在更新达尔文翼龙之前我们先来看看燕辽生物群的另一个地层——道虎沟“组”。
九龙山组以一套灰紫和灰绿色陆相碎屑及含火山碎屑沉积岩,包括砾岩、砂岩、粉砂岩、粘土岩,以及凝灰质砾岩、凝灰质砂岩和凝灰质粉砂岩。下与龙门组平行不整合接触,上与髫髻山组不整合接触。代表化石:瓣鳃类Ferganoconchacorta,介形虫Darwinulasarytimensis,侧羽叶、小异羽叶、石膜蕨型锥叶蕨。
费尔干蚌
中华异羽叶
但问题是,我们通常所指的“道虎沟组”,并不是真正意义上的道虎沟组(九龙山组)!
道虎沟组的规范说法应当是“Daohugou bed”而不是“Daohugou Formation”,而在这当中的所有物种,无一例外的现在在之前介绍的髫髻山组。
关于道虎沟“组”的年代问题:
He H., Wang X., Zhou Z., Zhu R., Jin F., Wang F., Ding X. & A. Boven: (^40)Ar/(^39)Ar dating of ignimbrite from Inner Mongolia, northeastern China, indicates a post-Middle Jurassic age for the overlying Daohugou Bed, Geophysical Research Letters 31, L20609 (whatever that means), 2004
The paper is short and concise, so I'll simply let it speak for itself:
"Since the name of the Daohugou Formation was not appropriately set up with a stratigraphic section, we would prefer to use the Daohugou Bed in this paper (Wang, 2000)."
"Some even went further to suggest that the Daohugou deposits could be correlated to the Middle Jurassic (Bathonian) Jiulongshan Formation (Ren et al., 2002)."
The age of that formation has been questioned, too (but only as a pers. comm. to the first author of the description of the Morrison salamander *Iridotriton*, and without a mention of what the actual age may be, other than "considerably younger"). It has yielded *Chunerpeton*, which was described as a cryptobranchid (not just -oid!) salamander... which may, rumor has it, not be correct either.
"The shale deposits obviously represent a later event than the ignimbrite; hence, the dating of the ignimbrite will provide a maximum age for the fossil-bearing sediments." The ignimbrite is 159.8 +- 0.8 Ma old. "Considering the vastly different isotope system behavior, this result is more or less concordant with the SHRIMP U-Pb zircon age" of 165.5 +- 1.5 Ma ago.
"The direct contact between the ignimbrite and the Daohugou sediments near the Daohugou village (119.22°E, 41.32°N) shows no baking of the sediments near the volcanic rocks, excluding the possibility of the ignimbrite having intruded later. Therefore, the overlying Daohugou Bed lake deposits clearly represent a geological event later than the ignimbrite."
"The Daohugou Biota is unambiguously different from that of the Jehol Biota despite the presence of the evidence of a close connection. As in the case of invertebrates and plants, the vertebrate assemblage of the Daohugou Biota shows more primitive appearance than the Jehol Biota. The Daohugou Biota is unambiguously different from that of the Jehol Biota despite the presence of the evidence of a close connection. As in the case of invertebrates and plants, the vertebrate assemblage of the Daohugou Biota shows more primitive appearance than the Jehol Biota."
"Despite the dating reported in this paper, the exact age of the Daohugou Bed in Inner Mongolia remains unsolved as there is yet no evidence showing how long the gap existed between the deposits and the volcanic base. It is, however, noteworthy that at the Daohugou locality, the fossil-bearing sediments are also found to overlie the Tuchengzi Formation in a few sites. The Tuchengzi Formation comprises gray, green to reddish sandstones, conglomerates and shales and is unconformably overlied by the Yixian Formation in many sites of Liaoning and its neighboring areas. Earlier (^40)Ar/(^30)Ar dating of the upper part of the Tuchengzi Formation in Beipiao, Liaoning Province gave a mean age of 139.4 +- 0.2 Ma (Swisher et al., 2002). Therefore, it appears that the Daohugou Bed is most likely younger than this age. Considering the abovementioned contacts and dating, as well as the primitive appearance of the Daohugou Biota compared to the typical Jehol Biota, we propose that the Daohugou Bed may represent the Early Cretaceous intermontaneous deposits between the Tuchengzi and the Yixian formations."
http://dml.cmnh.org/2005Sep/msg00329.html