摘要
前言节肢动物不仅是现代动物界中最为成功的生物类群,在长期演化历史中也一直是非常成功的类群。早期的节肢动物,除三叶虫外,大都骨骼化程度较差,不易保存为化石,只有在非常特异的地质条件下非三叶虫类节肢动物才可能保存成为化石。云南澄江下寒武统筇竹寺组玉案山段下部地层中最近发现了众多非三叶虫类节肢动物化石,这一发现填补了寒武纪早期非骨骼化节肢动物化石记录的缺环,其中部分化石已作了报道(张文堂、侯先光,1985;侯先光,
The preservation of soft parts in fossil is extremely rare. A notable exception is known from the basal Lower Cambrian Yuanshan Member (upper part of the Chiungchussu Formation) at the Maotian hill, Chengjiang, E. Yunnan, where completely soft-bodied groups, soft parts of creatures with resistant exoskeletons, and animals with fragile exoskeletons are beautifully preserved. Arthropods account for the largest part of the Chengjiang fauna in evidence of biomass and numbers of taxa. Information on their taxonomy accumulated in the last few years has resulted in the publications by Zhang and Hou (1985), Hou (1987 a, b, c) and Hou and Sun (1988). In addition, three new species belonging to three new genera are erected for non-trilobite arthropods, which are distinguished from one another by the broadly separate morphology of the exoskeleton; however, their body is unknown. These re Retifacies abnormalis gen. et sp. nov., Acanthomeridion serratum gen. et sp. nov. and Urokodia aequalis gen. et sp. nov., but all of them are rare. The first taxon bears a large, up to 75mm long, elliptically outlined exoskeleton covered with fine reticulate ornaments (Text-fig. 2). In contrast the second one is elongate, consisting of a cephalic shield,10 trunk tergites and a pair of fan-shaped telsons (Text-fig. 4), while the third one bears a relatively small exoskeleton, which consists of a cephalic shield, 14 trunk tergites nd a tail shield of cephalic shield analogy (Text-figs. 9, 10). All the specimens available for the present study were collected from the west (Coll. No. M_2, M_3 and M_4) and northwest (Coll. No. cf_3 and cf_6) slopes of the Maotian hill (Hou, 1987a, text-figs.4,5), and from 3 km southwest of the same hill, near the Dapotou village (Coll. No. Dj1 and Dj2) (Chen et al., 1989a, text-fig. 1). They are embedded in the moderately weathered, greyish-yellow mudstone beds in the lower part of the Yuanshan Member corresponding to the lower part of the Eoredlichia Zone in the trilobite succession. The Yuanshan Member is the unit with its name given to the upper part of the Lower Cambrian Chiungchussu Formation, in which a superbly preserved new fauna is encountered. Description of new genera and species Retifacies gen. nov. Type species: Retifacies abnormalis gen. et sp. nov. Etymology: Reti-and facies, Latin, net and surface, in reference to the reticulations on the surface of exoskeleton; abnormalis, Latin, different-shaped, in reference to the difference in reticulate outline between the cephalic (or tail) shield and trunk region. Diagnosis: Exoskeleton large and broad, sub-elliptical in outline, not trilobate, divided into cephalic shield, 10 trunk tergites and a tail shield. A pair of antennae projecting beyond the anterior margin of the cephalic shield. Tail shield large and broad, elliptical in outline, with a pair of posterior spines. External surface of exoskeleton covered with reticulate ornaments. Retifacies abnormalis gen. et sp. nov. (P1.Ⅰ, figs. 1—6; pl. Ⅱ, figs. 1—4; text-figs. 1, 2) Material: Four comparatively completely preserved exoskeletons and three isolated tail shields, all compressed dorsally. Holotype, Cat. No.108298; Paratype, Cat. No. 108299—108304. Description: Cephalic shield broad (transverse) and short (sagittal), semi-elliptical in outline of dorsal view, with pointed posterolateral angle (Pl. Ⅱ, figs. 1, 3; text-fig.1), A pair of poorly preserved antennae observed from the paratype counterpart (Pl.Ⅱ,figs.1, 3; text-fig. 1), but with their distal portion damaged; segmentations faintly visible in right antenna, with segment wider (transverse) than long (sagittal). Anterior three tergites probably straight (Pl.Ⅰ, fig. 2); remaining ones curved, with their curvature increasing posteriorly in successive order, and attaining to a maximum at the extreme.one (Pl. Ⅰ, figs. 1,2). Anterior 5 tergites slightly shorter (sagittal) than the remaining, posterior 5 ones, Tergites appearing progressively narrower from the fifth one anteriorly and posteriorly, thus with the fifth one being the broadest (Pl, Ⅰ, fig. 1). Trunk tergites, and trunk, cephalic or tail shield imbricating each other, with the posterior one telescoped beneath the anterior one to form an articulating flange observed as narrow band (Pl. Ⅰ, figs. l, 2; pl.Ⅱ, figs. 1, 3; text-fig. 1). Lateral extremities of each tergite, pointed and equal. Tail shield broad and large, elliptical in outline of dorsal view, bearing a pair of short posterior spines (Pl. Ⅰ, figs. 2, 3). Entire surface of exoskeleton covered with reticulate ornaments except the pointed lateral extremities and posterior tail spines with smooth surface. Reticulations smaller, on cephalic shield than on tail shield, irregularly pentagon- or hexagonshaped (Pl.1, figs. 1, 3, 6; pl. Ⅱ, figs. 1, 3, 4). Sculpture consisting of two transverse rows of reticulations on each of the trunk tergites; reticulations in the anterior row large and poorly preserved (pl.Ⅰ, fig, 1; pl. Ⅱ, figs. 1—3; text-fig. 1), corresponding to the area free from imbrication; those in the posterior row smaller but well-preserved (P1. Ⅰ, fig. 1; Pl.Ⅱ, figs. 1—3; text-fig. 1), covering an area roughly relevant to the imbricating area. An additional small circular pattern of reticulations observed on the trunk region restricted to the sagittal area, (Pl.Ⅰ, fig, 1). Three specimens available for measurement, with their length represented by the sagittal distance from the anterior margin of cephalic shield to the tip of the tail shield's posterior spines, and their maximum width represented by the fifth tergite measured across. Holotype 36mm long, 24 mm across; specimen. (Pl. Ⅱ, fig.1) about 43 mm long and 27 mm across, Specimen (Pl. Ⅰ, figs. 4, 5) represented by cephalic shield and trunk region; its original size estimated at about 75mm long and 50 mm across based on length-width ratio of the holotype. Discussion: The restoration of text-figure 2 is principally based on the observation of the specimens belonging to holotype (Pl.Ⅰ, fig. 1) and the counterpart of a paratype (Pl. Ⅱ, fig. 1) with a completely preserved exoskeleton consisting of a cephalic shield, 10 trunk tergites and a tail shield. The specimen (Pl.Ⅰ, figs. 4,5) possesses a cephalic shield and 10 trunk tergites; its 3rd to 10th tergites are strongly curved probably resulting from different distortions through the articulating flange during compaction. The specimen (Pl.Ⅰ, fig. 2) consists of 8 trunk tergites, but the 5th and 6th ones are absent, suggesting that probably each of them is separable. Our collection possesses separately preserved tail shields but none of separate cephalic shields, providing the evidence that the tail shield might be more easily separated from the trunk than the cephalic shield. The present new taxon possesses a large and broad non-trilobate exoskeleton with a large tail shield, showing a general similarity to the Lower Cambrian taxa Kuamaia lata Hou, 1987 and Rhombicalvaria acantha Hou, 1987 from Chengjiang, and to the Middle Cambrian Burgess Shale taxon Helmetia expansa Walcott, 1918 from British Columbia. It differs from the latter three ones in its reticulated exoskeleton consisting of semi-elliptical cephalic shield wtih no oval structures, 10 trunk tergites and a tail shield with no median terminal spines. Genus Acanthomeridion gen. nov. Type species: Acanthomeridion serratum gen. et sp. nov. Etymology: Acantho- and meridion, Greek, spine and somite, in reference to the extremely long lateral extremities of the 9th tergite; serratus, Latin, sawtooth, in reference to the sawtoothed posterior margins of the cephalic shield and trunk tergites. Diagnosis: Exoskeleton elongate, sub-rectangular in outline of dorsal view, divided into cephalic shield,10 trunk tergites and a pair of fan-shaped telsons; 9th tergite bearing exceedingly long lateral extremities which extend posteriorly far beyond the telson. Acanthomeridion serratum gen. et sp, nov. (Pl. Ⅲ, figs. 1—5; pl. Ⅳ, figs. 1—5; text-figs. 3, 4) Material: Five nearly complete exoskeletons and one separately preserved cephalic shield, all dorsally compressed. Holotype, Cat. No. 108305, consisting of incomplete part and relatively complete counterpart;Paratype, Cat. No.108306—108310. Description: Cephalic shield smooth, strongly arched anteriorly and straight posteriorly, elongate and semi-elliptical in outline of dorsal view (Pl. Ⅱ,fig. 5; pl.Ⅳ, figs.3—5); posterolateral angles in the shape of short spines (Pl. Ⅲ, fig. 3; pl. Ⅳ, figs. 4, 5). Thunk documented by holotype (Pl, Ⅲ, figs, 1, 2) and one of the paratypes (Pl. Ⅳ, figs. 4, 5) possessing 10 tergites, with the maximum width at the first one, and becoming narrower (transverse) posteriorly; curvature of each tergite increasing posteriorly; sagittal length stable; lateral extremities of tergites spine-shaped and equally developed in the anterior 6 tergites, but successively elongated in the next three ones, with the 9th one being the longest (Pl. Ⅲ , figs. 1,3; pl. Ⅳ, figs. 1, 4, 5). Lateral extremities of the 10th tergite likely turning into short spines (Pl. Ⅲ, figs. 1, 3; pl. Ⅳ, figs. 4, 5). Trunk tergites, and the first tergite with cephalic shield, imbricating with each other; overlapping area known as articulating flange crecentshaped (Pl. Ⅲ, figs. 1, 2; text-fig. 3). Posterior margin of trunk tergites and cephalic shield sawtooth-shaped (Pl.Ⅲ, figs. 1, 2; text-fig. 3). Telson less ascertained in morphological features so far, and tentatively interpreted as composed of two fan-shaped plates disposed like mirror image with each other (Pl.Ⅲ, figs. 3, 4; Pl.Ⅳ, figs. 4, 5). Three specimens available for measurement with their length represented by the sagittal distance from the anterior margin of cephalic shield to the end of the telson. Holotype measuring about 35 mm long, and 11 mm across; specimen (Pl. Ⅲ, fig. 3) about 29 mm long and 11 mm across; while specimen (Pl. Ⅳ, figs. 4, 5)about 28mm long and 8mm across. Discussion: Text-figure 4 provides a broad outline of dorsal view restored on the basis of observations on the holotype and the paratype (Pl. Ⅳ, figs. 4, 5). The outline of the cephalic shield is interpreted as elongatedly semi-elliptical based on observation of the paratypes (Pl. Ⅲ, fig. 5; pl. Ⅳ, figs. 3—5).The concave right side of the cephalic shield appeared in the holotype (Pl. Ⅲ, fig. 2) is considered to have resulted from compaction. The exoskeleton of the animal was evidently convex, thus it lies at slightly different levels of the bedding planes with its lateral extremities of trunk tergites often buried by the underlying layers of matrix. Morphology of the lateral extremities of the 10th tergite and features of the telson are uncertain because of poor preservation. However, the Specimens (Pl. Ⅲ,figs. 3, 4; pl.Ⅳ, figs. 4, 5), indicate that the lateral extremities of the 10th tergite seem to be characteristic of short but not long spines. The part following the 10th tergite is interpreted as the telsons rather than the 11th trunk tergite because it consists of two separated fanshaped plates. Acanthomeridion serratum gen. et sp. nov. is broadly similar to Leancholia superlata Walcott, 1912 (Simonetta, 1970; Simonetta & Delle Cave, 1975; Bruton & Whittington, 1983) in its elongated cephalic shield and outline of each trunk tergite, But it differs from the latter in the semi-elliptical rather than subtriangular cephalic shield without the up-turned snout in front, the trunk tergites with longer lateral extremities, the serrated poste- riol margin of the tergites and the two fan-shaped telsons. Genus Urokodia gen. nov. Type species: Urokodia aequalis gen. et sp.nov. Etymology: Uro- and kodia, Greek, telson and cephalon; aequalis; Latin, equal; referring to the subequal cephalic and tail shields in size and outline. Diagnosis: Exoskeleton elongate, rectangular in outline, consisting of a cephalic shield, 14 trunk tergites and a tail shield. Cephalic and tail shields resembling each other in size and morphological feature, bearing one pair of anterior or posterior spines and three pairs of lateral spines respectively. Trunk tergites also resembling one another in sagittal length and in transverse width. Urokodia aequalis gen.et sp.nov. (Pl.Ⅴ, figs. 1—3., pl, Ⅵ, figs. 1—5; text-figs. 5—10) Material: Twelve specimens available for study including relatively complete but poorly preserved exoskeletons and separately preserved cephalic and tail shields, mostly compressed dorsolaterally (Pl. Ⅴ, figs. 1, 2; pl. Ⅵ, figs. 1—4), with a few compressed dorso-ventrally (Pl. Ⅴ, fig. 3; pl.Ⅵ, fig. 5). Holotype, Cat. No. 108311; Paratype, Cat.No. 108312—108318. Description: Cephalic shield subsquare in darsal view, with one pair of anterior spines and three pairs of lateral spines. Four pairs of ridges stretching antero-laterally from middle of the cephalic shield for the latter three pairs of ridges to join with three pairs of lateral spines. Holotype (Pl.Ⅴ, fig. 1; text-fig. 5) with trank consisting of 14 tergites, which resemble one another in their sagittal length and in transverse width (Pl.Ⅴ, figs. 2, 3). Imbricating area between tergites, and between the first tergite and cephalic shield, represented by a narrow transverse ridge about one-seventh as long as each tergite (sagittal) (Pl. Ⅴ, figs. 2, 3). Anterior and posterior margins of each tergite laterally converged to form short spines (Pl. Ⅴ, figs. 2, 3). Tail shield resembling cephalic shield in size and morphological features, with one pair of posterior spines and three pairs of lateral spines. Holotype measures about 32 mm long; specimen (Pl. Ⅵ, fig. 1) about 29 mm long and specimen (Pl. Ⅵ; fig. 2) about 26 mm long from the tip of anterior cephalic spine to the end of posterior tail spine. Discussion: The present fossil material suggests that all the relatively complete exoskeletons were dorso-laterally,instead of laterally or dorso-ventrally compressed. The cephalic shields of complete exoskeletons are recognizable on the basis of imbricating pattern, showing one pair of anterior spines and three pairs of lateral ones in dorso-lateral view (Pl.Ⅴ, fig. 1; pl. Ⅵ, figs. 1, 2; text-figs. 5—7). Morphological features of the tail shield are illustrated by the specimen compressed dorso-late-rally (Pl. Ⅵ. fig. 1; text-fig. 6), which bears a strong resemblance to the cephalic shield. Thus it is difficult to identify the distinction between the cephalic and tail shields if they were preserved separately from the trunk (Pl. Ⅵ, figs. 3—5). Text-figure 8 shows the outlines of the cephalic or tail shield in different directions of compression changing from lateral to dorso-ventral views. Here A is an interpretative drawing of lateral view; B shows an outline of dorso-lateral view, as documented by the specimens (Pl. Ⅴ, fig. 1; pl, Ⅵ, figs. 1,4; text-figs. 5, 6); C shows an outline of dorso-lateral view, but closer to the dorso-ventral compression than in text-figure 8b, as documented by the specimens (Pl. Ⅴ, fig. 2; pl. Ⅵ, fig. 3); and D shows an outline of dorsoventral view, as documented by the specimen (Pl. Ⅵ, fig. 5). The trunk consists of 14 tergites as documented by the holotype (Pl. Ⅴ, fig. 1; text-fig. 5). The ridge on the tail shield may have created a false impression of the tergite because of poor preservation, as shown in the specimen (Pl. Ⅵ, fig. 2; text-fig. 7). Morphological features of the animal are illustrated in dorso-lateral and dorso-ventral views(Text-figs. 9, 10). Restoration of the dorso- lateral view is based on observation of the dorsolaterally compressed specimens (Pl.Ⅴ, fig. 1; pl. Ⅵ, figs. 1, 2, 4); that of dorso-ventral view on observation of the specimens represented by the separately preserved trunk (Pl. Ⅴ, fig. 3) and the cephalic or tail shield (Pl. Ⅵ, fig. 5), which were dorso-ventrally compressed. Urokodia aequalis gen. et sp. nov. resembles Mollisonia symmetrica Walcott, 1912 (Simonetta, 1975) in the elongate exoskeleton and subequal cephalic and tail shields. But it differs from the latter in its cephalic shield carrying one pair of anterior spines and three pairs of lateral spines, and in its trunk possessing 14 rather than 7 tergites.
出处
《古生物学报》
CAS
CSCD
北大核心
1989年第1期42-57,共16页
Acta Palaeontologica Sinica
