分支系统学与中国北部寒武纪高肌虫分类探讨

CLADISTICS AND CLASSIFICATION OF CAMBRIAN BRADORIIDS FROM NORTH CHINA

古生物学者在进行化石生物分类时,常以形态总体相似性或者某几个甚至某一个所谓的“主要性状”为分类依据。由此而产生的分类系统就常常不可避免地引起争议甚至产生错误。这是分类学者自觉或不自觉受到数值系统学(Numerical systematics)和进化系统学(Evolutionary systematics)影响的结果。究其原因,主要是由于不少分类学者对于生物分类的目的往往可能理解不全,或者即使完全了解也未予足够重视(当然,这与化石性状的局限性亦有一定关系)。许多分类学者可能以为,生物的分类就是同类相合,异类相分,如同图书馆的检索一样。其目的主要在于方便应用。

The principles and methods of cladistics are briefly introduced and reviewed in this paper, it is particularly worth notice that if one sister group is given the same rank of category as another according to Hennig's cladistics, some nominal taxa would be inevitably emerged. But, because a third taxon is either nearer to or farther from other two taxa in affinity relationship, it can be classified into a higher taxon including the latter or separately regarded to be the same higher taxon as that of latter. Thus, the unnecessary nominal taxa can be avoided although some necessary nominal taxa are still reserved. Consequentiy, this kind of classification could reflect the more unequivocal affinities among these taxa. Here the classification of Cambrian bradoriids from North China is dealt with based on the discussion. Of the bradoriids, Beyrichoninae and Houlongdongellinae are the sister groups constituting the family Beyrichonida. This family and Tsunyieilidae are other sister groups constituting the superfamily Beyrichona cea characterized by a depression or a groove developed on their carapace surfaces. All these taxa are grouped into the suborder Bradoriina owing to the bivalved, relatively small to middle-sized carapace, single hingeline, and developed ridge border which are the synapomorphies of the suborder. The phosphatocopinids have been split into two suborders, namely, Hesslandonina and vestrogothiina, mainly on the basis of the single or double hingeline (Müller, 1982). The latter as deemed with a single hingeline, however, has a double hingeline in reality. This can be testified by Hesslandona unisulcata Müller 1982 which may be a species of Falites with a double hingeline. It seems that Vestrogothia also has a double hingeline which has been hinted probably in V. spinata Müller 1964 (Müller, 1.964, p1. 2, fig. 7a). The refore the two suborders are tacitly synonyms of the suborder Phosphatocopina. In this suborder, the families Hesslandonidae and Falitidae are sister groups constituting the superfamily Hessian donacea. Another newly erected family, Dabashanellidae, can be given the same rank of category——superfamily—as that of Hesslandonacea because the former and the latter are sister groups. The former group is discriminated from the latter by the single hingeline, bivalved carapace and smooth carapace surface. In contrast with the former hesslandonaceans are characterized by the parabivalved carapace, double hingeline and the presence of a dorsal bar. The common features of the two groups lie in the very small (smaller than 1.5 mm in general) carapaces and the absence f the ridge border, which are the synapomorphies of Phosphatocopina. On the other hand, this suborder and Bradoriina are sister groups sharing the characters of small carapace, straight dorsal margins and symmetrical bivalves, which can be considered as synapomorphies of a monophyletic group—— the order Bradoriida (Tabs.Ⅰ—Ⅲ). In addition, the bradoriids with triangular or V-shaped grooves on their carapaces are also discussed. Such bradoriids have been assigned to the family Oepikalutidae and considered as phosphatocopinids by Jones and McKenzie (1980). With a well-developed ridge border and 'at least four partially preserved thoracic segments' (just as they have pointed out), these bradoriids obviously are not phosphatocopinids, for both ridge border and segment are all absent in the latter. Contrastively, the well-developed ridge border of these bradoriids shows that they belong to the suborder Bradoriina confirmably. However, the type genus of Oepikalutidae, Oepikaluta, is really a synonym of Houlongdongella established by Li Yuwen(1975). As a result, the name of Oepikalutidae becomes invalid and the bradoriids of this kind should be included in the subfamily Houlongdongellinae of Huo and Shu (1985). It must be further illustrated that the suborder name of Bradoriina was first used by Müller in 1964 who included the family Indianidae within the suborder besides Bradoriidae and Beyrichonidae. If so, the suborder would be untenable because indianids are close to Phyllocarida in affinity and thus the suborder would become a polyphyletic group that is unnatural and unreal in the light of cladistics. Therefore the Indianidae and other phyllocarid-like crustaceans should be driven out of Bradoriina (and also the Bradoriida) and the suborder should be revised as stated above. Another suborder, Lipabdomina of Huo and Shu (1985), with the same implication as Bradoriina also should be abolished. Finally, two new families Dabashanellidae (fam. nov.) and Liangshanellidae (fam. nov.) and two new genera Xichuanella (gen. nov.) and Pseudindiana (gen. nov.) are described, with the two genera separately grouped into Falitidae and Dabashanellidae (ram. nov.) of Phosphatocopina. Description of new families and generaFamily Liangshanellidae fam. nov. Type genus: Liangshanella Huo, 1956 Diagnosis: Carapace small; carapace surface smooth and ridge border developed. Family Dabashanelltdae faro. nov. Typic genus: Dabashanella Huo, Shu et Fu, 1983 Diagnosis: Carapace very small and bivalved; hingeline single and carapace surface smooth. Genus Xichuanella gen. nov. Type species: Monasterium bucerum Zhang, 1987 (P1. Ⅰ, fig. 3) Diagnosis: Carapace very small and hemicircular, hingeline probably in double structure; anterodorsal area of carapace developed with a horn-shaped node; ridge border absent. Discussion: This genus is similar to Monasterium in the presence of a node on the anterodorsal area, but differs in the absence of a ridge border, particularly, in the absence of a hemi-circular depression which is developed on the carapace surface of the latter. Genus Pseudindiana gen. nov. Type species: Indiana sipa Fleming, 1973. Diagnosis: Carapace very small; anterior and posterior ends with no spikes; carapace surface smooth and ridge border absent. Diseussion: The new genus is identical with Indiana in outline and ornament, but differs in the carapace which is much larger in the latter.