舌状体和犬科动物的祖先。所描述的四个现存属中的三个属之间的许多差异源于长期地质历史的转变。Pelagodiscus的特征是具有弹性的骨膜和可塑的初级层的规则的、同心的流变折叠（丝状）和零星发育的半球形的骨膜囊泡印记。这两个特征在古生代犬齿动物中更为突出。在最古老的物种奥陶系裂肢亚目和较年轻的圆肢亚目及其相关属中，囊泡是持久的六边形紧密排列的，在细丝上逐渐消失。然而，它们的组成一定不同，因为裂肢纲的大囊泡是简单的（可能是粘液），而圆尾纲和较年轻的属的小囊泡是由厚涂层的球体组成的，可能是脂蛋白，它们作为Pelagodiscus的分解残留物存活下来。在活犬科中，第二层内的小管套也不太发达，在Pelagodiscus中是早期的，仅限于Discinisca的背瓣。具有蛋白核的格状杆状体（杆状体）由松果状体或磷灰石棱柱状体组成，这取决于它们是由几丁质网还是由gag中的蛋白链支撑。这种分化发生在Schizotreta中，但在该种群（和Trematis）中，杆状集与杆状集对称分布在紧密的层板之间，而在较年轻和后古生代物种中，该集的外部边界层(e)通常是膜质和/或分层的。活的discinids中最显著的突触形态是在幼虫的外上皮内分泌具有结晶习惯的有机硅片，它们的胞吐作为幼虫封闭或开放的短暂生物矿物覆盖物。另一方面，运河与遍布扁壳的运河是同源的。这两个系统都与几丁质和蛋白质组相互连接，并且可能一直在支撑层状序列的有机支架中充当垂直支柱。一项主要基于壳结构的系统发育分析证实了discinoids是linguloids的姐妹类群，但与目前的分类实践相反，它也支持将acrotretoids包含在discinoid分支中，作为discinids的姐妹类群。
Stratiform shells of living discinids are composed of membranous laminae and variously aggregated, protein–coated granules of apatitic francolite supported by proteinaceous and chitinous nets in glycosaminoglycans (GAGs) to form laminae in rhythmic sets. The succession is like that of living lingulids but differs significantly in the structure of the periostracum, the nature of baculate sets and in its organic composition. In particular, discinids have a higher level of amino acids although with relatively lower acidic and higher basic concentrations; and their overall lower organic content is owing to lower levels of hydrophilic components, like GAGs and chitin. The organic constituents are not completely degraded during fossilization; but data are presently too meagre to distinguish between linguloid and discinoid ancestries. Many differences among three of the four described extant genera emanate from transformations with a long geological history. Pelagodiscus is characterized by regular, concentric rheomorphic folding (fila) of the flexible periostracum and the plastic primary layer and by sporadically developed hemispherical imprints of periostracal vesicles. Both features are more strikingly developed in Palaeozoic discinids. In the oldest discinid, the Ordovician Schizotreta , and the younger Orbiculoidea and related genera, vesicles were persistent, hexagonal close–packed arrays fading out over fila. They must have differed in composition, however, as the larger vesicles of Schizotreta were simple (possibly mucinous), whereas the smaller vesicles of Orbiculoidea and younger genera were composites of thickly coated spheroids, possibly of lipoproteins, which survive as disaggregated relicts in Pelagodiscus . Baculate sets within the secondary layer are also less well developed in living discinids, being incipient in Pelagodiscus and restricted to the dorsal valve of Discinisca . The trellised rods (baculi) with proteinaceous cores are composed of pinacoids or prisms of apatite, depending on whether they are supported by chitinous nets or proteinaceous strands in GAGs. This differentiation occurred in Schizotreta but in that stock (and Trematis ) the baculate set is symmetrical with baculi subtended between compact laminae, whereas in younger and post–Palaeozoic species the outer bounding lamina(e) of the set is normally membranous and/or stratified. The most striking synapomorphy of living discinids is the intravesicular secretion of organsiliceous tablets with a crystalline habit within the larval outer epithelium and their exocytosis as a close– or open–packed, transient, biomineral cover for larvae. Canals, on the other hand, are homologous with those pervading lingulid shells. Both systems interconnect with chitinous and proteinaceous sets and have probably always served as vertical struts in an organic scaffolding supporting the stratiform successions. A phylogenetic analysis based mainly on shell structure confirms the discinoids as the sister group of the linguloids but, contrary to current taxonomic practice, also supports the inclusion of acrotretoids within a ‘discinoid’ clade as a sister group to the discinids.