Longfengshania的形态学特征和归属

MORPHOLOGY AND SYSTEMICS OF LONGFENGSHANIA

一、前言自从杜汝霖、李培菊(1980)第一次在燕山怀来龙凤山南坡长龙山组(距今约900-850百万年)发现Longfengshania sp.以来,陆续又在燕山地区河北宽城、丰润、古冶、兴隆、蓟县、昌平、怀来、涿鹿等青白口系11个化石产地(插图1),计17个化石点发现大量的该类宏观植物化石(杜汝霖,1982;Du and Tian,1985;杜汝霖、田立富,1985,1986)。

The genus Longfengskania was first found by Du Ru-lin and Li Pei-ju, generally round, elliptical, heart-shaped, bean sprout-shaped or fan-shaped in appearance, mainly characterized by a stalk-like projection on the foliate's fringes. The fossils were collected from the fine-sandy shales and fine-sandy claystones in the second member of the Changlongshan Formation in Longfengshan Mountain of Huailai, Hebei which ranges from 850 to 900 Ma BP in age.This genus was established by Du Ru-lin (1982), occurring in the basal silty clay rocks in the second member of the Changlongshan Formation of the Qingbaikou System on the southern slope of the Longfengshan Mountain, northwestern Hualai.Du Rulin and Tian Lifu (1985) discussed the fossils collected from the midlower to basal greyish-green to greyish-black illitic shales of the second member of the Changlongshan Subformation in Xinglong, Hebei. They suggested that the fossil bodies of Longfengshania are composed of the latter with rhizoidal fixed organs at their bases- Such differentiation of the algal body provides an important evidence for the multicellular organism, marking the evolution of the primary metaphyte, algae. Based on the forms and contacts of their foliate and sub-leafstalk, the fossils were identified as a single genus, Longfengshania. Du and Tian (1985) also discussed the Longfengshania from the Qingbaikou System in the Yanshan Mountain Area of North China, and considered that the genus represents the fossils of marine sessile multicellular Metaphyta with possible relationships to the Phaeophyta and Chlorophyta, providing remarkable evidence for the evolution of higher algae in the late Precambrian and that these fossils are possibly the earliest benthonic algal macrofossils in the world. Longfengshania exhibits clear differentiation of organs (foliate and parastem), in addition, a rhizome (organ for fixation) is also observable at the base of some of the parastems. Hofmann (1985) described the Longfengshania exhibiting subradial arrangement and superposition from the Mid-Proterozoic Little Dal Group in the Mackenzie Mountains, Northwest Canada, and suggested that the genus was possibly an alga of Phaeophyte or Rhodophyte affinities.Du and Tian (1986) summed up the macroalgal fossils of the Qingbaikou Period in the Yanshan Range, and raised the Longfengshania to a family level. Longfengshaniaceae Duet Tian contain two genera, Longfengshania and Paralongfengshania; the latter genus differs in the contraction in the middle of foliate.By comparing the morphology of Longfengshania with that of early fossil bryophytes, lower Devonian fossil Sporogonites, Zhang Zhong-ying proposed a new interpretation that Longfengshania probably represents sporophytes of bryophytic affinities consisting of a slender sporangiophore (seta) with a terminal sporangium (capsule), and an occasionally preserved foot at its base. The common carbonaceous mass in Canadian specimens might be interpreted as representing the remainder from the gametophyte of bryophytes. He considered the Longfengshania as evolved from aquatic ancestors, probably from the Chlorophyta (or Phaeophyta or Rhodophyta), representing the transition from aquatic to terrestrial in habitat. Since 1979, the authors have observed all the specimens of Longfengshania from the Qingbaikou Period in the Yanshan area time and again. In morphology, Longfengshania should have been a thallophyte with three parts, vesicular body (pressed as foliate) and holdfast (rhizoid). The common carbonaceous mass in Canadian specimens (Hofmann, 1985, p1. 38, fig. 4, text-fig. 5) might be interpreted as representing a branch of the stalk base (or from the holdfast) of thallus, instead of a remainder from the gametophyte of bryophyte, because the branch is very distinct in preservation.The authors consider that Longfengshania should not be attributed to the Hepaticeae of Bryophyta, because there are still a lot of problems to be discussed. The life cycle of a bryophyte consists of a regular alternation of a sporophytic generation with a gametophytic generation. The gametophytes are developing and independent, but the sporophytes are all strictly limited in growth and wholly parasitic upon the gametophyte by foot which differs from the holdfast for fixation. Since the Longfengshania is a bryophyte, why couldn't its gametophytic fossil be preserved? The diameter of vesicular bodies in Longfengshania may be up to 10 mm, and that of capsules (special sporangia of Hepaticeae) is usually smaller than 1.5mm; the capsules have more complex structures and various shapes with special splitting function and characteristic tetrad spores, which should have been preserved. Although these structures could not be all preserved distinctly, yet some of their traces should be visible. In Longfengshania, the holdfasts for fixation on substrata differ from the feet of Hepaticeae in which they insert themselves into the gametophyte. The stalk (stem)of Longfengshania exhibits various winding shapes and lengths in close relationship with the depth and motive force of the water in their living environments. On the other hand, mineralogial analysis of fossiliferous rocks reveals that they are chiefly composed of illitic shale with organic matter, with a subsidiary amount of kaolinite, sericite, pyrite and quartz. The macrofossils are well-preserved in association with some other probably sessile organisms and trace fossils. The habitat of such fossils is inferred to have been a restricted, relatively quiet, weakly reducing, warm, tidal flat of marine facies which differs from the habitat of Hepeticeae belonging to terrestrial facies. Therefore, the authors suggest that the Longfengshania is a multicellular macroalga, a protometaphyte with simple tissue bodies which may be a group, meta-algae or a division of Metaphycophyta, with Longfengshaniales or two genera Longfengshania and Paralongfengshania.The discovery of Longfengshania in the late Precambrian implies the appearance of a large number of metaphytes, a macroalga probably much earlier than 900 Ma BP. This is an important event in organic evolution on the Earth, with even greater significance than the discovery of the Ediacaran animal population in Australia.