Taxonomy, evolution, and biostratigraphy of the Orthograptus quadrimucronatus species group (Ordovician, Graptolithina)
Taxonomy, evolution, and biostratigraphy of the Orthograptus quadrimucronatus species group (Ordovician, Graptolithina)
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DOI：
10.1017/s0022336000034909
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发表时间：
1995-05
影响因子：
1.4
通讯作者：
D. Goldman
中科院分区：
文献类型：
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作者：
D. Goldman
quadrimucronatus种群目前包括22个分类群，除O. ruedemanni外，它们与其他正形目的区别在于它们的鞘上存在成对的孔状刺。详细的形态计量学分析表明，这些分类群包括四种基本形态型，实际上只代表六个不同的物种：O. pageanus， O. quadrimucronatus， O. spinigerus， O. ruedemanni， O. eucharis和O. rivai。在头两个或四个囊上有一个迅速扩大的横纹肌和细长的刺。按时间顺序分为两个亚种，一个较老的，较小的形式，O. pageanus pageanus，和一个较年轻的，较大的形式，O. pageanus maximus n. subsp。棘棘草的标本在第6对和第13对之间的1对或2对鞘鞘上有细长的刺，近端很窄，横纹肌迅速扩大。四爪棘龙的标本具有更平行的侧面形状，没有异常长的刺。亚种O. quadrimucronatus richmondensis是由缺少中隔和第12棘来区分的。ruedemanni， O. rivai和O. eucharis都是矮形。在基于k -膨润土地层相关性的年代地层格架中，研究了O. quadrimucronatus物种群成员的进化变化模式和过程。在形态计量学上，每个物种都是一个连贯的实体，占据着自己独特的形态空间。每个物种形成事件可能发生得很快，排除了中间物种的保存，新物种在其存在期间保持了其基本形态。在这些谱系中，即使累积的时间间隔比预期的物种形成事件要长得多，种系变化要么不存在，要么只包含大小的变化。这些模式似乎与间断平衡的进化模型相一致。
The Orthograptus quadrimucronatus species group currently comprises 22 taxa, which, with the exception of O. ruedemanni, are distinguished from other orthograptids by the presence of paired apertural spines on their thecae. A detailed morphometric analysis of this group indicates that these taxa comprise four basic morphotypes that actually represent only six distinct species: O. pageanus, O. quadrimucronatus, O. spinigerus, O. ruedemanni, O. eucharis, and O. rivai. Specimens of Orthograptus pageanus have a rapidly widening rhabdosome with elongated spines on the first two or four thecae. Orthograptus pageanus is divided into two chronological subspecies, an older, smaller form, O. pageanus pageanus, and a younger, larger form, O. pageanus maximus n. subsp. Specimens of O. spinigerus have elongated spines on one or two thecal pairs between the sixth and thirteenth thecal pairs, a very narrow proximal end, and a rapidly widening rhabdosome. Specimens of O. quadrimucronatus have a more parallel-sided shape and no unusually long spines. The subspecies O. quadrimucronatus richmondensis is differentiated by the absence of both a median septum and a th 12 spine. Orthograptus ruedemanni, O. rivai, and O. eucharis are all dwarf forms. Within a chronostratigraphic framework based on K-bentonite bed correlations the pattern and process of evolutionary change in the members of the O. quadrimucronatus species group is examined. Morphometrically, each species remains a coherent entity through time, occupying its own distinct morphospace. Each speciation event probably occurred rapidly, precluding the preservation of intermediates, and the new species retained their basic morphology throughout their existence. Within these lineages, phyletic changes were either absent or encompassed only changes in size, even when accumulated for much longer intervals than would be expected for speciation events. These patterns appear to be consistent with the evolutionary model of punctuated equilibrium.