山东中新世山旺组紫荆属(豆科)叶化石的叶枕研究——兼论豆科植物叶枕的早期演化

提要叶枕作为叶活动的"马达器官",在豆科植物的分类、演化、发育和生理等方面都具有十分重要的研究意义,但在古植物学中似乎研究甚少。基于产自山东省临朐县中新世山旺组的叶印痕化石,结合现生紫荆叶结构和叶枕的形态观察,文中着重研究这些叶印痕化石中的叶枕形态,发现它们保存了倒卵形的上叶枕印痕和上叶枕组织腐烂分解后留下的半圆形印痕,其中一块还保存了卵形的下叶枕印痕。结合这些化石的叶结构特征,进一步确认了中新世山旺组紫荆属叶化石——华紫荆的存在,并指定一份保存于中国科学院南京地质古生物研究所的标本为后选模式,即No.39(H)。紫荆属隶属于云实亚科、紫荆族,为现存豆科植物分子系统发育树上最基部的类群之一,因此该属的化石历史对认识豆科植物的系统发育、早期演化和分异以及生物地理历史都至关重要。然而,由于过去缺乏详细的叶结构和叶枕研究,大多数以往报道的产自欧亚大陆和北美新生代的紫荆属叶印痕化石种都受到质疑、否定或亟需进一步研究。迄今为止,紫荆属最早的、具叶枕的叶化石发现于美国俄勒冈州晚始新世的约翰迪组,而最早的豆科叶枕则发现于阿根廷巴塔哥尼亚早古新世萨拉曼卡组中的一种可能属于含羞草亚科植物的小叶中。结合近来的比较形态解剖学研究结果,作者赞同先前的假说——紫荆属的单叶和叶枕类型实质上都是衍生的性状,而最原始的豆科植物长着羽状复叶,其复叶叶柄和小叶叶柄上可能都有各自的叶枕。

PULVINI OF CERCIS LEAVES FROM THE MIOCENE SHANWANG FORMATION OF SHANDONG PROVINCE AND THE EARLY EVOLUTION OF THE PULVINUS IN LEGUMINOSAE

Pulvini serving as ’motor organs’ for leaf movement play a very important role in the systematics,evolution,development,and physiology of the Leguminosae:the third largest group of flowering plants.Based on morphological observations of the leaf architecture and pulvini of extant redbuds(Cercis),this paper focused on pulvinus morphology in leaf impressions(Cercis miochinensis Hu and Chaney,1938) from the Miocene Shanwang Formation of Shandong Province.The fossils possess obovate upper pulvinus impressions which become semi-circular when degraded,as well as an ovate lower pulvinus impression in one of the specimens.Cercis belongs to the tribe Cercideae of the subfamily Caesalpinioideae,which is one of the basalmost clades in the molecular phylogenetic trees of living Leguminosae.Therefore the fossil history of Cercis is of fundamental importance in understanding legume phylogeny,particularly their early evolution,diversification and biogeography.However most species of the previously reported Cercis leaf impressions from the Cenozoic of Eurasia and North America have been questioned,rejected,or else need re-investigation because of a lack of sufficiently detailed studies on the leaf architecture and pulvini.So far,the earliest pulvinate leaves of Cercis have been discovered from the late Eocene of Oregon in the U.S.A.,but the oldest legume pulvinus has been reported in a leaflet possibly assigned to the subfamily Mimosoideae from the early Palaeocene of Central Patagonia in Argentina.Together with the results from comparative morphology and anatomy,the present study supports the view that the unifoliolate blade and pulvinus in the Cercis leaf are derived characters,and that the most primitive legumes had pinnately compound leaves probably with pulvini on each of their own petioles and petiolules(Text-fig.3).A lectotype of Cercis miochinensis is here designated:specimen No.39(H) deposited in the collections of Nanjing Institute of Geology and Palaeontology,Chinese Academy of Sciences.